We need to understand the conditions of earlier eras to make sense of climate change data. But past conditions were complex; the Jurassic world was, after all, not an unbroken vista of volcanoes and steamy jungle.
Timothy Myers, a palaeontologist at the Southern Methodist University in Dallas, collected 44 samples of ancient soils, or paleosols, from parts of the Morrison formation in the US south-west.
Researchers may assume that soil type gives a good indication of the prevailing climate. The reasoning, here, was that New Mexico, as today, would have been more arid than Wyoming and Montana further to the north.
Yet when Myers did the first detailed quantitative study, he found that mean average rainfall was 76cm (30in) for New Mexico and 114cm (45in) for north Montana, while being just 51cm (20in) in northern Wyoming.
There were big differences even between adjacent sites. “The apparently sudden shift from dry to relatively wet environments over such a short distance was perplexing,” said Myers. Normally this sort of pattern would only occur on opposite sides of a topographic feature, such as a mountain range.
Myers suggested that precipitation could have been highly seasonal at the Wyoming site, and this could have distorted the estimate for the total rainfall.
The findings indicate significant local variation in the Jurassic era climate, so we must be wary of generalising about entire areas from single sites. “Regions with broadly similar climates can have internal differences, even over short distances.”
