WATER MASS INFLUENCE ON BENTHIC FORAMINIFERAL ASSEMBLAGES IN THE MAASTRICHTIAN, SHATSKY RISE, NORTH PACIFIC

Food availability (organic matter flux) and dissolved oxygen content of water masses and interstitial waters are considered to be among the most important controls on the distribution of deep-sea benthic foraminifera. Shatsky Rise (northwest Pacific) records the benthic foraminiferal response to changes in water masses as ocean circulation ramped up towards the end of the Cretaceous. The Cretaceous Period (145.5 – 66 Ma) was generally a greenhouse world, and as a result was thought to have sluggish ocean circulation. A major source of bottom water formation at times during the Cretaceous has been postulated to be of low latitude high evaporative origin. By the Late Cretaceous, however, benthic foraminiferal δ¹⁸O and other proxies show that there was a long-term cooling trend. Studies of neodymium isotopes and benthic foraminiferal δ¹³C from different ocean basins and latitudes have documented multiple alternating intervals of deep water cooling and warming during the Maastrichtian (~72 Ma – 66 Ma) signaling reversals in thermohaline circulation, or other changes in deep water masses. A high resolution foraminiferal analysis of Shatsky Rise, focused on ODP Site 1209, reveals 7 bursts of first appearances of benthic taxa, each associated with isotopically-defined water mass changes. This site was never below the CCD during the Maastrichtian, but a severe dissolution event ~67.8-66.2 Ma indicates that the lysocline shoaled significantly. The findings from the Maastrichtian of Shatsky Rise suggest that deep-sea benthic foraminifera were likely sensitive to changing water mass characteristics, in addition to changes in organic matter flux (food availability) and dissolved oxygen.