evidence for the dissolution of magnetite in recent scottish peats

QUATERNARY RESEARCH 37, 171-182 (1992)

Evidence flor the Dissolution of Magnetite in Recent Scottish Peats

MARTIN WILLIAMS

British Geological Survey, Applied Geochemistry Unit, Keyworth, Nottingham NG 12 5GG, United Kingdom

Received November 13, 1990

Magnetic, hydrological, and geochemical data for 11 Scottish peat cores confirm previous re- ports that anthropogenic pollutants constitute the principal source of ferrimagnets to such environments, but indicate that postdepositional Fe,O, persistence may be more limited than formerly envisaged. In all cores, the lower stratigraphic limit of magnetic enrichment coincides with the water table, below which negative mV values and enhanced ratios of FeiMn and CulZn depict an abrupt transition to a strongly anoxic regime. Dissolution of Fe,O, in the reducing, inundated zone is also signified by the more rapid down-core loss of magnetic properties in cores from topographically depressed “Sphagnum lawns” than in those from more elevated “Calluna hummocks.” As a consequence, the peak and cumulative SIRM values for cores from the former settings appear disproportionately low, and the magnetostratigraphic records from adjacent, but topographically variant sites are frequently asynchronous. In contrast to most marine and lacustrine sediments, dissolution of Fe,O, in peat induces a refinement of magnetic mineral assemblages, highlighted by declining Sl.RM/ARM ratios with depth and a “hardening” of remanence near the base of the magnetically enhanced strata. SEM micrographs of magnetic extracts from one Scottish core indicate that this refinement results from preferential dissolution along crystallographic weak- nesses within multidomain phases, ultimately causing their disintegration and the adoption of “single domain-type” magnetic behavior. Rates of dissolution are considerably greater than those reported for marine sediments, probably due to the unusually high dissolved sulfide concentrations in mire waters. 0 1992 University of Washington

INTRODUCTION

The partially reduced iron oxide magnetite is present as a minor mineral compo- nent in most depositional environments (e.g., Kobayashi and Nomura, 1974; Levi and Banerjee, 1976; Thompson, 1986). It is derived from a variety of natural sources, including crustal denudation (e.g., Karlin, 1984; Thompson, 1986), bacterial mineral- ization (e.g., IBlakemore, 1975), and sedimentary authigenesis (e.g., Henshaw, 1978). In addition, hydrocarbon combustion processes are responsible for the mo- bilization of an “anthropogenic” magnetite flux which may enter depositional systems via the atmosphere (e.g., Raask and Goetz, 1981; Hunt et ul., 1984) or through water

I Throughout the text, the term “magnetic take-off’ is used as defined in previous studies (e.g., Jones, 1986) and refers to the lowest stratigraphic level showing significant magnetic enhancement.

bodies (e.g., Scoullos et al., 1979). This flux exerts little influence on the crystalline iron mineralogy of most lithogenous sediments, but may extensively control the ferrimagnetic properties of recently deposited biogenic sequences such as ombrotophic peats (e.g., Oldfield et al., 1981).

The conspicuous magnetic enhancement of naturally diamagnetic peat deposits by atmospheric deposition has been widely documented (Oldfield et al., 1978, 1979a, 1981; Thompson et al., 1980). Spatial studies throughout the United Kingdom and Scandinavia have shown that the saturation isothermal remanent magnetization (SIRM) values of surficial peats vary in direct ac- cordance with the proximity of industrial pollutant sources (Thompson et al., 1980), while down-core analyses of peat profiles have led to recurrent observations of a “magnetic take-off” in strata dating to ca. 150 yr B.P. (e.g., Oldfield et al., 1981). At virtually all sites studied thus far, SIRM

171 0033-5894/92 $3.00 Copyright Q 1992 by the University of Washington. All rights of reproductmn in any form reserved.

