Paleo-environmental effects of the mid-pleistocene transition in the tropical Atlantic and equatorial Africa

This thesis describes the environmental changes in the eastern tropical Atlantic and equatorial Africa during the Mid-Pleistocene Transition (MPT) as revealed by analyses of lipid biomarkers and their stable carbon isotope compositions. The MPT was the start of the Late Pleistocene ice ages, with an enlarged mean global ice volume varying in a predominant 100-kyr cyclicity. Before the MPT, global ice volume was, on average, smaller and had a prevalent 41-kyr cyclicity. The increase in mean global ice mass around 920 kyr BP significantly preceded the establishment of the 100-kyr cycle at 640 kyr 8P. A pronounced effect of the MPT was the temporary severe decrease of the Atlantic deepwater ventilation, caused by the strongest reductions of North Atlantic Deep-Water formation in the last 2.5 million years

Marine and terrigenous lipids in southeast atlantic sediments (leg 175) as paleoenvironmental indicators: initial results

Lipid compositions of sediments recovered during Ocean Drilling Program Leg 175 in the eastern South Atlantic reflect a variety of oceanographic and climatological environments. Most of the identified lipids can be ascribed to marine sources, notably haptophytes, eustigmatophytes, dinoflagellates, archaea, and diatoms. Elevated concentrations of cholesterol suggest zooplankton herbivory, characteristic for sites influenced by upwelling. At these sites, sulfurized highly branched isoprenoids from diatoms are also present in high amounts. Sterols, sterol ethers, hopanoids, and midchain hydroxy fatty acids could also be detected. Terrigenous lipids are n-alkanes, fatty acids, n-alcohols, and triterpenoid compounds like taraxerol and β-amyrine. n-Alkanes, fatty acids, and n-alcohols are derived from leaf waxes of higher land plants and transported to the sea by airborne dust or fresh water. Triterpenoid compounds are most probably derived from mangroves and transported solely by rivers. Lipid compositions below the Congo low-salinity plume are strongly influenced by terrigenous material from the Congo River. Elevated organic carbon contents and predominantly marine lipid distributions at the Angola margin may indicate a highly productive plankton population, probably sustained by the Angola Dome. Sedimentary lipids in the Walvis Basin contain an upwelling signal, likely transported by the Benguela Current. Sedimentary lipids off Lüderitz Bay and in the southern Cape Basin are dominated by plankton lipids in high to intermediate amounts, reflecting persistent and seasonal upwelling, respectively

High-resolution last deglaciation record from the Congo fan reveals significance of mangrove pollen and biomarkers as indicators of shelf transgression

High abundances of mangrove pollen have been associated with transgressive cycles on tropical margins, but the detailed relations between systems tracts and the taphonomy of the pollen are unclear. We report here the occurrence and high abundance of Rhizophora pollen, in association with taraxerol, a Rhizophora-sourced biomarker, from a high-resolution Congo fan core covering the last deglaciation. An age model based on 14C dates enables the temporal changes in taraxerol content and the percentage frequencies and flux (pollen grains (pg) cm-2 (10^3 yr)-1) of mangrove pollen to be compared quantitatively with the lateral rate of transgression across the flooding surface (derived from glacio-hydro-isostatic model output and the bathymetry of the margin). Rhizophora pollen concentrations and taraxerol content of the sediment are very strongly positively correlated with the lateral rate of transgression and indicate, independently of any sequence stratigraphic context, that mangrove pollen spikes are associated with the transgressive systems tract rather than the highstand systems tract or maximum flooding surface. Lower-resolution longer-term records from this margin indicate an association between taraxerol concentrations and transgressive rather than regressive phases. The flux of these materials to the Congo fan is interpreted as a function of the erosion of flooded mangrove swamp on the shelf and, less importantly, changing extent of mangrove habitat, during sea-level rise. Congo River palaeoflood events also result in reworking of mangrove pollen and supply to the fan, but this mechanism is subdominant. Rhizophora pollen has been underestimated in many palynological studies undertaken on cores from the African margin because of inappropriate sieve mesh size used during laboratory preparation.

