PAIRED ISOTOPE RECORDS OF CARBONATE AND ORGANIC MATTER, MIDDLE ORDOVICIAN (DARRIWILIAN) OF ARGENTINA: IMPLICATIONS FOR GLOBAL CLIMATE CHANGE

The Middle Darriwilian Isotope Carbon Excursion (MDICE), a globally recognized positive shift (+2 to +4‰) in the carbon-isotopic record of Darriwilian-Sandbian age marine carbonates, may represent the effect of changing ocean circulation patterns driven by long-term global cooling during the transition from greenhouse to icehouse climate conditions (Thompson and Kah, 2012). The timing of this event is coincident with conodont thermometry suggesting global cooling in the Middle-Ordovician that may have ultimately led to the End Ordovician glaciation (Trotter et al., 2008). The MDICE may thus represent ventilation of the Early Ordovician stratified ocean driven by invigorated thermohaline circulation, which led to upwelling of nutrient rich deep-waters, increased nutrient availability, enhanced organic productivity, and burial of organic carbon. The consequence of this event is recorded in the C-isotope record of authigenic marine carbonates (δ¹³C_carb), which are often used as a proxy for atmospheric pCO₂, as photosynthetic fractionation (ε_p) dominates the C-isotopic composition of carbonate and organic matter. However, for this proxy to be a useful record of Middle Ordovician climate it must be paired with the C-isotope record of sedimentary organic matter (δ¹³C_org) and the paired carbon isotopic record (Δ¹³C) must be evaluated against coeval geographically disparate sections.

Previous investigations have demonstrated the utility of the paired C-isotope record as a tool for estimating paleo-pCO₂ (Kump and Arthur, 1999; Cramer and Saltzman, 2007; Zhang et al., 2010). Recent biostratigraphic work on the Darriwilian-Sandbian Las Chacritas, Las Aguaditas, and Gualcamayo Formations located in the northeast of San Juan Province, Argentina make them ideal for global correlation of our isotopic profiles with records documented in North America, Europe, and China. Here we expand the carbon-isotopic record of the Middle-Ordovician of Argentina with new high-resolution δ¹³C_carb and δ¹³C_org profiles. The resulting Δ¹³C record of the Argentine Precordillera is utilized to approximate changes in pCO₂ during this interval of global climate change and oceanographic upheaval.