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) curve and inferred increase in nutrient delivery and enhanced primary productivity in nearshore settings, which led to high organic carbon burial. The timing and magnitude of this baseline shift, however, is still elusive in part because of the lack of high-resolution δ13C data that span this period. Much of the existing Ordovician δ13C literature is focused on isotopic excursions with less emphasis on identifying long-term shifts in baseline (pre- and post-excursion) values. This study presents new high resolution δ13C data from stratigraphic sections at Germany Valley (West Virginia) and Union Furnace (Pennsylvania) in the Central Appalachian Basin. These sections span the entire Sandbian Stage and continue into the lower Katian Stage. The δ13C data from both sections are characterized by relative stability carbon isotope values (mean = −0.61‰) in the lower Sandbian, followed by a + 1.2‰ shift in the upper Sandbian (mean = +0.62‰). Herein, we propose that the positive shift represents a long-term global shift in baseline δ13C values of dissolved inorganic carbon. The timing of this positive shift coinciding with the diversification of early land plants (i.e., bryophytes) supports earlier models that suggested enhanced organic carbon burial rates served as a mechanism for the stepwise oxygenation of the atmosphere during the Late Ordovician. •New high-resolution δ13C dataset of marine carbonate rocks from the Appalachian Basin•The δ13C dataset shows a 1.2‰ positive shift of baseline values•The shift reflects a fundamental change of global carbon cycles in the Sandbian.•The change is related to an O2 rise, driven by diversifications of land plants.