Distribution of glycerol dialkyl glycerol tetraether lipids along an altitudinal transect on Mt. Xiangpi,NE Qinghai-Tibetan Plateau,China

The distribution of glycerol dialkyl glycerol tetraethers (GDGTs) can reflect continental environmental changes. Recently, the distribution of branched GDGTs (bGDGTs) has been proposed as a novel tool for paleoelevation reconstructions. Here we report the variation in TEX₈₆ (tetraether index of 86 carbon atoms) of isoprenoidal GDGTs (iGDGTs) and MBT (methylation of branched tetraether index) of bGDGTs along an altitudinal transect on Mt. Xiangpi, NE Qinghai-Tibetan Plateau. Both TEX₈₆ and MBT values of surface soils showed significant linear decreases with altitude (TEX₈₆: R² = 0.65; n = 50; MBT: R² = 0.69; n = 24). We suggest that the apparent relationships between the two indices and altitude may be related to temperature. Our preliminary investigation suggests that the TEX₈₆ index can potentially be applied as a paleoelevation indicator in addition to the MBT index on the Qinghai-Tibetan Plateau.     

Distribution of glycerol dialkyl glycerol tetraether lipids in the water column of Lake Tanganyika

We studied the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) in suspended particulate matter from the water column of Lake Tanganyika (East Africa), where sediment studies had shown the applicability of the TEX₈₆ proxy for reconstructing surface lake water temperature. GDGTs, in particular crenarchaeol, showed maximum abundance within the suboxic zone (100–180 m), suggesting that this is the preferred niche of ammonia-oxidizing Thaumarchaeota. Despite evidence for anaerobic methane oxidation in deep anoxic water (300–1200 m) no unambiguous evidence for an imprint of methanotrophic archaea on GDGT distribution was found. Comparison of TEX₈₆ and BIT indices with those of surface sediments suggests that the sedimentary GDGTs are derived predominantly from the oxic zone and suboxic zone of the lake.     

Application of the TEX86 temperature proxy to the southern North Sea

A novel temperature proxy, the tetraether index of lipids with 86 carbon atoms (TEX₈₆), was applied to the suspended particulate organic matter (POM) and sediment core tops from eight sites in the southern North Sea in different seasons. The TEX₈₆-derived temperatures in many samples did not correlate with mean annual sea surface temperature (SST), but were shifted toward winter SST, apparently because Crenarchaeota are more abundant and metabolically active during periods of low primary production. This indicates that TEX₈₆-derived SST estimates do not necessarily reflect annual mean SST and may provide essential information about seasonal SST palaeoreconstruction. High TEX₈₆-derived SSTs were measured in the water of the river Rhine and in the sediment core tops and seawater from several stations in the southern North Sea. These sites were all characterised by important input of organic matter from soil and peat, as revealed by the relatively high values obtained with the new terrestrial proxy, the branched and isoprenoid tetraether (BIT) index. These data demonstrate that to reconstruct palaeotemperatures it is essential to estimate both TEX₈₆ and BIT indices to check that TEX₈₆ temperatures are not biased as a result of large terrestrial input. Important seasonal variations in TEX₈₆-derived SST were also evident for the surface sediments of several stations characterised by extremely low sedimentation rates, indicating temporary settlement of laterally transported organic matter with a warmer temperature signal. This implies that sediment core top correlations between TEX₈₆ and mean annual SST should not be carried out in areas characterised by transient sediment deposition.     

