aspects of marine geoscience: a review and thoughts on...

Phil. Trans. R. Soc. A (2012) 370, 5567–5612doi:10.1098/rsta.2012.0395

Aspects of marine geoscience: a review andthoughts on potential for observing active

processes and progress through collaborationbetween the ocean sciences

BY NEIL C. MITCHELL*

School of Earth, Atmospheric and Environmental Sciences, University ofManchester, Oxford Road, Manchester M13 9PL, UK

Much progress has been made in the UK in characterizing the internal structures of majorphysiographic features in the oceans and in developing understanding of the geologicalprocesses that have created or shaped them. UK researchers have authored articles of highimpact in all areas described here. In contrast to terrestrial geoscience, however, therehave been few instrumented observations made of active processes by UK scientists. Thisis an area that could be developed over the next decades in the UK. Research on activeprocesses has the potential ability to engage the wider public: Some active processespresent significant geo-hazards to populations and offshore infrastructure that requiremonitoring and there could be commercial applications of technological developmentsneeded for science. Some of the suggestions could involve studies in shallow coastal waterswhere ship costs are much reduced, addressing tighter funding constraints over the nearterm. The possibilities of measuring aspects of volcanic eruptions, flowing lava, turbiditycurrents and mass movements (landslides) are discussed. A further area of potentialdevelopment is in greater collaboration between the ocean sciences. For example, it is wellknown in terrestrial geomorphology that biological agents are important in modulatingerosion and the transport of sediments, ultimately affecting the shape of the Earth’ssurface in various ways. The analogous effect of biology on large-scale geomorphologyin the oceans is also known but remains poorly quantified. Physical oceanographicmodels are becoming increasingly accurate and could be used to study further thepatterns of erosion, particle transport and deposition in the oceans. Marine geological andgeophysical data could in turn be useful for further verification of such models. Adaptingthem to conditions of past oceans could address the shorter-period movements, such asdue to internal waves and tides, which have been barely addressed in palaeoceanography.

Keywords: mid-ocean ridge; sediment transport; continental rifting; continental margin;subduction zone; volcanic eruption

1. UK progress of the last 20 years

The wide scope of research in marine geosciences prevents a comprehensivereview. The following will therefore undoubtedly miss out many important*[email protected]

One contribution of 11 to a Theme Issue ‘Prospectus for UK marine science’.

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results and still be superficial, given the range of topics mentioned. However,it should at least illustrate the manner in which geoscientists have addressedgeological features or systems under the oceans. The science has been pursuedby a combination of geophysical methods and drilling to unravel internalstructure, whereas imaging with sonars and other shallow geophysical methodshas been used to study surface geomorphology, augmented with submersibleobservations and sampling. Geochemical and other studies of recovered sampleshave then unravelled aspects of geological processes. UK researchers have madesignificant contributions across a broad range of subject and global geographicalareas, and the following is written largely with an unashamedly UK focus.Funding has been provided by both individual project research grants from theNatural Environment Research Council (NERC) and other organizations and asefforts within major programmes. The latter have been provided by the NERC(BRIDGE, BOFS, MARGINS/LINK, NEAPACC, LOIS/SES, RAPID) and EUMAST (OMEX, OMEX-II, ENAM and STEAM). UK scientists have also beeninvolved in the Ocean Drilling Program (ODP) and Integrated Ocean DrillingProgram (IODP).

Through knowledge gained and training of PhDs, this work has underpinnedmany activities important to the UK economy and society, such as oil and gasfield exploration and development, delineation of Exclusive Economic Zones andidentification of geological hazards. Interested readers may also wish to read theIOC report [1] written before this period, anticipating the developments in marinegeosciences towards the year 2000. Looking back on the words presented therein,UK researchers have strongly progressed many of the areas anticipated at thattime.

Two key developments occurring over this period have greatly influenced thegeosciences and deserve mentioning at the start here. First, the GEOS-3 andSEASAT radar altimeters, followed, during the Clinton era, by release of datafrom GEOSAT and later satellite altimeters, revealed the detailed form of theocean’s surface. As the ocean’s surface is nearly an equipotential (water tendsto flow to its lowest level of potential energy), the measurements can be usedto derive the gravity field [2,3]. The largest density contrast influencing this fieldover 10–100 km spatial wavelengths is the rock–water interface; so the field can beused to interpolate its topography (bathymetry) between ship soundings, givencertain assumptions concerning the state of crustal isostasy. Although a form ofthe marine gravity field had been recovered earlier by this technique [4], the newGEOSAT data cover the oceans more nearly continuously, so that the gravityand bathymetry maps produced from this effort [3] reveal the physiography inmuch greater detail. This has transformed the way in which geoscientists work inthe oceans. Whereas previously the ocean’s physiography would be worked outpainstakingly from widely spaced echo soundings, the new data reveal features(such as the transform valleys, important for validating and applying the sea-floorspreading hypothesis) immediately. The context of any work in the deep oceansis often now not a major issue.

Second, a series of seabed imaging and bathymetry measuring sonars becamemore widely available, carrying the knowledge of physiography to a finer scale. Inthe UK, the community had access initially to the reconnaissance GLORIA [5]and later to the deeply towed TOBI [6] sidescan (seabed imaging) sonars. Overthe last two decades, the multibeam echo sounders, which became available

Phil. Trans. R. Soc. A (2012)



Marine geoscience: active processes 5569

to academic research in the late 1970s [7], were developed into more accuratesystems with many more beams, wider swaths and with acoustic backscatteringmeasurements [8]. In the UK, RRS Charles Darwin, RRS James Clark Ross andmore recently RRS James Cook were equipped with these systems, and severalUK groups have acquired smaller multibeam echo sounders for work in shelfand coastal seas. As with the satellite altimeters, deployment of this technologyhas enabled the context of any work on the ocean floor to be known in detail,facilitating better interpretation of many other kinds of acquired data, and greatlyhelping the planning of equipment deployments.

(a) Mid-ocean ridges

Mid-ocean ridge research has been funded over the past 20 years withseveral national programmes besides BRIDGE in the UK (RIDGE and its laterreincarnations in the USA, DORSALES in France and DeRIDGE in Germany,among others) as well as efforts under ODP and IODP. The geophysical effortin the UK included imaging of the magma lens underlying the East Pacific Riseand associated structure in two- and three-dimensional seismic reflection data[9–12], and similar efforts on other ridges [13,14]. Electromagnetic experimentshave revealed the low resistivity of the fractured and porous uppermost crust andconfirmed the limited across-ridge extent of the axial magma lens [15]. Freezingof magma in the lens and intrusion of dykes above it leave a transition fromgabbros in the lower crust, representing the former, to sheeted dykes above them.This transition was finally sampled by drilling at IODP Site 1256D [16]. Seismicrefraction experiments have been used to map out the velocity structure andthickness of oceanic crust [17–25]. Some of those data suggest that oceanic crustproduced at ridges of extremely slow spreading rate is anomalously thin, owingto reduced melting of a colder mantle beneath the ridge.

Oceanic sea floor formed at slow-spreading ridges has a more rugged, faultedmorphology than that produced at fast-spreading ridges, a result ultimately ofthe cooler thermal and consequently stronger mechanical structures of slow-ridgelithosphere. Generally, there is little evidence for persistent magma chambers,although a mid-crustal seismic reflector and associated low electrical resistivityhas been found at one site on the Reykjanes Ridge, a section of the slow-spreadingMid-Atlantic Ridge [26,27].

