Search

A high-resolution multi-proxy reconstruction of the Arabian Sea Summer Monsoon (ASSM) intensity over the past 90,000 years has been determined using two marine sediment cores: one from the Somali margin and one from the Indian margin. This reconstruction indicates that changes in monsoon- induced upwelling, primary productivity and denitrification have varied in synchrony with Dansgaard-Oeschger (D-O) cycles. Increased monsoon intensity correlates with warm climate events (interstadials) and decreased monsoon intensity, which coincides with stadials and Heinrich Events, is confirmed by elevated dust concentrations in the marine cores. A comparison of the Somali and Indian margin cores with previously reported studies from the Northern and Western Basin allows the identification of discrete sediment signals from the Indus River, the Arabian Peninsula and from local riverine runoff. Sedimentary deposition on the Indian margin during interglacials is dominated by local terrestrial runoff, whereas during glacial periods increased dust input from the Arabian Peninsula is evident. Both signals are related to changes in the intensity of the ASSM. Monsoon intensity has decreased during the Holocene as the Intertropical Convergence Zone (ITCZ) has moved to a more southerly position. The ASSM-ITCZ relationship (increased ASSM intensity and a northern ITCZ, decreased ASSM intensity and a southern ITCZ) has remained consistent over the last glacial cycle suggesting that global millennial scale climatic variability is in part driven by modulations in tropical hydrological cycle. This ASSM reconstruction provides evidence that rearrangements in the tropical convection system affected atmospheric dust concentrations as well as the concentration and location of atmospheric water vapour. In addition to modulating terrestrial and marine emissions of greenhouse gases, variation in the tropical hydrological cycle provides a mechanism of amplifying and perpetuating millennial-scale climatic changes.

Short-term fluctuations in the bottom water oxygen content of Santa Barbara Basin have been previously recognized from variations in a sedimentary bioturbation index (Behl and Kennett, 1996). A correlation between such anoxic events in the basin and Dansgaard-Oeschger interstadials, as measured from δ180ice in Greenland ice cores, was then used by the same authors to relate variations in bottom water oxygenation to high-frequency changes in the ventilation of the Santa Barbara Basin, driven by pan-hemispheric changes in climate transmitted through the atmosphere. An additional control on the oxygen content at depth is the local settling flux of metabolizable organic matter. Trace metal measurements from closely-spaced sediment samples are used here to distinguish oxygen depletion resulting from local increases in export production from oxygen depletion introduced by the importation of O2-poor water. Molybdenum, Re, U, Cd, and Ag and interelement ratios are used in conjunction with organic carbon concentrations, opal fluxes, and δ15N measurements to deduce into past redox conditions of the basin, variations in the depth of the redox boundary and the flux of organic material to the basin floor. Comparisons between ODP Holes 893A (Santa Barbara Basin), 1019 and 1017 (both California margin) allows one to distinguish of regional signals dominated by ventilation changes from local signals dominated by vertical organic flux. During the Holocene, variations in productivity appear indeed to have impacted the oxygen content of the Santa Barbara Basin bottom waters. However, anoxic events concurrent with the Bolling-Allerod and during the last glacial interval are regional events associated with intermediate water mass characteristics.