Late Cretaceous-Early Tertiary Palaeoclimates of Northern High Latitudes: a Quantitative View

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' & JUDITHTOTMAN 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 thepresent. 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 inthe 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), andthus 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 hasbeen 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

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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 rarelyfound. 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 example, experiments by Sea level had a minimal effect on global temperature; a Barron & Washington (1985) achieved 3 "C mean annual small temperaturedecrease was observedin very high temperature at the Cretaceous poles with CO2 four times latitudes in high-sea-level experiments (Barron & Washing- (4~)its present level; this approaches the observed value ton 1984). As discussed by Barron & Washington(1984), for the latest Cretaceous temperatures on the North Slope this is rather surprising because climatic indicators, such as (see later sections). However,the latest Cretaceous isotopic temperaturedeterminations and biogeographic temperature is also the coolest observed temperature for the changes, track the first-order sea-level curve (Frakes 1979; epoch, and the same experiment (4 X CO2) also produced Vail et al.1977), suggesting that sea level might be a tropical temperatures that approach the maximum observed first-order control on climate. Barron & Washington (1984) from 6l80 data(Barron & Washington 1985). Thus, speculated that the failure of sea-level changes to account increasing CO2to raise the polar temperaturesto for any climate change might reflect an as-yet-unresolved mid-Cretaceous levels would make the tropical temperatures weakness in the models. In addition, even relatively small too high (Barron & Washington 1985). changes in sea level, as expressed in changes in the area of exposed land, can result in large changes in albedo (Barron Seasonal cycle et al. 1980; Parrish 1985). Vegetation changes also can have Qualitatively, it has generally been assumed that more land an effect on albedo, and although the changes probably are at the poles means lower temperaturesand that lower small, they are not climatically insignificant (Barron 19366; temperatures are responsible for ice-cap formation (Donn & Dickinson 1984). A reliablecharacterization of the Shaw1977; Crowell 1982).Although this is true as a vegetation would help estimate this parameter. first-order approximation (Barron et al. 1981; Barron & The occurrence of fossils representing putative evergreen Washington 1982a, 1984), additional conditions must be met plants at high latitudes has prompted suggestions that the for ice-covered poles,just asconditions otherthan Earth'sobliquity was much less thanat present (Allard palaeogeography are apparently responsible for warm poles. 1948; Wolfe 1978, 1980; Douglas & Williams 1982). This Crowley et al. (1987) showed that, for a large polar reduction was deemed necessary to provide a moreeven continent, ice-cap formation may be inhibited in the

