Bacterial tetraether membrane lipids in peat and coal: Testing the MBT–CBT temperature proxy for climate reconstruction

Organic Geochemistry 42 (2011) 477–486

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Bacterial tetraether membrane lipids in peat and coal: Testing the MBT–CBT temperature proxy for climate reconstruction Johan W.H. Weijers a,b,⇑, Philipp Steinmann c,1, Ellen C. Hopmans b, Stefan Schouten a,b, Jaap S. Sinninghe Damsté a,b a b c

Utrecht University, Department of Earth Sciences – Geochemistry, Budapestlaan 4, NL-3584 CD, Utrecht, The Netherlands NIOZ Royal Netherlands Institute for Sea Research, Department of Marine Organic Biogeochemistry, P.O. Box 59, NL-1790 AB, Den-Burg – Texel, The Netherlands Institut de Géologie, Université de Neuchâtel, 2009 Neuchâtel, Switzerland

a r t i c l e

i n f o

Article history: Received 29 November 2010 Received in revised form 16 February 2011 Accepted 11 March 2011 Available online 21 March 2011

a b s t r a c t Peatlands are widespread and important natural archives of environmental change. Here we explore the potential of the recently introduced MBT–CBT proxy (methylation index and cyclisation ratio of branched tetraethers) to estimate past annual mean air temperature (MAT) based on the distribution of bacteriallyderived branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in peat and coal. To this end, branched GDGTs in an ombrotrophic peat bog from Switzerland and three coal deposits of increasing maturity were analysed. For the surface of the bog, reconstructed annual MAT is higher than both measured annual MAT and measured in situ pore water temperature. Changes in the CBT ratio, considered a proxy for pH, with depth in the bog do not match with present day in situ pore water pH, but coincide with a peat stratigraphic boundary. This indicates that GDGTs down the bog profile are predominantly fossil and not derived from extant biomass. The MBT–CBT derived annual MAT record also shows a large drop at this stratigraphic boundary, which likely relates to past change in trophic status of the bog. Branched GDGTs are abundant in an immature lignite (vitrinite reflectance, Ro 0.25%), but occur in low amount in a slightly more mature coal (Ro 0.32%). Annual MAT could be reconstructed for the lignite alone and is higher than other proxybased estimates from approximately the same time and location. Our results indicate potential for the application of the MBT–CBT proxy in peat and immature coals, but improved constraints on the effects of different types of peat on branched GDGT distributions as well as improved calibration of MAT estimates are needed before the method can be confidently applied. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Because of the waterlogged and therefore anoxic conditions, peatlands are excellent recorders of past climate change. For some time, palynologists have used the well preserved pollen, spores and macro remains (leafs, twigs, etc.) from these climate archives to reconstruct past changes in vegetation composition and thereby infer changes in climate (e.g. von Post, 1946; van Geel, 1978). Similarly, organic geochemists have used plant wax derived lipids as a tool for vegetation and climate reconstruction in peat bogs (e.g. Xie et al., 2000, 2004; Pancost et al., 2003; Bingham et al., 2010). Also, biomarkers that do not derive from the vegetation itself but from microorganisms living in the peat bog underneath could be a ⇑ Corresponding author at: Utrecht University, Department of Earth Sciences – Geochemistry, Budapestlaan 4, NL-3584 CD, Utrecht, The Netherlands. Tel.: +31 (0)30 2535068. E-mail address: [email protected] (J.W.H. Weijers). 1 Present address: Federal Office of Public Health, Schwarzenburgstrasse 165, CH3003 Bern, Switzerland. 0146-6380/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2011.03.013

useful source of information. One group of such compounds consists of glycerol dialkyl glycerol tetraether (GDGT) membrane lipids. Of these, the bacterially derived branched GDGT lipids, first detected in peat bogs (Sinninghe Damsté et al., 2000), have received increasing interest since their distribution in soils was shown to relate to soil pH and annual mean air temperature (MAT, Weijers et al., 2007a). Being fluvially transported to the ocean, these branched GDGT compounds become part of the marine sedimentary archive in which their distribution, expressed in the methylation index of branched tetraethers and cyclisation ratio of branched tetraethers (MBT–CBT), has been exploited as a recorder of past soil pH and temperature changes on land, both in geologically more recent times, including the last glacial – interglacial transition (Weijers et al., 2007b; Rueda et al., 2009; Peterse et al., 2009; Bendle et al., 2010) and in deep time like the early to mid Cenozoic (Weijers et al., 2007c; Schouten et al., 2008; Donders et al., 2009; Hren et al., 2010). In addition to applications in the marine realm, research is currently focussing on the potential application of this new proxy in lake sediments (Sinninghe Damsté et al., 2009; Zink et al., 2010; Tierney et al., 2010; Blaga et al., 2010;

