Cochran J. Kirk

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Cochran
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J. Kirk
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  • Article
    210Po and 210Pb distributions during a phytoplankton bloom in the North Atlantic: Implications for POC export
    (Elsevier, 2020-07-07) Horowitz, Evan J. ; Cochran, J. Kirk ; Bacon, Michael P. ; Hirschberg, David J.
    During the North Atlantic Bloom Experiment (NABE) of the Joint Global Ocean Flux Study (JGOFS), water column sampling for particulate and dissolved 210Po and 210Pb was performed four times (26 April and 4, 20, 30 May 1989) during a month-long Lagrangian time-series occupation of the NABE site, as well as one-time samplings at stations during transit to and from the site. There are few prior studies documenting short-term changes in 210Po and 210Pb profiles over the course of a phytoplankton bloom, and we interpret the profiles in terms of the classical “steady-state” (SS) approach used in most studies, as well as by using a non-steady state approach suggested by the temporal evolution of the profiles. Changes in 210Po profiles during a bloom are expectable as this radionuclide is scavenged and exported. During NABE, 210Pb profiles also displayed non-steady state, with significant increases in upper water column inventory occurring midway through the experiment. Export of 210Po from the upper 150 m using the classic “steady-state” model shows increases from 0.5 ± 8.5 dpm m−2 d−1 to 68.2 ± 4.2 dpm m−2 d−1 over the ~one-month occupation. Application of a non-steady state model, including changes in both 210Pb and 210Po profiles, gives higher 210Po export fluxes. Detailed depth profiles of particulate organic carbon (>0.8 μm) and particulate 210Po (>0.4 μm) are available from the 20 and 30 May samplings and show maxima in POC/Po at ~37 m. Applying the POC/210Po ratios at 150 m to the “steady state” 210Po fluxes yields POC export from the upper 150 m of 8.2 ± 1.5 mmol C m− 2 d−1 on 20 May and 6.0 ± 1.6 mmol C m−2 d−1 on 30 May. The non-steady state model applied to the interval 20 to 30 May yields POC export of 24.3 mmol C m−2 d−1. The non-steady state (NSS) 210Po-derived POC fluxes are comparable to, but somewhat less than, those estimated previously from 234Th/238U disequilibrium for the same time interval (37.3 and 45.0 mmol m−2 d−1, depending on the POC/Th ratio used). In comparison, POC fluxes measured with a floating sediment trap deployed at 150 m from 20 to 30 May were 11.6 mmol m−2 d−1. These results suggest that non-steady state Po-derived POC fluxes during the NABE agree well with those derived from 234Th/238U disequilibrium and agree with sediment trap fluxes within a factor of ~2. However, unlike the 234Th-POC flux proxy, non-steady stage changes in profiles of 210Pb, the precursor of 210Po, must be considered.
  • Dataset
    Activities of 210Po and 210Pb in aerosol samples collected on Leg 1 (Seattle, WA to Hilo, HI) of the US GEOTRACES Pacific Meridional Transect (PMT) cruise (GP15, RR1814) on R/V Roger Revelle from September to October 2018
    (Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2022-09-09) Cochran, J. Kirk ; Kadko, David C.
    This dataset includes activities of 210Po (Polonium-210) and 210Pb (Lead-210) in aerosol samples collected on Leg 1 (Seattle, WA to Hilo, HI) of the US GEOTRACES Pacific Meridional Transect (PMT) cruise (GP15, RR1814) on R/V Roger Revelle from September to October 2018. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/878689
  • Dataset
    Activities of 210Po and 210Pb in aerosol samples collected on Leg 2 (Hilo, HI to Papeete, French Polynesia) of the US GEOTRACES Pacific Meridional Transect (PMT) cruise (GP15, RR1815) on R/V Roger Revelle from October to November 2018
    (Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2022-09-09) Cochran, J. Kirk ; Kadko, David C.
