Description: Abstract Since the 1980s-1990s, international research efforts have augmented our knowledge of the physical and chemical properties of the Arctic Ocean water masses, and recent studies have documented changes. Understanding the processes responsible for these changes is necessary to be able to forecast the local and global consequences of these property evolutions on climate. The present work investigates the distributions of geochemical tracers of particle fluxes and circulation in the Amerasian Basin and their temporal evolution over the last three decades (from stations visited between 1983 and 2015). Profiles of 230-thorium (230Th) and 231-protactinium (231Pa) concentrations and neodymium isotopes (expressed as εNd) measured in the Amerasian Basin prior to 2000 are compared to a new, post-2000s data set. The comparison shows a large scale decrease in dissolved 230Th and 231Pa concentrations, suggesting intensification of scavenging by particle flux, especially in coastal areas. Higher productivity and sediment resuspension from the shelves appear responsible for the concentration decrease along the margins. In the basin interior, increased lateral exchanges with the boundary circulation also contribute to the decrease in concentration. This study illustrates how dissolved 230Th and 231Pa, with εNd support, can provide unique insights not only into changes in particle flux but also into the evolution of ocean circulation and mixing.

Description: Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 73 (2009): 2938-2960, doi:10.1016/j.gca.2009.02.029.

Description: The purpose of this research is to characterize the mobilization and immobilization processes that control the authigenic accumulation of uranium (U), rhenium (Re) and molybdenum (Mo) in marine sediments. We analyzed these redox– sensitive metals (RSM) in benthic chamber, pore water and solid phase samples at a site in Buzzards Bay, Massachusetts, U.S.A., which has high bottom water oxygen concentrations (230–300 mol/L) and high organic matter oxidation rates (390 mol C/cm2/y). The oxygen penetration depth varies from 2–9 mm below the sediment–water interface, but pore water sulfide is below detection (〈 2M). The RSM pore water profiles are modeled with a steady–state diagenetic model that includes irrigation, which extends 10–20 cm below the sediment–water interface. To present a consistent description of trace metal diagenesis in marine sediments, RSM results from sediments in Buzzards Bay are compared with previous research from sulfidic sediments (Morford et al., GCA 71). Release of RSM to pore waters during the remineralization of solid phases occurs near the sediment–water interface at depths above the zone of authigenic RSM formation. This release occurs consistently for Mo at both sites, but only in the winter for Re in Buzzards Bay and intermittently for U. At the Buzzards Bay site, Re removal to the solid phase extends to the bottom of the profile, while the zone of removal is restricted to ~2–9 cm for U and Mo. Authigenic Re formation is independent of the anoxic remineralization rate, which is consistent with an abiotic removal mechanism. The rate of authigenic U formation and its modeled removal rate constant increase with increasing anoxic remineralization rates, and is consistent with U reduction being microbially mediated. Authigenic Mo formation is related to the formation of sulfidic microenvironments. The depth and extent of Mo removal from pore water is closely associated with the balance between iron and sulfate reduction and the consumption of pore water sulfide via iron sulfide formation. Pore water RSM reach constant asymptotic concentrations in sulfidic sediments, but only pore water Re is constant at depth in Buzzards Bay. The increases in pore water U at the Buzzards Bay site are consistent with addition via irrigation and subsequent upward diffusion to the removal zone. Deep pore water Mo concentrations exceed its bottom water concentration due to irrigation–induced oxidation and remobilization from the solid phase. In sulfidic sediments, there is no evidence for higher pore water U or Mo concentrations at depth due to the absence of irrigation and/or the presence of more stable authigenic RSM phases. There are good correlations between benthic fluxes and authigenic accumulation rates for U and Mo in sulfidic sediments. However, results from Buzzards Bay suggest irrigation ultimately results in the partial loss of U and Mo from the solid phase, with accumulation rates that are 20–30% of the modeled flux. Irrigation can augment (Re, possibly U) or compromise (U, Mo) authigenic accumulation in sediments, and is important when determining burial rates in continental margin sediments.

