Clumped isotope temperature and salinity constrains for the Maastrichtian Chalk Sea based on planktonic and benthic foraminifera from Poland

Abstract. The Maastrichtian (~72–66 Ma), the final stage of the Cretaceous, experienced long-term cooling with high atmospheric CO₂ and weak latitudinal temperature gradients. Tectonic movements and variations in climate lead to sea-level changes and dynamic ocean conditions. This background probably affected the seawater circulation regime of the shallow epeiric Chalk Sea that covered a large portion of the Northern European continent. The connections to the evolving Northern Atlantic, the Arctic Basin and the tropical Thetis Ocean in the South and their impact on the seawater circulation and stratification in the open Chalk Sea is still not well understood. This study applies carbonate clumped-isotope thermometry (Δ₄₇) to well preserved planktonic and benthic foraminifera from the Polanówka UW-1 core (Poland) to reconstruct the local surface and bottom water conditions prevailing during the Maastrichtian in the Chalk Sea.

The results from planktonic foraminifera reveal dynamic surface water conditions of alternating warmer and more saline with colder and less saline surface waters compared to stable, warm, and saline bottom waters from the benthic foraminifera. Comparisons with previous studies indicate the new planktonic Δ₄₇-SST reconstructions align more with oxygen isotope-based SST than the SST based on organic proxies such as TEX₈₆-SST, with differences attributed to calibration, seasonality, and habitat depth. These findings suggest a stratified water column where the surface water is influenced by sporadic water entrainment with strong depleted ẟ¹⁸O that could be associated to freshwater runoff. The observed more stable and warm bottom water conditions and the range of ẟ¹⁸O may be associated with 2 scenarios: (1) a warm, saline bottom water periodically influenced by incursions of colder and fresher North Atlantic waters, or (2) bottom water conditions influenced by increased water input from the Tethys during periods of sea-level rise. Our reconstructions on the central European Chalk Sea conditions provide new insights into the thermal structure and water circulation in the Chalk Sea during the Mid-Maastrichtian Event and highlight the need for further research to refine the understanding of hydrology dynamics during this mild greenhouse climate interval.