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Bechtel et al., 2010; Tyler et al., 2010). In these lacustrine environments application of the MBT–CBT proxy seems, however, to be constrained by the ostensible in situ production of branched GDGTs in lake water or sediments (Sinninghe Damsté et al., 2009; Tierney and Russell, 2009; Tierney et al., 2010). Although peatlands are extensive in areal coverage in the boreal realm, i.e. over 3 million km2 (e.g. Laine et al., 2009 and references therein), and despite the fact that branched GDGTs generally occur in higher concentrations in peatlands than in soils (Weijers et al., 2006b), peatlands have received little attention in terms of application of the MBT–CBT proxy. Huguet et al. (2010) determined MBT– CBT indices at two different depth intervals at two different sites in a French peatland and Ballantyne et al. (2010) applied the proxy to a Pliocene peat deposit from Ellesmere Island, Canada. In addition to peatlands, lithified (or coalified) peat sequences, i.e. lignites, can be well preserved over geological time and could therefore provide unique windows into past times in terms of climate and peat ecosystem changes. Here, we investigate the applicability of the MBT– CBT proxy from a peat core obtained from a raised bog in the Swiss Jura mountains which covers the complete Holocene. In addition, three coals from the Argonne Premium Coal Series (Vorres, 1990) were analysed for the presence and distribution of branched GDGTs, to determine the potential for the MBT–CBT proxy in these old peat deposits. 2. Material and methods

(Steinmann et al., 2008). Daily mean temperature is 15 °C for the warmest month (July) and 5 °C for the coldest month (January), and a snow cover is present for 80 to 120 days a year (Eilrich, 2002). An extensive description of the bog is given by Steinmann and Shotyk (1997b). In short, Etang de la Gruère is a 22.5 ha, strongly domed bog with a peat accumulation of >6 m. The vegetation is dominated by Sphagnum (mainly S. magellanicum) and Eriophorum. The central part of the dome, where the core was taken, was treeless, but dispersed Pinus mugo grew at the fringes of the bog. The peat at the sampling site is dominated by Sphagnum in the uppermost part (0–60 cm depth), Sphagnum–Eriophorum from 60 to 250 cm, Sphagnum from 250 to 420 cm and by Carex below. At 650 cm there is a sharp boundary, with the underlying grey silty clay (part of the Oxfordian clays) and marls on which the bog developed. The water table seasonally fluctuates between 4 and 17 cm below the surface. A composite core was retrieved using a Belorussian peat sampler taking cylindrical cores at 1 m increments. In the laboratory the cores were cut alongside in half and sliced. Samples for GDGT analysis were immediately freeze-dried. Pore water temperatures have been reported by Eilrich and Steinmann (2003) and had been measured in situ at different depths at near monthly intervals between June 1999 and October 2001 using an electronic temperature probe mounted on extendible rods. Pore water pH values along a depth profile of the bog were reported by Steinmann and Shotyk (1997a) and were sampled using in situ diffusion equilibrium samplers.

2.1. Peat samples 2.1.1. Site and sampling The core was retrieved in July 2000 from the centre of the peat bog ‘Etang de la Gruère’ (EGr), located in the Franches Montagnes region of the Swiss Jura Mountains, ca. 50 km north of Neuchâtel (Fig. 1). The Franches Montagnes is a calcareous plateau ca. 1000 m above sea level. The climate of the region is characterised as moist continental, with average annual precipitation of 1600 mm and average annual mean air temperature of 5.5 °C

2.1.2. Age model The age model of the peat column at the centre of the bog is provided by Roos-Barraclough et al. (2004) and is based on conventional 14C AMS dating of 31 samples along the profile. These were calibrated to calendar years using CalibETH (Niklaus et al., 1992). The calendar year before present (BP) record was constructed using 11 separate linear regressions, which gave a better fit than a single regression. The accumulation of peat in the central part of EGr is very homogeneous as shown by the correlation of prominent ash

Fig. 1. Location of Etang de la Gruère (EGr), Jura Mountains, Switzerland.

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layers in several cores taken within several m of each other (Steinmann et al., 2006).

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and ultrasonically redissolved in hexane: propan-2-ol 99:1 (v/v) to a concentration of ca. 2 mg/ml and subsequently filtered over an 0.45 lm PTFE filter (Alltech) prior to analysis.

