EFFECTS OF COAL DEPOSITION ON CARBON ISOTOPES: MILANKOVITCH CYCLICITY, EUSTACY, AND THE CARBON CYCLE

Carbon isotope data were collected from 72 meters of Atokan age limestones in Arrow Canyon, Nevada. Samples were collected every 30 cm through cyclic carbonates that contained major and minor cycles. These data clearly show a periodicity close to the 400,000 year periodicity of the major coal units of the same age in the Appalachian Basin. The isotopic signal reflects the sequestration of organic matter as coal with highest values (~+4‰) occurring in lithologies indicative of deepest water, the same time coals were being deposited. Lowest δ¹³C values (~0‰) occur in silty limestone interpreted to be wind driven silt that accumulated during glacial maxima. These data show the impact coal deposition had on the function of the carbon cycle and may speak to the anomalously low CO₂ levels that have been modeled for the Pennsylvanian. These data also show how the function of the carbon cycle was altered by the presence of coal swamps. When compared to Cenozoic data the contrast is distinct. Modern ocean circulation dictates that glacial episodes have higher δ¹³C values associated with maximum ice, more vigorous upwelling and increased nutrient delivery. Interglacial episodes show the decrease in circulation and nutrient delivery as lower δ¹³C values. We attribute the difference between the Pennsylvanian example and the Cenozoic example to the inundation of the continents and a change in the rate of terrestrial sequestration.