A study of the chemistry of pore fluids and authigenic carbonates in methane seep environments: Kodiak Trench, Hydrate Ridge, Monterey Bay, and Eel River Basin

Analyses of the chemical and isotopic composition of carbonates rocks recovered from methane seepage areas of the Kodiak Trench, Hydrate Ridge, Monterey Bay Clam Flats, and the Eel River Basin, coupled with the studies of the chemistry of the pore fluids, have shown that these carbonates have grown within the sediment column. Geochemical profiles of pore fluids show that, in deep water seeps (Kodiak Trench—4450 m; Monterey Bay—1000 m; Hydrate Ridge—650 m), δ¹³C (DIC) values are low (isotopically light), whereas in the Eel River area ( 350–500 m), δ¹³C (DIC) values are much higher (isotopically heavier). In all cases, the δ¹³C values indicate that processes of methane oxidation, associated with sulfate reduction, are dominant in the shallow sediments. Data on the isotopic composition of authigenic carbonates found at sites in Kodiak Trench, Eel River Basin South, and Eel River Basin North indicate a variable composition and origin in different geochemical environments. Some of the authigenic carbonates from the study sites show a trend in their δ¹³C values similar to those of the pore fluids obtained in their vicinity, suggesting formation at relatively shallow depths, but others indicate formation at greater sediment depths. The latter usually consist of high magnesium calcite or dolomite, which, from their high values of δ¹³C (up to 23‰;) and δ¹⁸O (up to 7.5‰), suggest formation in the deeper horizons of the sediments, in the zone of methanogenesis. These observations are in agreement with observations by other workers at Hydrate Ridge, in Monterey Bay, and in the Eel River Basin.     

Causes of interannual variability in the sea ice cover of the Eastern Bering Sea

It has been shown that large-scale weather patterns in both the tropical South Pacific (El Niño-Southern Oscillation, or ENSO, events) and the North Pacific (Pacific-North American, or PNA, patterns) have strong teleconnection effects on the air, ice, and ocean environments of the Bering Sea. This signal apparently comes via the atmosphere and not the ocean. The connection between variability of the Bering Sea and the ENSO and PNA appears to be the winter position of the Aleutian Low. Interannual variability in air temperatures, ice cover, and surface winds in the Bering Sea generally are in phase with each other, whereas sea-surface temperatures (SST) tend to lag these variables by 1–3 months. These Bering Sea time-series are significantly correlated with the Southern Oscillation Index (SOI) time-series (an indicator of ENSO events) when the Bering sea data are lagged behind the SOI for up to 18 months. The correlations suggest that warming in the Bering Sea follows negative anomalies in the SOI (i.e., El Niño events). Cooling in the Bering Sea tends to follow positive anomalies (i.e., precursors of El Niños) in the SOI. Maximal correlations for the PNA also lag the SOI by a mouth or two.Analyses of variance indicate that the SOI can explain 30–40% of the variability in the Bering Sea. Stepwise multiple regressions can explain up to 54% of the variation in air temperatures, up to 39% of the variation in sea ice cover, and up to 46% of the variation in SST in the Bering Sea. PNA and SOI were significant variables only in the equation for air temperatures, indicating a close relationship between them and the atmosphere in the Bering Sea and suggesting that energy is transmitted to the water and ice via the atmosphere. The three variables airtemps, ice, and SST were significant each time they were used as independent variables, indicating a rapid and strong feedback relationship among them.Three ENSO events have occurred since the mid-1970s, but none have been typical. There have been either two positive SOI anomalies preceding an El Niño or there have been none preceding an El Niño. When there has been a positive anomaly, ice cover has been above normal, but neither a positive anomaly nor above-normal ice has occurred in the past two ENSO events. An ice retreat has occurred any time there has been an ENSO event, except in the case of the great El Niño of 1982–1983; the anomalous position of the Aleutian Low at that time explains the lack of response of the ice. Finally, one ice retreat occurred that was unrelated to an ENSO event, but was related to a PNA event.     

