Separating individual contributions of major Siberian rivers in the Transpolar Drift of the Arctic Ocean

The Siberian rivers supply large amounts of freshwater and terrestrial derived material to the Arctic Ocean. Although riverine freshwater and constituents have been identified in the central Arctic Ocean, the individual contributions of the Siberian rivers to and their spatiotemporal distributions in the Transpolar Drift (TPD), the major wind-driven current in the Eurasian sector of the Arctic Ocean, are unknown. Determining the influence of individual Siberian rivers downstream the TPD, however, is critical to forecast responses in polar and sub-polar hydrography and biogeochemistry to the anticipated individual changes in river discharge and freshwater composition. Here, we identify the contributions from the largest Siberian river systems, the Lena and Yenisei/Ob, in the TPD using dissolved neodymium isotopes and rare earth element concentrations. We further demonstrate their vertical and lateral separation that is likely due to distinct temporal emplacements of Lena and Yenisei/Ob waters in the TPD as well as prior mixing of Yenisei/Ob water with ambient waters

Alpha radiation from polymetallic nodules and potential health risks from deep-sea mining

In search for critical elements, polymetallic nodules at the deep abyssal seafloor are targeted for mining operations. Nodules efficiently scavenge and retain several naturally occurring uranium-series radioisotopes, which predominantly emit alpha radiation during decay. Here, we present new data on the activity concentrations of thorium-230, radium-226, and protactinium-231, as well as on the release of radon-222 in and from nodules from the NE Pacific Ocean. In line with abundantly published data from historic studies, we demonstrate that the activity concentrations for several alpha emitters are often higher than 5 Bq g−1 at the surface of the nodules. These observed values can exceed current exemption levels by up to a factor of 1000, and even entire nodules commonly exceed these limits. Exemption levels are in place for naturally occurring radioactive materials (NORM) such as ores and slags, to protect the public and to ensure occupational health and radiation safety. In this context, we discuss three ways of radiation exposure from nodules, including the inhalation or ingestion of nodule fines, the inhalation of radon gas in enclosed spaces and the potential concentration of some radioisotopes during nodule processing. Seen in this light, inappropriate handling of polymetallic nodules poses serious health risks

Grain size effects on Th-230(xs) inventories in opal-rich and carbonate-rich marine sediments

Excess Thorium-230 (230Thxs) as a constant flux tracer is an essential tool for paleoceanographic studies, but its limitations for flux normalization are still a matter of debate. In regions of rapid sediment accumulation, it has been an open question if 230Thxs-normalized fluxes are biased by particle sorting effects during sediment redistribution. In order to study the sorting effect of sediment transport on 230Thxs, we analyzed the specific activity of 230Thxs in different particle size classes of carbonate-rich sediments from the South East Atlantic, and of opal-rich sediments from the Atlantic sector of the Southern Ocean. At both sites, we compare the 230Thxs distribution in neighboring high vs. low accumulation settings. Two grain-size fractionation methods are explored. We find that the 230Thxs distribution is strongly grain size dependent, and 5090% of the total 230Thxs inventory is concentrated in fine material smaller than 10 μm, which is preferentially deposited at the highaccumulation sites. This leads to an overestimation of the focusing factor Ψ, and consequently to an underestimation of the vertical flux rate at such sites. The distribution of authigenic uranium indicates that fine organic-rich material has also been re-deposited from lateral sources. If the particle sorting effect isconsidered in the flux calculations, it reduces the estimated extent of sediment focusing. In order to assess the maximum effect of particle sorting on Ψ, we present an extreme scenario, in which we assume a lateralsediment supply of only fine material (b10 μm). In this case, the focusing factor of the opal-rich core would be reduced from Ψ=5.9 to Ψ=3.2. In a more likely scenario, allowing silt-sized material to be transported, Ψ is reduced from 5.9 to 5.0 if particle sorting is taken into consideration. The bias introduced by particle sorting is most important for strongly focused sediments. Comparing 230Thxs-normalized mass fluxes biased by sorting effects with uncorrected mass fluxes, we suggest that 230Thxs-normalization is still a valid tool to correct for lateral sediment redistribution. However,differences in focusing factors between core locations have to be evaluated carefully, taking the grain size distributions into consideration

