Dust variability and provenance in the Pacific and Atlantic sectors of the Southern Ocean

Abstract

The effect of dust on the global climate results from processes in the atmosphere and in the ocean. On the one hand, dust in the atmosphere scatters or absorbs incoming sunlight (a direct influence on climate) or modifies cloud properties (an indirect influence on climate). On the other hand, dust can stimulate the marine productivity by supplying iron to the ocean (iron fertilization). Increased dust fluxes, especially during glacial periods, may have enhanced the marine productivity in the Southern Ocean, which was likely responsible for an atmospheric CO2 reduction of 40 ppmv. Subsequently, the Southern Ocean is a crucial component of the global climate system. Analyzing marine dust records deposited downwind of the major Southern Hemisphere dust source areas provides valuable information on paleoclimate changes in source areas characteristics (e.g. aridity, precipitation, wind) and within the dust transporting winds. Based on these analyses, this dissertation aims to contribute to a better understanding of paleoclimate changes inferred from the dust variability of marine dust records from the Southern Ocean. To achieve this goal, four manuscripts are presented in chapters 3, 4, 5 and 6, which each utilizes different methodical approaches. A comprehensive set of sediment surface samples and sediment cores from the Pacific and Atlantic sectors of the Southern Ocean was used to conduct dust provenance, grain size and biomarker analyses. Manuscript one (chapter 3) presents the first combined lithogenic flux and dust provenance analysis extracted from sediment surface samples from the Pacific sector of the Southern Ocean, which allows to better understand Holocene dust variability. The lowest lithogenic fluxes detected in open ocean surface sediments are interpreted as solely derived from atmospheric dust. The provenance analysis reveals that sources in central and southeast Australia dominate the dust supply to the Pacific sector of the Southern Ocean during the Holocene. The second manuscript (chapter 4) aims to identify dust fluctuations over glacial-interglacial timescales. Grain size analysis were performed on two sediment cores located in the Pacific and Atlantic sectors of the Southern Ocean, respectively. The results reveal opposing grain size patterns with coarser glacial grain sizes in the Pacific sector compared to finer glacial grain sizes in the Atlantic sector. Additional iron and lithogenic flux records provide consistent results for both sectors with increased glacial values indicating that more dust was available in potential source areas. The inconsistent grain size patterns are the result of different responses to glacial climate conditions in the source areas coupled with variations within the Southern Westerly Winds favoring or hampering the erosion of coarse dust particles. A biomarker perspective on different terrigenous and marine proxies (dust, productivity and sea surface temperature) from the polar and subpolar zones of the Pacific sector of Southern Ocean is given in the third manuscript (chapter 5). Reconstructed sea surface temperatures reveal modern summer temperatures confirming that the analyzed surface samples are representative for the Holocene. The results inferred from n-alkanes and brGDGTs (organic tracers for terrigenous material) clearly reveal that the samples located in the open ocean were solely derived from atmospheric dust. In the fourth manuscript (chapter 6), the Drake Passage throughflow is reconstructed using the sortable silt mean grain size as proxy for changes in bottom current speed. The results illustrate a substantial glacial throughflow reduction, which is consistent with a decrease of the Southern Westerly Winds core zone over Patagonia. The weakened winds reduce, in addition to other factors (e.g. sea ice), the strength of the Antarctic Circumpolar Current, which in turn affects the bottom current speed. The Southern Westerly Winds are the main dust transport medium in the Southern Hemisphere, so that changes within this wind belt can affect the long-range dust transport. Ultimately, this thesis advances the understanding of the Southern Ocean dust variability with implications for paleoclimate reconstructions and clearly illustrates the versatile application of dust analyses in paleoclimate studies. The combination of different methods, the large spatial coverage and the temporal resolution of the surface samples and sediment cores used provide a unique dataset for the Southern Ocean.