Richard D. Pancost

Microbial lipid biomarkers preserved in geological archives can be used to explore past climate changes. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are unique bacterial biomarkers that have been used as molecular tools for the quantitative determination of terrestrial temperatures and the pH of depositional environments over a range of geological timescales. However, the exact biological source organisms – especially of the entire suite of brGDGTs found in the environment – remains unclear; by extension, so do the mechanisms that govern these proxies. Here, we identified a brGDGT-producing strain Candidatus Solibacter usitatus Ellin6076, by identifying archaeal tetraether synthase homologs in bacterial genomes. This strain synthesizes diverse brGDGTs, including regular C 5 -methylated and cyclic brGDGTs, and brGDGTs comprise up to 66 % of the major quantified lipids, far exceeding the proportions found in previous studies. The degree of C 5 -methylation in cultured strain Ellin6076 is primarily determined by temperature, whereas cyclization appears to be influenced by multiple factors, such as temperature, pH and oxygen availability. Consequently, culture-derived paleoclimate indices are in agreement with the global soil-derived MBT’ 5ME (methylation index of C 5 -methyl brGDGTs) proxy for temperature, albeit with a differed slope, but not the CBT 5ME (cyclization index of C 5 -methyl brGDGTs) proxy for pH. Our findings provide insights from a physiological perspective into the underlying mechanism of brGDGT-based proxies.

Glycerol dialkyl glycerol tetraethers (GDGTs) are widespread but unique membrane-spanning lipids of many Archaea and some Bacteria. However, their specific biological sources and the associated environmental controls on their distribution remain unclear, especially in lacustrine settings, hindering our understanding of these compounds and their application as environmental proxies. Here we investigated the GDGT distributions across a large bottom water dissolved oxygen (DO) gradient (0.10–7.20 mg/L) using surface sediments of Lake Yangzonghai, a warm monomictic lake in southwestern China. We show that the distributions of both branched (br) and isoprenoidal (iso) GDGTs co-vary significantly with bottom water DO concentration. The relative abundances of tetramethylated brGDGTs, 5-methyl penta- and hexamethylated brGDGTs and isoGDGT-0 show a stepwise increase as bottom water DO concentration decreases. On the other hand, the relative abundances of 6-methyl penta- and hexamethylated brGDGTs and crenarchaeol exhibit a stepwise decline with a decrease in bottom water DO concentration. Genetic data indicate these DO-induced changes in GDGT parameters are related to changes in the bacterial and archaeal communities across the oxycline in the lake. For example, the high abundance of isoGDGT-0 in low DO samples coincides with a high abundance of methanogenic archaea and Bathyarchaeota. We propose that brGDGT-producing bacteria might include a diversity of other phyla in addition to the proposed source organism acidobacteria; different groups of anaerobic and aerobic bacteria are likely to contribute to the increased abundance of 5-methyl brGDGTs and 6-methyl brGDGTs in low and high DO zones, respectively. Consequently, the MBT′ and MBT′ 6ME brGDGT indices display strong correlations with DO concentration. Importantly, the MBT′ 5ME index is not significantly influenced by changes in DO concentration, suggesting that this index might be more resilient to these impacts and more suitable to reconstruct temperature in lake systems. • One of the most systematic investigation on DO impact on all the GDGT compounds and the related paleoclimate proxies • A new ternary diagram was used to identity the sources of lacustrine brGDGTs • Anaerobic and aerobic bacteria contribute to the increased abundance of 5-methyl and 6-methyl brGDGTs, respectively • MBT′ 5ME might be more suitable to reconstruct temperature in lake systems than MBT′ and MBT′ 6ME

Wetlands play a crucial role in the carbon cycle as they are the largest natural source of methane, a potent greenhouse gas. Changes in wetland hydrology can alter the rate of greenhouse gas release from wetlands and have the potential to alter Earth’s carbon budget. However, the microbial dynamics underpinning these observations are poorly constrained. Here we combine monitoring of environmental parameters and greenhouse gas fluxes with monthly records of microbial phospholipid fatty acid (PLFAs) δ13C values to probe changes in microbial community and biogeochemistry in response to hydrological changes in a monsoon influenced subtropical wetland from central China. Our results show that water table depth is a key factor controlling the microbial community structure, with Gram-negative bacteria and actinobacteria increasing and fungi decreasing during dry and low water table periods. Meanwhile, the δ13C values of specific PLFAs decreased up to 12‰ during dry compared to wet periods. The extent of depletion varied, but PLFAs from Gram-negative bacteria were most depleted in 13C, indicative for a rapid increase in methanotrophy (methane consumption) during these dry periods. Furthermore, the methane emission of the wetland was drastically reduced and even had negative flux values during dry periods, suggesting that the increased methanotrophy led to a reduced methane flux and a temporary shift of the wetland from a methane source to a methane sink. Our results indicate that short-term hydrological variations lead to a rapid response in microbial community and carbon metabolic activity that directly influences wetland carbon dynamics and greenhouse gas emissions.

The Sahara is the largest hot desert on Earth. Yet the timing of its inception and its response to climatic forcing is debated, leading to uncertainty over the causes and consequences of regional aridity. Here we present detailed records of terrestrial inputs from Africa to North Atlantic deep-sea sediments, documenting a long and sustained history of astronomically paced oscillations between a humid and arid Sahara from over 11 million years ago. We show that intervals of strong dust emissions from the heart of the continent predate both the intensification of Northern Hemisphere glaciation and the oldest land-based evidence for a Saharan desert by millions of years. We find no simple long-term gradational transition towards an increasingly arid climate state in northern Africa, suggesting that aridity was not the primary driver of gradual Neogene expansion of African savannah C4 grasslands. Instead, insolation-driven wet–dry shifts in Saharan climate were common over the past 11 Myr, and we identify three distinct stages in the sensitivity of this relationship. Our data provide context for evolutionary outcomes on Africa; for example, we find that astronomically paced arid intervals predate the oldest fossil evidence of hominid bipedalism by at least 4 Myr. Pulses of Saharan dust have been entering the North Atlantic since at least 11 Ma, a result of astronomically paced cycles between arid and humid conditions in northern Africa, according to a terrigenous input record from an ocean core off west Africa.

The greenhouse-to-icehouse climate transition from the Eocene into the Oligocene is well documented by sea surface temperature records from the southwest Pacific and Antarctic margin, which show evidence of pronounced long-term cooling. However, identification of a driving mechanism depends on a better understanding of whether this cooling was also present in terrestrial settings. Here, we present a semi-continuous terrestrial temperature record spanning from the middle Eocene to the early Oligocene (~41–33 million years ago), using bacterial molecular fossils (biomarkers) preserved in a sequence of southeast Australian lignites. Our results show that mean annual temperatures in southeast Australia gradually declined from ~27 °C (±4.7 °C) during the middle Eocene to ~22–24 °C (±4.7 °C) during the late Eocene, followed by a ~2.4 °C-step cooling across the Eocene/Oligocene boundary. This trend is comparable to other temperature records in the Southern Hemisphere, suggesting a common driving mechanism, likely $$p{{\rm{CO}}_{2}}$$ . We corroborate these results with a suite of climate model simulations demonstrating that only simulations including a decline in $$p{{\rm{CO}}_{2}}$$ lead to a cooling in southeast Australia consistent with our proxy record. Our data form an important benchmark for testing climate model performance, sea–land interaction and climatic forcings at the onset of a major Antarctic glaciation. Terrestrial Southern Hemisphere cooling through the Eocene–Oligocene transition points to decreasing atmospheric CO2 dominantly driving global change, according to biomarker records from southeast Australian coals and palaeoclimate modelling.

