Bradley K. Esser

A possible link between the seawater osmium isotope record and weathering of ancient sedimentary organic matter

The Os isotopic composition of leachable Os from a North Pacific pelagic clay sequence (Pegram et al., 1992 ) and bulk sediment samples of metalliferous carbonates deposited near the East Pacific Rise (Ravizza, 1993) display similar patterns of temporal variation (Fig. 1 ). This similarity indicates that the composite record of t87Os/186Os ratio variations reflects changes in the Os isotopic composition of seawater. Temporal changes in the Sr isotopic composition of seawater record changes in the relative rates of continental weathering and oceanic crustal alteration. The Os isotopic composition of seawater should be influenced by similar processes. In both the Rb-Sr and Re-Os systems, the continental crust is characterized by large time-integrated parent/ daughter ratios, compared to the deep Earth, imparting relatively radiogenic Sr and Os signatures to continental material. However, the seawater Sr and Os records are decoupled from one another over the past 27 Ma. From 27 to 15 Ma 87Sr/86Sr increased rapidly while the 18VOs/186Os ratio of seawater remained nearly constant. At 15 Ma the ~87Os/tS6Os ratio of seawater began to increase rapidly but the rate

At-sea high-resolution chemical mapping: extreme barium depletion in North Pacific surface water

We have modified an inductively coupled plasma mass spectrometer (ICPMS) for use at sea, which permits mapping of dissolved Ba at high temporal and spatial resolution. While evaluating this system off the Pacific coast of Baja California, we discovered Ba depletion greater than 60% in surface waters over tens of kilometers. Observed Ba abundance is among the lowest reported in any ocean water. No corresponding depletion was seen in Sr, which strongly suggests that acantharians are not the primary source of biogenic Ba in these surface waters.

Isotope effect of mercury diffusion in air.

Identifying and reducing impacts from mercury sources in the environment remains a considerable challenge and requires process based models to quantify mercury stocks and flows. The stable isotope composition of mercury in environmental samples can help address this challenge by serving as a tracer of specific sources and processes. Mercury isotope variations are small and result only from isotope fractionation during transport, equilibrium, and transformation processes. Because these processes occur in both industrial and environmental settings, knowledge of their associated isotope effects is required to interpret mercury isotope data. To improve the mechanistic modeling of mercury isotope effects during gas phase diffusion, an experimental program tested the applicability of kinetic gas theory. Gas-phase elemental mercury diffusion through small bore needles from finite sources demonstrated mass dependent diffusivities leading to isotope fractionation described by a Rayleigh distillation model. The measured relative atomic diffusivities among mercury isotopes in air are large and in agreement with kinetic gas theory. Mercury diffusion in air offers a reasonable explanation of recent field results reported in the literature.

A membrane inlet mass spectrometry system for noble gases at natural abundances in gas and water samples

RATIONALE Noble gases dissolved in groundwater can reveal paleotemperatures, recharge conditions, and precise travel times. The collection and analysis of noble gas samples are cumbersome, involving noble gas purification, cryogenic separation and static mass spectrometry. A quicker and more efficient sample analysis method is required for introduced tracer studies and laboratory experiments. METHODS A Noble Gas Membrane Inlet Mass Spectrometry (NG-MIMS) system was developed to measure noble gases at natural abundances in gas and water samples. The NG-MIMS system consists of a membrane inlet, a dry-ice water trap, a carbon-dioxide trap, two getters, a gate valve, a turbomolecular pump and a quadrupole mass spectrometer equipped with an electron multiplier. Noble gases isotopes (4)He, (22)Ne, (38)Ar, (84)Kr and (132)Xe are measured every 10 s. RESULTS The NG-MIMS system can reproduce measurements made on a traditional noble gas mass spectrometer system with precisions of 2%, 8%, 1%, 1% and 3% for He, Ne, Ar, Kr and Xe, respectively. Noble gas concentrations measured in an artificial recharge pond were used to monitor an introduced xenon tracer and to reconstruct temperature variations to within 2 °C. Additional experiments demonstrated the capability to measure noble gases in gas and in water samples, in real time. CONCLUSIONS The NG-MIMS system is capable of providing analyses sufficiently accurate and precise for introduced noble gas tracers at managed aquifer recharge facilities, groundwater fingerprinting based on excess air and noble gas recharge temperature, and field and laboratory studies investigating ebullition and diffusive exchange.

Radiocesium in North San Francisco Bay and Baja California coastal surface waters

Radiocesium, 137Cs, and rare earth elements (REEs) were determined in suspended material and dissolved fractions of waters across the salinity gradient in North San Francisco Bay (estuary). We describe the variation of this conservative isotope tracer with salinity and sediment load. REE data are used to differentiate marine and terrigenous source terrains for suspended material and dissolved fractions. We estimate that about 1-4 x 10(10) Bq of 137Cs migrates annually on suspended material through the North Bay. In addition, 137Cs concentrations were measured in surface waters off Baja California. Combined in situ water density (sigma(t)) and 137Cs data distinguish between California Current and Gulf of California water, and delineate areas of upwelling, where nutrient-rich, deep Pacific Intermediate water, with little or no 137Cs, is brought to the surface off promontories along Baja California.