Global sediment core-top calibration of the TEX86 paleothermometer in the ocean

The TEX₈₆ (TetraEther indeX of tetraethers consisting of 86 carbon atoms) paleothermometer is based on the relative distribution of archaeal lipids, i.e. isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs), and is increasingly used to reconstruct past sea water temperatures. To establish a more extensive, global calibration of the TEX₈₆ paleothermometer, we analyzed GDGTs in 287 (in comparison with 44 in currently used calibration) core-top sediments distributed over the world oceans and deposited at different depths. Comparisons of TEX₈₆ data with (depth-weighted) annual mean temperatures of the overlying waters between 0 m and 4000 m as well as with different seasonal mean temperatures at 0 m water depth showed that the TEX₈₆ proxy reflects mostly annual mean temperatures of the upper mixed layer. The relationship between TEX₈₆ values and sea-surface temperatures (SSTs) was non-linear mainly because below 5 °C the change in TEX₈₆ values was minor with temperature. This suggests that the TEX₈₆ proxy might not be directly applicable for the Polar Oceans. Nevertheless, between 5 °C and 30 °C, the TEX₈₆ proxy has a strong linear relationship with SSTs. Here, we, therefore, propose a new linear calibration model (T = −10.78 + 56.2 ∗ TEX₈₆, r² = 0.935, n = 223) for past SST reconstructions using the TEX₈₆ palaeothermometer.     

Tectonics of the Afar Depression: A review and synthesis

This article outlines geomorphological and tectonic elements of the Afar Depression, and discusses its evolution. A combination of far-field stress, due to the convergence of the Eurasian and Arabian plates along the Zagros Orogenic Front, and uplift of the Afar Dome due to a rising mantle plume reinforced each other to break the lithosphere of the Arabian–Nubian Shield. Thermal anomalies beneath the Arabian–Nubian Shield in the range of 150 °C–200 °C, induced by a rising plume that mechanically and thermally eroded the base of the mantle lithosphere and generated pulses of prodigious flood basalt since ∼30 Ma. Subsequent to the stretching and thinning the Afar Dome subsided to form the Afar Depression. The fragmentation of the Arabian–Nubian Shield led to the separation of the Nubian, Arabian and Somalian Plates along the Gulf of Aden, the Red Sea and the Main Ethiopian Rift. The rotation of the intervening Danakil, East-Central, and Ali-Sabieh Blocks defined major structural trends in the Afar Depression. The Danakil Block severed from the Nubian plate at ∼20 Ma, rotated anti-clockwise, translated from lower latitude and successively moved north, left-laterally with respect to Nubia. The westward propagating Gulf of Aden rift breached the Danakil Block from the Ali-Sabieh Block at ∼2 Ma and proceeded along the Gulf of Tajura into the Afar Depression. The propagation and overlap of the Red Sea and the Gulf of Aden along the Manda Hararo–Gobaad and Asal–Manda Inakir rifts caused clockwise rotation of the East-Central Block. Faulting and rifting in the southern Red Sea, western Gulf of Aden and northern Main Ethiopian Rift superimposed on Afar. The Afar Depression initiated as diffused extension due to far-field stress and area increase over a dome elevated by a rising plume. With time, the lithospheric extension intensified, nucleated in weak zones, and developed into incipient spreading centers.     

Occurrence and distribution of tetraether membrane lipids in soils: Implications for the use of the TEX86 proxy and the BIT index

A diverse collection of globally distributed soil samples was analyzed for its glycerol dialkyl glycerol tetraether (GDGT) membrane lipid content. Branched GDGTs, derived from anaerobic soil bacteria, were the most dominant and were found in all soils. Isoprenoid GDGTs, membrane lipids of Archaea, were also present, although in considerably lower concentration. Crenarchaeol, a specific isoprenoid membrane lipid of the non-thermophilic Crenarchaeota, was also regularly detected and its abundance might be related to soil pH. The detection of crenarchaeol in nearly all of the samples is the first report of this type of GDGT membrane lipid in soils and is in agreement with molecular ecological studies, confirming the widespread occurrence of non-thermophilic Crenarchaeota in the terrestrial realm. The fluvial transport of crenarchaeol and other isoprenoid GDGTs to marine and lacustrine environments could possibly bias the BIT index, a ratio between branched GDGTs and crenarchaeol used to determine relative terrestrial organic matter (TOM) input. However, as crenarchaeol in soils is only present in low concentration compared to branched GDGTs, no large effect is expected for the BIT index. The fluvial input of terrestrially derived isoprenoid GDGTs could also bias the TEX₈₆, a proxy used to determine palaeo surface temperatures in marine and lacustrine settings and based on the ratio of cyclopentane-containing isoprenoid GDGTs in marine and lacustrine Crenarchaeota. Indeed, it is shown that a substantial bias in TEX₈₆-reconstructed sea and lake surface temperatures can occur if TOM input is high, e.g. near large river outflows.     

