Journal of Paleolimnology20: 163–173, 1998. 163c 1998Kluwer Academic Publishers. Printed in the Netherlands.

Magnetic properties of recent sediments in Lake Baikal,Siberia�

J. A. Dearing1, J. F. Boyle1, P. G. Appleby2, A. W. Mackay3 & R. J. Flower31 Environmental Magnetism Laboratory, Department of Geography, University of Liverpool, Liverpool L69 3BX,UK (e-mail: j.dearing@liverpool.ac.uk)2 Department of Applied Mathematics and Theoretical Physics, University of Liverpool, Liverpool L69 3BX. UK3 Environmental Change Research Centre, University College London, 26 Bedford Way, London, WC1H 0AP, UK

Received 23 June 1997; accepted 15 December 1997

Key words:Lake Baikal, mineral magnetism,210Pb, reductive diagenesis, erosion

Abstract

Mineral magnetic measurements of six210Pb-dated surface cores from different basins of Lake Baikal, Siberia,show temporal records controlled by a range of internal and external processes. With the exception of sedimentson the Academician Ridge, there is clear evidence for widespread reductive diagenesis effects on the ferrimagneticcomponent coupled with neo-formation of paramagnetic iron minerals. Greigite formation, bacterial magnetosomeaccumulation and turbidite layers may affect the properties of some sediment levels. Concentrations of cantedantiferromagnetic minerals (eg. haematite) appear to increase from the 19th century onwards. These mineralsare less affected by dissolution processes and probably represent detrital minerals delivered by catchment fluvialprocesses. The magnetic evidence for recent atmospheric pollution by fossil-fuel combustion processes is weak inall the cores, and supports the findings from studies of spherical carbonaceous particles (SCPs) and heavy metalsthat pollution is largely restricted to the southern basin. Correlations between recent sediments based on magneticdata may be insecure over long distances or between basins.

Introduction

Mineral magnetic measurements (Thompson & Old-field, 1986) have been applied to the study of lake sed-iments for more than two decades. Thompson et al.’s(1975) study of magnetic susceptibility profiles fromthe recent sediments of Lough Neagh, Northern Ire-land, was the first to demonstrate links between highconcentrations of detrital ferrimagnetic minerals andcatchment disturbance, but since then numerous stud-ies have shown the use of magnetic measurements tocorrelate cores (e.g. Dearing, 1986) and to reconstructhistories of erosion (e.g. Thompson & Morton, 1978;Oldfield et al., 1986; Higgitt et al., 1990; Snowball &Thompson, 1990; Dearing, 1992; Foster et al., 1996)and atmospheric pollution (e.g. Oldfield & Richard-

� This is the fifth in a series of seven papers published in thisspecial issue dedicated to the paleolimnology of Lake Baikal. Dr.Roger Flower collected these papers.

son, 1990; Williams, 1992). These different aspects ofsediment study are all important to the present LakeBaikal research programme (Flower, 1998). Large dis-tances require core correlation techniques to improvethe confidence of accepting regional rather than localinterpretation of sediment records and to help identifyanomalous sedimentation rates and patterns caused byturbidites (Lees et al., 1998b). Cultural activities whichare most likely to have altered the recent Lake Baikalecosystem are accelerated erosion from agriculturaland deforested landscapes and atmospheric pollutionfrom various industrial point sources. Mineral magnet-ic measurements also give information about climaticchange over glacial-interglacial timescales. Peck et al.(1994) have reconstructed palaeoclimates for the past250 kyr at Lake Baikal from slowly accumulating sed-iments sampled on the Academician Ridge (Figure 1).Interglacial periods are characterised by lower densitydiatomaceous sediment with small concentrations of

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Figure 1. Lake Baikal, site and location, showing coring locations of dated cores used in this study and other cores.

