Description: The Miocene expansion of C4 plants (mainly tropical grasses) between 8 and 4 million years (Ma) remains an enigma since regional differences in the timing of the expansion rules out decreased CO2 (pCO2) as a dominant forcing [e.g. Tipple and Pagani, 2007. The early origins of terrestrial C4 photosynthesis. Annu. Rev. Earth Planet. Sci. 35, 435–461]. Other environmental factors, such as low-latitude aridity and seasonality have been proposed to explain the low tree versus grass ratio found in savannahs and tropical grasslands of the world, but conclusive evidence is missing. Here we use pollen and stable carbon (δ13C) and hydrogen (δD) isotope ratios of terrestrial plant wax from a South Atlantic sediment core (ODP Site 1085) to reconstruct Miocene to Pliocene changes of vegetation and rainfall regime of western southern Africa. Our results reveal changes in the relative amount of precipitation and indicate a shift of the main moisture source from the Atlantic to the Indian Ocean during the onset of a major aridification 8 Ma ago. We emphasize the importance of declining precipitation during the expansion of C4 and CAM (mainly succulent) vegetation in South Africa. We suggest that the C4 plant expansion resulted from an increased equator-pole temperature gradient caused by the initiation of strong Atlantic Meridional Overturning Circulation following the shoaling of the Central American Seaway during the Late Miocene.

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: In this study, we obtained concentrations and abundance ratios of long-chain alkenones and glycerol dialkyl glycerol tetraethers (GDGTs) in a one-year time-series of sinking particles collected with a sediment trap moored from December 2001 to November 2002 at 2200 m water depth south of Java in the eastern Indian Ocean. We investigate the seasonality of alkenone and GDGT fluxes as well as the potential habitat depth of the Thaumarchaeota producing the GDGTs entrained in sinking particles. The alkenone flux shows a pronounced seasonality and ranges from 1 µg m-2 d-1 to 35 µg m-2 d-1. The highest alkenone flux is observed in late September during the Southeast monsoon, coincident with high total organic carbon fluxes as well as high net primary productivity. Flux-weighted mean temperature for the high flux period using the alkenone-based sea-surface temperature (SST) index UK’37 is 26.7°C, which is similar to satellite-derived Southeast (SE) monsoon SST (26.4°C). The GDGT flux displays a weaker seasonality than that of the alkenones. It is elevated during the SE monsoon period compared to the Northwest (NW) monsoon and intermonsoon periods (approximately 2.5 times), which is probably related to seasonal variation of the abundance of Thaumarchaeota, or to enhanced export of GDGTs by aggregation with sinking phytoplankton detritus. Flux-weighted mean temperature inferred from the GDGT-based TEXH86 index is 26.2°C, which is 1.8 °C lower than mean annual (ma) SST but similar to SE monsoon SST. As the time series of TEXH86 temperature estimates, however, does not record a strong seasonal amplitude, we infer that TEXH86 reflects ma upper thermocline temperature at approximately 50 m water depth.