Research Article| July 01, 2000 δ13C values and radiocarbon dates of microbial biomarkers as tracers for carbon recycling in peat deposits Richard D. Pancost; Richard D. Pancost 1Department of Marine Biogeochemistry and Toxicology, Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Netherlands Search for other works by this author on: GSW Google Scholar Bas van Geel; Bas van Geel 2Netherlands Centre for Geo-ecological Research, Department of Palynology and Paleo/Actuo-ecology, University of Amsterdam, Kruislaan 318, 1098 SM Amsterdam, Netherlands Search for other works by this author on: GSW Google Scholar Marianne Baas; Marianne Baas 1Department of Marine Biogeochemistry and Toxicology, Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Netherlands Search for other works by this author on: GSW Google Scholar Jaap S. Sinninghe Damsté Jaap S. Sinninghe Damsté 1Department of Marine Biogeochemistry and Toxicology, Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Netherlands Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (7): 663–666. https://doi.org/10.1130/0091-7613(2000)28<663:CVARDO>2.0.CO;2 Article history received: 30 Dec 1999 rev-recd: 20 Apr 2000 accepted: 24 Apr 2000 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Richard D. Pancost, Bas van Geel, Marianne Baas, Jaap S. Sinninghe Damsté; δ13C values and radiocarbon dates of microbial biomarkers as tracers for carbon recycling in peat deposits. Geology 2000;; 28 (7): 663–666. doi: https://doi.org/10.1130/0091-7613(2000)28<663:CVARDO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract This paper describes the first application of compound-specific stable carbon isotope and radiocarbon isotope analyses to the investigation of microbial processes in peat deposits. Carbon recycling in both modern and ancient peats is fundamental in assessing the release of methane to the atmosphere. Important relicts of carbon recycling observed in Holocene peat deposits are reservoir effects, in which the peat is as much as 200 14C yr older than its actual age. This appears to be related to either consumption of large quantities of microbially respired CO2 or smaller quantities of relatively older methane. We sought evidence for such recycling in two Holocene peat deposits. High abundances of methanogen biomarkers suggest that methanogenesis was a significant process. However, we found neither molecular nor isotopic evidence for methanotrophic activity; while this does not preclude such activity, it suggests that it was not sufficiently important to influence the 14C composition of the bulk peat. Likewise, compound-specific radiocarbon dates of bacterial and higher plant biomarkers were not significantly different from the dates of bulk peat, indicating that the reservoir effect was not focused in a specific subfraction of biomass. Instead, we propose that old carbon was incorporated directly into the peat-forming vegetation via fixation of old CO2. You do not currently have access to this article.

Bacteriohopanepolyols (BHPs) are lipid constituents of diverse bacteria and have great potential as taxonomically and environmentally diagnostic biomarkers. In order to examine their environmental behavior and potential for tracing biogeochemical processes, we analyzed BHPs and geohopanoids (the diagenetic products of BHPs) in soils and surface sediments from the middle Yangtze River catchment to the East China Sea (ECS) shelf. These data are compared to an up‐to‐date survey of BHP distributions in soils, including regions collectively covering the Arctic, temperate, subtropics, and tropics. Regional climatic differences, particularly temperature, likely exert an important control on BHP distributions in soils. In the aquatic (river–estuary–shelf) setting, BHP concentrations and structural diversity are substantially lower than in soils, suggesting that in aquatic environments either bacterial biodiversity is lower or there is not the same requirement for hopanoid synthesis. However, different aquatic regimes vary substantially: high BHP diversity and enhanced BHP production occur in the biogeochemically dynamic Yangtze estuary, whereas BHP distribution is uniform with much less structural diversity in the oligotrophic ECS open shelf. The R soil index, based on the relative abundances of soil‐marker BHPs against bacteriohopanetetrol, is suggested as a new approach to trace soil organic matter input into marine sediments. The R soil indices decrease from the river to the ECS, correlating strongly with branched and isoprenoid tetraether indices and moderately correlating with δ 13 C of organic carbon values and the concentrations of higher plant biomarkers, demonstrating their ability to trace soil organic matter inputs at least to the ECS.

Abstract The Cretaceous-Paleogene (K-Pg) boundary marks one of the five major mass extinctions of the Phanerozoic. The ways in which the climate system responded to a bolide impact and extensive volcanism at this time over different time scales are highly debated. We used the distribution of branched tetraether lipids (brGDGT) from fossil peats at two sites in Saskatchewan, Canada (paleolatitude ~55°N), to generate a high-resolution (millennial) record of mean annual air temperature (MAAT) spanning the last ~4 k.y. of the Cretaceous and the first ~30 k.y. of the Paleogene. Our study shows that MAATs ranged from 16 to 29 °C, with the highest value in the first millennia of the Paleogene. The earliest Paleogene averaged ~25 °C—maintaining or enhancing warmth from the latest Cretaceous—followed by a general cooling to ~20 °C over the following ~30 k.y. No abrupt postboundary cooling (e.g., an “impact winter”) or abrupt warming is evident in our data, implying that if such phenomena occurred, their duration was relatively short-lived (i.e., sub-millennial-scale). Further, no long-term impactor volcanism-driven warming is evident. The range of temperature change observed is considerably greater than that derived from marine proxy records over the same time interval. Our findings therefore more properly place bounds on the magnitude and duration of temperature change on land during this critical interval—the main setting for the demise of nonavian dinosaurs and the rise of mammals.

Abstract Methane-derived carbonates (MDCs) are common along modern and ancient continental margins, and the majority of such formations are associated with seafloor cold seeps. Here, we document petrographic, rare earth element + yttrium (REE + Y), carbonate clumped isotope temperature (TΔ47), and carbon-isotopic evidence from a shale succession in southern Tibet spanning a ~28 m.y. interval (ca. 113–85 Ma) that coincided with the mid-Cretaceous greenhouse event. At least nine limestone nodule-bearing horizons exhibit seep-associated sedimentary structures, including carbonate fabrics (e.g., micritic crusts, crystal fans, and botryoidal textures) and 13C-depleted isotopic compositions (δ13Ccarb &amp;lt; –32.3‰), which are indicative of methane-derived carbon sources. Along with sedimentary evidence, the patterns of TΔ47–δ13Ccarb–δ18Ocarb support precipitation of these MDCs over a large temperature range. The REE + Y compositions and europium (Eu) anomalies indicate that the release of methane was associated with hydrothermal fluids. Methane may have been derived from both thermogenic and biogenic sources based on the inorganic carbon isotopic signatures of the carbonate. These nodular carbonate horizons document multiple episodes of seafloor methane release during the mid-Cretaceous and represent exceptionally long-lived, active methane seepage. Massive methane release events may have played a role in generating the greenhouse climate of the mid-Cretaceous.

