Description: The past climate evolution of southwestern Africa is poorly understood and interpretations of past hydrological changes are sometimes The past climate evolution of southwestern Africa is poorly understood and interpretations of past hydrological changes are sometimes contradictory. Here we present a record of leaf-wax δD and δ13C taken from a marine sediment core at 23°S off the coast of Namibia to reconstruct the hydrology and C3 versus C4 vegetation of southwestern Africa over the last 140 000 years (140 ka). We find lower leaf-wax δD and higher δ13C (more C4 grasses), which we interpret to indicate wetter Southern Hemisphere (SH) summer conditions and increased seasonality, during SH insolation maxima relative to minima and during the last glacial period relative to the Holocene and the last interglacial period. Nonetheless, the dominance of C4 grasses throughout the record indicates that the wet season remained brief and that this region has remained semi-arid. Our data suggest that past precipitation increases were derived from the tropics rather than from the winter westerlies. Comparison with a record from the Congo Basin indicates that hydroclimate in southwestern Africa has evolved in antiphase with that of central Africa over the last 140 ka.

Description: The Indian Summer Monsoon (ISM) is a major global climatic phenomenon. Long-term precipitation proxy records of the ISM, however, are often fragmented and discontinuous, impeding an estimation of the magnitude of precipitation variability from the Last Glacial to the present. To improve our understanding of past ISM variability, we provide a continuous reconstructed record of precipitation and continental vegetation changes from the lower Ganges-Brahmaputra-Meghna catchment and the Indo-Burman ranges over the last 18,000 years (18 ka). The records derive from a marine sediment core from the northern Bay of Bengal (NBoB), and are complemented by numerical model results of spatial moisture transport and precipitation distribution over the Bengal region. The isotopic composition of terrestrial plant waxes (dD and d13C of n-alkanes) are compared to results from an isotope-enabled general atmospheric circulation model (IsoCAM) for selected time slices (pre-industrial, mid-Holocene and Heinrich Stadial 1). Comparison of proxy and model results indicate that past changes in the dD of precipitation and plant waxes were mainly driven by the amount effect, and strongly influenced by ISM rainfall. Maximum precipitation is detected for the Early Holocene Climatic Optimum (EHCO; 10.5 - 6 ka BP), whereas minimum precipitation occurred during the Heinrich Stadial 1 (HS1; 16.9 - 15.4 ka BP). The IsoCAM model results support the hypothesis of a constant moisture source (i.e. the NBoB) throughout the study period. Relative to the pre-industrial period the model reconstructions show 20% more rain during the mid-Holocene (6 ka BP) and 20% less rain during the Heinrich Stadial 1 (HS1), respectively. A shift from C4-plant dominated ecosystems during the glacial to subsequent C3/C4-mixed ones during the interglacial took place. Vegetation changes were predominantly driven by precipitation variability, as evidenced by the significant correlation between the dD and d13C alkane records. When compared to other records across the ISM domain, precipitation and vegetation changes inferred from our records and the numerical model results provide evidence for a coherent regional variability of the ISM from the Last Glacial to the present.

