The Carboniferous Period (359 to 299 million years ago) was a time of dramatic tectonic, climatic, and biologic change in Earth's history. Several aspects of the Carboniferous are distinctive from the standpoint of Earth Systems Science: the onset of significant glaciation, signaled by global regression; a major change in carbon cycling manifested by coal accumulation in the paleotropics; and reconfiguration of the continents resulting in the Alleghanian-Variscan orogeny, assembly of Pangea, and closure of the Rheic Ocean, leading to severing of the marine connection between the North American paleotropics and the Tethyan paleotropics. A wealth of paleobiologic, paleobiogeographic, and paleochemical data has been generated, with recent emphasis on applying marine geochemical proxies to unraveling the potential links between tectonics, carbon cycling, and climate. The marine paleochemical data, in theory, provide oceanic proxy records of temperature (Delta 18 Oxygen ), carbon cycling (Delta 13 Carbon), and weathering (34S/32S, 87Sr/86Sr), but the application of these proxies to the Paleozoic is confounded by the fact that they are derived from marine sediments deposited in epicontinental seas. These shallow, shelf-like environments must have had connections to open ocean basins (e.g., Panthalassa and Rheic Oceans) in order to support marine communities, yet there is evidence to suggest geochemical decoupling between shallow epicontinental seas and the open ocean. In an effort to better understand the causes and consequences of climate change during the Carboniferous, PIs must first establish what can be inferred from marine geochemical records. Thus they propose to address the question, To what degree does the Carboniferous North American epicontinental sea record global open-ocean conditions? They will use radiogenic and stable isotope tracers of varying residence times along a transect spanning the Mixteco terrane in present-day southwestern Mexico (most oceanic), Arrow Canyon, US Midcontinent, Illinois Basin, and Appalachian Basin (most interior) to constrain the degree of geochemical coupling between the epicontinental sea and the open ocean (Panthalassa and Rheic Ocean).
The evolution of the earth system through the Carboniferous represents an excellent analog for the major climate transition from overall warmth with little polar ice to a interval characterized by varying amounts of polar ice (nominally referred to as a "greenhouse to icehouse" transitions) that occurred during the Cenozoic. During the Carboniferous interval, sea level changes driven by changes in the volume of glacial ice at high latitudes resulted in expansion and contraction of the seaways that covered a significant portion of North America. To understand and reconstruct ancient climate, geologist use the distribution and chemical composition of fossils as proxies for environmental conditions. In this project we applied three proxy measurements generated from fossil brachiopods and teeth remains extracted from sediment sections spanning southern Mexico, northward into Nevada and then eastward to West Virginia to reconstruct the history of sea level and climate during the Carboniferous. Our proposed study addresses the question, To what extent were the epicontinental seas of the Carboniferous well connected (mixed) with the open ocean? Interpretations of Carboniferous marine geochemical data assume that they represent oceanic conditions, but is this assumption justified? Testing the entire system of Carboniferous epicontinental seas would not be feasible within the context of a single three-year project. Instead, we focused on remnants of the Carboniferous epicontinental sea that covered present-day U.S. and Mexico. We assessed the degree of connectedness/isolation by examining tracers with different residence times in the ocean. Radiogenic (Nd and Sr) and stable isotope analyses (brachiopod C and O, and clumped isotopes) from sections spanning the Mixteco terrane in present-day southwestern Mexico, Arrow Canyon, the U.S. Midcontinent, the Illinois Basin, and the Appalachian Basin constrained the degree of geochemical coupling between epicontinental seas and the open ocean. The Nd isotope values ranged from -8.6 to -4.5 in the US midcontinent basins and from -11.4 to -5.7 at Arrow Canyon (Nevada, USA), indicating the development of an east-west geochemical gradient throughout the Pennsylvanian. This apparent decoupling of the epicontinental seas from the open ocean was established by the latest Mississippian. Thus, circulation of waters between epicontinental seaways of western US and the basins to the east and south was periodically restricted. Shifts toward lower Nd isotope values at Arrow Canyon, Nevada correspond with glacio-eustatic changes across the mid-Carboniferous boundary. These Nd isotope variations may have been caused by changes in weathering rates/sources, marine circulation or a combination of both.
