Research paper
Calcareous nannofossils from the Great Australian Bight (IODP Site U1512) as a record of the peak and early decline of the mid-Cretaceous super greenhouse

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Highlights

  • Calcareous nannofossil biostratigraphy at Site U1512 (paleolatitude ~60°S) directly correlative to low latitude zonations.

  • Indications local paleoecological restrictions truncated some ranges and excluded some taxa.

  • Surface waters evolved from very warm, relatively fertile in Turonian to cooler, more oligotrophic in Coniacian–Santonian.

  • Surface water cooling from early-middle Turonian hot greenhouse began ~92 Ma with accelerated cooling evident ~91.2 Ma.

Abstract

International Ocean Discovery Program drilling in the Great Australian Bight (Site U1512) provides a record of Turonian to Santonian hemipelagic sedimentation deposited in an elongate epicontinental basin that opened westward to the Southern Ocean during the peak and the early decline of the Cretaceous hot greenhouse. Despite its relatively high paleolatitude (at about 60°S), the biostratigraphy of calcareous nannofossils from this succession is directly correlative to low-latitude zonation schemes throughout the Turonian to Santonian. These calcareous nannofossil assemblages are somewhat restricted in species composition, lacking some elements generally found in Late Cretaceous assemblages (e.g., holococcoliths). In addition, two biostratigraphically-important high latitude taxa (Thiersteinia ecclesiastica, and Zeugrhabdotus kerguelenesis) have biostratigraphic ranges notably shorter than elsewhere, indicating range truncation through local extirpation.

Quantitative paleoecological analysis of the calcareous nannofossil assemblages indicates that the major change in the paleoenvironment was the evolution from conditions that were quite warm and had relatively elevated surface water fertility in the Turonian to cooler and possibly more oligotrophic conditions in the Coniacian–Santonian. The first cooling began at about 92 Ma, accelerated at about 91.2 Ma and then continued to gradually cool during the Coniacian–Santonian. This paleotemperature scenario is consistent with those documented from other localities globally.

Introduction

The Late Cretaceous was a critical time in Earth history, including a peak of global temperatures (variously referred to as the “super greenhouse” or “hot greenhouse”) in which tropical sea surface temperatures exceeded 35 °C in the early Turonian (Huber et al., 2002; Wilson et al., 2002; Friedrich et al., 2012; O'Brien et al., 2017; Huber et al., 2018). Much of our knowledge of the nature and behavior of the oceanic surface waters and their calcareous nannofossil assemblages at high southern latitudes during the Late Cretaceous is currently based on sedimentary records from four areas, including Maud Rise (Campanian–Maastrichtian only) and the Falkland, Kerguelen, and Naturaliste plateaus. Mesozoic rifting between Australia and Antarctica resulted in an epicontinental basin on thinned continental crust in the Great Australian Bight (GAB) during the Turonian to Santonian that was flooded by a seaway which connected to the growing Indian Ocean to the west (Fig. 1). The International Ocean Discovery Program (IODP) drilled a single site (U1512) through much of the Upper Cretaceous to recover a record of the geological history of the GAB basin. The sedimentary succession at this site provides material to examine the evolution of the calcareous nannofossils from a new area of the Southern Ocean. The purpose of this study is to examine the biostratigraphic and paleoecologic character of these calcareous nannofossil assemblages to assess the paleoceanographic evolution in this area during the peak and subsequent early decline of the mid-Cretaceous hot greenhouse climate.

Section snippets

Geological setting and materials

The opening of the Australian-Antarctic Basin was a prolonged and episodic process. Rifting began in the Callovian on the western side of the basin and progressed eastward through the Tithonian, resulting in crustal thinning and extension, subsidence, and development of a sublinear epicontinental seaway between the Great Australian Bight (GAB) and the northern coast of Wilkes Land, Antarctica (Powell et al., 1988; Sayers et al., 2001; White et al., 2013; MacLeod et al., 2020). Basin rifting

Methods

The Upper Cretaceous of Hole U1512A were sampled extensively for calcareous nannofossils to establish a biostratigraphic framework and examine the paleoecological evolution of the assemblages. Calcareous nannofossil presence was not continuous through the cored interval, with a thick interval from approximately 220–292 m apparently barren of any calcareous microfossils (Fig. 2). Over 200 samples were examined and considered for biostratigraphy, and a subset of these were used for

Results

More than 160 samples were examined to biostratigraphically characterize the Upper Cretaceous section at Site U1512. This count does not include the many samples examined in a fruitless search for nannofossils from about 72 m of barren black mudstone (Fig. 2) from 220 to 292 m (Cores U1512A-23R to 31R). Assemblage counts and additional scans from 139 samples are reported in Watkins and Guerra, 2020.

Discussion

Site U1512 was located in an elongate, restricted basin at about 60°S latitude during the Turonian to Santonian, in which hemipelagic sedimentation was dominated by fine-grained clastic influx. Its depositional setting and paleogeographic location influenced the nature and preserved record of the calcareous nannofossils that inhabited the surface waters. Examination of the preserved calcareous nannofossil assemblages from this site indicates an interesting history of the development of this

Conclusions

Despite the geographically isolated nature of the Great Australian Bight basin at high paleolatitudes (near 65°S), nannofossil assemblages deposited during the lower and middle Turonian thermal maximum are compositional and structurally comparable with those from temperate and tropical regions, allowing direct application of low-latitude zonation schemes for biostratigraphic placement (in Zones CC10b - CC12 and correlatives). Thiersteinia ecclesiastica, a high latitude species, does appear at

Data availability

The dataset of calcareous nannofossil census counts for this article can be found at Watkins and Guerra, 2020, an open-source online data repository hosted at Mendeley Data.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research used samples provided by the Ocean Drilling Program (ODP), which is sponsored by the National Science Foundation (NSF) and supported by the U.S. Science Program (USSSP) and its international principal contractor, Joint Oceanographic Institutions (JOI), Inc. Funding for this project was provided by United States Science Support Program of the International Ocean Discovery Program and in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) -

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