Journal of the Geological Society, London, Vol. 147, 1990, pp. 329-341, 9 figs, 1 table. Printed in Northern Ireland Late Cretaceous-early Tertiary palaeoclimates of northern high latitudes: a quantitative view ROBERT A. SPICER' & JUDITH TOTMAN PARRISH2 Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK 2Department of Geosciences, Gould-Simpson Building, University of Arizona, Tucson, Arizona 85721, USA Abstract: Analyses of plant community structure, vegetational and leaf physiognomy, and growth rings and vascular systems in wood provide qualitative and quantitative data that can be combined to definenon-marine palaeoclimatic parameters with better resolution than is available from other, principallysedimentological, methods. Application of thesetechniques to Cenomanianthrough Paleocene floras from high palaeolatitudes (75"-85"N) indicates a polar light regime similar to that of the present. Plant data suggests Cenomanian sea level mean annual air temperatures (MATS) of 10 "C, and MATs of 13 "C, 5"C and 6-7 "C in the Coniacian, Maastrichtian, and Paleocene respec- tively. Evapotranspirational stresses at sea level were low and precipitation was in most part uniform throughout the growing season in the Cenomanian, with possible seasonal drying occurring by the Maastrichtian. Maastrichtian winter freezing was likely, but periglacial conditions did not exist at sea level. Permanent ice was likely above 1700m at 75"N in the Cenomanian, and above 1OOOm at 85"N in the Maastrichtian. These near-polar data provide critical constraints on global models of Late Cretaceous to early Tertiary climates. The Earth's climate is in a real sense defined by conditions Clark 1982), with reference to the Tertiary or Pleistocene, at the poles (Goody 1980). Changes in the equator-to-pole oron ice cap distribution and dynamics. Despiterecent thermal gradient, which is animportant control on suggestions tothe contrary (Frakes & Francis 1988), the atmospheric and oceaniccirculation, are expressedprin- weight of the geological evidence is that the Cretaceous was cipally as variations in polar temperatures (e.g., Shackleton ice-free (Frakes 1979; Hambrey & Harland 1981), at least at & Boersma 1981; Barron 1983a; Crowley 1988). At the NorthPole. However, as discussed in this paper, the present, the source of deep-ocean water is in high latitudes, geological and palaeontological evidence does permit the and this bottom water is cold and saline(Sverdrup et al. existence of montane glaciers and winter snow. 1942). By contrast, oceanic deep water in the past may have In recent comprehensive reviews of global palaeoclimatic been warm and saline and generated in low latitudes (Brass modelling and data (Barron 1983b; Lloyd 1984; Kutzbach et al. 1982). Significant ice forms most easily in the polar 1985; Covey & Barron 1988; Crowley 1983, 1988), particular regions (Goody 1980), and thus climate at the poles is a attention has been paid to climatic processes in the polar major determinant of global sea level. Seasonality of regions, in recognition of their importance to global climate. temperature is expressed most strongly at the poles (Barron Much of the effort has been directed toward explaining the 1983a; Kutzbach & Guetter 1986) and, by extension, must Cretaceous ice-free state. Part of this discussion has been have a powerful effect on the distribution and productivity concerned with understanding the boundary conditions for of global biota (e.g., Wolfe 1985). Thus, for any timein the formation of ice. Many of the processes involved are global palaeoclimatic history,understanding the climatic inadequately understood, partlybecause the models and conditions at the poles is a necessary part of understanding data are poorly constrained (Crowley 1988). the entire global climate system. The value of modelling exercises will dependon The purpose of this paper is to bring together data on quantitative data obtained from the geological record itself the palaeoclimate of theNorth Pole during theLate in order to either establish realistic boundary conditions or Cretaceous. The data are from fossil leaf and wood floras provide tests equal in resolution tothe model results. andsediments of theNorth Slope of Alaska andsome Because much of the numerical modelling has been in the elements are presented in greater detail in Spicer & Parrish form of sensitivity tests (e.g. Barron & Washington 1982a, (1986, 1987, 1990), Parrish et al. (1987), and Parrish & b, 1984, 1985; Rind 1986), having accurate palaeoclimatic Spicer (1988a,b). In order toput this information in context, data is particularly important in orderto evaluate the we will first briefly review some of the key features of polar applicability of the models to any given problem. To