Marine thorium and protactinium distributions: Tools for past and present chemical flux
Adsorption to sinking particulate matter, known as scavenging, is an important yet elusive term in the budgets of many bioactive and contaminant trace metals in the ocean. Scavenging is also involved in the cycling of other trace metals used in ocean sediments to learn about ocean processes, such as circulation rates or biological productivity, of the geologic past. This dissertation is an oceanographic exploration of the naturally-occurring radionuclides 230Th, 232Th and 231Pa aimed to better understand how their scavenging behavior can be utilized to trace chemical flux in the modern and the past ocean. While 230Th and 231Pa are produced by the radioactive decay of uranium dissolved in seawater, 232Th is added to the ocean via the input of continental material. Removal of Th by scavenging should be equivalent for both Th isotopes based on chemical principles. I use measured 232Th inventories from the North Pacific water column and apply a removal rate based on 234U:230Th disequilibria to estimate the flux of dissolved trace elements, such as the micro-nutrient Fe, to the ocean through the deposition of mineral dust (Chapter 2). Results from this study also raise the possibility of differing chemical behavior of the two Th isotopes in shallow water (<500 m depth) related to size-partitioning between truly dissolved and colloidal phases. Finally, results from the U.S. GEOTRACES North Atlantic Transect are used to investigate the relationship between 231Pa and 230Th in the water column and several of their proposed paleoceanographic purposes. The accuracy of 230Th as a proxy for constant sediment flux can be directly quantified to 40±10% with observations of boundary scavenging near the coast of Northwest Africa (Chapter 4). The 231Pa/230Th ratio in the water column is not clearly controlled by either ocean ventilation (Chapter 4) or particle composition (Chapter 5) alone. Near-bottom enhanced scavenging of both isotopes is also occurring over large regions, in benthic layers of resuspended sediments and in a hydrothermal plume emanating from the mid-Atlantic ridge. These novel observations must be reconciled in a new conceptual model, likely requiring future numerical modeling work, in order to clarify the use of the 231Pa/230Th ratio as a proxy for the Atlantic meridional overturning circulation.
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