S. Bradley Moran

Uranium and thorium isotopic and concentration measurements by magnetic sector inductively coupled plasma mass spectrometry

We have developed techniques by sector-field inductively coupled plasma mass spectrometry (ICP-MS) for measuring the isotopic composition and concentration of uranium and thorium, focusing on the rare isotopes, 230Th and 234U. These isotopes have been widely used as tracers in earth sciences, e.g., chronology, paleoclimatology, archeology, hydrology, geochemistry, and oceanography. Measurements made on reference materials demonstrate that the analytical precision approximates counting statistics and that the accuracy of the measurement is within error of accepted values. Routine measurement times are 20 min for U and 10 min for Th. The sensitivities (ions counted/atoms introduced) are 2–3‰ for U and 1.5–2‰ for Th. Samples of 10–40 ng of 238U (0.5–2.0 pg of 234U) give measurement precisions of 1–2‰ (2σ) for δ234U and U concentration ([U]). Only 0.4 pg of 230Th are needed to achieve [230Th] and 230Th/232Th data with errors less than 5‰ even for cases where 230Th/232Th is 10−5 or less. Our ICP-MS data, including uranium standards, thorium standards, 238U–234U–230Th–232Th dating of speleothems and 230Th–232Th in oceanic particulates, replicates measurements made by thermal ionization mass spectrometry (TIMS). Compared to TIMS, the ICP-MS method allows smaller sample size and higher sample throughput due to higher sensitivity, fewer sample preparation steps and shorter measurement times. However, mass biases, intensity biases, spectral interferences and instrumental blanks are significant and must be addressed.

An increasing CO2 sink in the Arctic Ocean due to sea‐ice loss

[1] The Arctic Ocean and adjacent continental shelf seas such as the Chukchi and Beaufort Seas are particularly sensitive to long-term change and low-frequency modes of atmosphere-ocean-sea-ice forcing. The cold, low salinity surface waters of the Canada Basin of the Arctic Ocean are undersaturated with respect to CO2 in the atmosphere and the region has the potential to take up atmospheric CO2, although presently suppressed by sea-ice cover. Undersaturated seawater CO2 conditions of the Arctic Ocean are maintained by export of water with low dissolved inorganic carbon content and modified by intense seasonal shelf primary production. Sea-ice extent and volume in the Arctic Ocean has decreased over the last few decades, and we estimate that the Arctic Ocean sink for CO2 has tripled over the last 3 decades (24 Tg yr 1 to 66 Tg yr 1 ) due to sea-ice retreat with future sea-ice melting enhancing air-to-sea CO2 flux by 28% per decade. Citation: Bates, N. R., S. B. Moran, D. A. Hansell, and J. T. Mathis (2006), An increasing CO2 sink in the Arctic Ocean due to sea-ice loss, Geophys. Res. Lett., 33, L23609, doi:10.1029/2006GL027028.

Rates of particle scavenging and particulate organic carbon export estimated using 234Th as a tracer in the subtropical and equatorial Atlantic Ocean

Abstract The export flux of particulate organic carbon and particle scavenging rates were calculated using a 234Th-based approach along a transect through the mid-Atlantic from 35°S to 10°N during May and June of 1996. The median residence time of dissolved 234Th along the transect was 135 days, with intense scavenging from 5°N to 6° N ( τ d =86±17 days). Vertical profiles and depth-integrated samples (surface-100 m) of dissolved ( 53- μm ) 234 Th activities were collected to estimate export fluxes of particulate organic carbon (POC). A steady-state model of the 234Th activity balance in the upper 100 m was combined with measurements of the POC/234Th ratio on >53-μm particles. POC export ranged from 1.5 mmol C m −2 d −1 in the central South Atlantic to 31 mmol C m −2 d −1 at 2.5°S. Export production along the Atlantic equator (5°S–5°N, 25°E–5°W) is estimated to be 0.22 Gt C yr−1. The 234Th-derived POC export estimates reported here and from other studies are comparable with model simulations of export production from the Princeton General Circulation Model (GCM) for the Atlantic Ocean.