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maximum continentality case, that is, when the continent is centred overthe pole, because,although winter temperatures are lower,summer temperaturesare too high for retention of snow cover. In addition, a supply of moisture is critical to ice-cap formation (Aksu et al. 1988); alarge, pole-centredcontinent inhibits thetransport of moisture into the coldest regions, further reducing snow cover. Even a smaller polar continent, such as Antarctica, might remain ice-free without some triggering mechanism other than polar position. North & Crowley (1985) calculated that with an ice-free Antarctica, polar mean annual temperatures would be 6-7°C; freezing temperatures would be realized only in the surrounding oceans. North et al. (1983), North & Crowley (1985) and :S RANGE A A Kutzbach & Guetter (1986) emphasized thatthe seasonal ~~~ cycle, not mean annual temperature per se, may be more -Braaed River m-- MeanderingRiver ? lop km important for ice-cap formation. Under certain regimes of ...... Present Shorellne orbital parameters, low winter temperaturesare offset by high summer temperatures, so that snow from the winter is 1. Palaeogeography of Corwin and Umiat deltas in always meltedduring the summer. Cool summers, on the Fig. Albian-Cenomanian times (After Huffmanet al. 1985). Approxi- otherhand, permitaccumulation of snow (North et al. mate palaeolatitude based on Smithet al. (1981). 1983). Thus, in simulations of the Pleistocene glaciations, the presence of land in thenorthern hemisphere is important (Donn & Shaw 1977), but the patchiness of that breaks in the non-marine sequence are in the Turonian and land, with extensive waterways moderatingsummer Santonian.Plant debris occurs ain wide range of temperatures (and providing a source of moisture), also is sedimentary facies throughout the non-marine sequences. important (North & Crowley 1985). Forthe Cretaceous, The Late Cretaceous rocks of the North Slope are well high sea level has only a slight cooling influence on mean exposed along the Colville River (Fig. 2) and consist of annual temperature (-0.1 "C; Barron & Washington 1984). intertonguing marine and non-marine units that are divided However, the model used by Barron & Washington (1984) into two groups: the Albian-Cenomanian Nanushuk Group, does not include a seasonal cycle, so the result of greater and the Turonian-Maastrichtian Colville Group (Fig. 3). patchiness brought about by higher sea level in the The Nanushuk group comprises the marine Tuktu, Cretaceous is difficult to evaluate. Grandstand and Ninuluk Formations, and the non-marine North & Crowley (1985) also emphasized the importance Killik and Niakogon Tongues of the equivalentChandler of thresholds and non-linearity in polar climate and ice-cap Formation (Gryc et al. 1951; Detterman in Gryc et al. 1956). formation. A climate close to a so-called 'critical point' may The Niakogon Tongue is Cenomanianin age based on be shifted into one of two stable states with only a small angiosperm leaf fossils (Smiley 1967; Spicer 1983) and on change in external forcing, such as CO,. Close to a critical Inoceramus dunveganensis McLearnfrom the Ninuluk temperature for glaciation, a change of as little as 1 "C might Formation, with which the Niakogon interfingers (Detter- result in ice-cap formation.Thus, for example, cooling man et al. 1963). The depositionalenvironments of the might proceed with no linear response in ice cap formu!ation Nanushuk Group have beensummarized in papers in until a critical point is reached. A small increment of cooling Ahlbrandt (1979) and Huffman (1985). past the critical point would establish the glacial stable state; The marine Seabee Formation represents the lowest part the same small increment of warming, however, would be of the Colville Group (Whittington in Gryc et al. 1956) and insufficient to return the system to the critical point. Thus, is dated as Turonian on the basis of marine invertebrates as emphasized by North & Crowley (1985), an ice-free pole is not necessarily a warm pole.

The palaeobotanical record

Geology The plant fossil record of Northern Alaska is particularly rich fromAlbian Eoceneto times (Spicer 1990; Detterman & Spicer 1981). The uplift of the Brooks Range began in Berriasiantime and continued intothe Aptian (Mull 1979, 1985). Further uplift took place between Albian SACAVAh'IRKTOK and Turonian times, and sediment was shed into the Colville Basin tothe north. Two large,river-dominated delta complexes developed: the Colville and Umiat Deltas (Fig. 1). These deltas prograded to the north and east throughout 1 / I the Late Cretaceous and into the Eocene, and vegetation Fig. 2. Map of Colville and Sagavanirktok River region of the flourished onthe extensivedelta plains. The only major North Slope of Alaska.

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Formation are exposed to the east, along the Sagavanirktok LITHOSTRATIGRAPHY River at Sagwon. The oldest exposed sediments appear to be of early Paleocene age based on pollen (T. Ager, pers. 5E commun. 1987), although when originally described, the .E L Sagwon Member Cretaceous/Tertiaryboundary was positioned within the exposed section. The positioning of the boundary was always in doubt, however, because it was based onthe degree of induration of the sediments (Detterman et al. 1975). Palynological work is continuing to define the stratigraphic range of the exposed units. It is possible that ...... most, if not all, of the Paleocene may be represented...... All of the Late Cretaceous and Paleocene rocks of the Y ...... North Slope consist of similar suites of sedimentary facies, e, ...... and represent a mosaic of depositionalenvironments 0 ...... Schrader Bluff ...... common to fluvio-marine deltas. Some differences do exist e, ...... _c0 ...... : between the Nanushukand Colville Groups.In general L ...... 4...""""""""""""""""""" a ...... v....,., Coniacian to Paleocene rocks have a higher frequency of occurrence of bentonites, and the Kogosukruk Tongue may represent an environment that was lower on the delta plain than the other units. It should be noted, however, that the ...... Ninuluk and Niakogon Tonguesinterbed on a finer scale than any units in the Colville Group. Plant megafosils have been recovered from all sediment types, which range from coals to fluvial sands and marginal marine facies. By E Killik Tongue sampling as many different sedimentaryenvironments as LL possible within a well-constrained stratigraphic framework, bothtemporal and spatialheterogeneity atthe plant community level canbe investigated, and taphonomic factors that may bias the climate signal may be understood.