On the flanks of the Reykjanes Ridge, ridges and troughs run sub-parallelto the ridge but angled towards the ridge axis, forming giant south-pointingV-shapes, also containing an unusual superimposed geomorphology of smalleren échelon volcanic ridges [28,29]. The Reykjanes Ridge shallows systematicallyover a distance of approximately 1000 km northwards to the Reykjanes Peninsula.Modelling of seismic refraction and gravity data has shown that the oceanic crustalso thickens northwards along the Reykjanes Ridge and beneath the off-axisV-shaped and other ridges of the region [24,25,30–32]. Along with informationfrom basalt geochemistry [33], these trends can be explained by enhanced meltingin the underlying mantle, which increases towards Iceland, whereas the off-axisridges have been produced by pulses of varying temperature that propagated awayfrom Iceland, influencing extents of melting beneath the Reykjanes Ridge axisand ultimately the thickness of crust produced. (Those pulses also correlate withchanges in ocean circulation recorded in North Atlantic sediment drifts, reflecting




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how ocean discharge between Greenland and Iceland has been modulated by theplume [34].) Variations in mantle temperature have also been hypothesized tovary the gross morphology of spreading ridges because of the effect of crustalthickness on the lithosphere’s mechanical properties [35].

Deployments of deeply towed seabed imaging (‘sidescan’) sonars have givenus detailed views of mid-ocean ridge terrains that are like aerial photographs,allowing basic geological mapping [36–38]. Along with magnetic anomaliesrecording the varied ages (from past magnetic field polarity) and magnetizationstructures of the volcanic materials, it is possible to work out an order of geologicalevents [36,39]. In areas of extreme pelagic productivity and sedimentation, butweak bottom currents, varied sediment cover revealed in deeply towed profilerrecords can also record sequences of lava flow emplacements [40].

Although an earlier geological model for slow-spreading ridges involved a simplegraben with steep border faults [41], the new sonar and geological sample datasetshave shown that some faults occur with large displacements (greater than tensof kilometres), exposing lower crustal and mantle rocks on the sea floor [42–48].The sidescan and multibeam sonar data, supplemented with field observationsfrom ophiolites (parts of oceanic crust that have been uplifted above sea level),have also allowed us to develop ideas on how the volcanic crust develops fromindividual volcanic eruptions [49–52].

Work on the geochemistry of rock samples has included using mid-ocean ridgesto sample the Earth’s underlying mantle. For example, whereas lava sodiumand iron concentrations, after correcting for fractionation effects during theirresidence in magma chambers, had previously been suggested to represent theextent and pressure of mantle melting [53], Niu and co-workers [54,55] updatedthat view, arguing that mantle composition exerts a more dominating influence.Also investigated have been how rare-earth elements reflect melting extent [56,57]and how other elements or isotopes mark distinct mantle provinces [58–61]. Atvery slow spreading rates, conductive cooling of the upper mantle reduces theamount of melting, leading to a thinner crust and distinct geochemistry [19,57,62].

Other work on seabed samples, cores from the scientific drilling programmesand outcrops on land analogous to oceanic crust (ophiolites) have addressedissues surrounding how those melts accumulate within the crust and subsequentlyintrude as dykes and sills, erupt at the seabed or solidify, releasing or interactingwith aqueous fluids as they do so. At fast-spreading ridges, the steady-statemagma chamber existing almost continuously along the ridge [9,10] presents aproblem of how the melt within the magma chamber solidifies to form the gabbroiclower crust while simultaneously allowing heat from below to escape [63–67].Calcium diffusion profiles between olivine and clinopyroxene suggest that coolingrates are orders of magnitude smaller in the lower crust than immediately belowthe magma chamber [68]. Areas where the crust has been exposed by deep riftinghave provided deep sections where the relationships between different crustalcomponents can be assessed, for example, where East Pacific Rise crust has beenrifted at Hess Deep [69].

The heat flux associated with volcanic centres drives hydrothermal circulationsthrough the crust and associated hot springs, as well as more abrupt heat infusionsto the oceans during eruptions [70]. Studies of fluids and sediments along withgeochemical modelling have addressed the interaction of aqueous fluids withthe rocks above magma chambers, hydration reactions during serpentinization





of peridotite and how expelled fluids influence the elemental balance of theoceans [63,71–74]. Rare-earth and other elements in hydrothermal sediments canrecord the chemistry of the circulating hydrothermal fluids or overlying oceanwaters [75,76]. Other studies have characterized the processes occurring withinthe exhaled plumes from the geochemical properties of particles within them andplume structure [77–81].

(b) Volcanic islands

Surveying with sidescan and multibeam sonars around the submarine slopesof volcanic islands has revealed widespread deposits of giant landslides [82,83].Before such data became available, the interpretation of their characteristicheadwall and sidewall escarpments on land was not so straightforward(escarpments could alternatively be caused by caldera collapse or coastal erosion,which is typically extreme around island coasts exposed to ocean waves). Thesonar data, however, resolved the issue conclusively and have allowed systematicstudy of their abundance and morphology suggesting aspects of emplacementprocesses. UK efforts have mainly concentrated on the Canary Islands [84–90], buthave also extended to the Cape Verde Islands [91], Stromboli [92], South SandwichIslands [93] and Lesser Antilles [94–96]. The hazards implications, includingpotential tsunami generation, have also been extensively debated [97,98].

Some ideas that have been put forward for the origins of oceanic volcanoflank collapse are given in the above publications and are summarized byKeating & McGuire [99]. The timing of collapse events correlates somewhat withglacial-induced eustatic sea-level change [100]. Furthermore, a transition occursfrom small oceanic volcanoes, which rarely show evidence for flank collapse,and those taller than 2500 m, where collapse structures are common [101,102].This observation hints that edifice permeability structure and pore pressurescould be important for initiating failure, because materials become compactedby loading in the larger edifices and many such edifices grew near or abovesea level, generating copious hyaloclastites, which also become less permeablewhen compacted. Unfortunately, as the original conditions leading up tofailure are poorly known, the wide range of suggested causes and triggeringmechanisms [99] are difficult to evaluate, though some progress has beenmade [103–105].

Surf erosion of coasts produces a coastal rock platform, whose shape can bemodelled by the effect of wave force varying over successive sea-level cycles [106].The platform can potentially form a useful reference surface for assessing stabilityof a volcanic edifice. For example, a slump identified on the south side of Topovolcano on Pico Island, Azores, is not associated with seismicity [107], but theapparent lack of movement could represent a long recurrence interval betweenevents. Geophysical data show that a lack of relief on normal faults continuedoffshore from the slump on land onto the rock platform, suggesting that thisfeature has been largely immobile over the Holocene since last modified by surfduring postglacial sea-level rise [108].

The compositions of the submarine parts of volcanic islands are still poorlyknown. Besides their clastic components formed from erosion and lava–waterinteractions, emplaced lava could originate either from direct eruption from dykesor from lava flows and tubes carrying lava from land [109]. Coastal multibeam




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sonars have been useful for revealing how Holocene lavas flowing from land havebehaved as they entered the sea, apparently spreading out in intricate dendriticpatterns of lava lobes [110].

Some volcanic islands provide extreme weights that distort the underlyingoceanic lithosphere. Seismic reflection as well as refraction and gravity data,coupled with mechanical modelling, have been used to estimate the effectiverigidity of the lithosphere from the resulting deflected sediments and oceaniccrust [111–119]. Compiling such results, Watts & Zhong [117] argued how theoceanic lithosphere deforms as a visco-elastic material. Such data have alsobeen used to address lithospheric uplift due to the buoyancy of underlyinglow-density mantle [120,121]. Seismic refraction experiments have furthermorerevealed seismic velocity anomalies associated with volcanic bodies within theedifices [94,95]. Over broader scales, techniques have been developed to isolateseamounts from echo-sounding data along ship tracks [122], and those results havehelped reveal how the oceanic lithosphere has locally subsided as it cooled [123].The deflection of the lithosphere caused by the weight of seamounts produces anobservable effect in the gravity field, which is detectable from satellite altimetrymeasurements. As the lithosphere’s rigidity varies systematically with age, thishas been used to date the original volcanism and thus study the history of volcanicemplacements across ocean basins [124–126]. Large igneous provinces are (in theoceans) regions of thickened crust that were produced by very large-scale volcanicextrusions generally in the Cretaceous; these features have also been extensivelyaddressed with geophysics and rock sample analysis [127].