Description: The relative abundance of the C32 1,15 long-chain alkyl diol (LCD) is an emerging proxy for the input of riverine aquatic particulate organic carbon (POC) into coastal oceans. This compound has the potential to complement other established proxies reflecting riverine terrestrial POC input and allows for a more nuanced assessment of riverine POC export to coastal seas. The current understanding of this proxy is, however, limited. In this study, we compare different indices for riverine sediment input to coastal marine waters (i.e. C32 1,15-LCD, BIT index and Fe/Ca ratio) in a source-to-sink assessment in the Amazon River drainage system and the northeast South American continental margin, and we test their down-core applicability in a marine gravity core containing late Pleistocene fluvial Amazonian sediments. We show that the relative abundance of the C32 1,15-LCD is highest in water bodies with low flow velocity and low turbidity such as the downstream portion of lowland tributaries and floodplain lakes. Relative C32 1,15-LCD abundance is lowest in Andean white water tributaries where autotrophic productivity is hindered by high turbidity and high flow velocity. We also find that suspended particulate matter from all major tributaries during the extreme 2015 dry season has a similar LCD distribution to that of floodplain lakes. This indicates that the chemical composition of the tributaries is less relevant for the LCD distribution than their physical properties such as flow velocity and turbidity. Results from marine surface sediments offshore the Amazon River estuary show significant positive correlations between all three studied proxies. In contrast, we find that the relative C32 1,15-LCD abundance in the down-core record is anti-correlated to the BIT index and Fe/Ca ratio. While BIT index and Fe/Ca ratio show high (low) values during Heinrich stadials (Dansgaard-Oeschger interstadials), the C32 1,15-LCD proxy shows the opposite signal. BIT values are also higher during Marine Isotope Stage (MIS) 2 than during MIS 3, in contrast to trends in the C32 1,15-LCD proxy. We posit that this pattern arises from a reduction in relative C32 1,15-LCD abundance and total LCD productivity in the Amazon River during MIS 2 when less-humid conditions and lower sea level led to reduced area of floodplains. During Heinrich stadials, Andean sediment input increased and led to higher turbidity that resulted in lower C32 1,15-LCD production. Our study shows that major changes in water discharge, sediment transport and river morphology can lead to discrepancies between the BIT index and the relative abundance of the C32 1,15-LCD. Thus, we suggest that Amazonian aquatic and terrestrial POC pools had contrasting responses to changes related to both climate (e.g. increased Andean precipitation) and river morphology (e.g. steeper along-channel slope due to falling and low stand sea level).

Description: Arctic rivers are known to export large quantities of carbon by discharge of dissolved and particulate organic carbon (DOC, POC), and in a warming and progressively moister Arctic, these exports may increase resulting in a reduction of arctic continental carbon stocks. These rivers have highly variable discharge rates with a pronounced maximum during the spring freshet associated with highest concentrations of DOC and POC. Most studies investigating the isotopic composition and quality of carbon exported by Arctic rivers rely on samples taken in summer during base flow, which is due to the logistical challenges associated with sampling in the remote Arctic permafrost regions. Here we present a record of δ13C and ∆14C of DOC and POC collected between late May during the freshet and late August 2014 in the Lena River Delta. POC ∆14C shows an initial trend towards older values in the spring samples, which is reversed in summer, associated with a shift towards more depleted δ13C values. We interpret this aging trend as reflecting progressive thawing throughout the ice-free season, resulting in mobilization of progressively older carbon from deeper thawed layers. The summer reversal indicates admixture of aquatic organic matter. DOC ∆14C, in contrast, remains at relatively modern levels with rather constant δ13C values throughout the sampling period. We furthermore analysed the biomarker composition of Lena Delta particulate OM collected in spring and summer. From spring to summer, we observe trends in abundance of individual leaf-wax derived biomarkers indicating higher abundance of algal biomass in the summer particles. Trends in soil microbial biomarkers and compound-specific δD of leaf-wax lipids suggest a shift in sources towards higher contributions from the southern catchment in summer. DOC composition investigated with FT-ICR-MS changes from spring with higher abundances of compounds with high H/C and low O/C ratios to late summer, when fewer compounds were found. Our results illustrate the seasonal variability in composition and sources of organic matter discharged by the Lena River. Paired with the strong seassonality of the hydrograph, this implies that total annual discharge of organic matter contains a disproportionally high contribution from the northern part of the catchment.