Scenario Evaluator for Electrical Resistivity Survey Pre‐modeling Tool

Geophysical tools have much to offer users in environmental, water resource, and geotechnical fields; however, techniques such as electrical resistivity imaging (ERI) are often oversold and/or overinterpreted due to a lack of understanding of the limitations of the techniques, such as the appropriate depth intervals or resolution of the methods. The relationship between ERI data and resistivity is nonlinear; therefore, these limitations depend on site conditions and survey design and are best assessed through forward and inverse modeling exercises prior to field investigations. In this approach, proposed field surveys are first numerically simulated given the expected electrical properties of the site, and the resulting hypothetical data are then analyzed using inverse models. Performing ERI forward/inverse modeling, however, requires substantial expertise and can take many hours to implement. We present a new spreadsheet‐based tool, the Scenario Evaluator for Electrical Resistivity (SEER), which features a graphical user interface that allows users to manipulate a resistivity model and instantly view how that model would likely be interpreted by an ERI survey. The SEER tool is intended for use by those who wish to determine the value of including ERI to achieve project goals, and is designed to have broad utility in industry, teaching, and research.     

Petrogenesis of basaltic shergottite Northwest Africa 8657: Implications for fO2 correlations and element redistribution during shock melting in shergottites

Northwest Africa (NWA) 8657 is an incompatible trace element-enriched, low-Al basaltic shergottite, similar in texture and chemistry to Shergotty, Zagami, and NWA 5298. It is composed of zoned pyroxene, maskelynite, merrillite, and Ti-oxide minerals with minor apatite, silica, and pyrrhotite. Pyroxene grains are characterized by patchy zoning, with pigeonite or augite cores zoned to Fe-rich pigeonite mantles. The cores have rounded morphologies and irregular margins. Combined with the low Ti/Al of the cores, the morphology and chemistry of the pyroxene grains are consistent with initial crystallization at depth (30–70 km) followed by partial resorption en route to the surface. Enriched rare earth element (REE) equilibrium melt compositions and calculated oxygen fugacities (fO₂) conditions for pigeonite cores indicate that the original parent melts were enriched shergottite magmas that staged in chambers at depth within the Martian crust. NWA 8657 does not represent a liquid but rather entrained a proportion of pyroxene crystals from magma chambers where fractional crystallization was occurring at depth. Variation between fO₂ and bulk-rock (La/Yb)_N of the enriched and intermediate shergottites suggests that oxidation conditions and degree of incompatible element enrichment in the source may not be correlated, as thought previously. Shock melt pockets are characterized by an absence of phosphates and oxide minerals. It is likely that these phases were melted during shock. REEs were redistributed during this process into maskelynite and to a lesser extent the shock melt; however, the overall normalized REE profile of the shock melt is like that of the bulk-rock, but at lower absolute concentrations. Overall, shock melting has had a significant effect on the mineralogy of NWA 8657, especially the distribution of phosphates, which may be significant for geochronological applications of this meteorite and other Martian meteorites with extensive shock melt.     

Highly siderophile element abundances and 187Re-187Os systematics in the Tafassasset carbonaceous-like primitive achondrite

Highly siderophile elements (HSE) strongly partition into metal phases over silicate minerals and so offer important constraints on nebular and core formation processes acting on early planetesimals. Abundances of the HSE are also an important tool for constraining relationships between metal-rich meteorites. The first bulk rock and in situ HSE abundance and ¹⁸⁷Re-¹⁸⁷Os data are reported for the ungrouped metal-rich achondrite Tafassasset to examine models of its petrogenesis and origin. Bulk rock and metal grain HSE abundances are elevated at ~2 and ~15 times CI chondrite abundances, respectively, and are largely unfractionated from one another. Metal within Tafassasset is therefore likely to have quenched shortly after partial melting without significant fractional crystallization. Metal grain HSE abundances can be used to calculate a metal fraction of 14 ± 4 wt%, overlapping with the parent bodies of CC iron meteorites, which have also been related to Tafassasset using nucleosynthetic isotope anomalies. Despite such similarities, HSE systematics of bulk rock Tafassasset are not equivalent to any known chondrites, and metal grains do not overlap with iron meteorites or chondrite metal grains, precluding a direct genetic relationship.     