Fig. 3. Stratigraphy of Late Cretaceous to early Tertiary rocks of Palaeogeography the central North Slope region, Alaska, based on BrosgC & Whittington (1966). The significance of the North Slope Cretaceous and early Tertiary sediments is not just that they are particularly rich in plant fossils and contain as much as one third of the total (Detterman et al. 1963) and a K-Ardate in bentonite US coal reserves (Sable & Stricker 1987), but also that they (Lanphere & Tailleur 1983). Overlying theSeabee were deposited close to the palaeo-North Pole. Throughout Formation is anothermarine unit, the Schrader Bluff theLate Cretaceous and early Tertiary,northern Alaska Formation,and its non-marineequivalents the Tuluvak was never farther south than7YN, and during the Tongue (Coniacian) andthe Kogosukruk Tongue(late Maastrichtian and Paleocene was about 85"N (Smith et al. Campanian to latest Maastrichtian), both subdivisions of the 1981; Ziegler et al. 1983). These positions are well Prince Creek Formation. The lower part of the Kogosukruk constrainedas there hasbeen little post-Cretaceous Tongue interbedswith the upper part of the marine Sentinel movement of the NorthSlope relative to cratonic North Hill Member of the Schrader Bluff Formation(Brosge & America (Jones 1983; Sweeney 1983). Based onthe Whittington 1966). Age diagnostic fossils are apparently distribution of coals andevaporites, Lottes (1987) places absent in the Sentinel Hill Member,but the underlying the Maastrichtian northern rotational pole within 4" of the BarrowTrail Member of the Schrader Bluff Formation palaeomagnetic pole. It is, of course, the rotational, not the contains Znocerarnus patootensis de Loriol, which is of palaeomagnetic, pole that influences atmospheric, oceanic, Santonian-early Campanianage (BrosgC & Whittington and biological systems. 1966). The upper part of the Kogosukruk Tongue is well exposed between Uluksrak Bluff and Ocean Point along the Colville River. The Kogosukruk Tongue hasbeen The plant record palynologically dated as mostly Maastrichtian, which the Prior tothe arrival of the angiosperms, the Corwin and CampanianIMaastrichtianboundary positioned within the UmiatDeltas supported a conifer-dominatedforest. Tongue just north of Sentinel Hill (Frederiksen 1986). Near Assessing true conifer diversity is difficult because several Ocean Point the Kogosukruk Tongue is overlain by, and taxa apparently exhibited polymorphic foliage. A number of possibly interbeds with, marginal marine sediments that ginkgophytes is known, with foliage forms such as have been dated as Paleocene on the basis of invertebrates Sphenobaiera or Sphenarion occuring in overbank deposits, and ostracodes (Marincovich et al. 1985, 1986) and as late whereas large, broadly digitateGinkgo-type leaves are Maastrichtian onthe basis of pollen (Frederiksen 1986; abundant in channel sands. Thick coals are generally devoid Frederiksen et al. 1986) and foraminifers (McDougall 1986). of megafossils except for compressed logs, but paper coals Apparently, theCretaceousfTertiary boundary is not yield leaves of the broadleaved conifer Podozarnites. exposed in the Ocean Point area. Rhizomes of Equiretites areabundant in fluvial margin, Paleoceneunits of the non-marineSagavanirktok overbank, lacustrine, and mire environments. A variety of

