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Soil Science Society of America Journal 63:1181-1187 (1999)
© 1999 Soil Science Society of America

DIVISION S-2-SOIL CHEMISTRY

Radiocarbon Dating of Aliphatic Hydrocarbons

A New Approach for Dating Passive-Fraction Carbon in Soil Horizons

Yongsong Huang{dagger},a, Baocai Lia, Charlotte Bryantb, Roland Bolc and Geoffrey Eglintona

a Biogeochemistry Research Center, Dep. of Geology, Univ. of Bristol, Bristol, BS8 1RJ, UK
b NERC Radiocarbon Lab., Scottish Enterprise Technology Park, East Kilbride, Glasgow G75 0QF, UK
c Inst. of Grassland and Environmental Research, North Wyke, Okehampton, Devon EX20 2SB, UK

yongsong{at}essc.psu.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Aliphatic hydrocarbons isolated from three types of British upland soils at different depths were 14C-dated by accelerator mass spectrometry (AMS) and compared with 14C ages of total organic C (TOC) of bulk soils and acid-hydrolyzed residues. In all cases, aliphatic hydrocarbons were significantly older than TOC but comparable with (in some cases older than) hydrolyzed residues, indicating that the 14C content of aliphatic hydrocarbons reflects the age of a passive-fraction C. The age differences between the aliphatic hydrocarbons and TOC increase with the degree of mineralization: thus, up to a 10000 yr difference in age is observed for highly mineralized horizons in podzol and acid brown earth. The leaf-wax n-alkanes (C25 to C33) isolated from a peaty gley core show a virtually linear relationship between their ages and the depth. In contrast to bulk soil organic matter that contains younger C deposited by plant roots and by water leaching, leaf wax n-alkanes are contributed at the soil surface by the leaves of dead plants and are of low mobility due to their extremely low water-solubility. The low biodegradability of long-chain n-alkanes leads to their persistence in the soil horizons where they were originally deposited. Therefore, their ages are ideal as chronological indicators for soils and peats.

Abbreviations: AMS, accelerator mass spectrometry • BP, before present • GC, gas chromatography • GC–MS, gas chromatography–mass spectrometry • TOC, total organic C


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
RADIOCARBON DATING is widely used in studying soil C dynamics (Jenkinson and Rayner, 1977; Jenkinson et al., 1991; Trumbore et al., 1989, 1990; Trumbore, 1996; Paul et al., 1997) and establishing chronology for paleoenvironmental reconstruction from soils (Nordt et al., 1994; Pendall et al., 1994; Boutton, 1996) and peats (Hillaire-Marcel et al., 1989; Aucour et al., 1994). Conventional radiometric 14C dating requires 1 to 2 g of C (Harkness and Wilson, 1972), which limits its application to large samples. Radiocarbon dating by accelerator mass spectrometry (AMS) requires only submilligram quantities of C, thus greatly extending the range of materials that can be precisely dated (Donahue, 1990).

Modeling soil C cycling requires an independent estimate of the 14C content of the passive-fraction C by identifying the most 14C-depleted (or the oldest) C in the soil column (Trumbore, 1996). One approach uses a 14C measurement from C in deep soil (O'Brien and Stout, 1978; Harrison et al., 1993). Another is based on 14C measurements of fractionated soil organic matter (Scharpenseel, 1977, 1993; Trumbore et al., 1989, 1990; Leavitt et al., 1996), such as acid-hydrolyzed residues which are enriched in hydrophobic components like lipids and other biopolymers (Lichtfouse et al., 1998). However, lipids are heterogeneous, containing compounds with significant difference in polarity and hence solubility in aqueous solutions. Individual compounds may have different ages depending on their mobility and degradability in soils. For example, carboxylic acids can be significantly younger than aliphatic hydrocarbons (Bol et al., 1996). The low 14C content of aliphatic hydrocarbons when compared with hydrolyzed residues in a peaty gley soil (Huang et al., 1996; Bol et al., 1996) suggests that aliphatic hydrocarbons are also part of the passive C pool.