An assessment of the relative importance of horizontal and vertical transport of particle-reactive chemicals in the coastal ocean

Abstract A two-dimensional transport and scavenging model has been developed and applied to a limited set of 238U–234Th disequilibria data in order to examine the relative significance of horizontal versus vertical removal of chemicals in coastal waters. During an intense scavenging episode in September 1993 (>95% 238U–234Th disequilibrium), vertical scavenging was found to be more important than horizontal transport in both Inner and Outer Casco Bay, Gulf of Maine. However, in May 1994 the two-dimensional model suggested that onshore horizontal dispersion of 234Th was substantial. Recognition of this horizontal flux required us to increase the net vertical scavenging flux in Inner Casco Bay by a factor of three over that obtained based only on the local 238U–234Th disequilibrium. The radionuclide (210Pbxs, 234Thxs, 7Be) record of the underlying sediments provided supporting evidence for onshore horizontal transport of chemicals. The highest sedimentary inventories for all three radionuclides were found at the stations nearest to the coast. As anticipated from their relative particle-affinities, the “regional boundary-scavenging” indicator 7Be/234Thxs was highest at the coastal boundary. The application of the two-dimensional 234Th-based transport model to assess the distributional fate of other chemicals was demonstrated for Casco Bay using simultaneously measured polycyclic aromatic hydrocarbons (PAHs). Based on limited PAH data, the model results suggest that about half of the pyrene and benzo[a]pyrene introduced to Portland Harbor, ME may be settling locally and that the remainder is exported to offshore locations. The approach introduced here, coupling information on particle-mediated vertical scavenging, chemical phase distribution, and tide-induced horizontal dispersion, should provide a useful mechanistic framework for elucidating quantitatively the dispersal of a wide range of geochemically and environmentally important chemicals in the coastal ocean.

Investigating the carbon cycle in the Gulf of Maine using the natural tracer thorium 234

The naturally occurring radionuclide 234Th (t1/2 = 24.1 days) was used to examine the organic carbon cycle in the Gulf of Maine. A seasonal study (March, June, and September 1995) was conducted in the central Gulf of Maine in Wilkinson and Jordan Basins, and a regional survey, which included the Scotian Shelf, was conducted during August-September 1997. Particulate organic carbon (POC) export (particulate export production) was estimated from a three-dimensional steady state model of 234Th flux combined with measurements of the POC/234Th ratio on >53-μm particles. The POC export for this region was seasonally variable; average values ranged from 15 to 34 mmol C m−2 d−1, between 11% and 25% of the regionally integrated primary production. The Gulf of Maine was a net source (to the Mid-Atlantic Bight) of dissolved organic carbon (2.4 mmol C m−2 d−1) amounting to ∼2% of carbon uptake rates. Organic carbon burial in the sediments was a minor fraction of the primary production, averaging 1.6 mmol C m−2 d−1. Though only a fraction of total export production was buried in the sediments, these estimates close the budget for organic carbon in the Gulf of Maine. An implication is that off-shelf export may not be as important as previously estimated in this shelf region.

Evaluation of two cross-flow ultrafiltration membranes for isolating marine organic colloids

