HR: 09:30h
AN: PP41A-07    [PDF]
TI: Enhanced Global Marine Denitrification Records the Initiation of the Last Deglaciation
AU: Altabet, M A
EM: maltabet@umassd.edu
AF: School for Marine Science and Technology, University of Massachusetts Dartmouth, 706 South Rodney French Boulevard, New Bedford, MA 02744 United States
AU: * Higginson, M J
EM: mhigginson@umassd.edu
AF: School for Marine Science and Technology, University of Massachusetts Dartmouth, 706 South Rodney French Boulevard, New Bedford, MA 02744 United States
AB: Denitrification is the predominant global loss term for combined nitrogen and can exert a major control on its oceanic inventory, as well as global productivity and atmospheric CO$_{2}$. It typically occurs in organic-rich continental margin sediments and in intermediate waters within intense oxygen-minimum zones when bacteria utilize NO$_{3}^{-}$ as an electron acceptor and in doing so convert it primarily to N$_{2}$ gas. Denitrification strongly fractionates nitrogen isotopes, leaving the remaining NO$_{3}^{-}$ enriched in $\delta^{15}$N. A paleoceanographic record for denitrification intensity is created when $\delta^{15}$N-enriched NO$_{3}^{-}$ is transported to surface waters, consumed by phytoplankton, transported downward with organic matter and preserved in the sediments. When there is good to excellent preservation of organic matter at the sea floor due to either suboxic bottom water and/or high sediment accumulation rates, sediment $\delta^{15}$N faithfully records the $\delta^{15}$N of sinking organic matter. Marine water-column denitrification is principally concentrated in three areas: the Arabian Sea, the Eastern Tropical North Pacific and Eastern Tropical South Pacific. The advent of well-dated, high-resolution records of sedimentary $\delta^{15}$N generated by us from each of these regions now facilitates comparison of the relative timing of their large changes in denitrification since the last glacial maximum ($\sim$25 Ka). By assigning a hypothetical equal contribution to global marine denitrification from well-constrained records from each region, we have calculated a marine denitrification composite (MDC) record which strongly resembles the Taylor Dome ice core Antarctic CO$_{2}$ record. Such early and coincidental changes in $\delta^{15}$N and CO$_{2}$ prompt us to hypothesize that changes in the inventory of marine nitrate caused by increased denitrification subsequently altered global marine carbon sequestration, especially in the vast oligotrophic regions of the oceans, at a time of concurrent reductions in atmospheric dust inventory. Since the process of denitrification is accelerated by low oxygen conditions, it is sensitive to both remote ventilation rates at the source of intermediate water and in situ respiration rates during the sinking of organic matter. We note that the timing of increased denitrification during deglaciation is earliest in the ETSP, followed by a peak in the ETNP, with maximum $\delta^{15}$N recorded in the Arabian Sea approximately 2.9 Kyr later. The ventilation of intermediate water originates in the Southern Ocean and is advected simultaneously to the denitrification regions. Therefore, regional processes such as export production must primarily determine denitrification rates, modulated by preconditioning of intermediate water masses. We conclude that although the initial trigger for deglaciation may lie with SST and ice-cover changes in the Southern Ocean, its progress to interglacial conditions was determined by regional effects such as rates of upwelling driven by low-latitude Walker/Hadley Circulation and the monsoon at each location.
UR: http://www.smast.umassd.edu/cmastweb/biohigginson.html
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1620 Climate dynamics (3309)
DE: 3339 Ocean/atmosphere interactions (0312, 4504)
DE: 3374 Tropical meteorology
DE: 4267 Paleoceanography
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting