Trace metal (Al, Mn, Fe) sources and cycling at continental margin and hydrothermal interfaces in the Pacific Ocean

Abstract

Trace metals, in particular iron (Fe), serve as essential nutrients for phytoplankton, which are at the base of almost all food webs in the ocean and are an important regulator of CO2 in Earth’s atmosphere. Thus, understanding the mechanisms by which the oceans are supplied with iron and other trace metals is of critical importance to identifying potential changes to trace metal fluxes in the future and improving our estimates of primary productivity. Margins (both continental and ocean/volcanic) can serve as important sources for trace metal delivery to the ocean. Here, we investigate Al, Mn, and Fe cycling in the Solomon Sea, the Gulf of Alaska, and the Mariana Back Arc to better understand the importance of margin input on open ocean chemistry. In the Solomon Sea, water flows past many land masses via strong currents and feeds the Equatorial Undercurrent, which upwells in the high nitrate low chlorophyll eastern equatorial Pacific Ocean. The EUC has a core of elevated trace metals, and so improving our understanding where these enrichments come from will help us better understand the global carbon cycle. In the Solomon Sea, we observe local enrichments of Al and Mn at stations within the basin, however a mass balance suggests that inflowing water is already enriched in trace metals, and the outflowing water has a similar net trace metal composition as the inflowing water. Ultimately, inputs of Al and Mn are short-lived and do not persist through the basin. The coastal Gulf of Alaska is an important source of trace metals to the iron-limited central Gulf of Alaska. Because climate change is likely impacting the hydrodynamics in this region, better understanding the seasonality of fluxes of Al and Mn will help constrain global metal budgets and improve models. Sampling in this region has been primarily focused during the summer months, when river runoff is high, and leaves gaps in our understanding of wintertime processes. We observed a strong seasonal cycle, where cycling of trace metals is driven by sediment resuspension and vertical mixing during the winter and spring and by glacial meltwater inputs during the summer. Despite the high inputs via rivers and sediment, concentrations are relatively low past the shelf break. In the Mariana Back arc, the shallow volcanos of the Mariana Arc could enrich surface waters in Fe and Mn via hydrothermal vents or sediment resuspension processes. To understand whether the Mariana arc volcanoes contribute significantly to the surface ocean Fe pool where primary productivity may be enhanced by addition of Fe, we sampled surface water (<1000 m) over the Mariana back-arc, along a North-South transect running parallel to the arc, for particulate and dissolved trace metals, including Fe and Mn. We also collected water samples for trace metals and dissolved Fe isotopes along an East-West transect between NW Rota-1 submarine volcano and our N-S transect. We also use Helium-3 to provide additional context for these data. We observed minimal evidence of hydrothermal input, and also minimal enrichment of trace metals in the Back arc, despite the rough bathymetry and strong flowing currents. “Hot spots” of local enrichment did exist, but these signals did not propagate between stations, suggesting inputs are short-lived and cycle quickly. Ultimately, we observe that despite evidence of local inputs, coastal/margin input to the open ocean is relatively low. This work provides new insights into margin and hydrothermal processes and their impact on global ocean budgets of trace metals.