Multiple monsoon-controlled breakdown of oxygen-minimum conditions during the past 30,000 years documented in laminated sediments off Pakistan

Late Holocene laminated sediments from a core transect centred in the oxyge n minimum zone (OMZ) impinging at the continental slope off Pakistan indica te stable oxygen minimum conditions for the past 7000 calendar years. High SW-monsoon-controlled biological productivity and enhanced organic matter p reservation during this period is reflected in high contents of total organ ic carbon (TOC) and redox-sensitive elements (Ni, V), as well as by a low-d iversity, high-abundance benthic foraminiferal Buliminacea association and high abundance of the planktonic species Globigerina bulloides indicative o f upwelling conditions. Surface-water productivity was strongest during SW monsoon maxima. Stable OMZ conditions (reflected by laminated sediments) we re found also during warm interstadial events (Preboreal, Bolling-Allerod, and Dansgaard-Oeschger events), as well as during peak glacial times (17-22 .5 ka, all ages in calendar years). Sediment mass accumulation rates were a t a maximum during the Preboreal and Younger Dryas periods due to strong ri verine input and mobilisation of fine-grained sediment coinciding with rapi d deglacial sea-level rise, whereas eolian input generally decreased from g lacial to interglacial times. In contrast, the occurrence of bioturbated in tervals from 7 to 10.5 ka (early Holocene), in the Younger Dryas (11.7-13 k a), from 15 to 17 ka (Heinrich event 1) and from 22.5 to 25 ka (Heinrich ev ent 2) suggests completely different conditions of oxygen-rich bottom water s, extremely low mass and organic carbon accumulation rates, a high-diversi ty benthic fauna, all indicating lowered surface-water productivity. During these intervals the OMZ was very poorly developed or absent and a sharp fa ll of the aragonite compensation depth favoured the preservation of pteropo ds. The abundance of lithogenic proxies suggests aridity and wind transport by northwesterly or northeasterly winds during these periods coinciding wi th the North Atlantic Heinrich events and dust peaks in the Tibetan Loess r ecords. The correlation of the monsoon-driven OMZ variability in the Arabia n Sea with the rapid climatic fluctuations in the high northern latitudes s uggests a close coupling between the climates of the high and low latitudes at a global scale. (C) 1999 Elsevier Science B.V. All rights reserved.