ALBERT

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Philosophy and the Woods Hole Oceanographic Institution November 1980

Description: Suspended particulate matter was collected by sediment traps deployed in the Sargasso Sea (Site S2), the north equatorial Atlantic (Site E), the north equatorial Pacific (Site P), and the Panama Basin (STIE Site). Additional samples of suspended particles were obtained by in situ filtration at Site F., at the STIE Site, and in the Guatemala Basin. Concentrations of dissolved Th and Pa were determined by extraction onto manganese dioxide adsorbers at Site P, at a second site in the Sargasso Sea (Site D), at the STIE Site and in the Guatemala Basin. Sediment samples were obtained from cores taken near Sites E and P. Results have shown unequivocally that suspended particulate matter in the open ocean preferentially scavenges Th relative to Pa. This behavior could not have been predicted from the known physical chemistry of Th and Pa. Dissolved 230Th/231Pa activity ratios were 3-5 at Sites P and D and 3-8 at the STIE Site. In contrast, unsupported 230Th/231Pa ratios were 22-35 (average 29.7 for 7 samples) in sediment-trap samples from greater than 2000 m at Sites S2, E and P. Ratios were lower in particulate matter sampled at shallower depths. Particles filtered at 3600 m and 5000 m at Site E had ratios of 50 and 40. In contrast to the open ocean samples described above, samples collected by six sediment traps at depths of 667-3791 m in the Panama Basin had unsupported 230Th/231Pa ratios of 4-8, and the deepest samples had the lowest ratios. Fractionation of Th and Pa that was observed at the three open ocean sites either does not occur or occurs to a very limited extent in the Panama Basin. Particulate 230Th/231Pa ratios were negatively correlated with the concentration of suspended particles. However, variable scavenging rates, as indicated by variable particle concentration, do not completely control the ratio at which Th and Pa are scavenged from solution. Major biogenic and inorganic components of trapped material were found in approximately the same proportions in the STIE samples and in samples from Sites E and S2. Lower 230Th/231Pa ratios found in the STIE samples must therefore result from subtle changes in the chemical properties of the particles. Consideration of 230Th/23lPa ratios in several depositional environments indicates that no single factor controls the ratio at which Th and Pa are adsorbed from seawater. Fluxes of 210Th and 231Pa were less than their rates of production in the overlying water column in every trap at Sites S2, E, and P. In the Panama Basin, fluxes measured with the same traps were greater than or equal to their rates of production. These results are a strong indication that even extremely reactive elements such as Th and Pa are redistributed within the oceans. Redistribution occurs because variable scavenging rates in different environments set up horizontal concentration gradients. Horizontal mixing processes produce a net horizontal transport of Th and Pa from areas of 1ow scavenging rates to areas of high scavenging rates. Protactinium is redistributed to a greater extent than Th. Fluxes of 230Th can be used to set lower limits for horizonttal transport of Pa even when absolute trapping efficiencies of the sediment traps are not known. Less than 50% of the Pa produced at the open ocean sites is removed from the water column by scavenging to settling particles. The remainder is removed by horizontal transport to other environments. At Sites E and P, 230Th/231Pa ratios were identical in the deepest sediment trap sample and in surface sediments. However, 230Th/232Th and 231Pa/232Th ratios were 2.5 times higher in trapped particles than in surface sediments. The 230Th/232Th ratios were 5.5 times higher in particles filtered at 3600 m and 5000 m at Site E than in surface sediments. This observation is best explained by dissolution of most of the 230Th and 231Pa scavenged by settling particles during remineralization of labile biogenic phases. The behaviors of certain other radioisotopes were also studied. 232Th is present only in detrital mineral components of trapped material. Concentrations of 232Th in trapped particles correlate closely with Al and K, at ratios approaching that of average shale or crustal abundances at Site E and P and basalts at the STIE Site. High specifìc activities of 228Th and 239+240Pu were found in sediment trap samples throughout the water column at Sites E and P and in the Panama Basin. The dominant source of these isotopes is near the sea surface and also near the sea floor in the case of 228Th. Thus it appears that the bulk of the trapped material is recently derived from the sea surface where it incorporates these isotopes,with little loss during rapid transit through the water column. A bioauthigenic form of particulate uranium is produced at the sea surface and remineralized in the deep ocean along with its labile carrier phase(s). This flux of uranium to the deep ocean is 0.25-1.0 dpm/cm2103 years, which is insufficient to cause a measurable concentration gradient in the uranium distribution within the mixing time of the oceans. Increased concentrations and fluxes of particulate uranium were not found in the eastern equatorial North Pacific under areas of an intense oxygen minimum. Therefore, reduction of uranium to the tetravalent state with subsequent scavenging to settling particles in oxygen minima is not a mechanism removing uranium from the oceans.