2.2. Coal and lignite samples 2.4. Analysis Three samples were selected from the Argonne Premium Coal Series, a set of 8 standard samples with different properties (Vorres, 1990). The coals are the Beulah Zap lignite, the Wyodak– Anderson coal and the Illinois #6 (or Herrin) coal. They are among the least mature coals, with H/C and O/C atomic ratios of 0.79 and 0.21 for Beulah Zap, 0.86 and 0.18 for Wyodak–Anderson, and 0.77 and 0.13 for Illinois #6, respectively and a mean maximum vitrinite reflectance (Ro) of 0.25%, 0.32% and 0.46%, respectively (Vorres, 1990). The Beulah Zap lignite derives from a coal zone in the Williston Basin, North Dakota, USA. The zone consists of five coal beds which are interbedded with fluvial deposits. The beds accumulated in interdistributary and abandoned delta peat swamps (Flores et al., 1999). The Beulah Zap coal zone is of late Palaeocene age. The Wyodak–Anderson coal derives from the Powder River Basin, Wyoming, USA, and is also of late Palaeocene age. The Wyodak–Anderson coal seam consists of raised peat deposits that accumulated in restricted parts of an inland floodplain, as evident from laterally interfingering narrow and elongate fluvial siltstone and shale bodies (Warwick and Stanton, 1988). Coal Illinois #6 is derived from the Herrin Coal in the Illinois Basin, Illinois, USA and was deposited during the Middle Pennsylvanian between 290 and 320 Myr ago in a peat swamp which was periodically flooded by nearby rivers, as evident form intertonguing siltstones (Dimichele and Phillips, 1988).

GDGTs were analysed using an Agilent 1100 series high performance liquid chromatography–atmospheric pressure chemical ionisation/mass spectrometry (HPLC–APCI/MS) instrument equipped with automated injector and HP-Chemstation software (Hopmans et al., 2000; Schouten et al., 2007). Injection volume was 10 ll and separation was achieved with an analytical Prevail Cyano column (150  2.1 mm, 3 lm) with hexane:propan-2-ol 99:1 (v/v) as eluent, isocratically for the first 5 min and linearly increasing to 1.8% propan-2-ol in 45 min. In addition to full scan mode, samples were run in selective ion monitoring (SIM) mode, screening for the compounds of interest, in order to increase sensitivity. GDGTs were quantified by integrating the surface area under the [M + H]+ peaks and comparing these with an external calibration curve constructed using known amounts of the GDGT crenarchaeol. The degrees of cyclisation and methylation of branched GDGTs, expressed in the CBT ratio and MBT index, were calculated using the following formulae (Weijers et al., 2007a):

CBT ¼  log

MBT ¼

  ½Ib þ ½IIb ½Ia þ ½IIa

ð1Þ

½Ia þ ½Ib þ ½Ic ½Ia þ ½Ib þ ½Ic þ ½IIa þ ½IIb þ ½IIc þ ½IIIa þ ½IIIb þ ½IIIc ð2Þ

2.3. Sample preparation Freeze dried peat samples (ca. 0.5 g) were ground, using liquid N2 if necessary, and extracted (3, 5 min each) using an accelerated solvent extractor (ASE 200,Dionex) with dichloromethane (DCM): MeOH 9:1 (v/v) at 7.6  106 Pa and 100 °C. The extracts were concentrated using a rotary evaporator and then evaporated to dryness under a continuous flow of N2. The extract was separated over an activated Al2O3 column into a nominal apolar fraction using DCM and a polar fraction with DCM: MeOH 95:5 (v/v). The polar fraction, containing the GDGTs, was dried under N2

The roman numerals refer to the structures in Fig. 2. The analytical error for both the CBT ratio and the MBT index is ca. 0.01. Based on a global soil calibration set, these values can be translated into soil pH and annual mean air temperature (MAT) using the following calibration formulae (Weijers et al., 2007a):

soil pH ¼ MAT ¼

3:33  CBT 0:38

ðMBT  0:122  0:187  CBTÞ 0:02

ð3Þ

ð4Þ

Fig. 2. Down core profile of bacterially-derived branched GDGT and archaeal-derived isoprenoid GDGT membrane lipid concentrations at EGr and corresponding structures. Isoprenoid GDGTs are dominated by GDGT-0, which is generally >70% of total isoprenoid GDGTs.

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3. Results 3.1. GDGT concentration The first 10 cm of the core consists of a considerable part of fresh, non-degraded, Sphagnum vegetation. As a result, the concentration of branched GDGTs in this horizon is, at 20 lg/g dry wt. peat, low relative to the deeper intervals. Down core, the branched GDGT concentration rapidly increases to a maximum of ca. 2700 lg/g dry wt. peat at 1.4 m (Fig. 2). Below 1.4 m, the concentration decreases again to ca. 500 lg/g dry wt. peat at 3.7 m. A small increase in branched GDGT concentration to ca. 900 lg/g dry wt. peat is apparent just below 4 m, after which it decreases to values around 100 lg/g dry wt. peat near the bottom of the bog at 5.6 m. Isoprenoid GDGTs in peat settings derived largely from methanogenic archaea (Pancost et al., 2000), are dominated by GDGT-0 (>70% of total isoprenoid GDGTs; Fig. 3A and B) and are present in large amounts, ranging from 10 to 700 lg/g dry wt. peat (Fig. 2). The concentration is, nevertheless, lower by a factor of 4–5 than the branched GDGTs (Fig. 3), a pattern more often observed in peat bogs (Weijers et al., 2009). Overall, in EGr, the GDGT concentration is high, i.e. up to an order of magnitude

