HIGH-FREQUENCY ISOTOPE EXCURSIONS WITHIN THE SILURIAN LAU POSITIVE CARBON ISOTOPE EXCURSION

The upper Silurian is characterized by positive excursions in marine carbonate carbon isotopes (δ¹³C_carb). 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 δ¹³C_carb, organic carbon isotopes (δ¹³C_org), 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 δ¹³C_carb and also δ¹³C_org that are contained within the larger (~5.0 ‰) δ¹³C_carb excursion. Two negative excursions in δ¹³C_carb (-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 δ¹³C_carb of ~5 ‰ over 0.5 m of stratigraphic height, marking the beginning of the Lau Excursion. Our high-resolution analysis identifies five smaller positive δ¹³C_carb excursions (magnitude 1.7 – 4.1 ‰) within the Lau Excursion, recorded in silty limestones with no consistent fluctuations in sedimentology. δ¹³C_org also demonstrates several smaller (> 1 ‰) negative excursions during the Lau Excursion interval, suggesting episodic increases in biologic productivity. δ¹³C_org versus total organic carbon (TOC) demonstrate a strong negative correlation, whereby increases in TOC have lower δ¹³C_org, that support the interpretation of intervals of high biologic productivity. However, δ¹³C_carb versus δ¹³C_org are not correlated, suggesting that the relationship between biologic productivity, organic carbon burial, and δ¹³C_carb is more complex than previous models suggest, especially when considered on a high-resolution scale.