Biogenic processes in crystalline bedrock fractures indicated by carbon isotope signatures of secondary calcite

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Origin of calcite in the glacigenic Virttaankangas complex

Groundwaters of the glacigenic Virttaankangas complex in southern Finland are characterized by high pH values ranging up to 9.5. These values are significantly higher than those observed in silicate-rich shallow groundwater formations in crystalline bedrock areas. TheVirttaankangas sediments were discovered to contain small amounts of fine grained, dispersed calcite, which has a high tendency to increase the pH of local groundwaters. The primary goal of this study was to determine the mode of occurrence of calcite and to identifyits sources. The mineralogy of the glacigenic Virttaankangas complex was studied using material from 21 sediment drill cores. Fine-grained calcite is present in trace amounts (<< 1.4 %) in the glaciofluvial and glaciolacustrine depositional units of the Virttaankangas complex. The topmost littoral sands were practically devoid of calcite. The isotope records of carbon and oxygen, the angular morphology of the grains and the uniform dispersion of calcite in the complex suggest a clastic origin for calcite, with no evidence for in-situ precipitation. In order to constrain the source of calcite, the isotopic composition of carbon and oxygen in five calcite samples was compared to the isotopic data from five carbonate rock erratics and eight crystalline bedrock samples from the region. Based on carbon and oxygen isotope ratios and chemical compositions, the dispersed calcite grains of the Virttaankangas complex appear to have been derived from the Mesoproterozoic Satakunta Formation, some 30 km NW from the Virttaankangas area. In sandstone, calcite is predominantly present as diagenetic cement in grain interspaces, concretions and interlayers. The source of detrital calcite was unexpected, as prior to this study the Satakunta sandstone hasnot been known to contain calcite

Travertine precipitation in the Paleoproterozoic Kuetsjärvi Sedimentary Formation, Pechenga Greenstone Belt, NE Fennoscandian Shield

PES was supported by Väisälä Foundation (Finnish Academy of Science and Letters) and the Finnish Doctoral Program in Geology. ATB was supported by NERC grant NE/G00398X/1. VAM was supported by NFR grant 191530/V30 (projects 331000 and 802795). This is a contribution (paper) # 18 to the ICDP FAR-DEEP project.Peer reviewedPublisher PD

Isotopic signatures of precent-day calcite and pyrite in low-temperature crystalline bedrock, Olkiluoto, SW Finland

Geochemical characteristics of precipitated fracture filling calcite and pyrite can provide much useful information about the deep bedrock environment at the time of their deposition. However, it has been difficult to identify fracture coatings precipitated from the present-day groundwater system. The aim of this study was to evaluate the relationship between the coexisting calcite and pyrite, and the groundwater present at the time of precipitation. Here we investigated fine-grained mineral precipitate deposited over a four-year period on the surface of groundwater monitoring equipment inserted into a drillhole at 530 m below sea level, at Olkiluoto, which is the planned site for a final repository of spent nuclear fuel. The experimental setting is also artificial in the sense that the drillholes have possibly affected groundwater circulation and a foreign object has been inserted into the drillhole. Combining the elemental and isotope geochemical composition of the precipitated calcite and pyrite with previously published compositional data on groundwater and evidence for microbial communities on this site, offered a possibility to get new insight of the precipitation and isotope fractionation processes taking place in deep crystalline bedrock. The concentration of the redox sensitive manganese in the precipitate gives supporting evidence for the influx of groundwater from overlying groundwater units. The delta C-13 (n = 13) and delta O-18 (n = 15) values of calcite vary from-13.2 to-9.7 parts per thousand and from-9.1 to-7.4 parts per thousand respectively. Comparison to the respective values in the local groundwater indicated that the precipitated calcite is in near isotopic equilibrium with its environment with respect to carbon and oxygen. The potential ultimate source of the carbon in the DIC and in the precipitate is likely in old fracture calcite coatings. The 834S values of pyrite (n = 9) show relatively small variation from-5.7 to 8.3 parts per thousand. This differs greatly from the huge span of 834S values from-50 to 80%o in fracture pyrites reported for the latest calcite fillings at Olkiluoto. The restricted range of 834S values is interpreted to result from open system conditions during precipitation, with new dissolved sulfate entering from the large brackish SO4-type groundwater unit above. The isotopic fractionation of sulfur between dissolved sulfate and sulfide is estimated to be 25 & PLUSMN; 10%o, which is in agreement with the results reported in laboratory experiments for bacterial sulfate reduction.Peer reviewe

Thriving or surviving? The isotopic record of the Wrangel Island woolly mammoth population

The world's last population of woolly mammoths (Mammuthus primigenius) lived on Wrangel Island persisting well into the Holocene, going extinct at ca. 4000 cal BP. According to the frequency of 'radiocarbon dated mammoth remains from the island, the extinction appears fairly abrupt. This study investigates the ecology of the Wrangel Island mammoth population by means of carbon, nitrogen and sulfur isotope analyses. We report new isotope data on 77 radiocarbon dated mammoth specimens from Wrangel Island and Siberia, and evaluate them in relation to previously published isotope data for Pleistocene mammoths from Beringia and lower latitude Eurasia, and the other insular Holocene mammoth population from St. Paul Island. Contrary to prior suggestions of gradual habitat deterioration, the nitrogen isotope values of the Wrangel Island mammoths do not support a decline in forage quality/quantity, and are in fact very similar to their north Beringian forebears right to the end. However, compared to Siberian mammoths, those from Wrangel Island show a difference in their energy economy as judged by the carbon isotope values of structural carbonate, possibly representing a lower need of adaptive strategies for survival in extreme cold. Increased mid-Holocene weathering of rock formations in the central mountains is suggested by sulfur isotope values. Scenarios related to water quality problems stemming from increased weathering, and a possibility of a catastrophic starvation event as a cause of, or contributing factor in their demise are discussed. (C) 2019 The Authors. Published by Elsevier Ltd.Peer reviewe