Investigating the ocean’s biological pump using thorium-234 and polonium-210

Samantha Jade Clevenger, Ph.D., 2024
Ken O. Buesseler, Advisor

Oceans play a critical role in the Earth’s ability to regulate atmospheric carbon dioxide, absorbing approximately one-third of all anthropogenically-emitted CO2. The biological pump is a mechanism which controls oceanic uptake of CO2 and sequestration of carbon into deeper waters. It is a complex web of processes starting with biota in the upper ocean which transform CO2 into particulate organic carbon (POC) via photosynthesis, a portion of which is ultimately sequestered from the atmosphere. Due to the massive scale of these processes, small changes in the strength of the biological pump can greatly impact atmospheric CO2 levels.

Tools used to quantify the flux of biologically relevant elements such as POC and biogenic silica via the biological pump are radioisotopes – naturally-occurring clocks which allow us to look back in time to study oceanic processes. In this thesis, the two radioisotope pairs ²³⁴Th/²³⁸U and ²¹⁰Po/²¹⁰Pb are utilized. Chapter 2 is an update of the ²³⁴Th/²³⁸U measurement method which simplifies the protocol, and allows for expansion of use. Chapter 3 presents an unprecedentedly extensive, non-steady state study of ²³⁴Th/²³⁸U to assess POC and biogenic silica flux through NASA’s EXPORTS project. The study involved tracking an eddy for one month in the North Atlantic Ocean to quantify export over the development of a dual-phase plankton spring bloom. In Chapter 4, this study is expanded to add a non-steady state application of the ²¹⁰Po/²¹⁰Pb tracer to look farther back in the history of POC and biogenic silica export. Extensive comparison of the two tracer systems results in new-to-the-field conclusions about the concurrent use of tracers. This thesis culminates in Chapter 5, which compares three ²³⁴Th/²³⁸U studies with community structure data to make inferences about POC export as it relates to who is present in the ecosystem. The EXPORTS North Atlantic dataset is a “strong” export endmember, WHOI’s Ocean Twilight Zone (OTZ) project offers an “intermediate” export case in the Northwest Atlantic Slope, and the EXPORTS North Pacific study is a “weak” export endmember. Taken together, this thesis greatly expands the use of ²³⁴Th/²³⁸U and ²¹⁰Po/²¹⁰Pb tracers to investigate the biological pump in diverse environments.