Dissolved and Particulate Barium Distributions Along the US GEOTRACES North Atlantic and East Pacific Zonal Transects (GA03 and GP16): Global Implications for the Marine Barium Cycle

Abstract

Processes controlling dissolved barium (dBa) were investigated along the GEOTRACES GA03 North Atlantic and GP16 Eastern Tropical Pacific transects, which traversed similar physical and biogeochemical provinces. Dissolved Ba concentrations are lowest in surface waters (∼35–50 nmol kg−1) and increase to 70–80 and 140–150 nmol kg−1 in deep waters of the Atlantic and Pacific transects, respectively. Using water mass mixing models, we estimate conservative mixing that accounts for most of dBa variability in both transects. To examine nonconservative processes, particulate excess Ba (pBaxs) formation and dissolution rates were tracked by normalizing particulate excess 230Th activities. Th‐normalized pBaxs fluxes, with barite as the likely phase, have subsurface maxima in the top 1,000 m (∼100–200 μmol m−2 year−1 average) in both basins. Barite precipitation depletes dBa within oxygen minimum zones from concentrations predicted by water mass mixing, whereas inputs from continental margins, particle dissolution in the water column, and benthic diffusive flux raise dBa above predications. Average pBaxs burial efficiencies along GA03 and GP16 are ∼37% and 17%–100%, respectively, and do not seem to be predicated on barite saturation indices in the overlying water column. Using published values, we reevaluate the global freshwater dBa river input as 6.6 ± 3.9 Gmol year−1. Estuarine mixing processes may add another 3–13 Gmol year−1. Dissolved Ba inputs from broad shallow continental margins, previously unaccounted for in global marine summaries, are substantial (∼17 Gmol year−1), exceeding terrestrial freshwater inputs. Revising river and shelf dBa inputs may help bring the marine Ba isotope budget more into balance.

Barium in seawater: Dissolved distribution, relationship to silicon, and barite saturation state determined using machine learning

“…This estimate of the total oceanic Ba inventory is approximately 10-21 % lower than previous estimates of 145 Tmol Ba (Dickens et al, 2003;Carter et al, 2020). Thus, given current estimates of global marine Ba fluxes of between 18 (Paytan & Kastner, 1996) and 44 Gmol Ba yr -1 (Rahman et al, 2022), the mean residence time of Ba in seawater is likely between 2,700-7,200 years.…”

Benthic-pelagic coupling and isotopic fractionation of barium in Kiel Bight, SW Baltic Sea

“…Baltic Sea water is generated by mixing of freshwater from the Baltic Sea drainage area and North Atlantic seawater entering via the Danish straits. Freshwater is generally characterized by higher Ba concentrations and a lower d 138 Ba than seawater (e.g., Cao et al, 2016;Cao et al, 2020;Bridgestock et al, 2021a;Bridgestock et al, 2021b;Rahman et al, 2022). Therefore, it is likely that the Ba concentration and isotopic distribution in Kiel Bight is to some extent a function of mixing of freshwater and seawater.…”

“…Given that the light Ba isotopes are preferentially adsorbed (Gong et al, 2020), river water is generally characterized by higher d 138 Ba (+0.1 to +0.35 ‰) (Cao et al, 2016;Bridgestock et al, 2021a;Bridgestock et al, 2021b;Cao et al, 2021) than crustal rocks (0.0 ± 0.03 ‰) (Nan et al, 2018) but a lower d 138 Ba than seawater. Dissolved Ba concentrations in rivers are generally higher than those in seawater (Rahman et al, 2022) and additional Ba is released from riverine particles by ion exchange during estuarine mixing (Hanor and Chan, 1977;Coffey et al, 1997;Bridgestock et al, 2021a). The differences in river water and seawater Ba concentrations and isotope compositions have been proposed as a (paleo-)proxy for freshwater input (Bridgestock et al, 2021b;Yu et al, 2022 and references therein).…”

“…Some of this imbalance could be explained by desorption of isotopically light particulate Ba (with a d 138 Ba ~0.2 ‰ lower than river water) within the estuarine mixing gradient (Bridgestock et al, 2021a). In addition, other Ba sources and sinks, which have not yet been characterized isotopically, could potentially close the oceanic Ba mass balance: Water column studies suggest that another important external flux of Ba to the ocean originates from the seafloor (Rahman et al, 2022;Whitmore et al, 2022). It is well established that marine sediments release Ba to the bottom water via diffusion or advection across the sediment-water interface (McManus et al, 1994;McManus et al, 1998).…”

High latitude controls on dissolved barium isotope distributions in the global ocean

“…Given that the oceanic residence time of dissolved Ba (∼3.5 to 5 kyr; Rahman et al, 2022) is longer than the global ocean mixing time, but short enough to prevent complete homogenisation, studies have focused on the importance of advection and mixing on determining dissolved δ 138 Ba within water masses in the deep Atlantic Meridional Overturning Circulation (AMOC; Bates et al, 2017;. The distribution of deep water δ 138 Ba is overall consistent with two end member mixing between North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW).…”