greater than other peat bogs where GDGTs have been analysed, like the Bolton Fell Moss and the Saxnäs Mosse (Weijers et al., 2006a, 2009). 3.2. CBT and MBT CBT values range from 1.7 at the top, decreasing to 1.2 around 1.5 m and increasing again to 1.7 at a 3.8 m (Fig. 4a). At 3.8 m, a sharp shift in the CBT record occurs, when values drop from 1.7 to 0.2 within a 1 m interval. At the bottom of the record, between 4.9 and 5.6 m, CBT values range from 0.2 to 0.4. The MBT record shows a rather smooth decline from a value of 0.70 in the uppermost horizon to 0.29 at the bottom of the record (Fig. 4b). The MBT record differs from the CBT record in that it does not show a pronounced shift starting at 3.8 m, but does show a small interruption in its declining trend between 4.2 and 5 m when MBT values slightly increase relative to the values around 4 m. 3.3. Coals GDGTs, both isoprenoid and branched, are abundant in the Beulah Zap lignite (Fig. 3C). The total concentration of branched GDGTs is ca. 14 lg/g dry wt. coal (dwc), with GDGT Ia by far the most abundant (ca. 10 lg/g dwc). Isoprenoid GDGT-0, in this setting most likely derived from methanogenic archaea, is also abundant at ca. 2.4 lg/g dwc. This value is clearly higher than that of other isoprenoid GDGTs, which vary from 0.7 to 1.0 lg/g dwc. The CBT and MBT values for the Beulah Zap lignite are 0.94 and 0.88, respectively. Duplicate sample processing resulted in similar values within 0.01 units for both. The Wyodak–Anderson coal does contain low amounts of branched GDGTs. With a concentration of ca. 0.03 lg/g dwc, their abundance is about a factor of 500 lower than in the Beulah Zap lignite. Moreover GDGT IIb was below detection limit, prohibiting calculation of the CBT ratio. The Illinois #6 coal did not contain branched or isoprenoid GDGTs. 4. Discussion 4.1. Fossil vs. extant signals

Fig. 3. LC-MS base peak chromatograms (full scan) of polar fractions of (A) EGr peat at 140 cm depth; (B) 250 cm depth; and (C) Beulah Zap lignite, showing relative abundance and distribution of both isoprenoid (light grey) and branched (dark grey) GDGTs. GDGT-0 to GDGT-4 refer to isoprenoid GDGTs containing 0 to 4 cyclopentane moieties, respectively; ‘cren’ is crenarchaeol (Sinninghe Damsté et al., 2002); roman numerals refer to structures of branched GDGTs in Fig. 2.

A major issue in interpreting an organic geochemical record based on compounds derived from microorganisms, in this case GDGT membrane lipids, is whether the compounds are fossil components or produced in situ. For peat records based on pollen or other vegetation biomarkers, the origin of the signal is easier to assess since vegetation always grows on top of the bog and wind delivered pollen is similar in age to the peat surface on which they are deposited. Microorganisms, on the other hand, live underneath the bog surface and, depending on their ecological niche, might live throughout the bog, particularly anaerobic microbes. Microbial ecological analysis of peat bogs generally shows the highest abundances of extant bacteria around or just below the water table (e.g. Dedysh et al., 2006; Weijers et al., 2009). Liu et al. (2010) analysed branched GDGT intact polar lipids (IPLs), i.e. including a functional polar head group and supposedly representative of extant biomass (e.g. Pitcher et al., 2009), in the 20– 50 cm interval (catotelm) of a 50 cm core from a German peat bog (water table at ca. 16 cm). They showed that branched GDGT IPLs, and thus potentially extant branched-GDGT producing microbes, were most abundant between 20 and 35 cm and decreased with depth below 35 cm, whereas branched GDGT core lipids (CLs, i.e. the fossil form of GDGTs as analysed in this study) seemed to increase in abundance over the 20 to 50 cm depth interval. Therefore, the horizon with highest concentrations of branched GDGT CLs in EGr at ca. 1.4 m likely does not reflect the zone of

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Fig. 4. Schematic representation of EGr core with depth profiles of (A) cyclisation ratio of branched tetraethers (CBT) and (B) methylation index of branched tetraethers (MBT).

