Chemical and mineral compositions of sediments from ODP Site 127-797

In order to reconstruct past variations in the aeolian dust (Kosa) contribution to the Japan Sea, and to establish a direct link between terrestrial and marine climatic records, we have applied statistical procedures to distinguish and quantify detrital subcomponents within the detrital fraction of the late Quaternary hemipelagic sediments in the Japan Sea. Q-mode factor analysis with varimax and oblique rotation of the factors followed by multiple-regression analysis between mineral composition and factor loadings was conducted using six ''detrital'' elements. Four detrital subcomponents were defined, which are attributed to Kosa derived from ''typical'' loess, Kosa from ''weathered'' loess, and fine and coarse arc-derived detritus, respectively, based on comparisons with the chemical and mineral compositions of probable source materials. Using these detrital subcomponents, the variation in Kosa fraction was reconstructed for the last 200 ky. The results reveal millennial-scale as well as glacial-interglacial scale variations in Kosa contribution. Especially, millennial-scale variability of Kosa contribution suggests the presence of high frequency variation in summer monsoon precipitation in the central to east Asia during the last 200 ky.

Bulk sediment element analysis of sediment cores GeoB10065-9 and GeoB10065-7, offshore northwest Sumba Island, Indonesia

The Australian-Indonesian monsoon has a governing influence on the agricultural practices and livelihood in the highly populated islands of Indonesia. However, little is known about the factors that have influenced past monsoon activity in southern Indonesia. Here, we present a ~6000 years high-resolution record of Australian-Indonesian summer monsoon (AISM) rainfall variations based on bulk sediment element analysis in a sediment archive retrieved offshore northwest Sumba Island (Indonesia). The record suggests lower riverine detrital supply and hence weaker AISM rainfall between 6000 yr BP and ~3000 yr BP compared to the Late Holocene. We find a distinct shift in terrigenous sediment supply at around 2800 yr BP indicating a reorganization of the AISM from a drier Mid Holocene to a wetter Late Holocene in southern Indonesia. The abrupt increase in rainfall at around 2800 yr BP coincides with a grand solar minimum. An increase in southern Indonesian rainfall in response to a solar minimum is consistent with climate model simulations that provide a possible explanation of the underlying mechanism responsible for the monsoonal shift. We conclude that variations in solar activity play a significant role in monsoonal rainfall variability at multi-decadal and longer timescales. The combined effect of orbital and solar forcing explains important details in the temporal evolution of AISM rainfall during the last 6000 years. By contrast, we find neither evidence for volcanic forcing of AISM variability nor for a control by long-term variations in the El Niño-Southern Oscillation (ENSO).

(Table T1) Long-chain alkenones, UK'37 and derived sea surface temperatures of ODP Sites 184-1147 and 184-1148

A reconnaissance study of alkenone stratigraphy for the past 35 m.y. in the northern South China Sea (SCS) using sediments from Sites 1147 and 1148 of Ocean Drilling Program (ODP) Leg 184 has been completed. Alkenones were not detected in sediment samples older than ~31 Ma. However, C37:2 appeared in the sedimentary record between ~8 and 31 Ma and both C37:2 and C37:3 were present between 0 and 8 Ma. These changes in alkenone occurrences may signal a response to global-scale Neogene cooling as well as to monsoon intensification and sea level changes over time as a result of Himalayan uplift and the opening of the SCS. Alternatively, they may be related to an evolutionary record of the development of temperature control on alkenone production in coccolithophores. The Uk'37 index for 0-8 Ma produces sea-surface temperatures (SST) of 19°-26°C, which are in the range of previously determined glacial-interglacial values for the northern SCS. Before the late Pleistocene (~1.2 Ma), the SST range is between 23° and 26°C with less variation. This change in variability may signify the early stage of intensified winter monsoons where cold wind and waters from the north may not yet have had a significant effect on SST or it may be the evolutionary link between the early development of unsaturated alkenones in coccolithophores and modern temperature control of alkenone production. We believe a long-term alkenone record is useful for further understanding of global-scale neogene cooling, the development of the East Asian monsoon system, and the evolutionary development of temperature control on alkenone unsaturation. Our data indicate that a high-resolution Uk'37 record for at least the last ~8 Ma is feasible for the northern SCS.

(Table T1) N-alkane, n-alcohol and n-fatty acid concentrations in ODP Site 184-1147 and 184-1148 sediments

Lipid compositions of sediments recovered during Ocean Drilling Program Leg 184 in the South China Sea have been identified and quantified. The identified lipids can be ascribed to terrigenous and marine sources. Terrigenous lipids are mainly C27, C29, C31 n-alkanes, C26, C28, C30 n-fatty acids, and n-alcohols, which were derived from leaf waxes of higher land plants and transported to the sea by airborne dust or fresh water. Marine lipids, mainly C37 and C38 alkenones, C30 diol, and C30 and C32 keto-ols, were from microalgae, notably haptophytes and eustigmatophytes. Elevated concentrations and accumulation rates of both terrigenous and marine lipids in the interval 202-245 meters composite depth (mcd) and 0-166 mcd were postulated to be related to the development of the East Asian monsoon at 6-8 Ma and enhanced variations of the developed East Asian monsoon after 3.2 Ma, respectively. The pronounced late Oligocene input of terrigenous lipids reflects the paleoenvironment of a newly opened, narrow basin, with restricted ocean waters and the proximity of continental runoff.