a simple model for the interpretation of magnetic records in lacustrine and ocean sediments

QUATERNARY RESEARCH 27, 160- 166 (1987)

A Simple Model for the Interpretation of Magnetic Records in Lacustrine and Ocean Sediments

JOHN HILTON

Freshwater Biological Association, The Ferry House, Ambleside, Cumbria LA22 OLP. United Kingdom

Received March 18. 1986

Many sources of magnetic minerals have been implicated as potential influences on the magnetic record of lake and ocean sediments. The “state-of-the-art” of our knowledge of these sources and their properties has been used to define a mode1 which indicates those sedimentary environments where specific magnetic mineral sources may influence the magnetic record. Authigenically formed magnetic minerals are only likely to be found in estuaries and eutrophic lakes. Cosmic deposition, eolian deposition, and magnetic bacteria may be important in low-accumulation-rate deep-ocean sediments. Whereas catchment-derived material is only likely to be the sole source of the magnetic record in high-accumulation-rate, low-productivity lakes, f ly ash is only likely to be a dominant source in very-low-accumulation-rate lake and ocean sediments. c 1987 university

of Washington.

INTRODUCTION

Magnetic properties of environmental samples can be measured quickly and simply using inexpensive equipment. Be- cause these measurements are very sensitive, responding to very subtle changes in low concentrations of magnetic minerals, they have become very powerful tools in environmental studies of peat, snow, lake sediments, and ocean sediments (Thompson et al., 1980; Oldfield et al., 1983; Oldfield and Thompson, 1986). Origi- nally changes in the magnetic properties of lake sediments, in particular, were inter- preted in terms of changes in the sources of magnetic minerals eroded from the catchment. This approach was used very suc- cessfully in several studies (Oldtield et al., 1979; Oldfield, 1983; Dearing, 1983). The only exception involved data from New Guinea, where discrete volcanic ash layers produced large magnetic signals within cores (Oldfield et al., 1980). Recently, Hilton and Lishman (1985) and Hilton et al. (1986) have shown that magnetic minerals formed in reducing conditions within the sediments of many lakes can dominate the magnetic properties, contributing 70% of

the signal intensity in one case. Similarly following the original work on ombrotrophic peat bogs (Oldfield et al., 1981; Thompson et al., 1980), several workers (Smith, 1985; J. Hilton, unpublished data; F. Oldfield, personal communication, 1985) have observed peaks at the top of lake-sediment profiles which have been attributed to the presence of large quantities of fly ash particles derived from atmospheric dis- posal of waste products from industrial sources, e.g., power stations, steel works.

To date, minerals from four distinct sources (catchment, volcanic, authigenic,’ fly ash) have been identified in lake sediments. However, when using magnetic properties it is important that they reflect only the sources of interest to that particular study, or that the component which re- flects that effect can be isolated. Contribu- tions from other sources would interfere

t Authigenic is used throughout the text to describe one particular set of magnetic minerals. This termi- nology has been chosen to differentiate the process of their formation from the loss of magnetic minerals by dissolution which is generally termed diagenesis by magnetic mineralogists. No specific process is implied by the term authigenic apart from a sense of formation within the sediments.

160 0033-.5894/87 $3.00 Copyright 0 1987 by the University of Washrngfon. All rights of reproduction in any form reserved.

LAKE AND OCEANIC MAGNETIC RECORDS 161

with the interpretation. For example, (i) at- tempts to study changes in sediment sources within a catchment would be se- verely hampered by the presence of signiti- cant non-soil-derived sources of magnetic minerals: (ii) Hilton (1986) has shown that normalized magnetic parameters (i.e., one magnetic measurement divided by a second) are only independent of concentration when a single group of minerals is present. Significant contributions from two or more sources of minerals which vary in- dependently are not concentration independent; (iii) the assumption that correlated peak maxima and minima represent the same point in time is not nescessarily cor- rect when the intensity of magnetic measurements is dominated by authigenically formed magnetic minerals. Hence, there is a pressing need to identify the likely sources of magnetic minerals in the sediments of a water body before any major survey is attempted.

Environmental magnetism is a young science and our knowledge is growing at a rapid rate. Many of the processes and variables controlling the transport to or formation of magnetic minerals in the sediments are unknown. In this paper, I will present a simple model, which uses our present state of knowledge and a few easily estimated parameters, to predict the most probable sources of magnetic minerals in sedimentary environments. The model is based on low-frequency magnetic susceptibility measurements and sets boundary limits by using realistic “worst-case” situations. The model will have a practical value if it indicates environmental conditions in which certain mineral sources will be important. However, its main purpose is to provide a framework within which our present position may be rationalized and future work directed.

