The Biogeochemical Cycling of Dissolved Iron, Manganese, and Lead in the Equatorial & North Pacific Ocean

Abstract

Trace elements and their isotopes are influenced by a range of oceanographic processes including biological activity, water mass circulation, and abiotic internal cycling processes. Here, we utilize the unique distributions of three trace elements (Fe, Mn, and Pb) to assign external metal sources and identify chemical transformations within the Pacific Ocean. The international GEOTRACES program has allowed for extensive sampling of the Pacific; however, most prior Pacific basin-scale studies have focused on the zonal distributions of these trace elements, which have focused on the influence of individual oceanographic processes and latitudinal metal source fluxes. The dataset presented in this dissertation was collected along the U.S. GEOTRACES Pacific Meridional Transect (GP15), representing the most expansive, full-depth meridional trace element sampling effort of the central Pacific Ocean and capturing the distribution of tracers throughout the Equatorial and North Pacific basins along 152ºW from the Alaskan coast (56N) to Tahiti (20ºS). The novel findings of this interdisciplinary research reveal how trace elements and their isotopes transform across varying conditions throughout diverse biogeochemical gradients.

This GP15 research is broken up into three chapters highlighting how the distribution of these trace elements are uniquely modulated within the Pacific Ocean. First, the GP15 section is the longest continuous meridional section of dissolved Pb and dissolved Pb isotopes (206Pb, 207Pb, and 208Pb) in the central Pacific, distinguishing the influence of anthropogenic inputs throughout the subsurface North Pacific. Specifically, we assess the role of reversible scavenging of anthropogenic Pb from sinking particles within two particle-rich ‘veils.’ We modeled how the full-water-column Pb isotope anomalies in those veils revealed the transport of anthropogenic Pb to the previously pristine abyssal Pacific. Second, we juxtapose the distributions of dissolved Fe and Mn while implementing stable dissolved Fe isotopes (56Fe) to further constrain the external sources (dry and wet atmospheric deposition, continental margins, and hydrothermal vents) and internal cycling process (scavenging, redox transformations, and biological uptake) acting on dissolved Fe. These are the first 56Fe measurements in the central Pacific basin, offering imperative basin-scale measurements that can strengthen systematic hypotheses that were only previously assumed: 1) biological uptake of Fe causes preferential ‘light’ fractionation of 56Fe and 2) scavenging does not cause fractionation of 56Fe. Lastly, we focus on the contribution of hydrothermal venting from previously neglected intraplate venting systems using the Kama’ehuakanloa (formerly Löi’hi) Seamount as a case study. This is the first study of its kind able to assess how dFe transforms from vent fluids to far-field hydrothermal plume samples (<500km, collected within two months of each other) utilizing a plethora of tracers including 56Fe measurements and the size partitioning of the dissolved Fe phase into colloidal and soluble size fractions. Overall, this dissertation aims to determine how the distributions of dissolved Fe, Mn, and Pb are influenced by major biogeochemical processes and water mass circulation on a basin-scale within the Pacific Ocean, one of the most remote and voluminous regions of the global ocean.