Primary production in Long Island sound

Daily and annual integrated rates of primary productivity and community respiration were calculated using physiological parameters measured in oxygen-based photosynthesis-irradiance (P-I) incubations at 8 stations throughout central and western Long Island Sound (cwLIS) during the summer and autumn of 2002 and 2003 and the late spring of 2003. Each calculation takes into account actual variations in incident irradiance over the day and underwater irradiance and standing stock with depth. Annual peak rates, ±95% confidence interval of propagated uncertainty in each measurement, of gross primary production (GPP, 1,730±610 mmol O₂ m⁻² d⁻¹), community respiration (R_c, 1,660±270 mmol O₂ m⁻² d⁻¹), and net community production (NCP, 1,160±1,100 mmol O₂ m⁻² d⁻¹) occurred during summer at the western end of the Sound. Lowest rates of GPP (4±11 mmol O₂ m⁻² d⁻¹), R_c (−50±300 mmol O₂ m⁻² d⁻¹), and NCP (−1,250±270 mmol O₂ m⁻² d⁻¹) occurred during late autumn-early winter at the outer sampled stations. These large ranges in rates of GPP, R_c, and NCP throughout the photic zone of cwLIS are attributed to seasonal and spatial variability. Algal respiration (R_a) was estimated to consume an average of 5% to 52% of GPP, using a literature-based ratio of R_a:R_c. From this range, we established that the estimated R_a accounts for approximately half of GPP, and was used to estimate daily net primary production (NPP), which ranged from 2 to 870 mmol O₂ m⁻² d⁻¹ throughout cwLIS during the study. Annual NPP averaged 40±8 mol O₂ m⁻² yr⁻¹ for all sampled stations, which more than doubled along the main axis of the Sound, from 32±14 mol O₂ m⁻² yr⁻¹ at an eastern station to 82±25 mol O₂ m⁻² yr⁻¹ at the western-most station. These spatial gradients in productivity parallel nitrogen loads along the main axis of the Sound. Daily integrals of productivity were used to test and formulate a simple, robust biomass-light model for the prediction of phytoplankton production in Long Island Sound, and the slope of the relationship was consistent with reports for other systems.     

Chemical and isotopic evidence for widespread Eoarchean metasedimentary enclaves in southern West Greenland

New mapping, geochemistry and zircon U-Pb ion microprobe geochronology of pre-3750 Ma rocks from West Greenland was used to identify sedimentary protoliths in a problematic high-grade metamorphic terrane. Samples were collected from southernmost part of the Itsaq Gneiss Complex where Akilia association supracrustal rocks have previously been noted. Supracrustal lithologies include laterally continuous and variably deformed units of amphibolite, ultramafics and ferruginous quartz-pyroxene rocks. Oxygen isotope and mass-independently fractionated sulfur isotopes, immobile trace elements and rare earth element patterns are consistent with origin of quartz-pyroxene rocks as chemical sediments deposited in a marine hydrothermal setting. We describe a further supracrustal lithology: Garnet-bearing quartz-biotite schists with elevated oxygen isotope values (δ¹⁸O_SMOW ? +16‰) and mass-independently fractionated S isotopes consistent with a low-temperature aqueous sedimentary origin. In several enclaves, granitoid gneisses within low-strain limbs transect lithologic contacts and contain inclusions of surrounding rocks. This supports the interpretation that some orthogneisses were originally emplaced as igneous veins that cut supracrustal lithologies. Zircon geochronology on orthogneisses that preserve intrusive relationships confirms minimum ages of ca. 3750 Ma for the supracrustals and pooled [Th/U]_zircon and δ¹⁸O_zircon values of older zircon populations are consonant with igneous growth in the bulk composition of the host rocks. Low [Zr]_WR and high Zr saturation temperatures further minimize the possibility of zircon inheritance. A >3750 Ma age and chemical sedimentary origin for various Akilia association lithologies underscores the widespread occurrence of rocks of this kind beyond the type locality on Akilia (island) at the southern limit of the Itsaq Gneiss Complex.     

Diel biogeochemical cycling in a hyperventilating shallow estuarine environment

A diel biogeochemical study was performed to assess the influence that periods of elevated biological activity have on the biogeochemical cycling of macronutrients and redox-sensitive elements in a natural estuarine environment. High-resolution data (15 min sampling) illustrates periodic extreme variations in dissolved oxygen (DO) in the shallow waters of Azevedo Pond, Elkhom Slough, California. During periods of low tidal flushing, DO values can range from highly oxic (>560 μM O₂: >250% saturation) during sunny days to suboxic conditions (<5 μM) at night. Nutrient cycling and redox-sensitive trace element biogeochemistry were evaluated in response to the extreme daily DO fluctuations. A diel sampling study was conducted over a 26-h period, where O₂ concentrations ranged from 346 μM to sustained non-detectable levels in the night hours. In concert with the DO fluctuations, diel phosphate cycling was on the order of 4 μM in response to tidal flushing events and biological assimilation and regeneration. The IO₃ ⁻/I⁻ redox couple quickly responded to suboxic conditions in the water column by a marked increase in I⁻ concentrations and corresponding depletion of IO₃ ⁻. The extreme fluctuations of the p∈ in the water column resulted in diel dissolved Mn²⁺ variations of nearly 5 μM, with observed dissolved Mn removal rates on the order of 1 μM h⁻¹. The elevated biogeochemical cycling of oxygen, nitrogen, phosphorus, iodine, manganese, and iron found in this shallow estuarine environment suggest that tidal restrictions and anthropogenic nutrient enrichments can amplify diel variations and potentially hinder the functional and ecological stability of these systems. These data suggest that accurate chemical monitoring of the health of an estuarine ecosystem must account for the diel variability inherent in these highly productive environments.     

