O'Sullivan, Edel Mary, Nägler, T. F., Turner, E. C., Kamber, B. S., Babechuk, M. G. and O'Hare, S. P. (2022) Mo isotope composition of the 0.85 Ga ocean from coupled carbonate and shale archives: Some implications for pre-Cryogenian oxygenation. Precambrian Research, 378 . Art.Nr. 106760. DOI 10.1016/j.precamres.2022.106760.

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Abstract

Highlights

• δ98Mo data of 0.85Ga microbial carbonate, lime mudstone and black shale fill temporal gap in Proterozoic Mo isotope record.
• Carbonates and black shales are compared as suitable seawater δ98Mo archives.
• Significant δ98Mo variation in carbonate samples likely related to porewater redox conditions.
• 0.4‰ discrepancy between heaviest δ98Mo value of carbonate and shale indicates shale is unsuitable as seawater estimate.
• Possible archive-dependent bias in δ98Mo interpretation may lead to underestimation of extent of Proterozoic oxic Mo sink.

This study addresses marine palaeoredox conditions of the mid-Neoproterozoic by analysing the Mo isotope, trace element, and U-Th-Pb isotope compositions of shallow water microbial carbonate, deep water pelagic carbonate, and shale from the Stone Knife Formation (SKF) in NW Canada. The U-Th-Pb isotope SKF systematics of reef microbialite carbonates, and the moderately expressed negative Ce anomalies are consistent with the presence of dissolved O-2 in the surface waters. Thirteen of 14 analysed samples yield a depositional Pb-206/U-238 regression age of 0.850 & PLUSMN; 0.028 Ga. The Mo isotope data (delta Mo-98) are distinct for the microbial and pelagic carbonates and the deeper water shales, with the isotopically heaviest black shales 0.4 parts per thousand lighter than the heaviest carbonate. The bulk digestion carbonate delta Mo-98 data scatter widely, ranging up to 1.64 parts per thousand, and are not reproducible between repeat digestions. The spread in shallow-water carbonate delta Mo-98 cannot be attributed to a single origin (e.g., admixture of silicate-hosted Mo) and probably reflects a combination of factors, including the complex pathway of Mo into microbial carbonates. Regardless, we propose a minimum delta Mo-98 of 1.64 parts per thousand for the 0.85 Ga ocean, similar to other Neo- and Mesoproterozoic estimates from studies of proxies other than black shale. Our new black shale delta Mo-98 data agree with most previous results from 1.8 to 0.7 Ga shales. If interpreted as reflecting seawater, this would mean a minimum oceanic delta Mo-98 composition of only 1.29 parts per thousand, implying a limited oxic reservoir compared to the modern Mo budget in agreement with previous studies. This study's results suggest that the discrepancy could also be explained by a systematic offset between delta Mo-98 compositions of black shales and the overlying water columns, regardless of depositional environment, akin to the relative depth distribution of delta Mo-98 in modern euxinic water columns such as the Black Sea. If valid, an implied heavier seawater delta Mo-98 throughout the Proterozoic would indicate that the magnitude of the Mo oxic sink remained relatively stable throughout the Proterozoic, shifting the apparent expansion of oxygen towards the younger boundary of the interpreted onset of the NOE (ca. 1.0-0.54 Ga).

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