Temporal variability of freshwater and pore water recirculation components of submarine groundwater discharges at Baffin Bay,Texas

Submarine groundwater discharges (SGDs) are an important source of freshwater as well as nutrients and other chemicals to bays and estuaries. SGDs are particularly important for coastal bodies in arid and semi-arid regions that are not fed by perennial streams. The Baffin Bay, TX is a shallow coastal water body that is weakly connected to the Gulf of Mexico and has no major rivers or streams draining into it. A year-long submarine groundwater discharge measurement study was carried out at the Loyola Beach of the Baffin Bay during the months of July 2005–June 2006. A total of 23 synoptic SGD sampling events were carried out with most events collecting SGD data continuously over a period of 24 h at a 1-min temporal resolution using an ultrasonic seepage meter. The median SGD was noted to be 3.83 cm/d with an inter-quartile range (IQR) of 11.36 cm/d. Four sampling events had anomalously high SGD values (~27–48 cm/d) which are hypothesized to be due to the geologic heterogeneity of the sea bed and meteorological effects. Eight of the 23 sampling events had a negative average SGD flux indicating landward flow. The short-term diurnal variability of SGD was comparable or sometimes higher than the longer-term and between-events variability. No long-term trend could be inferred. In the short-term, SGD measurements showed considerable persistence and the effective sample size analysis indicated each sampling event (consisting of over 1,000 samples) yielded only a handful of statistically independent measurements of SGD. The measured SGD values exhibited both negative (hydraulically controlled) and positive (wave set-up controlled) correlations with the bay water levels. Marine controls appeared to be the most significant SGD drivers and are in turn controlled by prevailing aeolian forcings. The salinity of the SGDs were estimated from measured sonic velocities and used in conjunction with the end-member mixing models to estimate fresh (meteoric) and re-circulated pore-water fractions. The freshwater fraction of the SGD was estimated to vary between nearly 4 and 89 % with a median value of 9.96 % and an IQR of 7.16 %. Three events were noted to have abnormally high freshwater fractions (~28, 50 and 84 %) which are likely artifacts caused by bay water freshening from rainfall and plausible thermal expansion. The meteoric and pore-water partitioning was sensitive to the assumed end-member concentrations. This study provides preliminary estimates for SGDs along the South Texas coast line and is useful for calibrating groundwater flow models and understanding the relative importance of terrestrial and marine controls on SGD. However, the heterogeneous nature of the sedimentary geology of the Texas Gulf Coast implies the SGD fluxes are likely to exhibit considerable spatial variation that has not been characterized yet. Therefore, the study provides useful insights for such future data collection and monitoring activities. The measured SGD values at Baffin Bay, TX are comparable to those reported at other parts of the Gulf of Mexico.     

Dependence of microbial magnetite formation on humic substance and ferrihydrite concentrations

Iron mineral (trans)formation during microbial Fe(III) reduction is of environmental relevance as it can influence the fate of pollutants such as toxic metal ions or hydrocarbons. Magnetite is an important biomineralization product of microbial iron reduction and influences soil magnetic properties that are used for paleoclimate reconstruction and were suggested to assist in the localization of organic and inorganic pollutants. However, it is not well understood how different concentrations of Fe(III) minerals and humic substances (HS) affect magnetite formation during microbial Fe(III) reduction. We therefore used wet-chemical extractions, magnetic susceptibility measurements and X-ray diffraction analyses to determine systematically how (i) different initial ferrihydrite (FH) concentrations and (ii) different concentrations of HS (i.e. the presence of either only adsorbed HS or adsorbed and dissolved HS) affect magnetite formation during FH reduction by Shewanella oneidensis MR-1. In our experiments magnetite formation did not occur at FH concentrations lower than 5 mM, even though rapid iron reduction took place. At higher FH concentrations a minimum fraction of Fe(II) of 25-30% of the total iron present was necessary to initiate magnetite formation. The Fe(II) fraction at which magnetite formation started decreased with increasing FH concentration, which might be due to aggregation of the FH particles reducing the FH surface area at higher FH concentrations. HS concentrations of 215-393 mg HS/g FH slowed down (at partial FH surface coverage with sorbed HS) or even completely inhibited (at complete FH surface coverage with sorbed HS) magnetite formation due to blocking of surface sites by adsorbed HS. These results indicate the requirement of Fe(II) adsorption to, and subsequent interaction with, the FH surface for the transformation of FH into magnetite. Additionally, we found that the microbially formed magnetite was further reduced by strain MR-1 leading to the formation of either dissolved Fe(II), i.e. Fe²⁺, in HEPES buffered medium or Fe(II) carbonate (siderite) in bicarbonate buffered medium. Besides the different identity of the Fe(II) compound formed at the end of Fe(III) reduction, there was no difference in the maximum rate and extent of microbial iron reduction and magnetite formation during FH reduction in the two buffer systems used. Our findings indicate that microbial magnetite formation during iron reduction depends on the geochemical conditions and can be of minor importance at low FH concentrations or be inhibited by adsorption of HS to the FH surface. Such scenarios could occur in soils with low iron mineral or high organic matter content.     

Trace element abundances in andesites

Abundance data for Cs, Rb, Tl, Ba, Pb, Sr, the rare earths, Th, U, Zr, Hf, Sn, Nb, Mo, Mn, Cu, Co, Ni, Sc, V, Cr, Ag, Sb and the major elements are reported for two andesites and a dacite from Saipan, nine andesites and a dacite from Bougainville and two andesites from Fiji. The Saipan rocks are low-K varieties and contain notably low abundances of Rb, Ba, Th and U and have rare earth patterns subparallel to chondritic patterns. The Bougainville andesites include low-Si and high-K varieties which have higher concentrations of the large cations. The Fijian samples are close to the average circum-Pacific andesite and have rareearth patterns sub-parallel to those of sedimentary rocks.All the andesites contain characteristically low (< 20 ppm) values for Ni and have Ni/Co ratios < 1, and V/Ni ratios > 10.These data preclude derivation of calc-alkaline rocks by mixing of upper crustal material or by fractional crystallisation from basaltic parents. A two stage model is proposed involving sea-floor spreading and transportation of the oceanic crust down the dipping seismic plane into the mantle where it is remelted to form andesites.