BENTHIC FORAMINIFERAL POPULATION AND ISOTOPE CHANGES AT THE MID-MAASTRICHTIAN EVENT, SHATSKY RISE, NORTH PACIFIC

During the Cretaceous, inoceramid bivalves were common in many benthic marine communities. They were found globally at all latitudes and wide paleodepths. Then, over a “geological second”, these organisms disappeared during what is known as the mid-Maastrichtian event (MME), a time of faunal turnover. This event did not happen catastrophically, but occurred over a 2-3 myr time span.

Two ODP sites were studied at Shatsky Rise, which was located in the tropical Pacific during the Late Cretaceous: Site 1209 (2357 m water depth) and 1210 (2573 m). We wanted to gain a better understanding of the inoceramid extinction by studying how benthic foraminiferal abundances changed around the time of the MME. In particular, did benthic foraminifers respond to changing conditions on the seafloor; were these changes synchronous with the short-lived occurrence of inoceramid bivalves; and were there significant differences as a function of water depth on Shatsky Rise?

Benthic foram populations were not stable between 70 and 68 Ma; they changed abruptly between epifaunal and infaunal dominance in multiple pulses leading up to, during, and after the MME. Changes were more extreme at Site 1210. The inoceramid event occurred during a period when epifaunal benthics dominated 60-70% of the foram community. N. truempyi and A. velascoensis abruptly decreased in abundance prior to the inoceramid interval, while Conorbina spp. and G. michelinianus dominated the foram assemblages when inoceramids were common.

The brief inoceramid interval on Shatsky Rise occurred during a warming trend from 70 Ma to 68 Ma based on a negative 1‰ shift in δ¹⁸O values. Deep-water cooling initiated after 68 Ma. Inoceramids appear within an interval of an abrupt +1‰ shift in benthic δ¹³C values. Cooling sea surface temperatures based on multi-species planktic oxygen isotopes and a reduction in planktic δ¹³C gradients suggest that there was increased productivity at this time.

The inoceramids were not found deeper than Site 1210 on the Shatsky Rise depth transect. In fact, the inoceramids only lived at Site 1210 for a very short period of time relative to Site 1209. It’s possible, therefore, that a warm intermediate water mass descended onto Shatsky Rise, but did not stay at Sites 1209 and 1210 very long, before disappearing along with the inoceramids.