Paper No. 4
Presentation Time: 8:00 AM-12:00 PM
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.