Description: The authors also acknowledge financial support from the National Science Foundation (JLM, WRM: OCE–0220892), Research Corporation (JLM, CMC), Franklin & Marshall College, and the Hackman Summer Research Program at F&M.

Description: Author Posting. © American Geophysical Union, 2015. 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 120 (2015): 3542-3566, doi:10.1002/2014JC010620.

Description: We present the results of a 6 week time series of carbonate system and stable isotope measurements investigating the effects of sea ice on air-sea CO2 exchange during the early melt period in the Canadian Arctic Archipelago. Our observations revealed significant changes in sea ice and sackhole brine carbonate system parameters that were associated with increasing temperatures and the buildup of chlorophyll a in bottom ice. The warming sea-ice column could be separated into distinct geochemical zones where biotic and abiotic processes exerted different influences on inorganic carbon and pCO2 distributions. In the bottom ice, biological carbon uptake maintained undersaturated pCO2 conditions throughout the time series, while pCO2 was supersaturated in the upper ice. Low CO2 permeability of the sea ice matrix and snow cover effectively impeded CO2 efflux to the atmosphere, despite a strong pCO2 gradient. Throughout the middle of the ice column, brine pCO2 decreased significantly with time and was tightly controlled by solubility, as sea ice temperature and in situ melt dilution increased. Once the influence of melt dilution was accounted for, both CaCO3 dissolution and seawater mixing were found to contribute alkalinity and dissolved inorganic carbon to brines, with the CaCO3 contribution driving brine pCO2 to values lower than predicted from melt-water dilution alone. This field study reveals a dynamic carbon system within the rapidly warming sea ice, prior to snow melt. We suggest that the early spring period drives the ice column toward pCO2 undersaturation, contributing to a weak atmospheric CO2 sink as the melt period advances.

Description: We acknowledge support from the Polar Continental Shelf Program (PCSP) of Natural Resources Canada, the Natural Sciences and Engineering Research Council of Canada, the Northern Scientific Training Program, Canada Economic Development, and Fisheries and Oceans Canada.

Description: 2015-11-19

Description: Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 70 (2006): 5104-5118, doi:10.1016/j.gca.2006.07.036.

Description: The oceanic cycle of cadmium is still poorly understood, despite its importance for phytoplankton growth and paleoceanographic applications. As for other elements that are biologically recycled, variations in isotopic composition may bring unique insights. This article presents i) a protocol for the measurement of cadmium isotopic composition (Cd IC) in seawater and in phytoplankton cells; ii) the first Cd IC data in seawater, from two full depth stations, in the northwest Pacific and the northwest Mediterranean Sea; iii) the first Cd IC data in phytoplankton cells, cultured in vitro. The Cd IC variation range in seawater found at these stations is not greater than 1.5 eCd/amu units, only slightly larger than the mean uncertainty of measurement (0.8 eCd/amu). Nevertheless, systematic variations of the Cd IC and concentration in the upper 300m of the northwest Pacific suggest the occurrence of Cd isotopic fractionation by phytoplankton uptake, with a fractionation factor of 1.6±1.4 eCd/amu units. This result is supported by the culture experiment data suggesting that freshwater phytoplankton (Chlamydomonas reinhardtii and Chlorella sp.) preferentially take up light Cd isotopes, with a fractionation factor of 3.4±1.4 eCd/amu units. Systematic variations of the Cd IC and hydrographic data between 300 and 700m in the northwest Pacific have been tentatively attributed to the mixing of the mesothermal (temperature maximum) water (eCd/amu=-0.9±0.8) with the North Pacific Intermediate Water (eCd/amu=0.5±0.8). In contrast, no significant Cd IC variation is found in the northwest Mediterranean Sea. This observation was attributed to the small surface Cd depletion by phytoplankton uptake and the similar Cd IC of the different water masses found at this site. Overall, these data suggest that i) phytoplankton uptake fractionates Cd isotopic composition to a measurable degree (fractionation factors of 1.6 and 3.4 eCd/amu units, for the in situ and culture experiment data, respectively), ii) an open ocean profile of Cd IC shows upper water column variations consistent with preferential uptake and regeneration of light Cd isotopes, and iii) different water masses may have different Cd IC. This isotopic system could therefore provide information on phytoplankton Cd uptake and on water mass trajectories and mixing in some areas of the ocean. However, the very small Cd IC variations found in this study indicate that applications of Cd isotopic composition to reveal aspects of the present or past Cd oceanic cycle will be very challenging and may require further analytical improvements. Better precision could possibly be obtained with larger seawater samples, a better chemical separation of tin and a more accurate mass bias correction through the use of the double spiking technique.

Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 19 (2004): PA1018, doi:10.1029/2003PA000939.

Description: There is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates. Notwithstanding its common incidence, this process (generally referred to as sediment focusing) is often difficult to recognize. If redistribution is near synchronous to deposition, the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked. Ignoring it, however, can lead to serious misinterpretations of sedimentary fluxes, particularly when past changes in export flux from the overlying water are inferred. In many instances, this problem can be resolved, at least for sediments deposited during the late Quaternary, by normalizing to the flux of 230Th scavenged from seawater, which is nearly constant and equivalent to the known rate of production of 230Th from the decay of dissolved 234U. We review the principle, advantages and limitations of this method. Notwithstanding its limitations, it is clear that 230Th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates.

Description: R. Francois and M. P. Bacon acknowledge support from the National Science Foundation. M. Frank thanks the Swiss Science Foundation for support.

Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 19 (2004): PA2018, doi:10.1029/2003PA000986.

Description: The eastern equatorial Pacific (EEP) is an important center of biological productivity, generating significant organic carbon and calcite fluxes to the deep ocean. We reconstructed paleocalcite flux for the past 30,000 years in four cores collected beneath the equatorial upwelling and the South Equatorial Current (SEC) by measuring ex230Th-normalized calcite accumulation rates corrected for dissolution with a newly developed proxy for “fraction of calcite preserved.” This method produced very similar results at the four sites and revealed that the export flux of calcite was 30–50% lower during the LGM compared to the Holocene. The internal consistency of these results supports our interpretation, which is also in agreement with emerging data indicating lower glacial productivity in the EEP, possibly as a result of lower nutrient supply from the southern ocean via the Equatorial Undercurrent. However, these findings contradict previous interpretations based on mass accumulation rates (MAR) of biogenic material in the sediment of the EEP, which have been taken as reflecting higher glacial productivity due to stronger wind-driven upwelling.

Description: This research was partly supported by NSF grant OCE-0095617 and funds from the Northern Illinois University Graduate School (Loubere); the NASA Michigan Space Grant Consortium Seed Grant for summer, 2001 for 230Th analyses at WHOI (Mekik); the French Ministere de l’Education Nationale, de la Recherche et de la Technologie, and a EURODOC grant from the Region Rhone-Alpes (Pichat).

Description: Author Posting. © The Author(s), 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 71 (2007): 895-917, doi:10.1016/j.gca.2006.10.016.

