Seasonal Alternation Between Groundwater Discharge and Benthic Coupling as Nutrient Sources in a Shallow Coastal Lagoon

Submarine groundwater discharge (SGD) is now recognized as an important source of nutrients and freshwater to some coastal environments. We studied a shallow coastal lagoon (Little Lagoon, AL, USA) in the northern Gulf of Mexico that lacks riverine inputs but has been suspected to receive significant SGD. We observed persistent salinity gradients between the east and west ends of the lagoon and the pass connecting it to the Gulf of Mexico. Covariance between salinity in the lagoon and the groundwater tracer ²²²Rn indicated that SGD was responsible for the salinity gradients and is the primary source of freshwater to the lagoon. Cluster analysis of 246 biweekly samples based on temperature, salinity, and two proxies of SGD revealed two hydrographic regimes with different drivers for nutrient inputs. In samples characterized by high discharge and low temperatures (generally December–April), total nitrogen (TN) was negatively correlated with salinity, while total phosphorus (TP) was positively correlated with temperature. Total nitrogen in the groundwater was very high (0.36–4.80 mM) while total phosphorus was relatively low (0.3–2.3 μM), consistent with SGD as the source of TN during the high-discharge periods. In periods with low discharge and higher temperatures (approx. May–November), TN and TP had strong positive correlations with temperature and are inferred to originate from benthic efflux. Seasonal changes in nutrient stoichiometry in the lagoon water column also indicate an alternation between low TN/TP sediments and high TN/TP groundwater as the primary sources of nitrogen in this system.     

Subsurface structure across the axis of the Tongariro Volcanic Centre,New Zealand

The relationship between structure and volcanism in the Tongariro Volcanic Centre, New Zealand, is largely masked by a mantle of young volcanic deposits. Here we report the results of an integrated geophysical investigation (using gravity, multi-level aeromagnetic and magnetotelluric methods) of subsurface deposits and structures in the upper 1–2 km across the axis of the Tongariro Volcanic Centre. Modelling of these data across the Tama Lakes saddle shows that the outcropping volcanic deposits are up to 800 m thick, underlain by Tertiary sediments (of a few 10's to a few 100 m in thickness) and in turn lying above a basement of probable Mesozoic greywacke. Basement faulting is shown to be concentrated in the centre of the rift, which is 18 km wide at this location, but no vertical offset is resolved at the rift axis. Vertical displacements on basement faults of 250–300 m are modelled giving a minimum total basement subsidence of 650 m. A 5 km-wide, deep low resistivity zone occurs at the axis of the rift which is interpreted as either resulting from extensive fracturing and/or hydrothermal alteration within the basement. Steep-sided volcanic bodies with a high proportion of lavas/dykes coincide with the Waihi fault and the rift axis. Coincidence with the Waihi Fault suggests that this fault system may have provided magma pathways to the surface and a focus for dyke emplacement, which could have contributed to rift extension. The lack of offset at the rift axis may reflect the juvenile nature of faulting at this location, which is consistent with the notion of a migration of faulting towards the centre of the graben, alternatively, rifting may have been entirely accommodated by dyke emplacement.     

Parasites of eastern oysters from subtidal reefs in a Louisiana estuary with a note on their use as indicators of water quality

A 12-month parasitological study in-volved examination of 1,242 eastern oysters,Crassostrea virginica, from two hydrologically different subtidal reefs in Calcasieu Lake, Louisiana. No evidence of morbidity attributed to infection with the protozoanPerkinsus marinus was detected among oysters at either site. Most oysters from the higher salinity site had shells infested with boring sponges,Cliona truitti. Oysters from both sites harbored infections with sporocysts and cercariae of the digenetic trematode,Bucephalus cuculus. However, average monthly incidence of infection was less than 0.5%.     

An instrument system for the investigation of particle fluxes

We present the rationale, design, and use of an instrument system to measure the variability of vertical and horizontal particle fluxes. The system features a new sequentially sampling sediment trap which collects and seals 10 separate samples during a single deployment. Horizontal particle fluxes are simultaneously monitored with a beam transmissometer interfaced to a standard Aanderaa current meter. Results from a 10-week deployment of instruments at several depths in a deep fjord estuary indicate that the trapping rate increases from 0.5g m⁻² day⁻¹ at 20 m to 150g m⁻² day⁻¹ at 200 m (5 m above bottom) because of frequent erosion in the deep waters. Periodic flushing of the deep water by intrusions of marine water over the seaward sill markedly enhances erosion and causes an up-estuary particle transport comparable to the vertical particle flux originating at the surface.     

Echelle spectroscopy of Ca ii HK activity in Southern Hemisphere planet search targets

We present the results of ultraviolet echelle spectroscopy of a sample of 59 F, G, K and M stars from the Anglo-Australian Planet Search target list. Ca  ii activity indices, which are essential in the interpretation of planet detection claims, have been determined for these stars and placed on the Mount Wilson R '_HK system.     

Determination of Selenium Concentrations in NIST SRM 610, 612, 614 and Geological Glass Reference Materials Using the Electron Probe, LA-ICP-MS and SHRIMP II

A combination of EMPA, sensitive high resolution ion microprobe (SHRIMP II) and/or LA-ICP-MS techniques was used to measure the concentration of selenium (Se) in NIST SRM 610, 612, 614 and a range of reference materials. Our new compiled value for the concentration of Se in NIST SRM 610 is 112 ± 2 μg g⁻¹. The concentration of Se in NIST SRM 612, using NIST SRM 610 for calibration, determined using LA-ICP-MS (confirmed using SHRIMP II) was 15.2 ± 0.2 μg g⁻¹. The concentration of Se in NIST SRM 614, using LA-ICP-MS was 0.394 ± 0.012 μg g⁻¹. LA-ICP-MS determination of Se in synthetic geological glasses BCR-2G, BIR-1G, TB-1G and the MPI-DING glasses showed a range in concentrations from 0.062 to 0.168 μg g⁻¹. Selenium in the natural glass, VG2, was 0.204 ± 0.028 μg g⁻¹.     

