1. School for Marine Science and Technology, University of Massachusetts Dartmouth, 706 S. Rodney French Boulevard, New Bedford, Massachusetts 02744-1221, USA
2. Center for Environmental Studies, Box 1943, Brown University, Providence, Rhode Island 02912, USA

Correspondence to: Mark A. Altabet¹ Correspondence and requests for materials should be addressed to M.A.A. (e-mail: Email: maltabet@umassd.edu).

Most global biogeochemical processes are known to respond to climate change, some of which have the capacity to produce feedbacks through the regulation of atmospheric greenhouse gases¹. Marine denitrification—the reduction of nitrate to gaseous nitrogen—is an important process in this regard, affecting greenhouse gas concentrations directly through the incidental production of nitrous oxide, and indirectly through modification of the marine nitrogen inventory and hence the biological pump for CO₂. Although denitrification has been shown to vary with glacial–interglacial cycles^{2, 3}, its response to more rapid climate change has not yet been well characterized. Here we present nitrogen isotope ratio, nitrogen content and chlorin abundance data from sediment cores with high accumulation rates on the Oman continental margin that reveal substantial millennial-scale variability in Arabian Sea denitrification and productivity during the last glacial period. The detailed correspondence of these changes with Dansgaard–Oeschger events recorded in Greenland ice cores⁴ indicates rapid, century-scale reorganization of the Arabian Sea ecosystem in response to climate excursions, mediated through the intensity of summer monsoonal upwelling. Considering the several-thousand-year residence time of fixed nitrogen in the ocean, the response of global marine productivity to changes in denitrification would have occurred at lower frequency and appears to be related to climatic and atmospheric CO₂ oscillations observed in Antarctic ice cores between 20 and 60 kyr ago⁵.