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Papers by Kirk Cochran

Deep Sea Research Part I: Oceanographic Research Papers, 2020

During the North Atlantic Bloom Experiment (NABE) of the Joint Global Ocean Flux Study (JGOFS), w... more During the North Atlantic Bloom Experiment (NABE) of the Joint Global Ocean Flux Study (JGOFS), water column sampling for particulate and dissolved 210 Po and 210 Pb 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 210 Po and 210 Pb 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 210 Po profiles during a bloom are expectable as this radionuclide is scavenged and exported. During NABE, 210 Pb profiles also displayed non-steady state, with significant increases in upper water column inventory occurring midway through the experiment. Export of 210 Po 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 210 Pb and 210 Po profiles, gives higher 210 Po export fluxes. Detailed depth profiles of particulate organic carbon (>0.8 μm) and particulate 210 Po (>0.4 μm) are available from the 20 and 30 May samplings and show maxima in POC/Po at ~37 m. Applying the POC/ 210 Po ratios at 150 m to the "steady state" 210 Po 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) 210 Po-derived POC fluxes are comparable to, but somewhat less than, those estimated previously from 234 Th/ 238 U 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 Poderived POC fluxes during the NABE agree well with those derived from 234 Th/ 238 U disequilibrium and agree with sediment trap fluxes within a factor of ~2. However, unlike the 234 Th-POC flux proxy, non-steady stage changes in profiles of 210 Pb, the precursor of 210 Po, must be considered. 210 Po (half-life = 138.4 d) to calculate POC fluxes for comparison with the sediment trap and Th-derived estimates. Although the samples were collected and processed some time ago, their publication is relevant now because of the increased use of the 210 Pb-210 Po proxy to estimate POC flux (e.g. Tang et al., 2019; Hayes et al., 2018). The data set is one among

Geochimica et Cosmochimica Acta, 2019

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Annual Review of Marine Science, 2012

Editors' Note Each year, the editorial board invites a distinguished member of the oceanograp... more Editors' Note Each year, the editorial board invites a distinguished member of the oceanographic community to contribute a prefatory chapter; this year, we were delighted when Karl Turekian, Sterling Professor of Geology and Geophysics at Yale, accepted our invitation. Over the course of a long and productive career, Dr. Turekian has pursued his interests in marine and atmospheric geochemistry by using natural radioactive and radiogenic isotopes to study Earth's evolution and the impacts of global change. He has also directed both the Center for the Study of Global Change at Yale and the Yale Institute for Biospheric Studies. In this interview, conducted by his former student Kirk Cochran, Dr. Turekian tells the story of his early career and discusses some of the major scientific challenges and opportunities faced along the way. His personal account of the rise of geochemistry is a charming story of how chance events and personalities impact scientific careers. His technical...

Deep Sea Research Part A. Oceanographic Research Papers, 1992

The dtsequdlbrtum between the particle-reactive tracer-'~4Th (q,'2 = 24 1 days) and its soluble p... more The dtsequdlbrtum between the particle-reactive tracer-'~4Th (q,'2 = 24 1 days) and its soluble parent, 23~U, was used to examine Th scavenging and export fluxes during the U S JGOFS North Atlantic Bloom Experiment (24 April-30 May 1989) at-47°N, 2I)°W Four profiles of dassolved and particulate 234Th in the upper 300 m and a non-steady state box model were used to quantity dissolved 234Th uptake and particle export rates The highest export fluxes occurred durmg the first half of May From POC/2~4Th and PON/2~4Th ratios, particulate organic C and N fluxes were calculated Results were 5-41 mmol C m 2 day-I and 0 9-6 5 mmol N m-2 day 1 from the 0-35 m layer The ratio of POC export flux to primary production ranged from 0 05 to 0 42, peaking in the first half of May The estimated fluxes agree with the observed losses of total C and N from the upper ocean during the bloom, but yield slgmficantly higher fluxes than were measured by floating traps at 150 and 300 m

Proceedings of the National Academy of Sciences, 1986

The 228 Ra/ 226 Ra ratios in a previously dated vesicomyid clam shell were used to determine that... more The 228 Ra/ 226 Ra ratios in a previously dated vesicomyid clam shell were used to determine that seawater was in contact with mid-oceanic-ridge basalt glass for 22-45 years prior to arrival to the surface at 350°C at the Galapagos Rise Spreading Center. The minimum rate of reaction for the 45-year sojourn time, based on a water/rock ratio of 2.8 derived from 226 Ra concentrations, is 8 g of basalt altered per kg of seawater per year.