A LREE-depleted component in the Afar plume: Further evidence from Quaternary Djibouti basalts

Major, trace element and isotopic (Sr, Nd, Pb) data and unspiked K–Ar ages are presented for Quaternary (0.90–0.95 Ma old) basalts from the Hayyabley volcano, Djibouti. These basalts are LREE-depleted (La_n/Sm_n = 0.76–0.83), with ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.70369 to 0.70376, and rather homogeneous ¹⁴³Nd/¹⁴⁴Nd (ε_Nd = + 5.9–+ 7.3) and Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 18.47–18.55, ²⁰⁷Pb/²⁰⁴Pb = 15.52–15.57, ²⁰⁸Pb/²⁰⁴Pb = 38.62–38.77). They are very different from the underlying enriched Tadjoura Gulf basalts, and from the N-MORB erupted from the nascent oceanic ridges of the Red Sea and Gulf of Aden. Their compositions closely resemble those of (1) depleted Quaternary Manda Hararo basalts from the Afar depression in Ethiopia and (2) one Oligocene basalt from the Ethiopian Plateau trap series. Their trace element and Sr, Nd, Pb isotope systematics suggest the involvement of a discrete but minor LREE-depleted component, which is probably an intrinsic part of the Afar plume.     

Lipids of marine Archaea: Patterns and provenance in the water-column and sediments

We measured archaeal lipid distributions from globally distributed samples of freshwater, marine, and hypersaline suspended particulate matter. Cluster analysis of relative lipid distributions identified four distinct groups, including: (1) marine epipelagic (<100 m) waters, (2) marine mesopelagic (200-1500 m) and upwelling waters, (3) freshwater/estuarine waters, and (4) hypersaline waters. A pronounced difference in lipid composition patterns is the near absence of ring-containing glycerol dialkyl glycerol tetraethers (GDGTs) at high salinity. Different archaeal communities populate marine (mesophilic Crenarchaeota and Euryarchaeota), and hypersaline environments (halophilic Euryarchaeota) and community shifts can regulate differences in lipid patterns between marine and hypersaline waters. We propose that community changes within meosphilic marine Archaea also regulate the lipid patterns distinguishing epipelagic and mesopelagic/upwelling zones. Changes in the relative amounts of crenarchaeol and caldarchaeol and low relative abundances of ringed structures in surface waters differentiate lipids from the epipelagic and mesopelagic/upwelling waters. Patterns of lipids in mesopelagic (and upwelling) waters are similar to those expected of the ammonia-oxidizing Group I Crenarchaeota, with predominance of crenarchaeol and abundant cyclic GDGTs; non-metric multidimensional analysis (NMDS) shows this pattern is associated with high nitrate concentrations. In contrast, limited culture evidence indicates marine Group II Euryarchaeota may be capable of producing mainly caldarchaeol and some, but not all, of the ringed GDGTs and we suggest that these organisms, along with the Crenarchaeota, contribute to lipids in epipelagic marine waters. Calculated TEX₈₆ temperatures in mesopelagic samples (reported here and in published data sets) are always much warmer than measured in situ temperatures. We propose lipids used in the temperature proxy derive from both Euryarchaeaota and Crenarchaeota, and observed values of TEX₈₆ are subject to changes in their ecology as influenced by nutrient fluctuations or other perturbations. Applications of published core-top TEX₈₆-SST correlations require that (1) the surface waters are always composed of similar communities with the same temperature response and (2) that deeper water GDGT production is not transported to the sediments. Our lipid distribution patterns demonstrate both surface-water archaeal community differences (which accompany greater nutrient influxes, shoaling of mesopelagic Crenarchaeota during upwelling periods, and possibly due to an influx of terrestrial Archaea), and changes in organic matter transport through the water column can affect the distribution of lipids recorded in sediments. We therefore suggest that reported temperature shifts in ancient applications indicate TEX₈₆ lipids recorded not only temperature changes, but also changes in archaeal ecology, nutrient concentrations, and possibly oceanographic conditions.     

