Distributions of total and size-fractionated particulate <sup>210</sup>Po and <sup>210</sup>Pb activities along the North Atlantic GEOTRACES GA01 transect: GEOVIDE cruise

Abstract

Abstract. Vertical distributions of total and particulate polonium-210 (210Po) and lead-210 (210Pb) activities in the water column were measured at 11 stations in the North Atlantic during the GEOTRACES GA01 transect: GEOVIDE cruise in May–June 2014. Total 210Po activity was on average 24 % lower than 210Pb activity in the upper 100 m, and it was closer to unity in the mesopelagic (100–1000 m). The partitioning coefficients (Kd) along the transect suggest the preferential association of 210Po relative to 210Pb onto particles. The prominent role of small particles in sorption was confirmed by the observation that over 80 % of the particulate radionuclide activity was on small particles. To account for the observed surface water 210Po ∕ 210Pb disequilibria, particulate radionuclide activities and export of both small (1–53 µm) and large (>53 µm) particles must be considered. A comparison between the GEOVIDE total particulate 210Po ∕ 210Pb activity ratios (ARs) and the ratios in previous studies revealed a distinct geographic distribution, with lower particulate ARs in the high-latitude North Atlantic (including this study) and Arctic in relation to all other samples. For the samples where apparent oxygen utilization (AOU) was calculated at the same depth and time as the 210Po ∕ 210Pb AR (40 stations including this study), there was a two-phase correlation between the total particulate AR and AOU, likely reflecting the nature of the particles and demonstrating the forces of remineralization and radionuclide decay from particles as they age.

Introduction to the French GEOTRACES North Atlantic Transect (GA01): GEOVIDE cruise

“…Almost all the stations displayed dFe minima in surface waters, in association with the chlorophyll maxima. The abundance of diatoms exerted a significant control on the surface concentrations of Fe and Al (Menzel Barraqueta et al, 2018b;Tonnard et al, 2018). Remineralization processes were also highlighted for some TEIs (see also Sect.…”

“…210 Pb, Al, Mn, Th isotopes, 7 Be; Baker et al, 2016;Hsieh et al, 2011;Measures and Brown, 1996); (ii) mixing rates of deep waters or shelf-to-open ocean (e.g. 231 Pa/ 230 Th, 14 C, Ra isotopes, 129 I, 236 U;van Beek et al, 2008;Casacuberta et al, 2016;Key et al, 2004); (iii) boundary exchange processes (e.g. ε Nd , Jeandel et al, 2011;Jeandel, 2001, 2005); and (iv) downward flux of organic carbon and/or remineralization in deep waters (e.g.…”

“…Some other TEIs were also strongly linked to water mass distribution: within GEOVIDE, silicon isotopes , lead (Pb) , mercury (Hg) (Cossa et al, 2018a), and particulate and dissolved Fe and Al Menzel Barraqueta et al, 2018a;Tonnard et al, 2018), for example. For instance, the Labrador Sea Water (LSW) is characterized by a relatively high silicon stable isotope composition for dissolved silicon (δ 30 Si DSi ) whose signature can be seen not only in the region where it is formed, but also throughout the mid-depth zone of the North Atlantic Ocean .…”

Reply to reviewer #2

“…Some other TEIs were also strongly linked to water mass distribution: within GEOVIDE, silicon isotopes (Sutton et al, 2018), lead (Pb) (Zurbrick et al, 2018), mercury (Hg) (Cossa et al, 2018a), and particulate and dissolved Fe and Al (Gourain et al, 2018;Menzel Barraqueta et al, 2018a;Tonnard et al, 2018), for example. For instance, the Labrador Sea Water (LSW) is characterized by a relatively high silicon stable isotope composition for dissolved silicon (δ 30 Si DSi ) whose signature can be seen not only in the region where it is formed, but also throughout the mid-depth zone of the North Atlantic Ocean .…”

“…One of the main sources of TEIs during GEOVIDE was sediment input (i) within the benthic nepheloid layers in the Iceland, Irminger and Labrador basins (Gourain et al, 2018), and (ii) above the Iberian, Greenland and Canadian margins, as well as fluvial and meteoric inputs (Benetti et al, 2017). This is notably the case for some dissolved TEIs, such as Fe (Tonnard et al, 2018), , and radium-226 ( 226 Ra) or barium (Ba) (Le Roy et al, 2018), as well as for particulate trace elements (Gourain et al, 2018). Overall, enhanced concentrations of TEIs close to the bottom suggest that continental shelves and margins acted as a source to adjacent waters.…”