Does N2 Fixation in the Oligotrophic SE Pacific Influence N Isotopic Signals in the Peru-Chile OMZ?

Altabet, M. A., Ryabenko, Evgeniya and Wallace, Douglas W.R. (2010) Does N2 Fixation in the Oligotrophic SE Pacific Influence N Isotopic Signals in the Peru-Chile OMZ? [Talk] In: AGU Fall Meeting 2010, 13.-17.12.2010, San Francisco, USA.

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residual nitrate. Through upwelling, phytoplankton assimilation, and downward particle flux, this signal is transferred to the underlying sediments and has been used to reconstruct past changes in denitrification and OMZ intensity in relation to climate change. However, there remain a number of impediments to quantitative interpretation of downcore δ15N records from OMZ’s with respect to past magnitude of N loss. One of these is knowledge of initial δ15N for nitrate prior to denitrification which cannot be assumed to be the modern oceanic average. In the case of the Peru-Chile OMZ, δ15N for nitrate in source waters from the Equatorial Undercurrent average 7‰ as compared to the oceanic average of ~5‰. This suggests processes external to the OMZ leading to isotopic enrichment such as partial phytoplankton nitrate utilization in the Subantarctic water mass formation region. In contrast, it has been surmised that initial δ15N for OMZ denitrification could be relatively low as a result of N2 fixation in geographically adjacent oligotrophic regions. High N2 fixation rates in the SE Pacifc Gyre are thought to be stimulated by low N/P waters upwelled from the Peru-Chile OMZ. As seen in the Sargasso Sea, subsurface remineralization of export production influenced by near-surface N2 fixation produces 15N-depleted nitrate in the subtropical mode water and the upper thermocline. Potentially, these SE Pacific gyre waters could mix back into the Peru-Chile OMZ at its southern boundary. We have investigated the relevance of such a phenomenon in the SE Pacific by examining samples collected during two CLIVAR repeat section (P6 and P18) which transected these highly oligotrophic water adjacent to the Peru-Chile OMZ in both the N-S and Ε-W directions. Surprisingly, nutrient data shows no positive subsurface nitrate anomaly as would be expected from significant rates of N2 fixation. Nitrate isotope data also shows no evidence of depleted values. In fact nitrate δ15N and δ18O in the upper 500 m are elevated (up to 10‰) and appear to be the result of subduction and advection of nitrate partially consumed at the surface in the vicinity of the subtropical front. At present, N2 fixation in the SE Pacific does not appear to be an important biogeochemical process and cannot be contributing a 15N-depleted signal to the Peru-Chile OMZ

Document Type: Conference or Workshop Item (Talk)
Keywords: Marine chemistry; Biogeochemistry; Isotopic composition and chemistry; Nitrogen cycling; Paleoclimatology and paleoceanography; Biogeochemical cycles, processes, and modeling; Paleoceanography and Paleoclimatology
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-CH Chemical Oceanography
Date Deposited: 02 Nov 2011 08:06
Last Modified: 23 Feb 2012 06:36

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