Snapshots of Antarctic Cretaceous Paleoclimate

The Cretaceous Austral-Antarctic microfossil record can be understood as paleoenvironmental archive documenting the vast variability of paleoclimatic settings from the middle Cretaceous hothouse to the gradual cooling towards the end of this period.

The research vessel JOIDES Resolution and the location of two sites drilled offshore western Australia during International Ocean Discovery Program Expedition 369. (© CC BY 4.0: JOIDES RESOLUTION: IODP/Bill Crawford, Erik Wolfgring)

Understanding biotic changes in bottom- and surface waters, in single celled marine organisms (particular in benthic and planktonic foraminifera), is in the focus of our studies. Based on the findings of the International Ocean Discovery Program Expedition 369 (Australia Climate and Tectonics), this FWF Project (J-4444) is evaluating the climate trends in the light of the breakup of the former supercontinent Gondwana and phases of climate deterioration and changing oceanic circulation patterns.

The breakup of the supercontinent of East Gondwana and the reconfiguration of continental landmasses together with a reorganization of oceanic circulation regimes, as well as a hothouse climate related to high CO2 levels most likely linked to immense volcanic outgassing, are the essential drivers for major changes in the Earth’s ocean and climate system during the Late Cretaceous.

Data emphasizing the Austral-Antarctic southern high latitudes (~60°S paleolatitude), that play a crucial role for climate modeling and are vital for enhancing the coherence of paleoclimate records, are underrepresented in the Cretaceous paleoclimate record. Understanding the greater geological and paleogeographic evolution of the southern hemisphere is essential for the reconstruction of ocean pathways and circulation patterns during Earth’s history. Continuous biostratigraphic and well constrained mid- to Upper Cretaceous benthic foraminiferal data are rare for this region, as is knowledge of paleoclimatic reconstructions. Few (ocean drilling) studies assessed benthic foraminiferal communities and put them into a broader stratigraphic and paleoecologic context. Expedition 369 drilled numerous localities offshore Australia that were located at a paleolatitude of about 60° during Cretaceous times. The closer evaluation of sediment samples and fossil evidence therein allowed the reconstruction of benthic paleoenvironments.

Scientists eagerly await the drill operators announcement – ‘core on deck’ signals another sediment core ready for inspection. (© CCBY 4.0: IODP)

We sampled sediment cores from offshore Australia to study the Oceanic Anoxic Event 2 (OAE 2), illustrating a major disturbance of the carbon cycle (Cenomanian/Turonian stages, ~94 Ma). This episode of global warming and oxygen depletion in bottom waters was accompanied by organic carbon burial and the deposition of black shales.

Initially, an increased hydrologic cycle, increased continental runoff, poorly stratified bottom water masses and a high availability of nutrients fostered increased organic productivity on the western Australian margin. These factors enforcing each other eventually led to a high consumption of oxygen and ultimately bottom water hypoxia.

We document the increase of foraminifera tolerant of oxygen-depleted environments, followed by the virtual disappearance of life in the bottom waters. The isotopic ratios preserved in benthic foraminiferal calcareous tests suggest seafloor temperatures of up to 20° Celsius. After the termination of OAE 2, ocean temperatures only slowly decreased and significant cooling pulses can be documented towards the Santonian stage of the Cretaceous period (~86 Ma).

Oxygen isotopes of planktonic foraminiferal tests reflect a cooling trend of ~ 5° Celsius in the surface waters. Benthic species indicate a cooling of ~2° Celsius in the bottom waters. The causes of the surface water cooling in the Santonian in the high latitudes are still uncertain and could invoke the reorganization of oceanic current patterns or the differentiation of water masses leading to increased bioprovincialism towards the end of the Cretaceous period.

The analysis of foraminiferal assemblages from sediment samples, obtained through scientific ocean drilling, offers the unprecedented opportunity of adding to the understanding of the causes and effect of major disturbances in the carbon cycle and ocean climate system in the southern high latitudes. Combining microfossil analysis with geochemical data offers the possibility to measure the pace and magnitude of past climate deteriorations – and their long-term consequences.