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low coercivity minerals whereas glacial periods showhigher clay contents and concentrations of all magnet-ic minerals especially high coercivity minerals, suchas haematite, which are attributed to increased aeolianinputs during arid phases. Validation of such proposedlinks between magnetic mineralogy and environmentalprocesses is often easier to achieve within recent sed-iments where environmental conditions and processesare more easily identified and more sharply defined.This is particularly important in the case of Lake Baikalwhere exceptionally deep water and slow accumulationrates could provide the conditions for reductive diage-nesis of magnetic minerals (Anderson & Rippey, 1988;Snowball, 1993), authigenic formation of ferrimagnet-ic iron sulphides (Hilton & Lishman, 1985; Hilton etal., 1986; Snowball & Thompson, 1988) and magneto-tactic bacteria (Oldfield, 1994; Snowball, 1994) to playmajor roles in controlling the mineral magnetic char-acteristics of sediment. Therefore the main aim of thispaper is to evaluate alternative controls on sedimentmagnetism in Lake Baikal.

Site and sampling

Cores of recent sediment were sampled between 1992and 1994 from a large number of locations using apurpose-designed box corer (Flower et al., 1995). Thepresent study presents results from the six210Pb-datedcores (Figure 1) extending to sediment depths rang-ing between 12 and 40 cm taken from the southernbasin (BAIK 6 and 38), the distal part of the Selen-ga delta (BAIK 19), the central basin (BAIK 22), theAcademician Ridge (BAIK 25) and the northern basin(BAIK 29), which are complemented by results fromanalyses of geochemistry (Boyle et al., 1998), micro-fossils (Flower et al., 1998) and spherical carbonaceousparticles (Rose et al., 1998). Preliminary results fromBAIK 6 have been published previously (Flower et al.,1995). All the cores comprise diatomaceous muds witha colour change from grey to brown within the depthrange 4–12 cm assumed to indicate the contemporaryoxic-anoxicboundary. Cores BAIK 19, 22 and 29 showevidence for the presence of discreet fine-grained anddiatom-rich turbidite layers.

Magnetic measurements and dating

The sediments were extruded at either 1 cm or 0.5 cmintervals and oven-dried (40�C). Each sample of oven-

dried material was gently ground from which an aliquotof � 0:3 g was taken for measurement in a Mol-spin vibrating sample magnetometer (VSM). The VSMmeasures the magnetisation of the sample through apre-set sequence of field strengths ranging from zeroto 1000 mT. Isothermal remanence is measured in zerofield (�0:1 mT) after each magnetisation step. TheVSM is calibrated to a palladium standard (moment at1000 mT= 31.23 mA m2) and field strengths are repeat-able to�0:1 mT at zero field and to�1 mT at highfields (> 100 mT). The precision of the equipment asshown by the coefficient of variation for repeated mea-surements of the calibration sample is� 0:1%. Thedetection limit is 0.01 mA m2 and the nylon sampleholders have a diamagnetic moment of� �0:2 mAm2 at 1000 mT. Small sample masses ruled out mea-surements of low field AC susceptibility, frequency-dependent susceptibility and anhysteretic remanentmagnetisation. Magnetisation and isothermal rema-nence data were used to calculate the following massspecific or ratio mineral magnetic parameters:

low field DC susceptibility (�LF :� @ 5 mT)high field paramagnetic susceptibility (�para:� @800 mT)ferrimagnetic susceptibility (�ferri :�LF � �para)percent paramagnetic susceptibility (%�para:�para=�LF � 100)percent ferrimagnetic susceptibility (%�ferri% :�ferri=�LF � 100)high field remanence (HIRM: IRM @ 1000 mT – IRM@ 100 mT)(saturation) remanence (Mrs : IRM @ 1000 mT)(saturation) magnetisation (Ms : M @ 1000 mT cor-rected for paramagnetism)ratio of saturation remanence to saturation magnetisa-tion (Mrs/Ms)ratio of low remanence to saturation remanence (Sratio : IRM @ 100 mT/IRM @ 1000 mT).

Sediment chronologies were obtained from analysesof 210Pb, 226Ra, 137Cs and241Am using gamma spec-trometry and both CIC and CRS models: results shownin this paper are described and evaluated in detail byAppleby et al. (1998). In summary, the last� 100 yearsis contained in the uppermost 15 cm and sedimentationrates rise in all cores during the 20th century. Dates of� 1900 and� 1950 are shown in the magnetic records.

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Figure 2. Mass specific concentration magnetic parameters plotted against depth for the six dated cores using spline fitting, showing210Pb-derived(Appleby et al., 1998) sediment dates� 1900 and� 1950 as horizontal lines.