    This dataset includes activities of 210Po (Polonium-210) and 210Pb (Lead-210) in aerosol samples collected on Leg 2 (Hilo, HI to Papeete, French Polynesia) of the US GEOTRACES Pacific Meridional Transect (PMT) cruise (GP15, RR1815) on R/V Roger Revelle from October to November 2018 For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/878703
  • Article
    Investigating boron isotopes for identifying nitrogen sources supplied by submarine groundwater discharge to coastal waters
    (Frontiers Media, 2020-08-11) Tamborski, Joseph ; Brown, Caitlin ; Bokuniewicz, Henry J. ; Cochran, J. Kirk ; Rasbury, E. Troy
    Stable isotopes of oxygen, nitrogen, and boron were used to identify the sources of nitrate (NO3–) in submarine groundwater discharge (SGD) into a large tidal estuary (Long Island Sound, NY, United States). Potential contaminants such as manure, septic waste and fertilizer overlap in δ15N and δ18O but have been shown to have distinctive δ11B in non-coastal settings. Two distinct subterranean estuaries were studied with different land-use up gradient, representative of (1) mixed medium-density residential housing and (2) agriculture. These sites have overlapping δ15N and δ18O measurements in NO3– and are unable to discriminate between different N sources. Boron isotopes and concentrations are measurably different between the two sites, with little overlap. The subterranean estuary impacted by mixed medium-density residential housing shows little correlation between δ11B and [B] or between δ11B and salinity, demonstrating that direct mixing relationships between fresh groundwater and seawater were unlikely to account for the variability. No two sources could adequately characterize the δ11B of this subterranean estuary. Groundwater N at this location should be derived from individual homeowner cesspools, although measured septic waste has much lower δ11B compared to the coastal groundwaters. This observation, with no trend in δ11B with [B] indicates multiple sources supply B to the coastal groundwaters. The agricultural subterranean estuary displayed a positive correlation between δ11B and [B] without any relationship with salinity. Binary mixing between sea spray and fertilizer can reasonably explain the distribution of B in the agricultural subterranean estuary. Results from this study demonstrate that δ11B can be used in combination with δ15N to trace sources of NO3– to the subterranean estuary if source endmember isotopic signatures are well-constrained, and if the influence of seawater on δ11B signatures can be minimized or easily quantified.
  • Preprint
    An assessment of particulate organic carbon to thorium-234 ratios in the ocean and their impact on the application of 234Th as a POC flux proxy
    ( 2005-06-18) Buesseler, Ken O. ; Benitez-Nelson, Claudia R. ; Burd, Adrian B. ; Charette, Matthew A. ; Cochran, J. Kirk ; Coppola, L. ; Fisher, Nicholas S. ; Fowler, Scott W. ; Gardner, Wilford D. ; Guo, L. D. ; Gustafsson, Orjan ; Lamborg, Carl H. ; Masqué, Pere ; Miquel, Juan Carlos ; Passow, Uta ; Santschi, Peter H. ; Savoye, Nicolas ; Stewart, G. ; Trull, Thomas W.
    Thorium-234 is increasingly used as a tracer of ocean particle flux, primarily as a means to estimate particulate organic carbon export from the surface ocean. This requires determination of both the 234Th activity distribution (in order to calculate 234Th fluxes) and an estimate of the C/234Th ratio on sinking particles, to empirically derive C fluxes. In reviewing C/234Th variability, results obtained using a single sampling method show the most predictable behavior. For example, in most studies that employ in situ pumps to collect size fractionated particles, C/234Th either increases or is relatively invariant with increasing particle size (size classes >1 to 100’s μm). Observations also suggest that C/234Th decreases with depth and can vary significantly between regions (highest in blooms of large diatoms and highly productive coastal settings). Comparisons of C fluxes derived from 234Th show good agreement with independent estimates of C flux, including mass balances of C and nutrients over appropriate space and time scales (within factors of 2-3). We recommend sampling for C/234Th from a standard depth of 100 m, or at least one depth below the mixed layer using either large volume size fractionated filtration to capture the rarer large particles, or a sediment trap or other device to collect sinking particles. We also recommend collection of multiple 234Th profiles and C/234Th samples during the course of longer observation periods to better sample temporal variations in both 234Th flux and the characteristic of sinking particles. We are encouraged by new technologies which are optimized to more reliably sample truly settling particles, and expect the utility of this tracer to increase, not just for upper ocean C fluxes but for other elements and processes deeper in the water column.