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ferns occur in crevasse splay andother overbank they may be ferns. In addition to the ubiquitous Equisetites environments, and leaves of the cycad Nilssonia are typically rhizomes, ferns are known from ponded environments and found in near-coastal settings. even found preserved three dimensionally within ash falls. The angiosperms first appear in the upper part of the Two seedforms andone largefruit form(up to 3 cm Lower Killik Tongue, in theform of leaves belonging to diameter) have also been found. the platanoid Protophyllum-Pseudoprotophyllum-Pseudo- In contrast to the thick (up to 5 m, but typically about uspidiophyllum complex. These replace Ginkgo-leaf produ- 1 m thick), mainly clastic-free coals in the Nanushuk Group, cers as riparian dominants. By late in the Cenomanian these the coals of the Kogosukruk Tongue rarely occur in seams platanoid leaves had expanded into lake-margin settings and thicker than 0.5 m and typically are about 20 cm thick. The were accompanied by a variety of entire-margined coals of the Nanushuk Groupare often woody, whereas angiosperm leaf forms. Lacustrinesediments also contain those of the Kogosukruk Tongue are usually devoid of logs both ferns and leaves of the delicate, highly digitate Ginkgo or woody remains. concinna. In the Sagwon Member of the Sagavanirktok Formation The Tuluvak Tongue of the Prince Creek Formation has angiosperm diversity rises (Table1) and angiospermous yielded two megafossil assemblages, one representing a vesseled wood occurs for the first time on the North Slope. small ponded environment and the other a fluvial channel. Although still conifer dominated,the vegetation was Platanoid leaves continued to dominate riparian situations, certainly richer in woody angiosperms than during but a variety of entire-marginedangiosperm leaves Kogosukruk and Nanushuk time. accumulated together with other platanoids in ponded environments.Although the sample size is small, cycadophytes and Podozamites both appear to be absent. Plant megafossils in the Kogosukruk Tongueare abundant but very limited in diversity. Only two leafy shoot The qualitative paleocliate signal forms of conifers have been recovered. The most abundant The application of Nearest Living Relative (NLR; see form is similar to modern Metasequoia, whereas the other is Chaloner & Creber this issue) techniquesfor the a very rare cupressoid form. Angiosperm leaves are interpretation of Cretaceous climates is a potentially flawed restricted to one specimen of a small lobed leaf referrable to exercise because it assumes stasis in environmental tolerance Hollickia. Otheraquatic leaves commonly referred to as in the face of demonstrable evolutionary change. Neverthe- Trapa but belonging tothe genus Quereuxia are usually less, with conservative groups such as the gymnosperms, considered angiospermous, but their true affinity is in doubt; NLRtechniques may be qualitatively employed to detect climatic trends if it can be demonstratedthat unrelated lineages are behaving in concert. In the case of the Late Cretaceous of the Alaskan NorthSlope, such trendsare seen in the progressive reduction of conifer diversity and Table 1. Diversity of Nanushuk Group, Kogosukruk the loss of certain elements such as cycadophytes and Tongueand Sagwon Member assemblages excluding ginkgophytes (Table 1) that persist at lower latitudes. Such fruits and seeds patterns suggest climatic deterioration. Climatic deterioration during the Late Cretaceous is also Nanushuk Kogosukruk Sagwon suggested by studies of structurally preserved wood. The use of tree ring studies is well established for the Quaternary Conifers (Fritts 1976) and is now becoming more widely used with 12 shootforms 2 shootforms 3 shoot forms more ancient material (Creber 1977; Jefferson 1982; Creber 4 male cones & Chaloner 1984, 1985, Chaloner & Creber this issue; 9 femalecones 1 femalecone 1 female cone Francis 1986; Parrish & Spicer 1988b). Woodsfrom the Ferns Nanushuk Group possess significantly wider rings than those 18 2 1 of the Kogosukruk Tongue, suggesting less benign growing conditions at the end of the Cretaceous (Spicer & Parrish Sphenophytes 1990). Furthermore, false rings, produced when growth is Equisetites EquisetitesEquisetites Equisetites temporarily interrupted by adverseconditions, are rare in Nanushuk Group woods but arecommon, with several Gikgopbytes occurring within eachring, in Kogosukruk woods. Inthe 4 0 0 Sagwon woods however there is a slight increase once more in mean ring width suggesting more benign summer Cycadophytes conditions (Fig. 4) (Parrish & Spicer in prep). 2 0 0 Using an NLR approach, the presence of cycad foliage at high palaeolatitudes has led to suggestions that the climate Livemorts was no cooler than ‘warm temperate’ (Smiley 1967; Scott & 1 0 0 Smiley 1979). HoweverKimura & Sekido (1975) have shown that the plants that produced Nilssonia foliage were Angiosperms quite unlike the modern, frost-sensitive evergreen cycads in 67 2 17 that they had slender vine-like stems (likely to have been Totals frost resistant) and were deciduous. Modem relict cycads 110 8 23 are thus a poor model for the morediverse Mesozoic forms. Taphonomicexperiments have shown that freshly

