Selective preservation of soil organic matter in oxidized marine sediments (Madeira Abyssal Plain)

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Geochimica et Cosmochimica Acta 72 (2008) 6061–6068 www.elsevier.com/locate/gca

Selective preservation of soil organic matter in oxidized marine sediments (Madeira Abyssal Plain) ¨ rjan Gustafsson c, Jack J. Middelburg b,d, Carme Huguet a,1, Gert J. de Lange b, O Jaap S. Sinninghe Damste´ a,b, Stefan Schouten a,* a

NIOZ Royal Netherlands Institute for Sea Research, Department of Marine Organic Biogeochemistry, P.O. Box 59,1790 AB Den Burg, Texel, The Netherlands b Faculty of Geosciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands c Stockholm University, Department of Applied Environmental Science, 10691 Stockholm, Sweden d Netherlands Institute for Ecology (NIOO-KNAW), Centre for Estuarine and Marine Ecology, P.O. Box 140, 4400 AC Yerseke, The Netherlands

Received 9 April 2008; accepted in revised form 22 September 2008; available online 30 September 2008

Abstract In ocean margin sediments both marine and terrestrial organic matter (OM) are buried but the factors governing their relative preservation and degradation are not well understood. In this study, we analysed the degree of preservation of marine isoprenoidal and soil-derived branched glycerol dialkyl glycerol tetraethers (GDGTs) upon long-term oxygen exposure in OM-rich turbidites from the Madeira Abyssal Plain by analyzing GDGT concentrations across oxidation fronts. Relative to the anoxic part of the turbidites ca. 7–20% of the soil-derived branched GDGTs were preserved in the oxidized part while only 0.2–3% of the marine isoprenoid GDGT crenarchaeol was preserved. Due to these different preservation factors the Branched Isoprenoid Tetraether (BIT) index, a ratio between crenarchaeol and the major branched GDGTs that is used as a tracer for soil-derived organic matter, substantially increases from 0.02 to 0.4. Split Flow Thin Cell (SPLITT) separation of turbidite sediments showed that the enhanced preservation of soil-derived carbon was a general phenomenon across the fine particle size ranges (<38 lm). Calculations reveal that, despite their relatively similar chemical structures, degradation rates of crenarchaeol are 2-fold higher than those of soil-derived branched GDGTs, suggesting preferential soil OM preservation possibly due to matrix protection. Ó 2008 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Burial of organic matter (OM) in marine sediments is the major sink for carbon on geological time scales, and the majority of this burial takes place on continental margins (Berner, 1982; Hedges and Keil, 1995; Burdige, 2007). In coastal sediments both marine and terrestrial OM are buried and it has been observed that terrestrial

*

Corresponding author. E-mail address: [email protected] (S. Schouten). 1 Present address: School of Oceanography, Box 355351, University of Washington, Seattle, Washington 98195-5351. 0016-7037/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2008.09.021

OM is often better preserved than marine OM (e.g. Hoefs et al., 2002; Cowie et al., 1995; Burdige, 2005). Different hypotheses have been offered to explain the differential degradation of terrestrial and marine OM. Compared to marine organic compounds, certain terrestrial organic molecules such as plant waxes and lignins are relatively difficult to degrade by microbes and thus have a greater preservation potential (de Leeuw and Largeau, 1993), suggesting that intrinsic chemical reactivity is an important factor. Alternatively, the physicochemical packaging within the sedimentary matrix could be different for terrestrial and marine OM (Keil et al., 1994a) and could make OM less accessible to microbial degradation (bio-availability) (Mayer, 1994; Rothman and Forney, 2007). This physical