maximum production of branched GDGTs, but is the combined result of absolute (cumulative production) and relative (preferential preservation) accumulation of branched GDGTs. This means that branched GDGTs at this depth in the peat are likely predominantly fossil species and not produced in situ. 4.2. Comparison of branched GDGT distributions with in situ pH and temperature 4.2.1. CBT and pore water pH An additional means of verifying whether branched GDGTs deeper down the profile are dominantly derived from extant bacteria

or represent an old fossil pool, is to translate the CBT values into pH estimates using Eq. (3) and compare these with in situ measured pore water pH values (Fig. 5a). The measured pore water pH gradually increased with depth from 4.2 at 0.5 m to 5.9 at 5.4 m. The measured pH in the top part of the bog (4.2–4.9) is about 0.6 units lower than the CBT-estimated pH (4.9–5.5). However, where the measured pH shows a steady increase with depth, the CBT-estimated pH first shows a similar increase down to ca. 1.5 m, but decreases between 2 and 3.8 m depth, followed by a sharp increase to estimates that are much higher (ca. 8) than the measured pH (Fig. 5a). A difference between estimated and measured pH of ca. 0.6 pH units for the top part of the bog is still within

Fig. 5. (A) Reconstructed pH (black dots) based on CBT values in EGr bog plotted vs. in situ pore water pH (solid line); (B) reconstructed annual mean air temperature (MAT, black dots) based on both the CBT and MBT values plotted vs. in situ pore water temperature profiles measured at near monthly intervals over two years (solid lines).

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the standard error of estimate of 0.8 pH units for the calibration formula (Weijers et al., 2007c). The large shift in pH estimates below 3.8 m is, however, by no means reflected in the measured pH. This shift in pH occurs at the point where the composition of the peat shifts from being Carex dominated at the base, which lies atop the calcareous plateau, to a Sphagnum dominated peat. The Carex dominated peat formed in a fen system (Steinmann and Shotyk, 1997b), which is not fully dependent on rain water alone and therefore has a pH value typically in the range 5–7, higher than values of 3–5 that are more typical for ombrotrophic Sphagnum dominated bogs that are disconnected from groundwater and consequently solely dependent on rain water (e.g. Galand et al., 2005; Merila et al., 2006). Thus, the change in trophic status will have an effect on pH, which is indeed reflected in the CBT ratio. The change in reconstructed pH is, however, rather large (ca. 4 pH units) and might be larger than the real shift. At present, the pH of the pore water near the surface of the bog is dominated by the presence of organic acids. On downward advection these become increasingly neutralised and the pH in the lower layers of the bog is largely determined by the partial pressure of dissolved CO2. This results in a relatively continuous trend of slightly increasing pH values with depth (Steinmann and Shotyk, 1997a). The fact that this stratigraphic boundary between fen peat and bog peat is still clearly reflected in the reconstructed pH record, indicates that the GDGT signal is most likely preserved at or around the time of deposition of the peat and therefore represents a predominantly fossil signal. 4.2.2. MBT–CBT and pore water temperature In situ pore water temperature in the bog shows the largest seasonal temperature variation at shallowest depth, i.e. 20 cm, varying between 1 °C (February) and 18 °C (August). Below 20 cm, pore water temperature quickly converges to a range between 5 and 10 °C at 2 m and reaches a constant value of 7–8 °C at 3 m depth (Fig. 5b). The reconstructed annual MAT, calculated using the soil-calibration derived equation, Eq. (4), is ca. 13 °C for the top 30 cm of the bog, which is clearly higher than the annual MAT recorded for the area (5.5 °C). Compared to the annual mean in situ temperature at this depth, which is ca. 8 °C (Fig. 5b), the difference is smaller, though still appreciable. Average in situ temperature slightly higher than average air temperature is a feature observed earlier in soils (Weijers et al., 2011) and is most likely a result of the heat capacity of the soil water and the insulating effect of vegetation. Certainly, in bogs which are wet and could have a dense cover of Sphagnum, the effect is likely to occur and may even be stronger. Also, in a few other peat bogs for which GDGTs have been measured, GDGT-derived MAT estimates seem slightly higher than measured MAT, although the magnitude of the difference between the two is different at different sites, i.e. in the Saxnäs Mosse (south Sweden) estimated MAT is ca. 10 °C and measured MAT ca. 6.5 °C (Weijers, unpublished results); in the Bullenmoor peat bog (northern Germany) estimated MAT is ca. 11 °C and measured MAT ca. 9 °C (Liu et al., 2010); and in Frasne peat bog, French Jura Mountains, estimated MAT is ca. 12 °C and measured MAT ca. 7 °C (Huguet et al., 2010). It has to be noted that all these data are from ombrotrophic peat bogs; for the Frasne peatland, a poor fen site was also analysed, which gave GDGT-based MAT estimates close to measured MAT (6 °C; Huguet et al., 2010). It has to also be mentioned that the estimates for the Frasne peatland were derived from a near surface sample (11 cm deep) and that a deeper sample (ca. 45 cm) from the catotelm gave lower estimates of MAT of 1 °C and 5 °C for the fen and the bog, respectively (Huguet et al., 2010). The deviation between reconstructed and measured MAT in the above peat bogs is appreciable but in most cases just within the standard error of estimate of the calibration (±5 °C), however, reconstructed MAT always deviates towards higher temperatures

