A weakness of my combined geological temperature record graph has always been the Royer / Veizer temperatures for the Cretaceous (66-145 My) and earlier. Unfortunately some people don’t read caveats and just take the thing as gospel, even citing actual numerical values from it (e.g. in the data box on the right here — must fix that sometime).
The problem is that the Royer / Veizer temperature estimates are derived from isotope ratios measured on the fossil shells of essentially tropical, shallow-water, marine organisms (mainly brachiopods). In the hothouse conditions of the mid-Cretaceous (80-100 My), tropical temperatures are thought to have increased relatively little (there is dispute about how much), while polar temperatures increased by a huge amount (maybe 20°C). The temperature gradient between the equator and the poles collapsed in response to massive atmospheric CO₂ levels — five times today’s. The result is that the Royer / Veizer tropical data seriously underestimates the global average change, even after my “x2” fudge (I multiply the plotted Royer / Veizer temperature anomalies by 2.0 as a crude attempt to globalise them).
To begin to address the problem, I’ve now added some Cretaceous data from a fairly recent paper by Oliver Friedrich and collegues¹, which the professor kindly provided for the purpose. Like the data used right through 0.02 to 66 My, the Freidrich et al collection comprises stable oxygen isotope ratios from the shells of tiny bottom-dwelling, deep-water marine organisms (benthic foraminifera). Because the Friedrich data is much older the spatial coverage is rather poor (a lot of sea floor from back then has been subducted away under drifting continents), and some data sites need to be excluded to counter extraneous effects². Nevertheless the result appears to provide a more realistic global picture than Royer / Veizer, consistent with the standard view of Cretaceous palaeoclimate³. For interpretation, I’ve just applied the Hansen et al prescription used for the more recent benthic records, which is perhaps a bit of a stretch but arguably not terrible. The result looks like this:
You can see that the little bit⁴ of Friedrich et al trace is substantially higher than Royer / Veizer, but much the same shape. The top of it is cut off (it spikes to nearly +23°C) but that doesn’t trouble me much because the extreme highs may be dubious (data from just a couple of sites).
Further details are given on the updated graph page, which is now here: https://gergs.net/?attachment_id=4308
Notes and references
- Friedrich, O., Norris, R.D. & Erbacher, J. (2012). Evolution of mid- to Late Cretaceous oceans – A 55 million year record of Earth’s temperature and carbon cycle. Geology 40: 107-110.
- I’m using the trimmed data from the supplementary figure 3.
- Wilson, P. A., Norris, R. D., & Cooper, M. J. (2002). Testing the Cretaceous greenhouse hypothesis using glassy foraminiferal calcite from the core of the Turonian tropics on Demerara Rise. Geology, 30(7), 607-610.
- It only looks little because of the varying timescales. Actually it covers about 50 My, nearly as long as the whole four panels to the right.