Real-time ocean chemistry for improved biogeochemical observation in dynamic coastal environments

We describe a new ocean observation system that integrates mass spectroscopy, hydrographic instrumentation, and satellite imagery (SeaWIFS). We used a quadrupole ICP mass spectrometer at sea to acquire continuous trace element data during separate surveys of the Baja California coastal margin, and the San Diego Bay and coastal environment. There is evidence for extreme Ba depletion in surface waters off the Baja coast, which is the result of biological productivity and marine barite precipitation. The synoptic data are used to elucidate the biochemical mechanism of barium removal; to constrain the spatial and temporal boundaries over which the phenomenon occurs; and to quantify surface flux to sediments. Further systems application led to mapping of the distribution of a suite of biologically active trace metals (Mn, Ni, Zn, Cu, and Cd) in San Diego Bay, and the Bay signature was tracked in tidal plumes into the coastal ocean. The continuous data provided chemical gradients within the Bay, with which we estimate the contaminant metal flux that is discharged from the Bay into the coastal ocean during tidal pumping.

Pre-concentration and measurement of low levels of gamma-ray emitting radioisotopes in coastal waters

We describe extensive testing of a large-volume, high-speed water sampler for the concentration and measurement of radionuclides using high-resolution gamma ray spectrometry. The sampler processed hundreds to thousands of liters of natural waters with variable suspended sediment and salinity loads at flow rates of 10-201/min. Extraction of most radionuclides in the water column was accomplished through the combination of physical filtration down to 0.1 microm particle size and chemical separation of dissolved species on cellulose-based inorganic sorbent beds without recourse to complex, or hazardous chemistry. Performance and extraction efficiencies for suites of radioisotopes were determined in the laboratory and in the field with river and coastal ocean water samples. Extraction and recovery efficiencies are better than 90% for most fission and activation product radioisotopes. This methodology has broad application to the study of the distribution and fate of radioisotopes in coastal waterways.

Mercury accumulation and attenuation at a rapidly forming delta with a point source of mining waste

The Walker Creek intertidal delta of Tomales Bay, California is impacted by a former mercury mine within the watershed. Eleven short sediment cores (10 cm length) collected from the delta found monomethylmercury (MMHg) concentrations ranging from 0.3 to 11.4 ng/g (dry wt.), with lower concentrations occurring at the vegetated marsh and upstream channel locations. Algal mats common to the deltas sediment surface had MMHg concentrations ranging from 7.5 to 31.5 ng/g, and the top 1 cm of sediment directly under the mats had two times greater MMHg concentrations compared to adjacent locations without algal covering. Spatial trends in resident biota reflect enhanced MMHg uptake at the delta compared to other bay locations. Eighteen sediment cores, 1 to 2 m deep, collected from the 1.2 km2 delta provide an estimate of a total mercury (Hg) inventory of 2500+/-500 kg. Sediment Hg concentrations ranged from pre-mining background conditions of approximately 0.1 microg/g to a post-mining maximum of 5 microg/g. Sediment accumulation rates were determined from three sediment cores using measured differences of (137)Cs activity. We estimate a pre-mining Hg accumulation of less than 20 kg/yr, and a period of maximum Hg accumulation in the 1970s and 1980s with loading rates greater than 50 kg/yr, corresponding to the failure of a tailings dam at the mine site. At the time of sampling (2003) over 40 kg/yr of Hg was still accumulating at the delta, indicating limited recovery. We attribute observed spatial evolution of elevated Hg levels to ongoing inputs and sediment re-working, and estimate the inventory of the anthropogenic fraction of total Hg to be at least 1500+/-300 kg. We suggest ongoing sediment inputs and methylation at the deltaic surface support enhanced mercury levels for resident biota and transfer to higher trophic levels throughout the Bay.

Real-time ocean chemical measurement: at-sea ICP-MS experiments

We describe the first at-sea deployment and operation of an inductively coupled plasma mass spectrometer (ICP-MS) for continuous measurement of isotopes and trace element concentrations in sea-water. The purpose of these experiments was to demonstrate that an ICP-MS can be operated in a harsh environment with no degradation in performance, and that accurate trace element data can be acquired on rapid analytical time scales. Evaluation at sea involved performance testing, characterization and calibration of a real-time sea-water analytical methodology, and continuous sea-water profiling over an extended three day transect from the Gulf of California to San Diego. We show that mass spectrometers can rapidly, precisely and accurately determine trace element concentrations in sea-water, thus allowing high-resolution mapping of large areas of surface sea-water. This analytical capability represents a significant advance toward real-time observation and characterization of water mass chemistry in dynamic coastal environments. While evaluating the ICP-MS off the coast of Baja California, we discovered barium depletion of 65% in surface waters over tens of kilometres.

Analytical Method for Measuring Cosmogenic 35S in Natural Waters

Cosmogenic sulfur-35 in water as dissolved sulfate ((35)SO4) has successfully been used as an intrinsic hydrologic tracer in low-SO4, high-elevation basins. Its application in environmental waters containing high SO4 concentrations has been limited because only small amounts of SO4 can be analyzed using current liquid scintillation counting (LSC) techniques. We present a new analytical method for analyzing large amounts of BaSO4 for (35)S. We quantify efficiency gains when suspending BaSO4 precipitate in Inta-Gel Plus cocktail, purify BaSO4 precipitate to remove dissolved organic matter, mitigate interference of radium-226 and its daughter products by selection of high purity barium chloride, and optimize LSC counting parameters for (35)S determination in larger masses of BaSO4. Using this improved procedure, we achieved counting efficiencies that are comparable to published LSC techniques despite a 10-fold increase in the SO4 sample load. (35)SO4 was successfully measured in high SO4 surface waters and groundwaters containing low ratios of (35)S activity to SO4 mass demonstrating that this new analytical method expands the analytical range of (35)SO4 and broadens the utility of (35)SO4 as an intrinsic tracer in hydrologic settings.