172 MARTIN WILLIAMS

signals have been found to peak at, or near. the present-day peat surface (Oldfield et al., 1984; Jones, 1985) where values are generally reported to exceed those of pre- industrial strata by 2 to 3 orders of magnitude. Consequently, it has been widely proposed that the down-core concentration gradients of ferrimagnetic minerals in peat accumulations may be valuable for deci- phering the history of atmospheric particulate deposition (e.g., Oldfield et al., 1984). Further, it has been suggested that the magnetic take-off may occur with sufficient reg- ularity in strata of early postindustrial age to facilitate its use in geochronology and core correlation programs (e.g., Jones, 1985).

Throughout most previous mineral magnetic studies of peats, the possible postdepositional alteration of magnetic phases has received little attention. However, following their observation of progressive magnetite dissolution with burial in Gulf of California and Oregon Shelf marine sediments, Karlin and Levi (1985) proposed that the declining down-core magnetic mineral concentrations in peats may constitute a predictable diagenetic phenomenon, given the severity of the prevailing pH/Eh regime. Here, an attempt is made to outline the theoretical and empirical basis for this hypothesis, and a combined geochemical/ mineral magnetic approach is used to exam- ine the likely extent of Fe,O, dissolution in peats from Scotland, UK.

THEORETICAL AND EMPIRICAL BACKGROUNDS

Early diagenesis in sediments is ultimately caused by the decomposition of organic matter. Following the exhaustion of 0, for direct oxidation, a series of microbi- ally mediated anaerobic processes is in- voked for this purpose. Their rates and concomitant effects on the substrate depend on the availability of reductants and the com- petitive efficiency of the microbial popula- tions present (Karlin and Levi, 1983). Be-

cause microbes preferentially reduce the chemical species which will yield the great- est energy for further organic breakdown. a well-defined sequence of MnO, ., NO, > Fe3 + oxide > SO4 reduction is typically identifiable (Karlin. 1984). Methanogenesis may subsequently occur under the most se- verely anoxic conditions.

Throughout the above reductive sequence, crystalline Fe oxides (particularly partially reduced phases such as Fe,O,) have often been considered to remain relatively inert. However, much of the thermodynamic data used to support this assump- tion (e.g., Watkins et al., 1974: Govatt. 1974: Garrels and Christ, 1975) has recently been shown to have misrepresented the Fe,O, stability field by failing to incorpo- rate the full range of hydrous phases into calculations (Henshaw. 1978). By varying the theoretical solid-solid interactions and by incorporating hydrous FeOOH into a thermodynamic model for the Fe-S-H,0 system, Henshaw and Merrill (1980) have shown that Fe,O, reactivity can extend to the pH/Eh conditions of most waterlogged peats, thus intimating that dissolution is plausible.

Observations of pristine magnetite under conditions which exceed the minerals’ stability field have often prompted the view that reaction kinetics preclude dissolution in the surface environment (e.g.. Huber. 1975; Krauskopf, 1967). However. Hen- shaw (1978) and James (1975) have cited kinetic controls to explain the authigenesis/ diagenesis of metastable Fe,O, in Jurassic and Holocene sediments, while Schwert- man and Taylor (1972) and Yaalon (1970) have shown that the high activation ener- gies required may slow, but will not neces- sarily stop, dissolutionary processes. On the basis of the data of Lindsay (1979). Fe,O, dissolution appears possible at kinet- ically controlled rates under conditions of pH 4/Eh- 106 mV through pH 5/Eh - 165 mV to pH 6/Eh - 224 mV. This entire range has been recorded in UK peatlands (Wil- liams, 1988).