Abstract Terrestrial methane (CH4) emissions may have increased during the Paleocene-Eocene Thermal Maximum (PETM; ca. 56 Ma) and promoted additional warming, especially in the high latitudes. Although there is evidence for increased CH4 cycling in a single Northern Hemisphere site, whether enhanced methane cycling was globally widespread is unknown because there have been no subsequent investigations. The mechanism of CH4 release is also unknown because a direct comparison between temperature and CH4 cycling has so far not been possible. Here we use biomarkers to reconstruct temperature change and CH4 cycling in a new PETM-aged succession in New Zealand. Our results indicate that the stable carbon isotopic composition (δ13C) of bacterial hopanoids decreased to very low values (−60‰) during the onset of the PETM, indicating enhanced consumption of CH4. These values are much lower than found in modern wetlands and suggest a major perturbation of the CH4 cycle during the onset of the PETM. Low hopanoid δ13C values do not persist into the early Eocene, despite evidence for elevated temperatures. This indicates that the terrestrial CH4 cycle operates differently during transient compared to gradual warming events. Enhanced CH4 cycling during the PETM may help to resolve the temperature data-model mismatch in the high latitudes and could yield higher estimates of Earth system sensitivity than expected from CO2 alone.

Extremely 13 C‐depleted sn ‐2‐hydroxyarchaeol is widely observed at marine methane seep sites. It is the first biosynthetic product that provides information about a previously unknown microorganism involved in anaerobic methanotrophy, a process that occurs widely in marine sediments. The mono‐ and di‐ TMS derivatives of this compound yield sharply differing and distinctive mass spectra. Depending on reaction conditions, either of these products can be formed in the course of conventional analyses.

The Cretaceous/Palaeogene Boundary (KPgB) represents one of the five major mass extinctions in Earth's history, and determining the nature of associated environmental change and biotic recovery is critical for understanding the history of life on our planet. To explore that, we examined the distributions of selected biomarkers (n-alkanes, acyclic isoprenoids, steranes and hopanes), organic and carbonate δ13C values, total organic carbon contents and major and trace elements in a distal section spanning the KPgB (Agost, SE Spain). The studied Agost section is an ~32 cm-thick bathyal sequence of marlstones, clays and marly limestones, and high sedimentation rates allow high (cm) resolution analysis. These analyses exhibit sample-to-sample variability, but there are few significant differences between pre- and post-KPgB biomarker assemblages, suggesting a rapid recovery of the non-fossilizing phytoplankton community after the KPgB. Despite the persistence of life, the organic matter assemblage is rather variable through the first 10 kyr after the impact event. This interval is associated with changing terrigenous and petrogenic inputs as well as varying redox conditions as reflected by the enrichment factor of uranium (UEF) vs that of molybdenum (MoEF) as well as biomarker indices (gammacerane and homohopane indices). Moreover, sterane distributions do differ between pre- and post-KPgB sediments. Thus, the KPgB impact did affect environmental conditions and non-fossilizing algal and bacterial communities even in distal sites, but these organisms appear to have rapidly recovered, within 10 kyr after the KPgB.

Abstract Recent time‐series from sediment traps show abnormally high chlorophyll‐ a concentrations and primary productivity in the oligotrophic central South China Sea (SCS), especially during wintertime. Here we present new insights from compound‐specific hydrogen isotopic analysis of leaf wax n ‐alkanes and Sr‐Nd isotope compositions extracted from four basin‐wide surface sediment transects. We find that the deepest surface sediments in the central basin contain the most depleted n ‐alkane hydrogen isotopes, suggesting inputs from higher latitude soils in northern China. This is supported by the Sr‐Nd isotope compositions of the same surface sediments. We propose that aeolian dust is transported by the winter monsoon and might fertilize the phytoplankton bloom in the central SCS. This process may have been enhanced in ancient times when the winter monsoon was stronger, driving both vertical mixing and dust transport to the central basin.

Abstract. Accurate estimates of past global mean surface temperature (GMST) help to contextualise future climate change and are required to estimate the sensitivity of the climate system to CO2 forcing during the geological record. GMST estimates from the latest Paleocene and early Eocene (~ 57 to 48 million years ago) span a wide range (~ 9 to 23 °C higher than pre-industrial) and prevent an accurate assessment of climate sensitivity during this extreme greenhouse climate interval. Here, we develop a multi-method experimental framework to calculate GMST during three target intervals: 1) the latest Paleocene (~ 57 Ma), 2) the Paleocene-Eocene Thermal Maximum (56 Ma) and 3) the early Eocene Climatic Optimum (EECO; 49.4 to 53.3 Ma). Using six independent methodologies, we find that average GMST estimates during the latest Paleocene and PETM are 11.7 °C (±0.6 °C) and 18.7 °C (±0.8 °C) higher than pre-industrial, respectively. GMST estimates from the EECO are 13.3 °C (±0.5 °C) warmer than pre-industrial and comparable to previous IPCC AR5 estimates (12.7 °C higher than pre-industrial). Leveraging the extremely large "signal" associated with these extreme warm climates, we combine estimates of GMST and CO2 from the latest Paleocene, PETM and EECO to calculate a gross estimate of the average climate sensitivity between the early Paleogene and today. This yields gross climate sensitivity estimates for the latest Paleocene, PETM and EECO which range between 2.8 to 4.8 °C (66 % confidence). These largely fall within the range predicted by the IPCC (1.5 to 4.5 °C per doubling CO2), but appear incompatible with low values (between 1.5 and 2.8 °C per doubling CO2).