The case for Archaean boninites

Rare Archaean light rare earth element (LREE)-enriched mafic rocks derived from a strongly refractory mantle source show a range of features in common with modern boninites. These Archaean second-stage melts are divided into at least two distinct groups—Whundo-type and Whitney-type. Whundo-type rocks are most like modern boninites in terms of their composition and association with tholeiitic to calc-alkaline mafic to intermediate volcanics. Small compositional differences compared to modern boninites, including higher Al₂O₃ and heavy REE (HREE), probably reflect secular changes in mantle temperatures and a more garnet-rich residual source. Whundo-type rocks are known from 3.12 and 2.8 Ga assemblages and are true Archaean analogues of modern boninites. Whitney-type rocks occur throughout the Archaean, as far back as ca. 3.8 Ga, and are closely associated with ultramafic magmatism including komatiites, in an affiliation unlike that of modern subduction zones. They are characterised by very high Al₂O₃ and HREE concentrations, and their extremely depleted compositions require a source which at some stage was more garnet-rich than the source for either modern boninites or Whundo-type second-stage melts. Low La/Yb and La/Gd ratios compared to Whundo-type rocks and modern boninites either reflect very weak subduction-related metasomatism of the mantle source or very limited crustal assimilation by a refractory-mantle derived melt. Regardless, the petrogenesis of the Whitney-type rocks appears either directly or indirectly related to plume magmatism. If Whitney-type rocks have a boninitic petrogenesis then a plume related model similar to that proposed for the modern Tongan high-Ca boninites might apply, but with uniquely Archaean source compositions and source enrichment processes. Second-stage melts from Barberton (S. Africa –3.5 Ga) and ca. 3.0 Ga rocks from the central Pilbara (Australia) have features in common with both Whundo- and Whitney-types, but appear more closely related to the Whitney-type. Subduction zone processes essentially the same as those that produce modern boninites have operated since at least ~3.12 Ga, while a uniquely Archaean boninite-forming process, involving more buoyant oceanic plates and very inefficient mantle-source enrichment, may have occurred before then.     

Evidence for heterogenes primary MORB and mantle sources,NW Indian Ocean

Basalts from 5 Deep Sea Drilling Project (DSDP) sites in the northwest Indian Ocean (Somali Basin and Arabian Sea) have general geochemical features consistent with a spreading origin at the ancient Carlsberg Ridge. However, compared to most MORBS from other oceans they have low normative olivine, TiO₂, and Zr contents. There is no evidence that the mantle source of these northwest Indian Ocean basalts was enriched in incompatible elements relative to the Atlantic and Pacific ocean mantles. In detail, incompatible element abundances in these DSDP basalts establish that they evolved from several compositionally distinct parental magmas. In particular, basalts from site 236 in the Somali Basin have relatively high SiO₂ and low Na, P, Ti, and Zr contents. These compositional features along with low normative olivine contents are similar to those proposed for melts derived by two-stage (or dynamic) melting. Published data also indicate there is no enrichment in incompatible elements at the southwest Indian Ocean triple junction, although southwest Indian Ocean basalts have slightly higher ⁸⁷Sr/⁸⁶Sr than normal Atlantic MORB. The data suggest that there are significant subtle geochemical variations in the Indian Ocean mantle sources, but are insufficient to show whether these variations have a systematic temporal or geographic distribution.     

Oxygen and hydrogen isotope studies of plutonic granitic rocks

The primary δD values of the biotites and hornblendes in granitic batholiths are remarkably constant at about ?50 to ?85, identical to the values in regional metamorphic rocks, marine sediments and greenstones, and most weathering products in temperate climates. Therefore the primary water in these igneous rocks is probably not “juvenile”, but is ultimately derived by dehydration and/or partial melting of the lower crust or subducted lithosphere. Most granitic rocks have δ¹⁸O = +7.0 to +10.0, probably indicating significant involvement of high-¹⁸O metasedimentary or altered volcanic rocks in the melting process; such an origin is demanded for many other granodiorites and tonalites that have δ¹⁸O = +10 to +13. Gigantic meteoric-hydrothermal convective circulation systems were established in the epizonal portions of all batholiths, locally producing very low δ¹⁸O values (particularly in feldspars) during subsolidus exchange. Some granitic plutons in such environments also were emplaced as low-¹⁸O magmas probably formed by melting or assimilation of hydrothermally altered roof rocks. However, the water/rock ratios were typically low enough that over wide areas the only evidence for meteoric water exchange in the batholiths is given by low D/H ratios (δD as low as ?180); for example, because of latitudinal isotopic variations in meteoric waters, as one moves north through the Cordilleran batholiths of western North America an increasingly higher proportion of the granitic rocks have δD values lower than ?120. The lowering of δD values commonly correlates with re-setting of K-Ar ages, and in the Idaho batholith two broad zones (10,000 km²) can be defined where δD biotite <?100 and K-Ar “ages” have all been re-set to values less than 60 m.y., suggesting that the Ar loss was caused by the meteoric-hydrothermal circulation systems. In certain Precambrian batholiths, a much different type of very low-temperature, regional alteration by surface-derived waters took place over an extended period long after emplacement, producing “brick-red” feldspars and markedly discordant Rb-Sr isochron “ages”.