(for the upper part of ombrotrophic bogs). This suggests that there may be a warm bias in peat bog derived MAT estimates based on branched GDGTs. A possible explanation would be that microbes are more active in warmer months and that, as a consequence, more branched GDGTs are produced in summer than in winter when many peat bogs freeze over. However, Harrysson Drotz et al. (2010) show that microbes can stay metabolically active in frozen boreal forest soil and continue producing membrane lipids. In addition, in a one year time series study of several mid-latitude soils, Weijers et al. (2011) did not observe a clear seasonal signal in the amounts and distributions of branched GDGTs. Ballantyne et al. (2010) estimated annual MAT based on branched GDGT distributions in a Pliocene peat deposit from Ellesmere Island (Canada) to be 0.6 °C, which was statistically identical to estimates based on d18O of tree ring cellulose (0.5 °C) and on a palaeovegetation coexistence approach (0.4 °C), also not pointing to a warm bias specific to the MBT–CBT proxy. Further analysis of peat bogs covering a wider temperature range is required to verify if this bias is consistent and whether or not a separate calibration for peat bogs is needed that is different from the soil-based calibration (Weijers et al., 2007c). 4.2.3. Down core temperature record Below 20 cm, MBT–CBT reconstructed temperature steadily decreases to 2 °C at 3.8 m, followed by a sharp rise to ca. 12 °C at 4.9 m, and a decrease again to ca. 5 °C at 5.5 m. These reconstructed temperatures clearly do not reflect in situ temperatures but instead deviate by as much as 10 °C to +5 °C from measured pore water temperatures. Thus, as with the pH profile, the pattern suggests that the reconstructed temperatures deeper down the peat bog represent a predominantly fossil signal. It is remarkable that the large shift in reconstructed temperature to some extent equals the shift in the CBT ratio and palaeo pH at this depth in the bog. Although the change in pH, as expressed by way of the CBT, is accounted for in the MBT–CBT temperature reconstruction, the change in CBT, and potentially palaeo pH, is unusually large and might somehow overprint the MBT temperature relationship, since MBT is related to both temperature and pH. Nevertheless, it could well be that the stratigraphic shift in peat composition was a response to a changing climate (temperature and/or humidity) and that at least part of the shift in reconstructed temperatures is real, although the overall amplitude is likely too large. Since most of the branched GDGTs in EGr reflect a fossil signal, the reconstructed temperature can be plotted vs. age. This shows that low temperatures of ca. 5 °C occurred around 12.5 ka, followed by an increase to ca. 12 °C at 10 ka. Subsequently, a 14 °C decrease from 12 to 2 °C occurred between 10 and 8.4 ka. This is then followed by a more or less continuous warming to ca. 13 °C at the present day (Fig. 6b). In terms of timing, the lower temperatures at the bottom of the record might be part of the Younger Dryas cold interval (YD, ca. 12.6–11.5 ka BP) followed by a warming of up to 7 °C into the early Holocene. Proxy evidence shows that at least summer temperatures for this part of Europe dropped by 3 to 5 °C during the YD (e.g. Renssen and Isarin, 1998; Isarin and Bohncke, 1999; Peyron et al., 2005; Larocque-Tobler et al., 2010), somewhat less than the ca. 6–7 °C drop in annual MAT reconstructed here. It should be acknowledged, however, that the sampling resolution at EGr is too low to properly cover the YD and that the record does not extend much prior to the YD, preventing estimation of the exact cooling into the YD. The large drop in reconstructed MAT occurs between 10 and 8.4 ka and coincides with the transition from the Carex dominated fen peat to Sphagnum dominated bog peat, which itself is dated at ca. 8.7 ka BP (Roos-Barraclough et al., 2004). The transitions occurs just before the 8.2 ka event, a ca. 300 yr cold event in the North Atlantic region caused by the release of large amounts of

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Fig. 6. MBT–CBT based reconstruction of (A) pH and (B) annual MAT in the EGr peat bog plotted vs. age (yrBP, years before present).