Differential degradation of intact polar and core glycerol dialkyl glycerol tetraether lipids upon post-depositional oxidation

Archaeal and bacterial glycerol dialkyl glycerol tetraether lipids (GDGTs) are used in various proxies, such as TEX₈₆ and the BIT index. In living organism, they contain polar head groups (intact polar lipids – IPLs). IPL GDGTs have also been detected in ancient marine sediments and it is unclear whether or not they are fossil entities or are part of living cells. In order to determine the extent of degradation of IPL GDGTs over geological timescales, we analyzed turbidite deposits, which had been partly reoxidized for several kyr after deposition on the Madeira Abyssal Plain. Analysis of core lipid (CL) and IPL-derived GDGTs showed a reduction in concentration by two orders of magnitude upon post-depositional oxidation, while IPL GDGTs with a mono- or dihexose head group decreased by 2–3 orders of magnitude. The BIT index for CL- and IPL-derived GDGTs increased substantially upon oxidation from 0.1 to up to 0.5. Together with changing MBT/CBT values, this indicates preferential preservation of soil-derived branched GDGTs over marine isoprenoid GDGTs, combined with in situ production of branched GDGTs in the sediment. The TEX₈₆ value for IPL-derived GDGTs decreased by 0.07 upon oxidation, while that of CL GDGTs showed no significant change. Isolation of IPLs revealed that the TEX₈₆ value for monohexose GDGTs was 0.55, while the that for dihexose GDGTs was substantially higher, 0.70. Thus, the decrease in TEX₈₆ for IPL-derived GDGTs was in agreement with the dominance of monohexose GDGTs in the oxidized turbidite, probably caused by a combination of in situ production as well as selective preservation of terrestrial isoprenoid GDGTs. Due to the low amount of IPL GDGTs vs. CL GDGTs, the impact of IPL degradation on CL-based TEX₈₆ paleotemperature estimates was negligible.     

http://www.sciencedirect.com/science/article/pii/S1674987112001260

Archaea have unique glycerol dialkyl glycerol tetraether (GDGT) lipids that can be used to develop paleotemperature proxies such as TEX₈₆. This research is to validate proposed GDGT-proxies for paleotemperature determination in the South China Sea (SCS). Samples were collected from core-top sediments (0–5 cm) in the northern SCS. Total lipids were extracted to obtain core GDGTs, which were identified and quantified using liquid chromatography-mass spectrometry (LC-MS). The abundance of isoprenoidal GDGTs (iGDGTs) ranged from 271.5 ng/g dry sediment to 1266.3 ng/g dry sediment, whereas the branched GDGTs (bGDGTs), supposedly derived from terrestrial sources, ranged from 22.2 ng/g dry sediment to 56.7 ng/g dry sediment. The TEX₈₆-derived sea surface temperatures ranged from 20.9 °C in the coast (water depth < 160 m) to 27.9 °C offshore (water depth > 1000 m). TEX₈₆-derived temperatures near shore (<160 m water depth) averaged 23.1 ± 2.5 °C (n = 4), which were close to the satellite-derived winter mean sea surface temperature (average 22.6 ± 1.0 °C, n = 4); whereas the TEX₈₆-derived temperatures offshore averaged 27.4 ± 0.3 °C (n = 7) and were consistent with the satellite mean annual sea surface temperature (average 26.8 ± 0.4 °C, n = 7). These results suggest that TEX₈₆ may record the sea surface mean annual temperature in the open ocean, while it likely records winter sea surface temperature in the shallower water.     