Abstract. ​​​​​​​Late Paleocene deposition of an organic-rich sedimentary facies on the continental shelf and slope of New Zealand and eastern Australia has been linked to short-lived climatic cooling and terrestrial denudation following sea level fall. Recent studies confirm that the organic matter in this facies, termed “Waipawa organofacies”, is primarily of terrestrial origin, with a minor marine component. It is also unusually enriched in 13C. In this study we address the cause of this enrichment. For Waipawa organofacies and its bounding facies in the Taylor White section, Hawke's Bay, paired palynofacies and carbon isotope analysis of heavy liquid-separated density fractions indicate that the heaviest δ13C values are associated with degraded phytoclasts (woody plant matter) and that the 13C enrichment may be partly due to lignin degradation. Compound-specific stable carbon isotope analyses of samples from the Taylor White and mid-Waipara (Canterbury) sections display similar trends and further reveal a residual 13C enrichment of ∼ 2.5 ‰ in higher plant biomarkers (long chain n-alkanes and fatty acids) and a ∼ 2 ‰–5 ‰ change in subordinate marine biomarkers. Using the relationship between atmospheric CO2 and C3 plant tissue δ13C values, we determine that the 3 ‰ increase in terrestrial δ13C may represent a ∼ 35 % decrease in atmospheric CO2. Refined age control for Waipawa organofacies indicates that deposition occurred between 59.2 and 58.5 Ma, which coincides with an interval of carbonate dissolution in the deep sea that is associated with a Paleocene oxygen isotope maximum (POIM, 59.7–58.1 Ma) and the onset of the Paleocene carbon isotope maximum (PCIM, 59.3–57.4 Ma). This association suggests that Waipawa deposition occurred during a time of cool climatic conditions and increased carbon burial. This relationship is further supported by published TEX86-based sea surface temperatures that indicate a pronounced regional cooling during deposition. We suggest that reduced greenhouse gas emissions from volcanism and accelerated carbon burial, due to tectonic factors, resulted in short-lived global cooling, growth of ephemeral ice sheets and a global fall in sea level. Accompanying erosion and carbonate dissolution in deep-sea sediment archives may have hidden the evidence of this “hypothermal” event until now.

• IsoGDGTs in the Lake Fuxian are primarily produced by Group I.1a Thaumarchaeota. • DO concentrations strongly affect the TEX 86 signal in the water column. • The warm-biased TEX 86 signal can be identified by % isoGDGT-2 with a threshold > 45%. The TEX 86 (tetraether index of 86 carbon atoms) temperature proxy has been widely applied to marine settings. The application of TEX 86 in lakes, however, is more complicated, as many other factors, such as dissolved oxygen (DO), also appear to influence the lacustrine TEX 86 signal. In order to better understand how DO content affects TEX 86 , we analyzed an annual cycle of isoGDGT distributions and abundances in the water column (0–100 m) of Lake Fuxian, in southwest China. It appears that isoGDGTs in suspended particulate matter (SPM) of the water column are mainly produced by aquatic Group I.1a Thaumarchaeota. The peak concentration of total isoGDGTs occurs mainly in June and December, which is probably driven by release of nutrients during phytoplankton blooms with a lag of about one month. About 95% of isoGDGTs occurred in the bottom water layer (hypolimnion) of this thermally stratified lake, indicating that Thaumarchaeota prefer to live in suboxic water conditions in Lake Fuxian. IsoGDGT-2 is the most sensitive component to monthly variations in DO contents. This relationship leads to a similar negative correlation between TEX 86 and DO. When TEX 86 values, especially at the hypolimnion, are lower than anticipated, this is caused by %isoGDGT-2 falling below 45%. Conversely, a warm bias of TEX 86 arising from a %isoGDGT-2 greater than 45% appears to be associated with lower DO contents. Therefore, this threshold of %isoGDGT-2 could be used in future paleoclimate reconstructions (of Lake Fuxian, and possibly similar lakes), especially since Thaumarchaeota typically reside at suboxic water layers.

Abstract. The complex tectonic evolution in the Tibetan region has impacted climate, the Asian monsoon system, and the development of major biodiversity hotspots, especially since the onset of the India-Eurasia continental collision during the early Paleogene. Untangling the links between the geologic, climatic, and ecological history of the broader region can provide insights into these Earth system mechanisms, relevant for the future of our rapidly changing planet. To better understand environmental conditions across this critical time and place, we reconstruct the climatic and environmental history from a key sedimentary repository within the Lühe Basin, Yunnan, China, uniquely located between high elevation Tibet and low elevation coastal China. We investigate a 340-m long section using a multi-proxy organic geochemistry approach, complemented with sedimentological interpretations and climate model simulations. The complementary organic geochemical proxies, including n-alkanes, terpenoids, and hopanes, suggest that these thermally immature sediments were deposited in a dynamic environment that fluctuated between low energy floodplains and high energy fluvial systems. Our branched glycerol diakyl glycerol tetraether-based proxies indicate terrestrial temperatures of around 17 °C ± 3 SD and our model-based temperatures indicate terrestrial temperatures for Chattian of around 19 °C, consistent with the literature palaeobotany-based temperatures from the nearby Lühe town section. These combined palaeotemperatures match present-day values, suggesting that this area has not undergone significant temperature change since the late Paleogene.

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The Cretaceous-Paleogene (K-Pg) boundary represents the most recent mass extinction in Earth history. Although it is widely accepted that a bolide impact in North America caused the extinction, many questions persist about how this impact affected Earth's climate and environment. Here we explore changes in organic matter inputs across the K-Pg in an impact-distal section in New Zealand (mid-Waipara River section, North Canterbury, New Zealand). The section is 21.2 m thick, including 1.2 m of uppermost Cretaceous and 20 m of lower Paleocene sediment, the latter spanning ∼1 Myr, albeit interrupted by at least one major unconformity. We examine the abundance and distribution of n-alkanes and n-alkanoic acids (mixed sources), acyclic isoprenoids (pristane and phytane), steranes (eukaryotes), hopanes and hopanoic acids (bacteria) and GDGTs (glycerol dialkyl glycerol tetraether lipids) to explore changes in the relative contribution of marine vs terrestrial sources. Minimal differences in the biomarker assemblages between sediments below and above the K-Pg boundary suggest a rapid recovery of the non-fossilizing phytoplankton community, similar to that observed in other distal settings. However, the organic matter source became variable immediately after the impact event. Variations in the concentrations and distribution of the high-molecular-weight n-alkanes and n-alkanoic acids, terrestrial-aquatic ratios, and GDGT branched to isoprenoidal tetraether indices indicate an increase in the absolute and proportional abundance of terrestrial soil- and plant-derived organic matter just after the impact event. This co-occurs with an apparently transient decline in marine productivity, indicated by a decrease in concentrations of low-molecular-weight n-alkanoic acids, concentrations of pristane + phytane (of putative algal chlorophyll origin), and sterane/hopane ratios. This suggests that the bolide did affect the mid-Waipara biomarker-producing bacterial and algal assemblages, distinct from what has been observed at other distal sites. However, it remains unclear if this was a direct consequence of the impact or the wider post-impact reorganisation of environment and oceanography as has been previously observed in this region.

Abstract The Toarcian Oceanic Anoxic Event (T-OAE, ca. 183 Ma) was one of the most intense perturbations of Earth’s System of the last 250 million year. It was associated with the large-scale emission of 12 C-enriched carbon, global warming, and increased organic carbon burial. Although the T-OAE and its impact on climate and biogeochemical cycles are well-documented for the marine realm, the impact on continental biogeochemical cycles that could provide powerful positive feedback mechanisms that exacerbate the initial perturbation is poorly understood. Here we show that the stable carbon isotopic compositions (δ 13 C) of bacterial lipids (hopanoids) became very depleted during the T-OAE in the Ordos Basin, a large inland lake in northern China. We interpret these data as reflecting a transient increase in aerobic methane consumption in the lake due to increased methane production in the lake sediments. The surplus depletion in hopanoid 13 C coincides with the appearance of biomarker evidence for photic zone euxinia, indicating a link between lakes’ water column stratification / deoxygenation and intensification of the methane cycle. Our results indicate that the T-OAE had a direct impact on terrestrial biogeochemical cycles that could have provided a positive feedback, enhancing and prolonging its duration.