freshwater in the North Atlantic, resulting in a slowing of the thermohaline circulation (e.g. Wiersma and Renssen, 2006 and references therein). Proxy evidence from the region suggests a potential drop in temperature of about 1 °C at the 8.2 ka BP event (e.g. Magny et al., 2001; Heiri et al., 2004), much less than the drop in annual MAT reconstructed here. However, the 8.2 ka event is also recognised as a turning point in climatatic conditions. Prior to this event, at ca. 9 ka, the central European climate was characterised by warm and dry summers and cold winters as a result of minimum winter and maximum summer solar irradiation (Tinner and Lotter, 2001 and references therein). Shotyk et al. (2001) indeed found elevated soil dust flux at 9 ka BP in the EGr record. Nearby Swiss vegetation reconstructions indicate that the central European climate never returned to these conditions again after the 8.2 ka event (Tinner and Lotter, 2001). In addition, on a more global scale, the 8.2 ka is also marked by a distinct drop in atmospheric CH4 (Chappellaz et al., 1993) and coincides with a dramatic change in the formation of peatlands in the northern hemisphere. The initiation of peat growth in western Siberia virtually stopped around this time (Smith et al., 2004) and a tentative reconstruction of palaeo net primary production (NPP) at EGr showed a rapid decrease around 8.2 ka (Steinmann et al., 2006). Thus, the observed change in the branched GDGT distribution between 10 ka and 8.4 ka, likely reflects not a change in MAT but a change in environmental conditions, i.e. wetness, which resulted in a shift in peat forming vegetation and trophic status of the bog. It is unclear if a change in humidity could be a causal factor behind the changing distribution of branched GDGTs in the core. The principal component analysis (PCA) performed on the global soil data by Weijers et al. (2007a) did show a significant loading of precipitation on the same PCA axis as the MBT index and annual MAT. The correlation between precipitation and MBT was, however, assumed to be an artefact of the correlation between precipitation and annual MAT. In contrast, Huguet et al. (2010) suggested that humidity might have an effect on the MBT index, i.e. the MBT at the (wetter) fen site was lower than at the (drier) bog site and also lower in the (wetter) catotelm than in the (drier) acrotelm layer. Also, in the EGr peat bog MBT (and CBT) values of the interval dominated by Carex peat, the supposedly wetter type of peatland, are lower than those obtained for the bog stage.

In the soil dataset, which does not contain any peat soil, the MBT index relates to both annual MAT and soil pH and, as a consequence, has to be corrected for pH to obtain estimates of past annual MAT (Weijers et al., 2007a). As discussed in Section 4.2.1., the change in trophic state is clearly reflected in the CBT ratio and can be explained as a change in palaeo pH. Although pH in the soil dataset has an effect on the MBT index, in the EGr record no clear signal is apparent in the MBT record related to this supposedly large shift in palaeo pH (Fig. 4). Since in the MBT–CBT proxy the MBT index is corrected for pH, we now see this correction back as a shift in reconstructed MAT. Overall, it is clear that analysis of more peatlands over a wider temperature gradient and pH gradient is required to see if the effect of pH on the MBT index is similar to that for the soil dataset and to see how important differences in humidity are with respect to branched GDGT distributions. Finally, irrespective of any causal relation between GDGT distribution and humidity in peatlands, it should be noted that an alkaline peat fen and an acidic peat bog are quite distinct environments, both in terms of pH and nutrients. Microbial ecological studies have shown clear differences in the microbial communities along such pH and nutrient gradients in peatlands (e.g. Juottonen et al., 2005; Merila et al., 2006). Thus, the observed shift in the distribution of branched GDGTs in the EGr profile, coinciding with the change in vegetation/trophic status of the peat, could at least partly also be a result of different branched GDGT producing communities that might have been present in the different types of peat. This suggestion remains, however, speculative as long as the precise species of branched-GDGT synthesising bacteria remain unknown. 4.3. Coals 4.3.1. Maturity and GDGT occurrence Branched GDGTs are clearly present in the Beulah Zap lignite (Fig. 3C). The Wyodak–Anderson coal is also of late Palaeocene age but only contains small amounts of branched GDGTs Ia and IIa, and traces of Ib and Ic, while others are below detection limit. Both age and depositional environment, i.e. peat swamps on river floodplains, for this coal are similar to the Beulah Zap lignite and

