biological mediation in ocean crust alteration: how deep is the deep biosphere?

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Page 1: Biological mediation in ocean crust alteration: how deep is the deep biosphere?

ELSEVIER Earth and Planetary Science Letters 166 (1999) 97–103

Biological mediation in ocean crust alteration: how deep is the deepbiosphere?

Harald Furnes a,Ł, Hubert Staudigel b

a Geological Institute, University of Bergen, Allegt. 41, 5007 Bergen, Norwayb Scripps Institution of Oceanography, University of California, La Jolla, CA 92093-0225, USA

Received 15 October 1998; revised version received 8 December 1998; accepted 7 January 1999

Abstract

Biological mediation has been suggested as a control of the chemical exchange between the oceanic crust and seawater,but very little is known about its distribution within the oceanic crust and the relative importance of biotic and abioticprocesses. Alteration textures in glassy pillow lava margins record the proportions of biotic and abiotic alteration, and thefraction of biotic alteration may be determined by point counting methods. We used this method at DSDP=ODP Sites 417Dand 418A (110 Ma crust south of Bermuda Rise) and Holes 504B and 896A (5.9 Ma Costa Rica Rift). Biotic alterationdominates glass alteration in the upper 250 m of the oceanic crust (60–85% of the total glass alteration) and steadilydeclines in importance down to 10–20% at 500 m. The consistency of data between two crustal sections of very differentage and tectonic setting suggest that microbially mediated glass alteration may be largely confined to the upper oceaniccrust. However, both sites studied are sealed by thick sedimentary layers and, thus, are typical for ocean crust underlyingthe oceanic basins, rather than crust at mid-ocean ridges with possibly deep and rapid hydrothermal circulation. Down-holetemperature measurements at the Costa Rica Rift suggest that glass-altering microbes are hyperthermophilic and thrive atleast up to temperatures of about 90ºC. Microbial activity does occur at higher temperatures (up to about 110ºC) but withreduced apparent abundance. 1999 Elsevier Science B.V. All rights reserved.

Keywords: oceanic crust; basaltic composition; volcanic glass; biogenic processes; biosphere; alteration

1. Introduction

Microbial processes are increasingly recognizedas a control of water–rock interactions at tempera-tures of about 110ºC, the presently accepted upperthermal limit of life [1]. The involvement of mi-crobes in such processes has been demonstrated inglassy basaltic rocks on the ocean floor and withinthe ocean crust using textural evidence and genetic

Ł Corresponding author. Fax: C47 5558 9416; E-mail: [email protected]

probes [2–7], and in experiments [8–10]. Despitethe abundant evidence for microbial alteration in theoceanic crust, it is effectively unknown how im-portant this process is relative to abiotic alteration,and where, and how deep it occurs in the oceaniccrust.

This paper presents the results of microscopic ob-servations on basaltic glass samples from two 500 msections of the oceanic crust, in 110 Ma old oceaniccrust (DSDP=ODP sites 417D and 418A), and in 5.9Ma old crust at the Costa Rica Rift (DSDP=ODPSites 504B and 896A). In these samples we dis-

0012-821X/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 0 0 5 - 9

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tinguish biotic from abiotic alteration, quantify thetwo alteration types, and relate our data to alterationminerals, porosity, permeability and temperatures.

2. Method and samples

Most studies of biological mediation of theoceanic crust alteration have focused on basalticglass. These textures of biotically altered glass al-teration have recently been described in some detail,including data from Sites 504B and 896A [2–6]. Bi-ological mediated alteration textures can be uniquelyidentified in alteration fronts into fresh glass, alongcracks or from the outer pillow margins (Fig. 1A–F).These textures appear not to be preserved, or at leastnot easily recognizable in the slightly birefringentpalagonite rinds. Two major types may be distin-guished, (1) isolated and=or more commonly, nearlycontinuous zones of coalescing spherical patcheswith diameters of 1–3 µm (Fig. 1A–D), or (2)distinct, irregular tubular features of 1–3 µm diam-eter and 70 µm length (Fig. 1D–F). Tubular bod-ies extend well beyond the alteration front into thefresh glass. Spherical patches or continuous zones ofbiologically mediated glass alteration are typicallyfound along cracks as well, but opposing sides ofcracks typically lack symmetry. These textures areinterpreted to be the result of microbial etching ofthe glass and subsequent precipitation of amorphousmaterial [2–4,6]. This interpretation has been furthersupported by the presence of DNA=RNA within thistype of alteration product [3–5,7].

Glass alteration also occurs abiotically, in whichcase increasing hydration causes the typical transi-tion from light yellow basaltic glass (sideromelane)to darker colored, slightly birefringent alteration ma-terial, known as palagonite [11]. This abiotic alter-ation type is distinct from biotic alteration and isdefined by regular zoned alteration bands of approx-imately equal thickness on both sides of joints, oras concentric alteration fronts. These concentric al-teration fronts are characterized by a smooth colorchange and they lack the typical bulbous protrusionsof the biotic alteration.

