ausblenden:
Schlagwörter:
monsoon climate , Arabian Sea , Holocene , sea surface temperature , Stable Nitrogen Isotopes
Zusammenfassung:
The Indian monsoon climate influences large parts of the world’s population. But relatively little is known about its decadal to centennial scale variation at time scales of societal relevance. The aim of this study was to reconstruct the Holocene history of summer and winter monsoon variability in high-resolution by analyzing sediment cores from different locations in the Arabian Sea (northern Indian Ocean). Oceanic properties and biogeochemical processes in the Arabian Sea, such as sea surface temperature (SST), primary productivity and the intensity of the mid-water oxygen minimum zone and water column denitrification are closely coupled to the seasonal monsoon cycle.
While primary productivity and SST in the northwestern Arabian Sea are mainly impacted by upwelling processes associated with the summer monsoon, in the northeastern Arabian Sea off Pakistan low SST and high primary productivity are driven by the north-easterly winds of the winter monsoon. Based on this modern setting, I analyzed alkenone-derived SST changes together with proxies of primary productivity (organic carbon, carbonate/opal, δ15N) in a well-laminated sediment core from the Pakistan continental margin to establish a high-resolution record of winter monsoon strength for the late Holocene (chapter 3). Over the last 2400 years reconstructed SST decreased whereas productivity increased, reflecting a long-term trend of winter monsoon strengthening. A comparison of my winter monsoon record with records of summer monsoon strength shows that an inverse relationship of summer and winter monsoon strength exists in the Asian monsoon region over the late Holocene. The linked variation of summer and winter monsoon strength most likely was caused by shifts in the long-term latitudinal position of the Intertropical Convergence Zone (ITCZ), forced by changes in solar output.
Reconstruction and comparison of alkenone-derived SST patterns from two sediment cores, one from the summer monsoon dominated northwestern Arabian Sea and one from the winter monsoon influenced northeastern Arabian Sea, reveal that this antagonistic behavior of summer and winter monsoon strength was also evident over the last 25,000 years (chapter 4). Strong upwelling at the coast of northern Oman reflects intensified summer monsoon activity during the early Holocene climate optimum, contemporaneous with a decline in winter monsoon strength as indicated by increasing SST off Pakistan. Strengthening of winter monsoon activity since the early Holocene was forced by a southward displacement of the ITCZ throughout the Holocene.
The late Holocene alkenone-based SST record from the northeastern Arabian Sea shows a close correlation to decadal to centennial scale climate variability recorded on the Asian continent and the high-latitude Northern Hemisphere. Colder climate conditions (as observed during the Little Ice Age) increase the strength of northeast monsoonal winds and lower SST in the northeastern Arabian Sea.
Chapter 5 deals with the temporal and spatial variability of the Arabian Sea oxygen minimum zone (OMZ) over the Holocene and its relation to varying monsoon strength. Proxies of mid-water oxygenation and southwest monsoon strength were analyzed in a sediment core from the northern Oman Margin representing the late and mid Holocene. The comparison of my δ15N and Mn/Al records with other records of denitrification and oxygenation from the northern Arabian Sea shows that the location of the core OMZ has shifted from the northwest (early Holocene) to the northeast (late Holocene) throughout the Holocene. This shift was caused by a reorganization of mid-water circulation (oxygen supply) in the northern Arabian Sea due to sea level rise together with spatial differences in the response of primary productivity (oxygen demand) to varying monsoon activity.
