Date of Award

Winter 2020

Project Type

Thesis

Program or Major

Oceanography

Degree Name

Master of Science

First Advisor

Robert T Letscher

Second Advisor

Kai Ziervogel

Third Advisor

Thomas Lippmann

Abstract

Preformed nitrate (PreNO3) was formulated to act as a conservative tracer of oceancirculation after accounting for the stoichiometry of marine biochemical reactions involving oxygen and nitrate. However, PreNO3 anomalies have been identified within the shallow subtropical ocean, that describe the biological consumption or production of oxygen without assumed stoichiometric changes in nitrate, which still have yet to be fully explained. The mechanisms proposed to drive the formation of PreNO3 anomalies are: vertically migrating phytoplankton (VMP), the export and subsequent remineralization of N-deficient transparent exopolymer particles (TEP, e.g. marine gels), and the remineralization of N-poor dissolved organic matter (DOM). Observations from the subtropical Pacific and Atlantic oceans taken by twenty Biogeochemical Argo (Bio-Argo) profiling floats which collectively span from 2007 to 2019 have been used to answer two questions related to PreNO3 anomalies. What is the seasonality and geographic extents of subsurface negative PreNO3 anomalies and euphotic zone positive PreNO3 anomalies in the global subtropical ocean? What biogeochemical processes capable of generating PreNO3 anomalies are consistent with the seasonality and spatial extents found in the Bio-Argo float records? Euphotic zone positive PreNO3 anomalies are consistently observed between 30˚S and 50˚N, whereas subsurface negative PreNO3 anomalies are absent in parts of the Equatorial West Pacific Ocean. The remineralization of N-poor DOM has been shown to contribute to PreNO3 anomaly formation by previous studies and has been accounted for in this study. The geographic extents of the remaining PreNO3 anomalies are consistent with previous observations of VMP taxa throughout the global subtropical ocean. The seasonal timing of euphotic zone positive PreNO3 anomalies in relation to subsurface negative PreNO3 anomalies indicates that both VMP and the export and remineralization of N-deficient TEP contribute to the formation of PreNO3 anomalies. Since VMP include large diatoms that produce ballasted organic matter while TEP may only sink slowly before remineralization, further investigation into the mechanisms generating PreNO3 anomalies is needed to assess their roles in the future biological carbon pump in an expanding subtropical ocean ecosystem.

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