Comprehensive glycerol ether lipid fingerprints through a novel reversed phase liquid chromatography–mass spectrometry protocol

Glycerol ether lipid distributions have been developed as proxies for reconstructing past environmental change or, in their intact polar form, for fingerprinting the viable microbial community composition. However, due to their structural complexity, full characterization of glycerol ether lipids requires separate protocols for the analysis of the polar head groups and the alkyl chain moieties in core ether lipids. As a consequence, the valuable relationship between core ether lipid composition and specific polar head groups is often lost; this limits understanding of the diversity of ether lipids and their utility as biogeochemical proxies. Here, we report a novel reversed phase liquid chromatography–electrospray ionization-mass spectrometry (RP-ESI-MS) protocol that enables the simultaneous analysis of polar head groups (e.g. phosphocholine, phosphoglycerol, phosphoinositol, hexose and dihexose) and alkyl moieties (e.g. alkyl moieties modified with different numbers of cycloalkyl moieties, hydroxyl and alkyl groups and double bonds) in crude lipid extracts without further preparation. The protocol greatly enhances detection of archaeal intact polar lipids (IPLs) and core lipids (CLs) with double bond- and hydroxyl group-bearing alkyl moieties. With these improvements, widely used ratios that describe relative distributions of the core lipids, such as TEX₈₆ and ring index, can now be directly determined in specific intact polar lipids (IPL-specific TEX₈₆ and ring index). Since IPLs are the putative precursors of the environmentally persistent core lipids, their detailed examination using this protocol can potentially provide new insights into diagenetic and biological mechanisms inherent to these proxies. In a series of 12 samples from diverse settings, core and IPL-specific TEX₈₆ values followed the order: 2G-GDGTs > core GDGTs > 1G-GDGTs > 1G-GDGT-PI and the ring indices followed: 1G-GDGTs ≈ core GDGTs > 2G-GDGTs > 1G-GDGT-P1G > 2G-OH-GDGTs ≈ 1G-OH-GDGTs (1G, monoglycosyl; 2G, diglycosyl; P1G, phosphomonoglycosyl; GDGT, glycerol dialkyl glycerol tetraether).     

Effects of long term oxic degradation on the U37K′, TEX86 and BIT organic proxies

One of the primary prerequisites for the application of organic proxies is that they should not be substantially affected by diagenesis. However, studies have shown that oxic degradation of biomarker lipids can affect their relative distribution. We tested the diagenetic stability of the $U_{37}^{K^{\prime }}$ and TEX₈₆ palaeothermometers upon long term oxygen exposure. For this purpose, we studied the distributions of alkenones and glycerol dialkyl glycerol tetraethers (GDGTs) in different sections of turbidites at the Madeira Abyssal Plain (MAP) that experienced different degrees of oxygen exposure. Sediments were deposited anoxically on the shelf and then transported by turbidity currents to the MAP, which has oxic bottom water. This resulted in partial degradation of the turbidite organic matter as a result of long term exposure to oxic bottom water. Concentrations of GDGTs and alkenones were reduced by one to two orders of magnitude in the oxidized parts of the turbidites compared to the unoxidized parts, indicating substantial degradation. High-resolution analysis of the Pleistocene F-turbidite showed that the $U_{37}^{K^{\prime }}$ index of long chain alkenones increased only slightly (0.01, corresponding to <0.5 °C) in the oxidized part of the turbidite, suggesting minor preferential degradation of the C_37:3 alkenone, in agreement with previous studies. TEX₈₆ values showed a small increase (0.02, corresponding to ～2 °C) in the F-turbidite, like $U_{37}^{K^{\prime }}$, while for other Pliocene/Miocene turbidites it either remained unchanged or decreased substantially (up to 0.06, corresponding to ～6 °C). Previous observations showed that the BIT index, a proxy for the contribution of soil organic matter to total organic carbon, was always substantially higher in the oxidized part in all the turbidites, as a result of preferential degradation of marine-derived GDGTs. This relative increase in soil-derived GDGTs affects TEX₈₆, as the isoprenoid GDGT distribution on the continent can be quite different from that in the marine environment. Our results indicate that the organic proxies are affected by long term oxic degradation to different extents; this should be taken into account when applying these proxies in palaeoceanographic studies of sediments which have been exposed to prolonged oxic degradation.     