Description: Polar regions are strongly affected by global climate change since warming is projected to be strongest in high latitudes. Understanding temperature changes is crucial to unravel the impact of climate change there. Rising sea surface temperatures (SST) modify oceanographic conditions of the polar and subpolar seas. In the northern hemisphere, increasing mean annual air temperatures (MAAT) lead to thawing of permafrost soils which may initiate release of vast amounts of fossil carbon to the environment. In order to study changes in SST, MAAT and the intensity of carbon export from East Siberia to the adjacent NW Pacific and Bering Sea over the last deglaciation we analyzed terrigenous and marine biomarkers (n-alkanes, branched GDGT & isoprenoid GDGTs) from two sediment cores recovered at the continental margin off Kamchatka peninsula (NW Pacific), and from the western Bering Sea. We test the applicability of TEX86 as a tool for SST-reconstructions over the last deglaciation and thereby produce a TEX86 based SST-record in the Bering Sea. The results are compared to Uk’37 and Mg/Ca based SST. The TEX86 record is interpreted to reflect summer subsurface temperatures. We further investigate the CBT/MBT indices calculated from the branched GDGTs as well as δD of n-alkanes as tools for the reconstruction of MAAT. MAAT based on CBT/MBT shows a pattern similar to Greenland ice core temperature records with cooling events during the Heinrich Event 1 (HE1) and the Younger Dryas (YD). The results for the late Holocene match the modern MAAT of Kamchatka peninsula. However, from the Last Glacial Maximum to the onset of the Bølling/Allerød interstadial (B/A) CBT/MBT produces unrealistic temperatures that are as high as during Holocene. Possibly the record shows summer temperatures during LGM and the early deglaciation and reflects the annual mean at the beginning of the B/A. When interpreting these findings one has to keep in mind that concentrations of branched GDGT are very low (BIT lower than 0.1). Thus it is questionable whether CBT/MBT can be used as temperature proxy since the signal may be affected by in-situ production. In contrast to the CBT/MBT our δD records have hardly no similarity with Greenland ice core data. The B/A does not differ from LGM conditions but there is a slight decrease during YD. While the record of the Bering Sea shows an abrupt increase of 40 ‰ during the early Holocene the NW-Pacific shows a gradual increase of the same magnitude over the whole Holocene period. During the HE1 both records show a sharp increase reaching the Holocene level. This indicates interglacial-like temperatures which is unrealistic for stadial conditions. Coeval changes in the CPI and n-C23/n-C27 indicate that δD is overprinted by a change in the carbon source. Potential explanations include redeposition of material mobilized during deglacial sea-level rise, or release of fossil carbon from permafrost. Terrigenous biomarkers were quantified and used to study the history of carbon export. Accumulation rates of n-alkanes and branched GDGT increase during the YD and are strikingly high during the Preboreal indicating enhanced carbon mobilization. Decomposing permafrost soils in East Siberia and on the Kamchatka peninsula is a likely carbon source.

Description: Long chain alkyl diols form a group of lipids occurring widely in marine environments. Recent studies have suggested several palaeoclimatological applications for proxies based on their distributions, but have also revealed uncertainty about their applicability. Here we evaluate the use of long chain 1,14-alkyl diol indices for reconstruction of temperature and upwelling conditions by comparing index values, obtained from a comprehensive set of marine surface sediments, with environmental factors such as sea surface temperature (SST), salinity and nutrient concentration. Previous studies of cultures indicated a strong effect of temperature on the degree of saturation and the chain length distribution of long chain 1,14-alkyl diols in Proboscia spp., quantified as the diol saturation index (DSI) and diol chain length index (DCI), respectively. However, values of these indices for surface sediments showed no relationship with annual mean SST of the overlying water. It remains unknown as to what determines the DSI, although our data suggest that it may be affected by diagenesis, while the relationship between temperature and DCI may be different for different Proboscia species. In addition, contributions from algae other than Proboscia diatoms may affect both indices, although our data provide no direct evidence for additional long chain 1,14-alkyl diol sources. Two other indices using the abundance of 1,14-diols vs. 1,13-diols and C30 1,15- diols have been applied previously as indicators for upwelling intensity at different locations. The geographical distribution of their values supports the use of 1,14 diols vs. 1,13 diols [C28 + C30 1,14-diols]/[(C28 + C30 1,13-diols) + (C28 + C30 1,14-diols)] as a general indicator for high nutrient or upwelling conditions.

