Reconstruction of late Quaternary marine and terrestrial environmental conditions of Northwest Africa and Southeast Australia A multiple organic proxy study using marine sediments

Abstract

NW Africa and SE Australia are regions which are particularly vulnerable to climate change. In this thesis, organic proxies are used from marine sediment cores to reconstruct past environmental conditions from these areas. In sediments from NW Africa, the UK'37 showed an efficient proxy for sea surface temperature (SST) reconstruction, while the TEXH86 reconstructed thermocline temperatures. The UK'37 and TEXH86 records for the last 192 ka showed that periods of reduced AMOC coincide with a reduction in the vertical temperature gradient. Thus, variations in AMOC strength is a driver of the thermocline structure in the tropical Atlantic. Three independent organic proxies (UK’37,TEXH86 and LDI) were used to reconstruct SSTs for the last 135 ka in sediments from offshore SE Australia. Comparison with SST estimates based on foraminiferal assemblages shows that LDI temperatures compared well with the temperature of the warmest month, TEXH86 with the temperature of the coolest month and UK’37 with mean annual temperature. Thus, the application of these 3 proxies together at this area enables the reconstruction of SST variations from different seasons. Accumulation rates and concentrations of biomarkers from marine plankton classes were used to examine the changes in productivity at the above mentioned sites during the late Quaternary. In NW Africa, a positive correlation was observed between TOC content, productivity proxies (for dinoflagellates, eustigmatophytes, haptophytes) and the sedimentary Fe, in particular during periods of high aridity on the continent. Thus, the productivity in this area is considerably influenced by the fertilization effect of the Sahara dust, while in SE Australia, Proboscia diatom productivity seems to be controlled by the transport of silicic acid to this area as increases in its productivity matched increased diatom productivity observed at some sites of the tropical equatorial Pacific. To assess changes in the vegetation of NW Africa and SE Australia, the d13C of the n-alkanes from plant leaf waxes were analysed. Sediments from the NW Africa reveal three periods (early Holocene, 50–45 and 120–110 ka) during the past 192 ka when the central Sahara/Sahel contained a higher percentage of C3 plants, indicating wetter conditions than at present. A remarkably close correlation between d13C of benthic foraminifera and n-alkanes indicates a connection between variability in AMOC strength and vegetation type in the Sahara/Sahel region. The n-alkane d13C record from SE Australia reveals an extensive period (68-31ka) of high C4 plant abundance that is punctuated by a sharp increase in C3 vegetation at ~43 ka. This sharp increase in C3 vegetation lasts ~5 ka and is characterized by increased levels of biomass burning. Thus, this vegetation change was likely caused by increased burning events and/or by reduced herbivory as it follows the main period of the late Quaternary megafauna extinction in Australia. This thesis demonstrates the benefit of applying multiple organic proxies on marine sediments to assess marine and terrestrial paleoenvironmental changes and it has thereby provided new insights in the cause and effect of environmental changes in marine and continental areas of NW Africa and SE Australia