Seasonal changes in glycerol dialkyl glycerol tetraether concentrations and fluxes in a perialpine lake: Implications for the use of the TEX86 and BIT proxies

To determine where and when glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in lakes are produced, we collected descending particles in Lake Lucerne (Switzerland) using two sediment traps (at 42 and 72 m water depth) with a monthly resolution from January 2008 to late March 2009. Suspended particulate matter (SPM) was monthly filtered from the water column at three different depths. The potential application of GDGTs in palaeoenvironmental and palaeoclimatic reconstructions was investigated by comparing core lipids and their relative GDGT distribution, with lake water temperatures throughout the year. Fluxes of GDGTs and their concentrations in the water column vary according to a seasonal pattern, showing a similar trend in the SPM and sediment traps. Fluxes and concentrations of isoprenoid GDGTs increase with depth, maximum values being observed in the deeper part of the water column, indicating production of isoprenoid GDGTs by Thaumarchaeota in the deep (∼50 m), aphotic zone of Lake Lucerne. The flux-weighted averages of the proxies TEX₈₆ (0.27) and BIT (0.03) based on the total extracted GDGTs are similar at both trap depths. A sediment core from the same location showed that in the first few centimetres of the core TEX₈₆ and BIT values of 0.29 and 0.07, respectively, are similar to those recorded for descending particles and SPM, indicating that the sedimentary TEX₈₆ records the annual mean temperature of deeper waters in Lake Lucerne. TEX₈₆ values are slightly higher below 20 cm in the core. This offset is interpreted to be caused by the present-day trophic state of the lake, which probably resulted in a deeper niche of the Thaumarchaeota. Branched GDGTs represent only a minor fraction of the total GDGTs in the lake and their origin remains unclear. Our data reveal that GDGTs in lakes have a large potential for palaeoclimatic studies but indicate that knowledge of the system is important for accurate interpretation.     

Warming and environmental changes in the eastern North Sea Basin during the Palaeocene–Eocene Thermal Maximum as revealed by biomarker lipids

Analysis of sediments deposited at different latitudes around the world during the Palaeocene–Eocene Thermal Maximum (PETM; ∼56 Ma) have revealed a globally profound warming phase, regionally varying from 5–8 °C. Such records from Europe have not yet been obtained. We studied the variations in sea surface and continental mean annual air temperatures (SST and MAT, respectively) and the distribution patterns and stable carbon isotopes of higher plant derived n-alkanes in two proximal PETM sections (Fur and Store Bælt, Denmark) from the epicontinental North Sea Basin. A negative carbon isotope excursion (CIE) of 4–7‰ was recorded in land plant derived n-alkanes, similar to what has been observed for other PETM sections. However, differences observed between the two proximal sites suggest that local factors, such as regional vegetation and precipitation patterns, also influenced the CIE. The presence of S-bound isorenieratene derivatives at the onset of the PETM and increased organic carbon contents points to a rapid shift in depositional environment; from well oxygenated to anoxic and sulfidic. These euxinic conditions are comparable with those during the PETM in the Arctic Ocean. SSTs inferred from TEX₈₆ show relatively low temperatures followed by an increase of ∼7 °C across the PETM. At the Fur section, a remarkably similar temperature record was obtained for MAT using the MBT′/CBT proxy. However, the MAT record of the Store Bælt section did not reveal this warming.     