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exclusion hypothesis suggests that OM decay due to microbial activity would only occur when the pores in the sediment matrix are large enough for OM-degrading bacterial enzymes to spread (Mayer, 1994). This process was recently modeled (Rothman and Forney, 2007) and the results provided an explanation for the general observation that OM degradation rates decrease logarithmically with the logarithm of time, the so-called power law of OM degradation (Middelburg, 1989). A combination of both intrinsic chemical resistance and physical protection has also been suggested to lead to enhanced preservation of terrestrial OM (Prahl et al., 1997). The main problem in determining the relative importance of these two preservation mechanisms is that the marine and terrestrial OM components studied so far have different chemical structures compared to each other (bulk organic carbon, biopolymers, biomarker lipids) and thus inherently different degradation rates (Cowie et al., 1995; Prahl et al., 1997). Hence, it is difficult to disentangle the impact of intrinsic chemical resistance from that of matrix protection on the preferential preservation of terrestrial OM. A large number of studies of OM degradation due to long-term oxygen exposure have used OM-rich turbidites from the Madeira Abyssal Plain (MAP) (Wilson et al., 1986; de Lange et al., 1987; Cowie et al., 1995, 1998; Hoefs et al., 1998; Hoefs et al., 2002; Prahl et al., 1997). These sediments were originally deposited on the northwestern African shelf and subject to anoxic diagenesis. However, due to slope instability, they were subsequently transported to the abyssal plain at 5000 m water depth by a turbidity current and deposited as well-mixed fine-grained OM-rich distal turbidites in the oxic deep sea (Cowie et al., 1995, 1998; Hoefs et al., 1998; Prahl et al., 1997). Once deposited at the abyssal plain, the contact with the oxic bottom waters caused a ‘‘burn down” effect resulting in degradation of the OM in the upper part of the turbidite by downward-diffusing oxygen while the lower part was left unaltered (Wilson et al., 1986). MAP turbidites are thus ideal to study the long-term effect of oxygen on OM degradation and preservation as the OM in the sediment was initially relatively homogenous and oxygen diffusion into the sediment has created ‘‘oxidation fronts” which are easily identifiable (Cowie et al., 1995, 1998; Hoefs et al., 1998). In this study, we analyzed the composition and concentrations of glycerol dialkyl glycerol tetraethers (GDGTs) in

MAP turbidites. GDGTs occurring in these turbidites, and many other aquatic environments, can be subdivided into two groups: those with an isoprenoid carbon skeleton and those with a branched alkane carbon skeleton (Fig. 1). Branched GDGTs are thought to be derived from anaerobic bacteria in soils (Weijers et al., 2006b) and are transported to ocean margin sediments via soil erosion and river transport (Hopmans et al., 2004; Kim et al., 2006; Herfort et al., 2006; Van Dongen et al., 2008). In contrast, the isoprenoidal GDGT crenarchaeol is thought to be mainly derived from group I Crenarchaeota (Sinninghe Damste´ et al., 2002a; Schouten et al., 2008), a group within the Archaea, which is one of the most abundant group of prokaryotes in today’s oceans (eg Karner et al., 2001; Herndl et al., 2005). Group I Crenarchaeota, and thus crenarchaeol, also occurs in terrestrial environments (Weijers et al., 2006a; Leininger et al., 2006) but in substantially lower amounts than the branched GDGTs (Weijers et al., 2006a). Therefore, the relative ratio of crenarchaeol versus branched GDGTs, expressed in the Branched Isoprenoid Tetraether (BIT) index, has been proposed as a tracer of the relative abundance of soil organic matter (Hopmans et al., 2004; Kim et al., 2006, 2007; Herfort et al., 2006). As both GDGT classes have similar chemical structures (Fig. 1) their (intrinsic) chemical reactivity will likely be similar, and thus the study of their degradation rates in MAP turbidites should potentially allow us to evaluate the effect of matrix protection on soil versus marine OM preservation. 2. MATERIALS AND METHODS 2.1. Turbidite sediments The Madeira Abyssal Plain has an average water depth of 5400 m and covers an area of 80000 km at the bottom of the Canary Islands shelf (Cowie et al., 1998). Sediments in this area comprise meter-thick distal turbidites intercalated by cm-thin organic-matter poor marls. The Pleistocene Fturbidite samples (Table 1) are from core 90P22, which was recovered at 32°03.00 N, 24°12.10 W during the 1990 R/ V Tyro cruise. The Pliocene/Miocene turbidites used in this study (see Table 1) were collected on board the R/V JOIDES Resolution cruise, ODP Leg 157. The samples were stored in geochemical LDPE bags (LGS, Leek, The

Crenarchaeol

Branched tetraether lipids HO

HO

O O

O

O

O

O OH

HO

O O

O

O

O

O

O

O

O

O

OH

OH

HO

OH

Fig. 1. The chemical structures of the investigated marine (crenarchaeol) and soil-derived (branched) GDGTs.