THE MODEL

Thompson et al. (1980) presented a list of six magnetic mineral sources: soil/rock erosion and transport by water, authigenic for-

mation, fly ash, atmospheric deposition of windblown soil (eolian deposition), cosmic particles, and volcanic ash. If to these we add local sources of pollution and magnetic bacteria (Blakemore, 1982), we have a comprehensive list of the possible sources of magnetic minerals in the lacustrine and oceanic environments. Table 1 indicates the importance of some of the major variables which influence the contribution of each source to the magnetic record.

These are only a few of the most obvious variables. The complete story is very com- plex, with numerous interlinked compo- nents. However, if we assume that volcanic inputs and local pollution sources will only be important on a local scale, then it is possible to use the present state of knowledge to give an indication of the sources which may contribute to the magnetic record in any particular aquatic sediment.

Different magnetic measurement tech- niques respond to different properties of a mineral assemblage, e.g., paramagnetic and diamagnetic minerals will only have an effect on susceptibility measurements; SIRM intensities are usually at least an order of magnitude more sensitive but the technique only reacts to minerals which re- tain a remanent magnetism. A mineral will influence the measureable bulk properties if its concentration and magnetic properties are such that the magnetic signal for that component from the whole of the measured sample is greater than the limit of detection of the method. In the present work, susceptibility will be used because most of the available data are in this form. The limit of detection will be taken as 0.1 scale reading on a Bartington meter with a MSlB sensor (= 0.001 x lop6 m3 kg-’ for a 10-g sample). However, it is unlikely that use of any other technique would substantially alter the final overall pattern.

Minerals which are transported via the atmosphere will be diluted by other mate- rials in the sediment. A high sediment deposition rate will dilute the minerals to such an extent that the bulk properties of

162 JOHN HILTON

TABLE 1. MAJORVARIABLESWHICHINFL~ENCETHEMEASURABLECONTRIBUTIONMADEBYDIFFERENT SOURCES TO THE BULK PROPERTIES OF SEDIMENTS

Soil erosion Diagenetic Fly ash Local pollution Volcanic Magnetic bacteria Eolion deposition Cosmic

Total deposition

rate Labile carbon

- - X

X - X

X X X X

Magnetic content of Dilution by catchment nonmagnetic

sources material

X X

X X

x x X X

Distance from

source

- X X X

x

Measurement method

the sediment are unaffected by their presence, whereas at low rates they can constitute a major part of the total magnetic intensity. An estimate of the sediment deposition rate is relatively easy to obtain in most cores and will constitute one input to the model. Olsen (1978) has presented a range of deposition rates, which cover six orders of magnitude, from very-low-rate deep-ocean sediments to high-rate glacial- lake and estuarine sediments.

Hilton et al. (1986) have shown that authigenic minerals only make a significant contribution to the bulk magnetic properties in anaerobic sediments. Hence, any predictive model must include an assess- ment of the redox state of the sediments. Unfortunately, it is not easy to obtain reli- able, absolute values of redox potential by direct measurement, and indirect measurement of NO,, Mn(II),Fe(II), and sulphide profiles in the interstitial water is time con- suming. Carbon is the most common me- tabolite which produces and sustains anaerobic conditions. The major source of this labile carbon is from algal production in the overlying water and the redox state of the sediments is related to this productivity. However, it is not a simple corre- spondence, as much of the carbon is me- tabolized before it even reaches the bottom (Wetzel et al., 1972; White and Wetzel, 1975) so that the water depth, its oxygen content, and temperature will also influence the redox state of the sediments. However, the productivity of a water body

is usually relatively easy to estimate and at this stage of our knowledge will suffice as the second variable in the model. Eppley et al. (1985) presented data for a range of ocean and lake sites covering three orders of magnitude from 10 to 10,000 mg C m-* d-‘.

Dilution

The proportion of intensity, due to atmospheric deposition from any particular source, in a susceptibility measurement on a dried sample is given by:

Intensity in scale units = area of sediment surface which contrib- uted deposition to the sample x number of years per slice of sediment x flux of atmospheric deposition x intensity per unit weight of deposit;

i.e.,

I = 5. P. F. SS . lo-3 , pS dr

where Z = meter reading due to x g DS from an S-cm slice of sediment x lo-* in dimensionless SI units,

X = weight of dry solid in grams taken for measurement,

P = bulk density of sediment in g DS crnM3 WS,

dr = total deposition rate of sediment (mm yr-9,

S = thickness of slice taken from the core (cm),


F = flux of any atmospheric deposition source (mg cm-* yr-l),

SS = specific susceptibility of deposited material (x 10e6 m3 kg-i).

