Diapycnal dissolved organic matter supply into the upper Peruvian oxycline.

Loginova, Alexandra N , Thomsen, Sören , Dengler, Marcus , Lüdke, Jan and Engel, Anja (2019) Diapycnal dissolved organic matter supply into the upper Peruvian oxycline. Open Access Biogeosciences (BG), 16 (9). pp. 2033-2047. DOI 10.5194/bg-16-2033-2019.

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Abstract

The eastern tropical South Pacific (ETSP) hosts the Peruvian upwelling system, which represents one of the most productive areas in the world ocean. High primary production followed by rapid heterotrophic utilization of organic matter supports the formation of one of the most intense oxygen minimum zones (OMZs) in the world ocean, where dissolved oxygen (O2) concentrations reach less than 1 µmol kg−1. The high productivity leads to an accumulation of dissolved organic matter (DOM) in the surface layers that may serve as a substrate for heterotrophic respiration. However, the importance of DOM utilization for O2 respiration in the Peruvian upwelling system in general and for shaping the upper oxycline in particular remains unclear so far. This study reports the first estimates of diapycnal fluxes and supply of O2, dissolved organic carbon (DOC), dissolved organic nitrogen, dissolved hydrolysable amino acids (DHAA) and dissolved combined carbohydrates (DCCHO) for the ETSP off Peru. Diapycnal flux and supply estimates were obtained by combining measured vertical diffusivities and solute concentration gradients. They were analysed together with the molecular composition of DCCHO and DHAA to infer the transport of labile DOM into the upper OMZ and the potential role of DOM utilization for the attenuation of the diapycnal O2 flux that ventilates the OMZ. The observed diapycnal O2 flux (50 mmol O2 m−2 d−1 at maximum) was limited to the upper 80 m of the water column; the O2 supply of ∼1 µmol kg−1 d−1 was comparable to previously published O2 consumption rates for the North and South Pacific OMZs. The diapycnal DOM flux (31 mmol C m−2 d−1 at maximum) was limited to ∼30 m water depth, suggesting that the labile DOM is extensively consumed within the upper part of the shallow oxycline off Peru. The analyses of DCCHO and DHAA composition support this finding, suggesting that DOM undergoes comprehensive remineralization within the upper part of the oxycline, as the DOM within the core of the OMZ was found to be largely altered. Estimated by a simple equation for carbon combustion, aerobic respiration of DCCHO and DHAA, supplied by diapycnal mixing (0.46 µmol kg−1 d−1 at maximum), could account for up to 38 % of the diapycnal O2 supply in the upper oxycline, which suggests that DOM utilization plays a significant role for shaping the upper oxycline in the ETSP.

Document Type: Article
Research affiliation: OceanRep > SFB 754 > B9
OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-PO Physical Oceanography
OceanRep > SFB 754
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-BI Biological Oceanography
Refereed: Yes
Open Access Journal?: Yes
DOI etc.: 10.5194/bg-16-2033-2019
ISSN: 1726-4170
Projects: SFB754, SPP 1403
Expeditions/Models/Experiments:
Date Deposited: 21 May 2019 09:44
Last Modified: 01 Jul 2019 12:23
URI: http://oceanrep.geomar.de/id/eprint/46761

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