1,721,045 research outputs found
The temperature of the deep ocean is a robust proxy for global mean surface temperature during the Cenozoic
No full textReconstructing global mean surface temperature (GMST) is one of the key contributions that paleoclimate science can make in addressing societally relevant questions and is required to determine equilibrium climate sensitivity (ECS). GMST has been derived from the temperature of the deep ocean (Td), with previous work suggesting a simple Td-GMST scaling factor of 1 prior to the Pliocene. However, this factor lacks a robust mechanistic basis, and indeed, is intuitively difficult to envisage given that polar amplification is a ubiquitous feature of past warm climate states and deep water overwhelmingly forms at high latitudes. Here, we interrogate whether and crucially, why, this relationship exists using a suite of curated data compilations and two sets of paleoclimate model simulations. We show that models and data are in full agreement that a 1:1 relationship is a good approximation. Taken together, the two sets of climate models suggest that (a) a lower sensitivity of SST in the season of deep water formation than high latitude mean annual SST in response to climate forcing, and moreover (b) a greater degree of land versus ocean surface warming are the two processes that act to counterbalance a possible polar amplification-derived bias on Td-derived GMST. Using this knowledge, we provide a new Cenozoic record of GMST. Our estimates are substantially warmer than similar previous efforts for much of the Paleogene and are thus consistent with a substantially higher-than-modern ECS during deep-time high CO2 climate states
A long-term, high-latitude record of Eocene hydrological change in the Greenland region
Full text linkA range of proxy approaches have been used to reconstruct short-term changes to Earth’s hydrological cycle during the early Eocene hyperthermals. However, little is known about the response of Earth’s hydrological and biogeochemical systems to long-term Cenozoic cooling, which began following the Early Eocene Climatic Optimum (53.3 – 49.4 million years ago; Ma). Here, we use the molecular distribution and isotopic composition of terrestrial biomarkers preserved in marine sediments of ODP Site 913, East Greenland, to develop a long-term record of high-latitude hydrological change between 50 and 34 Ma. There is a marked decline in the concentration of conifer-derived diterpenoids and angiosperm-derived triterpenoids during the Eocene. As the input of wind-blown conifer pollen remains stable during this interval, this implies that decreasing di- and triterpenoid concentrations reflect declining influence of fluvial inputs – and perhaps terrestrial runoff – throughout the Eocene. Branched GDGTs and bacterial-derived hopanes indicate an increased input of soil- and kerogen-derived organic matter, respectively, after 38 Ma. This coincides with evidence for ice rafted debris and suggests input of organic matter via glacial processes. This also implies some continental glaciation occurred on East Greenland in the middle-to-late Eocene. Leaf wax hydrogen isotopes extending throughout this section – the first such long-term record from the Paleogene - indicate that precipitation δ2H was persistently higher than that of modern coastal Greenland, consistent with warmer ocean source waters and enhanced poleward moisture transport. Non-intuitively, however, this effect appears to have been smallest during the warmest part of the record, and higher δ2H values occur in the middle Eocene. Although interpretation of these hydrogen isotope trends is unclear, they clearly indicate – alongside the changes in biomarker abundances – a perturbed hydrological cycle through the Eocene in coastal Greenland. More long-term records are required to ascertain if this represents regional or global hydrological reorganisation
Polyfunctionalised bio- and geohopanoids in the Eocene Cobham Lignite
Full text linkWe investigated the bacteriohopanepolyol (BHP) distribution in the Cobham Lignite sequence (SE England) deposited across the Palaeocene–Eocene boundary, including part of the Palaeocene–Eocene Thermal Maximum (PETM) as shown previously by a negative carbon isotope excursion (CIE). A variety of BHPs were identified, including the commonly occurring and non-source specific biohopanoid, bacteriohopanetetrol (BHT), and 32,35-anhydroBHT which was the most abundant polyfunctionalised geohopanoid in the majority of samples. BHPs with a terminal amine functionality, diagnostic biomarkers for methanotrophic bacteria, were found throughout the sequence, with similar distributions in both the lower laminated and upper blocky lignite except that 35-aminobacteriohopanepentol (aminopentol), indicative of Type I methanotrophs (gammaproteobacteria), was generally more abundant in the upper section within the CIE. The diagenetic fate of these compounds is currently poorly constrained; however, we also identified the recently reported N-containing transformation product, anhydroaminotriol, and several tentatively assigned novel N-containing structures potentially containing ketone functionalities. Although present throughout the section, there was a sharp peak in the occurrence of these novel compounds, which correlated with the onset of the CIE and highly isotopically depleted hopanes in the upper part of the laminated lignite, both also correlating well with aminopentol peak abundance. The significant abundance of these compounds suggests that 35-aminoBHPs have their own specific diagenetic pathway, potentially providing an alternative method allowing methanotroph activity to be traced in older samples even if the original biohopanoid markers are no longer present.At this time we cannot preclude the possibility that some or all of these BHPs have been produced by more recent subsurface activity, post deposition of the lignite; however, this would not be expected to generate the observed stratigraphic variability and we suggest that unprecedented observations of a range of highly functionalised biohopanoids in samples of this age could significantly extend the window of their known occurrence
δ<sup>13</sup>C values of bacterial hopanoids and leaf waxes as tracers for methanotrophy in peatlands