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Abstract. Oceanic Anoxic Event 2 (OAE 2) (∼ 93.5 Ma) is characterized by widespread marine anoxia and elevated burial rates of organic matter. However, the factors that led to this widespread marine deoxygenation and the possible link with climatic change remain debated. Here, we report long-term biomarker records of water-column anoxia, water-column and photic zone euxinia (PZE), and sea surface temperature (SST) from Demerara Rise in the equatorial Atlantic that span 3.8 Myr of the late Cenomanian to Turonian, including OAE 2. We find that total organic carbon (TOC) content is high but variable (0.41 wt %–17 wt %) across the Cenomanian and increases with time. This long-term TOC increase coincides with a TEX86-derived SST increase from ∼ 35 to 40 ∘C as well as the episodic occurrence of 28,30-dinorhopane (DNH) and lycopane, indicating warming and expansion of the oxygen minimum zone (OMZ) predating OAE 2. Water-column euxinia persisted through much of the late Cenomanian, as indicated by the presence of C35 hopanoid thiophene but only reached the photic zone during OAE 2, as indicated by the presence of isorenieratane. Using these biomarker records, we suggest that water-column anoxia and euxinia in the equatorial Atlantic preceded OAE 2 and this deoxygenation was driven by global warming.

Abstract. Oceanic Anoxic Event (OAE) 2 (~93.5 millions of years ago) is characterized by widespread marine anoxia and elevated burial rates of organic matter. However, the factors that led to this widespread marine deoxygenation and the possible link with climatic change remain debated. Here, we report long-term biomarker records of water column anoxia, water column and photic zone euxinia (PZE), and sea surface temperature (SST) from Demerara Rise in the equatorial Atlantic that span 3.8 million years of the late Cenomanian to Turonian, including OAE 2. We find that total organic carbon (TOC) contents are high but variable (0.41–17 wt. %) across the Cenomanian and increase with time. This long-term TOC increase coincides with a TEX86-derived SST increase from ~ 35 to 40 °C as well as the episodic occurrence of 28,30-dinorhopane (DNH) and lycopane, indicating warming and expansion of the oxygen minimum zone (OMZ) predating OAE 2. Water column euxinia persisted through much of the late Cenomanian, as indicated by the presence of C35 hopanoid thiophene, but only reached the photic zone during OAE 2, as indicated by the presence of isorenieratane. Using these biomarker records, we suggest that water column anoxia and euxinia in the equatorial Atlantic preceded OAE 2 and this deoxygenation was driven by global warming.

Because the chemical composition of organic matter (OM) varies among different organisms, the biological source of OM can exert an important control on its preservation and influence the formation of petroleum source rocks. Molecular compounds derived from specific organisms, biomarkers, are a useful tool in the evaluation of the biological sources of organic matter to ancient sediments. Previous work has shown that organic matter in the Kimmeridge Clay Formation derives from a range of organisms, represented in the biomarker record by isotopically distinct n-alkanes, branched alkanes, and isoprenoids. Here we report the abundances of these compounds and other select biomarkers in the extractable-organic-matter fraction and the distribution of n-alkanes and isoprenoids in the kerogen; these data are used to determine the stratigraphic variation in the occurrence of different organic-matter assemblages through the lower hudlestoni biozone in a sediment core from Dorset, southern England. The TOC-normalized abundances of n-alkanes and specific methyl and isopropyl branched alkanes in the extract are relatively constant through the studied interval, suggesting that their contribution to sedimentary organic matter did not vary. In contrast, the occurrence of isoprenoid-rich kerogen and bitumen is largely restricted to discrete horizons, three of which were clearly identified in this study. Because changes in the abundance of extractable isoprenoids correlate with changes in the relative abundances of kerogen- bound isoprenoids, the former does reflect a change in the predominant source of organic matter during these intervals. However, the proportional abundances of isoprenoids in both the bitumen and kerogen correlate poorly with both TOC contents and TOC contents calculated on a carbonate-free basis, indicating that such variation in organic-matter source did not influence organic-matter preservation. Instead, a strong correlation between TOC contents and the relative abundance of unresolved complex mixtures released during pyrolysis and inferred to consist of S-bound organic matter suggests that the availability of sulfide was the main control on OM accumulation regardless of the predominant sources of organic matter.

Abstract. Recent studies, utilising a range of proxies, indicate that a significant perturbation to global hydrology occurred at the Paleocene–Eocene Thermal Maximum (PETM; ~56 Ma). An enhanced hydrological cycle for the warm early Eocene is also suggested to have played a key role in maintaining high-latitude warmth during this interval. However, comparisons of proxy data to General Circulation Model (GCM) simulated hydrology are limited and inter-model variability remains poorly characterised, despite significant differences in simulated surface temperatures. In this work, we undertake an intercomparison of GCM-derived precipitation and P-E distributions within the EoMIP ensemble (Lunt et al., 2012), which includes previously-published early Eocene simulations performed using five GCMs differing in boundary conditions, model structure and precipitation relevant parameterisation schemes. We show that an intensified hydrological cycle, manifested in enhanced global precipitation and evaporation rates, is simulated for all Eocene simulations relative to preindustrial. This is primarily due to elevated atmospheric paleo-CO2, although the effects of differences in paleogeography/ice sheets are also of importance in some models. For a given CO2 level, globally-averaged precipitation rates vary widely between models, largely arising from different simulated surface air temperatures. Models with a similar global sensitivity of precipitation rate to temperature (dP/dT) display different regional precipitation responses for a given temperature change. Regions that are particularly sensitive to model choice include the South Pacific, tropical Africa and the Peri-Tethys, which may represent targets for future proxy acquisition. A comparison of early and middle Eocene leaf-fossil-derived precipitation estimates with the GCM output illustrates that a number of GCMs underestimate precipitation rates at high latitudes. Models which warm these regions, either via elevated CO2 or by varying poorly constrained model parameter values, are most successful in simulating a match with geologic data. Further data from low-latitude regions and better constraints on early Eocene CO2 are now required to discriminate between these model simulations given the large error bars on paleoprecipitation estimates. Given the clear differences apparent between simulated precipitation distributions within the ensemble, our results suggest that paleohydrological data offer an independent means by which to evaluate model skill for warm climates.