Particle-seawater interaction of neodymium in the North Atlantic

Dissolved neodymium (Nd) isotopes (expressed as εNd) have been widely used as a water mass tracer in paleoceanography. However, one aspect of the modern biogeochemical cycle of Nd that has been sparsely investigated is the interplay between dissolved and particulate phases in seawater. We here present the first regional data set on particulate Nd isotope compositions (εNdp) and concentrations ([Nd]p) from five stations in the western North Atlantic Ocean along the GEOTRACES GA02 transect, in conjunction with previously published dissolved Nd isotope compositions (εNdd) and concentrations ([Nd]d)1. Key observations and interpretations from our new particulate data set include the following: (1) A low fractional contributions of [Nd]p to the total Nd inventory per volume unit of seawater (~5%), with significant increases of up to 45% in benthic boundary layers. (2) Increasing Nd concentrations in suspended particulate matter ([Nd]SPM) and fractions of lithogenic material with water depth, suggesting the removal of Nd poor phases. (3) Different provenances of particulates in the subpolar and subtropical gyres as evidenced by their Nd isotope fingerprints reaching from εNdp ≈ -20 near the Labrador Basin (old continental crust), over εNdp ≈ -4 between Iceland and Greenland (young mafic provenance), to values of εNdp ≈-13 in the subtropics (similar to African dust signal). (4) Vertical heterogeneity of εNdp, as well as large deviations from ambient seawater values in the subpolar gyre, indicate advection of lithogenic particles in this area. (5) Vertically homogenous εNdp values in the subtropical gyre, indistinguishable from εNdd values, are indicative of predominance of vertical particulate supply. The process of reversible scavenging only seems to influence particulate signatures below 3 km. Overall, we do not find evidence on enhanced particle dissolution, often invoked to explain the observed increase in dissolved Nd in the North Atlantic

How well does wind speed predict air-sea gas transfer in the sea ice zone? A synthesis of radon deficit profiles in the upper water column of the Arctic Ocean

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 3696–3714, doi:10.1002/2016JC012460.We present 34 profiles of radon-deficit from the ice-ocean boundary layer of the Beaufort Sea. Including these 34, there are presently 58 published radon-deficit estimates of air-sea gas transfer velocity (k) in the Arctic Ocean; 52 of these estimates were derived from water covered by 10% sea ice or more. The average value of k collected since 2011 is 4.0 ± 1.2 m d−1. This exceeds the quadratic wind speed prediction of weighted kws = 2.85 m d−1 with mean-weighted wind speed of 6.4 m s−1. We show how ice cover changes the mixed-layer radon budget, and yields an “effective gas transfer velocity.” We use these 58 estimates to statistically evaluate the suitability of a wind speed parameterization for k, when the ocean surface is ice covered. Whereas the six profiles taken from the open ocean indicate a statistically good fit to wind speed parameterizations, the same parameterizations could not reproduce k from the sea ice zone. We conclude that techniques for estimating k in the open ocean cannot be similarly applied to determine k in the presence of sea ice. The magnitude of k through gaps in the ice may reach high values as ice cover increases, possibly as a result of focused turbulence dissipation at openings in the free surface. These 58 profiles are presently the most complete set of estimates of k across seasons and variable ice cover; as dissolved tracer budgets they reflect air-sea gas exchange with no impact from air-ice gas exchange.NSF Arctic Natural Sciences program Grant Number: 12035582017-11-0

Meridional circulation across the Antarctic Circumpolar Current serves as a double 231Pa and 230Th trap

Upwelling of Circumpolar Deep Water in the Weddell Gyre and low scavenging rates south of the Antarctic Circumpolar Current (ACC) cause an accumulation of particle reactive nuclides in the Weddell Gyre. A ventilation/reversible scavenging model that successfully described the accumulation of 230Th in this area was tested with other particle reactive nuclides and failed to adequately describe the depth-distributions of 231Pa and 210Pb. We present here a modified model that includes a nutrient-like accumulation south of the Antarctic Polar Front in an upper meridional circulation cell, as well as transport to a deep circulation cell in the Weddell Gyre by scavenging and subsequent release at depth. The model also explains depletion of 231Pa and 230Th in Weddell Sea Bottom Water (WSBW) by ventilation of newly formed deep water on a timescale of 10 years, but this water mass is too dense to leave the Weddell Gyre. In order to quantify the processes responsible for the 231Pa- and 230Th-composition of newly formed Antarctic Bottom Water (AABW) we present a mass balance of 231Pa and 230Th in the Atlantic sector of the Southern Ocean based on new data from the GEOTRACES program. The ACC receives View the MathML source6.0±1.5×106 dpms−1 of 230Th from the Weddell Sea, similar in magnitude to the net input of View the MathML source4.2±3.0×106 dpms−1 from the north. For 231Pa, the relative contribution from the Weddell Sea is much smaller, only 0.3±0.1×1060.3±0.1×106, compared to View the MathML source2.7±1.4×106 dpms−1 from the north. Weddell Sea Deep Water (WSDW) leaving the Weddell Gyre northward to form AABW is exposed in the ACC to resuspended opal-rich sediments that act as efficient scavengers with a Th/Pa fractionation factor F≤1F≤1. Hydrothermal inputs may provide additional removal with low F. Scavenging in the full meridional circulation across the opal-rich ACC thus acts as a double 231Pa and 230Th trap that preconditions newly formed AABW

Radium Isotopes Across the Arctic Ocean Show Time Scales of Water Mass Ventilation and Increasing Shelf Inputs

