ABSTRACT: The sedimentological, paleontological, and geochemical record of the Late Ordovician Laurentian epicontinental seaway shows a number of shifts that have been variously interpreted as indicating progressive cooling, episodes of upwelling, intervals of enhanced weathering, and times of organic carbon burial with seafloor anoxia, among other hypotheses. Determining the nature and cause of changes during this interval has general significance as these events are critical in understanding the evolution of the Late Ordovician ocean-climate system during the transition from a greenhouse to an icehouse world culminating in the brief end-Ordovician glacial event and mass extinction. Discriminating among alternative explanations, though, has proved difficult. The increasingly well-resolved dataset (taxonomically, spatially, and stratigraphically) of conodont oxygen isotopic measurements as well as a growing neodymium isotopic data set are providing independent constraints on temperature and circulation patterns. Both variables are invoked in many models, and these data can, thus, be used to test model predictions. In this study we show that there is no oxygen isotopic evidence for systematic cooling across the contact between the Dubuque Formation and the Maquoketa Formation in the Upper Mississippi Valley, there is neodymium isotopic evidence for expansion of waters with the signature expected of the Iapetus Ocean onto the Galena Platform, and there is a subtle, opposite neodymium isotopic shift in correlative deposits from the Cincinnati Region. From these results, we argue that sediment starvation resulting in condensed deposition is a better explanation for the formation of the phosphatic lower portions of the Maquoketa Formation than enhanced productivity driven by upwelling of cool waters sourced from the open ocean. Further, neodymium isotopes support circulation changes associated with the initiation of the Richmondian Invasive in the Cincinnati Region.