(c) Oceanic trenches/active margins

UK efforts have addressed the dynamics and kinematics of parts of variouskinds of plate subduction systems (both island arcs and continental arcs).These have included the Barbados accretionary prism [128–130], Central andSouth America [131], the South Sandwich Arc [93,132,133], the MediterraneanRidge [134], an accretionary prism west of the Gibraltar Straits [135], Japan [136–138], Indonesia [139,140] and Timor [141]. The changing physiography associatedwith development of subduction zones can significantly affect ocean circulationand thus climate [142]. The emergence of a closed circulation around the Antarcticcontinent, which has been suggested to have isolated Antarctica and led to theonset of the Neogene and Quaternary ice ages, has been linked to the opening ofthe Drake Passage [143,144].

The surface topographic gradients of accretionary wedges (the inclined pile ofmaterial built up above the subducting plate) can be remarkably small, implyinga low effective coefficient of friction of the deforming wedge material. There hasbeen some debate over whether this arises from elevated fluid pressures, of whichthere is evidence in seismic velocities, elevated heat flow from the depths of gashydrates (bottom-simulating seismic reflectors) and mud diapirism [129,145,146],or from low friction coefficients of the prism clay minerals [147].

The offshore source regions of the tsunamis generated by the giant Sumatran2004–2005 earthquakes have been surveyed with swath sonar [139]. The unusualrock mechanical properties at the décollement there implied by seismic reflectioncharacteristics may have been responsible for the large 2004 event [140]. Otherlarge earthquakes have been studied with regional geophysical data. For example,





the 2001 Peru earthquake rupture appears to have been modulated by asubducting fracture zone [148]. By contrast, seismic, bathymetry and geologicaldata from the source of the 1998 Papua New Guinea tsunami have beeninterpreted as showing evidence that a slump movement initiated the surfacewave there [149,150].

(d) Passive continental margin structure

Given the lack of space herein, the following text concentrates on activitieswhere UK research scientists have acquired data. However, significant parts ofpassive margin work are carried out commercially in the course of petroleumexploration, most of which does not appear in the open literature. Severalgroups within the UK have benefited from access to industry geophysical andcore datasets. Industry has also played an important role in the development ofgeophysical (particularly seismic) techniques and down-hole measurements.

Efforts to characterize the structure and mechanical behaviour of thelithosphere as continental margins develop from continental rifting to the laterlithospheric thermal subsidence (‘post-rift’) stage have become focused on pairsof continental margins that were originally formed at a common rift (so-calledconjugate margins). Such studies are needed to resolve two well-debated end-member models of continental break-up [151], which involve uniform stretchingof the crust and lithospheric mantle by extension on faults in the crust andductile thinning of the upper mantle (pure shear model [152]) and separationof the two halves of the continent along an inclined shear zone through thecrust and upper mantle (simple shear model [153]). Various forms of the latterhave been proposed to explain, for example, the distribution of lithologies andgeophysical characteristics of the Iberian–Newfoundland conjugate margin pair[154–158], the Rockall Plateau–Hatton Bank conjugate margin to Greenland [159]and the Amazon conjugate margin to Liberia [160]. In terms of UK efforts,the Iberian margin is arguably the best-characterized continental margin, withseismic, magnetic, gravity and scientific drilling data [161–170].

The temperature structure of the mantle is considered to influence thelithosphere’s mechanical behaviour and volcanism associated with rifting. Incontrast to the cool, magma-poor Iberian margin, the Seychelles–India pairhas been studied as a hotter, more magma-rich end-member [171–173]. Seismicrefraction data from the northwest European and Greenland margin pair revealevidence for lower crustal intrusions of magma in an area that was also ofelevated mantle temperature influenced by the Icelandic plume [159]. Othermargins have been studied with seismic and gravity data in the west of theBritish Isles [174], at Goban Spur [175], Edoras Bank [176], Rockall Trough [177],the Eastern Black Sea [178] and the Arctic [179]. Some margins were created bylateral movements of tectonic plates. These so-called transform margins have alsobeen investigated with seismic and gravity methods [180–183]. Complementingthe work on field datasets, numerical modelling has been applied to developunderstanding of the mechanical behaviour of the lithosphere during rifting andlater subsidence [160,184–188]. The structural evolution of young rift basinsprovides further clues to how continental margins develop. UK researchers havecollected and interpreted geophysical datasets from the Gulf of Corinth [189,190],west Iceland rift [191], Sea of Marmara [192] and Whakatane Graben, NewZealand shelf [193,194].




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(e) Shelf seas

The sediments and seabed morphology around the UK originate mainly fromthe Quaternary glaciations and, in shelf waters, the glacial sediments are variablymobilized by tidal currents and surface waves. Around parts of North Walesand Scotland, the new multibeam bathymetry, boomer and industry three-dimensional seismic reflection data collected over the past 20 years have improvedour knowledge of the extent, flow pattern and chronology of ice streaming from theBritish Ice Sheet [195–199], glacial tunnel valleys [200,201] and fluvial erosion bygiant glacial floods [202]. Modelling constrained by observations has shown howthe tidal currents and waves have changed as relative sea level generally rose inthe post-glacial period [203–206]. During that period of rising sea level, surfacewaves would have caused significantly greater bed shear stress than today onmany parts of the shelf [204]. This, and different tidal currents, may explain theexistence and asymmetry of some earlier-formed bedforms that are inconsistentwith modern conditions [207,208].

Our ability to model the modern transport of sediment on the shelf bycurrents and waves has been improving [203,204,209–212] so that we have a betterpotential to predict how the seabed will change with changing wind, sea leveland wave climate as well as response to man-made installations or commercialextraction of sediments. Experiments have better resolved the influences onthreshold of motion of sediments [213–217]. Attempts have been made toreconstruct sediment movements from grain size (sorting) trends [218,219].Recent observation and modelling effort has concerned understanding particlemovements in estuaries [219–224].

Much of bedload sediment transport occurs by bedform migration. Seismic dataand surveying with sonars (some of which has been repeated so that movementscan be quantified) are increasingly revealing the patterns of bedform dimensionsand their rates of migration [225–227]. Repeat measurements have shown thata surprising proportion of bedforms migrate in the direction opposite to thatexpected from their sense of asymmetry (migrating to their lee sides [227]),suggesting that much work needs to be done on the dynamics of bedforms frominstrumented observations of particles and currents, which are still relativelylimited [228–233].

(f ) Continental slopes and gravity-driven sediment transport

Sediments deposited on the northwest UK continental slope show variousmass-movement features [234–238]. The sediments there are variably affected bycontour-following (geostrophic) currents [239–241]. Periods in the past when thecurrents were anomalously fast have left layers of coarser particles (lags), whichmay form planes of weakness responsible for some of the large landslides in thenorthwest UK slope [236].

