ALBERT

Description: The modern Indian Ocean summer monsoon is driven by differential heating between the Asian continent and the Indian Ocean to the south. This differential heating produces a strong pressure gradient which drives southwest monsoon winds during June, July, and August. Satellite and meteorological observations, aerosol measurements, sediment trap studies, and mineralogical studies indicate an atmospheric mode of transport for modern lithogenic sediments in the northwest Arabian Sea. Analyses of lithogenic grain size and mass accumulation rate (MAR) records from the Owen Ridge indicate that eolian transport has been the primary mode of transport for the past 370 kyr. Visual inspection shows that the MAR record is positively correlated with global ice volume as indicated by the marine delta18O record. In contrast, the grain-size record varies at a much higher frequency, showing little correlation to either the MAR or the delta18O records. Spectral analyses confirm these relationships, indicating that the lithogenic grain-size and MAR records are coherent only over the precession band whereby the grain size leads the MAR by 124° (~8 kyr). We conclude that an eolian transport mechanism is the only mechanism that allows for this phase difference and at the same time is supported by comparison of the grain size and MAR with independent eolian records. We use lithogenic grain size as a paleoclimatic indicator of summer monsoon wind strength and lithogenic MAR as a paleoclimatic indicator of source-area aridity. These interpretations are supported by comparison of the lithogenic records to independent indicators of wind strength (Globigerina bulloides upwelling record) and aridity (a loess record from central China). Such comparisons indicate high coherence and zero phase relationships. Our work supports the findings of previous studies which have documented the link between monsoon strength and the Earth's axial precession cycles. Both the lithogenic MAR and the grain-size records have high coherency with precessional insolation. Maximum lithogenic MAR (source-area aridity) is in phase with delta18O (global ice volume) and leads maximum precessional insolation by 88° (~6 kyr). We attribute this lead to the influence of glacial conditions on the aridity, and therefore the deflation potential, of the source areas. Maximum lithogenic grain size (summer monsoon wind strength) lags maximum precession by 148° (~9 kyr). We attribute this lag both to the influence of global and/or local ice volume and to the availability of latent heat from the southern hemisphere Indian Ocean, the two of which combine to determine the strength of the Indian Ocean monsoon.

Description: The orbital-scale timing of South Asian monsoon (SAM) precipitation is poorly understood. Here we combine Mg/Ca measurements (a proxy for the temperature prevailing during calcification) and oxygen isotope (δ18O) analyses of the calcite shells of mixed-layer dwelling planktic foraminifera Globigerinoides sacculifer (present all year round in the Bay of Bengal26) in core NGHP 17 (Ref 27) to generate a unique orbital scale SAM precipitation record for the last ~1 million years.

Description: The 87Sr/86Sr ratio of sea water has increased gradually over the past 40 Myr, suggesting a concomitant increase in global chemical weathering rates (Raymo et al., 1988, doi:10.1130/0091-7613(1988)016〈0649:IOLCMB〉2.3.CO;2; Capo and DePaolo, 1990, doi:10.1126/science.249.4964.51; Hodell et al., 1990, doi:10.1016/0168-9622(90)90011-Z; Raymo and Ruddiman, 1992, doi:10.1038/359117a0; Caldeira, 1992, doi:10.1038/357578a0; Palmer and Edmons, 1992, doi:10.1016/0016-7037(92)90332-D). Recently, Dia et al. (1992, doi:10.1038/356786a0) analysed a 250-kyr 87Sr/86Sr record, and found superimposed on this gradual increase higher-frequency 87Sr/86Sr variations which appeared to follow a 100-kyr cycle; this periodicity corresponds to one of the prominent cycles in the Earth's orbital parameters, which are known to modulate the patterns of solar insolation and hence climate (Berger, 1978, doi:10.1016/0033-5894(78)90064-9; 1989, doi:10.1016/1040-6182(89)90016-5; Imbrie et al., 1992, doi:10.1029/92PA02253). The resolution of this record was, however, insufficient to establish the phase relationship between the 87Sr/86Sr variations and global climate cycles. Here we present a high-resolution seawater 87Sr/86Sr record spanning the past 450 kyr. We find that maxima and minima in 87Sr/86Sr coincide with minima and maxima, respectively, in continental ice volume (from the SPECMAP oxygen isotope record (Imbrie et al., 1984)), apparently suggesting that there was less chemical weathering in arid glacial periods than in the more humid interglacials. During glacial-interglacial transitions, however, seawater 87Sr/86Sr changes at a rate of ~1 p.p.m./kyr, approximately three times that evaluated by Dia et al. (1992, doi:10.1038/356786a0). Mass-balance calculations illustrate that simple changes in modern chemical weathering regimes cannot fully account for such rapid changes, suggesting that we need to revise current ideas about strontium reservoirs and the mechanisms for exchange between them.