RE-EVALUATING WASS MASS INFLUENCE ON BENTHIC FORAMINIFERA

The unique physical and chemical properties of water masses (e.g., NADW, AABW) were once thought to control the types of foraminiferal assemblages found in the deep-sea. Over time, that idea was abandoned in favor of more localized parameters, such as the flux of food to the seafloor and dissolved oxygen content. We re-evaluated this concept by looking at a ~13-myr late Campanian – early Danian benthic foraminiferal assemblage record at Shatsky Rise in the northwest Pacific. Stable carbon and oxygen isotopes reveal at least 12 water mass changes that likely originated from the northwest Pacific, Southern Ocean, and perhaps the Indian Ocean (Tethys).

Q-mode cluster analyses of the assemblages show 13 distinct and sharply defined groupings that are closely correlative with shifts in oxygen isotopes. First and last occurrences of taxa are uniform through the study interval and indicate that evolutionary turnover was not a driving factor in influencing the clustering. Of note is the start of long-term cooling beginning in the latest Campanian (~74 Ma), with major assemblage changes around the Campanian-Maastrichtian Biotic Event (CMBE, ~72 Ma) with a sharp increase in infaunal taxa (Praebulimina, Aragonia), followed by further cooling during the early Maastrichtian and an elevated abundance of inoceramid clams (71 Ma; Inoceramid Acme Event, IAE). The mid-Maastrichtian Event (MME; 70 Ma) was an abrupt warming that terminated the presence of inoceramids at Shatsky Rise followed by another rapid cooling event during the late Maastrichtian (68 Ma), culminating in a two-step warming at the end-Maastrichtian (~67 Ma). This is followed by a slight cooling at the K/Pg boundary and further warming in the early Danian.

One take away from this study is that specific taxa are not diagnostic of individual warming or cooling trends, although the changes in the cluster packages are highly indicative of changes in water mass. Many of the clusters are defined by abrupt shifts in taxa abundance, including the decline of P. elevata, O. umbonatus, and G. pyramidata at the MME, and the emergence of Tritaxia, sharp increases in G. becariiformis, and Reussella, followed by P. hillebrandti and Adercotryma. Water mass changes at these lower bathyal depths were subtle, but assemblage changes were associated with both cooling and warming events.