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Deoxygenation is a critical problem facing the ocean as the world warms, and has the potential to effect coastal upwelling zones, shelf areas influenced by high runoff and nutrification, and restricted and semi-restricted basins. The mechanisms that drive deoxygenation in these diverse environments are still not fully understood, in part because the modern record of redox change is short and anoxia is still relatively rare in the modern ocean. Here, we address this problem of scale by studying deoxygenation in the geologic past. We summarize decades of individual studies of benthic foraminifera to generate a record of bottom water oxygen change in the Cretaceous Western Interior Sea (WIS) of North America over ~13 myr (Cenomanian-Campanian), spanning two major sea level cycles. The WIS was prone to major changes in dissolved oxygen content throughout its long history, sometimes directly antiphase to trends in the global ocean. Presented as maps, our data show that bottom water oxygen within the WIS was controlled by a combination of water mass source and mixing moderated by sea level and basin restriction. Areas flooded by cool Boreal (northern-sourced) waters in the northern and western parts of the seaway were better oxygenated than the eastern and southern portions of the seaway, which were flooded by warmer Tethyan (southern-sourced) waters. Beyond east-west differences explained by water mass, the entire seaway was better oxygenated during periods of transgression, and more poorly oxygenated to anoxic during periods of peak transgression/highstand and regression. We suggest that this pattern was due to the formation and downwelling of Western Interior Intermediate Water by the mixing of Tethyan and Boreal waters. During transgressions, an increasing volume of these watermasses entered the 3 seaway, mixed, and downwelled well-oxygenated surface water to the seafloor. During late transgression/highstand, partial stratification and the encroachment of low oxygen waters from the open ocean caused dissolved oxygen levels to drop at the seafloor, but continued downwelling prevented anoxia. During the subsequent regression, a decline in the volume of outside watermasses entering the seaway caused a reduction in mixing and weakened downwelling which led to stratification and seafloor anoxia. As a model for other semi-restricted basins, the trends observed in the WIS show that local changes in relative sea level, mixing, and circulation are critical in controlling oceanic deoxygenation in these environments, in clear contrast to continental margins impinged by oxygen minimum zones, like the contemporaneous Demerara Rise in the southern Caribbean. Although the WIS is larger than most semi-restricted basins, it is characterized by quasi-estuarine circulation driving the interaction of normal marine and brackish watermasses, and thus serves a model for similar shallow epicontinental basins of any size. Understanding how these processes vary in different environments is key to predicting susceptibility of regional water ...

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Ocean Drilling Program Hole 803D (Leg 130) from the western tropical Pacific (Ontong Java Plateau) and Hole 628A (Leg 101) from the western subtropical North Atlantic (Little Bahama Bank) contain rich assemblages of planktonic foraminifers. The uppermost Eocene-basal Miocene section of Hole 803D is apparently complete, whereas the Oligocene section of Hole 628A contains three unconformities based on planktonic foraminiferal evidence. Anomalous ranges are recorded for Chiloguembelina cubensis and Globigerinoides primordius. C. cubensis is found to range throughout the upper Oligocene of both sites, and G. primordius first occurs near the base of upper Oligocene Zone P22 in Hole 628A. Paleomagnetic stratigraphy provides constraints on the last occurrence (LO) of Subbotina angiporoides, the first occurrence (FO) of Globigerina angulisuturalis, the FO of Globigerinoides primordius, the FO of Paragloborotalia pseudokugleri, and the LO of Chiloguembelina cubensis.In general, taxon ranges, total diversity, and the composition of the planktonic foraminiferal assemblages from Holes 628A and 803D are similar. Differences in the composition of planktonic foraminiferal assemblages between the two sites are interpreted to be primarily the result of enhanced dissolution at Site 803 (e.g