GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 36-2
Presentation Time: 9:00 AM-5:30 PM


BOBKA, Alana Marie, Geosciences, The University of Tulsa, 800 S Tucker Dr, Tulsa, OK 74104 and THEILING, Bethany P., Geosciences, University of Tulsa, 800 S Tucker Dr, Tulsa, OK 74104

The upper Silurian is characterized by positive excursions in marine carbonate carbon isotopes (δ13Ccarb). The Lau Excursion takes place during the Ludlow epoch (427.4 - 423 Ma) and is identified in marine carbonate deposits globally. Suggested paleo-oceanographic and climatic conceptual models have not been able to adequately resolve presumed increases in organic carbon burial with Silurian deposits that lack black shale deposition. This study uses marine δ13Ccarb, organic carbon isotopes (δ13Corg), and high-resolution stratigraphic analysis to evaluate the relationship between carbon budgets, organic productivity and inferred sea-level fluctuations. 10 cm sampling of the Henryhouse Formation in the Arbuckle Mountains of south-central Oklahoma illuminates smaller (> 1.5 ‰) but significant fluctuations in δ13Ccarb and also δ13Corg that are contained within the larger (~5.0 ‰) δ13Ccarb excursion. Two negative excursions in δ13Ccarb (-4.5 ‰ and -3.4 ‰) are observed in the lower portion of the sampled section, suggesting meteoric diagenesis, yet are recorded in alternating beds of carbonate cemented blue-green shale and siltstone. These negative excursions are followed by an increase in δ13Ccarb of ~5 ‰ over 0.5 m of stratigraphic height, marking the beginning of the Lau Excursion. Our high-resolution analysis identifies five smaller positive δ13Ccarb excursions (magnitude 1.7 – 4.1 ‰) within the Lau Excursion, recorded in silty limestones with no consistent fluctuations in sedimentology. δ13Corg also demonstrates several smaller (> 1 ‰) negative excursions during the Lau Excursion interval, suggesting episodic increases in biologic productivity. δ13Corg versus total organic carbon (TOC) demonstrate a strong negative correlation, whereby increases in TOC have lower δ13Corg, that support the interpretation of intervals of high biologic productivity. However, δ13Ccarb versus δ13Corg are not correlated, suggesting that the relationship between biologic productivity, organic carbon burial, and δ13Ccarb is more complex than previous models suggest, especially when considered on a high-resolution scale.