DID EARLY LAND PLANTS PRODUCE A STEPWISE-CHANGE IN ATMOSPHERIC OXYGEN DURING THE LATE ORDOVICIAN (SANDBIAN ~ 458 MA)?

A stepwise-change in atmospheric oxygen (O₂) levels during the Ordovician has been attributed to the emergence of land plants. This phenomenon is tied to a major baseline shift in the stable carbon isotope (δ¹³C_carb) curve, and an inferred increase in nutrient delivery, and enhanced primary productivity in nearshore settings, which led to enhanced organic carbon burial. The timing and magnitude of this baseline shift, however, is still elusive in part due to the lack of high-resolution δ¹³C data that span this period.

This study presents new high-resolution δ¹³C data from Middle to Upper Ordovician sections at Germany Valley (West Virginia) and Union Furnace (Pennsylvania) in the Central Appalachian Basin. The δ¹³C curves of the data from both sections are characterized by relative stability and low carbon isotope values (mean δ¹³C = -0.61 ‰) in the lower Sandbian, followed by a ~ 1.2 ‰ shift toward more positive values (mean δ¹³C = +0.62 ‰) in the upper Sandbian. We propose that the positive shift represents a fundamental state change in the global carbon cycle that is coincident with a major diversification of early land plants (i.e., bryophytes). This shift in δ¹³C baseline supports a growing number of studies that interpret enhanced organic carbon burial rates served as a mechanism for the stepwise oxygenation of the atmosphere during the Late Ordovician.