No full textMethane emissions from peatlands contribute significantly to atmospheric CH4 levels and play an essential role in the global carbon cycle. The stable carbon isotopic composition (δ13C) of bacterial and plant lipids has been used to study modern and past peatland biogeochemistry, especially methane cycling. However, the small number of recent peatlands that have been characterised and the lack of consistency between target compounds means that this approach lacks a rigorous framework. Here, we undertake a survey of bacterial and plant lipid δ13C values in peatlands from different geographic regions, spanning a wide range of temperature (−8 to 27 °C) and pH (∼3 to 8), to generate a reference dataset and probe drivers of isotopic variability. Within our dataset, the carbon fixation pathway predominantly determines leaf wax (n-alkane) δ13C values. Bacterial-derived C31 hopane δ13C values track those of leaf waxes but are relatively enriched (0 to 10‰), indicating a heterotrophic ecology and preferential consumption of 13C-enriched substrates (e.g. carbohydrates). In contrast, ≤C30 hopanoids can be strongly 13C-depleted and indicate the incorporation of isotopically light methane into the bacterial community, especially at near neutral pH (∼5–6 pH). Previous analysis of Eocene sediments has suggested isotopic decoupling between C31 and ≤C30 hopanoid δ13C values. Our work suggests a globally widespread decoupling in recent peatlands; this persists despite the profound diversity of hopanoid producing bacteria and associated controls on their δ13C values and it has significant implications for future work. Re-analysis of published data from: (1) the (mid-to-early) Holocene and late Glacial, and (2) latest Paleocene and earliest Eocene in this revised context highlights that perturbations to the peatland methane cycle occurred during the past, and we envisage that this approach could provide unique (qualitative) insights into methane cycling dynamics throughout the geological record
Terrestrial environmental change across the onset of the PETM and the associated impact on biomarker proxies: a cautionary tale
Full text linkThe following supplementary information includes one dataset which contains 3 tables:
Biomarker distributions and proxies at Cobham, UK
Bulk and compound specific isotope data at Cobham (UK)
Model-derived mean annual surface temperature and precipitation estimates as a function of CO2 at Cobham (UK)
Origin and preservation of bacteriohopanepolyol signatures in <i>Sphagnum </i>peat from Bissendorfer Moor (Germany)
Full text linkDistributions of bacteriohopanepolyols (BHPs) were investigated in a peat core from the Bissendorfer Moor (Germany) in order to test the utility of BHPs as indicators of microbial processes in peats. Between 13 and 22 BHPs were identified in each sample (23 structures in total), with total concentrations ranging from 160 – 2800 µg g-1 19 TOC. We have tentatively ascribed sources of most BHPs observed at this site via comparison of known BHPs source organisms with recent microbiological studies on the peat microbiome. Members of the Alpha-, Beta- and Gammaproteobacteria and specifically the genera Burkholderia, Bradyrhizobium and Rhodoblastus, as well as other phyla including the cyanobacteria, Acidobacteria and Acetobacteria are amongst the most likely sources. Additionally, BHP signatures which could be assigned directly to methane oxidising bacteria (35-aminobacteriohopanepentol and 35-aminobacteriohopanepentol) were present only at very low levels, supporting previous studies which have shown that the majority of precursor organisms biosynthesising hopanoids in peat environments are heterotrophs. The surface layers also contained a highly unusual signature comprising high concentrations of unsaturated compounds, including unsaturated bacteriohopanetetrol pseudopentose, which has previously only been reported in Gloeocapsa cyanobacteria. This genus is known to occur in symbiotic association with host Sphagnum species, and has the ability to fix atmospheric nitrogen which is a well known trait amongst members of the peat microbiome and amongst hopanoid producing microorganisms. The apparent capacity for hopanoids to protect organisms from external stresses such as low pH is therefore likely to be a significant factor accounting for the high BHP contributions from heterotrophs, methanotrophs and phototrophic organisms in Sphagnum peats
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Ocean variability drives a millennial-scale shift in South Pacific hydroclimate
Full text linkThe South Pacific Convergence Zone is a band of intense austral summer rainfall in the tropical Pacific. Changes in the South Pacific Convergence Zone are linked to Pacific sea surface temperatures on decadal timescales, but its behaviour and impacts over longer timescales remain poorly understood due to limited proxy records and model uncertainties. We combine new plant wax hydroclimate records with existing proxy evidence and climate model simulations to investigate South Pacific Convergence Zone changes over the past 1500 years. Our findings indicate that between 1000 and 200 years ago, the eastern South Pacific Convergence Zone became wetter while the western part became drier. Model simulations suggest that these centennial-scale changes were driven by Pacific sea surface temperature gradients. This eastward shift coincides with Polynesian colonisation, implying hydroclimate shifts both ‘pushed’ migration eastward and ‘pulled’ successful eastern settlement
Insoluble prokaryotic membrane lipids in continental shelf sediments offshore Cape Town:implications for organic matter preservation
Full text linkThe largest organic carbon (OC) reservoir on Earth is in the geosphere, mainly comprising insoluble organic matter (IOM). IOM formation, therefore, plays an important role in the short and long-term carbon cycle, carbon bioavailability and formation of source rocks. To explore the mechanism of insolubilization of organic matter (OM), we have analysed soluble and IOM fractions of continental shelf marine sediments. We have applied sequential solvent-extractions followed by a selective chemical degradation of the post-extraction residue, specifically targeting prokaryotic membrane lipids (branched fatty acids – FAs, hopanoids, archaeol and glycerol dialkyl glycerol tetraethers – GDGTs). Up to 80% ofprokaryotic membrane lipids are not solvent-extractable, and we observe compound-specific differences in partitioning between soluble and IOM fractions. Based on these observations, we propose a variety of mechanisms for the incorporation of prokaryotic lipids into IOM in marine sediments: First, OM association with authigenic carbonates; second, cross-linking via esterification reactions with time, which could be particularly relevant for FAs; third, competition between reactivity and loss of polar head groups, the latter rendering the OM less susceptible to incorporation; and finally, inherent solvent-insolubility of some lipids associated with prokaryotic cells
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