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Figure 3. Interparametric ratios%�ferri=�para, Mrs/Ms, S-ratio plotted against depth for the six dated cores, showing210Pb-derived (Applebyet al., 1998) sediment dates� 1900 and� 1950 as horizontal lines.

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Results

In broad terms, the profiles of percent dry weight, massspecific concentration parameters (Figure 2) and inter-parametric ratios (Figure 3) for the set BAIK 6, 19,22 and 29 show some similarities in terms of trendsand data variance, but a lack of correlation betweenthe details of dated sections indicates that the role oflocal or in situ processes is probably strong. Decreas-ing upcore percent dry weight values in the lower partsof BAIK 25 and 29 are paralleled by decreasing con-centrations of magnetic minerals (Figure 2) but thisrelationship breaks down in the upper parts of coreswhere low densities in all cores probably reflect rel-atively high concentrations of labile organic matterrather than real reductions in minerogenic matter.

Profiles for�LF (Figure 2) and�ferri (not shown)correlate well in each core showing that�LF is con-trolled largely by the ferrimagnetic component. How-ever, the paramagnetic (�para) contribution to�LF (Fig-ure 3) is significant across the lake, ranging from� 10–15% in BAIK 6 to � 20–80% in BAIK 38. Thoughthe general trends in mass specific ferrimagnetic com-ponents (�LF, Ms, Mrs) are often parallel in each core,there are many examples of non-parallel peaks suggest-ing that ferrimagnetic minerals and grain-sizes maydiffer between samples in a core. With the exceptionsof BAIK 22 and BAIK 25, there is a tendency forthe concentrations of ferrimagnetic minerals to risetowards the sediment surface, and all cores show atleast one ferimagnetic ‘spike’ in the upper 4 cm. With-in sediments accumulated since 1900 the ferrimagnet-ic concentrations are highest in BAIK 6, followed byBAIK 22 and 19, and then by BAIK 25, 29 and 38.

S-ratios (Figure 3) decrease in the upper 10 cm inBAIK 6, 19, 22 and 38 indicating a gradual increasein the proportion of canted antiferromagnetic miner-als compared with ferrimagnetic minerals. This trendis seen only in the upper 2 cm of BAIK 29 and isreversed in BAIK 25; in both of these cores lowerS-ratios suggest a higher proportion of canted antifer-romagnetic minerals than in the other cores. Mrs/Ms

ratios (Figure 3) are indicative of the relative impor-tance of ‘hard’ magnetic phases, such as stable singledomain magnetite, greigite and haematite, and showtrends in BAIK 6, 19, 29 and 38, which are general-ly the inverse of those for S-ratios. In general, cantedantiferromagnetic minerals, such as haematite, seemto dominate changes in the ‘hard’ components morethan either stable single domain magnetite or greig-ite. Increasing trends in the HIRM data (Figure 2)

since 1900, indicative of the concentration of cantedantiferromagnetic minerals, are strongest for BAIK 6followed by BAIK 19, 22 and 29, but the trend isdecreasing in BAIK 25 and ill-defined in BAIK 38.

Trends for paramagnetic minerals show no com-mon trends with sediment depth (Figures 2 and 3) foreither mass concentrations (�para ) or relative propor-tions (%�para) or clear correlations with other parame-ters. In BAIK 6 and 22 significant peaks in the con-centrations of paramagnetic minerals occur just belowthe mud-water interface (0–3 cm) and in BAIK 22 at8–12 cm.

Discussion

1. Post-depositional changes

There are four types of post-depositional effects onthe magnetic properties of detrital minerals: reductivediagenesis of ferrimagnetic minerals; authigenic for-mation of ferrimagnetic greigite; authigenic formationof paramagnetic/canted antiferromagnetic oxyhydrox-ides, such as ferrihydrite and goethite; and incorpora-tion of ferrimagnetic magnetosomes from magnetotac-tic bacteria.