MAGNETITE DISSOLUTION IN PEAT 173

Conclusive empirical evidence of magnetite dissolution in recent deposits was first provided by Karlin (1984) and Karlin and Levi (1983, 1985) who interpreted progressive down-core losses of NRM intensity and accompanying nonvolatile sulfide accumulation through the surticial 2 m of sediment from the: California Gulf and Oregon Shelf as indicating rapid Fe,O, dissolution and pyritization. Falling median demagnetizing field (MDF) values were also observed immediately beneath the surface, suggesting the preferential removal of fine superparamagnetic (SP) and single domain (SSD) ferrimagnets. Additional work on marine sediments (Canfield and Berner, 1987), lake muds (Anderson, 1986), and gleyed soils (Maher, 1984) has shown such granulometrically selective dissolution to be widespread and Canfield and Berner (1987) have attempted to quantify the process by equating the rate of Fe,04 loss to the specific surface area of the magnetite assemblage and the ambient concentration of dissolved sullide. The inclusion of this latter factor in Canfield and Berner’s model has important implications for Fe,O, stability in Scottish peats, as total sulfur concentrations are frequently in excess of 1.5% (Williams, 1988).

FIELD SAMPLING PROCEDURE

During the period from 1984 to 1986, 11 sample cores were collected from ombrotrophic settin,gs in Galloway, Glen Quoich (Invernesshire), Glen Muich (Aberdeen- shire), and the Rannoch Moor area (Argyll) by use of a l-m-long by 22-cm-diameter barrel corer. Galloway cores Gl-G2, G3- G4, and Rannoch cores Rl-R2 represent “pairs” from adjacent “Sphagnum lawn” and “Calluna hummock” sites (less than 5 m apart) and are ideal for evaluating the effects of microtopography and local groundwater variability on the magneto- stratigraphy of the peat. Each core tube was equipped with sealable access points for a Pt electrode, allowing down-core Eh measurements to be made in the field im-

mediately after core extraction. An AGB- 50 redox meter was used for this purpose. Field measurements of water table depth were taken by inserting a 1 m x 7 cm- diameter tube (with “flow holes” drilled at 5-cm intervals along the walls) into the peat and noting the height of the water column within it after a brief equilibration period.

LABORATORY PROCEDURE

In the laboratory, all cores were cut into contiguous slices l-cm thick. The subsamples were then dried at 37°C and split into three independent aliquots for use in magnetic, geochemical, and mineral extraction analyses. On account of the nondestructive nature of magnetic analyses, this aliquot was subsequently used for palynological and radiometric dating.

Magnetic Analyses

Samples used in magnetic analyses were packed into precleaned IO-cc polythene cylinders and weighed to an accuracy of 0.001 g (thus facilitating accurate mass- specific computation of magnetic results). Anhysteretic remanent magnetizations (ARM) were produced for selected core samples in a custom-built AC demagnetization unit during the progressive decay of an alternating field from 100 to 0.1 mT in the presence of a steady (DC) field of 0.1 mT. Saturation isothermal remanent magnetization values (SIRM) were acquired for all samples following their subjection to a 1.0-T field. All “saturated” samples were subsequently placed within reversed polarity fields of 40 and 100 mT with the loss remanence being recorded in each case. The values of IRM- ,,,/SIRM presented here can be considered analogous to the “S” quotient proposed by Thompson et al. (1975) for the determination of magnetic mineralogy and granulometry. All remanence measurements were made using a Molspin portable flux-gate magnetometer.

Methods of magnetic mineral extraction followed those of Canfield and Berner (1987). Approximately 5 g of dry material

174 MARTIN WILLIAMS

was placed in 500 ml of water and retained in suspension for 24 hr using an electrical stirrer. A Rare Earth magnet in a latex sheath was immersed in the fluid throughout the stirring period, after which it was removed and any particulates on the sheath evaporated to dryness. The extracts were then washed in acetone and examined under a Cambridge Instruments scanning electron microscope.

Chemical Analyses

A Unicam AAS system was used to de- termine the concentration of Fe, Mn, Cu, and Zn (along with some 20 other elements) in all core subsamples following the diges- tion of 2 g of dry material with HFIHClO,/ HNO,/HCl. Data for the four specified elements are presented here as Fe/Mn and Cui Zn ratios. A detailed discussion of the distribution of a wider range of elements is provided elsewhere (Williams, 1988).