Net primary production and decomposition of salt marshes of the Ebre delta (Catalonia,Spain)

Net primary production was measured in three characteristic salt marshes of the Ebre delta: anArthrocnemum macrostachyum salt marsh,A. macrostachyum-Sarcocornia fruticosa mixed salt marsh andS. fruticosa salt marsh. Above-ground and belowground biomass were harvested every 3 mo for 1 yr. Surface litter was also collected from each plot. Aboveground biomass was estimated from an indirect non-destructive method, based on the relationship between standing biomass and height of the vegetation. Decomposition of aboveground and belowground components was studied by the disappearance of plant material from litter bags in theS. fruticosa plot. Net primary production (aboveground and belowground) was calculated using the Smalley method. Standing biomass, litter, and primary production increased as soil salinity decreased. The annual average total aboveground plus belowground biomass was 872 g m⁻² in theA. macrostachyum marsh, 1,198 g m⁻² in theA. macrostachyum-S. fruticosa mixed marsh, and 3,766 g m⁻² in theS. fruticosa biomass (aboveground plus belowground) was 226, 445, and 1,094 g m⁻², respectively. Total aboveground plus below-ground net primary production was 240, 1,172, and 1,531 g m⁻² yr⁻¹. There was an exponential loss of weight during decomposition. Woody stems and roots, the most recalcitrant material, had 70% and 83% of the original material remaining after one year. Only 20–22% of leafy stem weight remained after one year. When results from the Mediterranean are compared to other salt marshes dominated by shrubbyChenopodiaceae in Mediterranean-type climates, a number of similarities emerge. There are similar zonation patterns, with elevation and maximum aboveground biomass and primary production occurring in the middle marsh. This is probably because of stress produced by waterlogging in the low marsh and by hypersalinity in the upper marsh.     

Pattern and process of land loss in the Mississippi Delta: A Spatial and temporal analysis of wetland habitat change

An earlier investigation (Turner 1997) concluded that most of the coastal wetland loss in Louisiana was caused by the effects of canal dredging, that loss was near zero in the absence of canals, and that land loss had decreased to near zero by the late 1990s. This analysis was based on a 15-min quadrangle (approximately 68,000 ha) scale that is too large to isolate processes responsible for small-scale wetland loss and too small to capture those responsible for large-scale loss. We conducted a further evaluation of the relationship between direct loss due to canal dredging and all other loss from 1933–1990 using a spatial scale of 4,100 ha that accurately captures local land-loss processes. Regressions of other wetland loss on canal area (i.e., direct loss) for the Birdfoot, Terrebonne, and Calcasieu basins were not significant. Positive relationships were found for the Breton (r²=0.675), Barataria (r²=0.47), and Mermentau (r²=0.35) basins, indicating that the extent of canals is significantly related to wetland loss in these basins. A significant negative relationship (r²=0.36) was found for the Atchafalaya coastal basin which had statistically lower loss rates than the other basins as a whole. The Atchafalaya area receives direct inflow of about one third of the Mississippi discharge. When the data were combined for all basins, 9.2% of the variation in other wetland loss was attributable to canals. All significant regressions intercepted the y-axis at positive loss values indicating that some loss occurred in the absence of canals. Wetland loss did not differ significantly from the coast inland or between marsh type. We agree with Turner that canals are an important agent in causing wetland loss in coastal Louisiana, but strongly disagree that they are responsible for the vast majority of this loss. We conclude that wetland loss in the Mississippi delta is an ongoing complex process involving several interacting factors and that efforts to create and restore Louisiana’s coastal wetlands must emphasize riverine inputs of freshwater and sediments.