Sedimentology and chemostratigraphy of the terminal Ediacaran Dengying Formation at the Gaojiashan section, South China.

The late Ediacaran Dengying Formation (ca. 551.1-538.8 Ma) in South China is one of two successions where Ediacara-type macrofossils are preserved in carbonate facies along with skeletal fossils and bilaterian animal traces. Given the remarkable thickness of carbonate-bearing strata deposited in less than 12.3 million years, the Dengying Formation holds the potential for a relatively continuous chemostratigraphic profile for the terminal Ediacaran stage. In this study, a detailed sedimentological and chemostratigraphic (δ13Ccarb, δ18Ocarb, δ13Corg, δ34Spyrite, and 87Sr/86Sr) investigation was conducted on the Dengying Formation at the Gaojiashan section, Ningqiang County of the southern Shaanxi Province, South China. Sedimentological results reveal an overall shallow marine depositional environment. Carbonate breccia, void-filling botryoidal precipitates, and aragonite crystal fans are common in the Algal Dolomite Member of the Dengying Formation, suggesting that peritidal facies were repeatedly karstified. The timing of karstification was likely early, probably soon after the deposition of the dolomite sediments. The presence of authigenic aragonite cements suggests high alkalinity in the terminal Ediacaran ocean. Geochemical analysis of micro-drilled samples shows that distinct compositions are registered in different carbonate phases, which should be considered when constructing chemostratigraphic profiles representative of true temporal variations in seawater chemistry. Integrated chemostratigraphic data suggest enhanced burial of organic carbon and pyrite, and the occurrence of extensive marine anoxia (at least in the Gaojiashan Member). Rapid basinal subsidence and carbonate accumulation during a time of elevated seawater alkalinity and increased rates of pyrite burial may have facilitated the evolutionary innovation of early biomineralizing metazoans.

Trans enantiomeric separation of MESA and MOXA, two environmentally important metabolites of the herbicide, metolachlor.

Complete separation of the trans-enantiomers of the two most abundant, persistent polar metabolites of metolachlor, metolachlor ethane sulfonic acid (MESA) and metolachlor oxanilic acid (MOXA), was achieved using UPLC equipped with a reverse phase chiral column and trace detection with an electrospray triple quadrupole mass spectrometer. Various conditions that influenced the separation and instrumental signal were investigated to achieve the optimum separation and instrument response within an analysis time of less than 30 minutes. Different eluting solvent compositions for each metabolite were required for optimized separation of of the 4 enantiomers. Standard curves were responsive to less than 13 ng/mL and 8 ng/mL for the least plentiful MOXA and MESA enantiomers, respectively with a linear coefficient of determination greater than 0.998. Suitability of the method for quantification of the 4 mixed enantiomers of each was demonstrated using natural surface water samples collected from the Choptank River watershed in Eastern Maryland.•LC chiral separation parameters were varied to achieve optimal separation of the major enantiomers of the two metolachlor metabolites.•LC/MS-MS parameters were adjusted to maximize response and minimize analysis time.•Finished methods were used to quantitate enantiomers in archived stream water extracts from agricultural watersheds with corn/soybean production.

Method to Evaluate the Age of Groundwater Inputs to Surface Waters by Determining the Chirality Change of Metolachlor Ethanesulfonic Acid (MESA) Captured on a Polar Organic Chemical Integrative Sampler (POCIS).

We previously discovered a method to estimate the groundwater mean residence time using the changes in the enantiomeric ratio of metolachlor ethanesulfonic acid (MESA), (2-[(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2-oxoethanesulfonic acid), a metabolite of the herbicide metolachlor. However, many grab samples would be needed for each watershed over an extended period, and this is not practical. Thus, we examined the use of a polar organic chemical integrative sampler (POCIS) deployed for 28 days combined with a modified liquid chromatography-mass spectrometry LC-MS/MS method to provide a time-weighted average of the MESA enantiomeric ratio. POCISs equipped with hydrophilic-lipophilic-balanced (HLB) discs were deployed at five sites across the United States where metolachlor was used before and after 1999 and compared the effectiveness of the POCIS to capture MESA versus grab samples. In addition, an in situ POCIS sampling rate (Rs) for MESA was calculated (0.15 L/day), the precision of MESA extraction from stored POCIS discs was determined, and the effectiveness of HLB to extract MESA was examined. Finally, using molecular modeling, the influence of the asymmetric carbon of metolachlor degradation on the MESA enantiomeric ratio was predicted to be negligible. Results of this work will be used in projects to discern the groundwater mean residence times, to evaluate the delivery of nitrate-N from groundwater to surface waters under various soil, agronomic, and land use conditions, and to examine the effectiveness of conservation practices.

Geomicrobial characterization of gas hydrate-bearing sediments along the mid-Chilean margin.

Bacterial diversity in eight sediment cores from the mid-Chilean margin was studied using length heterogeneity (LH)-PCR, and described in relation to in situ geochemical conditions. DNA from the sulfate-methane transition (SMT) of three cores [one containing methane gas; two proximal to a gas hydrate mound (GHM)] was cloned and sequenced. Clones related to uncultured relatives of Desulfosarcina variabilis were found in all clone libraries and dominated one. Desulfosarcina variabilis related clones were similar to phylotypes observed at the SMT in association with anaerobic methane oxidation in the Eel River basin, Cascadia margin and the Gulf of Mexico. The LH-PCR amplicon associated with D. variabilis clones matched the amplicon that dominated most SMT samples, indicating environmental selection for D. variabilis relatives. Clones related to the Verrucomicrobia dominated the library for the methane gas-containing core. Uncultured Treponema relatives dominated the library for the core obtained on the edge of a GHM. Statistical analysis using geochemical data to describe variance in LH-PCR data revealed that stable carbon isotope ratios of dissolved inorganic carbon are the principal structuring factor on SMT communities. These data suggest that D. variabilis relatives are involved in anaerobic oxidation of methane at the SMT in Chilean margin sediments.