Geochronology

A crude geochronologic framework was produced for four cores using a combina- tion of 210Pb and palynological methods. Pollen data for the “paired” Galloway cores, Gl and G2 illuminated markedly increased frequencies of Pinus and Picea up- ward of I1 and 7 cm. respectively. corre- sponding to the onset of afforestation over the Cairnsmore and Loch Doon areas in 1962. A synchronous reduction of Plantago/ arboreal pollen ratios was observed at 29 cm in these cores, and negligable “unsupported ” “‘Pb assays of 0.07 and 0.03 pCi/g at the same depth facilitated a tentative age determination of 140 yr B.P. for this hori- zon. Rannoch Moor core R2 was subjected to detailed “‘Pb analysis, allowing the determination of mean accumulation rates of 0.21 cm/yr-’ for the 0- to 3-cm sequence and 0.11 cm/yr-i for peats at 4- to 1 l-cm depth. More limited “‘Pb analyses of core Rl showed unsupported concentrations to decline to zero at 14 cm, suggesting an age of ca. 150 yr B.P. for this level.

RESULTS AND DISCUSSION

General Magnetic Trends

The results of SIRM and S analyses for all cores are presented in Figure 1. In ac- cordance with most previous studies (e.g., Oldfield et al.. 1978, 1979a, 1981). all cores show SIRM increments within the upper 5-30 cm, with ferrimagnetic concentrations appearing to exceed those of the deeper peat by 2 to 3 orders of magnitude. Addi- tional analyses of longer (3 m) cores from Galloway and Rannoch Moor have con- firmed that such enhancement is unique to the near-surface (Williams, 1988). Accord- ingly the SIRM values at ca. 30- to 35cm depth can be regarded as reflective of magnetic mineral concentrations throughout the entire underlying peat column.

The peak and cumulative SIRM values for the 11 cores (Table 1) illustrate a persistently high degree of ferrimagnetic enrichment in the Galloway peats compared to that within the cores from more remote locations in central and northern Scotland. For example, the cumulative SIRMs for cores Gl-G6 (Galloway) exceed those of any Rannoch Moor core by factors of 1.8- 6.0. This trend is consistent with previous claims regarding both the predominance of anthropogenic magnetite fluxes and the tendency for deposition rates to decline with increasing distance from major industrial centers (e.g., Thompson et al., 1980).

The S quotient data provide a basic in- sight into the magnetic mineralogy of the enriched peat strata (Fig. 1). All enhanced samples attain between 60 and 100% magnetization reversal in response to place- ment in a lOO-mT demagnetizing field. Al- though antiferromagnetic phases such as hematite may also be present, such trends signify the predominant control of ferrimagnets on the overall magnetic properties of the surficial peat. The coercivity profiles for cores Gl, G2, and G5 (Fig. 2) confirm this point, the most enriched samples un- dergoing complete remanence losses in reversed polarity lields of ca. 40 mT. The


R5

GQl

FIG. 1. SIRM, S, Fe/Mn, and Cu/Zn gradients for sample cores from ombrotrophic settings in Scotland. Cores Gl/G2, G3/G4, and Rl/R2 represent pairs from adjacent Sphagnum lawn and Calluna hummock microtopographies (Table 1 gives further details). Dates are provided where possible on cores Gl, G2, Rl, and R2. The level of water inundation at each coring site is depicted by the horizontal dashed lines. These lines are generally coincident with the major down-core redox boundary. The ratios Fe&In and Cu/Zn and the quotient S are dimensionless. All SIRM values are given in mAm’kg- ‘.

