Description: The Amazon River transports large amounts of terrestrial organic carbon (OCterr/ from the Andean and Amazon neotropical forests to the Atlantic Ocean. In order to compare the biogeochemical characteristics of OCterr in the fluvial sediments from the Amazon drainage basin and in the adjacent marine sediments, we analysed riverbed sediments from the Amazon mainstream and its main tributaries as well as marine surface sediments from the Amazon shelf and fan for total organic carbon (TOC) content, organic carbon isotopic composition (δ13CTOC/, and lignin phenol compositions. TOC and lignin content exhibit positive correlations with Al = Si ratios (indicative of the sediment grain size) implying that the grain size of sediment discharged by the Amazon River plays an important role in the preservation of TOC and leads to preferential preservation of lignin phenols in fine particles. Depleted δ13CTOC values (-26.1 to -29.9 ‰) in the main tributaries consistently correspond with the dominance of C3 vegetation. Ratios of syringyl to vanillyl (S = V) and cinnamyl to vanillyl (C = V) lignin phenols suggest that non-woody angiosperm tissues are the dominant source of lignin in the Amazon basin. Although the Amazon basin hosts a rich diversity of vascular plant types, distinct regional lignin compositions are not observed. In the marine sediments, the distribution of δ13CTOC and 38 (sum of eight lignin phenols in organic carbon (OC), expressed as mg/100 mg OC) values implies that OCterr discharged by the Amazon River is transported north-westward by the North Brazil Current and mostly deposited on the inner shelf. The lignin compositions in offshore sediments under the influence of the Amazon plume are consistent with the riverbed samples suggesting that processing of OCterr during offshore transport does not change the encoded source information. Therefore, the lignin compositions preserved in these offshore sediments can reliably reflect the vegetation in the Amazon River catchment. In sediments from the Amazon fan, low lignin content, relatively depleted δ13CTOC values and high (Ad = Al)V ratios indicating highly degraded lignin imply that a significant fraction of the deposited OCterr is derived from petrogenic (sourced from ancient rocks) sources.

Description: Due to the high sensitivity of southern Africa to climate change, a reliable understanding of its hydrological system is crucial. Recent studies of the regional climatic system have revealed a highly complex interplay of forcing factors on precipitation regimes. This includes the influence of the tropical easterlies, the strength of the southern hemispheric westerlies as well as sea surface temperatures along the coast of the subcontinent. However, very few marine records have been available in order to study the coupling of marine and atmospheric circulation systems. Here we present results from a marine sediment core, recovered in shallow waters off the Gouritz River mouth on the south coast of South Africa. Core GeoB18308-1 allows a closer view of the last ∼4 kyr. Climate sensitive organic proxies, like the distribution and isotopic composition of plant-wax lipids as well as indicators for sea surface temperatures and soil input, give information on oceanographic and hydrologic changes during the recorded time period. Moreover, the micropaleontology, mineralogical and elemental composition of the sediments reflect the variability of the terrigenous input to the core site. The combination of down-core sediment signatures and a catchment-wide provenance study indicate that the Little Ice Age (∼300–650 cal yr BP) was characterized by climatic conditions favorable to torrential flood events. The Medieval Climate Anomaly (∼950–650 cal yr BP) is expressed by lower sea surface temperatures in the Mossel Bay area and humid conditions in the Gouritz River catchment. These new results suggest that the coincidence of humid conditions and cooler sea surface temperatures along the south coast of South Africa resulted from a strengthened and more southerly anticyclonic circulation. Most probably, the transport of moisture from the Indian Ocean by strong subtropical easterlies was coupled with Agulhas Bank upwelling pulses, which were initiated by an increase in Agulhas Current strength.

Description: Intense debate persists about the climatic mechanisms governing hydrologic changes in tropical and subtropical southeast Africa since the Last Glacial Maximum, about 20,000 years ago. In particular, the relative importance of atmospheric and oceanic processes is not firmly established. Southward shifts of the intertropical convergence zone (ITCZ) driven by high-latitude climate changes have been suggested as a primary forcing whereas other studies infer a predominant influence of Indian Ocean sea surface temperatures on regional rainfall changes. To address this question, a continuous record representing an integrated signal of regional climate variability is required, but has until now been missing. Here we show that remote atmospheric forcing by cold events in the northern high latitudes appears to have been the main driver of hydro-climatology in southeast Africa during rapid climate changes over the past 17,000 years. Our results are based on a reconstruction of precipitation and river discharge changes, as recorded in a marine sediment core off the mouth of the Zambezi River, near the southern boundary of the modern seasonal ITCZ migration. Indian Ocean sea surface temperatures did not exert a primary control over southeast African hydrologic variability. Instead, phases of high precipitation and terrestrial discharge occurred when the ITCZ was forced southwards during Northern Hemisphere cold events, such as Heinrich stadial 1 (around 16,000 years ago) and the Younger Dryas (around 12,000 years ago), or when local summer insolation was high in the late Holocene, that is, during the past 4,000 years.