Basaltic glass contains the most reliable texturalrecord of biotic versus abiotic alteration, and pointcounting methods may be used to quantify these

two alteration types. For the purpose of this study,we selected 37 basaltic glass samples; 19 from theupper 550 m from DSDP Sites 417D, 418A, and18 from the upper 500 m of DSDP=ODP Sites504B and 896A. Typical sample sizes allowed usto count 500–2000 points for determining volumeproportions of the three textural categories, freshglass, biotically altered glass, and abiotically alteredglass. Our samples from DSDP Sites 417D and 418Atend to be larger than the samples from Sites 504Band 896A allowing for better counting statistics.

3. Results

All our samples come from the ‘fresh’ portion ofthe glassy margins just inside the alteration front. Inthese samples biotic, and abiotic alteration is foundalong cracks that permeate the glass. We did notinclude samples from the palagonite rinds, partlybecause poor recovery limited their abundance (inparticular in 504B and 896A), and palagonite it-self tends not to preserve biological textures. Thisprocedure may bias our observations, and the abso-lute estimates of biotic vs. abiotic alteration almostcertainly has a relatively large error (25%?). Mostof this error is probably in an overestimate of abi-otic alteration, because of the fragility of microbialtextures. Such features may be destroyed by recrys-tallization of layer silicates or the swelling associ-ated with hydration of glass. However, our study isinternally consistent, where we consistently chosesamples of similar overall degree of alteration, weapplied the same visual criteria to all samples and wenormalized our data to the total amount of alterationfound. Thus, we consider the down-hole variationin biotic alteration counts as a rather robust dataset. In Fig. 2A,B, we plotted the fraction of bioticalteration as a percentage of total alteration for thefour drill sites studied in the Atlantic Ocean and atthe Costa Rica Rift. Biotic alteration dominates inglass alteration in the upper 250 m of all holes, com-prising 65–90% of all alteration. Below this depth,the amount of biotic alteration decreases to about10% at about 550 m depth. Below this depth noadditional information may be obtained because oflimits in basement penetration (417D, 418A, 896A)or preservation of fresh glass (504B).

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Fig. 1. Microphotographs showing a general overview (A, C, E) and details (B, D, F) of fresh and altered basaltic glass (FG andAG, respectively) from the ocean crust of the Costa Rica Rift and the Atlantic Ocean. Altered glass displays textures that are herecharacterized as biotically and abiotically altered glass (B and AB, respectively). Microphotographs (A) and (B): from DSDP sample51-53-418A, 56-5, 129–132 cm; Microphotographs (C) and (D): from DSDP sample 51-53-418A, 30-3, 4–6 cm. Microphotographs (E)and (F): from ODP sample 148-896A, 11R-01, 73–75 cm.

4. Discussion

Overall, our data follow a consistent, well de-fined trend, whereby biotic alteration dominates in

the upper 300 m, but it extends to about 550 m, de-creasing in importance. On basis of this observation,one might suggest that the ‘Deep Biosphere’ extendsat least 500 m into the oceanic crust, whereby most

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Fig. 2. Percentage biotic alteration of total (biotic C abiotic) alteration related to depth (m) into the volcanic basement of samples fromthe Atlantic Ocean (Holes 417D and 418A) and the Costa Rica Rift (Holes 504B and 896A). Most microbial activity can be found in theupper 250 m of the oceanic crust, and the influence of microbial activity decreases to a very minor effect at about 500 m.

of the activity may be limited to the upper 300 m.However, before such a generalization can be madewith any confidence, we have to discuss the limi-tations of our data set, and whether there are anyother controls other than depth. Such controls couldinclude, in particular chemical boundary conditionslike the availability of oxygen, and physical controlslike temperature, porosity and permeability.

4.1. Limitations

Our data set is limited by concentrating on partic-ular sample types (fresh glass portion of the glassymargin) and by the limited number of holes in thewide range of types of ocean crust found in variousocean basins. Our focus on the fresh glassy por-tions of the oceanic crust limits our observations toa small, but important portion of the oceanic crust.Glass is the least stable phase in the oceanic crustthat probably makes the largest contributions to thebudget of seawater–ocean crust chemical exchange(e.g. [10,12]). Concentrating on the alteration frontsinto the fresh glass gives us a view of the most recentactivity in the oceanic crust, but it ignores the earlierphases of alteration that led to the formation of thickpalagonite rinds and possibly in the interior portions

of thick veins. It may also have to be pointed outthat observations of biotic alteration textures natu-rally reflect only the type of biological activity thatinvolves colonization. Microbial activity may occurin solutions and not leave any detectable traces in thegeological record. In the absence of microbial sam-ples from pore waters in these sites, our geologicalrecord cannot be related to these types of activity.

The data presented here come from two oceancrust sections that are quite different in terms oftheir tectonic environment and age, but they alsoshare common features. Both sections are sealedby overlying sediments that inhibit free circulationof seawater through the oceanic crust and for thisreason, they are typical only for ocean crust as itunderlies ocean basins and ridge flanks that are olderthan 5.9 Ma. Young crust near ridges is not repre-sented, where water can freely circulate, driven byactive magmatic systems. In this kind of environ-ment, we expect extreme variability in a number ofparameters, including the chemical composition offluids, porosity, or the thermal gradient in the down-or upwelling limbs of hydrothermal systems. There,it is conceivable that microbial activity penetratesto greater depth, at least in downwelling systems.Thus, our data focus on by far the largest frac-

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tion of the ocean basins by volume, but they arenot representative for active submarine hydrothermalsystems.