  • Preprint
    Th-234 sorption and export models in the water column : a review
    ( 2005-10-10) Savoye, Nicolas ; Benitez-Nelson, Claudia R. ; Burd, Adrian B. ; Cochran, J. Kirk ; Charette, Matthew A. ; Buesseler, Ken O. ; Jackson, George A. ; Roy-Barman, Matthieu ; Schmidt, Sabine ; Elskens, Marc
    Over the past few decades, the radioisotope pair of 238U/234Th has been widely and increasingly used to describe particle dynamics and particle export fluxes in a variety of aquatic systems. The present paper is one of five review articles dedicated to 234Th. It is focused on the models associated with 234Th whereas the companion papers (same issue) are focused on present and future methodologies and techniques (Rutgers van der Loeff et al.), C/234Th ratios (Buesseler et al.), 234Th speciation (Santschi et al.) and present and future applications of 234Th (Waples et al.). In this paper, we review current 234Th scavenging models and discuss the relative importance of the non steady state and physical terms associated with the most commonly used model to estimate 234Th flux. Based on this discussion we recommend that for future work the use of models should be accompanied by a discussion of the effect that model and data uncertainty have on the model results. We also suggest that future field work incorporate repeat occupations of sample sites on time scales of 1-4 weeks in order to evaluate steady state versus non steady state estimates of 234Th export, especially during high flux events (> ca. 800 dpm m-2 d-1). Finally, knowledge of the physical oceanography of the study area is essential, particularly in ocean margins and in areas of established upwelling (e.g. Equatorial Pacific). These suggestions will greatly enhance the application of 234Th as a tracer of particle dynamics and flux in more complicated regimes.
  • Preprint
    Dissolved strontium in the subterranean estuary- implications for the marine strontium isotope budget
    ( 2013-01-13) Beck, Aaron J. ; Charette, Matthew A. ; Cochran, J. Kirk ; Gonneea, Meagan E. ; Peucker-Ehrenbrink, Bernhard
    Submarine groundwater discharge (SGD) to the ocean supplies Sr with less radiogenic 87Sr/86Sr than seawater, and thus constitutes an important term in the Sr isotope budget in the modern ocean. However, few data exist for Sr in coastal groundwater or in the geochemically dynamic subterranean estuary (STE). We examined Sr concentrations and isotope ratios from nine globally-distributed coastal sites and characterized the behavior of Sr in the STE. Dissolved Sr generally mixed conservatively in the STE, although large differences were observed in the meteoric groundwater end-member Sr concentrations among sites (0.1 – 24 μM Sr). Strontium isotope exchange was observed in the STE at five of the sites studied, and invariably favored the meteoric groundwater end-member signature. Most of the observed isotope exchange occurred in the salinity range 5-15, and reached up to 40% exchange at salinity 10. Differences in fresh groundwater Sr concentrations and isotope ratios (87Sr/86Sr = 0.707-0.710) reflected aquifer lithology. The SGD end-member 87Sr/86Sr must be lower than modern seawater (i.e., less than 0.70916) in part because groundwater Sr concentrations are orders of magnitude higher in less-carbonate and volcanic island aquifers. A simple lithological model and groundwater Sr data compiled from the literature were used to estimate a global average groundwater end-member of 2.9 μM Sr with 87Sr/86Sr = 0.7089. This represents a meteoric-SGD-driven Sr input to the ocean of 0.7-2.8 × 1010 mol Sr y-1. Meteoric SGD therefore accounts for 2-8% of the oceanic Sr isotope budget, comparable to other known source terms, but is insufficient to balance the remainder of the budget. Using reported estimates for brackish SGD, the estimated volume discharge at salinity 10 (7-11 × 1015 L y-1) was used to evaluate the impact of isotope exchange in the STE on the brackish SGD Sr flux. A moderate estimate of 25% isotope exchange in the STE gives an SGD Sr end-member 87Sr/86Sr of 0.7091. The brackish SGD Sr flux thus accounts for 11-23% of the marine Sr isotope budget, but does not appear sufficient to balance the ~40% remaining after other known sources are included. Substantial uncertainties remain for estimating the SGD source of Sr to the global ocean, especially in the determination of the volume flux of meteoric SGD, and in the paucity of measurements of groundwater Sr isotope composition in major SGD regions such as Papua New Guinea, the South America west coast, and West Africa. Consequently, our global estimate should be viewed with some caution. Nevertheless, we show that the combined sources of meteoric SGD and brackish SGD coupled with isotope exchange in the STE may constitute a substantial component (~13-30%) of the modern oceanic 87Sr/86Sr budget, likely exceeding less radiogenic Sr inputs by sedimentary diagenesis and hydrothermal circulation through the mid-ocean ridge system. Temporal variation in SGD Sr fluxes and isotope composition may have contributed to fluctuations in the oceanic 87Sr/86Sr ratio over geologic time.