Description: This study examined the removal of U, Mo, and Re from seawater by sedimentary processes at a shallow-water site with near-saturation bottom water O2 levels (240-380 μmol O2/L), very high organic matter oxidation rates (annually averaged rate is 870 μmol C/cm2/y), and shallow oxygen penetration depths (4 mm or less throughout the year). Under these conditions, U, Mo, and Re were removed rapidly to asymptotic pore water concentrations of 2.2–3.3 nmol/kg (U), 7–13 nmol/kg (Mo), and 11–14 pmol/kg (Re). The order in which the three metals were removed, determined by fitting a diffusion-reaction model to measured profiles, was Re 〈 U 〈 Mo. Model fits also suggest that the Mo profiles clearly showed the presence of a near-interface layer in which Mo was added to pore waters by remineralization of a solid phase. The importance of this solid phase source of pore water Mo increased from January to October as the organic matter oxidation rate increased, bottom water O2 decreased, and the O2 penetration depth decreased. Experiments with in situ benthic flux chambers generally showed fluxes of U and Mo into the sediments. However, when the overlying water O2 concentration in the chambers was allowed to drop to very low levels, Mn and Fe were released to the overlying water along with the simultaneous release of Mo and U. These experiments suggest that remineralization of Mn and/or Fe oxides may be a source of Mo and perhaps U to pore waters, and may complicate the accumulation of U and Mo in bioturbated sediments with high organic matter oxidation rates and shallow O2 penetration depths. Benthic chamber experiments including the nonreactive solute tracer, Br-, indicated that sediment irrigation was very important to solute exchange at the study site. The enhancement of sediment-seawater exchange due to irrigation was determined for the nonreactive tracer (Br-), TCO2, NH4 +, U and Mo. The comparisons between these solutes showed that reactions within and around the burrows were very important for modulating the Mo flux, but less important for U. The effect of these reactions on Mo exchange was highly variable, enhancing Mo (and, to a lesser extent, U) uptake at times of relatively modest irrigation, but inhibiting exchange when irrigation rates were faster. These results reinforce the observation that Mo can be released to and removed from pore waters via sedimentary reactions. The removal rate of U and Mo from seawater by sedimentary reactions was found to agree with the rate of accumulation of authigenic U and Mo in the solid phase. The fluxes of U and Mo determined by in situ benthic flux chamber measurements were the largest that have been measured to date. These results confirm that removal of redoxsensitive metals from continental margin sediments underlying oxic bottom water is important, and suggest that continental margin sediments play a key role in the marine budgets of these metals.

Description: We appreciate the financial support from the National Science Foundation (OCE-0220892). Funding for this work was also provided to JLM by the Postdoctoral Scholar Program at WHOI courtesy of the Cabot Marine Environmental Science Fund and the J. Seward Johnson Fund. Financial support to IMK was given by The Swedish Foundation for International Cooperation in Research and Higher Education.

Description: Author Posting. © Elsevier B.V., 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 54 (2007): 557-585, doi:10.1016/j.dsr.2007.01.002.

Description: An inverse finite-difference model of the abyssal circulation in the North Atlantic Ocean is developed in order to evaluate the dynamical information contained in measurements of thorium-230 (230Th). The model has a very coarse resolution and is based on lowest order balances for planetary flows. The naturally occurring 230Th differs from more conventional oceanic tracers in several respects, e.g., its production (by 234U radioactive decay) is globally uniform to a good approximation and its removal can be understood in terms of a simple reversible exchange with particles sinking slowly to the seafloor. The time required for 230Th to reach steady state with respect to particle exchange is estimated to increase with depth, reaching O(10) yr below 1000 m. In the North Atlantic 230Th activities at distant locations share a similar increase with depth in the upper 1000m—a pattern consistent with a reversible exchange—but show drastic differences in the abyssal interior. Two inversions are conducted in order to determine whether the 230Th differences reflect the effects of the circulation—by preventing the slow attainment to steady state w.r.t. particle exchange in deep water—and provide complementary information about the abyssal flow. In a first inversion, observations of density from a hydrographic compilation and of volume transports at specific locations are combined with the dynamical balances in order to infer the basin-scale flow. The inferred flow displays the western boundary current and coherent structures in the abyssal interior with low statistical significance. In a second inversion, the flow is further constrained by the 230Th measurements and the condition that 230Th divergence by the flow field and particle sinking must be locally balanced by 230Th production from 234U decay. The addition of 230Th leads to the estimation of a larger amplitude of the integrated meridional transports below 1000 m (by 2–9 Sv), where the range reflects the uncertainties in the large scale 230Th distribution and in the radiochemical balance. This result is interpreted as a correction by 230Th for the tendency of inverse geostrophic models to lead to the inference of a vanishing circulation when horizontal density gradients are insignificant.

