SGER: Sequence Stratigraphic Architecture of a Middle Orodovician Carbon Isotope Excursion

University Of Iowa, Iowa City IA

Investigators

Abstract

Sequence Stratigraphic Architecture of a Middle Ordovician Carbon Isotope Excursion Gregory Ludvigson EAR- 0000741 A late Middle Ordovician (453 million year-old) shift in the stable carbon isotopic ratios (13C/12C) of sedimentary carbonate rock strata in the mid-continent United States has now been identified in time-equivalent rocks in Pennsylvania and Estonia. Recent research results suggest that expression of this geochemical event was global in extent, and recorded a short-term decrease in the concentration of atmospheric CO2. This event is related to an increase in the rate of burial of marine organic matter that resulted from short-term paleoceanographic changes. Published depositional interpretations of the rock strata in which the carbon isotope shift was originally discovered in Iowa suggest that this time interval was characterized by non-deposition in more offshore settings over much of the eastern United States. However, this idea has never actually been tested regionally with carbon isotope profiles of rock stratigraphic sections. New studies of time-equivalent nearshore green calcareous shales in northwest Iowa show that the carbon isotope shift is also present in these rock strata, and suggest that this geochemical event might have applications for tracing this particular interval of geologic time across geographic boundaries separating major sedimentary rock types. The aims of this project are to: 1) develop stratigraphic profiles of the 13C/12C ratios of the mineral calcite (CaCO3) in the Decorah Formation and overlying/underlying rock strata from six drillcores along a transect perpendicular to the ancient Ordovician shoreline, extending from southeast Minnesota through eastern Iowa into central Illinois; 2) develop matching stratigraphic profiles measuring the amount of organic carbon contained in these rocks; and 3) develop profiles of the 13C/12C ratios of organic matter in a selected subset of those drillcores. The larger scientific goals of the project are to test the idea that the carbon isotope excursion interval was characterized by non-deposition in much of the eastern United States, and to further test ideas on the causes of the excursion event. The aims of this project are to: 1) develop stratigraphic profiles of the 13C/12C ratios of the mineral calcite (CaCO3) in the Decorah Formation and overlying/underlying rock strata from six drillcores along a transect perpendicular to the ancient Ordovician shoreline, extending from southeast Minnesota through eastern Iowa into central Illinois; 2) develop matching stratigraphic profiles measuring the amount of organic carbon contained in these rocks; and 3) develop profiles of the 13C/12C ratios of organic matter in a selected subset of those drillcores. The larger scientific goals of the project are to test the idea that the carbon isotope excursion interval was characterized by non-deposition in much of the eastern United States, and to further test ideas on the causes of the excursion event.