Geosciences - Seminar Series ( PRO-SEM )

Following the mid-Cretaceous “super-greenhouse”, the Late Cretaceous was characterized with overall global cooling that was punctuated with two episodes of intermediate water mass cooling correlated with two third order sea level falls and two episodes of global warming. These events, driven by changes in thermohaline circulation and other environmental controls, are thought to be responsible for significant biotic changes during this time. Notable mid-Maastrichtian biologic events include the diachronous extinction of the wide-ranging and successful inoceramid clams, the collapse of rudistid reefs and shifts in the latitudinal distributions of calcareous nannoplankton and planktic foraminifera. These environmental and faunal changes are known as the mid-Maastrichtian Event (MME).

Environmental changes associated with the MME are not well understood particularly for the Pacific Ocean due to poor preservation, diagenetic alteration and sparse sample coverage. ODP tropical Pacific sites 1209 and 1210 provide an unusual record of the Mid-Maastrichtian Event (MME). Clusters of large Inoceramus prisms are restricted to several meters of section before abruptly disappearing. The significance of the short range of inoceramid bivalves over the shallowest sites of Shatsky Rise is currently not understood.

New evidence and isotopic and benthic foraminiferal population data from these sites may provide a possible explanation to the limited stratigraphic range of inoceramids on Shatsky Rise. These data suggest that this limited range was rather an Inoceramid Acme Event (IAE) where inoceramids, rare but present in the Late Cretaceous at Shatsky Rise dramatically increased before suddenly going extinct. The IAE, visible in both sites at Shatsky Rise, is indicated in surface-dwelling planktic foraminifera by a 0.50‰ positive shift in ∂18O values (~2oC cooling) and a 0.24‰ negative shift in ∂13C values occurring at ~69.3 Ma. This cooling event is in contrast to a previous study of these sites by Frank et al. (2005) which showed a warming in sea surface temperatures at the MME (IAE). Increased seasonality could account for differences in SSTs as recorded by two different species P. costulata (this study) and R. rugosa (Frank et al. 2005). Simultaneous is a decrease of both the ∂18O and ∂13C gradient between surface and thermocline dwellers. The decrease of the surface ∂13C gradient records higher (seasonal) productivity due to the upwelling of cooler, nutrient-rich (12C-rich) waters.

There are no significant shifts in the ∂18O and ∂13C values of planktic foraminifera at the time inoceramids abruptly disappear from the record at Shatsky Rise (~69.1 Ma). After this time the ∂13C and ∂18O gradient in the surface waters stayed high but variable. We propose that the Inoceramid Acme Event at Shatsky Rise was a regional event brought on by changes in surface productivity and greater flux of food out of the surface waters, or by a change in deep or intermediate waters. The inoceramid extinction at Shatsky Rise may have been a regional expression of a diachronous event.