On other slopes away from the UK, researchers have been interestedin morphologies and origins of mass movements or landslides [86,242–246].Coasts around the North Atlantic, including Scotland, were inundated bytsunamis generated by the massive Storegga Slide off Norway about 8000 yearsago [247,248], a landslide of area similar to Scotland [249]. Given that the Atlanticsuffers fewer earthquake-generated tsunamis than the Pacific Ocean, and hencethere is little societal experience of tsunamis around the Atlantic, there may be an





10 km

Figure 1. Three-dimensional view of multibeam sonar data from the continental slope off the USAtlantic coast (from data shown in Mitchell [257] originally provided by NOAA). Depths in theimage vary from 1700 m (green) to 100 m (pink).

under-appreciation by the general public and policy makers of the potential threatthat they pose, but the possibility of submarine landslides setting off tsunamismay be a significant hazard [250].

Landslides in continental slopes can be much larger than those on land, aresult of the physiography providing much larger slopes of unstable materialin general and because sediment accumulation on the slopes has left layeredstructures prone to failure along strata [251]. Beyond the latter general statement,the origins of slope failure are still controversial—in particular, the role ofelevated pore pressures [249]. Earthquakes are often suggested to be a cause offailure, but their role is difficult to test, except in cases where sediment flowson otherwise unconnected slope areas can be shown to be coincident, implyinga common triggering event [252]. Few examples of these have been documentedin the literature. Dissociation of gas hydrates has been suggested to lead to highpore pressures and reduced sediment strength leading to failure, and indeed amodal depth of landslide headwalls occurs at 1000–1300 m in the North Atlantic,consistent with such an idea [242]. (Such dissociation has been a concern asa source of potential greenhouse gas emissions, as failure could release largequantities of methane into the overlying water [253].) However, thermal modellingof the Storegga Slide suggests that the gas hydrate stability depth would havelain below the slide plane immediately before failure [254]; so the role of hydratesis still unresolved.

There has been revived interest in the geomorphology of slopes, promptedpartly by the expansion in quantitative terrestrial geomorphology. Althoughthe processes by which sediments are transported and deposited in the oceansare different from those in terrestrial environments, and although continentalslopes are generally aggrading landscapes over long time scales associated withprogradation of the margins, sonar images of continental slopes can be remarkablysimilar to those of eroding terrestrial landscapes [255,256]. An example fromthe US central Atlantic coast is shown in figure 1. This has motivatedefforts to characterize submarine slopes in terms of similar geometrical parametersto those used in terrestrial geomorphology as a step towards understandingthe processes shaping these terrains. For example, slope canyons can havesimilar long-profile geometries, confluence structures, knickpoint morphologiesand channel branching structures [257–261]. Furthermore, the regions betweencanyons can have rounded forms, much like terrestrial hillslopes undergoing soil




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creep. Where the uppermost slope areas are sandy and have been strongly affectedby oscillating currents, such as beneath waves and tides during glacial conditionsof lowered sea level, the gravity effect on bedload particles could potentiallyproduce a ‘diffusive-like’ evolution of the seabed as on land [262,263].

Fans of thick sediment lying beyond continental slope channels and canyons areformed of the deposits of sedimentary flows. This is an important area of interestin petroleum geology. When first mapped with sonars and seismic systems, theirsurface channels appeared superficially similar to river systems, with braided ormeandering channels, commonly with levées [256,264]. Their sinuosities vary withchannel gradient somewhat like meandering rivers [265]. However, in detail theyare not identical, suggesting differences in flow and depositional processes [266].For example, the channel and its levées are commonly super-elevated abovethe surrounding fan, and the stacking patterns of successive channels can bedifferent, suggesting that they tend not to meander laterally as much as rivers.Peakall et al. [267] proposed that the latter arises from, among other effects,flow stripping of turbidity currents (overtopping the levées) changing the flowpressure distribution and leading to a reversal in the helical secondary circulation,in turn leading to deposition on the outside of bends rather than erosion astypical in rivers. Turbidity currents on the fans were originally considered tooriginate from dilution of debris flows, themselves perhaps originating from slopefailure. However, detailed fieldwork on land and in the oceans by correlatingbeds across large distances has shown that the opposite can also happen, withdensification of turbidity currents or embedded debris flow layers leading todepositing denser flows such as debris flows [268,269]. The high density of debrisflows may also explain how some large boulders can be carried as far as the distalparts of fans [270]. These and many other issues have been addressed by UKgroups with laboratory and numerical simulation of sedimentary flows [271–276].The combination of marine fieldwork and theoretical work is a UK strength inthis area.

(g) Pelagic sedimentation

Many of us may have started off as undergraduates with an idea of the deepocean filling very slowly with pelagic sediments (the particles formed mainly ofskeletons of pelagic organisms) accumulating on the seabed like a simple drapingblanket, falling there as ‘marine snow’ from surface waters. From sedimentarymoats around basement outcrops and the presence of abyssal bedforms [277–279], it became clear several decades ago that significant currents can transportparticles and vary their net accumulation.

Understanding particle transport is important for palaeoceanography inparticular, as lateral movements may imply mixing of particles of differentage and that particle deposition does not represent the immediately overlyingwater column. Unfortunately, measurements of the currents in deep abyssalareas are rare, and progress in this area has remained modest. Deposition islikely to be modulated by both steady (e.g. geostrophic) currents and higher-frequency motions (e.g. surface and internal tides) varying with depth, locationand over time. Some ocean surface large-scale eddies of the Gulf Stream, forexample, have been found to penetrate as far as the seabed 4.9 km belowin short-lived events resuspending sediments [280]. Deep-tow sediment profiler





records from the Mid-Atlantic Ridge crestal mountains show the sedimentsvariably containing moats around outcrops (indicating erosion or non-depositionby currents) or forming curved deposits as though produced from particlestransported intermittently downslope [281].

(h) Measurements constraining deep ocean sediment transport by currents andsediment drifts

Two important technical developments have helped quantify particle transport.First, McCave and co-workers [282–284] have attempted to characterize the sizesorting of silt particles deposited by geostrophic currents to assess varying flowstrength. Bianchi & McCave [285] compared silt distributions with hydrographicdata to assess how modern deposition in the Iceland Basin varies with currents,and Bianchi & McCave [286] used grain size data from a core to infer variationsin current speed over the Holocene.

Second, deposition rates of 230Th [287] and 3He from extra-terrestrial dustparticles [288] have been used as tracers of anomalous particle accumulationor ‘focusing’. 230Th is produced at a predictable rate in the water column,thanks to the dissolved uranium being well mixed in the oceans. Becausethorium is insoluble, it is absorbed rapidly onto falling particles. This has ledto researchers using the ratio of deposition rate of 230Th to that predictedfrom uranium decay in the immediately overlying water column to assess howmuch the particles have been advected laterally before finally depositing [287].The accuracy of the method has been debated [289,290]. Anomalously high230Th accumulation along the Pacific equator has been suspected to originatefrom enhanced stripping of thorium by falling particles [291]. The method alsodoes not provide knowledge of the particle transport vector (where particles aretransported from). Nevertheless, how the local 230Th accumulation varies spatiallycould be useful for assessing accumulation variability [292], as the enhancedstripping described by Broecker [291] should be a longer-wavelength effect varyingwith the scale of the enhanced productivity region.

Most recently, Turnewitsch and co-workers [293–296] have measured thedistribution and fluxes of the shorter half-life (24.1 day) isotope 234Th inbottom waters, falling particles and deposited sediments, with some novel results.Discrepancies between 234Th fluxes and those expected from in situ 238U decaywithin the bottom mixed layer are evidence for stripping of 234Th by resuspendedsediments [296]. The effect is greater in abyssal areas of the Atlantic Ocean, wheretidal currents are stronger [295] than in the abyssal Mediterranean [293]. Similarmeasurements have revealed a sedimentary effect of asymmetric flow around adeep seamount expected from theory [294].