Description: In this study we reconstruct sea surface temperatures (SSTs) using two lipid-based biomarker proxies (alkenone unsaturation index View the MathML sourceU37K′ and TEX86 index based on glycerol dibiphytanyl glycerol tetraethers) in 36 surface sediment samples from the Indonesian continental margin off west Sumatra and south of Java and the Lesser Sunda Islands. Comparison of measured temperatures (World Ocean Atlas 09) to reconstructed temperatures suggests that SST estimates based on View the MathML sourceU37K′ reflect the SE monsoon SST in the upwelling area south of Java and the Lesser Sunda Islands. Estimates based on TEX86 using the calibration for temperatures 〉20 °C (View the MathML sourceTEX86H) are up to 2 °C lower than View the MathML sourceU37K′-based SSTs. This offset is possibly related to either one or a combination of two factors: (i) the depth habitats of the source organisms and (ii) different seasonal production and/or seasonality of export associated with phytoplankton blooming triggered by primary productivity. In the non-upwelling area off west Sumatra, the alkenone-based SSTs are cooler than measured temperatures during the entire year, likely reflecting the limitations of the View the MathML sourceU37K′ proxy beyond 28 °C, while reconstructed temperatures based on View the MathML sourceTEX86H are consistent with mean annual SST.

Description: The past climate evolution of southwestern Africa is poorly understood and interpretations of past hydrological changes are sometimes contradictory. Here we present a record of leaf-wax dD and View the MathML source taken from a marine sediment core at 23°S off the coast of Namibia to reconstruct the hydrology and C3 versus C4 vegetation of southwestern Africa over the last 140 000 years (140 ka). We find lower leaf-wax dD and higher View the MathML source (more C4 grasses), which we interpret to indicate wetter Southern Hemisphere (SH) summer conditions and increased seasonality, during SH insolation maxima relative to minima and during the last glacial period relative to the Holocene and the last interglacial period. Nonetheless, the dominance of C4 grasses throughout the record indicates that the wet season remained brief and that this region has remained semi-arid. Our data suggest that past precipitation increases were derived from the tropics rather than from the winter westerlies. Comparison with a record from the Congo Basin indicates that hydroclimate in southwestern Africa has evolved in antiphase with that of central Africa over the last 140 ka.

Description: The Indian Summer Monsoon (ISM) is a major global climatic phenomenon. Long-term precipitation proxy records of the ISM, however, are often fragmented and discontinuous, impeding an estimation of the magnitude of precipitation variability from the Last Glacial to the present. To improve our understanding of past ISM variability, we provide a continuous reconstructed record of precipitation and continental vegetation changes from the lower Ganges-Brahmaputra-Meghna catchment and the Indo-Burman ranges over the last 18,000 years (18 ka). The records derive from a marine sediment core from the northern Bay of Bengal (NBoB), and are complemented by numerical model results of spatial moisture transport and precipitation distribution over the Bengal region. The isotopic composition of terrestrial plant waxes (dD and d13C of n-alkanes) are compared to results from an isotope-enabled general atmospheric circulation model (IsoCAM) for selected time slices (pre-industrial, mid-Holocene and Heinrich Stadial 1). Comparison of proxy and model results indicate that past changes in the dD of precipitation and plant waxes were mainly driven by the amount effect, and strongly influenced by ISM rainfall. Maximum precipitation is detected for the Early Holocene Climatic Optimum (EHCO; 10.5-6 ka BP), whereas minimum precipitation occurred during the Heinrich Stadial 1 (HS1; 16.9-15.4 ka BP). The IsoCAM model results support the hypothesis of a constant moisture source (i.e. the NBoB) throughout the study period. Relative to the pre-industrial period the model reconstructions show 20% more rain during the mid-Holocene (6 ka BP) and 20% less rain during the Heinrich Stadial 1 (HS1), respectively. A shift from C4-plant dominated ecosystems during the glacial to subsequent C3/C4-mixed ones during the interglacial took place. Vegetation changes were predominantly driven by precipitation variability, as evidenced by the significant correlation between the dD and d13C alkane records. When compared to other records across the ISM domain, precipitation and vegetation changes inferred from our records and the numerical model results provide evidence for a coherent regional variability of the ISM from the Last Glacial to the present.