Palaeogeography, Palaeoclimatology, Palaeoecology, 2003

Well-preserved fossils of the Late Cretaceous Western Interior Seaway (WIS) of North America have... more Well-preserved fossils of the Late Cretaceous Western Interior Seaway (WIS) of North America have been analyzed for Sr concentration and Sr and O isotopes in order to decipher paleosalinities and paleotemperatures. The samples are from four biofacies within the Seaway (late Maastrichtian): offshore Interior (Pierre Shale), nearshore Interior (Fox Hills Formation), brackish (reduced salinity; Fox Hills Formation) and freshwater (Hell Creek Formation). Samples were also obtained from the Severn Formation of Maryland (considered to be representative of the open ocean). All biofacies (except the freshwater) are demonstrably within the Jeletzkytes nebrascensis ammonite zone (6 1 Ma duration). The 87 Sr/ 86 Sr ratios show significant and systematic decreases from marine (mean þ 1 S.D. = 0.707839 þ 0.000024) to brackish facies (mean þ 1 S.D. = 0.707677 þ 0.000036), consistent with dilution by freshwater with a lower 87 Sr/ 86 Sr ratio than seawater. Such variation disallows using the 87 Sr/ 86 Sr ratios of fossil shell material to assign ages to fossils from the Late Cretaceous WIS without knowledge of the salinity in which the organism grew. The Sr isotope ratios for scaphitid ammonites within a single biofacies are similar to each other and different from those for scaphites in other biofacies, implying that these organisms are restricted in their distribution during life. The 87 Sr/ 86 Sr values of freshwater unionid mussels range widely and are not compatible with the freshwater endmember 87 Sr/ 86 Sr ratio required by the trend in 87 Sr/ 86 Sr vs. biofacies established from the other samples. Paleosalinities for the biofacies are estimated to range from 35x in the open marine to a minimum of 20x in the brackish, based on the presence of cephalopods in all four facies and the known salinity tolerance of modern cephalopods. Producing reasonable 87 Sr/ 86 Sr values for the freshwater endmember of a 87 Sr/ 86 Sr vs. 1/[Sr] plot requires a Sr concentration 0.2^0.5 that of seawater for the dominant freshwater input to the WIS. Such high Sr concentrations (relative to seawater) are not observed in modern rivers, and we suggest that the brackish environment in the WIS arose through the mixing of freshwater and seawater in a nearshore aquifer system. Reactions of the solution with aquifer solids in this 'subterranean estuary' [Moore, Mar. Chem. 65 (1999) 111^125] produced brackish water with the Sr concentration and isotopic composition recorded in the brackish biofacies. N 18 O values of the fossils

Marine Environmental Research, 1998

Zn, PCBs in all samples. Contaminant distributions in the Hudson River estuary show two types of ... more Zn, PCBs in all samples. Contaminant distributions in the Hudson River estuary show two types of trends: Ag, Cu and Pb show an increasing trend down-estuary with maximum values in New York Harbor sediments; in contrast, Cd, Zn and total PCBs display a decreasing trend toward New York Harbor where urban sources are also apparent. Silver is a useful tracer of urban sources of contaminants in the Hudson River estuary and polychlorinated biphenyls (PCBs) are useful source indicators of upriver sources. Correlations of Cu and Pb with Ag suggest that Ag, Cu and Pb are dominated by down-estuary sources such as wastewater efluent. The history of their inputs suggests that they have been progressively transported downstream. Correlations of Cd and Zn with total PCBs indicate that these contaminants are dominated by upriver sources, where they are removed and diluted downstream along with the sediment transport.