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3 I H Mean Ring Width (mm) €l Ratio of Latewood to Earlywood

Jul Aug Sept Oct NOV Dec Jan F& Mar Apr May Jun Months

-r Fig. 5. Percentage of days in the month with total darkness at Nanushuk Kogosukruk Sagwon Kogosukruk Nanushuk present day 85". Data from The Air Almanac (Anon 1983). Fig. 4. Comparison of Nanushuk, Kogosukruk and Sagwon tree ring characteristics based on Parrish& Spicer (19886), Spicer and Parrish (1990) and Parrish& Spicer (in prep). The ratioof latewood The quantitative signal to earlywood (100% earlywood = 1) was calculated on the basis of cell numbers. Angiosperm leaf margin analysis In addition to the qualitative palaeoclimate signal inherent abscized leaf material is remarkablyrobust (Spicer 1981; in plant fossils so far discussed, it is possible to use the Ferguson 1985). The observation that even large platanoid palaeobotanicalrecord to obtain quantitativeestimates of leaves arefound intact in fluvial sandsrepresenting ancient temperatures.The relationshipbetween the high-energy environments, and that overall very few leaves physiognomy of leaves fromarborescent dicotyledonous show any signs of mechanical or biological degradation, angiosperms and mean annual temperature wellis suggests strongly that almost all plant material was fresh at established (Bailey & Sinnot 1915; Wolfe 1979). However, burial and was therefore likely to have enteredthe the relationship is only fully developed for vegetation in its depositional system synchronously. Autumnal leaf fall climax state and when it is not growth-limited by lack of (deciduousness) is also suggested by the thin texture of all water. Of particular value for taxon-independent quantita- angiosperm leaves, and the morphology of conifer shoots tive determinations is the ratio of entire (smooth) margined shed as discrete units (Spicer & Parrish 1986; Spicer 1987~). leaves to those with toothed margins (Fig. 6). In general in With the exception of a microphyllous conifer that could thenorthern hemispherea 3% change in this ratio have entereddormancy, all LateCretaceous and early corresponds with a change in the mean annual temperature Tertiary North Slope taxa appear to have been deciduous, of 1 "C. In thesouthern hemisphere, which has ahigher to have died back to underground perennating organs, or to have survived the winter as seeds (Spicer & Parrish 1986; Spicer 1987~). A wholly deciduous flora precludes the necessity of invoking areduction in obliquity to provide a more even distribution of light throughout the year (cf. Allard 1948; Wolfe 1978, 1980; Douglas & Williams 1982). Evidence that 1 the Late Cretaceous light regime was similar to that of the present day comes from tree ring records. Tree rings from structurallypreserved Nanushuk Group woods exhibita high ratio of early wood to late wood and a rapid transition to the late wood (Fig. 4). An average ring width of 2.8 mm (maximum ring width of 12.9mm), coupled with rare false rings suggests thatthere were few constraints on growth throughout the summer and that the transition to dormancy 1 O( I I I I 1 80 60 40 20 was achieved rapidly (Pamsh & Spicer 1988b). Such a ring pattern is consistent with a light regime not demonstrably 1 Percent Entire-Margined Species different fromthat of today, which meansthat the Late Fig. 6. Relationship between the percentageof entire-margined Cretaceous and Paleocene North Slopeforest experienced woody dicotyledonous angiosperm leaves and mean annual rapid changes in spring and autumn daylength and between temperature for the northern and southern hemispheres using threeand four monthswinter darkness (Spicer 19876; climax vegetation in humid environments (based on Wolfe (1979) Parrish & Spicer 1988b) (Fig. 5). and Wolfe & Upchurch (1987~).

