59th Annual Report on Research 2014

Shales and tillites have been traditionally used to deduce the role of continental weathering and sediment recycling through geologic time and to constrain the average chemical composition of the continental crust and for prospecting this reservoir for oil and gas. We have performed compilation of published analyses of oxygen isotope systematics of shales through time, topics which have been insufficiently studied so far, even though unaltered shales are preserved since ~3.2 Ga. During the Archean-Paleoproterozoic transition, several major environmental changes could have happened, including the appearance of oxygen in the atmosphere, increases in the continental landmass, decreases in CH₄ concentrations, Snowball Earth glaciations, and evolution of aerobic microbial life, all affecting continental weathering rates and attainment of isotopic fractionation. Furthermore, inferred variable d¹⁸O values of seawater could have left an imprint on these lithologies during diagenesis.

We performed oxygen and hydrogen isotope data for 100 bulk shales samples that were collected from drillholes on all continents, using collection of Dr. Anrdey Bekker. These samples have also been analyzed for total organic and inorganic carbon, total sulfur, d¹³C_org values, and are currently being analyzed for modal % of quartz. Having uncompromised fresh samples from drillcores is a must for this kind of investigation. We have a particularly good coverage for the ca. 2.7-2.2 Ga time interval when Earth experienced three to four Snowball Earth glaciations associated with the rapid rise in atmospheric O₂ and fluctuations in CO₂, thus affecting weathering cycle. The shales were also analyzed by XRF for major and trace elements to calculate chemical index of alteration (CIA). When compared to published analyses of Phanerozoic shales, Precambrian shales have comparable ranges in d¹⁸O values (+7 to +20‰), with slightly decreasing means with increasing age. Shales in some drill holes display wide ranges over short stratigraphic intervals comparable to the full range in our dataset and suggesting significant variability in the provenance. All units have similar to Phanerozoic ranges in d¹³C_org values (-23 to -33‰ PDB) and C_org content (0.1 to 10 wt. %). We however observe a significant, several permil downward shift and decrease in the range of d¹⁸O values (7-9‰) in 2.2-2.5 Ga shales from several continents that are associated with the Paleoproterozoic glaciations. This likely indicates either changes in contribution of products of physical vs. chemical weathering of igneous sources with normal d¹⁸O values or contact with glacial meltwater having ultra-low-d¹⁸O values during deposition or diagenesis of these shales. Scattered negative correlation of CIA with d¹⁸O, for some of these shales broadly associated with the Paleoproterozoic glaciations suggest the latter possibility. The dD values of shales range from -50 to -75‰, and are comparable to Phanerozoic values, with the exception of the ~2.5-2.2 Ga shales that reach to -100‰. D¹⁷O values of Precambrian shales are indistinguishable from those of modern analogues, suggesting that any photolytically-induced ¹⁷O excess or mass-dependent kinetic fractionations are not resolvable over the large range of d¹⁸O values in our dataset. We are also comparing O isotope values of ultra-low-d¹⁸O, +8 to -27‰ SMOW rocks recently discovered in Karelia (Russia), quartz amygdules in mafics and their relations to the global weathering cycle represented by our shale dataset. The overall conclusion is that despite first-order changes in the above mentioned parameters, weathering cycle was not dramatically different before and after the rise of atmospheric oxygen at ~2.3-2.4 Ga.

We are beginning to prepare a paper on these subjects (Bindeman, Bekker, Zakharov, 2015). The grant supports work of graduate student David Zakharov and a postdoc Matt Loewen, positively contributes to maintaining analytical infrastructure of the University of Oregon Stable Isotope Laboratory. This grant also provides support for my transition to low-T geochemistry and petroleum science, topics that I have not been exposed before, through active research in these fields we are learning basics and contributing new fresh insights. We will present results of our investigation at the Annual Meeting of the American Geophysical Union in San Francisco, Dec 15-19, 2014. The abstract is already published in the conference proceedings volume.