Marine Chemistry, 2007

A mass balance for the naturally-occurring radium isotopes (224 Ra, 223 Ra, 228 Ra, and 226 Ra) i... more A mass balance for the naturally-occurring radium isotopes (224 Ra, 223 Ra, 228 Ra, and 226 Ra) in Jamaica Bay, NY, was conducted by directly estimating the individual Ra contributions of wastewater discharge, diffusion from fine-grained subtidal sediments, water percolation through marshes, desorption from resuspended particles, and water exchange at the inlet. The mass balance revealed a major unknown source term accounting for 19-71% of the total Ra input, which could only be resolved by invoking a source from submarine groundwater. Shallow (b 2 m depth) groundwater from permeable sediments in Jamaica Bay was brackish and enriched in Ra relative to surface bay waters by over two orders of magnitude. To balance Ra fluxes, a submarine groundwater input of 0.8 × 10 9-9.0 × 10 9 L d − 1 was required. This flux was similar for all four isotopes, with individual estimates varying by less than a factor of 2. Our calculated groundwater flux was 6-to 70-fold higher than the fresh groundwater discharge to the bay estimated by hydrological methods, but closely matched direct flow rates measured with seepage meters. This suggests that a substantial portion of the discharge consisted of recirculated seawater. The magnitude of submarine groundwater discharge varied seasonally, in the order: summer N autumn N spring. Chemical analyses suggest that the recirculated seawater component of submarine groundwater delivers as much dissolved nitrogen to the bay as the fresh groundwater flux.

Marine Chemistry, 2006

Over the past few decades, the radioisotope pair of 238 U / 234 Th has been widely and increasing... more Over the past few decades, the radioisotope pair of 238 U / 234 Th 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 234 Th. It is focused on the models associated with 234 Th whereas the companion papers (same issue) are focused on present and future methodologies and techniques (Rutgers van der Loeff et al.), C / 234 Th ratios (Buesseler et al.), 234 Th speciation (Santschi et al.) and present and future applications of 234 Th [Waples, J.T., Benitez-Nelson, C.R., Savoye, N., Rutgers van der Loeff, M., Baskaran, M., Gustafsson, Ö , this issue. An Introduction to the application and future use of 234 Th in aquatic systems. Marine Chemistry, FATE special issue]. In this paper, we review current 234 Th scavenging models and discuss the relative importance of the non-steady state and physical terms associated with the most commonly used model to estimate 234 Th 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 234 Th export, especially during high flux events (N ca. 800 dpm m À 2 d À 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 234 Th as a tracer of particle dynamics and flux in more complicated regimes.

Limnology and Oceanography: Methods, 2012

Journal of Geophysical Research, 1982

Concentrations of lead, 21øpb, and 21øpo were measured in rain selected for least influence by lo... more Concentrations of lead, 21øpb, and 21øpo were measured in rain selected for least influence by local sources of contamination at several tropical and subtropical islands (Enewetak; Pigeon Key, Florida; and American Samoa) and shipboard stations (near Bermuda and Tahiti). Ratios expressed as ng Pb/dpm 2•øpb in rain were 250-900 for Pigeon Key (assuming 12% adsorption for 21øpb and no adsorption for lead), depending on whether the air masses containing the analyzed rain came from the Caribbean or from the continent, respectively; about 390 for the northern Sargasso Sea downwind from emissions of industrial lead in North America; 65 for Enewetak, remote from continental emissions of industrial lead in the northern hemisphere; and 14 near Tahiti, a remote location in the southern hemisphere where industrial lead emissions to the atmosphere are much less than in the northern hemisphere. (The American Samoa sample yielded a higher ratio than Tahiti; the reason for this is not clear but may be due to local Pb sources.) The corresponding fluxes of lead to the oceans, based on measured or modeled 2•øpb precipitation fluxes, are about 4 ng Pb/cm2y for Tahiti, 10 for Enewetak, and 270 for the Sargasso Sea site, and between 110 to 390 at Pigeon Key. Sample Collection Wind Number Date Times, EDT Direction Collection Volume Water Collected or Volume Acid Wash, 1 ng Pb/kg dpm 21øpb/kg dpm 21øpo/kg Solution Solution Solution '

Geochimica et Cosmochimica Acta, 2013

Submarine groundwater discharge (SGD) to the ocean supplies Sr with less radiogenic 87 Sr/ 86 Sr ... more Submarine groundwater discharge (SGD) to the ocean supplies Sr with less radiogenic 87 Sr/ 86 Sr 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 (87 Sr/ 86 Sr = 0.707-0.710) reflected aquifer lithology. The SGD end-member 87 Sr/ 86 Sr 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-2 radiogenic 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-30 member of 2.9 µM Sr with 87 Sr/ 86 Sr = 0.7089. This represents a meteoric-SGD-driven Sr input to the ocean of 0.7-2.8 × 10 10 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 × 10 15 L y-1) was used to evaluate the impact of isotope exchange 35 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 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr ratio over geologic time.