The first full transarctic section of 228Ra in surface waters measured during GEOTRACES cruises PS94 and HLY1502 (2015) shows a consistent distribution with maximum activities in the transpolar drift. Activities in the central Arctic have increased from 2007 through 2011 to 2015. The increased 228Ra input is attributed to stronger wave action on shelves resulting from a longer ice-free season. A concomitant decrease in the 228Th/228Ra ratio likely results from more rapid transit of surface waters depleted in 228Th by scavenging over the shelf. The 228Ra activities observed in intermediate waters

An assessment of the use of sediment traps for estimating upper ocean particle fluxes

Author Posting. © Sears Foundation for Marine Research, 2007. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 65 (2007): 345–416, doi: 10.1357/002224007781567621This review provides an assessment of sediment trap accuracy issues by gathering data to address trap hydrodynamics, the problem of zooplankton "swimmers," and the solubilization of material after collection. For each topic, the problem is identified, its magnitude and causes reviewed using selected examples, and an update on methods to correct for the potential bias or minimize the problem using new technologies is presented. To minimize hydrodynamic biases due to flow over the trap mouth, the use of neutrally buoyant sediment traps is encouraged. The influence of swimmers is best minimized using traps that limit zooplankton access to the sample collection chamber. New data on the impact of different swimmer removal protocols at the US time-series sites HOT and BATS are compared and shown to be important. Recent data on solubilization are compiled and assessed suggesting selective losses from sinking particles to the trap supernatant after collection, which may alter both fluxes and ratios of elements in long term and typically deeper trap deployments. Different methods are needed to assess shallow and short- term trap solubilization effects, but thus far new incubation experiments suggest these impacts to be small for most elements. A discussion of trap calibration methods reviews independent assessments of flux, including elemental budgets, particle abundance and flux modeling, and emphasizes the utility of U-Th radionuclide calibration methods.WG meetings and production of this report was partially supported by the U.S. National Science Foundation via grants to the SCOR. Individuals and science efforts discussed herein were supported by many national science programs, including the U.S. National Science Foundation, Swedish Research Council, the International Atomic Energy Agency through its support of the Marine Environmental Laboratory that also receives support from the Government of the Principality of Monaco, and the Australian Antarctic Science Program. K.B. was supported in part by a WHOI Ocean Life Institute Fellowship

Comment on "Do geochemical estimates of sediment focusing pass the sediment test in the equatorial Pacific?" by M. Lyle et al.

Accurately estimating the vertical flux of material reaching the seafloor from the overlying surface waters is essential for the paleoceanographic reconstruction of a wide variety of oceanic processes. Two approaches are currently being used. One consists of estimating mass accumulation rates (MAR) between dated horizons as the product of linear sedimentation rates, sediment dry bulk densities, and concentrations. One pitfall with this approach is that sediments can be redistributed on the seafloor by bottom currents, and their accumulation may not necessarily reflect the true vertical rain rate originating from the overlying water column. To address this problem, the method of ²³⁰Th normalization was developed [Bacon, 1984]. This method is based on the assumption that the rapid scavenging of ²³⁰Th produced in the water column by decay of dissolved uranium results in its flux to the seafloor always being close to its known rate of production. To the extent that this assumption is correct, scavenged ²³⁰Th can be used as a reference to estimate the settling flux of other sedimentary constituents and to correct for sediment redistribution on the seafloor [Henderson and Anderson, 2003; Francois et al., 2004]

Radium Isotopes across the Arctic Ocean show Time Scales of Water Mass Ventilation and Increasing Shelf Inputs

The first full transarctic section of 228Ra in surface waters measured during GEOTRACES cruises PS94 and HLY1502 (2015) shows a consistent distribution with maximum activities in the Transpolar Drift. Activities in the central Arctic have increased from 2007 through 2011 to 2015. The increased 228Ra input is attributed to stronger wave action on shelves resulting from a longer ice‐free season. A concomitant decrease in the 228Th/228Ra ratio likely results from more rapid transit of surface waters depleted in 228Th by scavenging over the shelf. The 228Ra activities observed in intermediate waters (< 1500m) in the Amundsen Basin are explained by ventilation with shelf water on a time scale of about 15‐18 years, in good agreement with estimates based on SF6 and 129I/236U. The 228Th excess below the mixed layer up to 1500m depth can complement 234Th and 210Po as tracers of export production, after correction for the inherent excess resulting from the similarity of 228Ra and 228Th decay times. We show with a Th/Ra profile model that the 228Th/228Ra ratio below 1500m is inappropriate for this purpose because it is a delicate balance between horizontal supply of 228Ra and vertical flux of particulate 228Th. The accumulation of 226Ra in the deep Makarov Basin is not associated with an accumulation of Ba and can therefore be attributed to supply from decay of 230Th in the bottom sediment. We estimate a ventilation time of 480 years for the deep Makarov‐Canada Basin, in good agreement with previous estimates using other tracers