OPENING AND CLOSING OF A SHORT-LIVED (13 M.Y.) SEAWAY: FAUNAL RESPONSE TO SEA LEVEL AND CLIMATE CHANGE

Understanding how regional ecosystems respond to sea level and environmental perturbations is a main challenge in paleoecology. Even if environments may change over time, not all changes cause turnover, raising the question of which environmental conditions promote relative ecologic stability versus turnover. Here we present a study on marine benthic communities from the Middle–Upper Jurassic (Bajocian–Oxfordian) of the Western Interior Seaway (U.S.A.), a long and shallow sea that cyclically expanded and contracted for about 13 m.y. We collected quantitative faunal abundance estimates in Wyoming, South Dakota and Montana, and placed them in a sequence stratigraphic framework that includes eight third-order depositional sequences.

Three patterns in the faunal data are noteworthy. (1) The strongest differences in faunal composition are among sequences characterized by carbonate deposition versus those dominated by siliciclastic deposition, which generally parallels the history of the basin. Relative few eurytopic species are shared among these two systems. (2) Faunal turnover is low among carbonate sequences, where communities vary along a water depth gradient from deep subtidal through restricted shallow subtidal facies. Turnover is greater among siliciclastic sequences, which are dominated by offshore facies. (3) Atypical from other Jurassic settings, most communities present low richness (S: 2–6) and evenness (1–2 dominant species).

The shift from a warm-arid climate to humid-temperate conditions in the Seaway at the Middle–Upper Jurassic transition caused the main change in the depositional regime, which most likely caused the main faunal turnover. This climate shift, superimposed on third-order sea level cycles, was also the cause of turnover in faunas from siliciclastic settings, whereas climate stability during carbonate deposition promoted the recurrence of biofacies through third-order cycles. The unique physiography of the Seaway, with a sole connection to the proto-Pacific Ocean about 2000 km north of its southern end, most likely promoted strong gradients in salinity and temperature, so that only a few species were able to successfully colonize it.