Much UK effort in particle transport over the past 20 years has concentrated onthe European continental margins, funded by the NERC and EU programmes:Land–Ocean Interaction Study (LOIS) off the Outer Hebrides, Scotland, andOcean-Margin Exchange (OMEX) I and II off the SW Approaches to the UK andWestern Iberia, respectively. This effort has involved measurements of particleconcentration and current velocities [297–300] coupled with numerical modellingof the water motions [301–304]. It has revealed how particles are suspended andtransported off-shelf by a combination of wind waves, internal waves and wind-driven upwelling and downwelling.




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The sediment drifts left by particles transported by deep currents are importantarchives of changing flow velocity and climate linked to planetary-scale oceanheat transport. Analyses on samples recovered by scientific ocean drilling andother cores have looked at changing circulation at various places such asnear New Zealand [305,306] and in the Iceland Basin [285]. Past changes inwater properties have commonly led to regional changes in sediment densityand thus acoustic impedance, which produce regionally correlatable seismicreflections [307]. Seismic stratigraphy has therefore turned out to be usefulfor mapping the structures of drifts, and the changing patterns of depositionand erosion within them are linked to changing flow properties. Exampleshave been studied in seismic datasets south of the Falkland Islands [308],south of Greenland [309,310], off Hatton Bank [311] and west of the ShetlandIslands [240,312]. Near to the UK, varied flow through the Faroe–Shetlandchannel has left a variety of sedimentary features, in places overprinted orunderlain by the products of gravity-driven transport observable in surfacegeology [313–315]. More recently, UK researchers have been involved in the effortto characterize sediments deposited under the outflow of the Mediterranean [316–318]. The effects of evaporation on Mediterranean salinity have varied the outflowdischarge, and hence the sediments form an important archive of regional climatechange, which has recently been recovered during IODP Leg 339.

2. Prospects for the next 20 years

Marine geoscience has traditionally developed by tackling practical problems, byadopting new technologies or techniques and by curiosity-driven enquiry. So thiscombination can be expected in the future, with practical research promotedparticularly where it involves major sponsors such as the oil and gas industry.Besides that general statement, however, predicting future outcomes is difficult.The following should be considered only as a personal suggestion of how the fieldcould develop rather than as an accurate prediction.

The summary of achievements demonstrates how progress has been madeby geological and geophysical work on the major physiographic features.Outside coastal and deep-water sedimentology, there has been less instrumentedmeasurement and analysis of active geological processes by UK researchers, incontrast to the terrestrial geosciences. This is understandable, because, wheregeological events are isolated, they can be difficult to detect in the oceans.Volcanic eruptions cannot be easily located from infrared anomalies [319], forexample. Many of the measurements that can be made on land are also difficultto make in the deep oceans even where events are detected. However, somenew ocean monitoring techniques may help improve our ability to detect andrespond to events, and in some shallow-water (more accessible) sites, geologicalevents can be more frequent and predictable. As an example of the former,hydrophone arrays have been installed within the sound fixing and ranging(SOFAR) channel to detect seismic events of much lower magnitudes than hasbeen possible previously with seismometers on land [320]. Volcanic earthquakeswarms have unique characteristics, which allow them to be discriminated fromtectonic earthquakes in the acoustic data [321,322]. Some active processes presentmajor hazards to human activity—shallow eruptions, active faults near major





cities, and slump and sediment movements over or near oil and gas fields, forexample. Those hazards could spur greater interest in this area in the future, inparticular where they involve threats to offshore industries and where they alsopresent the ability to engage with the broader public in outreach activities.

The geological sciences have traditionally adopted expertise from other sciencesin pursuing objectives within the subject. In oceanography, the large datasetsand modelling capability developed from activities outside traditional geosciencescould also prove useful. Some ideas reflecting the author’s own interests aregiven in §2b, which are intended to illustrate only the possibilities that mightbe achieved through a broader interaction.

(a) Active processes

(i) Volcanic processes

Direct eruption into the ocean. Submarine volcanic eruptions have largely onlybeen observed remotely from the sea surface, where they are located by pumiceand other floating particles [323,324]. Process has been inferred indirectly fromanalysis of the particles deposited from the erupting vents [325–329] or recoveredfrom the water surface [324].

To observe an eruption directly, the vent needs to be in deep water andhave high ambient pressure; otherwise exsolving magmatic gases and steam fromseawater can expand explosively [330]. There have been two such observationsreported to this author’s knowledge. Chadwick et al. [331] described videoobservations from a remotely operated vehicle (ROV) of a gaseous eruptionjet from a vent at 550 m depth in the Mariana Arc, along with measurementsof acoustic noise. They estimated eruption discharge cycling from zero to 10–100 m3 h−1. Explosive bursts of 2–6 min alternated with non-eruptive pauses of10–100 s. Landslides in the flanks of the cone were recorded in repeat bathymetryand acoustic noise measurements [332]. In contrast, Gaspar et al. [333] observeda more effusive eruption on a shallow ridge to the west of Terceira Island inthe Azores in which expanding gas produced balloons of basaltic lava. Thebuoyant rocks were also observed by the author reaching the sea surface duringan expedition on the Italian RV Urania in 1999. Besides the acoustic noisemeasurements of Chadwick et al. [331,332], these eruptions were observed onlyvisually; so quantitative data are still lacking.

The measurement of venting jets formed during eruptions presents majorchallenges. The acoustic data of Chadwick et al. [331] suggest that this is a noisyenvironment and the jet may be strongly attenuating or scattering to acousticsignals. The proximity of the hot jet containing solid particles also presentsobvious hazards to any instrumentation. Nevertheless, careful choice of acousticfrequency and signal processing techniques may enable measurements of the flowvelocity distribution of such features to be made. A vertical array of horizontallyoriented acoustic Doppler current profilers (ADCPs) could potentially image themixing structure around the edges of the jet. Instruments could potentially beinstalled using bottom crawlers as demonstrated at the Neptune observatorysystem (www.neptunecanada.ca). Such technology could find other applications(such as in the oil and gas industry or in effluent monitoring), making this morefinancially efficient.




5580 N. C. Mitchell

Flowing lava. The prospect of observing flowing lava from isolated vents in thedeep ocean is modest realistically, given that the lavas are probably emplaced overdays and their eruption is unpredictable (although the SOFAR acoustic arraysnow offer some hope of detecting seismic swarms associated with volcanism intime to respond to them). To date, recent flows have only been inferred fromweathered or sedimented surfaces, fauna or instruments over-ridden by lava flows[334–336] or from changes in bathymetry [337,338]. An exception occurred whena lava flow on the Juan de Fuca Ridge over-ran a pressure sensor in 1998. Foxet al. [339] were able to recover data from the instrument and deduced verticalmovements associated with the lava emplacement and, from those movements,inferred the flow effusion rate.

Observations of moving lava could be made more practically around volcanicislands, where lava flows into the sea. Moore et al. [340] reported SCUBA diverobservations (ciné film and stills) of lava tongues and lobes produced during the1969 and 1971 eruptions of Kilauea, Hawaii. They included one ’a’a lava flow thatpassed beyond 70 m depth and several hundred metres offshore, while sourcingseveral finger-like extrusions laterally. Pahoehoe lava was found mostly to builda slope of volcanic particles from lava–water interactions at the shoreline, butsome lobes penetrated below water and sourced pillow lavas and lobes duringperiods of greater flow discharge, where cliffs at the water edge were unusuallylow or where lava was protected from surf. In 1989, 0.7–1.5 m wide lava streamson the delta front were seen by divers emanating from surf-traversing flows ortubes [341].

The types of structures that might be expected are illustrated in figure 2. Itis derived from a multibeam sonar survey carried out across a site where an a’a’lava flow entered the sea in 1718 on the north side of Pico Island, Azores [110].The figure shows a curious dendritic pattern of lava lobes in that location, whichis similar to lava structures observed near Mt Fuji, Japan, where a lake has beendrained, exposing a subaqueous lava flow [342].

