THE MID-MAASTRICHTIAN EVENT on SHATSKY RISE, ODP LEG 198

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 isotopic and benthic foraminiferal population data from these sites may provide a possible explanation to the limited stratigraphic range of inoceramids on Shatsky Rise. We suggest that this was a regional expression of a diachronous event.

This study shows the MME is indicated in surface-dwelling planktic foraminifera by a 0.50⁰/₀₀ positive shift in ∂¹⁸O values (~2 ⁰C cooling) and a 0.24⁰/₀₀ negative shift in ∂¹³C values occurring at ~69.3 Ma. This is coincident with the FO of inoceramids and changes in benthic foraminiferal populations. This cooling event is in contrast to a previous study of these sites by Frank et al. (2005; Paleoceanography) which showed a negative shift in surface ∂¹⁸O values at the MME. Increased seasonality could account for differences in surface temperatures as recorded by two different species P. costulata (this study) and R. rugosa (Frank et al.) and may indicate a preference for cooler water and warmer water, respectively. Simultaneous is a collapse of both the ∂¹⁸O and ∂¹³C gradient between surface (P. costulata) and thermocline (G. stuartiformis) dwellers. The collapse of the surface ∂¹³C gradient records higher 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. After this time the ∂¹³C and ∂¹⁸O gradient in the surface waters stayed high but variable before a sudden warming event in the planktic record at 68.7 Ma a date which is in good agreement for the global extinction of inoceramid bivalves in the North Pacific (Nifuku et al., 2009; Palaeo3). This supports our hypothesis that the short range of inoceramid bivalves 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 waters.