4.2. Availability of oxygen

Availability of oxygen (or lack thereof) may bedetermined on basis of mineral equilibria, or di-rect measurements of dissolved O2 in pore waters.While the latter is probably the best indicator of the

Fig. 3. Alteration minerals, porosity, permeability and temperature data as a function of depth for the ODP=DSDP Holes 417D, 418A,504B and 896A. (after [13–19]). All data compilation for 504B has been terminated at 500 m depth into volcanic basement, beyondwhich depth we do not have data due to the lack of fresh glass. (A) The down-hole distribution of celadonite. Celadonite is a relativelyearly mineral phase and indicates oxidative alteration. (B) Distribution of pyrite. Pyrite is formed from relatively reducing solutions asthey are characteristic for late stage (current?) hydrothermal processes. (C) Variation in apparent bulk porosity for Hole 504B determinedby applying Archie’s Law to large-scale electrical resistivity logs. (D) Effective porosity for Holes 417D and 418A as determined byair-dried and water-saturated weighing of samples. (E) Bulk permeabilities measured over intervals spanned by the vertical bars. (F)Down-hole temperatures. Note that the currently, microbially most active region has an early oxidizing history that is overprinted withreducing hydrothermal solutions, in a porous and permeable upper ocean crustal environment. Most microbial activity occurs between60–90ºC, but significant hyperthermophile activity can be found up to approximately 115ºC at 500 m depth in Site 504B.

present-day oxygenation conditions, uncontaminatedpore water samples are not always available and onehas to use secondary mineral phase assemblages toinfer the oxygenation of pore waters. The distribu-tion of some key secondary phases are indicated inFig. 3A,B (following [13–16]). Celadonite and rela-tively oxidized clay minerals were found in the upperportions of all sites studied (Fig. 3A). Abundance ofthese phases indicates rather oxidizing conditions, inparticular in the upper 300 m and decreasing with

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depth. Celadonite is typically a relatively early phaseto precipitate, and thus, oxidizing conditions mayhave prevailed mostly in early phases of alteration.A decrease in dissolved oxygen at a later alter-ation stage may be indicated by pyrite that may befound throughout most of the sections (Fig. 3B). Theabundance of pyrite increases with depth, and, thus,the lower part of the ocean crust is more reducingthan the upper part. Oxidizing conditions are also inagreement with a predominance of low δ13C valuesfound in glass samples from Hole 896A, pointingtoward the activity of bacteria, which was also iden-tified by RNA probes [7]. Local reducing conditionsmay also be in general agreement with the recogni-tion of Archaea in the biotically altered glass fromHole 896A [7].

4.3. Porosity=permeability, temperature

Porosity, permeability and temperature are plottedin Fig. 3C–F (following [15–21]). The upper 200–300 m of these ocean crustal sections have generallyhigher porosity and permeability, providing spaceand transport paths for water, the most importantingredient of life. The highest fraction of biologicallymediated glass alteration can be found in this zone(see Fig. 2A,B)

Down-hole temperatures were measured at Hole504B and 896A, whereby temperatures at Hole 896Aare slightly lower than at Hole 504B. The present-day temperatures in the upper 250 m at Hole 504Brange from approximately 60–90ºC, increasing to115ºC at 500 m [17]. This temperature calibrationsuggests that some of the microbial activity responsi-ble for glass alteration may be caused by hyperther-mophilic microbes, without any apparent decrease inactivity up to 90ºC. At higher temperatures, micro-bial activity appears to become less significant, eventhough it is still found up to temperatures of 115ºC.We believe that the microbial activity is currentlystill active at least at the Costa Rica sites because wehave measured microbial activity at the most recentalteration fronts and some of these features allowedpositive identifications of some microbes by usingselective molecular probes in alteration fronts fromHole 896A [7]. These observations are consistentwith current concepts about the upper thermal limitof life [1].

5. Conclusions

This study shows that biotic alteration is the dom-inant process of glass alteration in the upper 250m of the oceanic crust at two very different oceancrustal sections in the Western Atlantic Ocean andthe Eastern Pacific Ocean. Both regions show de-creasing importance of microbial alteration textureswith depth, down to about 10% at 500 m. Thissuggests that the bulk of microbial glass alterationoccurs at shallow depths, while some may be founduntil about 500 m. This distribution can be related todepth, but it is also correlated with temperatures <110ºC and high permeability=porosity of the uppercrust.

Acknowledgements

We thank the National Science Foundation, theDeep Sea Drilling Program and the Ocean DrillingProgram for supplying samples. Financial supportwas provided by the Research Council of Norway(grant no. 110833=410). We appreciate very helpfulconstructive reviews by Jeff Alt and Martin Fiskand the constructive discussions with I. Thorseth,T. Torsvik, O. Tumyr, K. Muehlenbachs and A.Yayanos. [RV]

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