Establishing the correct depositional ages for the soil and peat profiles is essential for paleoclimatic interpretations (e.g., Pendall et al., 1994). Chronologies for soil and peat are often determined by radiocarbon dating of charcoal and plant remains (e.g., Nordt et al., 1994; Hillaire-Marcel et al., 1989) and TOC (Aucour et al., 1994). However, charcoal may be absent from key horizons. Absorption of recent organic C by charcoal and plant remains could introduce errors in 14C ages (Gillespie et al., 1992). Dating of TOC is more problematic because soil organic matter is highly heterogeneous, with different turnover rates (Jenkinson and Rayner, 1977; Trumbore et al., 1989). The "date" of TOC in soil represents only a weighted average age of the different (fast, slow, and passive) pools of organic matter. Moreover, plant roots can penetrate to significant depth, depositing younger C in the older soil horizons. Organic compounds can be transported vertically by water, either in soluble or colloidal form, especially in soils subjected to intense leaching processes (Huang et al., 1996, 1998a). Additional problems exist for dating TOC or operationally defined fractions that consist of organic matter derived from different sources, for example, higher plants, algae, soil fauna, and microorganisms, which may utilize C pools of different ages. In situ biosynthesis by microorganisms using modern C pools is known to contribute to the TOC in deep soil horizons (Huang et al., 1998a). Such inhomogeneity in 14C ages has been demonstrated by radiocarbon dating of individual compounds isolated from marine sediments (Eglinton et al., 1997).

The objectives of this study were to extend our data set on the 14C dating of lipid fractions from one soil core (Bol et al., 1996; Huang et al., 1996). We demonstrate by studying three additional cores from three types of British upland soils that aliphatic hydrocarbons represent a fraction of the passive soil C that persists in soil mineral horizons, and infer that their ages correspond with the time of soil deposition. We have also refined the isolation procedure for higher plant leaf waxes (long-chain n-alkanes with strong odd–even predominance) by selective removal of unsaturated hydrocarbons of more recent bacterial origin.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Soils
Soil samples used in this study came from Moor House Nature Reserve (54°65' N, 2°45' W), North Yorkshire in the United Kingdom. The soils were obtained by coring to a maximum depth of 28 cm (Huang et al., 1996, 1998a) from the rough grassland pastures at the 760- to 775-m elevation with sheep fescue (Festuca ovina L.) or (Juncus squarrosus L.) as the main species. Hornung (1968) and Heal and Smith (1978) describe the soils of the area.

The three types of soils chosen for this study were described in detail elsewhere (Huang et al., 1996, 1998a). Briefly, acid brown earth, micropodzol, and peaty gley were chosen because they represent an average of 40% of British upland soil types. Acid brown earth is similar to Typic Dystrocrepts in USDA or Dystric Cambisol in FAO classifications. Micropodzol is similar to a Placic Aquod in the USDA or the Placic Podzol in the FAO/UNESCO classifications. Acid brown earth generally occurs on steeper slopes (>20°) than podzol ({approx}10–20°). Peaty gley belongs to cambic stagnohumic gley soil from Wilcocks or Onecote association, similar to Typic Humaquept in USDA or Humic gleysol in FAO classifications. The following data have been reported previously: molecular characterizations, compound-specific {delta}13C analyses of aliphatic hydrocarbons, and 14C ages of TOC in the three soils (PG1, MP5, and ABE 5 in Table 1) (Huang et al., 1996); 14C ages of aliphatic hydrocarbons in PG1 except for the 5.5- to 7.5-cm depth (Huang et al., 1996); comparison of 14C and 13C contents of hydrolyzed residues, carboxylic acid fractions, aliphatic hydrocarbon fractions, and TOC in PG1 (Bol et al., 1996). We present new 14C dates on aliphatic hydrocarbons and hydrolyzed residues from MP5 and ABE5; aliphatic hydrocarbons from the 5.5- to 7.5-cm depth of PG1 (Table 1); and 14C dates on TOC, aliphatic hydrocarbons, and saturated aliphatic hydrocarbons from PG6 (Table 2) .


