Vegetation and Soil Dynamics of a Louisiana Estuary Receiving Pulsed Mississippi River Water Following Hurricane Katrina

We monitored wetland biomass, decomposition, hydrology, and soil porewater chemistry at the Breton Sound estuary, which receives Mississippi River water from the Caernarvon river diversion structure. The estuary was in the direct path of hurricane Katrina in 2005, which caused a dramatic loss of wetlands in the upper basin. From March 2006 to October 2007, we made duplicate measurements at three distance classes from the diversion structure along the estuarine gradient as well as at a reference area, designated Near (N_1&2), Mid (M_1&2), Far (F_1&2), and Ref (R_1&2). Above- and belowground live biomass, porewater nutrients (NO_x, NH₄, and PO₄), salinity, sulfide, and soil Eh were measured every 2 months. Water level was monitored with gauges. Above- and belowground decomposition was measured using the litterbag (both) and cotton strip (belowground only) methods. Analysis of porewater parameters showed that stress factors affecting biomass production (porewater salinity, sulfide, flooding, and redox potential) were generally low to moderate, while measurable porewater nutrient concentrations occurred at all sites. Aboveground end of season live (EOSL) standing crop in October ranged from 423 g/m² at site M₂ to 1,515 at site F₁, and was significantly greater at site N₁ than at sites N₂, M₁, or M₂. Aboveground EOSL biomass during this study was significantly lower than previously measured in 1999, 2000, and 2001. Peak belowground biomass ranged from 8,315 g/m² at site R₂ to 17,890 g/m² at site N₁, which is among the highest reported in the literature, and there were significant increases throughout the study, suggesting recovery from hurricane Katrina. The decomposition bag data did not indicate any significant differences; however, the cotton strip decomposition rate was significantly lower at the lowest depth. Wetland surface vertical accretion ranged from 0.49 cm/year at N₂ to 1.24 cm/year at N₁, with site N₁ significantly greater than N₂, M₁, F₂, and R₁, and site N₂ significantly less than all other sites except site R₁. These findings show that marsh productivity and stability is related to a number of factors and no one factor can explain the impacts of the hurricanes.     

Origin of late Archean granite: geochemical evidence from the Vermilion Granitic Complex of northern Minnesota

The 2,700-Ma Vermilion Granitic Complex of northern Minnesota is a granite-migmatite terrane composed of supracrustal metasedimentary rocks, mafic rocks, tonalitic and granodioritic plutonic rocks, and granite. The metasedimentary rocks are predominantly graywacke, which has been regionally metamorphosed to garnet-sillimanite-muscovite-bearing biotite schist, and has locally undergone anatexis. The mafic rocks form early phases within the complex and are of two types: (1) basaltic amphibolite, and (2) monzodiorite and essexite rich in large ion lithophile elements (LILE). The members of the early plutonic suite form small bodies that intrude the metasedimentary rocks and mafic rocks, producing an early migmatite. The granite is of two distinct varieties: (1) white garnet-muscovite-biotite leucogranite (S-type; Chappell and White 1974) and (2) grayish-pink biotite-magnetite Lac La Croix Granite (I-type). The leucogranite occurs in the early migmatite and in paragneissic portions of the complex, whereas the Lac La Croix Granite is a late-stage intrusive phase that invades the early migmatite and metasediment (producing a late migmatite) and forms a batholith. This study focuses specifically on the origin of granite in the Vermilion Granitic Complex. Chemical mass-balance calculations suggest that the S-type two-mica leucogranite had a metagraywacke source, and that the I-type Lac La Croix Granite formed via partial fusion of calc-alkaline tonalitic material, which may have been similar to rocks of the early plutonic suite. This model is satisfactory for petrogenesis of similar Late Archean post-kinematic granites throughout the Canadian Shield.     

Oxygen isotope evolution of biogenic calcite and apatite during the Middle and Late Devonian

Oxygen isotope ratios of well-preserved brachiopod calcite and conodont apatite were used to reconstruct the palaeotemperature history of the Middle and Late Devonian. By assuming an oxygen isotopic composition of –1 V-SMOW for Devonian seawater, the oxygen isotope values of Eifelian and early Givetian brachiopods and conodonts give average palaeotemperatures ranging from 22 to 25 °C. Late Givetian and Frasnian palaeotemperatures calculated from ¹⁸O values of conodont apatite are close to 25 °C in the early Frasnian and increase to 32 °C in the latest Frasnian and early Famennian. Oxygen isotope ratios of late Givetian and Frasnian brachiopods are significantly lower than equilibrium values calculated from conodont apatite ¹⁸O values and give unrealistically warm temperatures ranging from 30 to 40 °C. Diagenetic recrystallization of shell calcite, different habitats of conodonts and brachiopods, as well as non-equilibrium fractionation processes during the precipitation of brachiopod calcite cannot explain the ¹⁸O depletion of brachiopod calcite. Moreover, the ¹⁸O depletion of brachiopod calcite with respect to equilibrium ¹⁸O values calculated from conodont apatite is too large to be explained by a change in seawater pH that might have influenced the oxygen isotopic composition of brachiopod calcite. The realistic palaeotemperatures derived from ¹⁸O _apatite may suggest that biogenic apatite records the oxygen isotopic composition and palaeotemperature of Palaeozoic oceans more faithfully than brachiopod calcite, and do not support the hypothesis that the ¹⁸O/¹⁶O ratio of Devonian seawater was significantly different from that of the modern ocean.     

Magma intrusion as a driving mechanism for the seismic clustering following the 9 May 1989 earthquake swarms at the Canary Islands

On 9 May 1989 a M _L = 5.2 earthquake struck a region between the islands of Tenerife and Gran Canaria. We investigated the time-spatial evolution of seismic patterns affecting the Canary Islands region during 1989–1995, using a quantitative spatial fractal analysis method. This method allows quantitative investigation of subtle trends in seismicity distribution through time. The fractal analysis indicates that epicenters clustered around a large zone during the May 1989 sequence affected narrow zones during 1991–1993, but then larger zones during 1993–1995 with an overall trend to shallower focal depths. The spatial localisation of seismic data and its time evolution appear to be related to magmatic rather than tectonic activity. Spatial clustering properties of seismicity are consistent with a major intrusive episode in 1989, followed by a period of quiescence and renewed deep intrusive activity from 1993 onwards. This interpretation suggests an increasing probability of future volcanic hazard in the region investigated.