Abstract. The response of the Earth System to greenhouse-gas driven warming is of critical importance for the future trajectory of our planetary environment. Hypethermal events – past climate transients with significant global-scale warming – can provide insights into the nature and magnitude of these responses. The largest hyperthermal of the Cenozoic was the Palaeocene-Eocene Thermal Maximum (PETM ~ 56 Ma). Here we present a new high-resolution cyclostratigraphy for the classic PETM section at Zumaia, Spain. With this new age model we are able to demonstrate that detrital sediment accumulation rates within this continental margin section increased more than four-fold during the PETM, representing a radical change in regional hydrology that drove dramatic increases in terrestrial to marine sediment flux. During the body of the PETM, orbital-scale variations in bulk sediment Si/Fe ratios are evidence for the continued orbital pacing of sediment erosion and transport processes, most likely linked to precession controls on sub-tropical hydroclimates. Most remarkable is that detrital accumulation rates remain high throughout the body of the PETM, and even reach peak values during the recovery phase of the characteristic PETM carbon isotope excursion (CIE). Using a series of Earth System Model inversions, we demonstrate that the silicate weathering feedback alone is insufficient to recover the PETM CIE, and that active organic carbon burial is required to match the observed dynamics of the CIE. Further, that the period of maximum organic carbon sequestration coincides with the peak in detrital accumulation rates observed at Zumaia. Based on these results, we hypothesize that precession controls on tropical and sub-tropical hydroclimates, and the sediment dynamics associated with this variation, play a significant role in the timing of the rapid climate and CIE recovery from peak-PETM conditions.

Abstract One of the key differences between Bacteria and Archaea are their canonical membrane phospholipids, which are synthesized by distinct biosynthetic pathways with non-homologous enzymes. This “lipid divide” has important implications for the early evolution of cells and the type of membrane phospholipids present in the last universal common ancestor (LUCA). One of the main challenges in studies of membrane evolution is that the key biosynthetic genes are ancient and their evolutionary histories are poorly resolved. This poses major challenges for traditional rooting methods because the only available outgroups are distantly related. Here, we address this issue by using the best available substitution models for single gene trees, by expanding our analyses to the diversity of uncultivated prokaryotes recently revealed by environmental genomics, and by using two complementary approaches to rooting that do not depend on outgroups. Consistent with some previous analyses, our rooted gene trees support extensive inter-domain horizontal transfer of membrane phospholipid biosynthetic genes, primarily from Archaea to Bacteria. They also suggest that the capacity to make archaeal-type membrane phospholipids was already present in LUCA.

The UK government recently committed to achieving net-zero carbon emissions by 2050. We asked a selection of UK-based experts to reflect upon this commitment, the challenges ahead, and the actions required to make it a reality.

Isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs) are archaeal biomarkers. In many settings, the degree of cyclization of isoGDGTs is correlated with temperature, forming the basis of the TEX86 paleothermometer that is widely used to reconstruct sea surface temperature (SST) across a range of time scales. However, the application of TEX86 to the polar regions is relatively limited and there is currently no consensus on which calibration is best suited for polar environments. In addition, application of TEX86 to the polar regions is complicated by uncertainty regarding the source of organic matter input in coastal polar environments. We tested five different calibrations for TEX86 in marine sediments from the Antarctic coastal region of Admiralty Bay near King George Island, using four short cores that span the second half of the 20th century. We also explored the possible sources of organic matter in these cores using sterol biomarkers. Best results for TEX86 were obtained using a quadratic calibration. The TEX86 signal presented a strong seasonal signal and best matched reanalysis temperatures of the austral spring season (Oct-Nov-Dec). The most abundant compounds observed in the sediments were the sterols cholest-5-en-3β-ol and 24-ethylcholest-5-en-3β-ol, the fatty alcohols C16 and phytol, and isoGDGT-0, indicating a dominant marine origin of the organic matter. Differences in their vertical distributions suggests that some compounds (such as cholest-5-en-3β-ol and phytol) may have had different sources over the evaluated period. Together our results indicate that TEX86 can be used to reconstruct SSTs in the Antarctic coastal region.

Abstract. Late Paleocene deposition of an organic-rich sedimentary facies on the continental shelf and slope of New Zealand and eastern Australia has been linked to short-lived climatic cooling and terrestrial denudation following sea-level fall. Recent studies have confirmed that the organic matter in this facies, termed Waipawa organofacies, is primarily of terrestrial origin, with a minor marine component. It is also unusually enriched in δ13C. In this study we aim to determine the cause or causes of this enrichment. For Waipawa organofacies and its bounding facies in the Taylor White section, Hawkes Bay, paired palynofacies and δ13C analysis of density fractions indicate that the heaviest δ13C values are associated with degraded phytoclasts (woody plant matter) and that the 13C enrichment is partly due to lignin degradation. Compound specific δ13C analyses of samples from the Taylor White and mid-Waipara (Canterbury) sections confirms this relationship but also reveal a residual 13C enrichment of ~ 2.5 ‰ in higher plant biomarkers (n-alkanes and n-alkanoic acids) and 3–4 ‰ in the subordinate marine component, which we interpret as indicating a significant drawdown of atmospheric CO2. Refined age control for Waipawa organofacies indicates that deposition occurred between 59.2 and 58.4 Ma, which coincides with a Paleocene oxygen isotope maximum (POIM) and the onset of the Paleocene carbon isotope maximum (PCIM). This timing suggests that this depositional event was related to global cooling and carbon burial. This relationship is further supported by published TEX86-based sea surface temperatures that indicate a pronounced regional cooling during deposition. We suggest that reduced greenhouse gas emissions from volcanism and accelerated carbon burial related to several tectonic factors and positive feedbacks resulted in short-lived global cooling, growth of ephemeral ice sheets, and a global fall in sea level. Accompanying erosion and carbonate dissolution in deep sea sediment archives may have hidden the evidence of this "hypothermal" event until now.

Atmospheric CO 2 is a critical component of the Earth’s exogenic carbon system, and is seen by many as the major “knob” controlling Earth’s climate past, present and future. Accurate and precise reconstructions of its concentration through geological time are therefore seen as one of the holy grails of the Earth Sciences. This has never been more important than now, as Earth System modellers, policy makers and the wider public increasingly rely on palaeo-CO 2 reconstructions to ground-truth and inform climate models capable of predicting future conditions. Two long established methods of reconstructing atmospheric CO 2 , one based on the boron isotopic composition of planktic foraminifera, the second from the carbon isotopic fractionation between DIC and the alkenones produced by some haptophytes, have received increasing attention in recent years. Both may be generated from ocean sediments, allowing, in theory, high temporal resolution datasets to be generated at a global scale from the repositories of the (I)ODP. However as more records become available discrepancies between the two are beginning to appear, with often reduced amplitudes of variability in alkenones vs. boron based reconstructions Here we present new and existing records from ODP Site 999 from identical samples from the Plio-Pleistocene. These data may help to unravel the emerging differences, and go some way to improving our understanding of how to correctly reconstruct this critical greenhouse gas.

