Time-series observations of upper ocean iron cycling in the North Pacific Subtropical Gyre

Abstract

Iron is an essential micronutrient for marine phytoplankton, with low dissolved iron (dFe) concentrations limiting primary productivity, and subsequently marine carbon uptake, in approximately one-third of the global surface ocean. Significant work over the past few decades has drastically improved our understanding of the global distribution of iron and the biogeochemical processes controlling this distribution. In the North Pacific Subtropical Gyre (NPSG), atmospheric deposition is thought to be the primary source of Fe, while internal cycling between dissolved and particulate forms is driven by biological uptake and remineralization, abiotic scavenging and dissolution, and the formation of authigenic minerals. However, our understanding of seasonal and interannual variability of iron cycling remains limited, particularly in how temporal changes in source magnitudes and biological demand influence iron availability. This dissertation represents the first multi-year, monthly resolved time-series study of iron and other trace metals in the upper ocean. Samples were collected on 21 Hawaii Ocean Time-series cruises to Station ALOHA in the NPSG over three years, capturing temporal variability on monthly to interannual timescales. The first study focuses on the drivers of particulate trace metal cycling and export, including of Fe, lithogenic elements Al and Ti, and biologically-important metals Co, Ni, Cu, Mn, and Cd. Lithogenic material appears to be exported quickly (within ~1 week), while biogenic trace elements spend month(s) in the upper ocean due to biological recycling. The second study investigates seasonal variability in dFe, finding mixed layer dFe varied 5-fold throughout the year and unexpectedly peaked in winter. Coincident increases in dissolved Ti and particulate Ti:Al ratios suggest basaltic inputs from the Hawaiian Islands are the source of wintertime dFe. The third study leverages biotic Fe uptake incubations to differentiate the labile particulate Fe (pFe) pool into biogenic and authigenic components. Bulk Fe uptake rates were generally highest in spring, and both biogenic and authigenic pFe were found to be important contributors to the labile pFe pool throughout the year. Overall, this research reveals the seasonal variability of iron cycling at Station ALOHA. Iron is brought to the system by local inputs in winter, followed by springtime Asian dust deposition. With an estimated residence time of ~5 months in the euphotic zone, dFe from these sources persists long enough to sustain biological activity throughout the year. Prochlorococcus and picoeukaryotes appear to drive biotic iron uptake and biogenic pFe concentrations, and labile particulate Fe is then recycled several times in the euphotic zone before export. Together, these processes highlight the roles of seasonal inputs and internal recycling in regulating iron cycling in the North Pacific Subtropical Gyre.