Origins of archaeal tetraether lipids in sediments: Insights from radiocarbon analysis

Understanding the supply and preservation of glycerol dibiphytanyl glycerol tetraethers (GDGTs) in marine sediments helps inform their use in paleoceanography. Compound-specific radiocarbon measurements of sedimentary alkenones from multiple environments have been used to gain insight into processes that affect paleotemperature reconstructions. Similar analyses are warranted to investigate how analogous processes affecting GDGTs impact TEX₈₆ paleotemperatures. Here we present radiocarbon measurements on individual GDGTs from Bermuda Rise and Santa Monica Basin sediments and discuss the results in the context of previous studies of co-depositional alkenones and foraminifera. The ¹⁴C contents of GDGTs and planktonic foraminifera in Bermuda Rise are very similar, suggesting a local source; and TEX₈₆-derived temperatures agree more closely with foraminiferal temperatures than do temperatures. In contrast, GDGTs in Santa Monica Basin are depleted in ¹⁴C relative to both alkenones and foraminifera, and TEX₈₆ temperatures agree poorly with known surface water values. We propose three possible factors that could explain these results: (i) GDGTs may be labile relative to alkenones during advective transport through oxic waters; (ii) archaeal production deep in the water column may contribute ¹⁴C-depleted GDGTs to sediments; and (iii) some GDGTs also may derive from sedimentary archaeal communities. Each of these three processes is likely to occur with varying relative importance depending on geographic location. The latter two may help to explain why TEX₈₆ temperature reconstructions from Santa Monica Basin do not appear to reflect actual sea surface temperatures. Terrigenous GDGTs are unlikely to be major contributors to Bermuda Rise or Santa Monica Basin sediments, based on values of the BIT index. The results also indicate that the crenarchaeol regioisomer is governed by processes different from other GDGTs. Individual measurements of the crenarchaeol regioisomer are significantly depleted in ¹⁴C relative to co-occurring GDGTs, indicating an alternative origin for this compound that presently remains unknown. Re-examination of the contribution of crenarchaeol regioisomer to the TEX₈₆ index shows that it is a significant influence on the sensitivity of temperature reconstructions.     

Sea level and shoreline reconstructions for the Red Sea: isostatic and tectonic considerations and implications for hominin migration out of Africa

The history of sea level within the Red Sea basin impinges on several areas of research. For archaeology and prehistory, past sea levels of the southern sector define possible pathways of human dispersal out of Africa. For tectonics, the interglacial sea levels provide estimates of rates for vertical tectonics. For global sea level studies, the Red Sea sediments contain a significant record of changing water chemistry with implications on the mass exchange between oceans and ice sheets during glacial cycles. And, because of its geometry and location, the Red Sea provides a test laboratory for models of glacio-hydro-isostasy. The Red Sea margins contain incomplete records of sea level for the Late Holocene, for the Last Glacial Maximum, for the Last Interglacial and for earlier interglacials. These are usually interpreted in terms of tectonics and ocean volume changes but it is shown here that the glacio-hydro-isostatic process is an additional important component with characteristic spatial variability. Through an iterative analysis of the Holocene and interglacial evidence a separation of the tectonic, isostatic and eustatic contributions is possible and we present a predictive model for palaeo-shorelines and water depths for a time interval encompassing the period proposed for migrations of modern humans out of Africa. Principal conclusions include the following. (i) Late Holocene sea level signals evolve along the length of the Red Sea, with characteristic mid-Holocene highstands not developing in the central part. (ii) Last Interglacial sea level signals are also location dependent and, in the absence of tectonics, are not predicted to occur more than 1–2 m above present sea level. (iii) For both periods, Red Sea levels at ‘expected far-field’ elevations are not necessarily indicative of tectonic stability and the evidence points to a long-wavelength tectonic uplift component along both the African and Arabian northern and central sides of the Red Sea. (iv) The observational evidence is consistent with tectonic and isostatic processes both operating over the past 300,000 years without requiring changes in the time averaged (over a few thousand years) tectonic rates. (v) Recent bathymetric data for the Bab al Mandab region have been compiled to confirm the location and depth of the sill controlling flow in and out of the Red Sea. Throughout the last 400,000 years the Red Sea has remained open to the Gulf of Aden with cross sectional areas at times of glacial maxima about 2% of that today. (vi) The minimum channel widths connecting the Red Sea to the Gulf of Aden at times of lowstand occur south of the Hanish Sill. The channels are less than 4 km wide and remain narrow for as long as local sea levels are below ?50 m. This occurs for a number of sustained periods during the last two glacial cycles and earlier. (vii) Periods suitable for crossing between Africa and Arabia without requiring seaworthy boats or seafaring skills occurred periodically throughout the Pleistocene, particularly at times of favourable environmental climatic conditions that occurred during times of sea level lowstand.     

