Controls on Deglacial Changes in Biogenic Fluxes and Authigenic Uranium in the North Pacific Ocean
Abstract
The re-organization of high-CO2 deep waters in the North Pacific Ocean may have played a crucial role in the degassing of carbon dioxide to the atmosphere during the last deglaciation. This reorganization would leave an imprint on productivity and on oxygen concentrations. We present 230Th-normalized biogenic fluxes and aU concentrations from an intermediate depth sediment core in the Northwest Pacific (RC10-196, 54.7N, 177.1E, 1007 m) and place them within the context of a synthesis of previously-published biogenic flux and aU data from the North Pacific Ocean. We evaluate several mechanisms as possible drivers of deglacial change, including changes in preservation, sediment focusing, sea ice extent, iron inputs, stratification, and circulation shifts initiated in the North Atlantic and North Pacific. Biogenic fluxes were lowest during the last glacial period (LGM, 19-23 ka). With the exception of a site at 5500 m, all sites including RC10-196, 1007 m, had higher concentrations of aU during the LGM, which implies lower pore water oxygen. Since organic carbon accumulation rates were generally lower during the LGM, these results support the idea that export production and deep-water oxygen were both reduced during the LGM in response to physical changes. Biogenic fluxes increased marginally at some locations in the NW Pacific during Heinrich Event 1 (H1; 15-18 ka) relative to the LGM. This increase may be consistent with an enhancement of ventilation to 2500-3000 m during H1, which may have helped to destratify the glacial ocean and increase nutrient delivery to surface waters. Although consistent with aU data at 2980 m water depth in the far NW Pacific, this interpretation is at odds with the relatively high concentration of aU measured during H1 at RC10-196 in the NW Pacific at 1007 m. High aU concentrations, in the absence of elevated biogenic flux, imply low oxygen bottom waters, which is inconsistent with increased ventilation. Similarly high values of aU are found at sites below 3000 m in the NW Pacific, suggesting that this area remained poorly ventilated below 1007 m during H1. Finally, biogenic fluxes reached maxima across the subarTctic North Pacific during the B/A period (15-12.5 ka). The aU was also high at all sites during the B/A, but in this case elevated organic carbon rain is the likely cause. We argue that paleo-observations are most consistent with ventilation increases in the North Atlantic (during B/A) as a primary driver of increases in biogenic flux during the deglaciation, as they were likely to bring nutrients to the surface via increased vertical mixing and shoaling of the global thermocline.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMPP41C1786K
- Keywords:
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- 4900 PALEOCEANOGRAPHY;
- 4912 PALEOCEANOGRAPHY / Biogeochemical cycles;
- processes;
- and modeling;
- 4926 PALEOCEANOGRAPHY / Glacial;
- 4999 PALEOCEANOGRAPHY / General or miscellaneous