Dissertations, Theses, and Capstone Projects

Date of Degree

9-2019

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor

Gillian Stewart

Committee Members

Haydee Salmun

Gary Hemming

J. Kirk Cochran

Phoebe J. Lam

Pere Masqué

Subject Categories

Oceanography

Keywords

210Po, 210Pb, Distribution, Fractionation, Application, GEOTRACES, GA03, GP16, GA01

Abstract

The sinking flux of particles is an important pathway for the removal of carbon and other elements from the surface ocean via the biological pump. The 210Po/210Pb disequilibrium method can be used to study particle export at high spatial resolution over the time scale of months. The distribution of 210Po and 210Pb activity was measured during two GEOTRACES transects of the North Atlantic and one GEOTRACES transect of the eastern tropical South Pacific. This dissertation aimed to advance the knowledge and improve methods of the application of the 210Po/210Pb pair to quantify particle export in a variety of marine environments.

First, we describe the influence of particle concentration and composition on the fractionation of 210Po and 210Pb in the subtropical North Atlantic (Chapter 2). There was a strong relationship between 210Po and both CaCO3 and particulate organic carbon (POC), as well as between 210Pb and both opal and lithogenic material. We developed an end-member mixing model to predict the bulk partition coefficient (Kd) for 210Po and 210Pb. The model could accurately predict the Kd of 210Pb but could not predicate the Kd of 210Po with as much accuracy, indicating that some important scavenging agent or process for 210Po were missing from the model. These results support the known complicated cycling of 210Po in particles due to its bio-reactive behavior.

Second, we investigated the relationship between the particulate 210Po/210Pb activity ratio (AR) and apparent oxygen utilization (AOU) in the subpolar North Atlantic (Chapter 3). Two distinct relationships suggested the source of the particles (e.g., fresh particles vs. refractory particles), and the main biogeochemical processes (e.g., respiration, remineralization, oxidation) and time that were governing the 210Po/210Pb AR in the particles. This result may provide insights into the missing scavenging process for 210Po in the end-member mixing model that have been proposed in the subtropical North Atlantic.

Third, we calculated the export flux of particulate organic carbon (POC) from the upper water of the subpolar North Atlantic by using the 210Po/210Pb disequilibrium method (Chapter 4). We then compared the POC flux derived from the 210Po/210Pb pair to the POC flux derived from the 234Th/238U pair. POC fluxes estimated from the two pairs agreed within a factor of 3 and the differences were attributed to integration timescales of the two nuclide pairs and the history of bloom events.

Lastly, we evaluated the assumptions of the traditional 1-D steady-state 210Po/210Pb export model across the three GEOTRACS transects (Chapter 5). The validation of each assumption for the individual transects was studied, and some possible bias introduced by the assumptions was discussed, resulting in recommendations for both sampling and model construction.

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