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Table 1 TOC and d13C and preservation factor (PF) of TOC, crenarchaeol and branched GDGTs in the F-turbidite with depth, the 951A/7H, 951A/ 25X, 952A/15H and 952A/27X MAP turbidites and the SPLITT separated F-turbidite sediments. An oxidized sediment (a mixture of sediments from 1068.5 and 1085.75 cm depth) and an unoxidized sediment (a mixture of sediments at 1111.5 and 1099.5 cm depth) from the F-turbidite were separated by SPLITT. The PF for each turbidite sample was calculated as a percentage relative to the average concentration of that compound in the unoxidized part of the turbidite. In the case of the SPLITT separated sediment analysis the PF was calculated as the percentage relative to the average concentration of that compound in the same SPLITT fraction of the unoxidized sample. TOC and d13C data of the SPLITT sediments, 951A/7H, 951A/25X, 952A/15H and 952A/27X MAP turbidites are from the oxic parts. nd, not determined. Depth (cm)/sample code

TOC (%)

d13 C (&)

PF TOC (%)

PF crenarchaeol (%)

PF branched GDGTs (%)

F-turbidite 1068.50 1072.50 1078.50 1081.50 1085.25 1085.75 1086.50 1091.50 1092.50 1094.50 1095.75 1097.00

0.18 0.19 0.19 0.19 0.24 0.33 0.28 0.33 0.34 0.59 0.73 0.86

22.1 21.7 21.6 21.4 21.7 21.0 21.1 21.2 20.6 20.6 20.3 20.2

20.0 21.2 21.2 21.2 26.7 36.7 31.2 36.7 37.8 65.7 81.3 95.7

0.2 0.3 0.5 0.8 1.3 3.4 2.7 3.2 7.9 37.2 69.3 96.4

7.1 10.5 18.3 22.8 19.0 22.0 22.9 32.4 33.5 63.3 105.5 117.0

ODP 951 and 952 MAP turbidites 951A/7H 951A/25X 952A/15H 952A/27X

0.16 0.26 0.25 0.25

nd nd nd nd

22.9 20.6 15.1 17.9

4.9 0.8 1.2 1.7

25.5 3.2 7.2 5.5

SPLITT fractions F-turbidite Fine (<38 lm) Gravitoids (<38 lm; >1 m/d) Colloids (<38 lm; <1 m/d)

0.20 0.20 0.19

21.0 20.7 21.0

20.5 22.9 22.8

1.4 1.3 1.3

15.8 14.5 16.8

Netherlands) and were kept and transported dark and frozen from the ship to the laboratory (de Lange, 1998). The five organic-rich turbidites used for this study are from different times of deposition: The F-turbidite emplacement took place 127 Ka, whereas turbidites 951A/7H, 951A/ 25X, 952A/15H and 952A/27X have emplacement times of up to 14 Ma. The F-turbidite was sampled in higher resolution (18 samples) over the redox front, while 3 or 4 samples were taken from turbidites 951A/7H, 951A/25X, 952A/15H and 952A/27X, typically representing one oxidized turbidite layer and 2–3 unoxidized turbidite layers. 2.2. Split Flow Thin Cell (SPLITT) separation of turbidite sediments SPLITT fractionation was done on a sediment mixture of oxidized sediments at 1068.5 and 1085.75 cm depth and a sediment mixture of unoxidized sediments at 1111.5 and 1099.5 cm depth from the F-turbidite. Prior to SPLITT fractionation, the bulk sediments were separated into successively smaller grain size fractions by wet sieving through 250, 100, 63 and 38 lm stainless steel sieves. For the MAP F-turbidite samples, 98% of the sediment mass passed through the smallest mesh sieve (<38 lm) and was subsequently separated without dispersion chemicals using Extra High Capacity-SPLITT fractionation (Coppola et al., 2005, 2007) into hydrodynamic fractions using a cutoff settling

