J. Kirk Cochran

Carbon and nitrogen export during the JGOFS North Atlantic Bloom experiment estimated from 234Th: 238U disequilibria

Abstract The disequilibrium between the particle-reactive tracer 234 Th ( t 1 2 = 24.1 days) and its soluble parent, 238 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, 20°W. Four profiles of dissolved and particulate 234 Th in the upper 300 m and a non-steady box model were used to quantify dissolved 234 Th uptake and particle export rates. The highest export fluxes occured during the first half of May. From POC/ 234 Th and PON/ 234 Th ratios, particulate organic C and N fluxes were calculated. Results were 5–41 mmol C m −2 day −1 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 significantly higher fluxes than were measured by floating traps at 150 and 300 m.

Particle mixing rates in sediments of the eastern equatorial Pacific: Evidence from 210Pb, 239,240Pu and 137Cs distributions at MANOP sites

Abstract Particle mixing rates (DB) calculated from excess 210Pb gradients in sediments of the east equatorial Pacific range from 0.04 to 0.5 cm2/y, with variation of a factor of 3–4 at a single site. Diffusion of the 236Ra daughter 222Rn may affect 210Pb distributions under conditions of slow mixing and low 210Pb flux to the seafloor, as shown by a siliceous ooze-clay core which contained the fallout radionuclides 239,240Pu and 137Cs but no excess 210Pb (relative to 226Ra). There is no clear relationship between 210Pbderived mixing rates and sediment type, accumulation rate or organic carbon flux to the sediments. Comparison of 210Pb mixing rates with those calculated from 239,240Pu and 137Cs distributions reveals better agreement for a pulse input of the fallout radionuclides (DB = 0.03−0.4 cm2/y) than for continuous input at a constant rate (DB = 0.1−1.6 cm2/y), although the Pu and 137Cs data are better fit by the latter model. The agreement may be fortuitous because 239,240Pu and 137Cs appear significantly deeper than 210Pb in at least one core. Tracer separation could be caused by particle size-selective mixing by the benthic fauna or by chemical mobilization. If the fallout radionuclides are scavenged from surface waters by large, organic-rich particles such as fecal pellets, their release and migration may result from decomposition of the carrier in surface sediments. Either a relatively unreactive form of Pu (e.g. oxidized Pu) has been released by this process or a one-dimensional model is inadequate to explain its observed penetration into the sediments. Activity ratios of 239,240 Pu 137 Cs in the sediments decrease with increasing north latitude, and the trend reflects higher fluxes of 239,240Pu near the weapons test site at Christmas Island (2°N). The 239,240 Pu 137 Cs ratios and fluxes to the sediment (assuming constant input) at the siliceous ooze-red clay site are consistent with published sediment trap data from a nearby site. Thus if fallout radionuclide fluxes to the sea floor were higher in the past, both 239,240Pu and 137Cs have been released from sinking particles.

Uranium geochemistry in estuarine sediments: Controls on removal and release processes

Abstract Porewater uranium profiles from Long Island Sound (LIS) and Amazon shelf sediments and LIS sediment incubation experiments indicate that both removal and release processes control U geochemistry in estuarine sediments. Release of U from sediments occurs in association with Fe reduction. A correlation between U and Fe (and Mn) observed in sediment incubation experiments suggests that there is release of U from Fe-Mn-oxides as they are reduced, consistent with data from the Amazon shelf. In both sediment porewater profiles (LIS and Amazon) and sediment incubation experiments (LIS), there is removal of U from porewater under conditions of sulfate reduction. Sediment incubation experiments indicate that the removal rate is first-order with respect to U concentration, and the rate constant is linearly correlated to sulfate reduction rates. The link between U removal and sulfate reduction (a measure of diagenetic microbial activity) is consistent with a microbial mediation of U reduction. The diffusional flux of U into LIS sediments is estimated from porewater profiles. The inclusion of this estuarine removal term in the oceanic U balance increases the importance of the sediment sink.

DISTRIBUTION OF HEAVY METAL AND PCB CONTAMINANTS IN THE SEDIMENTS OF AN URBAN ESTUARY : THE HUDSON RIVER

Abstract Surficial sediments obtained from sediment cores were collected over 100km along the axis of the lower Hudson River in June 1994, November 1994, May 1995 and April 1996 and showed the presence of anthropogenic Ag, Cd, Cu, Pb, 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 effluent. 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.