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they were deposited geographically rather close to each other. This suggests that the difference in GDGT abundance relates mainly to the higher level of maturity. The mean maximum Ro of the Wyodak–Anderson coal is, at 0.32%, indeed higher than that of the Beulah Zap lignite (0.25%). No branched GDGTs were detected in the Illinois #6 coal, in agreement with the even higher Ro value of 0.46%. However, given the age of the Illinois #6 coal of ca. 300 Ma, it cannot be excluded that branched-GDGT synthesising bacteria had not yet evolved at that time. It has to be noted, though, that also GDGT-0, derived from methanogenic archaea, has not been detected in the Illionois #6 coal, supporting the idea that the high maturity of the coal is the reason for the absence of branched GDGTs. This is supported by the results of Schouten et al. (2004), who artificially matured a sediment from a relatively immature outcrop section from the Gesosso–solfifera Formation (Messinian, Italy) using hydrous pyrolysis at 160–300 °C. The initial Ro value of the sediment, which contained abundant GDGTs, was 0.25%, similar to the Beulah Zap lignite studied here. It was found that GDGTs started to disappear above a hydrous pyrolysis temperature of 240 °C and were below detection limit above 280 °C. A hydrous pyrolysis temperature of 280 °C corresponds to 17a,21b(H)-hopane 22S/(22S + 22R) values of ca. 0.2 (determined for the same samples; Koopmans et al., 1996). This in turn compares with an Ro value of ca. 0.35% (Killops and Killops, 2005) which is just higher than that of the Wyodak–Anderson coal in which only small amounts of GDGTs could be detected.

real annual MAT. Further research is needed to determine the extent of this ostensible bias and to verify whether it also occurs in peat systems other than bogs. Down core analysis of the EGr bog showed that the CBT ratio does not match with present day in situ pore water pH, but, on the contrary, matches with a peat stratigraphic boundary, representing a change from Carex dominated peat to Sphagnum dominated peat. This indicates that GDGTs deeper down the water table predominantly represent fossil biomass. The reconstructed annual MAT shows lower temperatures during the YD interval, in agreement with other proxy data. An unusually large decline in reconstructed MAT occurs, however, between 10 ka and 8.4 ka and is likely a result of a change from dry to wet conditions concomitant with a change in trophic status of the peat. Branched GDGTs could only be detected in very immature coal (lignite) with Ro < 0.32%. Reconstructed annual MAT for the Beulah Zap lignite is higher than those based on leaf margin and oxygen isotope analyses from roughly the same age and area. Despite the uncertainties associated with the reconstructed MAT in the EGr peat bog and the Beulah Zap lignite, the results show that there may be potential for the application of the MBT–CBT proxy in peat cores and lignites. It is clear, nevertheless, that improved constraints on the effects of different peat environments on the branched GDGT distribution, as well as an improved calibration towards annual MAT, are needed before the method can be applied with confidence. Acknowledgements

4.3.2. Palaeoclimatic interpretation The CBT and MBT values of the Beulah Zap lignite translate in a pH estimate of 6.2 and an estimate of past annual MAT of ca. 29 °C. This pH value is not highly indicative for a specific environment, but could well fit with the depositional environment, i.e. peat swamps in a river delta plain (Flores et al., 1999), which often show pH values in the range of 5–7 (e.g. Nanson, 2009). The reconstructed annual MAT based on the branched GDGT distribution in the Beulah Zap lignite is much higher than current annual MAT at this location of 5.3 °C [Williston, North Dakota, USA (KNMI, 1997)] and is therefore most likely representative of the time of active peat formation, i.e. the late Palaeocene. Comparison of our MAT estimate with the few other reported MAT estimates shows that it is considerably higher. For late Palaeocene North America, Wing et al. (2000) and Wilf (2000) estimated MATs between 13 and 19 °C for the Wyoming and Green River basins based on leaf margin analysis of fossil leafs from channel deposits, while oxygen isotope analysis of biogenic phosphate generally shows MAT estimates slightly higher than estimates based on leaf margin analysis for the late Palaeocene – early Eocene interval (Fricke and Wing, 2004). However, both estimates are known to potentially underestimate MAT (Kowalski and Dilcher, 2003; Pucéat et al., 2010). Alternatively, the MBT–CBT proxy overestimates past annual MAT because of unknown factors. It does, however, agree with the generally warm conditions in late Palaeocene North America. Further calibration of the MBT–CBT proxy in peat is required to improve MAT estimates and enable more confident application of the proxy. 5. Conclusions MBT–CBT-derived annual MAT estimates from the top part of a Swiss peat bog are ca. 7 °C higher than annual MAT at the EGr site and ca. 5 °C higher than annual mean in situ pore water temperature. As such differences are also present in estimates from a few other peat bogs, it suggests that MBT–CBT-derived MAT estimates based on branched GDGT distributions in peat bogs overestimate

Two anonymous reviewers are thanked for providing useful comments and suggestions, which helped to improve the manuscript. This study was made possible by a VENI grant to J.W.H.W. by the Netherlands Organisation for Scientific Research. J.S.S.D. acknowledges funding from the European Research Council.

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