  • Article
    Flux of particulate elements in the North Atlantic Ocean constrained by multiple radionuclides
    (American Geophysical Union, 2018-11-22) Hayes, Christopher T. ; Black, Erin E. ; Anderson, Robert F. ; Baskaran, Mark ; Buesseler, Ken O. ; Charette, Matthew A. ; Cheng, Hai ; Cochran, J. Kirk ; Edwards, R. Lawrence ; Fitzgerald, Patrick ; Lam, Phoebe J. ; Lu, Yanbin ; Morris, Stephanie O. ; Ohnemus, Daniel C. ; Pavia, Frank ; Stewart, Gillian ; Tang, Yi
    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.
  • Article
    Quantifying Po-210/Pb-210 disequilibrium in seawater: a comparison of two precipitation methods with differing results
    (Frontiers Media, 2021-06-22) Roca-Martí, Montserrat ; Puigcorbé, Viena ; Castrillejo, Maxi ; Casacuberta, Nuria ; Garcia-Orellana, Jordi ; Cochran, J. Kirk ; Masqué, Pere
    The disequilibrium between lead-210 (210Pb) and polonium-210 (210Po) is increasingly used in oceanography to quantify particulate organic carbon (POC) export from the upper ocean. This proxy is based on the deficits of 210Po typically observed in the upper water column due to the preferential removal of 210Po relative to 210Pb by sinking particles. Yet, a number of studies have reported unexpected large 210Po deficits in the deep ocean indicating scavenging of 210Po despite its radioactive mean life of ∼ 200 days. Two precipitation methods, Fe(OH)3 and Co-APDC, are typically used to concentrate Pb and Po from seawater samples, and deep 210Po deficits raise the question whether this feature is biogeochemically consistent or there is a methodological issue. Here, we present a compilation of 210Pb and 210Po studies that suggests that 210Po deficits at depths >300 m are more often observed in studies where Fe(OH)3 is used to precipitate Pb and Po from seawater, than in those using Co-APDC (in 68 versus 33% of the profiles analyzed for each method, respectively). In order to test whether 210Po/210Pb disequilibrium can be partly related to a methodological artifact, we directly compared the total activities of 210Pb and 210Po in four duplicate ocean depth-profiles determined by using Fe(OH)3 and Co-APDC on unfiltered seawater samples. While both methods produced the same 210Pb activities, results from the Co-APDC method showed equilibrium between 210Pb and 210Po below 100 m, whereas the Fe(OH)3 method resulted in activities of 210Po significantly lower than 210Pb throughout the entire water column. These results show that 210Po deficits in deep waters, but also in the upper ocean, may be greater when calculated using a commonly used Fe(OH)3 protocol. This finding has potential implications for the use of the 210Po/210Pb pair as a tracer of particle export in the oceans because 210Po (and thus POC) fluxes calculated using Fe(OH)3 on unfiltered seawater samples may be overestimated. Recommendations for future research are provided based on the possible reasons for the discrepancy in 210Po activities between both analytical methods.
