Glacial‐interglacial variability in denitrification in the World's Oceans: Causes and consequences

The late Quaternary history of water‐column denitrifcation of the eastern Pacific margins and the Arabian Sea is reconstructed using sedimentary δ 15 N measurements. The δ 15 N values in six piston cores raised from these regions show remarkably similar cyclic variations, being heavy (9–10.5‰) during the interglacials and 2–3‰ lighter during the glacials. This implies that denitrification in these regions decreased substantially during the glacial periods. The glacial decline in denitrification is attributed to reduced upwelling and flux of organic material through the oxygen minimum zone. Since water‐column denitrification in these areas accounts for about half of the fixed‐nitrogen loss in the modern ocean, the inferred decrease in denitrification should have increased the oceanic nitrate inventory during glacial periods. Because nitrate is a limiting nutrient, oceanic productivity and attendant changes in CO 2 may therefore have been modulated on glacial‐interglacial timescales by variations in the oceanic NO 3 content.