velocity of 1 m/d. The two SPLITT fractions were collected and concentrated by ultra-centrifugation (1 h at 10,000 rpm) into a ‘‘gravitoid” <38 lm (settling velocity 1 m/d) and a ‘‘colloid” <38 lm (settling velocity 1 m/d) fraction. After separation, all SPLITT particle fractions were air dried (40–50 °C, 48 h) and analyzed for organic carbon content and GDGT abundances. 2.3. TOC and d13C TOC and d13C were determined on decarbonated (twice in a 1 N HCl solution) samples, using a Carlo Erba NA1500 CNS analyzer coupled via a Conflo II interface to a Finnigan Deltaplus isotope mass spectrometer. Reported d13C values have a precision better than 0.1 for unoxidized samples, but lower and variable precision for oxidized samples. 2.4. GDGT extraction and analysis Freeze-dried sediment samples were extracted using an Accelerated Solvent Extractor 200 (ASE 200, DIONEX) with a mixture of dichloromethane (DCM) and methanol (MeOH) (9:1, vol:vol) at 100 °C and 7.6  10 Pa. A known amount (1–2 g) of d13C GDGT internal standard was added to the total extract to measure absolute GDGT abundances (Huguet et al., 2006). An aliquot of each total

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stant TOC content in the unoxidized part, low TOC levels in the oxidized turbidite, and a transition interval characterized by a substantial drop in TOC content (Fig. 2). A d13C shift towards more 13C-depleted values with prolonged oxidation is also observed (Fig. 2), suggesting a preferential preservation of terrestrial, 13C-depleted OM, in agreement with previous studies (Prahl et al., 1997; de Lange, 1998). Analysis of the unoxidized part of the F-turbidite shows that the concentration of crenarchaeol, the marine GDGT, is two orders of magnitude higher than the soil-derived branched GDGTs, and BIT values are well below 0.1 (Fig. 2) in agreement with a dominant marine imprint. The constant levels of crenarchaeol, branched GDGTs and TOC, in the unoxidized part of the F-turbidite confirm the homogeneity with respect to OM composition upon emplacement of the turbidite. GDGT concentrations show, parallel to TOC content, a strong decline upon subsequent oxic degradation caused by the downward diffusion of oxygen (Fig. 2). The decline is strongest near the oxidation front (transition zone) with a loss of 70–90% of the original concentration of GDGTs and TOC (Fig. 2 and Table 1), suggesting extensive and rapid degradation upon long-term (10 kyr for the F-turbidite; Buckley and Cranston, 1988) exposure to oxygen. The magnitude of the decline in GDGT concentrations strongly differs between the marine-derived crenarchaeol and soil-derived branched GDGTs. To visualize this distinction, we calculated a preservation factor (PF; cf. Hoefs et al., 1998, 2002) to establish the relative degree of preservation of the different GDGTs and TOC in the oxidized part and the transition zone of the F-turbidite (Fig. 2 and Table 1). The PF for each compound in the oxidized samples was calculated as a percentage relative to the average compound concentration in the unoxidized part (Hoefs et al., 2002). The PF for branched GDGTs and crenarchaeol in the uppermost oxidized part of the turbidite (1072.5– 1068.5 cm) are substantially lower than that of TOC, indicating that they are less well preserved than bulk OM

extract was separated into an apolar and a polar fraction using a glass pipette column filled with activated alumina eluting with hexane/DCM (1:1, vol:vol) and DCM/MeOH (1:1, vol:vol), respectively. The polar fraction was analyzed for GDGTs according to the procedure described by Schouten et al. (2007). Analyses were performed in triplicate with an HP 1100 Series Liquid Chromatography–Mass Spectrometer (LC–MS) equipped with an auto-injector and ChemStation chromatography manager software. Separation was achieved on a Prevail Cyano column (2.1  150 mm, 3 lm; Alltech, Deerfield, Illinois, USA), maintained at 30 °C. GDGTs were eluted isocratically first with hexane/isopropanol (99:1, vol:vol) for 5 min, then using a linear gradient up to 1.8% vol of isopropanol over 45 min. Flow rate was 0.2 mL/min. After each analysis the column was cleaned by back flushing hexane/isopropanol (90:10, vol:vol) at 0.2 mL/min for 10 min. Detection was achieved using atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) of the eluent using the following conditions; nebulizer pressure 60 psi, vaporizer temperature 400 °C, Ndrying gas flow 6 L/min at 200 °C, capillary voltage 3 kV, corona 5 A (3.2 kV). Single Ion Monitoring (SIM) was used instead of full mass scanning because SIM increases the signal-to-noise-ratio and thus improves reproducibility (Schouten et al., 2007). SIM was set to scan the five [M + H] ions of the GDGTs and the three [M + H] ions of the branched tetraethers with a dwell time of 237 ms for each ion. 3. RESULTS AND DISCUSSION 3.1. OM and GDGT degradation in turbidites We investigated the upper part of the 4 m thick Pleistocene F-turbidite at relatively high resolution across the oxic–anoxic transition. The sharp oxidation front is clearly revealed by the TOC profile with a relatively high and con-

B- δ13C TOC (% VPDB) -22.5

-21.5

D- Branched GDGTs (µg/g sed.)

