Redox fluctuations in the Early Ordovician oceans: An insight from chromium stable isotopes

JOAN D´ARCY; GEOFFREY J. GILLEAUDEAU; SILVIO PERALTA; CLAUDIO GAUCHE; ROBERT FREI

CHEMICAL GEOLOGY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2016 vol. 1 p. 1 - 58

0009-2541

Chromium (Cr) stable isotopes are a useful tracer of changes in redoxconditions because changes in its oxidation state are accompanied by anisotopic fractionation. Recent co-precipitation experiments have shown thatCr(VI) is incorporated into the calcite lattice, suggesting that carbonates are apotential sink for dissolved Cr in the ocean and could effectively capture the Crisotope composition of ambient seawater. Thus, the Cr isotopic composition(δ53Cr) of carbonates has the potential to record changes in the oxygenationstate of ancient oceans. To test the potential of this system, we measured theδ53Cr values of limestone and dolostone from an Early Ordovician marinecarbonate platform (Cerro La Silla section of the Precordillera, San Juan,Argentina). An increasing number of studies support the hypothesis that theEarly Ordovician oceans were stratified with respect to oxygen and that theglobal ocean experienced multiple episodes of anoxic expansion andcontraction, making this time period ideal for testing the usefulness of the Crisotope system. The most prominent feature of the Cerro La Silla Cr isotopeprofile is a positive excursion (of ~ +0.5?) during the Late Tremadocian,accompanied by exceptionally low Cr concentrations (