New constraints on global geochemical cycling during Oceanic Anoxic Event 2 (Late Cretaceous) from a 6-million-year long molybdenum-isotope record. / Dickson, Alexander; Jenkyns, Hugh C.; Idiz, Erdem; Sweere, Tim; Murphy, Melissa; van den Boorn, Sander H.J.M.; Porcelli, Donald.

In: Geochemistry, Geophysics, Geosystems, 27.10.2020.

Research output: Contribution to journalArticle

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  • Alexander Dickson
  • Hugh C. Jenkyns
  • Erdem Idiz
  • Tim Sweere
  • Melissa Murphy
  • Sander H.J.M. van den Boorn
  • Donald Porcelli

Abstract

Intervals of extreme warmth are predicted to drive a decrease in the oxygen content of the oceans. This prediction has been tested for the acme of short (<1 million years) episodes of significant marine anoxia in the Phanerozoic geological record known as Oceanic Anoxic Events (OAEs). However, there is a paucity of data spanning prolonged multi-million-year intervals of geological time before and after OAEs. We present a Mo-isotope record from limestones and marlstones of the Eagle Ford Group, South Texas, which was deposited in the southern Cretaceous Western Interior Seaway of North America during a 6-million-year period encompassing Oceanic Anoxic Event 2 (Late Cenomanian–early Turonian: ~94 Ma). Mo-isotope compositions from deposits that formed in euxinic (sulfidic) conditions before OAE 2 allow the paleo-seawater composition to be constrained to 1.1–1.9 ‰. This range of values overlaps previous estimates of up to ~1.5 ‰ for the peak of OAE 2 determined from similarly sulfidic sediments deposited in the restricted proto-North Atlantic Ocean. Mo-isotopes thus varied by less than a few tenths of per mil across one of the most extreme intervals of global deoxygenation in the Late Phanerozoic. Rather than a limited change in oceanic deoxygenation, we suggest that the new data reflect changes to global iron cycling linked to basalt-seawater interaction, terrestrial weathering and expanded partially oxygenated shallow shelf-seas that played a key role in the burial of isotopically light molybdenum, thus acting as a counterbalance to its removal into sulfidic sediments.
Original languageEnglish
JournalGeochemistry, Geophysics, Geosystems
Publication statusAccepted/In press - 27 Oct 2020

ID: 39431647