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  		Table 1 Radiocarbon content data measured for the peaty gley, micropodzol, and acid brown earth soils from Moorhouse Nature Reserve

 

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  		Table 2 Radiocarbon content data measured for the peaty gley Core 6 (PG6); comparison of 14C ages for TOC, aliphatic hydrocarbons, and saturated aliphatic hydrocarbons.{dagger}{ddagger}

 
Lipid Fractions
The procedure for lipid extraction, separation, and isolation of aliphatic hydrocarbon fractions for AMS dating have been described previously (Huang et al., 1996). Extreme care was taken to avoid contamination, since even small amounts of contamination by modern C will render the 14C data unusable. All the glassware was combusted at 500°C overnight and rinsed with solvents prior to use. Solvents used were HPLC or spectroscopic grade with no stabilizers and 14C-dead. For soil samples with TOC in the range of 30 to 50%, {approx}5 to 10 g was used, or {approx}10 to 15 g for TOC in the range of 10 to 30% and up to 30 to 50 g for mineral horizons with TOC < 5%. Samples were ultrasonically extracted using dichloromethane. The aliphatic hydrocarbons were obtained by eluting total extracts with hexane using silica gel flash chromatography (Huang et al., 1996).

We performed RuO4 oxidation of aliphatic hydrocarbons by a modified procedure as reported by Boucher et al. (1989). Fresh RuO4 in CCl4 was prepared by dissolving RuO2 · H2O (10 mg) in 10 mL of distilled water in a separatory funnel, followed by adding 6 mL of CCl4 and {approx}50 mg of sodium periodate, and then shaking rigorously for 5 min. To remove alkenes (hopenes) derived from bacteria, drops of the RuO4 solution were added to the aliphatic hydrocarbons and the product then dried under a stream of N2; this step was repeated 2 to 3 times until the bright yellow color (of RuO4) persisted. Subsequently the reaction products were passed though a small silica gel column using hexane as eluant, yielding saturated aliphatic hydrocarbon fractions. The aliphatic hydrocarbon samples (dissolved in CH2Cl2) were then filtered through a 0.45-µm Millipore filter (Millipore Corp., Bedford, MA), transferred to quartz tubes (precleaned by heating to 900°C), and dried under vacuum ({approx}5–11 Pa; 4–8 torr) and centrifugation until a constant weight ({approx}1–3 mg) was achieved ({approx}24 h). (Huang et al., 1996; Bol et al., 1996). About 5 mg of a C23 n-alkane standard was dissolved in CH2Cl2 and dried in parallel with authentic samples. No weight gain (or loss) was recorded for this standard, indicating a complete removal of solvent in the drying process (Huang et al., 1996).

Hydrolysis of Soils
Acid-insoluble residues of soil were recovered from {approx}50 g of raw soil that had been air-dried and sieved through a 2-mm steel mesh. Each soil sample was refluxed for 18 h with 250 mL of 6 M HCl. After cooling, the flask contents were diluted with distilled water and the solid residues were separated by centrifugation and retained. The residues were then washed until they started to disperse, collected by filtration through a no. 3 glass sinter, and they were then washed further with cold distilled water until the filtrate was free of Cl-. The prepared samples were then freeze-dried.

Radiocarbon (14C) Dating
The 14C dates for aliphatic hydrocarbons, hydrolyzed residues (except for peaty gley at 7.5–12.5 cm depth) and bulk soils with TOC <3% were determined by AMS (Tables 1 and 2). The samples were sent to the NERC Radiocarbon Lab., East Kilbride, in quartz tubes plugged with precleaned quartz wool. These tubes were inserted into larger quartz combustion tubes, together with {approx}2 g of copper oxide and 100 mg of silver metal. The combustion tubes were evacuated to 0.0133 Pa ({approx}10-3 torr), flame-sealed, and combusted to CO2 by heating to 900°C in a muffle furnace (Boutton et al., 1983). Carbon dioxide was purified by cryogenic separation from other combustion products. A subsample of CO2 was used to measure &d13CPDB using a dual-inlet mass spectrometer (VG OPTIMA, Micromass, UK) so that 14C dates can be normalized to &d13C = -25{per thousand} (Donahue, 1990). Carbon dioxide was converted to an iron–graphite mixture (Fe/C < 3:1 by weight) by Fe/Zn reduction (Slota et al., 1987) and sent to the NSF-AMS Facility University of Arizona, Tucson for 14C analysis (Donahue, 1990). For bulk soil samples with TOC > 3%, 14C dates were determined by liquid scintillation counting (publication codes SRR- in Tables 1 and 2). Samples (containing 1–2 g of C) were converted into CO2 in a high-pressure combustion bomb charged with pure oxygen. Subsequently, CO2 was converted to benzene in a routine procedure at the NERC Radiocarbon Laboratory (Harkness and Wilson, 1972). Radiometric and AMS results are reported as conventional radiocarbon years before present (BP; before 1950) or percentage modern 14C in Tables 1 and 2. Due to insufficient soil sample and low concentrations of organic C in the mineral horizons for micropodzol (Core 5) and acid brown earth (Core 5), aliphatic hydrocarbons from wider depth intervals were combined to give sufficient C for 14C dating.