Description: OM acknowledges the support from the Ocean and Climate Change Institute at WHOI and from the US National Science Foundation. The IAEA (JS) is grateful for the support provided to its Marine Environment Laboratory by the Government of the Principality of Monaco. JS is grateful to Jan Fietzke for ICPMS measurements and for support from the ‘Deutsche Forschungsgemeinschaft’ (grant no. SCHO752/ 2-1).

Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 19 (2004): PA4023, doi:10.1029/2003PA000994.

Description: The (231Pa/230Th)xs,0 records obtained from two cores from the western (MD97-2138; 1°25′S, 146°24′E, 1900 m) and eastern (Ocean Drilling Program Leg 138 Site 849, 0°11.59′N, 110°31.18′W, 3851 m) equatorial Pacific display similar variability over the last 85,000 years, i.e., from isotopic stages 1 to 5a, with systematically higher values during the Holocene, isotopic stage 3, and isotopic stage 5a, and lower values, approaching the production rate ratio of the two isotopes (0.093), during the colder periods corresponding to isotopic stages 2 and 4. We have also measured the 230Th-normalized biogenic preserved and terrigenous fluxes, as well as major and trace elements concentrations, in both cores. The (231Pa/230Th)xs,0 results combined with the changes in preserved carbonate and opal fluxes at the eastern site indicate lower productivity in the eastern equatorial Pacific during glacial periods. The (231Pa/230Th)xs,0 variations in the western equatorial Pacific also seem to be controlled by productivity (carbonate and/or opal). The generally high (231Pa/230Th)xs,0 ratios (〉0.093) of the profile could be due to opal and/or MnO2 in the sinking particles. The profiles of (231Pa/230Th)xs,0 and 230Th-normalized fluxes indicate a decrease in exported carbonate, and possibly opal, during isotopic stages 2 and 4 in MD97-2138. Using 230Th-normalized flux, we also show that sediments from the two cores were strongly affected by sediment redistribution by bottom currents suggesting a control of mass accumulation rates by sediment focusing variability.

Description: SP funding for this research was provided by grants from the French Minister of Research and a EURODOC grant of the Re´gion Rhoˆne-Alpes (SAFIR-980065327). SP also gratefully acknowledges the financial support of the WHOI Geology and Geophysics Dept. This work was also supported by a CNRS-NSF grant (SP and KWWS). The contribution of JFM to this study was supported in part by the US NSF and by WHOI OCCI and Mellon awards.

Description: Author Posting. © American Geophysical Union, 2016. 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 121 (2016): 2918–2936, doi:10.1002/2015JC011535.

Description: We examine the dissolved inorganic carbon maximum in the Canada Basin halocline using a suite of geochemical tracers to gain insight into the factors that contribute to the persistence of this feature. Hydrographic and geochemical samples were collected in the upper 500 m of the southwestern Canada Basin water column in the summer of 2008 and fall of 2009. These observations were used to identify conservative and nonconservative processes that contribute dissolved inorganic carbon to halocline source waters, including shelf sediment organic matter remineralization, air-sea gas exchange, and sea-ice brine export. Our results indicate that the remineralization of organic matter that occurs along the Bering and Chukchi Sea shelves is the overwhelming contributor of dissolved inorganic carbon to Pacific Winter Water that occupies the middle halocline in the southwestern Canada Basin. Nonconservative contributions from air-sea exchange and sea-ice brine are not significant. The broad salinity range associated with the DIC maximum, compared to the narrow salinity range of the nutrient maximum, is due to mixing between Pacific and Atlantic water and not abiotic addition of DIC.

Description: NSERC; Fisheries and Oceans Canada; US National Science Foundation Office of Polar Programs Grant Number: OPP-0424864; Canadian International Polar Year Office

Description: 2016-11-04