Millennial-scale oscillation of denitrification intensity in the Arabian Sea during the late Quaternary and its potential influence on atmospheric N2O and global climate

The intensity of denitrification in the Arabian Sea during the last 65 kyr is reconstructed using high-resolution delta N-15 records of three sediment cores in conjunction with other geochemical tracers for water column oxyge nation and productivity. The results reveal a close link to the Greenland i ce core record with low or absent water column denitrification during the L ast Glacial Maximum, the stadials, and at the time of the Heinrich Events i ncluding the Younger Dryas. In contrast, denitrification was high during th e Holocene and the interstadials. The intensification of denitrification is related to stronger SW monsoonal upwelling, which enhances organic matter flux and degradation, resulting in a strengthening of the midwater oxygen d eficiency, Such a combination of enhanced upwelling and denitrification has also been implied for the Eastern Tropical North Pacific (ETNP), where the se events occur during the Holocene and to some extent during the interstad ials, too. Today, the Arabian Sea and the ETNP together contribute substant ially to the global marine water column denitrification, and a significant fraction of the ocean-atmosphere N2O flux originates from these areas. Chan ges in N2O emissions from these areas could thus have effected the recently described stadial/interstadial variations in the atmospheric concentration of this greenhouse gas as deduced from ice cores. Moreover, denitrificatio n is the major sink for oceanic nitrate and provides a primary control for the oceanic nutrient inventory, which in turn influences global primary pro ductivity and CO2 sequestration by the biological pump. Short-term switches between a nondenitrification mode and a denitrification mode in these mari ne regions therefore have an impact on global climate.