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proportion of evergreen leaves (possibly as a result of the Thirdly,although relationships with palaeolatitudecan Cretaceous/Tertiary boundary event (Wolfe 1987)), the be demonstrated throughout the Late Cretaceous, calibra- ratio is slightly different in that a 4% change is equivalent to tion of thethermal gradientpresents a problem. 1"C (Fig. 6) (Wolfe & Unpchurch 1987a). Because most Nevertheless temperatures in the high southern latitudes modern tooth typeshad evolved by theCenomanian obtainedfrom oxygen isotopestudies (Pirrie & Marshal1 (Spicer & Parrish 1986; Wolfe & Upchurch1987b) and 1988; Rich et a1 1988; Rich & Rich 1989) are comparable Cenomanian leaf margin ratios correlate with palaeolatitude with thosepredicted from similar northern latitudes using (Fig. 7) (Wolfe & Upchurch1987a), leaf margin analysis plants. Although more data are needed,we tentatively infer appears to have some validity for application to assemblages from this that the calibration of the plant-derived climatic as old as Cenomanian. signal is more or less correct. In addition, it is also worth Notwithstanding the early establishment of a relationship noting that leaf margin ratios track the qualitative climatic between angiosperm leaf margin ratios andmean annual signal throughout theLate Cretaceous and earlyTertiary temperature severalfactors may influence the reliable derived from diversity comparisons and tree ring data. application of thistechnique to theNorth Slope Of the 67 angiosperm leaf formsconsidered to be assemblages. First,as on all delta floodplains, not all the consistently recognizable in the Upper Killik and Niakogon vegetation sourcing the assemblages would have been Tongues about one third are entire-margined. This equates climax. Riparian communities, those that contribute greatly to mean annualtemperature of 10 "C (Spicer & Parrish to the fossil record, would have been subject to disturbance 1986; Parrish & Spicer 1988a) (Fig. 6).Although the by river migration (Spicer & Greer 1986). This problem has, deciduous nature of the vegetation demands the use of the in large part, beenovercome by examining assemblages northern hemisphere curve, at these low ratios of entire to from as many depositional environments as possible in order non-entire margins thesouthern hemisphere curve, if to detect ubiquitous background elements representative of extrapolated, would give a similar result. the climax state, as well as incorporating riparian elements Coniacian assemblages havenot been sampled tothe that contribute to the species count overall. same extentas those of theCenomanian, and therefore Secondly, the polar light regime selects for deciduous- temperature estimates are subject to greater error (Parrish ness, and because extant deciduousangiosperms tend to & Spicer 1988a). Of the 18 dicotyledonous angiosperm leaf have toothed leaves, the possibility exists thatthe light types found in Coniacian fluvial channeland lacustrine regime might bias interpretation of the leaf assemblages environments, seven were entire-margined. Simple applica- towards a cooler regime than really existed. Modem tion of the leaf-margin technique yields a meanannual deciduous vegetation also tends to be that which grows in temperature of 13 "C. Subject to the errors introduced by a cold climates, andthe link between light-induced de- small samplesize, this could be a minimum temperature ciduousness and toothed margins may have been different in estimate if there were a strong bias towards river and lake the Cretaceous. However there is no suggestion of any sharp margin plants which typically tendto produce toothed decline in entire margin representationat, or near,the leaves. However above 13 "C, the vegetational physiognomy inferred palaeo-Arctic Circle which suggests that any light would beexpected to be different (Wolfe 1979.). The effect was insignificant even in the Cretaceous. vegetation is still mixed coniferous and as 13 "C is the maximum for that physiognomy, the estimate based on leaf margin analysis is probably robust. 100 ______~ a 0 Angiosperm diversity in the Maastrichtian assemblages is 2 > y = 109.69 - 1.0005X R = 0.836 too low for the application of leaf-margin ratios. However, 0 -l in the early Tertiary there is an increase in diversity such thatcautious use of the technique is possible. Of the 15 E ; 80 angiosperm leaf forms present only one is entire-margined. n Although the errors are large, this suggests a mean annual 0 -m temperature below 10 "C; probably around 6-7°C. If we aC assume thatthe Kogosukruksediments contain as representative a sample of the sourcevegetation as other p 60 -c non-marine North Slopesediments, then this estimate P provides a ceiling forthe Maastrichtian meanannual 5 temperature. Based on the low diversity, Parrish & Spicer -!! (1988a) estimated the Maastrichtian mean annual tempera- 2 40 w ture to have been as low as 2 "C and no higher than 6 "C.

