Accepted geoscientific ‘wisdom’ is that the Cretaceous Period was so warm that forests reached polar latitudes and so too did cold-blooded reptiles. Planktonic foram oxygen isotopes indicate that the Cretaceous ‘hothouse’ in the Turonian (93.5-89.3 Ma) produced tropical sea-surface temperatures up to 37°C; warmer than human blood temperature. It also saw sea level reach an all time high. Both features have been attributed to the rate of ocean-floor volcanism being at its highest. It has, however, been difficult to model the warmth at high latitudes without fudging the input to general circulation models.
Measuring d18O in both planktonic and benthonic (ocean-floor) forams at centimetre spacings in Turonian ocean-floor sediments seems to have truly bamboozled specialists in the Cretaceous. They reveal a period of ~200 ka at around 91.2 Ma where both show a sharp increase (Bornemann, A. and 8 others 2008. Isotopic evidence for glaciation during the Cretaceous supergreenhouse. Science, v. 319, p. 189-192). Respectively, the peaks reflect decreased sea-surface temperature (but only down to 32°C in the tropics) and an increase in the extraction of light 16O from the oceans; only likely when ice caps build up on land. The size of the benthonic d18O increase suggests ice caps about half the size of that now blanketing Antarctica. Other evidence includes rapid decreases in Turonian sea level in Europe, North America and Russia; only likely on such a scale as a result of glacio-eustasy. However, direct evidence in the form of tillites, striated pavements and glacio-marine sediments has yet to turn up
Until these convincing data emerged, it seemed that sufficient post-Permian frigidity for large-scale glaciation had not developed until Oligocene times. However, the paradox of high-latitude ice caps and low-latitude balmy seas is resolvable. Evaporation from the tropical sea surface would have been much greater than nowadays. Transport of moisture to cooler areas may have resulted in such immense winter snowfall at high latitudes that sufficient remained unmelted after winter darkness for its albedo to further cool the polar region. Almost certainly the site for the ice cap would have been Antarctica, which in the Cretaceous, as now, sat over the South Pole. Remove the present ice, and that continent would have had an average surface height of between 1 and 2 km that would have encouraged snow build up were sufficient to have fallen during the Turonian. Yet without the direct evidence for glaciation in sediments – much would be buried by the present Antarctic ice cap, if not eroded away – the scenario is difficult for some to believe.
Holocene cold spell and glacial lake burst
The most startling event during the gradual warming after the last glacial maximum was the millennium of icy conditions between 12.5 and 11.5 ka; the Younger Dryas. Long after Holocene warmth seemed well established and agriculture had been underway for two millennia, with perhaps increased human population, a smaller cold ‘snap’ took place, between 8.21 and 8.17 ka; i.e. for about 70 years. Its main effect was around the North Atlantic, but it was felt over the whole hemisphere. It must have been devastating for early farmers and new migrants into higher latitude lands. High-resolution records of many kinds are possible for such a young event, from both ice and marine cores, and also terrestrial pollen records. Norwegian, French and Dutch climate researchers have gleaned a great deal from a sea-floor core from between southern Greenland and Labrador (Kleiven, H.F. et al. 2008. Reduced North Atlantic deep water and the glacial Lake Agassiz outburst. Science, v. 319, p. 60-64). Their combined fossil, oxygen-isotope and mineralogical study shows anomalies from about 170 years before to 100 years after the drop in regional temperatures. These include signs of decreased saltiness of the water in the Labrador Basin and a reduction in production of deep water in the North Atlantic. This is exactly the predicted signature for a shut-down of the Gulf Stream, similar to those implicated in Dansgaard-Oeschger events through the last Ice Age and the Younger Dryas itself.
The Younger Dryas has been linked to sudden drainage of huge glacially dammed lakes that once surrounded the ice cap of the Canadian Shield. One scenario for that is a huge, protracted flood down the St Lawrence River into the North Atlantic, another being one down the MacKenzie River into the Arctic Ocean. Freshening of surface waters by such means would have reduced the formation of the dense cold brines that sink to form North Atlantic Deep Water today. In so doing these down-wellings drag surface waters northwards from low latitudes to form the Gulf Stream that makes the western side of the North Atlantic unusually warm. If they stop or slow significantly regional air temperatures fall, as they did again around 8.2 ka. In this case the likely cause was escape of water melted from the last dregs of the North American ice sheet that had been held in a glacial lake south of Hudson Bay: Lake Agassiz.