Nutrient conditions in the North Atlantic during the last glacial period
Abstract
One proposed mechanism for lower atmospheric CO2 during past glacial periods is reduced overturning in the Southern Ocean, which reduces the leakage of deeply sequestered CO2 back into the atmosphere from this region. This mechanism relies on the assumption that nutrient consumption in the North Atlantic surface has remained relatively constant across glacial/interglacial cycles. Due to isotope fractionation during nitrate assimilation, nitrate in the sunlit surface waters is progressively elevated in 15N/14N as it is consumed by phytoplankton. The 15N/14N of organic matter bound within the walls of planktonic foraminiferal tests covary with the 15N/14N of nitrate consumed in surface waters across the global ocean and is thus expected to record past changes in nutrient consumption in high latitude surface waters, an assertion currently being tested with core top measurements. We have measured a down-core record of foraminifera-bound 15N/14N in the subpolar North Atlantic in an effort to reconstruct nutrient conditions in this region during the last glacial period. The foraminifera species N. pachyderma (sin.) was picked from regular depth intervals in core VM28-73 (57.2°N, 20.9°W). After chemical cleaning of the tests, the test-bound organic N was released by acid digestion of the tests, oxidized to nitrate with potassium persulfate, converted bacterially to nitrous oxide, and finally analyzed on an isotope ratio mass spectrometer. Foraminifera-bound 15N/14N is generally lower during the last glacial period than during the Holocene, suggesting less complete nutrient consumption during the former. However, at least two single-point maxima in 15N/14N are observed within the glacial section; these coincide with distinct sedimentological changes in the core and may be associated with Heinrich events. Full interpretation of the data awaits an improved age model for the core as well as a suite of ancillary measurements.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.B41B0213T
- Keywords:
-
- 0428 BIOGEOSCIENCES / Carbon cycling;
- 0469 BIOGEOSCIENCES / Nitrogen cycling;
- 0473 BIOGEOSCIENCES / Paleoclimatology and paleoceanography