176 MARrlN WILLIAMS

TABLE 1. PEAK ANDCUMULATIVE SIRM VALUES FOR IISAMPLF.CORES FROM OMBRoTRoPtit( SWTINGS IN SCOTLAND

Core Site Microtopography Peak SIRM mAm’kg ’ Cumulative SIRM mAm’kg ’

Gl Galloway Calluna hummock G2 Galloway Sphagnum lawn G3 Galloway Calluna hummock G4 Galloway Sphagnum lawn G6 Galloway Calluna hummock RI Rannoch Calluna hummock R2 Rannoch Sphagnum lawn R5 Rannoch Sphagnum lawn Ml Glen Muich Calluna hummock

GQI Glen Quoich Calluna hummock

GQ2 Glen Quoich Sphagnum lawn

Nore. Topographic information is also given.

SIRM/ARM data presented for 5 cores (Fig. 3) provide additional granulometric information, with peak values of over 100 indicating large, pseudo-single domain (PSD) or multidomain (MD) magnetite presence. Whitby and Cantrell(1975) have proposed a scheme whereby Fe&-Fe oxide transformations during hydrocarbon combustion produce mineral assemblages rich in ferrimagnetic spinels of PSD or MD size. Ac- cordingly, the high surficial SIRM/ARM values reported here can be considered consistent with the view that peatlands re- ceive influxes of magnetic phases from pre- dominantly anthropogenic sources (e.g., Oldfield et al., 1981).

Magnetite Persistence during Burial

Unlike the marine and lacustrine sediments in which the persistence of Fe,O, has previously been examined (e.g., Karlin, 1984; Anderson, 1986), decreasing magnetic response with peat depth cannot, in itself, be considered indicative of dissolution; such profiles primarily resulting from increased postindustrial deposition. How- ever, several features of the data presented in Figure 1 strongly indicate that the down- core SIRM gradients of the 11 sample cores are not solely controlled by allogenic mineral fluxes, but are perpetually subject to postdepositional modification. These can be summarized as follows:

2.0 10 4.1 44 I.1 I4 0.8 I: 2.4 IX I.0

-1

0.9 0.3 3

I.0 h 0.39 1

0.30 2

(1) In all cores, the decline of SIRM values toward background is stratigraphically coincident with the level of the water table. Field analyses of down-core Eh gradients showed that this level coincides with a sharp redoxycline, below which values as low as - 200 mV typically prevail. Such Eh values are quite sufficient for pyritization (Brookins, 1988) and it appears likely that the rapid dissolution of magnetite grains following their translocation from aerated surficial peats into waterlogged anoxic strata constitutes an important diagenetic control on the precise location of the magnetic take-off. The apparent strength of the relationship between down-core magnetic and hydrological variations at the 11 sampling stations suggests that adjustments of magnetic stratigraphy occur relatively rap- idly in response to major (e.g., seasonal) water table movements. However, detailed research into this matter remains to be un- dertaken.

(2) The constraint imposed by the peat Eh regime on Fe,O, persistence is further emphasized by the relationship of SIRM with certain redox-sensitive geochemical parameters. In most instances, the stratigraphic level of the magnetic take-off is characterized by a sharp decrease of FelMn values to below 100. In cores Gl, G2, G3, G5, Ml, GQL, and GQH, the lower point of magnetic enhancement also correlates with

MAGNETITE DISSOLUTION IN PEAT

. f

.I.

..: .::. .

.I ‘Oj

. . ,.:

5 f

F h 20: : 0 .

. . .

Gl G2 G-5

FIG. 2. Typical variation of coercivity with depth in -40-mT demagnetization curves through cores Gl, G2, and. GS. Levels where the profile deflects to the right of the zero line are characterized by coercivities of less than 40 mT, while deflection to the left indicates a hardening of magnetic remanence and increasing coercivity. The dashed lines mark the level of the magnetic take-off in the respective cores.

an up-core reduction of Cu/Zn values by 80400%. Such changes generally signify a transition from anoxic to oxic conditions (e.g., Mackereth, 1965; Hirons and Thomp- son, 1986) and hence confirm the limited persistence of Fe,O, beneath the peat redoxycline.