date, palaeoclimates to which the North Slope data are pertinent. evaluation of even the crudest models has been limited to In addition, we hope to show that the North Slope data fill a qualitative tests, forexample, matching the known critical gap in our knowledge of the Cretaceouswarm Earth. distributions of climate-relateddeposits, such ascoal and evaporites, with model predictions. Quantitativeevaluation of sedimentological data has Polar and global climate remained an intractable problem, althoughconsiderable Reviews specifically of polar palaeoclimates have tended to effort is now underway to overcome it (e.g. Parrish 1988). focus on either simply the presence or absence of ice (e.g., The problem has been particularly acute for the terrestrial 329 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/2/329/4890457/gsjgs.147.2.0329.pdf by guest on 01 October 2021 330 SPICER A. R. & J. T. PARRISH realm, where data are relatively scarce, and for the polar distribution of light throughout the year in order to maintain regions, where material suitable for isotopic determinations the evergreen state. Numerical models of this condition are rarely found. The North Slope data can help provide predict a reduction of polar temperatures compared to some of the necessary constraints to define accurately the present (Barron 1984); a result that demands a re-evaluation climatic stateto be modelled. The following discussion of either the models, the palaeovegetation, or both. outlines some of the key problems in understanding polar palaeoclimates. Heat transport A persistent problem in understanding warmer polar regions Palaeogeography and,therefore, lower equator-to-pole temperature gradi- A priori, a polar continental ice sheet cannot form unless ents, is the problem of heattransport (Barron & there is land near the pole. Polar land reduces the efficiency Washington 1985; Schneider et al. 1985; Rind 1986; Covey of oceanic heattransport and provides a sitefor the & Barron 1988; Crowley 1988). Theamount of heat accumulation of snow, both of which lead to cooling (Covey transported by atmosphere and oceans varies with latitude & Barron 1988). Thus, hypotheses about continental (Newell 1974), and heat transport in the atmosphere can be glaciation through time have concentrated on continentality increased through latent heat transport, possibly resulting in as a key factorin ice capformation (Cox 1968; Crowell an increasein rainfall at higher latitudes (Manabe & 1982), and the role of palaeogeography in climatic change Wetherald 1980). However, models that specify warm polar has been a focal point for modelling experiments (Donn & sea-surface temperatures (Barron & Washington 1984), or Shaw 1977; Barron et al. 1984; Barron & Washington 1982b, that allow for infinitely efficient oceanic heat transport (that 1984; Barron 1985). Donn & Shaw's (1977) models is, no thermal gradient in the oceans; Schneider et al. 1985), predicted a temperature decrease during the Mesozoic and resulted in temperatures that were low in the continental Cenozoic resulting from northward movement of land into interiors,as discussed above.Changes in latent versus the polar regions, and although flaws in the model made this sensible heattransport (e.g. Manabe & Wetherald, 1980) cooling a foregoneresult (Barron 1983b), the general also may be insufficient (Barron & Washington 1985; Covey conclusion has beensupported in subsequentstudies & Barron 1988; Crowley 1988). (Barron et al. 1984; Barron & Washington 1982a, 6, 1984; Barron 1985). Temperature Higher than present temperatures in theCretaceous werepredicted by changes in continental positions. In Barron & Washington's (1982~)models, the predicted Cretaceouscontinental positions resulted in temperatures thermal gradient in January on the North Slope was very 4.8 "C above present global average. However this increase steep, >20"C within a few degrees latitude, and the 7 "C was countered by -1.1 "C (3.7 "Cabove present temperature) isotherm was parallel to the coast.Barron & Washington when Cretaceoustopography was added(Barron & (1984), using a newer version of their global circulation Washington 1984, 1985). Inaddition, the coldestregions model, reported a mean annual temperature of about 0-2 "C predicted for the continental interiors tendedto occur on or for the North Slope region, much lower than the estimates near topographic highs (Barron & Washington 1984; we report here. (However, it should be borne in mind that Schneider et al. 1985). Removal of topographyraised the these models were sensitivity tests, and not necessarily temperatures in these areas 6 "C or less (Barron & intended to be realistic in detail).Increased CO, may Washington 1985). contain part of the answer.For