Description: Organic geochemical and micropaleontological analyses of surface sediments collected in the southern Drake Passage and the Bransfield Strait, Western Antarctic Peninsula, enable a proxy-based reconstruction of recent sea ice conditions in this climate-sensitive area. We study the distribution of the sea ice biomarker IPSO25, and biomarkers of open marine environments such as more unsaturated highly branched isoprenoid alkenes and phytosterols. Comparison of the sedimentary distribution of these biomarker lipids with sea ice data obtained from satellite observations and diatom-based sea ice estimates provide for an evaluation of the suitability of these biomarkers to reflect recent sea surface conditions. The distribution of IPSO25 supports earlier suggestions that the source diatom seems to be common in near-coastal environments characterized by annually recurring sea ice cover, while the distribution of the other biomarkers is highly variable. Offsets between sea ice estimates deduced from the abundance of biomarkers and satellite-based sea ice data are attributed to the different time intervals recorded within the sediments and the instrumental records from the study area, which experienced rapid environmental changes during the past 100 years. To distinguish areas characterized by permanently ice-free conditions, seasonal sea ice cover and extended sea ice cover, we apply the concept of the PIP25 index from the Arctic Ocean to our data and introduce the term PIPSO25 as a potential sea ice proxy. While the trends in PIPSO25 are generally consistent with satellite sea ice data and winter sea ice concentrations in the study area estimated by diatom transfer functions, more studies on the environmental significance of IPSO25 as a Southern Ocean sea ice proxy are needed before this biomarker can be applied for semi-quantitative sea ice reconstructions.

Description: The hydrogen isotopic composition of plant leaf-wax n-alkanes (dDwax) is a novel proxy for estimating dD of past precipitation (dDp). However, vegetation life-form and relative humidity exert secondary effects on dDwax, preventing quantitative estimates of past dDp. Here, we present an approach for removing the effect of vegetation-type and relative humidity from dDwax and thus for directly estimating past dDp. We test this approach on modern day (late Holocene; 0-3 ka) sediments from a transect of 9 marine cores spanning 21°N-23°S off the western coast of Africa. We estimate vegetation type (C3 tree versus C4 grass) using d13C of leaf-wax n-alkanes and correct dDwax for vegetation-type with previously-derived apparent fractionation factors for each vegetation type. Late Holocene vegetation-corrected dDwax (dDvc) displays a good fit with modern-day dDp, suggesting that the effects of vegetation type and relative humidity have both been removed and thus that dDvc is a good estimate of dDp. We find that the magnitude of the effect of C3 tree - C4 grass changes on dDwax is small compared to dDp changes. We go on to estimate dDvc for the mid-Holocene (6-8 ka), the Last Glacial Maximum (LGM; 19-23 ka) and Heinrich Stadial 1 (HS1; 16-18.5 ka). In terms of past hydrological changes, our leaf-wax based estimates of dDp mostly reflect changes in wet season intensity, which is complementary to estimates of wet season length based on leaf-wax d13C.

Description: Reconstructing terrestrial water budgets is of prime importance for understanding past climate and environment. To shed more light on how plant-wax derived n-alkanes may be used for this purpose we investigated the distribution and stable isotopic compositions of hydrogen (dD) and carbon (d13C) of plant-wax derived n-C29 and -C31 alkanes in terrestrial, coastal and offshore surface sediments in relation to hydrology along a NW-SE transect east of the Italian Apennines from the Po River to the Eastern Gulf of Taranto. The plant wax average chain length increases southward and may relate to increasing temperature and/or aridity. The plant wax dD of the terrestrial and coastal samples also increases southward and mainly reflects changes in the dD of precipitation. The d13C of plant waxes is primarily interpreted in terms of C3 vegetation changes rather than varying contributions by C4 plants. The plant wax d13C-dD composition of the Po River and Apennine rivers differs considerably from that in southern Italy, and suggests a mainly southern source for plant waxes in marine sediments of the Gulf of Taranto. This calibration provides a basis for the reconstruction of past changes in the Italian water balance and n-alkane source areas.

Description: Pollen and stable carbon (d13C) and hydrogen (dD) isotope ratios of terrestrial plant wax from the South Atlantic sediment core, ODP Site 1085, is used 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 emphasise 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 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.