Tectono-sedimentary evolution of transverse extensional faults in a foreland basin: Response to changes in tectonic plate processes

Late Paleocene to Middle Eocene strata in the easternmost part of the Southern Pyrenees, up to 4 km thick, provide information on tectono-sedimentary evolution of faults transversal to the Pyrenean chain. To know how changes in tectonic plate processes control the structural evolution of transverse faults and the synchronous thickness and lithological distribution of sedimentary strata in a foreland basin, field observations, interpretation of 2D seismic lines tied to lithostratigraphic data of exploration wells and gravity modelling constrains were carried out. This resulted in the following two tectono-sedimentary phases in a foreland basin: first phase, dominated by transverse extensional faulting, synchronous with deposition of marine carbonates (ca. 57 to 51 Ma); and second phase, characterized by transverse contractional faulting, coeval to accumulation of marine and transitional siliciclastics (51 to 44 Ma). During the first phase, Iberia and Adria were moving to the east and west respectively. Therefore, lithospheric flexure in the easternmost part of the Iberian plate was developed due to that Sardinia was over-thrusting Iberia. Consequently, activation of E-dipping normal faults was generated giving rise to thick-deep and thin-shallow carbonate platform deposits across the hanging walls and footwalls of the transverse structures. During the second phase, a shearing interaction between Iberia and Sardinia prevailed re-activating the transverse faults as contractional structures generating thin-shelf and thick-submarine fan deposits across the hanging walls and footwalls of the transverse structures. In the transition between the first and second phases, evaporitic conditions dominated in the basin suggesting a tectonic control on basin marine restriction. The results of our study demonstrate how thickness and lithology distribution, controlled by transverse faulting in a compressional regimen, are influenced by phases related to processes affecting motions and interactions between tectonic plates and continental blocks.     

Late Mesoproterozoic rifting in Arctic Canada during Rodinia assembly: impactogens,trans‐continental far‐field stress and zinc mineralisation

This paper challenges the conventional interpretation of a major, economically important Mesoproterozoic intracratonic rift system as a group of aulacogens, proposing instead that they are rifts that developed in response to far‐field stress caused by continent–continent collision (impactogens) during supercontinent assembly. The tectonostratigraphic evolution of the Bylot basins (Arctic Canada) records dramatic alternations between extensional and compressional stress regimes, precluding an aulacogen interpretation and favouring an impactogen interpretation. New geochronological data (U–Th–Pb whole‐rock depositional age; detrital zircon signatures) provide a record of impactogens that developed in Laurentia's interior as a result of Grenvillian (~1.1 Ga) far‐field stress during assembly of the supercontinent Rodinia. Formation of the world‐class Nanisivik zinc deposit in one of the rifts is temporally associated with the Grenvillian orogeny, and consequently represents both (i) a rare exception to the global pattern of few ore deposits forming during supercontinent episodes, and (ii) a hitherto unrecognised tectonic setting for carbonate‐hosted zinc deposits.     

Insights about the formation of a complex impact structure formed in basalt from numerical modeling: The Vista Alegre structure,southern Brazil

We present the outcomes of simulations of the formation of the Vista Alegre impact structure, Paraná Basin, Brazil. The target comprised a thick sequence of volcanic rocks of predominantly basaltic composition of the Serra Geral Formation that had been deposited on top of sedimentary rocks (sandstones) of the Pirambóia/Botucatu formations. The cratering process was modeled using the iSALE shock physics code. Our best‐fit model suggests that (1) the crater was originally ~10 km in size; (2) it was formed in ~115 s by a stony projectile of 1000 m in diameter, for an assumed impact velocity of 12 km s^?1; (3) target rocks underwent a peak pressure of ~20 GPa, in agreement with previous petrographic investigations of shock deformation. Furthermore, the model points out that the sedimentary strata below the layer of volcanic rocks were raised by ~650 meters at the central part of the crater, which resulted in the current partial exposure of the sandstones at the surface. The outcomes of our modeling suggest that parameters like cohesion and strength of the target rocks, after shock compression, determined the final morphology of the crater, especially the absence of a topographically prominent central peak. Finally, the results of the numerical modeling are roughly in agreement with gravity data over the structure, in particular with respect to the presence of the uplifted sedimentary strata, which are responsible for a low gravity signature at the center of the structure.     