Determination of thorium isotopes in seawater by nondestructive and radiochemical procedures

Abstract Procedures have been developed for the analyses of dissolved and particulate 234Th, 228Th, 230Th and 232Th in seawater. Large volume samples (>1000 1) are collected using in situ pumps. Seawater is pumped sequentially through a filter cartridge and two MnO2 adsorbers for the collection of particulate and dissolved Th, respectively. Both filters adsorbers are analysed for 234Th using a simple gamma counting technique. This newly developed 234Th procedure can be conducted at sea, and thus provides an easy and efficient method for 234Th analyses on large volume samples. Subsequent radiochemical purification procedures and low-level alpha counting techniques are used in the laboratory for the analyses of 228Th, 230Th and 232Th on these same samples.

210Pb scavenging in the North Atlantic and North Pacific Oceans

The radionuclide210Pb 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 of210Pb in the water column, relative to its supply from the atmosphere and from in situ decay of dissolved226Ra, and (2) inventories of excess210Pb in deep-sea sediments. The ratio of210Pb 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 of210Pb onto sinking particles and lateral transport of210Pb to areas of more intense removal. Sediment inventories of excess210Pb, normalized to the210Pb deficiency in the overlying water column, permit evaluation of the relative importance of these effects. In the North Equatorial Pacific virtually all of the210Pb 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 the210Pb removed. The deficiencies of210Pb 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 of210Pb to bottom sediments in those areas.

Natural and anthropogenic radionuclide distributions in the northwest Atlantic Ocean

We have used in-situ pumps which filter large volumes of sea water through a 1 μm cartridge prefilter and two MnO2-coated cartridges to obtain information on dissolved and particulate radionuclide distributions in the oceans. Two sites in the northwest Atlantic show subsurface maxima of the fallout radionuclides137Cs,239,240Pu and241Am. Although the processes of scavenging onto sinking particles and release at depth may contribute to the tracer distributions, comparison of predicted and measured water column inventories suggests that at least 35–50% of the Pu and241Am are supplied to the deep water by advection. n nThe depth distributions of the naturally occurring radionuclides232Th,228Th and230Th reflect their sources to the oceans.232Th shows high dissolved concentrations in surface waters, presumably as a result of atmospheric or riverine supply. Activities of232Th decrease with depth to values ⩽ 0.01 dpm/1000 l.228Th shows high activities in near surface and near bottom water, due to the distribution of its parent,228Ra. Dissolved230Th, produced throughout the water column from234U decay, increases with depth to ∼ 3000 m. Values in the deep water (> 3000 m) are nearly constant (∼ 0.6–0.7 dpm/1000 l), and the distribution of this tracer (and perhaps other long-lived particle-reactive tracers as well) may be affected by the advection inferred from Pu and241Am data. n nThe ratio of particulate to dissolved activity for both230Th and228Th is ∼ 0.15–0.20. This similarity precludes the calculation of sorption rate constants using a simple model of reversible sorption equilibrium. Moreover, in mid-depths228Th tends to have a higher particulate/dissolved ratio than230Th, suggesting uptake and release of230Th and228Th by different processes. This could occur if228Th, produced in surface water, were incorporated into biogenic particles formed there and released as those particles dissolved or decomposed during sinking.230Th, produced throughout the water column, may more closely approach a sorption equilibrium at all depths.230Th,241Am and239,240Pu are partitioned onto particles in the sequence Th > Am > Pu with ∼ 15% of the230Th on particles compared with ∼ 7% for Am and ∼ 1% for Pu. Distribution coefficients (Kd) are 1.3–1.6 × 107 for Th, 5–6 × 106 for Am and 7–10 × 105 for Pu. The lower reactivity for Pu is consistent with analyses of Pu oxidation states which show ∼ 85% oxidized (V + VI) Pu. However, theKd value for Pu may be an upper limit because Pu, like228Th, may be incorporated into particles in surface waters and released at depth only by destruction of the carrier phase.