  • Article
    Radium mass balance sensitivity analysis for submarine groundwater discharge estimation in semi-enclosed basins: the case study of Long Island Sound
    (Frontiers Media, 2020-07-17) Tamborski, Joseph ; Cochran, J. Kirk ; Bokuniewicz, Henry J. ; Heilbrun, Christina ; Garcia-Orellana, Jordi ; Rodellas, Valenti ; Wilson, Robert
    Estimation of submarine groundwater discharge (SGD) to semi-enclosed basins by Ra isotope mass balance is herein assessed. We evaluate 224Ra, 226Ra, and 228Ra distributions in surface and bottom waters of Long Island Sound (CT-NY, United States) collected during spring 2009 and summer 2010. Surface water and bottom water Ra activities display an apparent seasonality, with greater activities during the summer. Long-lived Ra isotope mass balances are highly sensitive to boundary fluxes (water flux and Ra activity). Variation (50%) in the 224Ra, 226Ra, and 228Ra offshore seawater activity results in a 63–74% change in the basin-wide 226Ra SGD flux and a 58–60% change in the 228Ra SGD flux, but only a 4–9% change in the 224Ra SGD flux. This highlights the need to accurately constrain long-lived Ra activities in the inflowing and outflowing water, as well as water fluxes across boundaries. Short-lived Ra isotope mass balances are sensitive to internal Ra fluxes, including desorption from resuspended particles and inputs from sediment diffusion and bioturbation. A 50% increase in the sediment diffusive flux of 224Ra, 226Ra, and 228Ra results in a ∼30% decrease in the 224Ra SGD flux, but only a ∼6–10% decrease in the 226Ra and 228Ra SGD flux. When boundary mixing is uncertain, 224Ra is the preferred tracer of SGD if sediment contributions are adequately constrained. When boundary mixing is well-constrained, 226Ra and 228Ra are the preferred tracers of SGD, as sediment contributions become less important. A three-dimensional numerical model is used to constrain boundary mixing in Long Island Sound (LIS), with mean SGD fluxes of 1.2 ± 0.9 × 1013 L y–1 during spring 2009 and 3.3 ± 0.7 × 1013 L y–1 during summer 2010. The SGD flux to LIS during summer 2010 was one order of magnitude greater than the freshwater inflow from the Connecticut River. The maximum marine SGD-driven N flux is 14 ± 11 × 108 mol N y–1 and rivals the N load of the Connecticut River.
  • Dataset
    Dissolved and total water column 210Po and 210Pb from samples collected on Leg 1 (Seattle, WA to Hilo, HI) of the US GEOTRACES Pacific Meridional Transect (PMT) cruise (GP15, RR1814) on R/V Roger Revelle from September to October 2018
    (Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2023-01-31) Cochran, J. Kirk
    This project is part of the international GEOTRACES program, which was created to allow a comprehensive, coordinated study of trace elements and isotopes (TEIs) in the oceans. This project uses the radionuclide pair 210Pb and its grand-daughter, 210Po, to provide important biogeochemical rate information pertinent to the trace elements and isotopes (TEIs) measured during the US GEOTRACES Meridional Transect in the Pacific from Alaska to Tahiti in late 2018. Many processes in the ocean cannot be directly observed and, as such, tracers such as 210Po and 210Pb can be used to provide important constraints on their rates and pathways. 210Po (half-life = 138 d) and 210Pb (half-life = 22.3 y) decay on timescales that are useful to study 1) atmospheric deposition of trace elements, 2) scavenging of particle-reactive trace elements, 3) export of particulate organic carbon (POC) from the photic zone, and 4) the fate of hydrothermal plumes dispersed from the mid-ocean ridge system. The present data set primarily includes measurements of 210Po and 210Pb on filtered water samples (i.e., "dissolved" activities) collected with CTD casts. Surface water samples were collected separately and were unfiltered, so represent "total" activities. Particulate samples also were collected using in situ pumps and those data are reported separately. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/883724
  • Dataset
    Dissolved and total water column 210Po and 210Pb from samples collected on Leg 2 (Hilo, HI to Papeete, French Polynesia) of the US GEOTRACES Pacific Meridional Transect (PMT) cruise (GP15, RR1815) on R/V Roger Revelle from October to November 2018
    (Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2023-01-31) Cochran, J. Kirk
    This project is part of the international GEOTRACES program, which was created to allow a comprehensive, coordinated study of trace elements and isotopes (TEIs) in the oceans. This project uses the radionuclide pair 210Pb and its grand-daughter, 210Po, to provide important biogeochemical rate information pertinent to the trace elements and isotopes (TEIs) measured during the US GEOTRACES Meridional Transect in the Pacific from Alaska to Tahiti in late 2018. Many processes in the ocean cannot be directly observed and, as such, tracers such as 210Po and 210Pb can be used to provide important constraints on their rates and pathways. 210Po (half-life = 138 d) and 210Pb (half-life = 22.3 y) decay on timescales that are useful to study 1) atmospheric deposition of trace elements, 2) scavenging of particle-reactive trace elements, 3) export of particulate organic carbon (POC) from the photic zone, and 4) the fate of hydrothermal plumes dispersed from the mid-ocean ridge system. The present data set primarily includes measurements of 210Po and 210Pb on filtered water samples (i.e., "dissolved" activities) collected with CTD casts. Surface water samples were collected separately and were unfiltered, so represent "total" activities. Particulate samples also were collected using in situ pumps and those data are reported separately. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/883797