Description: Financial support for parts of this work have come from many sources, including: National Science Foundation Grants OCE-7826318, OCE-7825724, and OCE-7727004; Department of Energy Contract EY-76-S-02-3566; a Cottrell Research Grant from the Research Corporation; the WHOI Ocean Industries Program; a fellowship from the WHOI Education Office, and the Paul Fye Fellowship

Description: Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 32(12), (2019): 1738-1758, doi:10.1029/2018GB005994.

Description: Sinking particles strongly regulate the distribution of reactive chemical substances in the ocean, including particulate organic carbon and other elements (e.g., P, Cd, Mn, Cu, Co, Fe, Al, and 232Th). Yet, the sinking fluxes of trace elements have not been well described in the global ocean. The U.S. GEOTRACES campaign in the North Atlantic (GA03) offers the first data set in which the sinking flux of carbon and trace elements can be derived using four different radionuclide pairs (238U:234Th ;210Pb:210Po; 228Ra:228Th; and 234U:230Th) at stations co‐located with sediment trap fluxes for comparison. Particulate organic carbon, particulate P, and particulate Cd fluxes all decrease sharply with depth below the euphotic zone. Particulate Mn, Cu, and Co flux profiles display mixed behavior, some cases reflecting biotic remineralization, and other cases showing increased flux with depth. The latter may be related to either lateral input of lithogenic material or increased scavenging onto particles. Lastly, particulate Fe fluxes resemble fluxes of Al and 232Th, which all have increasing flux with depth, indicating a dominance of lithogenic flux at depth by resuspended sediment transported laterally to the study site. In comparing flux estimates derived using different isotope pairs, differences result from different timescales of integration and particle size fractionation effects. The range in flux estimates produced by different methods provides a robust constraint on the true removal fluxes, taking into consideration the independent uncertainties associated with each method. These estimates will be valuable targets for biogeochemical modeling and may also offer insight into particle sinking processes.

Description: This study grew out of a synthesis workshop at the Lamont‐Doherty Earth Observatory of Columbia University in August 2016. This workshop was sponsored by the U.S. GEOTRACES Project Office (NSF 1536294) and the Ocean Carbon and Biogeochemistry (OCP) Project Office (NSF 1558412 and NASA NNX17AB17G). The U.S. National Science Foundation supported all of the analytical work on GA03. Kuanbo Zhou measured 228Th in the large size class particles (NSF 0925158 to WHOI). NSF 1061128 to Stony Brook University supported the BaRFlux project, for which Chistina Heilbrun is acknowledged for laboratory and field work. The lead author acknowledges support from a start‐up grant from the University of Southern Mississippi. Two anonymous reviewers are thanked for their constructive comments. All GEOTRACES GA03 data used in this study are accessible through the Biological and Chemical Oceanography Data Management Office (http://data.bco‐dmo.org/jg/dir/BCO/GEOTRACES/NorthAtlanticTransect/), and derived parameters are reported in the supporting information.

Description: 2019-05-22

Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Black, E. E., Kienast, S. S., Lemaitre, N., Lam, P. J., Anderson, R. F., Planquette, H., Planchon, F., & Buesseler, K. O. Ironing out Fe residence time in the dynamic upper ocean. Global Biogeochemical Cycles, 34(9), (2020): e2020GB006592, doi:10.1029/2020GB006592.

Description: Although iron availability has been shown to limit ocean productivity and influence marine carbon cycling, the rates of processes driving iron's removal and retention in the upper ocean are poorly constrained. Using 234Th‐ and sediment‐trap data, most of which were collected through international GEOTRACES efforts, we perform an unprecedented observation‐based assessment of iron export from and residence time in the upper ocean. The majority of these new residence time estimates for total iron in the surface ocean (0–250 m) fall between 10 and 100 days. The upper ocean residence time of dissolved iron, on the other hand, varies and cycles on sub‐annual to annual timescales. Collectively, these residence times are shorter than previously thought, and the rates and timescales presented here will contribute to ongoing efforts to integrate iron into global biogeochemical models predicting climate and carbon dioxide sequestration in the ocean in the 21st century and beyond.

Description: We would like to thank S. Albani for providing the dust model results (Community Atmosphere Model, C4fn) and the three anonymous reviewers for their constructive comments. The U.S. GEOTRACES work was supported by the National Science Foundation (OCE‐1232669 and OCE‐1518110) and E. Black was also funded by a NASA Earth and Space Science Graduate Fellowship (NNX13AP31H) and the Ocean Frontier Institute. The GEOVIDE work was funded by the Flanders Research Foundation (G071512N), the Vrije Universiteit Brussel (SRP‐2), the French ANR Blanc GEOVIDE (ANR‐13‐BS06‐0014), ANR RPDOC BITMAP (ANR‐12‐PDOC‐0025‐01), IFREMER, CNRS‐INSU (programme LEFE), INSU OPTIMISP, and Labex‐Mer (ANR‐10‐LABX‐19).