Alenka E. Črne

Isotopic Evidence for Massive Oxidation of Organic Matter Following the Great Oxidation Event

Analysis of two-billion-year-old rocks reveals an extreme carbon-cycle disruption after atmospheric oxygen increased. The stable isotope record of marine carbon indicates that the Proterozoic Eon began and ended with extreme fluctuations in the carbon cycle. In both the Paleoproterozoic [2500 to 1600 million years ago (Ma)] and Neoproterozoic (1000 to 542 Ma), extended intervals of anomalously high carbon isotope ratios (δ13C) indicate high rates of organic matter burial and release of oxygen to the atmosphere; in the Neoproterozoic, the high δ13C interval was punctuated by abrupt swings to low δ13C, indicating massive oxidation of organic matter. We report a Paleoproterozoic negative δ13C excursion that is similar in magnitude and apparent duration to the Neoproterozoic anomaly. This Shunga-Francevillian anomaly may reflect intense oxidative weathering of rocks as the result of the initial establishment of an oxygen-rich atmosphere.

A biocalcification crisis at the Triassic-Jurassic boundary recorded in the Budva Basin (Dinarides, Montenegro)

Volcanic activity in the Central Atlantic magmatic province, resulting in an increased flux of CO 2 , SO 2 , and CH 4 into the oceans and atmosphere, has been proposed as one of the mechanisms causing the biotic crisis at the Triassic-Jurassic boundary. Oceanic uptake of CO 2 due to extreme greenhouse conditions should have had an impact on ocean chemistry and the position of the calcite compensation depth. In this study, we chose two pelagic sections from the Budva Basin as archives for paleoceanographic change across the Triassic-Jurassic boundary in deep-water settings. Our record represents the first documentation of a sudden termination of carbonate deposition across the Triassic-Jurassic boundary in a pelagic deep-water environment. Based on radiolarian biostratigraphy, the system boundary is placed at the sharp lithological contact between two pelagic formations, the Upper Triassic limestones and Lower Jurassic siliceous limestones alternating with shales. A rapid drop of carbonate content from 90% to less than 10% occurred contemporaneous with a negative anomaly in the stable carbon isotope record measured in both bulk carbonate (1.3‰) and bulk organic matter (1.1‰). The abrupt reduction of carbonate content in the Budva Basin was the result of either increased carbonate dissolution causing shoaling of the calcite compensation depth or reduced carbonate input due to biocalcification crisis. Both nonexclusive scenarios support the hypothesis of decreased ocean saturation with respect to calcium carbonate, which could be a direct consequence of increased CO 2 , SO 2 , and CH 4 fluxes.

Methanotrophy in a Paleoproterozoic oil field ecosystem, Zaonega Formation, Karelia, Russia.

Organic carbon rich rocks in the c. 2.0 Ga Zaonega Formation (ZF), Karelia, Russia, preserve isotopic characteristics of a Paleoproterozoic ecosystem and record some of the oldest known oil generation and migration. Isotopic data derived from drill core material from the ZF show a shift in δ(13) C(org) from c. -25‰ in the lower part of the succession to c. -40‰ in the upper part. This stratigraphic shift is a primary feature and cannot be explained by oil migration, maturation effects, or metamorphic overprints. The shift toward (13) C-depleted organic matter (δ(13) C(org) < -25‰) broadly coincides with lithological evidence for the generation of oil and gas in the underlying sediments and seepage onto the sea floor. We propose that the availability of thermogenic CH(4) triggered the activity of methanotrophic organisms, resulting in the production of anomalously (13) C-depleted biomass. The stratigraphic shift in δ(13) C(org) records the change from CO(2) -fixing autotrophic biomass to biomass containing a significant contribution from methanotrophy. It has been suggested recently that this shift in δ(13) C(org) reflects global forcing and progressive oxidation of the Earth. However, the lithologic indication for local thermogenic CH(4) , sourced within the oil field, is consistent with basinal methanotrophy. This indicates that regional/basinal processes can also explain the δ(13) C(org) negative isotopic shift observed in the ZF.

Impact spherules from Karelia, Russia: Possible ejecta from the 2.02 Ga Vredefort impact event

Spherule beds of possible impact origin have been discovered in two drill cores in the Paleoproterozoic Zaonega Formation, Karelia, northwest Russia. Spherules are found within the dolomite matrix of in-situ brecciated sedimentary dolostones. Spherules are millimeter size and generally round, although teardrop and dumbbell morphologies are present. Spherules contain up to 0.75 ppb Ir, with a Ru/Ir of 2, indicating a mixing of target rocks with a minor chondritic component. The age of the Zaonega Formation is constrained between limits of 1.975 ± 0.024 Ga (Sm-Nd) and 1.980 ± 0.057 Ga (Pb-Pb), and 2.050 Ga (Re-Os), which brackets the age of the 2020 Ma Vredefort impact structure in South Africa, and suggests that the spherule beds could represent ejecta from that event. If the link is confirmed, the size of the spherules and thickness of the beds suggest that the distance from the impact site was <2500 km, thereby constraining the paleogeographic distance between the Fennoscandian Shield and Kaapvaal craton during the late Paleoproterozoic.

Enhanced accumulation of organic matter: the Shunga event

A number of sedimentary formations deposited globally around 2.0 Ga ago are characterised by high abundances of organic carbon. These formations often contain occurrences of highly concentrated, matured organic material representing metamorphosed oil, now pyrobitumen. Apart from their common names pyrobitumen or anthraxolite, different terminology has been used for these rocks within the pertinent literature, including shungite, thucolite, or Precambrian “coal”. Given their long and frequently complex geologic history, these sedimentary formations exhibit a variable and sometimes substantial degree of metamorphic (thermal) overprint. Consequently, many of them show undisputable signs of thermal mobilisation, migration and likely loss of hydrocarbons/bitumen. This includes the so-called shungite rocks on the Fennoscandian Shield.