Although methane is a critical greenhouse gas (GHG), there are no proxy methods for reconstructing its ancient atmospheric concentration. This is especially important because biogenic methane emissions are controlled by environmental conditions, such as temperature and precipitation, such that methane could be a significant positive or negative feedback on global climate. Understanding how methane emissions and cycling acted in the high pCO2 greenhouse worlds of the Paleogene potentially bridges the gap between our understanding of other, better (although arguably still poorly-) constrained GHGs and global temperature. We apply an advanced three dimensional global modelling strategy to the problem of Eocene trace GHG concentrations and show how important these may be in high-CO2 worlds, with as much as 2.7 oC of global warming contributed by increased trace GHGs. We compare the model results to an indirect proxy for Paleogene methane cycling afforded by the distributions and carbon isotopic compositions of hopanoid lipid biomarkers with 13C-depleted isotopic compositions indicative of enhanced methane cycling. Examination of literature-derived and new hopanoid carbon isotopic analyses supports the spatial relationships between temperature, precipitation and methane cycling observed in the biogeochemical model. Together, the biogeochemical model and organic proxy data apply new constraints on ancient methane emissions in high-CO2 worlds.

Greenhouse gases (GHGs) are one of the fundamental controls of Earth’s climate past, present and future. Significant research effort has been expended attempting to produce accurate and precise reconstructions of GHG concentrations through the geologic past. As Earth System modellers, policy makers and the wider public increasingly rely on paleo-CO 2 reconstructions to ground-truth and inform climate models capable of predicting future conditions, reliable estimates have never been more important. Recently doubts have begun to emerge about the accuracy and precision of the key biomarker-based tool for CO 2 reconstruction - based on measuring compound specific alkenone δ 13 C values - in part due to discrepancies between values calculated using this proxy and records generated utilising the boron isotopic composition of planktic foraminifera. Meanwhile, increasing efforts are being expended to investigate other potentially important GHGs such as CH 4 during critical intervals in the Cenozoic. Here we present new, coupled alkenone δ 13 C – boron δ 11 B CO 2 records across a full glacial-interglacial cycle from identical samples at ODP Site 999 in the Caribbean Sea. This allows comparison between the two palaeo-CO 2 proxies currently most frequently used, with more direct measurements of atmospheric GHGs from ice cores. Our results suggest that current alkenone-based CO 2 estimates may require reassessment – especially at lower levels of atmospheric pCO 2 . We discuss the implications of this work for our understanding of atmospheric pCO 2 evolution through the Cenozoic in the context of new CO 2 records from the Eocene. We also introduce new efforts to ground-truth numerical model-derived changes in CH 4 with lipid biomarker evidence for methane processing from globally-distributed terrestrial Eocene deposits, and discuss the impact and importance of CH 4 on global climate.

The early Eocene ( 55-48 Ma) was punctuated by multiple hyperthermal events in which atmospheric CO2 and global temperatures rose with unprecedented and as of yet unexplained rapidity. Relative temporal spacing of three such events ‐ the Paleocene-Eocene Thermal Maximum (PETM), Eocene Thermal Maximum 2 (ETM2, aka Elmo) and Eocene Thermal Maximum 3, (ETM3, aka “X” event) ‐ have led to the hypothesis that Eocene hyperthermals are paced by million-year-scale orbital cyclicity. Here we present evidence of a fourth early Eocene carbon cycle perturbation, identified through high resolution carbon isotopic analysis of Green River Formation lacustrine strata. A new age model for the Green River Formation based on frequency analysis supports the hypothesis that Eocene carbon isotope excursions are paced by the 1.2 Myr cycle that modulates eccentricity. We propose that this cycle is a significant forcing mechanism for hyperthermals not only because it modulates eccentricity, but because it also dictates the placement of nodes in obliquity. Each Eocene hyperthermal occurs shortly (< 400kyr) after a node in obliquity, suggesting that reorganization of high-latitude carbon pools when moving from low-seasonality to high-seasonality regimes (ie, out of an obliquity node) control the timing of extreme climate events.

ABSTRACTSaline alkaline sediments and soils are widespread in arid and semiarid regions, but their occurrence in ancient dry periods remains unknown due to the lack of a suitable proxy. On the basis of investigations of modern Chinese soils with a wide pH range of 3.5–9.1, we sug-gest that the microbial lipid ratio R i/b , i.e., the abundance ratio of archaeal isoprenoid GDGTs (glycerol dialkyl glycerol tetraethers) to bacterial branched GDGTs, indicates the presence of drought-induced alkaline deposits in terrestrial settings. The R i/b is invariant in modern soils with pH 600 mm, but it increases sharply at higher pH values and lower mean annual precipitation (<600 mm). In contrast, the CBT index (the cyclization ratio of branched GDGTs), which has been proposed to refl ect envi-ronmental pH in other contexts, appears to be relatively stable in the highly alkaline Chinese soils from semiarid and arid regions investigated. We further explore the R

espanolEl Evento Anoxico Oceanico del Aptiense inferior fue un intervalo de tiempo caracterizado por el aumento del enterramiento de carbono organico en los sedimentos marinos, notables cambios sedimentarios y bioticos, y bruscas excursiones en los isotopos del carbono indicativo de cambios paleoambientales importantes. Los estudios bio- y quimioestratigraficos realizados en la seccion de La Frontera (sur de Jaen) han permitido reconocer y caracterizar la expresion local del OAE1a en el Subbetico. La sucesion esta constituida por black shales (Fm. Carbonero) sobre margas y margocalizas (Fm. Carretero). Un hiato de edad Barremiense terminal-Aptiense basal separa ambas unidades litoestratigraficas. En el intervalo de black shales de la Fm. Carbonero se ha registrado una brusca excursion negativa en δ13Corg seguido por una pronunciada excursion positiva. El inicio de la excursion positiva (segmento C4), que representa el comienzo del OAE1a, esta situado en la parte superior de la zona de Hayesites irregularis de nanofosiles y proxima al final de la zona de Globigerinelloies blowi de foraminiferos planctonicos. El deposito de black shales con anterioridad al OAE1a y la asociacion de biomarcadores confirma la importancia de los factores paleogeograficos locales en el desarrollo de condiciones anoxicas antes y durante el OAE1a. EnglishThe early Aptian Oceanic Anoxic Event was a time interval characterized by increased organic carbon accumulation in marine sediments, notable sedimentary and biotic changes, and abrupt carbonisotope excursions indicative of significant major palaeoenvironmental changes. Detailed bio- and chemostratigraphic studies (TOC, δ13C, biomarkers) carried out in La Frontera Section (south of Jaen) allowed us to recognize and characterize the local expression of the OAE1a in the Subbetic of Jaen. The succession consists of black shales (Carbonero Fm.) overlying marls and marly limestones (Carretero Fm.). A hiatus, uppermost Barremian-earliest Aptian, separates both lithostratigraphic units. An abrupt negative δ13Corg excursion followed by a pronounced positive excursion is recorded in the black shales of the Carbonero Fm. The beginning of this positive excursion (C4 segment), representing the onset of the OAE1a, is located in the upper part of Hayesites irregularis nannofossil zone and near to the top of the Globigerinelloides blowi planktonic foraminifera zone. The deposit of black shales previously to OAE 1a and the biomarker association confirm the importance of the local factors in the development of anoxic conditions before and during the OAE1a at this site.