Values for�ferri are not reduced to zero in any ofthe cores, indicating that reductive diagenesis acting onferrimagnets, if present, is partial. The strongest evi-dence that reductive diagenesis has affected some sec-tions of sediment is in the relationship between the fer-rimagnetic, paramagnetic and total Fe records (Boyleet al., 1998). In BAIK 6 and 22,�para and total Fe arelinearly associated (Figure 4) suggesting that peaks intotal Fe concentration in these cores are largely con-trolled by the presence of paramagnetic iron oxyhy-droxides. The source of paramagnetic oxyhydroxidesmay be either detrital or authigenic, but in Lake Baikalthe evidence suggests an authigenic source derivedfrom the partial reduction of previously deposited ironoxides. In both cores and to a lesser extent others, thepeaks in total Fe and�para are preceded (i.e. lower inthe core) by reductions in the ferrimagnetic componentand followed (i.e. higher in the core) by peaks in Mn(Boyle et al., 1998; Figure 1). This is consistent witha reducing front in the upper few centimetres causingthe partial dissolution of ferrimagnetic minerals andthe subsequent migration of Fe and Mn upwards to theoxic zone and precipitation as paramagnetic minerals.The one exception is BAIK 25 (Academician Ridge)which shows no evidence for Fe and Mn mobility in

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Figure 4. Mass specific paramagnetic susceptibility (�para) plotted against total Fe (Boyle et al., 1998) for the six dated cores.

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the uppermost sediments, and the weakest associationbetween�para and total Fe (Figure 4) suggesting thatthe coring site lies in a deep oxidised zone of the lakebed where�para is controlled by the concentration ofdetrital clay minerals.

The sharp peaks in�ferri in the uppermost sedimentsof all cores except BAIK 6 could be evidence for thepresence of either greigite (Fe3S4) or magnetite in theform of bacterial magnetosomes. Each peak is associ-ated with a sharp rise in the S-ratio confirming a shifttowards dominantly low coercivity (soft) ferrimagneticproperties. Magnetic discrimination between the twominerals is through their remanence properties and theredox conditions of their formation. Greigite exhibitshigh coercivity (hard) ferrimagnetic properties withcharacteristically (very) high Mrs values relative to�LF

but is a product of strictly reducingconditions in organ-ic and sulphur-rich sediments. Magnetotactic bacteriaproduce stable single domain magnetite grains withrelatively hard magnetic properties, but the live bacte-ria are restricted to microaerophilic environments. Theapparently soft magnetic behaviour coupled with thecontinuous presence of well-oxygenated bottom waterover a large part of the Lake Baikal bed, allowing thewidespread precipitation of Fe and Mn in surface oxi-dised sediments, suggests that the sharp peaks of ferri-magnetic concentrations are possibly zones of magne-tosome production and accumulation rather than greig-ite formation. This argument is supported by three fur-ther observations: the absence of ferrimagnetic peaksin more strongly reducing deeper sediments; the lackof very high Mrs values diagnostic of greigite; andthe lack of evidence for greigite in or near those sed-iment zones where reductive diagenetic processes arebelieved to have operated. None of the lower sectionsof cores shows peaks in�ferri of comparable magnitudeto those at or near the surface which implies that accep-tance of a magnetosomeexplanationhas also to includetotal or partial magnetosome dissolution in sedimentslying below 5 cm or alternatively that the existence ofmagnetotactic bacteria is a recent phenomenon. Snow-ball (1994) confirmed that magnetosomes dominatedthe ferrimagnetic properties of the upper� 50 cm ofsome Swedish lake sediments, but that these were sub-ject to reductive diagenesis as they became buried toleave coarser multidomain grains dominating the fer-rimagnetic properties of old sediment. Microscopic ormolecular methods are required to confirm these alter-native explanations.

2. Turbidites

Turbidites are rapidly deposited layers derived fromextreme events such as floods, or the reworking andslumping of upslope sediments. In some cores fromLake Baikal they are characterised by minerogenicand diatomaceous layers fining upwards from sandyto silty, with an initial rise in�LF values which declineup-core (Lees et al., 1998b). Turbidite sedimentationbetween 1.2–1.4 cm and 5.75–6.75 cm in BAIK 22 isindicated from sediment accumulation rates (Apple-by et al., 1998) and lithology descriptions (Mackay etal., 1998).�LF values show a decreasing trend overthe depth range 1–7 cm and are matched by similardecreasing trends in�ferri, Ms and Mrs, low valuesof paramagnetic parameters (�para and %�para), butbroadly constant values of HIRM. Close inspection ofthe density record of BAIK 22 show that some peaksin density are associated with small minima in magnet-ic concentration parameters, and a similar associationcan be found in the turbidite layer of BAIK 29 (0.6–0.8 cm). The evidence suggests that turbidites in thesecores are composed of slumped diatomaceous sedi-ments rather than reworking of minerogenic sedimentfrom the margins with higher density detrital minerals(cf. Lees et al., 1998b) and that they may dilute theconcentration of catchment-derived detrital minerals.