Geochemical evidence of the renewal of volcanic activity inferred from CO<Subscript>2</Subscript> soil and SO<Subscript>2</Subscript> plume fluxes: the 2007 Stromboli eruption (Italy)

On 27 February 2007, a new eruption occurred on Stromboli which lasted until 2 April. It was characterized by effusive activity on the Sciara del Fuoco and by a paroxysmal event (15 March). This crisis represented an opportunity for us to refine the model that had been developed previously (2002–2003 eruption) and to improve our understanding of the relationship between the magmatic dynamics of the volcano and the geochemical variations in the fluids. In particular, the evaluation of the dynamic equilibrium between the volatiles (CO₂ and SO₂) released from the magma and the corresponding fluids discharged from the summit area allowed us to evaluate the level of criticality of the volcanic activity. One of the major accomplishments of this study is a 4-year database of summit soil CO₂ flux on the basis of which we define the thresholds (low–medium–high) for this parameter that are empirically based on the natural volcanological evolution of Stromboli. The SO₂ fluxes of the degassing plume and the CO₂ fluxes emitted from the soil at Pizzo Sopra la Fossa are also presented. It is noteworthy that geochemical signals of volcanic unrest have been clearly identified before, during and after the effusive activity. These signals were found almost simultaneously in the degassing plume (SO₂ flux) and in soil degassing (CO₂ flux) at the summit, although the two degassing processes are shown to be clearly different. The interpretation of the results will be useful for future volcanic surveillance at Stromboli.     

Insights into the origin of primitive silica-undersaturated arc magmas of Aoba volcano (Vanuatu arc)

Aoba picrites in Vanuatu arc (Southwestern Pacific) offer the opportunity to address the question of the origin of Si-undersaturated arc magmas, through the geochemical study of their olivine-hosted melt inclusions. These latter delineate a differentiation trend of calc-alkaline silica-undersaturated basalts, with typical trace-element patterns of arc magmas. The most primitive melt inclusions, preserved in olivines with Fo ≥ 89, have normative nepheline compositions with CaO/Al₂O₃ > 0.8, but belong to three distinct populations differing in their enrichment or depletion in LILE, Cl, and alkalis (Rb, K). The dominant population is characterized by medium-LILE concentrations (La/Yb ~ 7–8) and represents the parental magma of the Aoba lavas. The two others (La/Yb ~ 20 and 2) are either significantly enriched or extremely depleted in LILE, Cl, and alkalis. This compositional variability of primitive magma batches requires the multi-stage mixing between melts generated by partial melting of both peridotite and clinopyroxene-rich lithologies. Medium-LILE magma derives from the mixing between peridotite- and clinopyroxenite-derived melts, whereas the high- and low-LILE melts involve amphibole-bearing and amphibole-free clinopyroxenite sources, respectively.     

Geogas transport in fractured hard rock – Correlations with mining seismicity at 3.54 km depth, TauTona gold mine, South Africa

An on-site gas monitoring study has been conducted in the framework of an earthquake laboratory (The International NELSAM–DAFGAS projects) at the TauTona gold mine, South Africa. Five boreholes up to 60 m long were drilled at 3.54 km depth into the highly fractured Pretorius Fault Zone and instruments for chemical and seismic monitoring installed therein. Over the span of 4 years sensitive gas monitoring devices were continuously improved to enable the direct observation of geogas concentration variations in the DAFGAS borehole. The major gas concentrations are constant and air-like with about 78% N₂, 21% O₂, 1% Ar. The geogas components CO₂, CH₄, He and H₂ show the most interesting trends and variations on the minute-by-minute basis and significantly correlate with seismic data, while the ²²²Rn activity remains constant. Time series and cross correlation analysis allow the identification of different gas components (geogas and tunnel air) and the identification of two processes influencing the borehole gas composition: (1) pumping-induced tunnel air breakthrough through networks of initially water-saturated fault fractures; and (2) seismicity induced permeability enhancement of fault fractures to above ∼5 × 10^-10 m². The current set-up of the gas monitoring system is sensitive enough to quantify the resulting geogas transport during periods of intense blasting activities (including recorded blasts with seismic moment ?1 × 10⁹ Nm, located within 1000 m of the cubby) and, it is suggested, also during induced earthquakes, a final goal of the project.     

A performance based approach to dolomite risk management

Urban development commonly disturbs the meta-stable conditions in the dolomite environment which can lead to sinkhole formation. 650 sinkholes, which manifested from 1984 to 2004 in an approximately 3,700 ha urbanized environment, located on dolomite land south of Pretoria in South Africa, was analyzed in the absence of risk mitigation measures. A 4 year period post the implementation of a comprehensive risk management system was also analyzed. This research permitted a timely review of the Buttrick et al. (J South African Inst Civil Eng 43(2):27–36, 2001) methodology for dolomite land hazard identification which is commonly used in stability analysis and the development of risk mitigation strategies in Southern Africa. The research confirmed this methodology and demonstrated that the approach to the mitigation of hazards through risk management can improve the hazard rating of dolomite land. The research provided much needed data and insights to refine and expand upon the current South African methodology to cover the full spectrum of developments in a manner which is capable of being included in legislative frameworks governing the development of dolomite land, both locally and internationally.