Contributions to bulk sedimentary nitrogen isotope variability in the Eastern Equatorial Pacific
Abstract
The surface waters of the Eastern Equatorial Pacific contain a standing nitrate pool due to iron limitation. As a consequence, nitrogen isotopes records from sediments underlying the Eastern Equatorial Pacific have been interpreted as an archive of relative nitrate utilization in the upwelling region and thus, a potential record of changing Fe availability. The nitrogen isotopic composition of the upwelled nitrate was assumed to be constant on glacial-interglacial timescales. However, some proportion of the nitrate in the Eastern Equatorial Pacific passes through the oxygen minimum zone of the Eastern Tropical South Pacific. Active water column denitrification in the oxygen minimum likely imparts a large isotopic signature upon the advecting nitrate pool. Here, a high resolution, well-dated sedimentary N isotope record from Eastern Equatorial Pacific site ODP 1240 is compared to records from the Eastern Pacific margins where variation in sedimentary 15N/14N is primarily attributed to changes in regional denitrification rates. Similarity in the magnitude and timing of the N isotope variations suggest that the large-scale glacial-interglacial pattern observed in the sedimentary 15N/14N of Eastern Equatorial Pacific sediments reflects variation in regional denitrification rates rather than large-scale changes in demand for nutrients driven by fluctuating availability of iron. Underlying the glacial- scale variations in the Eastern Equatorial Pacific is a longer timescale shift in sedimentary 15N/14N that cannot be directly attributed to regional changes in denitrification rate. This shift may reflect an important biogeochemical change within the Equatorial Pacific however, diagenetic artifacts must first be ruled out.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFMPP32B..08R
- Keywords:
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- 4912 Biogeochemical cycles;
- processes;
- and modeling (0412;
- 0414;
- 0793;
- 1615;
- 4805);
- 4926 Glacial;
- 4964 Upwelling (4279)