3. Catchment erosion

Apart from the sand particles in flood and shallow waterslump turbidites, catchment-derived material whichreaches the deep water zones is likely to consist ofclay-silt sized minerogenic particles eroded from soilsand terrestrial sediments. An influx of minerogenicparticles within this size range could be expected togive a variety of magnetic signals depending uponthe mineralogy of the geological source, the soil typeand particle-size. However, most clay and silt-sizedminerogenic sediments, unless dominated by quartzwill include ferrimagnetic, paramagnetic and cant-ed antiferromagnetic minerals. All the lake sedimentsamples contain measurable concentrations of ferri-magnetic, paramagnetic and canted antiferromagnet-ic minerals, indicating that a consistent detrital min-eral contribution from catchment sources is a pos-sibility. However the evidence for widespread post-depositional changes makes interpretation in termsof detrital sources of ferrimagnetic and paramagneticdata problematical. Canted antiferromagnetic minerals(haematite and goethite) are the iron oxyhydroxides

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most resistant to reductive dissolution which suggeststhat any record of long term (i.e. excluding extremeflood events) shifts in the delivery of eroded catch-ment material will be best recorded in the HIRM pro-files. The consistency in the trends of HIRM, increas-ing since at least 1900 in the three deep water cores(BAIK 6, 22 and 29) and BAIK 19 opposite the Selen-ga inflow suggests that catchment erosion particular-ly from igneous rock sources has increased since the19th century; Lees et al.’s (1998a) magnetic data forsediment sources in the catchment show that igneousrocks have at least 2� the HIRM value of sedimentaryrocks and topsoil. The trigger for increased erosion isnot known, but could include changes in farming andforestry practices, increased construction or climatechanges leading to, for example, higher flood frequen-cies or faster snowmelt. The post-1950 increase in sed-imentation rates (Appleby et al., 1998) coupled with arising trend of HIRM values argues for a recent accel-eration in erosion, but the dilution effects of biogenicsilica and organic matter on the magnetic records needto be evaluated before this argument can be accept-ed. The S-ratio profiles support an interpretation ofhigher HIRM in terms of increased canted antiferro-magnetic mineral concentrations, but caution is neededbecause the S-ratio profiles are clearly influenced bythe absolute changes in the concentrations of paramag-netic and ferrimagnetic concentrations brought aboutby dissolution, authigenesis and magnetosome produc-tion.

4. Atmospheric pollution

Magnetic spheres produced in predominately coal-fired combustion processes contain both magnetite andhaematite and are therefore detectable by ferrimag-netic and canted antiferromagnetic parameters. Post-depositional changes to the ferrimagnets would indi-cate that, as with eroded catchment material, the opti-mum magnetic parameter is HIRM. Values of HIRMin catchment samples are as high for modern pollutedsnow samples as for igneous rock samples (Lees et al.,1998a). Discrimination of the effects of atmosphericpollution from erosion on the magnetic record is there-fore difficult but the weight of evidence points to thelatter. Studies of heavy metals (Boyle et al., 1998)and SCPs (Rose et al., 1998) in these cores concludesthat the major control on heavy metal concentrationis sediment supply and only in the southern basin isatmospheric pollution detected. The highest post-1950HIRM values are recorded in BAIK 6 from the south-

ern basin and the trend parallels the curve for SCPsfrom the 1930s when they are first recorded in the sed-iments, but the rising HIRM values start in this and inother cores during the 19th century.

