Iron and Carbon Speciation in Non-Buoyant Hydrothermal Plumes along the East Pacific Rise: A Chemistry Love Story

Abstract

In the ocean, iron (Fe) is an important micronutrient for phytoplankton growth. Phytoplankton play a vital role in the global carbon (C) cycle, accounting for 50% of the total photosynthesis on the planet (Field et al. 1998; Moore et al. 2013; Fitzsimmons et al. 2014). When they die, phytoplankton sink and can become buried in the sediments of the deep ocean, removing C from the atmosphere and surface water. While Fe is an abundant element overall in the Earth’s crust (Edwards et al. 2004), it is extremely diluted in the surface ocean. Iron-poor surface waters limit phytoplankton growth (Vraspir and Butler 2009) and their ability to remove C from the atmosphere and surface ocean. Over the past few decades, research has focused on constraining the global Fe cycle and its impacts on the global C cycle (Tagliabue et al. 2010). Hydrothermal vents have become a highly debated potential source of Fe to the surface ocean. Initially, the hypothesis stated that hydrothermal Fe would all be oxidized and deposited locally upon being expelled from the vent. Therefore, hydrothermal vent would have a negligible effect on global biogeochemical cycles (Elderfield and Schultz 1996). However, as a variety of new sampling techniques were developed to preserve reduction-oxidation (redox) states and increase the ability to collect trace-metal clean samples (Johnson et al. 1997; Johnson et al. 2007; Breier et al. 2009), it was discovered that hydrothermal Fe could be protected from oxidation and removal to the sediments and be a potential source of Fe to the deep ocean and surface waters in some locations (Toner et al. 2009a; Toner et al. 2012a). With the amount of Fe released through hydrothermal venting to the ocean per year being similar in magnitude to that delivered by global riverine run-off (Elderfield and Schultz 1996), hydrothermal vents could be an unrecognized nutrient source to the surface ocean and play a role in global C cycling. Two main mechanisms, nanoparticles with slow settling rates, and complexation reactions with organic functional moieties, have been hypothesized to transport solid and aqueous phase Fe over long distances (Bennett et al. 2008; Toner et al. 2009a; Yücel et al. 2011). During the time interval of this dissertation, studies have investigated a large hydrothermal Fe source emanating from the East Pacific Rise (EPR; Resing et al. 2015; Fitzsimmons et al. 2017; Lee et al. 2018). This has informed current working geochemical models about the complexity of reaction pathways and transport mechanisms active in hydrothermal plumes with implications for basin-scale transport and bioavailability of hydrothermal Fe (Tagliabue and Resing 2016; Tagliabue et al. 2017). This dissertation will focus on investigating the chemical speciation and transport mechanisms of Fe in non-buoyant plume particles along the East Pacific Rise. Therefore, further informing the global impact of hydrothermal venting within ocean basins.