Interpretation of transmissivity estimates from single-well pumping aquifer tests

Interpretation of single-well tests with the Cooper-Jacob method remains more reasonable than most alternatives. Drawdowns from 628 simulated single-well tests where transmissivity was specified were interpreted with the Cooper-Jacob straight-line method to estimate transmissivity. Error and bias as a function of vertical anisotropy, partial penetration, specific yield, and interpretive technique were investigated for transmissivities that ranged from 10 to 10,000 m(2)/d. Cooper-Jacob transmissivity estimates in confined aquifers were affected minimally by partial penetration, vertical anisotropy, or analyst. Cooper-Jacob transmissivity estimates of simulated unconfined aquifers averaged twice the known values. Transmissivity estimates of unconfined aquifers were not improved by interpreting results with an unconfined aquifer solution. Judicious interpretation of late-time data consistently improved estimates where transmissivity exceeded 250 m(2)/d in unconfined aquifers.     

The stable carbon isotope biogeochemistry of acetate and other dissolved carbon species in deep subseafloor sediments at the northern Cascadia Margin

Ocean drilling has revealed the existence of vast microbial populations in the deep subseafloor, but to date little is known about their metabolic activities. To better understand the biogeochemical processes in the deep biosphere, we investigate the stable carbon isotope chemistry of acetate and other carbon-bearing metabolites in sediment pore-waters. Acetate is a key metabolite in the cycling of carbon in anoxic sediments. Its stable carbon isotopic composition provides information on the metabolic processes dominating acetate turnover in situ. This study reports our findings for a methane-rich site at the northern Cascadia Margin (NE Pacific) where Expedition 311 of the Integrated Ocean Drilling Program (IODP) sampled the upper 190 m of sediment. At Site U1329, δ¹³C values of acetate span a wide range from −46.0‰ to −11.0‰ vs. VPDB and change systematically with sediment depth. In contrast, δ¹³C values of both the bulk dissolved organic carbon (DOC) (−21.6 ± 1.3‰ vs. VPDB) and the low-molecular-weight compound lactate (−20.9 ± 1.8‰ vs. VPDB) show little variability. These species are interpreted to represent the carbon isotopic composition of fermentation products. Relative to DOC, acetate is up to 23.1‰ depleted and up to 9.1‰ enriched in ¹³C. Broadly, ¹³C-depletions of acetate relative to DOC indicate flux of carbon from acetogenesis into the acetate pool while ¹³C-enrichments of pore-water acetate relative to DOC suggest consumption of acetate by acetoclastic methanogenesis. Isotopic relationships between acetate and lactate or DOC provide new information on the carbon flow and the presence and activity of specific functional microbial communities in distinct biogeochemical horizons of the sediment. In particular, they suggest that acetogenic CO₂-reduction can coexist with methanogenic CO₂-reduction, a notion contrary to the hypothesis that hydrogen levels are controlled by the thermodynamically most favorable electron-accepting process. Further, the isotopic relationship suggests a relative increase in acetate flow to acetoclastic methanogenesis with depth although its contribution to total methanogenesis is probably small. Our study demonstrates how the stable carbon isotope biogeochemistry of acetate can be used to identify pathways of microbial carbon turnover in subsurface environments. Our observations also raise new questions regarding the factors controlling acetate turnover in marine sediments.     

A simple guideline to apply excitation-emission matrix spectroscopy (EEMs) for the characterization of dissolved organic matter (DOM) in anoxic marine sediments

Marine sediments represent a major carbon reservoir on Earth. Dissolved organic matter(DOM) in pore waters accumulates products and intermediates of carbon cycling in sediments. The application of excitation-emission matrix spectroscopy(EEMs) in the analysis of subseafloor DOM samples is largely unexplored due to the redoxsensitive matrix of anoxic pore water. Therefore, this study aims to investigate the interference caused by the matrix on EEMs and propose a guideline to prepare pore water sam...     

