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The effect of millennial-scale changes in Arabian Sea denitrification on atmospheric CO2


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 cycles2,3, 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.

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Figure 1: Location of sediment cores on the Oman margin, and (inset) the location of the Oman margin within the Arabian Sea.
Figure 2: Time series of sediment-core data along with the GISP2 δ18O record for the past 65,000 years.
Figure 3: The 3-kyr moving average of the δ15N record compared to Antarctic ice-core records of climate and atmospheric CO2 (ref. 5).


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We thank R. Singh and X. Huang for technical assistance. Sediment core material was obtained from the collection and curating facilities of the Lamont–Doherty Earth Observatory core collection with support from the US NSF and ONR. We thank S. Clemens, L. Labeyrie and J. Kennett for discussions. This work was supported by the US NSF ESH program and the Camille and Henry Dreyfus Foundation.

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Correspondence to Mark A. Altabet.

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Altabet, M., Higginson, M. & Murray, D. The effect of millennial-scale changes in Arabian Sea denitrification on atmospheric CO2. Nature 415, 159–162 (2002).

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