Geochimica et Cosmochimica Acta, 1985

Particle mixing rates (D B ) calculated from excess 210 Pb gradients in sediments of the east equ... more Particle mixing rates (D B ) calculated from excess 210 Pb gradients in sediments of the east equatorial Pacific range from 0.04 to 0.5 cm 2 /y, with variation of a factor of 34 at a single site. Diffusion of the 236 Ra daughter 222 Rn may affect 210 Pb distributions under ...

Geochimica et Cosmochimica Acta, 1987

Remobilization of Mn resulting from the,diagenetic oxidation of organic carbon causes steep adsor... more Remobilization of Mn resulting from the,diagenetic oxidation of organic carbon causes steep adsorption gradients within hemipelagic sediments of the eastern equatorial Pacific. Transport of Ra along these gradients is capable of maintaining marked 226Ra-2?h disequilibrium in the surface sediments. Pore water and solid phase 226Ra profiles measured in cores from two sites in this area are modeled by adjusting the Ra adsorption coefficient as a function of the depth-varying Mn content. At MANOP site M, the high organic carbon accumulation rate causes a sharp redox gradient within the upper 10 cm of sediment. Biological mixing is constant and rapid (-500 cm*/kyr) in this upper layer, below which the Mn content, the Ra adsorption coefficient. and the 226Ra/2qh ratio are all shamlv reduced. At MANOP site H. the oraanic carbon accumulation rate is less and therefore the redox gradient is expanded over a greater depth scale. In the surface sediment, biological mixing is slower, decreases with depth, and gradients in the adsorption coefficients and 22"Ra/23r'Th ratio are less marked than at site M.

Geochemistry, Geophysics, Geosystems, 2008

Estuarine, Coastal and Shelf Science, 1998

Deposition chronologies of six high saltmarsh cores collected in New York City and Long Island, N... more Deposition chronologies of six high saltmarsh cores collected in New York City and Long Island, New York, determined using excess 210 Pb, show that accretion rates range from 0•1 to 0•4 cm year 1. On average the marshes are accreting within about a factor of two of the rise in mean sea level for the area. Excess 210 Pb inventories in the marshes average 31 14 dpm cm 2 , comparable to values expected from atmospheric deposition of 210 Pb. Chronologies of fluxes of 137 Cs and excess Pb, Cu, Zn and Cd (defined as values in excess of the pre-anthropogenic background at depth in a core) indicate significant migration of 137 Cs and Cd in the marsh cores. Lead shows maximum fluxes between 1970 and 1980, consistent with the historical record of consumption of leaded gasoline in the U.S.A. Copper shows recent decreases in flux at four of the six sites and Zn shows variable patterns. Fluxes of Pb, Cu and Zn are generally less than or equal to directly measured atmospheric fluxes of these metals, indicating the importance of atmospheric supply of the metals to the marsh sites. Assuming that the atmospheric pathway is dominant for both metal and 210 Pb supply to the marshes, the regional pattern of atmospheric metal fluxes and the relative importance of the atmosphere in contributing metals to adjacent Long Island Sound can be determined. The marsh data indicate the atmospheric fluxes of Pb, Cu and Zn are greatest in New York City and decrease eastward along the north shore of Long Island. Comparison of the ratios of the inventories of excess metals to excess 210 Pb in the marshes and in the subtidal sediments of adjacent Long Island Sound suggests that atmospheric supply can contribute large fractions of Pb (at most 70-90% of integrated input). The atmospheric contributions of Cu and Zn are <50%, indicating the dominance of non-atmospheric sources for these metals. For all the metals, lower contributions of atmospheric input are evident in western Long Island Sound, suggesting that non-atmospheric sources of metals (point and non-point) are of greater importance in this portion of the estuary.

Estuaries and Coasts, 2009

Jamaica Bay, NY, is a highly urbanized estuary within the boroughs of New York City conspicuously... more Jamaica Bay, NY, is a highly urbanized estuary within the boroughs of New York City conspicuously lacking published information on dissolved trace metal concentrations. The current study examines the distribution and cycling of trace metals in that embayment with data gathered during cruises in November 2004, April 2005, and June 2006. Most of the metal distributions (Fe, Zn, Co, Ag, Cu, Pb, Ni) in the water column are explained by the input of substantial volumes of treated wastewater effluent. However, several lines of evidence suggest that submarine groundwater discharge (SGD) is also an important source of dissolved Fe, Zn, Co, Ni, and isotopically distinct stable Pb ratios (206 Pb, 207 Pb, 208 Pb) in the Bay. Conversely, the recirculated seawater component of SGD is an apparent sink for dissolved Mo. This study provides the first measurements of dissolved trace metals in the Jamaica Bay water column and subterranean estuary and provides evidence for trace metal input due to SGD.