-20.5

0.00

0.04

F-BIT index

0.08

1070

Oxic

Depth (cm)

1080

1090

Transition 1100

Anoxic

1110

1120 0.0

0.5 A-TOC (%)

1.0

0

1

2

3

4

C- Crenarcheol (µg/g sed.)

-1

10

0

1

2

10 10 10 10

3

E- PF(% Log)

Fig. 2. Profile of (A) TOC (black diamonds), (B) d13C (open diamonds), (C) crenarchaeol abundance (black circles), (D) branched GDGTs abundance (open circles), (E) preservation factor (PF) for branched GDGTs (open circles), crenarchaeol (black circles) and TOC (black diamonds) plotted on a log scale across the oxidation front, (F) BIT index in the F-turbidite. The oxidized part and the transition zone of the turbidite are indicated by the dark grey and light grey area, respectively.

Selective preservation of soil organic matter

(Fig. 2, Table 1) as observed for many other lipids (Hoefs et al., 2002; Sinninghe Damste´ et al., 2002b). Remarkably, the PF of crenarchaeol is generally an order of magnitude lower than that of the terrestrial GDGTs (Fig. 2). In the uppermost section of the oxidized part, which has been exposed to oxygen for the longest time, only 0.2% of the original concentration of crenarchaeol is left while ca. 7% of the branched GDGTs remain (Fig. 2, Table 1). Noticeably, the relative concentration of crenarchaeol declines much more rapidly in the transition zone (1097–1094.5 cm) and to a much larger extent in the oxidized part than that of soil-derived branched GDGTs (Fig. 2 and Table 1). As a consequence, the BIT index changes from values 0.02 in the anoxic part to values up to 0.40 in the uppermost section of the oxidized part (Fig. 2). To confirm the results from the F-turbidite, we also analyzed four other OM-rich MAP turbidites of Pliocene/Miocene age, but in these cases only one slice from the oxidized part and two or three slices from the unoxidized part were analyzed. These turbidites were deposited at different times, with different deposition times and likely different oxygen exposure times (de Lange, 1998). Nevertheless, they show the same trends as observed in the F-turbidite; branched GDGTs have a slightly lower degree of preservation as TOC but an order of magnitude higher PF than that of crenarchaeol (Fig. 3, Table 1). Similarly, BIT values are substantially higher in the oxidized parts (0.23–0.35) of the turbidite as compared to the anoxic parts (0.05–0.12). This confirms the results obtained for the F-turbidite, i.e.

0.0 Crenarchaeol Branched GDGTs

-0.5

TOC

Log10 ('k Cren') = -0.57*Log10 (dx )2 -0.34 r2 = 0.94

Log10 ('k')

-1.0

-1.5

-2.0

Log10 ('kBranched')= -0.58*Log 10 (dx)2 -0 .70 r2 = 0.89

-2.5

2

Log10 ('kTOC') = -0.67*Log10 (dx) -0.66 r2 = 0.96

-3.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Log10(distancex)2

Fig. 3. Bar plot showing the PFs (in %) of TOC, crenarchaeol and branched GDGTs in the F-turbidite (average for all samples from the oxidized part), selected MAP turbidites from ODP sites 951 A and 952 A and different particle size classes of the F-turbidite. Samples representing an oxidized sediment (a mixture of sediments from 1068.5 and 1085.75 cm depth) and an unoxidized sediment (a mixture of sediments from 1111.5 and 1099.5 cm depth) from the F-turbidite were separated according to particle size by SPLITT. The PF for each turbidite was calculated as a percentage relative to the average concentration of that compound/organic carbon in the unoxidized part of the turbidite. In the case of the SPLITT separated sediment analysis the PF was calculated as the percentage relative to the average concentration of that compound in the same SPLITT fraction of the unoxidized sample. In nearly all cases TOC has the highest PF, with branched GDGTs having slightly lower PFs. In all sediments the PF of crenarchaeol is an order of magnitude lower.