Abstract Laboratory and field studies were performed to evaluate two 1 kilo-Dalton (kD) cross-flow ultrafiltration (CFF) membranes (a Millipore Prep-scale CFF membrane constructed primarily from regenerated cellulose and an Amicon CFF polysulfone membrane) to isolate colloids (operationally defined as particles or macromolecules between 1 kD and 0.2–1 μ m) from sea water. We focused on three crucial aspects when applying the CFF technique: retention characteristics, sorptive potential and ultrafilter breakthrough. Lab results showed that both CFF systems retained ≥91% of a 3000 nominal molecular weight (NMW) dextran standard, consistent with the manufacturers rated cutoff. The Millipore membrane showed essentially no loss of a dextran standard, while 33% was lost for the same molecule onto the Amicon CFF membrane. Both membranes showed higher losses of a protein standard (Lactalbumin) added to sea water. For bulk organic carbon (OC), both membranes usually had reasonable recovery (100±10%) as long as the membranes were preconditioned. This was true for both lab experiments and field investigations in open ocean water off Bermuda. However, data from 234 Th and 230 Th analysis of samples from a station off Bermuda showed very large losses and hence low recovery from CFF. Results of these fractionated OC and 234 Th distributions are also discussed in the context of prior studies. Ultrafilter breakthrough of both high molecular weight (HMW) and low molecular weight (LMW) compounds may occur throughout the CFF process, especially when processing coastal sea water where COC is relatively enriched. A permeation coefficient model provides an overall reasonable fit to the data characterizing the permeation behaviour of CFF; the retentate prediction based on the model indicates that breakthrough becomes more significant after the concentration factor (cf) is higher than 5, which implies that fractionation of organic components increases at higher cf.

Below the disappearing marshes of an urban estuary: historic nitrogen trends and soil structure

Marshes in the urban Jamaica Bay Estuary, New York, USA are disappearing at an average rate of 13 ha/yr, and multiple stressors (e.g., wastewater inputs, dredging activities, groundwater removal, and global warming) may be contributing to marsh losses. Among these stressors, wastewater nutrients are suspected to be an important contributing cause of marsh deterioration. We used census data, radiometric dating, stable nitrogen isotopes, and soil surveys to examine the temporal relationships between human population growth and soil nitrogen; and we evaluated soil structure with computer-aided tomography, surface elevation and sediment accretion trends, carbon dioxide emissions, and soil shear strength to examine differences among disappearing (Black Bank and Big Egg) and stable marshes (JoCo). Radiometric dating and nitrogen isotope analyses suggested a rapid increase in human wastewater nutrients beginning in the late 1840s, and a tapering off beginning in the 1930s when wastewater treatment plants (WWTPs) were first installed. Current WWTPs nutrient loads to Jamaica Bay are approximately 13 995 kg N/d and 2767 kg P/d. At Black Bank, the biomass and abundance of roots and rhizomes and percentage of organic matter on soil were significantly lower, rhizomes larger in diameter, carbon dioxide emission rates and peat particle density significantly greater, and soil strength significantly lower compared to the stable JoCo Marsh, suggesting Black Bank has elevated decomposition rates, more decomposed peat, and highly waterlogged peat. Despite these differences, the rates of accretion and surface elevation change were similar for both marshes, and the rates of elevation change approximated the long-term relative rate of sea level rise estimated from tide gauge data at nearby Sandy Hook, New Jersey. We hypothesize that Black Bank marsh kept pace with sea level rise by the accretion of material on the marsh surface, and the maintenance of soil volume through production of larger diameter rhizomes and swelling (dilation) of waterlogged peat. JoCo Marsh kept pace with sea-level rise through surface accretion and soil organic matter accumulation. Understanding the effects of multiple stressors, including nutrient enrichment, on soil structure, organic matter accumulation, and elevation change will better inform management decisions aimed at maintaining and restoring coastal marshes.

Arrival of the Fukushima radioactivity plume in North American continental waters