Carbon and Chlorine Stable Isotope Fractionation during Anaerobic Degradation of α-Hexachlorocyclohexane by a Mixed Culture Enriched from a Contaminated Site

During the Pleistocene, large continental ice sheets episodically apperead in the Northern Hemisphere, covering large parts of Europe and North America. Besides the results based on benthic oxygen isotope records, which predominantly represent variations in global ice volume, little is know about the timing of and astronomical control on the advances and retreats of the continental ice sheets in the Northern Hemisphere during the late Pliocene and early Pleistocene (3-2 million years (Ma) ago). Here we therefore present the first orbitally-resolved records of terrestrial higher plant leaf wax input to the North Atlantic covering the last 3.5 Ma, based on the accumulation of long-chain nalkanes and n-alkanl-1-ols at IODP Site U1313 [1]. These lipids are a major component of dust, even in remote ocean areas, and have a predominantly aeolian origin in distal marine sediments. Our results demonstrate that around 2.7 Ma, coinciding with the intensification of the Northern Hemisphere glaciation (NHG), the aeolian input of terrestrial material to the North Atlantic increased drastically. Since then, during every glacial the aeolian input of higher plant material was up to 30 times higher than during interglacials. We argue that the increased aeolian input at Site U1313 during glacials is predominantly related to the episodic appearance of continental ice sheets in North America and the associated strengthening of glaciogenic dust sources. The records thus reflect the timing of the advances and retreats of the North American ice sheets. Evolutional spectral analyses of the n-alkane records were therefore used to determine the dominant astronomical forcing in North American ice sheet advances over the last 3.5 Ma. These results demonstrate that during the early Pleistocene North American ice sheet dynamics responded predominantly to variations in obliquity (41 ka), which argues against previous suggestions of precession-related variations in Northern Hemisphere ice sheets during the early Pleistocene [2].

The Mid-Pliocene is the most recent time in Earth's history when mean global temperatures were substantially warmer and sea levels higher than they are today. The subsequent intensification of the Northern Hemisphere Glaciation (iNHG) represents the key final step in the climatic transition from the warm Pliocene to the current “icehouse” climate. Recent modeling and proxy based results suggest that this climate shift was forced by a reduction in atmospheric CO2 concentrations (pCO2), which highlights the relationship between climate and this important greenhouse gas. Hence, there is significant potential in the use of the Pliocene as an analogue for future global warming in modeling studies and as a key period to study the role of CO2 in driving major climatic shifts. Despite recent advances in the reconstruction of pCO2 change during this important period, detail at the orbital scale is currently lacking. Boron isotopes in planktic foraminifera are a proven proxy for surface oceanic pH, which has been shown to provide valuable insights into past changes in the ocean carbonate system and ultimately into past atmospheric pCO2. Here we will provide foraminiferal δ11B-based records to determine the temporal evolution of pCO2 for an interval spanning the Pliocene Warm Period and the iNGH at orbital scale temporal resolution. Our record provides valuable insights into the causes and consequences of the changes in the atmospheric concentration of this important greenhouse gas.

(1) University Of Bristol, School of Geographical Sciences, Bristol BS8 1SS, United Kingdom (David.Strong@bristol.ac.uk), (2) Durham University, Department of Geography, Durham DH1 3LE, United Kingdom, (3) University of Bristol, School of Chemistry, Bristol BS8 1TS, United Kingdom, (4) British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, United Kingdom, (5) NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, United Kingdom

The Pliocene period is the most recent time when the Earth was globally significantly (~3°C) warmer than today. However, the existing pCO2 data for the Pliocene are sparse and there is little agreement between the various techniques used to reconstruct palaeo-pCO2. Moreover, the temporal resolution of the published records does not allow a robust assessment of the role of declining pCO2 in the intensification of the Northern Hemisphere Glaciation (INHG) and a direct comparison to other proxy records are lacking. For the first time, we use a combination of foraminiferal (delta11B) and organic biomarker (alkenone-derived carbon isotopes) proxies to determine the concentration of atmospheric CO2 over the past 5 Ma. Both proxy records show that during the warm Pliocene pCO2 was between 330 and 400 ppm, i.e. similar to today. The decrease to values similar to pre-industrial times (275-285 ppm) occurred between 3.2 Ma and 2.8 Ma - coincident with the INHG and affirming the link between global climate, the cryosphere and pCO2.

The reconstruction of ancient atmospheric carbon dioxide concentrations is essential to understanding the history of the Earth and life. It is also an important guide to identifying the sensitivity of the Earth system to this greenhouse gas and, therefore, constraining its future impact on climate. However, determining the concentration of CO2 in ancient atmospheres is a challenging endeavour requiring the application of state-of-the-art analytical chemistry to geological materials, underpinned by an understanding of photosynthesis and biochemistry. It is truly an interdisciplinary challenge.

Nearly 250,000 sites with past and present potentially polluting activities need urgent remediation within Europe. Major pollutants include organochlorines (OCls), e.g. chlorinated ethenes (CEs) and hexachlorocyclohexanes (HCHs), mainly used as industrial solvents and pesticides, respectively. Due to improper handling and disposal, OCls contaminants are present in the soil or groundwater surrounding sites, where they have been produced or used. CEs and HCHs can undergo degradation by microorganisms indigenous to the soil or groundwater. Therefore natural attenuation (NA), relying on the in situ biodegradation of pollutants, is considered as a cost effective remediation strategy, yet it requires accurate monitoring methods. Compound specific isotope analysis (CSIA) is a powerful tool to provide information on the extent of degradation and, when combining two isotope systems (2D-CSIA), such as carbon (δ13C) and chlorine (δ37Cl), on reaction mechanisms.The diagnostic reaction-specific isotope enrichment factors (eC and eCl) were determined in laboratory experiments for the anaerobic degradation of PCE, TCE (Paper II) and α-HCH (Paper III) by mixed bacterial cultures enriched from CEs and HCHs contaminated sites, respectively. The related mechanism-specific eCl/eC ratios were calculated as 0.35 ± 0.11 (PCE), 0.37 ± 0.11 (TCE) and 0.52 ± 0.23 (α-HCH). These values are smaller than previously reported values for pure cultures. This is explained by the microbial community composition changes observed during degradation of PCE and α-HCH, which also reflect the variability of the microbial community at the field level. Furthermore, eCl/eC ratio might be bacteria specific.These values allowed the estimation of the extent of contaminant degradation at the respective study sites (Paper III and IV). Application of both isotope systems (δ13C and δ37Cl) led to comparable estimates. However the choice of representative e values is crucial for an accurate assessment.These studies show that CSIA is useful to quantify in situ degradation of OCls contaminants and identify reaction pathways, by combining δ13C and δ37Cl.

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