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.
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.
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.
Figure: 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.
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.