COUPLING OF THE GLOBAL CARBON CYCLE AND SEA-LEVEL CHANGE DURING THE EARLY MISSISSIPPIAN

A well-documented positive carbon isotope (δ¹³C) excursion with a magnitude of ≥ 5‰ is present at the Kinderhookian-Osagean (K-O) transition in North America and other successions globally. This positive δ¹³C excursion has been attributed to enhanced organic carbon burial that removed ¹³C-depleted carbon from the atmosphere and ocean. If this interpretation is correct, there should be evidence present in the sedimentary successions that support climate-driven sea-level fall at or near peak values of the K-O δ¹³C excursion. A well-exposed Early Mississippian section near Alamo, Nevada, records the K-O δ¹³C excursion in two limestone formations, the Joana Limestone and Limestone X, but evidence for sea-level fall at or near the peak of the δ¹³C excursion has not been well documented. An integrated sedimentological and isotope geochemical study of the K-O strata in the Great Basin reveals the occurrence of a sequence boundary near the spike of the δ¹³C excursion. The sequence boundary is expressed by laterally discontinuous, thinly laminated calcareous siltstone and cherty-silty packstone that penetrate downward into the underlying crinoidal wackestone and packstone. These laterally discontinuous layers contain evidence of bioturbation, pedogenic cavity fill cements, and coated grains along an undulated erosional surface with strong cementation, recording a carbonate hardground that developed above a karstic surface. Together, these features support significant global cooling and sea-level fall at the peak of the K-O δ¹³C excursion, consistent with enhance organic carbon burial and lowering of atmospheric CO₂. These findings provide evidence for the coupling of the Early Mississippian carbon cycle and paleoclimate change that may have set the clock for the late Paleozoic ice age.