The Inorganic Geochemical Record of the Northwest Arabian Sea: A History of Productivity Variation over the Last 400 k.y. from Sites 722 and 724

Abstract

High-resolution sampling from late Pleistocene (last 400 k.y.) sediments of Site 722 (upper 16 m) and Site 724 (upper 70 m), and subsequent inorganic geochemical analysis, has defined the history of productivity in the northwest Arabian Sea. Eolian dust input from the Arabian Peninsula and Somalia is characterized by the record of Ti/Al and Cr/Al. This dust record displays strong precessional periodicity (cycles at 25 k.y.) suggesting the Southwest Monsoon and associated winds play a key role in transporting terrigenous material from the land. High biological productivity results in the accumulation of biogenic CaCO 3 and opal in the sediments, the latter having an unexpectedly minor contribution to the total mass flux. Due to dilution of the CaCO 3 record by the terrigenous component, the record of biological productivity is best exemplified by Ba. Its record, together with that of other metals recording biological association and redox variability (Cu, Ni, Zn, V, U) clearly identifies the interglacial episodes as being more biologically productive. The striking agreement between Ba and the δ 18 θ record in planktonic foraminifers suggests that the supply of nutrients during these periods of high productivity is linked to ocean-wide changes in ocean fertility, and not just local upwelling conditions. High levels of phosphate accumulation in interglacial sediments is attributed to both diagenetic phosphorite formation and biogenic skeletal debris. This study provides a detailed record of productivity variation in the northwest Arabian Sea during the late Pleistocene.

Paleoclimatic and diagenetic history of the late quaternary sediments in a core from the Southeastern Arabian Sea: Geochemical and magnetic signals

“…3). The planktonic δ 18 O excursion to low values correlates well with the increase in Ba/Al ratio, a feature also observed by several others in sediment cores from different parts of the Arabian Sea (see Shimmield and Mowbray, 1991;Reichard, et al 1997;Shenegar et al, 2000;Pattan et al, 2003). As Ba, excess Ba or Ba/Al ratio have been used as a proxy for productivity in the Arabian Sea, the elevated Ba/Al ratio has been attributed to high productivity due to increased upwelling and/or increased runoff and nutrient supply during interglacial periods (see Weedon and Shimmield, 1991;Shimmield, 1992;Reichard et al, 1997;Shenetgar et al 2000).…”

“…Following the seminal works of other investigators on the geochemistry of the sediments of the Arabian Sea (Hermelin and Shimmield, 1990;Shimmield and Mowbray, 1991;Reichart et al, 1997;Shenetger et al, 2000;Van der Weijden et al, 2006) …”

“…Schnetger et al (2000) showed that the total organic carbon is not a tracer, but Ba/Al serves as a proxy for paleoproductivity in the sediments of deep Arabian Sea. Sediment trap experiments showed that Ca, Sr and Ba associate with biogenic particles and, particulate organic matter is an effective scavenger for metals such as Cu, Ni, Zn and V, whose concentrations are controlled by seasonally driven changes in primary productivity (Jickells et al, 1984;Shimmield and Mowbray, 1991;Schnetger et al, 2000). Ba in the offshore precipitates as barite by the living phytoplankton cells and thus barite content in sediments is a function of biological productivity.…”

“…For more than two decades, numerous sedimentary records have been investigated from its northwestern and northeastern margins and deep Arabian Sea (Van Campo et al, 1982;Clemens and Prell, 1990;Shimmield et al, 1990;Shimmield and Mowbray 1991;Prell et al, 1992;Sirocko et al, 1996Sirocko et al, , 2000Reichart et al, 1997;Schulz et al, 1998;von Rad et al, 1999;Shenetger et al, 2000). The records reveal the history of upwelling related productivity driven by the monsoons.…”

Metals in Sediment Cores from Nine Coastal Lagoons in Central Vietnam

“…Finally, factor 4 (7.28% of the variance) is controlled by silt and negatively correlated with geometric sorting, thus indicating the influence of course content patterns. The geochemistry of Central Vietnam is dominated by calc-alkaline rocks with high grade metamorphism (Al-Shawi and Dahl, 1999;Thuy et al, 2000) that can evidently influence the geochemistry of major elements in sediments (Shimmield and Mowbray, 1991). Actually, the good relation Al versus fines or clay content (Table 3) and many metals (Table 2) confirm the hypothesis of a prevailing alumina-silicate input into four of the nine lagoons (AK, NM, CR, TN).…”