Significance The radionuclide results in this report represent the first systematic study, to our knowledge, of the arrival of the Fukushima radioactivity signal in continental waters off North America. The present time series results are critical to an understanding of the circulation of Fukushima tracers in the eastern North Pacific and to the tuning and validation of ocean circulation models that are being used to predict the future evolution of this signal. They are also important for informing the public of the magnitude of the Fukushima radioactivity signal in North American continental waters and enabling a science-based assessment of the significance of its potential effects on human health and the environment. The large discharge of radioactivity into the northwest Pacific Ocean from the 2011 Fukushima Dai-ichi nuclear reactor accident has generated considerable concern about the spread of this material across the ocean to North America. We report here the first systematic study to our knowledge of the transport of the Fukushima marine radioactivity signal to the eastern North Pacific. Time series measurements of 134Cs and 137Cs in seawater revealed the initial arrival of the Fukushima signal by ocean current transport at a location 1,500 km west of British Columbia, Canada, in June 2012, about 1.3 y after the accident. By June 2013, the Fukushima signal had spread onto the Canadian continental shelf, and by February 2014, it had increased to a value of 2 Bq/m3 throughout the upper 150 m of the water column, resulting in an overall doubling of the fallout background from atmospheric nuclear weapons tests. Ocean circulation model estimates that are in reasonable agreement with our measured values indicate that future total levels of 137Cs (Fukushima-derived plus fallout 137Cs) off the North American coast will likely attain maximum values in the 3–5 Bq/m3 range by 2015–2016 before declining to levels closer to the fallout background of about 1 Bq/m3 by 2021. The increase in 137Cs levels in the eastern North Pacific from Fukushima inputs will probably return eastern North Pacific concentrations to the fallout levels that prevailed during the 1980s but does not represent a threat to human health or the environment.

Sources and transport of anthropogenic radionuclides in the Ob River system, Siberia

Abstract The potential sources of anthropogenic radionuclides to the Ob River system of western Siberia include global stratospheric fallout, tropospheric fallout from atomic weapons tests and releases from production and reprocessing facilities. Samples of water, suspended and bottom sediments collected in 1994 and 1995 have been used to characterize the sources and transport of 137Cs, Pu isotopes, 237Np and 129I through the system. For the radionuclides that associate with particles, isotope ratios provide clues to their sources, providing any geochemical fractionation can be taken into account. Activity ratios of 239,240Pu/137Cs in suspended sediments are lower than the global fallout ratio in the Irtysh River before its confluence with the Ob, comparable to fallout in the central reach of the Ob, and greater than the fallout values in the lower Ob and in the Taz River. This pattern mirrors the downriver decrease in dissolved organic carbon (DOC) concentrations. Laboratory adsorption experiments with Ob River sediment and water show that Kd values for Am (and presumably other actinides) are depressed by two orders of magnitude in the presence of Ob DOC concentrations, relative to values measured in DOC-free Ob water. Iodine and cesium Kd values show little or no (less than a factor of 2) dependence on DOC. Mixing plots using plutonium isotope ratios (atom ratios) show that Pu in suspended sediments of the Ob is a mixture of stratospheric global fallout at northern latitudes, tropospheric fallout from the former Soviet Union test site at Semipalatinsk and reprocessing of spent fuel at Tomsk-7. Plutonium from Semipalatinsk is evident in the Irtysh River above its confluence with the Tobal. Suspended sediment samples taken in the Ob above its confluence with the Irtysh indicate the presence of Pu derived from the Tomsk-7 reprocessing facilities. A mixing plot constructed using 237Np/239Pu vs. 240Pu/239Pu shows similar mixtures of stratospheric and tropospheric fallout, with the likely addition of inputs from reprocessing facilities and reactor operations. As with Pu/Cs ratios, Np/Pu ratios could be modified by differential geochemical behaviors of Np and Pu. Dissolved 129I only weakly interacts with particles in the Ob; size-fractionated sampling shows that the colloidal 129I fraction (defined as 1 kDa–0.2 μm) contains generally

Ground water input to coastal salt ponds of southern Rhode Island estimated using 226Ra as a tracer.

The naturally occurring radionuclide 226Ra (t1/2 = 1600 years) was used as a tracer to determine ground water input to Point Judith, Potter, Green Hill and Ninigret ponds in southern Rhode Island. Measurements of 226Ra activity were made in samples collected from salt ponds, pore waters, sediments, and local ground water wells during June-August, 1997. These results were combined with a simple box model to derive ground water input fluxes of 0.1-0.3 cm3 cm-2 d-1 (2-5 x 10(7) L d-1), which are comparable to previous estimates of ground water input to these ponds.