87SR/86SR AND εND STRATIGRAPHY OF MIDDLE ORDOVICIAN CARBONATE SEDIMENTS IN ARGENTINA: A POTENTIAL LINK BETWEEN BIOGEOCHEMICAL AND PHYSIOCHEMICAL CHANGES IN OCEAN CHEMISTRY

Dramatic changes in the biogeochemical cycles of carbon and sulfur have been recognized throughout the Ordovician. Specifically, initiation of the Middle Darriwilian Isotope Carbon Excursion (MDICE) corresponds to a sharp change in the isotopic composition of carbonate-associated sulfate (CAS) and extends through an interval marked by deposition of “superheavy” pyrite (−∆³⁴S values). These geochemical changes have been attributed to potential changes in marine circulation, deep-water ventilation, and shifting nutrient fluxes. Biogeochemical changes also appear coincident with a globally recognized shift in the ⁸⁷Sr/⁸⁶Sr value of seawater, which decrease from ~0.7090 in the Middle Darriwilian (ca. 462 Ma) to ~0.7078 by the early Sandbian (ca. 458). The drop in ⁸⁷Sr/⁸⁶Sr occurs on timeframe close to the residence time of Sr in the modern ocean (5×10⁶ years), and has been attributed to decreased continental weathering, increased weathering of island arcs, and even changes in ocean circulation.

Here we evaluate the potential for an oceanic driver for the observed changes in carbon, sulfur, and strontium isotope compositions of Darriwilian seawater. The middle Darriwilian Las Chacritas and Las Aguaditas formations of the Argentine Precordillera span the Yangtzeplacognathus crassus to the Eoplacognathus suecicus zones and the Y. crassus to the E. pseudoplanus zones, respectively, across shallower- and deeper-water depositional environments. In both sections, the uppermost Darriwilian is represented by a disconformity. Preliminary ⁸⁷Sr/⁸⁶Sr data from these formations fit within the global framework for Sr-isotope change with values near 0.709050 falling rapidly to 0.708637 prior to the late Darriwilian disconformity. To test whether upwelling of seawater with less radiogenic Sr-isotope compositions may be responsible for this shift, we have examined neodymium isotope compositions of these carbonate units. Neodymium, which has a much shorter residence time than Sr, has been used to distinguish discrete oceanic water masses. The resulting ε_Nd values increase from approximately −10 to −4 through this interval, suggesting that marine strata may record the upwelling of hydrothermally influenced deep water into shallow water environments in the middle Darriwilian.