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soil-derived GDGTs are preferentially preserved over marine GDGTs upon long-term oxygen exposure. 3.2. Degradation rates of GDGTs and OM Our high resolution data for the F-turbidite potentially allows for the assessment of relative degradation rates of the different GDGTs by calculating apparent first order degradation rate constants (k) (cf. Middelburg, 1989; Canuel and Martens, 1996). As the time of oxygen exposure is difficult to constrain for these paleo oxidation fronts we calculated a pseudo rate constant ‘k’ by assuming that the time of oxygen exposure is proportional to the inverse of the square of the distance of a depth interval to the oxidation front (cf. Buckley and Cranston, 1988) at 1097 cm. Then ‘k’ can be calculated as: ‘k’  ½1=ðd xþ1  d x Þ2   In½ðGx Þ=ðGxþ1 Þ

ð1Þ

where dx, distance from oxidation front at depth x, Gx, concentration of compound at depth x, and x + 1 is the depth interval overlying depth x. In the oxidized part of the turbidite, the log ‘k’ of TOC, branched GDGTs and crenarchaeol, correlate well with log distance squared (i.e. time; Fig. 4) in accordance with the power law of OM degradation (Middelburg, 1989). However, there is a significant offset between branched GDGTs and crenarchaeol, i.e. the slopes of the correlations lines are identical (0.57±0.05 vs. 0.58±0.07 for crenarchaeol and branched GDGTs, respectively) but the intercepts differ by 0.36 (0.34±0.10 and 0.70±0.14 for crenarchaeol and branched GDGTs, respectively). This difference corresponds to a degradation rate that is 2-fold faster for crenarchaeol than for branched GDGTs branched GDGTs (i.e., 100.36 = 2.3). The point closest to the oxidation front (1095.75 cm) seems to be slightly off the general trends probably because lower oxygen concentrations were a limiting factor for degradation. These data strongly indicate different rates and degrees of degradation for the two types of GDGTs with the soil-derived GDGTs degrading slower and thus being better preserved than the marine-derived GDGT. A 2-fold difference in degradation rate as determined here may seem relatively minor but on geological time scales it will result in major differences, up to orders of magnitude, in preservation (Hedges and Prahl, 1993; Hedges and Keil, 1995), as reflected in the preservation factors (Fig. 2 and Table 1). 3.3. Impact of particle size on GDGT preservation It has been shown that in marine sediments, much of the preserved OM is either adsorbed to mineral surfaces or enclosed in particle aggregates (Mayer, 1994; Keil et al., 1994b; Hedges and Oades, 1997), presumably attenuating the bioavailability of the OM to enzymatic degradation. This so-called sorptive preservation is related to the mineral surface area, with clay-size particles having a relatively higher percentage of associated OM (Keil et al., 1994b). To investigate the impact of particle sizes, the relative preservation of organic carbon in the fine fraction (<38 lm) of selected turbidite sediments, i.e. one pooled sediment from the anoxic part and one pooled sediment from an oxic part,

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Preservation factor (%)

25

20

15

10

5

0 F-turb.

951A-7H 951A-25X 952A-15H 952A-27X

Turbidite sediments

Fine

Gravitoids

Colloids

SPLITT fractions

Fig. 4. Double logarithmic cross plot of the pseudo first order degradation rate, ‘k’, for TOC, branched GDGTs and crenarchaeol versus the square of the distance to the oxidation front at 1097 cm. The rate ‘k’ was calculated according to Eq. (1)where it was assumed that the inverse of the square of the distance to oxidation front is roughly proportional to the time of oxygen exposure (Buckley and Cranston, 1988). In the oxidized part of the turbidite, the log ‘k’ of TOC, branched GDGTs and crenarchaeol, correlate well with log square distance (i.e. time) in accordance with the power law of OM degradation (Middelburg, 1989).