A key observation needed to develop an understanding of how subaqueousflows evolve and develop their morphologies is the distribution of heat flux fromthem. This is because the viscosity of lava strongly depends on temperature [343],so that cooling lava becomes progressively viscous and then visco-elastic, beforesolidifying. Pahoehoe lava on land tends to develop by a series of inflating lobes,where the internal fluid has low viscosity, while the outer surface appears asthough expanded under surface tension—in reality, a strong outer layer witha visco-elastic rheology [344]. In terrestrial lava flows, the surface solid crust(outside the visco-elastic layer) is usually weak because of pervasive fractures.Mitchell et al. [110] suggested that the dendritic patterns of subaqueous lavas asshown in figure 2 arise because cooling by water circulating through irregularlyspaced cracks forcibly opened by the movement of the lava leads to a penetrationof isotherms and thinning of the visco-elastic layer and thus a weakening. Thoseweak points then form the locations where new lobes develop from breakoutswhen the flow interior becomes pressurized from episodic supply of lava fromupslope. Therefore, although it might seem logical superficially for lava to bestronger where it is more strongly cooled (and this is of course the case when theflow has more nearly solidified), enhanced cooling at cracks could instead lead topoints of weakness in the visco-elastic layer and mark locations where new lavalobes develop.





100 m

80 60 40 20depth (m)

Figure 2. Surface morphology of the seabed immediately adjacent to where an a’a’ lava flow enteredthe sea in 1718. Image was generated from multibeam sonar data collected on the north coast ofPico Island, Azores [110]. Top-right inset locates the dataset on a map of the island. Copyright2008 American Geophysical Union. Reproduced by permission of American Geophysical Union.

Advective heat flux could potentially be measured by monitoring the waterflow and temperature fields above the lava. The flow field could be imagedwith an array of ADCPs, while a mesh of thermistors provide the temperaturedistribution. Given the hazards of working near active lava deltas [345], theequipment may need to be emplaced using robotic surface vehicles, while theevolving morphology would be imaged with a sonar on an autonomous underwatervehicle. Such equipment may be useful for detecting or measuring other kindsof geophysical flows where there are significant temperature differences betweenwater bodies, some of which may have practical or commercial applications, e.g.for evaluating a foundered nuclear submarine or other salvage target and in hoteffluent discharge from oil and gas reservoirs.

(ii) Turbidity currents and other sedimentary flows

Turbidity currents are flows formed of suspensions of particles in water, wherethe suspension leads the sediment-laden water to have a greater density than itssurroundings, causing a turbulent gravity flow [346]. Our knowledge of their flowbehaviour is generally indirect, for example, based on the grain size distribution ofsediments deposited from the flows [347–349], geometry of channel levées [350] orchannel deposits [267]. The average velocities of some turbidity current fronts areknown from the timing of successive breaks of telephone cables [351]. A turbiditycurrent in mine tailing discharge has been imaged acoustically [352]. Over the pastfew decades, the ability to measure flow properties has improved, as reviewed byXu [353]. Turbidity currents can occur frequently in some canyons if fed by rivers




5582 N. C. Mitchell

capable of transporting high sediment concentrations during floods. MontereyCanyon is one such feature [354], and measurements with ADCPs suspendedover the canyon have been used to record flow velocity profiles [353,355].

Laboratory experiments have been employed to gain insights into flowbehaviour, but it is not possible to scale all the similarity numbers (in particular,Reynolds numbers) of the laboratory experiments simultaneously to the naturalflows (prototypes), leading to some uncertainty in the accuracy to whichthe results correspond to the prototypes [346,356]. Before developing scaledexperiments, information is in any case needed on the prototype. There istherefore a strong argument for collecting more field observations of turbiditycurrents. This could involve studying submarine channels known to experienceregular hyperpycnal currents from rivers in flood [357,358] and more extensivemeasurements of the types made in Monterey Bay [355]. Measuring the soliddensities of these flows has only been possible for the most dilute suspensions offiner particles because instruments are typically destroyed by these events [359],but the distribution of particles could be interesting for dynamical calculations ifit could be recovered.

There are other kinds of flows that could also be of interest, though manypresent even more extreme measurement conditions. Active volcanoes canproduce hot gravity flows of rock particles in air (pyroclastic flows). The depositsleft by such flows after entering the ocean around Montserrat have been thesubjects of several studies involving UK scientists, based on the geophysicalcharacter of the deposits and their structures in sediment cores [360–364].Hyperconcentrated and other similarly dense flows entering sea water have beeninterpreted as responsible for chutes and other features on fan deltas [365].These events present challenges in terms of achieving useful measurements offlow velocity structure, density, etc., while also protecting the instruments used,although these can have the advantage of occurring at sites where flow events arerelatively frequent and therefore somewhat predictable.

(iii) Seabed tectonic and other displacements

Geodesy is a field within the geosciences addressing the movements of theEarth’s solid surface associated with the rupture of faults during earthquakes,subsidence over mine workings or regions where fluid has been extracted, slumpmovements and surface displacements caused by subterranean movements ofmagma. A number of factors suggest that this field could become important inthe oceans also. Many deep-water oil and gas reserves are located in regionswhere the continental slope is mobile, because of weak shale or rock salt deposits.Major landslides occur commonly in the continental slopes [242] and many willprobably overlie or occur near to oil and gas prospects. The Ormen Langefield underlying the Storegga Slide [243], for example, has been monitored formovement by the oil companies operating there (P. Bryn, Statoil, 2011, personalcommunication). Many of the world’s largest faults, and by inference potentiallygreatest earthquakes, lie offshore major population centres, as illustrated by theMarch 2011 Japanese giant earthquake and tsunamis. The displacement aroundsuch faults may not necessarily be understood through studying analogues onland, given the different mechanical behaviour of continental crust; so there is aneed for in situ measurements.





The measurement of deformation fields by the Global Positioning System(GPS) and comparison of repeated satellite radar scenes (i.e. InSAR [366]) arenot achievable underwater, as electromagnetic radiation is strongly attenuated.If an acoustic version of InSAR can be made to work, there would still not bethe advantage of global coverage afforded by the satellite measurements. Largechanges in seabed elevation have been characterized using repeated bathymetrymeasurements with multibeam and other echo sounders [226,331,338,361,367].Those studies have mainly addressed changes due to sedimentary movements,landslides and lava emplacements, but not as yet tectonic movements. Thepotential to detect opportunistically a change due to a geological event isa strong argument in favour of keeping the multibeam sonars on researchvessels in continual operation, even if the resources may not be availablefor routine processing of the data. Accurate measurements of change are notstraightforward [368], as they require rigorous calibration of the sonars andfrequent measurement of water sound velocity structure, but some assessmentof changes found opportunistically would nevertheless be possible even withoutthose procedures.

Seabed movements can be measured with acoustic transponders measuringranges to surface buoys or research vessels, which are themselves positioned withGPS, or ranges between seabed transponders. Most of the work has been carriedout by American and Japanese groups. On the Juan de Fuca Ridge’s AxialSeamount, an acoustic extensometer recorded a volcanic deflation event [369],and pressure sensors have recorded a more recent steady inflation of the calderafloor [370]. The immobility of a recent landslide off Santa Barbara, California,was confirmed by acoustic ranging across its headscarp [371]. The submarineHilina Slump of Kilauea was investigated using pressure sensors by Phillipset al. [372], who found the slump’s mid-slope bench to be uplifting moderatelywhile other parts were more stable. Sato et al. [373] described measurementsof vertical (up to 3 m) and horizontal (up to 24 m) displacements of the seabedimmediately overlying the March 2011 earthquake epicentre, recorded with seabedtransponder locations. As these measurements lacked data from the Pacific Plate,the deformation field of a major subduction earthquake has yet to be determined.In an earlier paper [374], they described the discovery of the locking of a faultoff Japan after the 2005 earthquake off Miyagi Prefecture. Seabed measurementsare also needed where there is no land available for characterizing plate tectonicmovements, illustrated by Chadwell et al.’s [375] measurements across the Juande Fuca Ridge.