Earth and Planetary Science Letters, 1990

The radionuclide 21°pb shows significant geographic variations in the extent of its removal from ... more The radionuclide 21°pb shows significant geographic variations in the extent of its removal from the open ocean water column. This "texture of scavenging" is defined by mapping: (1) the integrated deficiency of 21°pb in the water column, relative to its supply from the atmosphere and from in situ decay of dissolved 226Ra, and (2) inventories of excess 21°pb in deep-sea sediments. The ratio of 21°pb deficiency to its supply, termed the scavenging effectiveness, is-20% in the North Equatorial Pacific and-50% in the North Atlantic. This variation is related to the combined effects of uptake of 21°pb onto sinking particles and lateral transport of 21°pb to areas of more intense removal. Sediment inventories of excess 21°pb, normalized to the 21°pb deficiency in the overlying water column, permit evaluation of the relative importance of these effects. In the North Equatorial Pacific virtually all of the 21°pb removed from the water column is present in the underlying sediments but in the mid-latitude North Atlantic, the sediments comprise only about 50% of the 2a°pb removed. The deficiencies of 21°pb in the mid-latitude North Atlantic sediments south of 50°N are qualitatively offset by surpluses in high-latitude sediments north of 50°N. Higher primary productivity and new production in the surface waters of the high-latitude North Atlantic and North Equatorial Pacific, relative to the oligotrophic central North Atlantic, may account for the greater fluxes of zmPb to bottom sediments in those areas.

Deep Sea Research Part II: Topical Studies in Oceanography, 1995

Determination of the naturally occurring radionuclides 232Th 23sTh "*Th and 2'sPb, and the anthro... more Determination of the naturally occurring radionuclides 232Th 23sTh "*Th and 2'sPb, and the anthropogenic radionuclides "'Am 23g*2?'u, '34Cs and r3'Cs in water samples collected across the Nansen Basin from the Barents Se: slope to the Gakkel Ridge provides tracers with which to characterize both scavenging rates and circulation timescales in this portion of the Arctic Ocean. Large volume water samples (-1500 1) were filtered in situ to separate particulate (> 0.5 pm) and dissolved Th isotopes and 241Am. Thorium-230 displays increases in both particulate and dissolved activities with depth, with dissolved *??I greater and particulate a3'?h lower in the deep central Nansen Basin than at the Barents Sea slope. Dissolved "*Th activities also are greater relative to ***Ra, in the central basin. Residence times for Th relative to removal from solution onto particles are-1 year in surface water,-10 years in deep water adjacent to the Barents Sea slope, and-20 years in the Eurasian Basin Deep Water. Lead-210 in the central basin deep water also has a residence time of-20 years with respect to its removal from the water column. This texture of scavenging is reflected in distributions of the particle-reactive anthropogenic radionuclide 241Am, which shows higher activities relative to Pu in the central Nansen Basin than at the Barents Sea slope. Distributions of 13'Cs show more rapid mixing at the basin margins (Barents Sea slope in the south, Gakkel Ridge in the north) than in the basin interior. Cesium-137 is mixed throughout the water column adjacent to the Barents Sea slope and is present in low but detectable activities in the Eurasian Basin Deep Water in the central basin. At the time of sampling (1987) the surface water at all stations had been labeled with '34Cs released in the 1986 accident at the Chernobyl nuclear power station. In the-1 year since the introduction of Chernobyl '34Cs to the Nansen Basin, it had been mixed to depths of-800 m at the Barents Sea Slope and to-300 m in the central basin. "Pre-Chernobyl" inventories of r3'Cs (as well as 23g*2@Pu) are 10 times those expected from global atmospheric fallout from nuclear weapons testing and are derived principally from releases from the Sellafield, U.K., nuclear fuel reprocessing facility on the Irish Sea. Based on the sources of r3'Cs to the Nansen Basin, mixing time scales are %18 years for the upper water column (to 1500 m) and-40 years for the deep water. These mixing time scales, combined with more rapid scavenging at the basin margin relative to the central basin, produce residence times of particle-reactive radionuclides in the Nansen Basin comparable to other open ocean areas (e.g. northwest Atlantic) despite the presence of permanent ice cover and long periods of low-light levels that limit productivity in the Arctic.