DEFINING THE RELATIONSHIP BETWEEN MILANKOVITCH CYCLICITY AND THE CARBON CYCLE IN THE PENNSYLVANIAN AT ARROW CANYON, NEVADA

Studies have shown that the d¹³C of foraminifera shift by approximately 1‰ between glacial and interglacial episodes within the Pleistocene. We have observed similar changes in preliminary data from a 72 meter Pennsylvanian-aged (Atokan) carbonate section in Arrow Canyon, NV. Values range from 3.19‰ to 4.04‰ in the cycle occurring 5.5 meters above the base of the section and from 2.50‰ to 3.95‰ in the two-cycle set starting 12 meters above the base of the section and extending 9 meters upwards. This section displays distinctive cycle sets with Milankovitch periodicity ratios and has been interpreted as having recorded the rise and fall of sea level during ~1.8 million years of the late Paleozoic Ice Age. The d¹³C shifts to heaviest values at the point of maximum regression within each meter scale cycle, which is marked by a lithologic shift from a nodular argillaceous wackestone as the deepest water lithology to fossil wackestones and packstones when water has shallowed. This maximum regression is the point at which the seas are furthest off the shelves and there is the maximum amount of continental exposure, erosion and nutrient runoff. This variation in nutrient flux would have caused an increase in biomass. In turn, this organic matter would have caused an increase in the fractionation and burial of ¹²C. This correspondence suggests that there is a link between the carbon cycle and the glacial-interglacial cycles cased by Milankovitch variation.