Gas Chromatography and Gas Chromatography–Mass Spectrometry
Small aliquots of aliphatic hydrocarbons were analyzed by gas chromatography (GC) and gas chromatography–mass spectrometry (GC-MS) for identification of the compounds, as described previously (Huang et al., 1996).


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Comparison of Carbon-14 Ages of Total Organic Carbon and Aliphatic Hydrocarbon Fractions
The core number, sample depth, soil horizons, 14C ages of TOC, aliphatic hydrocarbon fractions, and hydrolyzed residues from peaty gley (Core 1), podzol (Core 5), and acid brown earth (Core 5) are listed in Table 1. In all three cores, the ages of TOC increase with depth. The maximum age in peaty gley Core 1 is at the 21.5- to 24.5-cm depth (10445 ± 85 yr BP), whereas micropodzol and acid brown earth at comparable depths are significantly younger ({approx}4000 yr BP) (Table 1, Fig. 1) . In contrast, the ages of the aliphatic hydrocarbon fractions in all soil horizons studied are consistently older than the TOC from the same depth. In peaty gley, the differences between the ages of TOC and aliphatic hydrocarbons were about 50% (5–8 cm), 34% (13.5–15.5 cm), and 18% (21–25 cm). The differences are much greater for acid brown earth and micropodzol. The ages for aliphatic hydrocarbons from 4 to 14 cm (the Ah or organic-rich mineral horizon) and 16 to 24 cm (the B or mineral horizon) in acid brown earth are about four to five times greater than TOC ages averaged for the same depth ranges. A similar phenomenon is also observed for the lower part of the mineral (B) horizon in micropodzol. The younger TOC ages are consistent with the TOC containing a mixture of organic C from different ages and different pools (Jenkinson and Rayner, 1977; Sharpenseel, 1977; Jenkinson et al., 1991; Trumbore et al., 1989), while the older ages for aliphatic hydrocarbons are in agreement with these compounds having a low turnover rate.



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  		Fig. 1 Gas chromatograms of the aliphatic hydrocarbon fraction from peaty gley (Core PG6) before and after the treatment by RuO4 oxidation. Alkenes (primarily hopenes of bacterial origin) were oxidized and then removed chromatographically. The dotted peaks are leaf-wax n-alkanes that are untouched by RuO4 oxidation

 
The differences in 14C ages between aliphatic hydrocarbons and TOC for the three types of soils appear to correspond with the extent of mineralization and decomposition. The smallest difference is seen in the peaty gley soil, which has the lowest degree of mineralization, as indicated by the absence of the B horizon (Table 1). Both the acid brown earth and micropodzol cores show a well-developed eluvial horizon (E) and mineral horizon (B), indicating a high degree of water leaching and a high rate of organic matter decomposition. The lower soil horizons in the acid brown earth and the micropodzol cores apparently contain a higher percentage of younger C than does the peaty gley. Frequent water logging in peaty gley retards microbial decomposition and limits the vertical movement of organic matter (Huang et al., 1998b).

Estimating the Carbon-14 Content of the Passive Pool of Organic Carbon by Dating Aliphatic Hydrocarbon Fractions
In ecological models of soil organic C cycling (Jenkinson et al., 1991; Trumbore, 1996), it is necessary to divide the soil organic matter into fast (<=1 yr), slow (decadal–centennial), and passive or inert (millennial) cycling pools. When applying 14C measurements to the study of soil C dynamics, an independent estimate of the passive-fraction 14C content is essential to the data interpretation (Trumbore, 1996). One important approach is based on the physical and chemical fractionation of soils. Many data have shown for all soil horizons that the acid-hydrolyzed residues have older ages than TOC (Scharpenseel, 1977, 1993; Leavitt et al., 1996) and hence are more representative of the 14C contents of the passive pool (Trumbore et al., 1989). However, the molecular compositions for acid-hydrolyzed residues are poorly known and hence the causes for their old ages are unclear.

