Quantifying trace element and isotope fluxes at the ocean–sediment boundary: a review

Abstract

Quantifying fluxes of trace elements and their isotopes (TEIs) at the ocean's sediment–water boundary is a pre-eminent challenge to understand their role in the present, past and future ocean. There are multiple processes that drive the uptake and release of TEIs, and properties that determine their rates are unevenly distributed (e.g. sediment composition, redox conditions and (bio)physical dynamics). These factors complicate our efforts to find, measure and extrapolate TEI fluxes across ocean basins. GEOTRACES observations are unveiling the oceanic distributions of many TEIs for the first time. These data evidence the influence of the sediment–water boundary on many TEI cycles, and underline the fact that our knowledge of the source–sink fluxes that sustain oceanic distributions is largely missing. Present flux measurements provide low spatial coverage and only part of the empirical basis needed to predict TEI flux variations. Many of the advances and present challenges facing TEI flux measurements are linked to process studies that collect sediment cores, pore waters, sinking material or seawater in close contact with sediments. However, such sampling has not routinely been viable on GEOTRACES expeditions. In this article, we recommend approaches to address these issues: firstly, with an interrogation of emergent data using isotopic mass-balance and inverse modelling techniques; and secondly, by innovating pursuits of direct TEI flux measurements. We exemplify the value of GEOTRACES data with a new inverse model estimate of benthic Al flux in the North Atlantic Ocean. Furthermore, we review viable flux measurement techniques tailored to the sediment–water boundary. We propose that such activities are aimed at regions that intersect the GEOTRACES Science Plan on the basis of seven criteria that may influence TEI fluxes: sediment provenance, composition, organic carbon supply, redox conditions, sedimentation rate, bathymetry and the benthic nepheloid inventory.This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column

“…The true magnitude of dissolved Fe species released to bottom waters, will further reflect the influence of biological and physical disturbances to the sediment-water interface and transport mechanisms within the benthic boundary layer, which may further enhance exchange rates, but are neglected in our assessment more rigorous scaling-up of our findings across the Celtic Sea, to quantify the impact of ligand-mediated benthic fluxes to oxic shelf seas. Such a result, once seasonal perturbations to benthic oxygen and to carbon dynamics and ligand-sustained fluxes of Fe are properly accounted for, is likely to reveal that a larger amount of Fe is released from oxic shelf sediments, which are estimated to typify most of the ocean-continent boundary, especially in large areas of the Atlantic, Arctic and Southern Oceans (Homoky et al 2016), than previously assumed.…”

“…Impact of water column oxygen on release of benthic Fe(II) Large benthic fluxes of dFe to the water column are widely reported in oxygen deficient zones and are on the order of 100-1000 lmol m -2 day -2 (Homoky et al 2016 and references therein). These observations enable an empirical assessment of the impact of oxygen concentration on the release of Fe(II) from seafloor sediments (Dale et al 2015).…”

“…To a first approximation, this is a favourable comparison, and it is not unreasonable that individual study sites will have benthic Fe fluxes that deviate from the averaged relationships described by Dale et al (2015). However, it is also clear that well-oxygenated ocean margins have been largely absent from the compiled benthic Fe flux data used to parameterise ocean biogeochemical models to date (Homoky et al 2016), hence there is potential for an underestimated contribution of dFe from oxic ocean margins.…”

“…Benthic chamber measurements have shown that the largest fluxes of dFe are released from continental margin sediments with high inventories of organic carbon and reactive iron minerals underlying oxygen-depleted waters (Severmann et al 2010;Dale et al 2015). However, few studies have examined the Fe cycle in oxic shelf seas, even though they represent a large fraction of the ocean's continental boundaries (Homoky et al 2016). There are many reports of oxic shelf waters with higher dissolved Fe (dFe) than the open ocean (Charette et al 2007;Cullen et al 2009;de Jong et al 2012;Marsay et al 2014) and hence they may also be a significant source to the ocean in the global Fe cycle.…”

“…However, to date, there are no data that document the seasonal changes in porewater Fe in such systems, or the controls on dFe exchange between sediments and the water column. It is crucial to obtain estimates of Fe fluxes from different types of shelf environments to better constrain global supply rates in biogeochemical models that feed into global climate models (Homoky et al 2016). …”