Given UK commercial interests in deep-water oil and gas, this area seems themost promising for seabed geodetic research within the UK, and it is thereforeinteresting to speculate on the possible range of slump movements that could bemeasured. Figure 3 is an example of a region of bed that is strongly suspected tobe mobile because of deformation of weak halite beds in evaporites. This figurewas constructed from multibeam echo-sounder data collected on the Italian RVUrania [376]. Although movement rates of similar features reported in the Gulfof Mexico are more modest, movements of namakiers (subaerial salt flows) inIran have been measured at 1–8 m yr−1 [377,378]; so the possibility of significantmobility exists. Whereas seasonal variations in pore fluid pressures can leadto seasonal movements in slumps on land [379,380], it might seem not so obviousthat seasonal movements could occur in submarine slumps. However, in situ pore




5584 N. C. Mitchell

1000

2000

10 km

10 km

A

B

Figure 3. Three-dimensional view of multibeam sonar data from the central Red Sea showingmassive flows of evaporites entering the volcanic axial valley of the spreading centre [376]. Copyright© 2008 by Geological Society of America.

water measurements in the Rhone delta slope suggest that its aquifer is connectedto fresh water supplied from land and that chemical effects in clay mineralsassociated with fresh water and varying pore pressures may have been partlyresponsible for the 1979 Nice airport landslide disaster there [381]. Evidence forfreshwater expulsions have been found at other sites too [382]. Such effects, aswell as tidal, storm and temperature changes, suggest that interesting non-steadymovements may emerge from seabed geodetic studies.

(b) Collaboration between the sciences

(i) Biosciences

There are already many areas where information from the biosciences hasbeen used in the geosciences (interpreting the palaeoenvironments of planktonicmicro-fossils, for example) but much further interaction could be productive.In the terrestrial geomorphological literature, organisms are well known to beimportant to various Earth surface processes. The action of tree roots and micro-organisms helps to break down rock into soil; trees and bushes can stabilize riverbanks and slopes or, if shallow-rooted and on clay-rich slopes, their weight cancontribute to slope failure. Vegetation can strongly affect water penetration too,also affecting slope stability [380]. The effects of some of these processes arediscussed quantitatively by Dietrich & Perron [383].

Biological agents are known to be important in the oceans too. Crabs andother organisms burrow into consolidated or indurated sediments within canyons[384–388]. This process may be important for preparing the beds of canyons for





2 km 0

500

wat

er d

epth

(m

)

Figure 4. The typical convex-upwards morphology of the uppermost continental slope is illustratedin a seismic reflection image from the US Geological Survey [263].

erosion by energetic sedimentary flows. In mountain river beds, weathering (frostaction, for example) breaks up exposed rocks so that they can be mobilized duringhigh flow conditions of the spring snow melt. In submarine canyons, the depthof erosion by sedimentary flows might be expected to be limited to the depthat which the flows can overcome sediment shear strength [259], which typicallyincreases with burial depth and consolidation [389]. It is still unclear how deepexcavation of canyons occurs, but boring and burrowing activity may play a rolein preparing substrates for deep excavation by sedimentary flows of strata incontinental slopes.

Sediment resuspension by organisms, leaving the suspensions to deposit undergravity or be carried by currents, may also have a role in shaping the seabed’smorphology. For example, the uppermost continental slope has a simple upward-convex rounded shape in profile, which mimics a decline observed in moderncurrents with water depth [390]. This characteristic convex profile shape has beenvariously explained as originating from how the bed shear stresses due to currentsdecline with depth and modulate sediment deposition [263,391,392]. An examplefrom the US central Atlantic slope is shown in figure 4. However, over this samedepth interval, rapid changes in biological mixing rates have been determinedfrom radiometric tracers in sediment cores [393–397]. The curved shape couldtherefore also partly originate from greater biological activity in the shallowerwater. More generally, pelagic sediment deposits in the deep ocean commonly havesimple curved surfaces as though transported downslope according to a diffusiontransport model scheme, such as from repeated resuspension and depositionduring downslope movement of those suspensions [281].

Resuspension by benthic organisms and their effect on the threshold of motionof sediment have been known for some time to be important sediment transportagents [398–400] but their net effect on the larger-scale geomorphology of theseabed is still relatively poorly known. Quantifying the effects of biology onseabed geomorphology presents difficult challenges, as it would require measuringthe lateral sediment flux arising from the biological activity. (Deposition and




5586 N. C. Mitchell

2.0

(a) (b)

2.5

0.028 0.022 5 kmsite 865

0

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)

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0 20 40 60 80age (Ma)

Figure 5. (a) Seismic reflection image across the summit of Allison Guyot in the central Pacific(adapted from Winterer & Sager [405]). (b) Depth–age plot derived from sediment dates inODP Site 865 cores updated in the Geomap database [406]. Red and yellow represent sectionswith sedimentation rates less than 0.1 and 1.0 m Myr−1, respectively, highlighting the reducedaccumulation or erosion over the past few million years at this site.

erosion rate are then determined by conservation of mass from lateral gradientsin flux [401].) To quantify the effects of borings in preparing more resistantsubstrates for erosion, rates of excavation or excavation history would be required.

(ii) Physical oceanography

Geologists already work closely with physical oceanographers, for example, inunderstanding how the distribution and geometries of sand dunes relate to tidalcurrents [208], in modelling isotope distributions [294] or, at a more practicallevel, in determining ocean sound velocity structure for multibeam echo-soundingrefraction corrections. More recently, geophysicists have found ways to createacoustic reflectivity images from seismic reflection data to reveal the fine densitystructure of the oceans [402,403]. The following are two ideas for possible furthercollaboration.

Physical oceanography models and particle entrainment and transport.Numerical models of physical oceanographic movements are becomingincreasingly sophisticated and accurate. In the UK, modelling efforts haveaddressed internal wave and other motions on the European Atlantic margins[301–304,404]. As has already been demonstrated in the LOIS and OMEXprogrammes, such models are useful for assessing the movements of particlesacross margins and hence help in evaluating the palaeoceanographic significanceof information from sediment cores. After several decades of surveying andscientific drilling, geoscientists have many datasets revealing where differentareas of the ocean floors have been eroding and where particles have beendepositing. Such data could be combined with the ocean model predictions, asillustrated here.

Figure 5 shows a seismic reflection record and associated depth–age graph fora site drilled on Allison Guyot in the central Pacific Ocean in 1518 m of water.Seismic reflections are truncated at the seabed and the depth–age graph suggestslow net deposition rates of pelagic sediments over the past few million years at thissite (either erosion or non-deposition). In contrast, the sediment profiler recordin figure 6a shows sub-bottom reflections roughly mimicking the seabed overLo-En Guyot. The drill site was located in 1083 m depth, and its depth–age graph(figure 6b) shows continual accumulation of pelagic sediments for the past 6 Myr.





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1.6

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(m)

150

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30

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0.021

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Figure 6. (a) Seismic reflection image across the summit of Lo-En Guyot in the central Pacific(adapted from Premoli Silva et al. [407]). (b) Depth–age plot derived using sediment datesconstrained by biostratigraphy from Pearson [408]. Hiatuses are highlighted by grey bars. (Theshallowest hiatus below 40 m sediment depth is marked by a red horizontal bar in (a) andcorresponds roughly to an unconformity observable in that figure.)

