Foraminiferal stratigraphy and paleoenvironments of a high latitude marginal marine basin – a Late Cretaceous record from IODP Site U1512 (Great Australian Bight)

Abstract

The 690 m core recovered at IODP Site U1512 offers unparalleled insights into climate during the peak and demise of the Cretaceous hothouse from the southern high latitudes. A new high-resolution micropaleontological record (notably deep water agglutinated foraminifera, DWAF) combined with additional geochemical data was produced from this locality, with the goal of recognizing benthic foraminiferal biozones, their relationship to the Tethyan realm and associated paleoenvironmental changes. We document a diverse benthic foraminiferal assemblage consisting of 162 taxa (110 agglutinated and 52 calcareous). The most common elements of the DWAF assemblage are tubular (i.e., Kalamopsis grzybowskii, Bathysiphon spp.) and planispiral forms (i.e., Ammodiscus spp., Haplophragmoides spp., Buzasina sp., Labrospira spp.). The Turonian strata yield abundant Bulbobaculites problematicus and piroplectammina navarroana. The presence of Uvigerinammina jankoi provides a tie point to the Tethyan DWAF biozonation of Geroch and Nowak (1984). The calcareous foraminiferal assemblage is composed of cosmopolitan deep-water forms including Lenticulina, Dentalina, Gavelinella/Anomalinoides, Praebulimina and Pseudobolivina. Organic geochemical data (notably steranes/hopanes ratios), foraminiferal assemblages, together with an increasing abundance of radiolaria reveal a complex and constantly changing marginal marine paleoenvironment. The prevailing regime (oxic or dysoxic, marine, or terrestrially influenced) was strongly influenced by paleobathymetry, unstable oceanic circulation and terrestrial runoff. These factors caused the paleoenvironment to change five times over 10 million years, each lasting <2 Ma. A major increase in the abundance of radiolaria (>50%) after the mid-Turonian followed a peak in the abundance of tubular epifaunal foraminifera during the lower to mid-Turonian. This is possibly related to an increase in bathymetry, that could correspond to a preceding sea level lowstand, and associated changes in ocean chemistry, such as changes in organic flux or salinity. Foraminiferal and geochemical evidence from Site U1512 allow for far-reaching interpretations of the paleoenvironments of the Great Australian Bight during the Late Cretaceous.