If we normalize all data with respect to a 10-g sample from a l-cm slice, and take a typical value of 1.2 for the bulk density (bulk densities range from 1 for organic matter to approx. 2.6 for quartz (Ha- kanson, 1977)), then the intensity is di- rectly related to the specific susceptibility times the flux of deposited material, and in- versely related to the total rate of deposition on the sediment surface.

Fly ash. The flux of magnetic spherules from fly ash is dependent on the distance from the source (Puffer et al., 1980; Nriagu and Bowser, 1969; Thompson et al., 1980). Thompson et al. (1980) gave total spherule deposition rates for five sites in the United Kingdom and Finland. Four of these were from relatively unpolluted sites, and deposition rates differed by a factor of 7 from the largest to the smallest within this group. If we take the flux for the largest deposition rate at a site which is relatively free of local pollution (viz. S. Lake Dis- trict), then the total deposition since the start of the industrial revolution has been measured in lake sediments as 2.4 g m-* (J. Hilton, unpublished data) which is close to the estimate from the Thompson et al. figure from ombrotrophic peat bogs. Data from Karpansuo Bog (Thompson et al., 1980) show that deposition rates have changed by at least an order of magnitude over the last century. By scaling the recent Karpansuo flux in the ratio of the total fluxes for the two sites, the recent fly ash flux in the southern Lake District of the United Kingdom can be estimated as approximately 130 mg m-* yr-‘. This flux and a specific susceptibility of 316.5 x lO-‘j m3 kg-‘, obtained in this laboratory for cleaned magnetically extracted fly ash, can be substituted into Eq. (1). Results in- dicate that fly ash will not make a measur-

able contribution to the susceptibility (i.e., ~0.1 scale reading) above an accumulation rate of 0.4 mm yr-‘.

Eolian deposition. Data for atmospheric deposition of soil-derived particulate material (as opposed to atmospheric concentrations of particulate material) are scarce. Deposit guage data are available for a limited number of sites in the United Kingdom (Warren Spring Laboratory, 1980- 1982). These data are at best semiquantitative but show a mean deposition rate for 22 “unpolluted” sites in Britain of 12,800 mg m-* yr-i (35 mg m-* d-l). However, all these “unpolluted” sites are actually quite close to major sources of air pollution and must be a considerable overestimate of the soil- derived component alone. Hence a value of 0.366 mg cm-* yr-i (10 mg m-* d-l. the lowest annual mean value at any site) was taken as a more realistic estimate. The specific susceptibility (SS) of this material will vary from site to site, but clean clay minerals (Mehra and Jackson, 1960) from the southern Lake District are relatively highly magnetic, with a SS of 0.17 m3 kg-‘. Chemical cleaning removes magnetic oxides, hence the SS of eolian material in the southern Lake District will be greater than this value. Estimates of the catchment contribution to Esthwaite Water, United Kingdom (J. Hilton, unpublished data) suggest that a value of 0.3 m3 kg-’ would be more realistic. These values for flux and SS give a limit of 0.009 mm yr-l, above which atmospheric deposition of soil is unlikely to make any contribution to the magnetic record.

Cosmic particles. Parkin et al. (1980) gave a flux of lo+ mg m-* yr-l (14 tons d-i total flux to earth) for cosmic particles. They are very similar in size and shape to fly ash particles. Using the same specific susceptibility as for fly ash, an upper limit of 0.0003 mm yr-l is obtained.

Microbiological Processes

Magnetic bacteria. The sediments of

164 JOHN HILTON

many Lake district lakes are very productive. Typically they contain lOlo bacteria per ml (Jones, 1980) wet sediment. Al- though the weight of a bacterium is variable, a value of 0.2 pg bacterium-l is com- monly accepted (Luria, 1960), and Blake- more et nl. (1979) have stated that one type fo magnetic bacterium contains 2% by weight of magnetite. Visual observation of sediments from Blelham tarn (J. G. Jones, unpublished data) suggest that magnetic bacteria are very abundant, constituting approximately 0.1% of the total numbers. Using 679 m3 kg-’ specific susceptibility values for 50-nm cube particles (Bloe- mendal, 1982), 10 g of dry solid from Blelham Tarn, where accumulation rates are 10 mm yr-i, would contain approximately 0.04 ug Fe,O,, a magnetically un- measurable quantity. Assuming that this quantity of bacteria is produced each year in any lake of equivalent productivity, then magnetic bacteria would only make a measurable contribution at accumulation rates less than 0.1 mm yr-‘. If we assume a linear relationship between organic matter and growth rate, then the magnetic bacterial region would be bounded by a diagonal line. In reality a much lower proportion of the surface water production reaches the sediment in the deep, low-accumulation- rate oceans than in a shallow productive lake. Taking account of this fact puts a lower boundary on this region at about 300 mg C rnp2 d-l.

