Description: 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 of 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-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-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.

Description: Rising temperatures in the Arctic Ocean region are responsible for changes such as reduced ice cover, permafrost thawing, and increased river discharge, which, together, alter nutrient and carbon cycles over the vast Arctic continental shelf. We show that the concentration of radium-228, sourced to seawater through sediment-water exchange processes, has increased substantially in surface waters of the central Arctic Ocean over the past decade. A mass balance model for 228 Ra suggests that this increase is due to an intensification of shelf-derived material inputs to the central basin, a source that would also carry elevated concentrations of dissolved organic carbon and nutrients. Therefore, we suggest that significant changes in the nutrient, carbon, and trace metal balances of the Arctic Ocean are underway, with the potential to affect biological productivity and species assemblages in Arctic surface waters.

Description: We investigate the time scales of magma genesis, melt evolution, crystal growth rates and magma degassing in the Erebus volcano magmatic system using measurements of 238 U– 230 Th– 226 Ra– 210 Pb– 210 Po, 232 Th– 228 Ra– 228 Th and 235 U– 231 Pa– 227 Ac. These are the first measurements of 231 Pa– 227 Ac in volcanic samples and represent the first set of data in a volcanic system to examine the entire suite of relevant 238 U, 235 U and 232 Th decay series nuclides. Our sample suite consists of 22 phonolite volcanic bombs, erupted between 1972 and 2005, and five anorthoclase megacrysts separated from bombs erupted in 1984, 1989, 1993, 2004 and 2005. The 238 U– 230 Th, 230 Th– 226 Ra and 235 U– 231 Pa systems are uniform over the 34 years examined. The anorthoclase megacrysts and phonolite glasses show complementary 226 Ra/ 230 Th disequilibria with ( 226 Ra/ 230 Th) ~40 in the anorthoclase and ~0·75 in the phonolite glass. In all samples, ( 210 Pb/ 226 Ra) is in radioactive equilibrium for both phases. In two phonolite glass samples ( 227 Ac/ 231 Pa) is unity. For the phonolite glasses ( 228 Ra/ 232 Th) is in equilibrium, whereas in the anorthoclase megacrysts it is significantly greater than unity. Instantaneous crystal fractionation, with magma residence times greater than 100 years and less than 10 kyr, can account for the measured 238 U– 230 Th– 226 Ra– 210 Pb and 235 U– 231 Pa– 227 Ac. However, the significant 228 Ra/ 232 Th disequilibria in the anorthoclase megacrysts preclude this simple interpretation. To account for this apparent discrepancy we therefore developed an open-system, continuous crystallization model that incorporates both nuclide ingrowth and decay during crystallization. This open-system model successfully reproduces all of the measured 238 U and 232 Th disequilibria and suggests that the shallow magma reservoir at Erebus is growing. The implication of this modeling is that when the time scale of crystallization is comparable with the half-life of the daughter nuclide of interest (e.g. 226 Ra) the simple isochron techniques typically used in most U-series studies can provide erroneous ages. The observation that ( 210 Pb/ 226 Ra) and ( 227 Ac/ 231 Pa) are in radioactive equilibrium suggests that the residence time of the magmas is 〉100 years. When considering the effect of 222 Rn degassing on 210 Pb/ 226 Ra, the data indicate that the majority of magma degassing is deep and long before eruption, consistent with melt inclusion data. Additionally, for the 2005 lava bomb, whose eruption date (16 December 2005) is known explicitly, 210 Po was not completely degassed from the magma at the time of eruption. Incomplete degassing of 210 Po is atypical for subaerially erupted lavas and suggests that the Erebus shallow magma degasses about 1% of its Po per day. The combined 238 U and 232 Th data further indicate that the pyroclasts ejected by Strombolian eruptions at Erebus have compositions that are close to what would be expected for a near-steady-state system, reflecting inmixing of degassed magmas, crystal fractionation, and aging.

Description: AbstractWe conducted an intercomparison of methods for the determination of 234Th in seawater. Samples were collected either from a shore-based 600 m water source, or from standard bottle casts collected in deep waters off Hawaii and in the Southern Ocean. We compared large volume techniques which rely upon Mn cartridges for the collection of dissolved 234Th and its detection via gamma counting (〉200 liter samples) with small volume methods that employed either direct beta counting, or beta counting after radiochemical purification (2-20-Liter samples). Unique to this study are the presentation of a novel 2 and 5 liter 234Th methods. This new method is an adaptation of 20-liter methods which are based on a coprecipitation of thorium with Mn oxides and the direct beta counting of the precipitate. These Mn coprecipitation methods were found to be superior to other methods due to ease of sample collection and processing and low overall analytical uncertainties.

Description: Thorium-234 is a naturally occurring radionuclide that has been widely studied in ocean sciences. These studies use the disequilibrium between the soluble parent uranium-238 (half-life = 4.5 x 109 years), and its particle reactive daughter, 234Th (half-life = 24.1 days), to quantify the in-situ removal rates of 234Th on sinking particles. Here, we present additional experiments that test a new 2-L procedure in which 234Th is co-scavenged with a MnO2 precipitate. Unlike other techniques, this method can be easily applied at-sea with an overall precision and accuracy of £ 5 %. Our experiments have sought to elucidate the effects of delaying reagent addition and precipitate filtration, differences in sample bottle types, and issues related to sample backgrounds and 234Th particulate sampling. Most of these experiments were conducted using water collected on repeated occupations of station ALOHA (22°45.0’N, 158°00.0’W), 100 km North of Oahu, Hawaii.