Geochemical and microbiological fingerprinting of airborne dust that fell in Canberra, Australia, in October 2002

During the night of 22–23 October 2002, a large amount of airborne dust fell with rain over Canberra, located some 200 km from Australia's east coast, and at an average altitude of 650 m. It is estimated that during that night about 6 g m−2 of aeolian dust fell. We have conducted a vast number of analyses to “fingerprint” some of the dust and used the following techniques: grain size analysis; scanning electron microscope imagery; major, trace, and rare earth elemental, plus Sr and Nd isotopic analyses; organic compound analyses with respective compound‐specific isotope analyses; pollen extraction to identify the vegetation sources; and molecular cloning of 16S rRNA genes in order to identify dust bacterial composition. DNA analyses show that most obtained 16S rRNA sequences belong mainly to three groups: Proteobacteria (25%), Bacteriodetes (23%), and gram‐positive bacteria (23%). In addition, we investigated the meteorological conditions that led to the dust mobilization and transport using model and satellite data. Grain sizes of the mineral dust show a bimodal distribution typical of proximal dust, rather than what is found over oceans, and the bimodal aspect of size distribution confirms wet deposition by rain droplets. The inorganic geochemistry points to a source along/near the Darling River in NW New South Wales, a region that is characteristically semiarid, and both the organic chemistry and palynoflora of the dust confirm the location of this source area. Meteorological reconstructions of the event again clearly identify the area near Bourke‐Cobar as being the source of the dust. This study paves the way for determining the export of Australian airborne dust both in the oceans and other continents

Multi-isotopic constraints on the origin and fate of n-alkyl lipids in recent sediments

Recent analytical advances now make it feasible to determine multiple isotopic characteristics of individual organic compounds within complex mixtures. In particular, hydrogen isotopic compositions and radiocarbon contents can now be readily determined to complement information derived from stable carbon isotopic analysis. When used in concert, these lines of geochemical information can yield important new insights into the origins and histories of the biomarker compounds, and lead to new and refined interpretation of molecular proxy records of paleoenvironmental variability. We present multi-isotopic data on n-alkyl lipids isolated from a range of recent aquatic sediments in order to demonstrate both the complexity of geochemical signals preserved in environmental matices, and the utility of multi-isotopic information in constraining inputs. In particular, we focus on long-chain lipids derived from terrestrial and marine precursors. Marked radiocarbon age and stable isotopic variability is apparent, even within closely related homologous series of compounds recovered from the same sample. Based on multi-isotope characteristics, we interpret these variations in terms of differences in source versus reactivity of individual organic compounds, and discuss implications for interpretation of the sedimentary record

Late Holocene Precipitation Fluctuations in South America Triggered by Variability of the North Atlantic Overturning Circulation

Historic droughts document the strong spatio-temporal variability of the South American Monsoon System, which currently provides more than two thirds of the rainfall in tropical South America. The drivers of this variability have remained not well understood due to the lack of continuous, high-resolution paleorecords, especially from the more arid regions of tropical South America. Here we present a novel record of moisture availability across eastern South America for the past similar to 5,000 years from a sediment core retrieved off eastern Brazil. We document distinct decadal- to millennial-scale spatial shifts of major atmospheric convection centers that caused increasingly pronounced droughts in eastern South America over the past similar to 2,000 years. These fluctuations were triggered by climate anomalies in the high northern latitudes and propagated into equatorial latitudes via fluctuations in North Atlantic Overturning Circulation strength. As global warming is expected to decrease oceanic overturning due to enhanced meltwater input into the North Atlantic while at the same time reducing precipitation over eastern South America, an increasing risk for long-lasting droughts can be expected for this region, posing severe socio-economic challenges