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Science Highlights

Barium isotope measurements help constraining the oceanic barium cycle

Hsieh and Henderson (2017, see reference below) propose a compilation of the oceanic barium (Ba) concentrations together with its isotopic profiles measured so far. Their review covers the main oceanic basins, comparing data obtained in the North and South Atlantic, North Pacific and the Southern Oceans.

Their main conclusions are: near-surface Ba isotope values are controlled by basin-scale balances rather than by regional or short-term processes; isotope Ba fractionation during its removal from the surface is significant: the global Ba isotope data can be fit by mixing and removal/addition of Ba with a single isotope fractionation of 1.00058 ±0.00010; the resulting Ba isotope composition of the upper ocean waters is correlated with the fraction of Ba utilization at the basin scale; in the deep waters, it is suspected that external inputs of Ba (released by sediments or hydrothermal sources) can be traced by their specific isotopic signatures.

17 Hsieh
Figure:
 Seawater Ba isotope compositions versus 1/[Ba] in the global ocean. The data are fitted with three curves generated by a steady-state (open) model, a Rayleigh fractionation (closed) model and a mixing model, each constrained using an initial composition equal to the average value in the deep Southern Ocean and a final value equal to the surface values in the Pacific Ocean. The results show that seawater Ba isotope compositions are controlled by basin-scale Ba utilization, remineralisation, and ocean mixing during the internal oceanic Ba cycle. External Ba inputs also play important roles in the oceanic Ba isotope budget. For example, riverine input introduces light Ba isotopic signatures to the surface ocean; and sediment or hydrothermal inputs may introduce heavy Ba isotopic compositions to the deep water, which have been identified with the non-conservative behaviour of Ba isotopes during the N-S Atlantic deep water mixing. Such distinct Ba isotope signatures from these sources can become useful tracers for constraining Ba inputs in the present and past ocean. Click here to view the figure larger.

Reference:

Hsieh, Y.-T., & Henderson, G. M. (2017). Barium stable isotopes in the global ocean: Tracer of Ba inputs and utilization. Earth and Planetary Science Letters, 473, 269–278. http://doi.org/10.1016/j.epsl.2017.06.024

 

Short-Term Variability of Dissolved Rare Earth Elements and Neodymium Isotopes in the Entire Water Column of the Panama Basin

Patricia Grasse and co-workers (2017, see reference below) present new dissolved neodymium isotope compositions (εNd) and rare earth element (REE) concentrations from the Panama Basin.

REE concentrations peak at the surface reflect high lithogenic inputs from the nearby Central American Arc (CAA) resulting in highly radiogenic εNd signatures, with the observation of the most radiogenic value measured for seawater to date (+4.3). Intermediate and deep waters of the Panama Basin (mean εNd value = 0) are significantly more radiogenic than the inflowing water masses from the Peruvian Basin (−1.1 to −6.6 εNd). This demonstrates that the highly radiogenic Nd isotope compositions must result from release of radiogenic Nd through partial dissolution of volcanic material of the CAA. The re-occupation of one station demonstrates that the high amounts of radiogenic Nd released from these particles can reset water mass Nd isotope and REE signatures of the entire Panama Basin water column within 3.5 years, clearly an area where water masses acquire their signatures before they are advected to the open ocean.

Large amounts of REEs readily released from volcanic particles on such short time scales may require a different parameterization of the Nd isotope signal acquisition processes of water masses in models of the oceanic Nd isotope distribution and are important for the seawater Nd budget.

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Figure: (a) Map of sampling locations during Meteor Cruise M90 (Oct./Nov. 2012) in the Panama Basin together with the location of re-occupied station 160 sampled previously during Cruise M77-4 in February 2009. The dashed grey line indicates the approximate position of the Intertropical Convergence Zone (ITCZ) in spring and late summer (b) Water column distribution of St. 1555 eNd (filled black circles) and Nd concentrations (open black circles) together with previously occupied St.160 eNd (filled red squares) and Nd concentrations (open red squares) (Grasse et al., 2012). Please click here to view the figure larger.

Reference

Grasse, P., Bosse, L., Hathorne, E. C., Böning, P., Pahnke, K., & Frank, M. (2017). Short-term variability of dissolved rare earth elements and neodymium isotopes in the entire water column of the Panama Basin. Earth and Planetary Science Letters, 475, 242–253. DOI: 10.1016/j.epsl.2017.07.022

 

Shelf sediment dissolved iron source via non-reductive dissolution in the Gulf of Alaska

Crusius and co-workers (2017, see reference below), reveal temporal and spatial variability in the sources of iron (Fe) to the northern Gulf of Alaska, based on data from cruises from three different seasons from the Copper River (AK) mouth to beyond the shelf break.  April data are the first to describe late winter Fe behavior before surface-water nitrate depletion began.  Sediment resuspension during winter and spring storms generated high “total dissolvable Fe” (TDFe) concentrations of ~1000 nmol kg-1 along the entire continental shelf, which decreased beyond the shelf break.  In July, high TDFe concentrations were similar on the shelf, but more spatially variable, and driven by low-salinity glacial meltwater.  Conversely, dissolved Fe (DFe) concentrations in surface waters were far lower and more seasonally consistent, ranging from ~4 nmol kg-1 in nearshore waters to ~0.6-1.5 nmol kg-1 seaward of the shelf break during April and July, despite dramatic depletion of nitrate over that period. The April DFe data can be simulated using a simple numerical model that assumes a DFe flux from shelf sediments, horizontal transport by eddy diffusion, and removal by scavenging.  Calculations suggest dust is an important Fe source beyond the shelf break.