Description: During the last glacial termination atmospheric carbon dioxide (CO2atm) rose about 100 ppm and atmospheric radiocarbon activity (Δ14C) concurrently dropped by about ca. 400 ‰. Global warming likely triggered large-scale thawing of permafrost soils in the northern hemisphere resulting in release of 14C-depleted carbon which may have contributed to the changes in CO2atm and Δ14C1. However, the timing and duration of the thawing as well as regional differences regarding these points are poorly understood. In order to unravel the evolution of permafrost decomposition and its role within the glacial-interglacial climate change a profound understanding of the deglacial carbon-turnover and temperature development in subarctic and arctic regions are of great importance. Working with two sediment cores from the continental margin off Kamchatka Peninsula, western Bering Sea (WBS, site SO201-2-114KL) & Northwest Pacific (NW Pacific, site SO201-2-12KL) we establish Glacial to Holocene records in sea surface temperature (SST) and mean air temperature (MAT) using the TEXL86 (SST) and CBT/MBT (MAT) temperature proxies that are based on marine and terrigenous biomarkers (Glycerol dialkyl glycerol tetraethers). The hydrogen isotopic composition (δD) of plant-wax derived lipid biomarkers (long-chain n-alkanes and long-chain n-fatty acids) is another tool for reconstructing air temperature but can also provide information of the provenance of terrigenous organic matter (OM). The radiocarbon activity of the lipid biomarkers is applied to reconstruct changes in terrestrial residence times of terrigenous OM. Our records in SST and CBT/MBT-based MAT are interpreted as summer temperatures. From the Bølling/Allerød interstadial (B/A) to the present they show temperature fluctuations similar to Greenland ice core records including a warming at the onset of the B/A and a cooling during the Younger Dryas (YD) followed by a warming into the Preboreal (PB) suggesting an atmospheric coupling between N-Atlantic and N-Pacific, East Siberian/Kamchatka temperature development. However, during Heinrich Stadial 1 (HS1) where WBS SST and Kamchatka MAT cool down synchronously with Greenland temperatures, the NW-Pacific SST rises gradually and seems to be decoupled from the N-Atlantic. Since the gradual warming trend and the subsequent synchronization with Greenland ice core data during B/A is known from SST records from the Gulf of Alaska2 (GOA) surface conditions in the NW Pacific likely are controlled by the Alaskan Stream overprinting the atmospheric effect and the influence of the East Kamchatka Current. Asynchronous development of our TEXL86 record and the GOA records during the Holocene indicate that the AS weakenes over the deglaciation. For the time-span of the Last Glacial Maximum to the onset of the B/A interstadial, the CBT/MBT temperature proxy may be seasonally biased as it produces improbably high summer temperatures that level Holocene conditions. In contrast to the CBT/MBT-based temperatures the δD of n-fatty acids does not show clear stadial-interstadial fluctuations and remains on a stable level during the deglaciation instead. During the Holocene, δD increases progressively, which may have resulted from a gradual warming trend. Comparing the radiocarbon activity of the n-fatty acids to the Δ14C-signature of the atmosphere at the time of deposition the dimension of the terrestrial residence time prior to the deposition (ΔΔ14C) can be estimated. High ΔΔ14C values during deglaciation indicate that the plat-wax biomarkers are strongly pre-aged which may bias the δD-temperature record. Gradually decreasing ΔΔ14C imply declining terrestrial residence times from late glacial to late Holocene and argue for major changes in the relative contribution of weakly and strongly pre-aged OM. Possible sources for strongly pre-aged OM include permafrost decomposition and the congruent mobilization of 14C-depleted carbon but also the erosion of basal tills from the North-American or Kamchatka ice sheets that bear OM dating back to times prior to the glaciation. The n-C25/n-C25+n-C27 ratio, an indicator for the relative contribution of sphagnum spp.3, is on a stable level over most parts of the deglaciation indicating a constant composition of the vegetation and/or a constant carbon source. Between 16.5-14.6 kaBP the record shows a sharp excursions towards higher values congruent with a sharp increase in the δD of n-alkanes that is not evident in the δD of n-fatty acids. Coevally, lowered CPI-values (carbon preference index) point to a major contribution of fossil carbon at that time. Thus, our sites may be subject to a change in the carbon source. It coincides with melting events of the N-American ice sheets4 which may indicate that large amounts of OM from eastern Beringia accumulated in the WBS/NW Pacific during HS1. 1. S. A. Zimov, E. A. G.Schuur & F. S. Chapin III (2006). Permafrost and the global carbon budget.- Science, 312, p. 1612-1613 2. J. E. Vonk & Ö. Gustaffsson (2009). Calibrating n-alkanes Sphagnum proxies in sub-Arctic Scandinavia.- Organic Geochemistry, 40, p. 1085-1090. 3. S. K. Praetorius & A. C. Mix (2014). Synchronization of North Pacific and Greenland climates preceded abrupt deglacial warming.- Science, 345, p. 444-448. 4. I. L. Hendy & T. Cosma (2009).Vulnerability of the Cordilleran Ice Sheet to iceberg calving during late Quaternary rapid climate change events.- Paleoceanography, 23, p. PA2101