(3) In circumstances where magnetic profiles solely reflect temporal trends of Fe,O, deposition, good chronological agreement should be evident between the magnetic records of cores from proximal sampling sites. Yet radiometric and palynological data obtained for selected cores (Williams, 1988) indicate that the magnetic profiles from different microtopographic facets of individual mires are, in fact, markedly asynchronous. In the dated Galloway Cal-

luna hummock core G 1, the magnetic take- off is located at the 30-cm depth in strata ca. 150 yr old, while the adjacent Sphagnum lawn core G2 displays no evidence of magnetic enrichment in peat deposited prior to ca. 100 yr B.P. Similar trends are identifiable on examination of paired cores from Rannoch Moor, with the magnetic take-off dating to around 150 yr B.P. at the raised hummock site Rl and no magnetic enhancement occurring in levels exceeding 90 yr old at the proximal lawn site R2. While acknowledging that 2’0Pb chronologies for ombrotrophic peats may be prone to significant error (e.g., Oldfield et al., 1979b), such dramatic asynchrony of magnetic trends is consistent with the restricted sur- vival of Fe,O, during burial in Sphagnum

178 MARTIN WILLIAMS

YRM/ARM

0 150 0

3

iF x

r

40-1 64

0 IS” 0

3

E

% cl

P 40 R1

FIG. 3. Down-core variations of the ratio SIRMi ARM through cores Cl, G2, G3, G4, and RI. Four of the cores yield values in excess of 100 at or near the peat surface, depicting a predominance of large, pseudo-single domain or multidomain fertimagnets in the magnetic assemblages at these levels. The progressive decline of SIRM/ARM ratios with depth to values below 10 is consistent with a down-core refinement of magnetic mineral phases. SIRM/ARM values are dimensionless.

lawn microenvironments relative to more elevated Calluna hummocks on account of the higher level of inundation associated with the former. The more rapid dissolution of Fe30, in areas of inundated microtopog-

rdphy also explains the persistently low cumulative SIRM values given for lawn relative to hummock cores in Table 1

Mechanisms of Dissolution

In a recent model of magnetite dissolution, Canfield and Berner (1987) proposed that the process adheres to the rate law

dcm, - = dt

- 1 .I x lo-’ Cs’.” C mag A mag 9

where Cmag is the concentration of magnetite, C, is the concentration of dissolved sulfide and Amag is the specific surface area of magnetite grains. This model yields half- life values of 50-1000 yr for magnetite grains across a SP-PSD size range in marine environments, a much lower rate of dissolution than that occurring in Scottish peats where complete removal of PSD/MD size anthropogenic magnetites has occurred since their deposition no more than 150 yr ago. This discrepancy may reflect the low Eh potential of inundated ombrotrophic peats relative to most marine sediments, but is more probably the result of a higher reactive H,S loading in the mire waters.

Significantly, the tendency for the selective removal of tine Fe,O, phases and the concomitant coarsening of magnetic assemblages during dissolution implied by the Canfield and Berner model and verified by numerous down-core SIRM/ARM and MDF profiles for marine and lacustrine sediments (e.g., Karlin, 1984; Anderson, 1986) is not apparent from the magnetic data for the peat cores described here. Rather, the leftward deflection of the S and coercivity profiles (Figs. 1 and 2) at the base of the zone of ferrimagnetic enhancement signi- fies a hardening of magnetic remanence often associated with a refinement of the sig- nal-carrying minerals. The progressive decrease of SIRM/ARM ratios with depth in the five cores for which data are available (Fig. 3) further confirms this trend.

