Torrid times in the Cretaceous Arctic

Despite its latitude (above the Arctic Circle) the sedimentary depocentre of northern Alaska is becoming famous for its Cretaceous terrestrial flora and fauna.  Plant remains indicate luxuriant vegetation cover, and high excitement greeted the discovery of 8 species of dinosaurs (4 herbivores and 4 theropod predators (Fiorillo, A.R. 2004.  The dinosaurs of Arctic Alaska.  Scientific American, v. 291(6), p. 60-67).  How dinosaurs were able to survive the darkness of the Arctic winter is a bit of a mystery, unless the migrated as do modern caribou – Fiorillo cites evidence for small juveniles that would have been unlikely to have migrated far, because compared with adults they were much smaller than young caribou.  There would have been sufficient winter biomass for survival during the Cretaceous, but seeing and being active as cold-blooded reptiles pose problems.  At least one of the species had unusually large eyes, so one of the conditions for dinosaur’s remaining year-round seems established.  New data regarding climatic conditions in the far north have turned up after an most unusual and intrepid programme of drilling through a drifting island of pack ice over the Arctic Ocean’s Alpha Ridge, not far short of the geographic North Pole.  An extraordinary feature of the programme is that it took place between 1963-74, the core having only been examined in detail in the last year (Jenkyns, H.C. et al. 2004.  High temperatures in the Late Cretaceous Arctic Ocean.  Nature, v. 432, p. 888-892).  The Late Cretaceous part of the cores is black mud rich in terrestrial vegetation remains and marine diatoms, and totally lacking in evidence for dropstones and other debris from floating ice shelves.  Unfortunately, the Arctic sediments lack carbonate-shelled plankton remains,  so the now standard method of sea-surface temperature measurement is not possible.  However, Jenkyns et al. were able to use a method based on the fatty acids that survive in plankton membranes, results from which match oxygen-isotope palaeo-temperature measurements in Cretaceous cores from lower latitudes.  Astonishingly, even at polar latitudes, the Cretaceous Arctic Ocean seems to have been as warm as 15°C.  Climate modelling based on lower latitude data and estimates of CO2 concentration in the Late Cretaceous atmosphere falls around 10° short of these levels.  The conventional modelling requires 3 to 6 times more “greenhouse” warming than generally accepted, to account for Arctic sea temperatures in which we could swim in moderate comfort.  Possibly the modelling is awry.  One of the most important features of Late Cretaceous palaeogeography was a major seaway across North America that connected the Arctic with tropical latitudes.  It existed because global sea level was far higher than now, probably due to the oceans’ volume having been substantially reduced by huge magmatic outpourings on the floor of the West Pacific basin (the Ontong-Java Plateau), earlier in Cretaceous times, together with higher rates of sea-floor spreading.  The seaway would have been shallow, and thereby easily warmed.  Had poleward currents been possible in it, their flow would have acted very like the modern Gulf Stream to warm high latitudes.  Despite palaeoclimatologists reliance on models of heat circulation, it needs to be remembered that they are based on grossly simplified geographic features.  If they get it very wrong indeed for the well-studied Cretaceous, that casts doubts on climate modelling’s predictive powers for the course of current climate evolution.

See also: Poulsen, C.J. 2004.  A balmy Arctic.  Nature, v. 432, p. 814-815

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