Experimental simulation of plagioclase diagenesis atP-T conditions of 3.5 km burial depth

Dissolution of plagioclase under the physical conditions at shallow to intermediate burial depths is a prime candidate for secondary porosity generation in feldspathic siliciclastic sediments. The diagenetic behavior of granular aggregates of plagioclase feldspar and quartz has been investigated by experimentation performed in a Bridgeman-type pressure vessel. The experiments, each of two weeks duration, simulated pressure-temperature conditions approximating 3.5 km burial depth. By using a double-acting pore-fluid reservoir, solutions of various chemistries were cycled through samples composed of oligoclase or labradorite feldspar and quartz (90:10 wt% respectively).Scanning electron microscope analysis of the post-experiment samples reveals dissolution features and precipitated products. Dissolution voids of 10 microns occur typically in areas of maximum stress such as crack-tips and grain contacts. Dissolution on a larger scale is exemplified by topographical smoothing of grain su faces. The dissolved species are subsequently reprecipitated as Ca-enriched overgrowths (possibly zeolites) and clays. These precipitates are found individually on the scale of 10 microns and collectively as surface coatings on both feldspar and quartz grains. Atomic absorption spectroscopic analyses of the pore fluid suggest that the fluid chemistry is consistent with the observed experimental precipitates.These experiments show that clay coatings are unnecessary precursors to grain surface dissolution and that the diagenetic precipitation is not mineral selective. Also, the mass transfer of the dissolved species appears to be localized because grains displaying both dissolution and precipitation features are commonplace. Volume changes due to mineral transformation/alteration may increase secondary porosity if the dissolved species produced from dissolution are only partially involved in reprecipitation and the remaining dissolved material is flushed out by the pore fluids. However, if the mass transfer is primarily local then permeability would significantly decrease as precipitates may choke the pore throats.     

Vesteris Seamount: An enigma in the Greenland Basin

Vesteris Seamount is a solitary submarine volcano located at 73°30 N, 9°10W in the Greenland Basin. Steeply rising from a base depth of 3100 m to a minimum depth of ~ 130 m and striking 030°/210°, the feature lies ~ 300 km east of the east Greenland margin on an otherwise nearly flat and featureless seafloor. The main body of the seamount appears to have been formed episodically, the last of which culminated about 110 000 years ago. Subsequent, lower intensity volcanic activity continued sporadically until about 25 000 years ago, as evidenced by ash layers found in cores near the base of the feature. The smoothed surfaces at the summit make it likely that the seamount actually broached the surface during the Weichselian glacial period, between 8000 and 13 000 years ago. Two multibeam bathymetric investigations aboardPFS Polarstern during ARKTIS II/4 (1984) and ARKTIS VII/1 (1990), combined with geologic sampling, single-channel seismic profiling and underwater television coverage, have resulted in a new interpretation of both the morphology and origins of the seamount. Data collected aboardPolarstern from ARKTIS II/4 (1984) have been previously reported by Hempelet al. (1991), however, when combined with the ARKTIS VII/1 (1990) data set, a more detailed interpretation of the morphology and structure was feasible. This included the elongated shape of the feature and showed the existence of several small volcanic cones on the seamount flanks.The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

The Drivers of Market Integration Among Indigenous Peoples: Evidence From the Ecuadorian Amazon

Knowledge of the driving forces behind indigenous participation in the market is essential for practitioners intending to integrate conservation and development policies in indigenous territories. Nevertheless, empirical research on the determinants of market integration among indigenous peoples is still scarce. This article uses household survey data and multivariate techniques to examine the drivers of market integration among indigenous groups in the Ecuadorian Amazon. We use multiple measures of market integration, including the sale of crops, timber, and wildlife; the use of credit; and participation in wage labor. The results show that the way in which indigenous peoples integrate into the market depends on their endowments of human, financial, and physical capital. More educated households are able to engage in commercial agriculture and nonagricultural wage work, whereas uneducated poor households in communities in conflict with outsiders are pushed to engage in poorly paid agricultural wage work and (often illegal) timber operations.