60th Annual Report on Research 2015

Shales 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 ~3.5 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 now performed oxygen, hydrogen, and carbon isotope analyses, and total carbon concentration investigation, for 200 bulk shales samples that were collected from drillholes on all continents, using collection of Dr. Anrdey Bekker, and have done fieldwork in northern Russia last summer. A paper summarizing our discoveries has been just submitted to the Earth and Planetary Science Letters, the abstract of the paper, outlining our results, is attached below.

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 presented results of our investigation at the Annual Meeting of the American Geophysical Union in San Francisco, Dec 19, 2014. The abstract is already published in the conference proceedings volume.

Oxygen isotope perspective on crustal evolution on early Earth: A record of Precambrian shales with emphasis on Paleoproterozoic glaciations and Great Oxygenation Event; Bindeman I.N.^1*, Bekker A.², Zakharov D.¹; ¹Department of Geological Sciences, 1272 University of Oregon, Eugene OR 97403; ²Department of Earth Sciences, University of California, Riverside, CA 92521; author: Bindeman@uoregon.edu; Word count: 6479; Abstract: 387; Figures: 10; Tables: 4 (Supplementary); STATUS: SUBMITTED TO EPSL: MS Ref number: EPSL-S-15-00979

Abstract

We present stable isotope and chemical data for 200 Precambrian bulk shale and tillite samples that were collected mostly from drillholes on all continents and span the age range from 0.7 to 3.5Ga with a dense coverage for 2.7-2.2Ga time interval when Earth experienced four Snowball Earth glaciations and the irreversible rise in atmospheric O₂. We observe significant, downward shifts of several ä and a decreased range of d¹⁸O values (7 to 9ä) in 2.2-2.5Ga shales that are associated with the Paleoproterozoic glaciations. These samples consist of more than 50% mica minerals and have equal or higher chemical index of alteration than overlying and underlying formations and thus underwent equal or greater degrees of chemical weathering. Their pervasively low-d¹⁸O and also low dD values (down to -85ä) provide strong evidence of alteration and diagenesis in contact with ultra-low d¹⁸O glacial meltwaters in lacustrine, deltaic or sikussak-type environments associated with the Paleoproterozoic glaciations. The dD_silicate values for the rest of Precambrian shales range from -75 to -50ä and are comparable to Phanerozoic and Archean values. Likewise, these samples have similar ranges in d¹³C_org values (-23 to -33ä PDB) and C_org content (0.0 to 10 wt.%) to Phanerozoic shales.

Precambrian shales have a large range of d¹⁸O values comparable to that in the Phanerozoic shales in each age group and formation, suggesting similar variability in the provenance and intensity of chemical weathering. Moreover, 2.7-2.5Ga shales that were deposited before the Great Oxidation Event (GOE) and post-GOE Paleoproterozoic shales overlap in d¹⁸O values. Absence of a step-wise increase in d¹⁸O, dD, and d¹³C values suggests that despite the first-order change in the composition of the atmosphere, weathering cycle was not dramatically different before and after the GOE at ~2.4-2.3 Ga. Shales do not show comparable d¹⁸O rise in the early Phanerozoic as is observed in the coeval d¹⁸O trends for cherts and carbonates and attributed to changes in seawater d¹⁸O composition. There is however a sharp increase in the average d¹⁸O value from the Early Archean to the Late Archean followed by a progressively decelerating increase into the Phanerozoic. This decelerating increase with time likely reflects declining contribution of mantle-extracted, normal-d¹⁸O crust and lends support to crustal maturation and increasing ¹⁸O sequestration into the crust and recycling of high-d¹⁸O sedimentary rocks. This secular increase in the d¹⁸O composition of the continental crust could have also had a mild effect on seawater d¹⁸O composition.