e E e Vegetational physiognomy 20 Reconstruction of the vegetationaswhole, including 30 40 50 60 70 non-angiosperm elements, can also be used as a quantitative Latitude guide to mean annual temperature and mean annual range Fig. 7. Relationship between the percentage of entire-margined of temperature. Thistechnique relies onthe fact that angiosperm leaves in North American assemblagesof Ceonomanian structural aspects of the vegetation, not its composition at age. The graph, including the regression line, is modified from that the species, genus or even family level, is constrained by of Wolfe & Upchurch (1987~)in that leaves of unknown climate. Because the climate signal is derived from a large provenance but attributed to the Dunvegan flora of British number of unrelatedtaxa and in large part is based on Columbia have been omitted. physiological constraints, it is a robusttechnique that has

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application to the fossil record. However because so much foresttype, the most likely thermaldomain forthe modern vegetation has a significant or dominant angiosperm Cenomanianforest is shown in Fig. 8. While the mean component it is bestreserved forLate Cretaceous or annual temperature estimate of 10 "C is reasonably well younger assemblages. constrained, the estimate of meanannual range of Wolfe (1979) presented quantitative a nomogram temperature is less secure due to the breadthof the domain defining thermaldomains occupied by extant climax parallel to the horizontal axis of the nomogram. Taken at its vegetation in humid SE Asia (Fig. 8). Because this maximum, the mean annual range of temperature is almost technique includes all taxa,not just angiosperms, and 20 "C, implying a possible warm month mean temperature because it is based on all parts of the plant, not just leaves it around 20 "C, and a cold month mean temperature around is independent of leaf margin analysis. Like leaf margin freezing. analysis, 'calibration' of thesedomains forCretaceous The Coniacian assemblages yield anabundance of vegetation is arguable and can only be properly evaluated specimens representingconifers, angiosperms, and ferns. against other independent thermal indicators. Nevertheless, Equisetites is still present, but the ginkgophytes are reduced there is a considerable degree of concurrence between the toone large form of Ginkgo, and the cycadophytes are temperature estimates obtained by this approach and that apparentlyabsent altogether. In spite of these losses the inferred from the independent technique of angiosperm leaf physiognomy of the vegetation is compatible with that of the margin analysis. directA modern analogue forthe low montane mixed coniferousforest. Above 13 "C mean Cenomanian North Slope deciduous forests does not exist, annual temperature thisforest type gives way to but the closest equivalent is the 'low montane mixed microphyllous and notophyllousbroadleaved evergreen coniferousforest' (sensu Wolfe 1979). Thisforest type is forests in which the coniferous component is much reduced essentially conifer-dominated,but hasit diverse a (Wolfe 1979), Thus, without additional evidence it appears subordinate angiosperm component. Taking account of the that the Coniacian mean annual temperature is unlikely to leaf margin evidence for a slight warming in the Coniacian have greatly exceeded 13 "C. without any strongevidence foran associated change in The Campanian and Maastrichtian forests were distinctly impoverished in tree species compared to those that preceded them. Only two conifers have been identified on Thermal Domains of LivingForest Types the basis of leafy shoots although pollen types show more diversity (Frederiksen et al. 1988). Coniferous logs and (After Wolfe. 1979) stumps,some in life position,averaged 15-25cm in diameter and thus were considerably smaller than those of the Nanushuk Group. Like the conifers,angiosperm 30 9 megafossil diversity is less than that in the pollen record. In 25 the case of angiosperms, part of the reason for this disparity -E may be that many angiosperms could have been herbaceous, f 20 possibly annuals, which produce leaves with a poor E preservation potential. Ginkgophytes and cycadophytes are 15 absentfrom the Kogosukruk Tongueand there is alsoa - m major drop in fern diversity. Overall, the vegetation was 10 conifer-dominated but with smaller treesand perhaps a more open structure. Fusain is abundant throughout the Kogosukruk Tongue indicating that frequent fires may have contributed to maintaining anopen structure with a high proportion of weedy species. An open structure may also havebeen maintained by 10 4b 20 30 low summer temperatures.Tree rings are significantly Mean Annual Range of Temperature ("c) thinner than those of the Nanushuk Group, and there is a higher proportion of late wood to early wood (Fig. 4). We Tropical Rain Forest @ Paratropical Rain Forest 0 interpret this to be the result of a slowing of early wood @ Microphyllous Broadleaved Evergreen Forest production brought about by coolersummers where late @ Notophyllous Broadleaved Evergreen Forest summer temperatures became limiting to photosynthesis 0Tundra @ Mixed Coniferous Forests earlier than light loss. Mixed Broadleaved Evergreen andDeciduous Early Paleocene vegetation appears to have been of the 0 mixed coniferoustype but with a more modernaspect. @ Mixed Mesophytic Forests Ferns and Equbetites persisted as the dominantground @ Mixed Broadleaved DeciduousForests cover probably in conjunction with herbaceous angiosperms, @ Mixed Northern HardwoodForests 0Taiga as one again angiosperm pollen diversity exceeds that of the @Simple Broadleaved Deciduous Forest megafossils. Abundant fusain layers several centimetres thick occur in well developed bituminous coals thatare common throughout the unit. This suggests that the climate ClosestModern Analogue for Cenomanian N. SlopeForests was commonly humid enough to allow substantial peat Fig. 8. Nomogram showing the thermal domains, expressed in formation, but that periodic drying led to burning of the terms of mean annual temperatureand mean annual range of mire vegetation and the peat itself. Preserved mud cracks temperature, occupied by extant humid forests, with the closest are lacking however.Wood studies show that summer analogue to the Cenomanian North Slope vegetation plotted. growing conditions were quite variable (Parrish & Spicer in

