GLACIO-EUSTATIC INFLUENCE ON THE CARBON CYCLE; CHEMOSTRATIGRAPHY OF CYCLOTHEMS IN THE ATOKAN, ARROW CANYON, NEVADA, USA

Previous carbon isotope stratigraphy done on Pennsylvanian-aged rocks has shown a noisy, non-trending δ¹³C signal that contrasted sharply with the slightly older Mississippian-aged rocks that showed trends and even excursions. At higher resolution, however, carbon isotope stratigraphy from the Atokan-aged (Pennsylvanian) sequence of carbonates in Arrow Canyon, Nevada shows a pattern that tracks the lithologic sequence. The lithologic pattern, a repetitive, shallowing-upward sequence, is a result of the glacially driven rise and fall of sea level that was occurring repeatedly at the time. The sequence is typified by a deep-water heterozoan faunal association followed by photozoan faunal associations displaying more shoreward lithologies upward in each cycle. The repetition of this type of sequence follows a periodicity calculated to be very close to the estimated length of the obliquity signal of Milankovitch cyclicity during the mid-Pennsylvanian time. The carbon isotopes (δ¹³C) show a shift from deep to shallow water of approximately 1 ‰(VPDB), but as much as 1.45 ‰. The lightest isotopic values are recorded in the deepest water and progressively become heavier as the water shallows above the depositional surface in each cycle. For example, in one shallowing-upward cycle the deepest water lithology, a nodular wackestone, recorded a δ¹³C value of 2.50 ‰, shifting gradually through the cycle to record a value of 3.95 ‰ in a lagoonal wackestone. There are several factors that contribute to the values seen through the cycles. One factor is the local circulation patterns inherent of being in shallow water on a very wide platform, as these rocks were when deposited. Another factor is the global carbon cycle. Lastly, orbital variation clearly plays a role in driving our δ¹³C values.