Sequence stratigraphy and carbon isotopes from the Trenton and Black River Groups near Union Furnace, PA: Constraining the role of land plants in the Ordovician world

Sea-level change influences carbon isotopic trends in both modern and ancient carbonate depositional environments. Generally, this relationship is manifested as a positive carbon isotopic excursion where the rising limb of the excursion is associated with transgression. These excursions exist because sea level can influence 1) local/ regional/global carbon cycling, 2) basin restriction and meteoric influence, and 3) carbonate sedimentation. The first two processes are influenced, in part, by the large carbon reservoir represented by the terrestrial biosphere. Our goal is to explore the relationship between carbon isotopes and sea level prior to the evolution of vascular plants at a time when the terrestrial carbon reservoir was far smaller. This study focuses on the Upper Ordovician Union Furnace section in Pennsylvania, where a previously documented similar to 1 parts per thousand increase in delta C-13 values in the upper Sandbian (458.4-453.0 Ma; referred to as the baseline shift) has been interpreted to reflect a perturbation to the global carbon cycle due to the proliferation of non-vascular plants. By focusing on the interval deposited before the baseline shift, we were able to examine the relationship between sea level, carbon isotopes, and some of the earliest terrestrial ecosystems. We present a high-resolution delta C-13 record directly tied to the sedimentological and sequence stratigraphic framework. By sampling genetically related packages of rock we were able to document a relationship between carbon isotopes and sea level at the sequence and parasequence level. Our results show that carbon isotopic values decreased during transgressions and increased during regressions. This seemingly inverse relationship between delta C-13 values and sea level is interpreted to reflect increased terrestrial fluxes during regression in a world devoid of vascular plants. Our results suggest that the early colonization of land by non-vascular plants impacted regional carbon cycling in the Appalachian Basin and eventually the global carbon cycle in the Late Ordovician.

Automated summary

Sea-level change influences carbon isotopic trends in both modern and ancient carbonate depositional environments. Generally, this relationship is manifested as a positive carbon isotopic excursion where the rising limb of the excursion is associated with transgression. These excursions exist because sea level can influence local/regional/global carbon cycling, basin restriction and meteoric influence, and carbonate sedimentation. Our goal is to explore the relationship between carbon isotopes and sea level prior to the evolution of vascular plants at a time when the terrestrial carbon reservoir was far smaller. Our results suggest that the early colonization of land by non-vascular plants impacted regional carbon cycling in the Appalachian Basin and eventually the global carbon cycle in the Late Ordovician.