The 14C ages measured in this study for aliphatic hydrocarbons and hydrolyzed residues are similar in the peaty gley and micropodzol cores (Table 1). In the acid brown earth core, the aliphatic hydrocarbons are older than hydrolyzed residues (although the soil samples used to prepare these fractions were not from exactly the same depth range) (Table 1). Hence, we infer that the aliphatic hydrocarbon fractions can be used to estimate the end-member 14C ages for the passive pool of organic matter in soils. Persistence of the aliphatic hydrocarbons in mineral (B) horizons of soils may result from protection by noncrystalline minerals (Torn et al., 1997) and by macromolecules in humic fractions (Schnitzer and Neyroud, 1975; Lichtfouse et al., 1998).

Bol et al. (1996) has shown that carboxylic acids in soil horizons are significantly younger than the corresponding TOC and aliphatic hydrocarbons. Therefore, despite lipids being among the recalcitrant C pool (Schulten, 1996), the total lipid fractions are inhomogeneous and hence unsuitable for dating passive-fraction C. Total lipid fractions are obtained by extraction with organic solvents (usually dichloromethane and methanol) and hence contain various types of organic compounds, such as aliphatic hydrocarbons, carboxylic acids, and alcohols with very different polarities. For instance, in relatively alkaline soils, the salts of carboxylic acids can be soluble in water and mobilized by water leaching.

Carbon-14 Ages of Aliphatic Hydrocarbons and Soil Depositional Ages
A reliable chronology sequence is essential for paleoclimatic and paleoenvironmental reconstruction of the late Quaternary soil or peat deposits. Aliphatic hydrocarbon fractions offer advantages over TOC and other fractions for determining the soil deposit ages. First, the individual aliphatic hydrocarbons can be readily characterized by GC and GC-MS. For example, the aliphatic hydrocarbons in the acid upland soils used in this study are mainly higher-plant derived (leaf waxes) and <5% microbially derived (Huang et al., 1996; Fig. 1). Secondly, the aliphatic hydrocarbons with C number range from C23 to C33 (most abundant in these soils) are virtually insoluble in water, thus precluding their vertical transport in aqueous solution. These compounds' low solubility in water, probably aided by physical inaccessibility from noncrystalline minerals (Torn et al., 1997) and macromolecules (Lichtfouse et al., 1998), would reduce their biodegradability in soils. Thirdly, and probably most importantly, the C23 to C33 n-alkanes with strong odd–even predominance, which originate from the surface of plant leaves (Eglinton and Hamilton, 1963), are actually deposited on the surface of the soil after litter falls. In contrast, TOC or other operationally defined fractions inevitably contain materials delivered to lower horizons by plant roots and water leaching. Leaf-wax n-alkanes form sequential layers of deposits on the surface of soil and peat, thus their ages are most likely to represent the true depositional ages. Finally, the aliphatic hydrocarbon fractions in soils are relatively easy to prepare in a pure state.

The greatest 14C ages of the aliphatic hydrocarbon fractions isolated from all three types of soil are strikingly consistent (ranging from 13120 to 13670 yr BP, Table 1). The predominance of n-alkanes of higher-plant leaf wax origin in these fractions (Huang et al., 1996) indicates that vegetation had developed at the study site by that time, consistent with a period of climate amelioration 14000 and 13000 14C years ago (the Windermere Interstadial) (Lowe and Gray, 1980). Although the data are few, the sequential increases in the 14C ages of the aliphatic hydrocarbon fractions with depth in the three types of soils (Table 1) are in strong support of their use as chronological indicators in soil profiles.

Saturated Aliphatic Hydrocarbon Fractions: Removing Alkenes of Bacterial Origin
The presence of abundant alkenes (Fig. 1), especially hopenes derived from bacteria (Huang et al., 1996), may result in the ages of total aliphatic hydrocarbon fractions being perturbed by anachronistic contamination. Bacteria living in lower soil horizons may utilize the organic C dissolved and carried downward by water from the surface (litter and fermentation or LF horizon), thus producing biomass that is of significantly younger 14C age than the indigenous soil organic matter. Pyrolysis–mass spectrometry (Py-MS) and pyrolysis–gas chromatography–mass spectrometry (Py-GC-MS) studies have demonstrated that microbes can contribute significant amounts of C to the mineralized soil horizons (Schulten, 1996; Huang et al., 1998a). Thus, to obtain an age that is more representative of the soil development, the alkenes (mainly hopenes from bacteria) are removed from alkanes by RuO4 oxidation. The RuO4 quantitatively oxidizes double bonds and aromatic structures into carboxylic acids, ketones, or aldehydes, but does not react with saturated hydrocarbons (Carlsen et al., 1981). The saturated hydrocarbons can then be quantitatively recovered by column chromatography.

