Paper No. 32-4
Presentation Time: 2:35 PM
RECORD OF MIDDLE DEVONIAN (EIFELIAN-EARLY GIVETIAN) CLIMATE AND PALEOCEANOGRAPHIC CHANGE AND ASTRONOMICAL FORCING: SOUTHERN ILLINOIS BASIN-CENTRAL NORTH AMERICA
We present proxy records that cover the entire Eifelian, and Early Givetian Stage (Lower-Upper Middle Devonian) in order to assess climatic and environmental variability during that interval. To do so, we sampled the Grand Tower and St. Laurent Formations (southern Illinois Basin) for bulk magnetic susceptibility, δ13Ccarb δ18Ocarb, and δ18Oapatite from Icriodid P1 elements. Biostratigraphic investigation of the Lower and Middle Devonian strata in the southern Illinois Basin yield diagnostic conodont faunas. The study of conodont samples of the Grand Tower and St. Laurent Formations provide biostratigraphic control on the position of the Emsian-Eifelian and Eifelian-Givetian stage boundaries in the Grand Tower type section along the Mississippi river in Jackson County, and in the Illinois State Geological Surveys’ White County core in White County, Illinois. We analyzed the bulk magnetic susceptibility (MS) and δ13Ccarb δ18Ocarb and δ18Oapatite geochemical proxy records against a robust biostratigraphic framework and we identify the Kačák-otomari positive δ13Ccarb excursion beginning in the interval of the ensensis Zone in the lower St. Laurent Formation. The conodont apatite δ18O values of Icriodid P1 elements range between 17.4 and 21.2 ‰VSMOW through the outcrop and core sections and indicate considerable changes in subtropical Sea Surface Temperatures (SSTs) and/or changes in the isotopic composition of local seawater in the southern Illinois Basin during the Eifelian and Early Givetian. Within the upper Grand Tower, conodont δ18O values decrease by 4‰ then increase by 2.5‰. In the overlying St. Laurent Formation, values decrease by 1‰. These variations, if due to marine water mass temperature, yield estimated oxygen isotope paleothermometery values indicating warming of up to 16°C during the mid-Eifelian followed by a 11°C cooling episode in the Late Eifelian and 4°C warming in the Early Givetian. Moreover, we evaluate a possible imprint of astronomical climate forcing and interpret the persistent 2.3 ft thick cyclic variations in the stable isotopic and MS proxy records as the imprint of 100-kyr eccentricity forcing. However, the imprint of 405-kyr eccentricity or other astronomical parameters (precession, obliquity) cannot be observed, making this astronomical interpretation tentative.