17 Crusius lFigure:  Seasonal and spatial variability in Fe in the northern Gulf of Alaska: a) Sampling region in the northern Gulf of Alaska extending from the Copper River Mouth to ~50 km beyond the shelf break.  The surface water transect was carried out along the line defined by the green dots (which define sampling stations).  This is superimposed upon a MODIS image from 9 April, 2010 that shows resuspended sediments (light blue) landward of the 500-m depth contour (orange line).  b) Surface water total dissolvable Fe (TDFe) concentrations and salinity plotted versus distance from shore during April, May and July.  c) Dissolved Fe (DFe) data (blue squares) from April, along with several time-dependent model simulations that bracket the data, with varying flux of DFe from the shelf sediments, horizontal eddy diffusion, and removal by chemical scavenging. Click here to view the figure larger.

Reference:

Crusius, J., A. W. Schroth, J. A. Resing, J. Cullen, and R. W. Campbell (2017), Seasonal and spatial variabilities in northern Gulf of Alaska surface-water iron concentrations driven by shelf sediment resuspension, glacial meltwater, a Yakutat eddy, and dust, Global Biogeochem. Cycles, 31, doi:10.1002/2016GB005493.

Manganese in the west Atlantic Ocean in the context of the first global ocean circulation model of manganese

Marco van Hulten and co-workers (2017, see reference below) ran a global ocean model to understand manganese (Mn), a biologically essential element. The model shows that:

(i) in the deep ocean, dissolved [Mn] is mostly homogeneous ~0.10—0.15 nM. The model reproduces this with a threshold on MnO₂ of 25 pM, suggesting a minimal particle concentration is needed before aggregation and removal become efficient.

(ii) The observed distinct hydrothermal signals are produced by assuming both a strong source and a strong removal of Mn near hydrothermal vent.

17 vanHulten l2Figure: (A) The modelled dissolved [Mn] (nM) at the Zero-Meridian section component of the GIPY5 cruise dataset, and the West Atlantic GA02 GEOTRACES section cruise (annual average). Observations at the transects are presented as coloured dots. (B) Worldmap showing cruise transects for GA02 (red) and GIPY5 (green, in the Atlantic sector of the Southern Ocean). Please click here to view the figure larger. Modified from Biogeosciences.

Reference:

van Hulten, M., Middag, R., Dutay, J.-C., de Baar, H., Roy-Barman, M., Gehlen, M., Tagliabue, A., and Sterl, A. (2017) Manganese in the west Atlantic Ocean in the context of the first global ocean circulation model of manganese, Biogeosciences, 14, 1123-1152. DOI: 10.5194/bg-14-1123-2017.

Surprising cadmium isotope results north of the Subantarctic Front in the South West Atlantic Ocean

Xie and co-workers (2017, see reference below) report cadmium (Cd) isotopic compositions from five stations and 15 tow-Fish surface waters from 50ºS to the equator along GEOTRACES GA02 Leg 3. Along this transect, the coupled Cd concentrations and Cd isotopes reflect classical behaviour dominated by preferential uptake of light Cd by the biological species at the surface, release in the twilight zone and water mass mixing deeper. Surprisingly, ε112/110Cd displays a "flattening off" pattern in the surface and subsurface waters of stations north of the Subantarctic Front, while Cd concentrations decrease to low levels; this observation can be extended to the global Cd isotope dataset at hand for Cd concentrations below a nominal value of 0.1 nmol kg-1. Two explanations are proposed for this behaviour: 1) either Cd is bound by organic detritus, colloids or ligands and passes the 0.2μm filtration of the samples, products which could dominate ε112/110Cd over that of the dissolved pool; or 2) the ε112/110Cd values result from a simple open system, steady-state model for the (sub)surface layer, fed with an in-flux of Cd from deeper waters.

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Figure:
Map of the five super stations (color circles) and tow-Fish surface sites (crosses) for Cd isotopes along GA02 Leg 3 (Left), and Cd isotope systematics in the western South Atlantic (Middle) and the global ocean (right). Color circles for profile samples, and open circles for tow-Fish seawater samples (this study). In the right-hand panel: grey diamonds – North Atlantic (Boyle et al., 2012; Conway and John 2015a; John and Conway, 2014); open triangles – North (Conway and John, 2015b; Ripperger et al., 2007) and South Pacific (New Zealand (Gault-Ringold et al., 2012), South China Sea and Philippine Sea (Yang et al., 2012, 2014)); open squares – Southern Ocean (Abouchami et al., 2011, 2014; Xue et al., 2013). Red dashed lines in the middle and right-hand panels schematically highlight the evolution of seawater e112/110Cd toward low Cd concentrations. Error bars (2s) are shown. Click here to view the figure larger.

Reference:

Xie, R. C., Galer, S. J. G., Abouchami, W., Rijkenberg, M. J. A., de Baar, H. J. W., De Jong, J., & Andreae, M. O. (2017). Non-Rayleigh control of upper-ocean Cd isotope fractionation in the western South Atlantic. Earth and Planetary Science Letters (Vol. 471). DOI: 10.1016/j.epsl.2017.04.024

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