Particulate Trace Element Export in the North Atlantic (GEOTRACES GA01 Transect, GEOVIDE Cruise)

Abstract

Vertical export of particulate trace elements (pTEs) is a critically underconstrained aspect of their biogeochemistry. Here, we combine elemental analyses on large (>53 μm) particles and 234Th measurements to determine downward export fluxes from the upper layers (40-110 m) of pTEs (Al, Cd, Co, Cu, Fe, Mn, Ni, P, Ti, V, Zn) and mineral phases (lithogenic, Fe-and Mn-oxides, calcium carbonate, and opal) in the North Atlantic along the GEOVIDE transect (Portugal-Greenland-Canada; GEOTRACES GA01 cruise). The role of lithogenic particles in controlling TE fluxes is obvious at proximity of the Iberian margin where the highest pTE export fluxes were estimated (up to 3912 μg/m2/d for pFe). However, high lithogenic and pTE fluxes are also observed up to 1700 km off this margin in the west European and Icelandic basins (up to 931 μg/m2/d for pFe). The lowest pTE export fluxes are determined in the Labrador Sea (as low as 501 μg/m2/d for pFe). High Mnand Fe-oxide fluxes are estimated at the open ocean stations, suggesting that authigenic particles are an important vector of pTEs. All along the transect, biogenic particles also drive the pTE export fluxes, as shown by the similar pTE/POC ratios between exports and phytoplankton quotas. The shortest residence times (dissolved + particulate) are generally observed where lithogenic particles control the pTE fluxes (as low as 2 days for Fe) whereas pTEs seem to be longer retained when the contribution of biogenic particles become greater (residence times up to 147 days for Fe).

Revisiting five decades of <sup>234</sup>Th data: a comprehensive global oceanic compilation

“…Overall, Fig. 7a shows at a glance the number of data points in the compilation that could be used to evaluate processes in the upper ocean such as export flux and export efficiency (e.g., Buesseler et al, 2020b), scavenging rates of trace metals (e.g., Black et al, 2018;Lemaitre et al, 2020), or particle sinking velocities (e.g., by using "deficit" ratios, as well as those that could be used to study processes such as particle rem-E. Ceballos-Romero et al: Revisiting five decades of 234 Th data ineralizations (e.g., Usbeck et al, 2002) by using the "excess" ratios. Recent studies have highlighted the role that disaggregation and fragmentation could play in setting the magnitude of flux attenuation (Baker et al, 2017;Briggs et al, 2020;Cavan et al, 2017) and pointed to it as the most important currently unaccounted for process for improving modern-day export flux simulations (Henson et al, 2021).…”

“…The most widespread application of the 234 Th approach is to estimate the gravitational settling of carbon as particulate organic carbon (POC) out of the surface layer, which results in a downward flux of POC (see, e.g., review by Le Moigne et al, 2013a, and references therein). To a lesser extent, this radionuclide is also used to estimate the downward flux of other elements to the deep ocean, such as particulate inorganic carbon (e.g., Le Moigne et al, 2013b;Wei et al, 2011), biogenic silica (e.g., Buesseler et al, 2005;Lemaitre et al, 2016;Le Moigne et al, 2013b), particulate organic nitrogen (PON) (e.g., Buesseler et al, 1992;Charette and Buesseler, 2000;Murray et al, 1996), or trace metals fluxes (e.g., Black et al, 2018;Lemaitre et al, 2016Lemaitre et al, , 2020Weinstein and Moran, 2005).…”

Authigenic Iron Is a Significant Component of Oceanic Labile Particulate Iron Inventories

“…Other studies have quantified mineral phases that are selective to specific chemical leaches, but these may include only a portion of the authigenic minerals (e.g., oxalate wash (Tovar‐Sanchez et al., 2003) or 25% acetic acid (Hurst & Bruland, 2007)) and/or may include a portion of both authigenic and biogenic minerals (Berger et al., 2008). Recent approaches to quantifying the authigenic fraction calculate it from difference (Black et al., 2020; Lemaitre et al., 2020; Marsay et al., 2017). While lithogenic and biogenic particulate fractions can often be estimated using direct elemental proxies (e.g., Al or Ti and C or P, respectively), authigenic phases like Mn‐ and Fe‐oxides and any associated sorbed TMs typically lack clear “end member” signatures and therefore cannot be easily directly estimated from bulk elemental analyses.…”

Vertical Flux of Trace Elements Associated With Lithogenic and Biogenic Carrier Phases in the Southern Ocean

“…Particle sinking is one of the mechanisms of downward transport of chemical elements (Boyd et al., 2019). Large biogenic particles such as phytoplankton aggregates or fecal pellets are major vectors in the downward transport of particulate organic carbon (POC), but lithogenic particles can also play a role by ballasting aggregates and reducing remineralization (Lemaitre et al., 2020). Due to their role in the control of atmospheric CO 2 (Antia et al., 2001), carbon vertical export fluxes have been extensively studied, yet TE export fluxes have been considerably less investigated.…”