Taking the Temperature of a Late Cretaceous Greenhouse World
A recent study by researchers at Oxford Earth Sciences has discovered new proxy data indicating sea-surface temperatures at high southern latitudes reached over 35°C during a period of extreme greenhouse climate that began about 100 million years ago. These findings were published in February 2019 in Paleoceanography and Paleoclimatology, and summarised in an Eos article by Freelance Writer Terri Cook, adapted excerpts of which are included below. For the full article, please click here.
During the Late Cretaceous, the Earth was a greenhouse world characterised by extreme temperatures and high concentrations of atmospheric carbon dioxide. Records from the Falkland Plateau in the southern Atlantic Ocean suggest that during the period of peak warmth, which lasted from about 100 to 90 million years ago, sea surface temperatures at middle to high southern latitudes exceeded 30°C, conditions that were significantly hotter than today’s mean annual average of 0°C. However, because these data have been difficult to reconcile with climate models and other proxy records, scientists have debated whether they truly reflect global climatic conditions.
In a recent study, Oxford Earth Sciences Researcher Dr Lauren O’Connor and colleagues, further explore this question using TEX86, an organic palaeothermometer regularly used to reconstruct past sea-surface temperatures. The team applied this technique to Upper Cretaceous sediment samples collected at Deep Sea Drilling Project drilling Sites 327 and 511 on the Falkland Plateau as well as Ocean Drilling Program Sites 1138 and 1135 on the Kerguelen Plateau in the southern Indian Ocean. All four locations lay between 50°S and 60°S from about 100 to 66 million years ago.
Palaeogeographic reconstruction of the continents during the Turonian (Late Cretaceous; adapted from Blakey, 2016). Stars represent the Deep Sea Drilling Project/Ocean Drilling Program Sites used in the O’Connor et al. (2019) study. Circles are previously published data from other sites for deep sea drilling locations. Base map shows late Turonian (90 Ma) paleogeography (adapted from Blakey, 2016).
The data indicate that Late Cretaceous sea-surface temperatures ranged from 27°C to 37°C at these locations. Since these results are comparable to other local proxy data as well as the global TEX86 records, the authors conclude their data accurately reflect long-term global trends and therefore corroborate previous interpretations of extreme warmth during the Late Cretaceous at Earth’s subpolar latitudes. The results demonstrate that after sea-surface temperatures peaked during the Cenomanian-Turonian time interval, around 94 million years ago, Earth’s climate experienced a slow and steady cooling trend that lasted for at least 16 million years.
These findings also highlight the discrepancy between proxy records and climate model simulations, which struggle to reproduce such warm conditions at middle to high latitudes. Although modelling studies suggest that a combination of drivers was necessary to sustain these high temperatures, the relative influence of volcanic carbon dioxide emissions, water vapour, methane, and other potential agents is still hotly debated. These high temperatures have important implications for global heat transport and polar ice in a high-carbon dioxide world, an understanding of which is important for modelling both past and future climate change.
Citations:
Cook, T. (2019), Late cretaceous extreme warmth at high southern latitudes, Eos, 100, https://doi.org/10.1029/2019EO120391. Published on 22 April 2019
O’Connor, L. K., Robinson, S. A., Naafs, B. D. A., Jenkyns, H. C., Henson, S., Clarke, M., & Pancost, R. D. (2019). Late Cretaceous temperature evolution of the southern high latitudes: A TEX86 perspective. Paleoceanography and Paleoclimatology, 34. https://doi.org/ 10.1029/2018PA003546.
Blakey, R. (2016). Global paleogeography. Retrieved from www2.nau.edu/rcb7/globaltext2.html