Phytoplankton biomass, production and potential export in the North Water

Abstract The seasonal patterns of phytoplankton biomass and production were determined in the North Water, located between Greenland and Ellesmere Island (Canadian Arctic), in August 1997, April–July 1998, and August–September 1999. The patterns differed among the four defined regions of this large polynya, i.e. North (>77.5°N), East (>75°W), West ( 5xa0μm) fraction dominated the biomass and production during the bloom. During July, August, and September, biomass and production decreased over the whole region, with the highest biomass, dominated by large cells, occurring in the North. The annual particulate and dissolved phytoplankton production were the highest ever reported for the high Arctic, reaching maximum values of 254 and 123xa0gxa0Cxa0m−2xa0yr−1, respectively, in the East. Rates in the North and West were considerably lower than in the East (ca. two- and three-fold, respectively). The f-ratios (i.e. ratio of new to total production), derived from the size structure of phytoplankton, were high north of 76°N (0.4–0.7). Regionally, this indicated a high potential export of particulate organic carbon (EPOC) from the phytoplankton community to other trophic compartments and/or downwards in the East (155xa0gxa0Cxa0m−2xa0yr−1), with lower values in the North and West (i.e. 77 and 42xa0gxa0Cxa0m−2xa0yr−1, respectively). The seasonal and spatial patterns of EPOC were consistent with independent estimates of potential carbon export. Phytoplankton biomass and production were generally dominated by the large size fraction, whereas EPOC seemed to be dominated by the large size fraction early in the season and by the small size fraction (

234TH AND 7BE AS TRACERS FOR THE TRANSPORT AND DYNAMICS OF SUSPENDED PARTICLES IN A PARTIALLY MIXED ESTUARY

The sources of the natural radionuclides {sup 234}Th (half-life = 24 d) and {sup 7}Be (half-life = 53 d) to the partially mixed Hudson River estuary are distinctly different: {sup 234}Th is produced from decay of dissolved {sup 238}U that varies conservatively as a function of salinity, and {sup 7}Be is added directly from the atmosphere. {sup 234}Th and {sup 7}Be activities were measured in suspended and bottom sediments under varying conditions of river flow to evaluate the relative importance of local sediment resuspension and advection of suspended sediments through the estuary. Both radionuclides are removed from the estuarine water column on time scales of from <1 to 13 d. The {sup 234}Th{sub xs}/{sup 7}Be activity ratio is used to eliminate variations in the specific activities caused by grain size or sediment compositional changes. A simple steady-state model is used to set limits on the activity ratios expected if local processes (in situ production of {sup 234}Th, atmospheric supply of {sup 7}Be, scavenging, particle settling, and resuspension) dominate the activity ratio. During low river discharge, observed {sup 234}Th{sub xs}/{sup 7}Be ratios exceed the model upper limits in the higher salinity portions of the estuary, suggesting that particles labeled with Thmorexa0» and Be in higher salinities are transported to lower salinities with the estuarine circulation. Distance scales of particle transport are estimated to be from 10 to 20 km, comparable to the tidal excursion. Sampling during a high-flow event of short duration shows that the activity ratios tend toward the lower limit. Such a pattern could be produced by down-river transport of particles or by enhanced suspension. Similarities between the {sup 234}Th{sub xs}/{sup 7}Be activity ratios of suspended and bottom sediments suggest that increases in resuspension during high flow events dominate the activity ratio in the suspended sediment reservoir.«xa0less

234Th/238U disequilibrium as an indicator of the seasonal export flux of particulate organic carbon in the North Water

Abstract Water-column deficits of 234Th (half-life=24.1xa0d) relative to its parent 238U were used to estimate fluxes of 234Th and particulate organic carbon (POC) through the euphotic zone of the North Water, a polynya in northern Baffin Bay. Samples collected in May 1998, early in the seasonal development of the polynya, showed small deficits of 234Th in the upper 100xa0m. Deficits were greater in July 1998 and August–September 1999. Sinking fluxes of POC were calculated from the water-column 234Th deficits by multiplying the latter by the POC/234Th ratio on sinking particles (assumed to be the >70-μm filterable fraction). POC fluxes were greatest in July 1998 (mean 27.4±13.4xa0mmolxa0Cxa0m−2xa0d−1) and declined in August 1999 (mean 8.7±1.7xa0mmolxa0Cxa0m−2xa0d−1), largely due to a decrease in the POC/Th ratio in the 1999 samples. Regionally, POC fluxes were greatest in the western and northern portions of the North Water in July 1998 and markedly lower in the west in August 1999. This pattern resembles spatial and temporal changes in primary production in the polynya determined independently. POC fluxes determined from water-column 234Th profiles agreed within factors of 2–5 with values determined from floating sediment traps deployed in the upper 100xa0m. Thorium-based indices of new production (ThE ratios) obtained within the polynya encompassed a wide range (0.1–0.9), with western areas showing substantially higher transfer efficiency of carbon fixed by phytoplankton at the surface. Thorium-derived POC fluxes at 100xa0m for both July 1998 and August 1999 correlated positively with biomass of the dominant diatom in the North Water, Chaetoceros socialis, suggesting that regional variations in carbon export can be further understood by factors controlling the distribution of this species.