Such discrepancies between the granulo-


FIG. 4. SEM micrographs of magnetically extracted cenospheres from core Gl. Micrograph (A) shows a particulate from aerated strata at l-cm depth, consisting of a silicaceous. magnetite-rich bead with a superficial armoring of ferric oxide crystals. Micrograph (B) shows the morphology of a sphere from 24-cm depth and illustrates the apparent alteration of such particles resulting from progressive burial and inundation. The surface oxide coatings are completely absent and fractures are evident within the cenosphere structure.

metric changes resulting from dissolution in Scottish peats and most other reducing sediments are almost certainly attributable to differences of initial magnetic mineral influx. Studies, of Gulf of California sediments (Karlin, 1984) have shown that the selective removal of line Fe,O, grains may induce down-core coarsening only until their supply is effectively exhausted. Be- yond this point, dissolution tends to occur along lines of weakness in large PSD/MD structures with resultant fragmentation and the adoption of single domain-type magnetic behavior (Karlin and Levi, 1983, 1985). However, in peats, the initial magnetite influx is principally anthropogenic

and comprises mainly PSD/MD phases with a negligable SP/SSD fraction (Whitby and Cantrell, 1975). In the absence of fine phases, the fragmentation of MD grains and the associated refinement of magnetic assemblages seen only during the latter stages of Fe,O, diagenesis in other sedimentary environments are likely to constitute the initial stages of dissolution in peats. The results of SEM analyses of magnetic extracts from core Gl support this hypothesis, showing the contrasting morphology of pristine Fe-bearing cenospheres from the peat surface (Fig. 4A) and surface oxide de- pleted, structurally fractured particulates from buried strata at 24-cm depth (Fig. 4B).

180 MARTIN WILLIAMS

SUMMARY AND CONCLUSIONS

The data presented for peat cores from four Scottish localities indicate that hydrocarbon-derived particulates constitute the principal ferrimagnetic input to ombrotrophic peats. The data also confirm previous claims (e.g., Thompson et al., 1980) that the surface magnetic properties of such peatlands may reliably reflect the magnitude of the contemporary pollutant influx. However, the contention that a reliable chronological record of atmospheric deposition is retained during the progressive accumulation of peats appears applicable only in circumstances where the entire postindustrial sequence is permanently elevated above the water table (e.g., in sample cores described by Oldfield et al., 1981). Beneath this critical level, the down-core concentra-

tion gradients of magnetic oxides appear more closely related to Eh-controlled dissolution processes than to historical variations of allogenic mineral influx.

In the light of the observations made here, certain interpretations that were formerly placed on magnetic profiles through ombrotrophic peats may warrant reap- praisal. In previous studies, the persistently lower peak and cumulative SIRM values recorded in sample cores from Sphagnum lawn locations relative to adjacent Calluna hummocks have been attributed to the dif- fering efficiencies with which the respective microtopographic facets of peat surfaces filter particulates from the atmosphere (e.g., Oldfield et al., 1979a). This explanation has always been at odds with the limited variability of pollen catchment across peat surfaces (Williams, 1988). From


the evidence presented here, it now seems equally likely that such trends reflect the more rapid dissolution of magnetic phases in the water-inundated conditions typifying Sphagnum lawn microtopographies. In similar fashion, the high coercivity (50-60 mT) assemblages frequently found in strata de- marking the magnetic take-off may have formerly been misinterpreted as reflecting the deposition of hematite-rich particulates from Bessemer fuel combustion systems during the early postindustrial period (e.g., Jones, 1985). lnstead, it now appears possible that this trend could reflect the dissolutionary alteration of MD magnetites by etching along domain walls and the concomitant production of an SD-like magnetic assemblage.

ACKNOWLEDGMENTS

The author thanks Dr. R. Thompson (Department of Geophysics, University of Edinburgh) and Dr. P. A. Furley (Department of Geography, University of Ed- inburgh) for valuable advice throughout the undertak- ing of this work. The staff of the Department of Geo- physics, University of Edinburgh, provided important technical assistance during field work. The research was funded by the UK Natural Environment Research Council.

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evidence for the dissolution of magnetite in recent scottish peats

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