Authigenic minerals. Hilton et al. (1986) have shown that authigenic magnetic minerals only make a major contribution to the total signal in anaerobic sediments. How- ever, even aerobic sediments show very small, approximately IO%, changes in magnetic properties with time under oxidizing conditions, suggesting that the small anaerobic spaces within organic floes are sufti- cient to produce magnetic minerals (Hilton et al., 1986). Obviously, highly productive systems produce large quantities of magnetic minerals, whereas the aerobic lakes in

which losses are less than 10% of the total must be near the boundary for the formation of diagenetic minerals. The diagenetic boundary must lie just below the value for Buttermere (diagenetic fraction = 8% (Hilton et al., 1986); accumulation = 2 mm yr-i (Pennirgton, 1981); productivity = 1100 mg C rnp2 d-l (Jones, 1972)). At the present state of knowledge no direct influence of accumulation rate on the authigenic production of magnetic minerals has been identified. However, in high-accumulation- rate systems organic matter is rapidly re- moved from the easily available oxygen in the water column, creating anaerobic sediments with much less carbon input. Con- versely, in the deep oceans only a few per- cent (<I%) of the algal carbon reaches the sediments compared with the 30-50% which is more typical in shallow lakes. Simple mathematics suggest that an order of magnitude increase in accumulation rate will bring about anaerobic conditions at an order of magnitude lower productivity, but the accompanying dilution of organic matter will also increase the availability of oxygen. In the absence of more detailed in- formation a slope of Y3 will be used to indi- cate this effect.

Catchment

Catchment effects are only likely to be important in sediments where detrital soil and bedrock particles can be transported to them, i.e., lacustrine and and near-shore coastal sediments. Olsen (1978) puts the lower limit of accumulation at about 0.05 mm yr- ’ . However, at high productivities carbon will contribute a large proportion of the sediment, diluting the measurable catchment contribution. Assuming 100% transfer of C to the sediment, the catchment component will be unmeasureable at a productivity of 10,000 mg C mW2 d-i when the accumulation rate is below I mm yr- *. Hence the lower boundary slopes to lower accumulation rates with higher productivity.


All this data can be combined into a single diagram as shown in Figure 1A.

DISCUSSION

With the present state of our knowledge the model (Fig. IA) is, of necessity, very crude. However, its prime function is to draw attention to the likely contributory sources in the sedimentary record of any aquatic environment. This aim has been approached by choosing boundary conditions which create the worst case situation. Any site which is outside a particular boundary is unlikely to be affected by that source. The converse is not true, however, if a site lies within a particular boundary there is a high probability that the source in question will contribute to the total record and should be considered, but a variety of

0.000, 0.001 0.01 0.1 1.0 10 100

Total Deposition Rale mm y-1

FIG. I. (A) Regions influenced by different magnetic mineral sources. (B) Demarkation of productivity-accumulation rate combinations which are likely to be encountered in different aquatic environments.

situations may arise to reduce the effect of that source within a particular system, e.g., lower atmospheric deposition rates than used in the boundary calculations which are not compensated for by lower accumulation rates; dilution by nonalgal carbon, biogenic silica or calcium carbonate.

Using typical ranges for deposition rates (Olsen, 1978) and productivity (Eppley el al., 1983, the regions of different aquatic environments can be transcribed onto the productivity/accumulation rate diagram (Fig. 1B). A pattern emerges, suggesting that individual sources are only likely to be important in a limited number of sedimentary environments. Authigenically formed minerals will only contribute in estuaries, eutrophic lakes, and possibly in ocean sediments below upwelling areas. Cosmic deposition will only provide a significant contribution to the magnetic record in ex- tremely low-accumulation-rate oceanic sediments. Eolian deposition may well be important in most oceanic sediments, but not in the lacustrine environment. Simi- larly, magnetic bacteria are only likely to be important in low-accumulation-rate oceanic sediments below productive upwelling regions although they could make a contribution in low-accumulation-rate, high-productivity lakes. Finally, the catchment component will only exist, uncluttered by other sources, in relatively high-accumulation-rate, low-productivity lakes.

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a simple model for the interpretation of magnetic records in lacustrine and ocean sediments

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