In figure 7, ocean drilling sites showing evidence for erosion or non-deposition(solid circles) and deposition (open circles) are plotted over a map of internal wavekinetic energy calculated [409] using a global bathymetry dataset [3]. Althoughthe energy map has some irregularities, the eroding or non-depositing sites areall associated with a kinetic energy flux above 103 W m−1, and the depositingsites are all associated with flux below 103 W m−1. A thorough assessment wouldrequire more work on local flow dynamics and evaluation of whether the predictedbed stress exceeds the threshold of motion, but this example illustrates howgeophysical and geological data might be usefully compared with ocean models, anapproach being carried out in more detail with isotopic measurements [293–296].

Numerical physical ocean models could potentially help us to study short-period motions in past oceans also. Unfortunately, our knowledge of theirbathymetry becomes less accurate further back in time, but this should be lessof an issue for the more recent past. Thus Egbert et al.’s [410] model of deep-water tides of the Last Glacial Maximum should be reasonably accurate andprovides a start that can be updated with improved palaeo-bathymetry. Similarefforts have been made locally around the UK [206] and have been used toexplain the distribution and orientations of moribund bedforms in the CelticSea [208]. This work is important because, whereas palaeoceanographers havebeen able to uncover many details of the chemical characteristics and broaddirection of palaeoceanographic flows, those movements are relatively long induration. The modelling, in contrast, could potentially unfold aspects of higher-frequency motions too. The seismic and drilling datasets in figures 5 and 6




5588 N. C. Mitchell

12060

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Figure 7. Scientific drilling sites where the bed has been locally depositing or eroding/non-depositing are shown by open and solid circles, respectively. The eroding/non-depositing sites are:H, Horizon; A, Allison; O, Ojin. The depositing sites are: I, Ita Mai Tai; Lo, Lo-En; Li, Limalok;S, Suiko; W, Wodejebato guyot. Colours in the map show the internal wave kinetic energy fluxobtained in a global numerical model [409].

suggest that Allison Guyot experienced continuous deposition prior to about20 Ma, whereas Lo-En Guyot experienced a pronounced hiatus up until around6 Ma. In other datasets not shown here, the hiatuses terminate at differenttimes [408], suggesting that they have a physical rather than a chemical origin.Such observations may point to a different Pacific internal wave field from themodern day.





Incorporating form drag in tidal and other flow modelling. Flow is resisted bypressure variations caused by large-scale roughness of the bed, otherwise called‘form drag’. From current measurements made over sand dunes in Torres Strait,Australia, Hughes et al. [411] suggested that as much as 65 per cent of the beddrag coefficient there was due to form drag. Spatially varied form drag is nottypically incorporated in models of tides and other flows, but regional multibeamsonar datasets show significant variations in bedform geometry [225], which couldpotentially be incorporated into tidal models.

Kean & Smith [412] developed an analytical solution for the form dragresistance due to a series of Gaussian-shaped roughness elements in the flowboundary. They suggested that the flow resistance should be given by

t0f = 12

rCdHl

u2ref , (2.1)

where H is the protruding height of a roughness element, l is the lengthscale of the wake produced by that element and uref is a reference velocity.The bedform effective friction coefficient Cd was found empirically to equal1.79 exp(−0.77s/H ), where s is the Gaussian width parameter of the roughness.The inverse of the ratio s/H is analogous to the steepness index of sedimentarybedforms [413]. Because of the effect of the exponential (and as l should increasewith H so H /l varies less), the form drag will increase with increasing steepnessof the bed protuberances. Van Landeghem et al. [225] found that the bedformheight H on average varies according to 0.0692 L0.8 in the Irish Sea (L is bedformspacing measured over three orders of magnitude), similar to a more globalstudy [414]. These relationships suggest that the ratio H /L, while not varyinggreatly, is not constant. Furthermore, individual values of H vary by more thanone order of magnitude for a given L in the compilation of van Landeghemet al. [225]. On a shallow sandbank crest, Schmitt et al. [226] found that therelationship varied according to water depth, with the exponent decreasing toonly 0.02 shallower than 15 m depth. As form drag affects flow over bedforms, thebedform development and form drag are probably inter-related in a potentiallyinteresting feedback.

3. Conclusions

The post-World War II era of expanding marine geological research has beenextended over the past 20 years in large part by concerted efforts in fundedprogrammes. UK researchers made significant contributions as part of BRIDGE(mid-ocean ridges), MARGINS/LINK (continental margins), OMEX, LOIS,BOFS and RAPID (particle transport) and were actively engaged in the ODPand IODP. Much of this effort has involved the ‘forensic’ style of geoscientificresearch, where processes are deduced from analysis of the structures they haveleft behind. Less effort has been expended in studying active processes in the UK,except in coastal and some deep-water sedimentary transport.

Despite a presently uncertain financial backdrop to academic research, thereare many grounds for optimism. The study of active processes could becomea new theme, particularly where it addresses threats to human populationsand infrastructure (e.g. offshore oil and gas) and offers opportunities for public




5590 N. C. Mitchell

engagement. Some such work presents technological challenges, such as measuringadvective heat loss from flowing volcanic lava, or the flow and acoustic backscatterstructure of erupting vents or sedimentary flows. Accurate measurement of solid-body movements of the seabed through acoustic geodesy and pressure sensorscould help address threats from major offshore faults near population centres andstability of submarine slopes being exploited for oil and gas reserves. Unlike inother ocean sciences, active geological events are usually rare and so are difficult tocapture, present challenging engineering problems and are costly to instrument,but some progress is possible by targeting sites where the processes are morecommon or detectable (a lava flowing from land into the sea or river floodsproducing hyperpycnal currents offshore). Many of these observations need notrequire expensive deep-water research vessels. Indeed, some such as lava flowmonitoring would require robotic placement of instruments and others couldinvolve less costly coastal vessels.

Geology has frequently benefited from involvement of researchers from theother scientific disciplines. Although greater collaboration can be anticipatedsimply for cost sharing and practical reasons, there are also potential intellectualgains to be made. For example, biological processes are important in facilitatingerosion (particle suspension and borings preparing bedrock for erosion by varioustypes of flows), but their large-scale geological effect is largely unquantified. Inphysical oceanography, numerical modelling of the physical state of the oceans isgreatly improving because of increased computing resources, advances in softwareand better knowledge of bathymetry, salinity and temperature. For geologists,accurate predictions of water movements and bed shear stresses could be usefulresources for understanding modern erosion, transport and deposition of particles.For example, the pattern of internal wave energy in the Pacific Ocean has beenrevealed in remarkable detail by recent global numerical modelling [409]. Thatpattern compares well with the incidence of longer-term erosion and depositionfound in scientific drilling of the tops of guyots as shown here. Modelling canhelp geologists in understanding the origin of hiatuses within sediment deposits,while the geologists’ work can help to validate the ocean numerical models.Whereas palaeoceanographers have been mainly concerned with relatively long-period changes in the chemical or physical state of the oceans, such numericalmodelling, coupled with improving knowledge of bathymetry, temperature andsalinity structure of the past oceans, offers the potential for investigating theshorter-period physical states as well.

I wish to thank Harper Simmons for kindly providing the data for figure 7, two anonymousreviewers and Tony Watts for their helpful comments, and the meeting organizers Harry Bryden,Carol Robinson and Gwyn Griffiths for this highly stimulating Challenger meeting.

References

1 Intergovernmental Oceanographic Commission. 1984 The ocean floor. In Ocean science for theyear 2000: a report on an inquiry by the Scientific Committee on Oceanic Research and theAdvisory Committee on Marine Resources Research. Paris, France: UNESCO.

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