Distribution of intact and core GDGTs in marine sediments

We conducted a survey of archaeal GDGT (glycerol dibiphytanyl glycerol tetraether) distributions in marine sediments deposited in a range of depositional settings. The focus was comparison of two pools presumed to have distinct geobiological significance, i.e. intact polar GDGTs (IP GDGTs) and core GDGTs (C GDGTs). The former pool has been suggested to be related to living communities of benthic archaea in marine sediments, while the latter is commonly interpreted to consist of molecular fossils from past planktonic archaeal communities that inhabited the surface ocean. Understanding the link between these two pools is important for assessment of the validity of current molecular proxies for sedimentary archaeal biomass and past sea surface temperatures. The relative distributions of GDGTs in the two pools in a core at a CH₄ rich site in the Black Sea provide evidence for in situ production of glycosidic IP GDGTs and their subsequent degradation to corresponding C GDGTs on timescales that are short in geological terms. In addition, we monitored the relationship between the IP GDGT and C GDGT pools in a sample set from various ocean basins with subseafloor depth from a few cm to 320 m and 0 to 4 Myr in age. Notable differences between the two pools can be summarized as follows: the GDGT with acyclic biphytanes, GDGT-0, and its analogues with two and three cyclopentane moieties (GDGT-2 and -3) are generally more abundant in the pool of IP GDGTs, while crenarchaeol tends to be more abundant in the C GDGT pool. Consequently, the ring index is generally higher for the C GDGTs while TEX₈₆, a molecular proxy ratio not considering the two major GDGTs, tends to be higher in the IP GDGT pool. These differences in the proportion of individual GDGTs in the two pools are probably due to in situ production of IP GDGTs with distributions differing from those of C GDGTs. Despite these differences, we observed significant correlation of these two ratios between the two pools. Specifically, in both pools TEX₈₆ is high in sediments from warm oceanic regimes and low in cold regimes. We discuss these relationships and suggest that recycling of core GDGTs by benthic archaea is an important mechanism linking both molecular pools.     

Glacial/interglacial sea surface temperature changes in the Southwest Pacific ocean over the past 360 ka

The phase relationship between climate parameters during terminations gives insight into deglaciation mechanisms. By combining foraminiferal Mg/Ca and alkenone thermometers with planktonic and benthic foraminiferal δ¹⁸O, we determined the phase relationship between local sea surface temperature (SST) and global seawater δ¹⁸O changes in the Coral Sea in the Southwestern Pacific over the last 360 ka. The onset of the SST warming preceded the seawater δ¹⁸O change by several ka for Termination I, II and III. During Termination I, the SST warming started at 20 ka BP, earlier than atmospheric CO₂ rise suggesting that the greenhouse effect was not the main trigger of this early warming. Compilation of ¹⁴C-dated SST records from the whole Pacific during Termination I reveals that the onset of the warming is generally earlier in the Southern and the tropical Pacific than in the North Pacific. This spatio-temporal warming pattern suggests linkage between the southern ocean and tropical Pacific. The early tropical warming could provide heat and moisture to the northern high latitudes, modifying radiative balance and precipitation over ice sheets at the onset of deglaciation.