5. Glacial-interglacial mineral magnetic records

In their study of glacial-interglacial sediments fromthe Academician Ridge, Peck et al. (1994) used HIRMto infer aeolian inputs of iron-stained grains duringglacial periods. The present study shows that dur-ing interglacial times, HIRM may indicate fluvially-transported soils and sediment, raising the questionof how records of HIRM during glacial periods maybe confirmed as showing the effects of long distancedust transport. The dominance of glacial sediments byangular quartz grains (Flower pers. comm.) supportsa ‘mountain loess’ origin (Smalley, 1995), but fluvialtransport within a periglacial catchment over relativelyshort distances could also deliver sediment with highconcentrations of canted antiferromagnetic minerals.Long interglacial mineral magnetic records of detritalminerals in cores from the southern basin may pro-vide information about the growth and function of theSelenga Delta as it responds to major lake level changes(cf. Romashkin & Williams, 1997).

6. Core correlations

The recent magnetic records from Lake Baikal demon-strate the array of potential mineral sources, processesand transformations encompassing the catchment, thelake and the sediments. Intuitively, the likelihood oftracing synchronous layers between cores will becomeincreasingly difficult as the distance increases. Leeset al. (1998b) show convincing evidence for core cor-relations based on magnetic signatures of turbiditesover distances of up to tens of kilometres, but on theevidence of the present study it must remain doubtfulwhether magnetic-based correlations in recent sedi-ments between basins or even sub-basins are possibleeverywhere. Indeed, given the strong morphologicaland hydrodynamic contrasts between basins, the largecatchment size and the numerous inflows, it is unlike-ly that short term influxes of fluvially-derived detritalsediments to the lake are driven by synchronous andcatchment-wide processes. For studies of recent sedi-ments, the best means for core correlation is likely tobe based on atmospherically-derived properties (hencethe success of210Pb dating) such as SCPs (Rose et al.,1998). In older sediments, signals of autochthonous

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processes such as the widespread� 18th century risein one species of diatom (Mackay et al., 1998) mayrepresent a sounder basis for core correlation betweenbasins. In long glacial-interglacial sediment sequences,major climate-mediated shifts in the whole lake’s sed-iment system will be useful for correlation purposesas demonstrated by Julius et al.’s (1997) analysis ofdiatom-rich interglacial and diatom-poor glacial sedi-ments in the southern basin and Peck et al.’s (1994) useof susceptibility to identify glacial-interglacial bound-aries across large distances on the Academician Ridge.

Conclusions

Mineral magnetic measurements of six recent sedi-ment cores from across Lake Baikal show the pos-sible effects of a variety of controls including reduc-tive diagenesis, authigenesis of paramagneticminerals,bacterial magnetosome production, atmospherically-derived magnetic particulates and catchment-deriveddetrital minerals. With the exception of a core fromthe Academician Ridge, the evidence is strongest forthe presence of widespread reductive diagenesis ofiron-containing minerals including ferrimagnets withupward migration of Fe to form paramagnetic miner-als in the seasonally oxic zone beneath the mud-waterinterface. Sediments on the Academician Ridge showevidence for magnetosome accumulation but signals ofreductive diagenesis are weak. There is evidence fromHIRM data that there has been a widespread increase inthe delivery of catchment-derived minerogenic materi-al to the deep parts of the northern and southern basinssince the 19th century. The opposite trend found on theAcademician Ridge may be explained by its isolationfrom sediment-carrying bottom currents and the dilut-ing effect of diatomaceous silica on magnetic concen-tration parameters. Further diagnostic tests of mineral-ogy and sediment composition are required to confirmthese findings.

Acknowledgements

The work presented here has been carried out in collab-oration with other colleagues in the Lake Baikal Pro-gramme. Field work facilities at Irkutsk and on LakeBaikal were kindly provided by Prof. Grachev, Direc-tor of the Limnological Institute in Irkutsk. Thanksare due to Dr Ye. V. Lihkoshway who organised themain sediment collection expedition, to Don Mon-

teith who helped to collected most of the sedimentcores, to Bob Jude who measured some of the samplesand produced the diagrams, and to Dr J. A. Lees foroffering comments on a first draft. The Lake Baikalprogramme received financial support from the Roy-al Society (BICER), the Leverhulme Trust (ProjectF134AZ), and ENSIS Ltd (University College Lon-don). Thanks are due to both reviewers for makingconstructive comments on the text.

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