Late eocene—early oligocene integrated isotope stratigraphy and biostratigraphy for paleoshelf sequences in southern Australia: paleoceanographic implications

Abstract

A late Eocene—early Oligocene planktonic foraminiferal oxygen-isotope stratigraphy derived for two overlapping sections (Browns Creek and Castle Cove) through paleoshelf sequences in southern Victoria, Australia, contains a pronounced late Eocene depletion (warm) peak (∼1.2%PDB) centred at about 41 Ma, followed by a two-stage enrichment (cooling) pattern, the later one coinciding with the well known Terminal Eocene Event (∼37 Ma), exhibiting ∼1.0%PDB) erinchment. The early Oligocene isotope stratigraphy shows fluctuations of about 1.0%PDB, with prominent depletion peak immediately following the terminal Eocene δ 18O enrichment. The start of the terminal Eocene δ 18O enrichment coincides with the LAD of the nannofossil indicator of the Eo-Oligocene boundary, Discoaster saipanensis (early NP21 Zone). On this basis the traditional southern Australia planktonic foraminiferal Eocene-Oligocene boudary markers (LAD Globigerapsis index, LAD Subbotina linaperta Subbotina linaperta) locally survived into Zone NP22, well into the early Oligocene. Sea-surface paleotemperature estimates derived from the δ 18O data assuming a δW value of −1.2%PDB, show that the late Eocene ingression of the tropical planktonic foraminiferal genus Hankenina coincided with peak surface-water tempertures of ∼24°C. The temperature decline in the southern Australia shelf waters associated with the Terminal Eocene Event may have been as much as 7°C, reaching a minimum value of ∼13°C. The later survival of G. index and S. linaperta coincides with a rapid recovery of water temperatures over the south Australian shelf following the Terminal Eocene Event. This is attributed to the local warming of waters in shallow broad embayments where circulation patterns were decoupled from the developing Circum-Antartic Current to the south.