For this purpose, an additional peaty gley core (PG6, Table 2) was selected. This core has a well-developed Oh horizon but no Ah horizon and a consistently high organic C content throughout the core (45.4–56.3%, Table 2). This section of the core is thus a peat. Good chronologies for peat cores are of vital importance because peatbogs preserve large amounts of paleoenvironmental information. Frequently, TOC is used for 14C dating of peat cores (Hillaire-Marcel et al., 1989; Aucour et al., 1994). Here we compare the 14C ages of TOC, total aliphatic hydrocarbons and saturated aliphatic hydrocarbons obtained from various levels of Core PG6.

The GC traces for aliphatic hydrocarbons before and after RuO4 oxidation demonstrate the effectiveness of the procedure (Fig. 1). The aliphatic hydrocarbons prior to alkene removal contain hopenes and smaller amounts of n-alk-l-enes. After the RuO4 oxidation followed by silica gel column chromatography, the saturated aliphatic hydrocarbon fractions contain primarily n-alkanes with extremely high C preference index values, which are derived from higher plant leaf waxes (Eglinton and Hamilton, 1963).

Table 2 gives the 14C ages of TOC, total aliphatic hydrocarbons, and saturated aliphatic hydrocarbons measured in this study. The data are plotted in Fig. 2 . Only two aliphatic hydrocarbon fractions (without RuO4 oxidation) were dated by AMS; the ages are similar or slightly older than the corresponding TOC. The four saturated aliphatic hydrocarbon fractions measured are consistently older than the corresponding TOC, with the difference varying in the range of 11 to 30%, lying on a straight line against the depth (Fig. 2). Although we have not dated deeper horizons, the saturated aliphatic hydrocarbon fractions clearly offer promise for the dating of soils and peats and the study of soil processes.



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  		Fig. 2 Radiocarbon ages for saturated aliphatic hydrocarbons, and total organic matter in peaty gley (Core PG6). Depth–age functions are shown on the right-hand diagram. The errors are smaller than symbol sizes

 

   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
Aliphatic hydrocarbons (especially saturated aliphatic hydrocarbons of leaf wax origin) appear to be well suited to the 14C dating of soils and peats. It is now possible to date individual hydrocarbons (Eglinton et al., 1997), but the procedure is time-consuming, and requires expensive, nonroutine, trace-quantity ({approx}0.2 mg) radiocarbon dating by AMS. In contrast, the saturated aliphatic hydrocarbon fractions can be readily isolated in milligram quantities, thus increasing the reliability and precision of the 14C ages. Dating of resistant higher plant leaf-wax–alkanes thus offers a convenient method for establishing chronology for late Quaternary soils and peats. The consistently low 14C contents found in the aliphatic hydrocarbons compared with other soil fractions (e.g., hydrolyzed residues) indicate that they can be used to determine the end-member 14C ages of the passive C pool in soils.Boucher Standen Eglinton 1991


   ACKNOWLEDGMENTS
 
We acknowledge the financial support provided by the NERC through its TIGER (Terrestrial Initiative in Global Environmental Research) program, grant numbers GST/02/613 and T91/I.2/813, as well as the support of IGER of BBSRC. We thank Dr. P. Ineson and Dr. D.D. Harkness for advice, J.A.Meredith for performing the hydrolysis of the soil samples, and Dr. R.P. Evershed for access to the GC/MS facilities of the Organic Geochemistry Unit (grant numbers GR3/2951 and GR3/3758). We thank staff at the NERC Radiocarbon Laboratory, East Kilbride for radiometric dates and AMS graphite preparation and the 14C AMS analyses of graphites carried out at the NSF Arizona AMS Facility in Tucson, University of Arizona.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
{dagger} Yongsong Huang, current address: Dep. of Geosciences, 407 Deike Building, Pennsylvania State Univ., University Park, PA 16802. Back

Received for publication June 22, 1998.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 




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