were investigated by Split Flow Thin Cell (SPLITT) fractionation (Keil et al., 1994b; Gustafsson et al., 2000; Coppola et al., 2007). The SPLITT instrument uses gravity to separate particles based on their characteristic settling velocities during passage of the suspended sediment through a laminar flow. The settling-velocity-based separation of particles at the exit of the SPLITT thus depends on size, shape, density of the particles, and on the flow rates set with the instrument. The turbidite fine fractions (98% of total sediment) of the two pooled sediments were each separated by SPLITT into an ultrafine colloidal particle fraction (<38 lm and <1 m/d settling velocity) hosting 57–62% of fines with the remaining fraction in a gravitoid fraction (<38 lm and >1 m/d settling velocity). Calculation of the PF of the different size fractions showed that the relative preservation of TOC and GDGTs were identical among the SPLITT fractions (Fig. 3, Table 1), i.e. PF of TOC is 21–23%, 15– 17% for branched GDGTs and 1% for crenarchaeol. These results suggest that the enhanced preservation of soil-derived carbon was a general phenomenon across the fine size ranges (<38 lm) of the turbidite sediment. 4. IMPLICATIONS Our results indicate that branched GDGTs derived from the continents are degraded slower by a factor of 2 than crenarchaeol, which is predominantly derived from the marine environment. Potentially, branched GDGTs could be more resistant to chemical degradation than isoprenoid GDGTs despite their similar chemical structure. However, microbial degradation of compounds with ether-bonds by aerobic bacteria involves the enzymatic oxidation and subsequent hydrolysis of the ether-bond rather than the breakdown of aliphatic chains (White et al., 1996). Even in the unlikely case that GDGT degradation rates are for some reason determined by the interior carbon skeleton struc-

ture, it has been observed that compounds with branched alkyl chains degrade faster than those with isoprenoid chains (Peters et al., 2005) opposite to that which is observed here. Therefore, our results are unlikely to be explained by differences in intrinsic chemical reactivity and alternative mechanisms must have governed the differential degradation and preservation of these GDGTs. Different matrices to which the branched GDGTs and crenarchaeol are associated, i.e. soil-derived matter versus marine-derived matter might have caused differences in degradation. As the 13C-contents of bulk OM shows a similar trend towards the selected concentration of terrestrial OM due to prolonged oxic degradation, it appears that soil OM in general might also be better preserved than marine OM. Alternatively, soil-derived OM may have been exposed longer to degradation than marine OM before emplacement in the turbidite: i.e. the initial age and reactivity (sensu Middelburg, 1989) of soil-derived OM are higher and lower, respectively, than that of marine OM. Interestingly, the relative preservation of soil GDGTs (7.1–22.8%) is higher than observed for pollen grains (0%; Keil et al., 1994c) and lignin (9%; Prahl et al., 1997) in the same turbidites. The higher lability of lignin and pollen compared to branched GDGT may be due to the fact that the former are OM derived from vegetation while the branched GDGTs represents OM derived from soils. Possibly the type of matrix association for vegetation OM is such that it is less protective against oxidative degradation than that for soil OM. Our results also put constraints on the use of the BIT index as a proxy for soil organic matter input. The large increase in the BIT index upon long-term oxygen exposure is concomitant with a large decrease in the isotopic composition of the bulk organic matter, suggesting a substantial relative increase in terrestrial (soil) organic matter. Hence, the BIT index still seems to work well as a proxy for the actual contribution of soil OM to marine sediments. How-

Selective preservation of soil organic matter

ever, it is less suitable as a proxy for estimating the original contribution of soil organic matter during sediment deposition, at least in sediments which have gone through a prolonged period of oxidative degradation after sediment burial. 5. CONCLUSIONS The post-depositional oxidation of GDGTs derived from soil and marine environments in Madeira Abyssal Plan turbidites shows that the soil-derived GDGTs are preserved to a much larger extent than marine-derived GDGTs and degrade ca. 2-times slower. The concomitant decrease in 13C of bulk organic matter suggests that soil organic matter is in general better preserved upon prolonged exposure to oxygen, possibly due to some form of matrix protection. ACKNOWLEDGMENTS We thank Dr. Burdige and two anonymous reviewers for their constructive comments. This project was supported by NWO-ALW (Project No. 152911) through a grant to S.S. NWO-ALW is also thanked for their financial contribution to core 90P22 recovery and ODP 157 sampling; A. Mets, E.C. Hopmans (NIOZ), G. Nobbe and H. de Waard (UU) are thanked for analytical support.

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