Bioactive trace metals and their isotopes as paleoproductivity proxies: An assessment using GEOTRACES-era data

Abstract

The ocean plays host to three carbon 'pumps' that redistribute climatically-significant quantities of carbon dioxide (CO2) from the atmosphere to the ocean interior and seafloor (Volk & Hoffert, 1985). These ocean carbon pumps-biological, carbonate, and solubility-influence Earth's climate over timescales ranging from decades to millions of years (e.g., Volk & Hoffert, 1985; Sigman et al., 2010; Khatiwala et al., 2019). The biological pump is of particular interest as it connects the cycles of C to those of O2, (micro)nutrients, and marine biology, and today accounts for as much as 70 % of the 'contribution ' of all three carbon pumps (Sarmiento & Gruber, 2006). The biological pump redistributes atmospheric carbon in two steps. First, phytoplankton, photoautotrophic microbes, use sunlight to transform ambient DIC (dissolved inorganic carbon) into POC (particulate organic carbon), represented here by CO2 and glucose, respectively, by the simplified reaction: CO2 + H2O + hv → CH2O + O2 [1] The second step requires that some fraction of the newly-formed POC sinks into the ocean interior through a combination of biological and physical aggregation processes (e.g., Alldredge & Silver 1988). The resulting surface ocean DIC deficit promotes the invasion of atmospheric CO2 into seawater to maintain air-sea CO2 equilibrium, driving an overall reduction in atmospheric pCO2. (This definition of the biological pump neglects dissolved organic carbon export, which is comparatively understudied, though may account for as much as one-third of C export; e.g., Carlson et al., 2010; Giering et al., 2014.) Importantly, Reaction [1] requires sunlight and can only occur in the euphotic layer of the ocean. In contrast, aerobic heterotrophic respiration can occur wherever POC and O2 are present: CH2O + O2 → CO2 + H2O [2] (There are a number of O2-independent respiration pathways that are reviewed in detail elsewhere; e.g., Froelich et al., 1979.) While the representation of all POC as glucose (CH2O) is instructive for illustrating an important biotic transformation in the ocean, it is also simplistic; microbial biomass consists of dozens of bioactive elements that serve many essential functions (e.g., da Silva & Williams, 1991). The elemental stoichiometry of POC can thus be expanded to include a number of major and micronutrient elements, as illustrated by the extended Redfield ratio reported by Ho et al.

Global Ocean Sediment Composition and Burial Flux in the Deep Sea

“…Paytan and Kastner (1996) based these fluxes on data available at the time from the Congo, Amazon and Mississippi Rivers (Edmond et al, 1978) Pacific Rise (Von Damm et al, 1985). The magnitude of Ba sources is currently being assessed (e.g., Horner et al, 2020), including evidence for other sources such as submarine groundwater discharge or cold seeps. Because of the uncertainty in our Ba burial estimate, it is premature to conclude whether or not there is a steady-state balance between burial and known sources.…”

“…Paytan and Kastner (1996) based these fluxes on data available at the time from the Congo, Amazon and Mississippi Rivers (Edmond et al, 1978) and hydrothermal vent observations from the East Pacific Rise (Von Damm et al, 1985). The magnitude of Ba sources is currently being assessed (e.g., Horner et al, 2020), including evidence for other sources such as submarine groundwater discharge or cold seeps.…”