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prep) and greater mean ring widths, coupled with decreased maintained winter temperatures within a few degrees of ratios of latewood to earlywood, suggest more benign freezing. growing conditions with higher summer temperatures than in Kogosukruk time. Frederiksen et al. (1988) infer from Cretaceous ice NorthSlope pollen studies that angiospermswere co-dominant with the conifersduring the Paleocene,and The air temperatures discussed above all relateto those from this deducethat the Paleocene was cooler thanthe experienced at Cretaceous sea level and it follows therefore Maastrichtian. However, the abundance of angiosperms in thatthere is little likelihood of sea ice evenduring the the megafossil assemblages seems to indicate a shift towards Maastrichtian of 85"N. Althoughplant fossils seldom the mixed mesophytic, mixed northern hardwood, or mixed represent montane vegetation, crude mean annual tempera- broadleaved deciduousforest domains (Fig. 8)and thus, ture estimates at altitude may be made by applying the perhaps, an increase in mean annual range of temperature global average saturated adiabatic lapse rate.For the ratherthandecreased a mean annualtemperature. present day this represents a drop of 6°C per 1000 m of Furthermore, a fall meanin annualtemperature is altitude. Applying this tothe Cenomanian we may inconsistent with the tree-ring and leaf margin studies. reasonably expect a mean annual temperature of 0 "C or Nevertheless more data are required to better understand less 1700m aboveCenomanian sea level. In Kogosukruk these post K/T boundary forests, and until these are carried and Sagwon times, when the North Slope was at 8YN, the out any interpretations must remain speculative. 0 "C isotherm would have dropped to about loo0 m above sea level. It follows from this that permanent snowfields, if notmontane glaciation, probablyoccurred in the newly- Winter temperatures uplifted BrooksRange, although there is no evidence for Assessing winter temperaturesand in particular the cold glaciers reaching the coastal plain. month mean is extremely difficult in situations where cold or light, or a combination of these, induces dormancy in plants (Parrish et al. 1987). Plants donot providecontinuous Discussion records of adverse conditions while dormant. Nevertheless, By adopting an integrated multidisciplinary approach it is in as much asthey provide accurate estimates of mean possible to constrain climate data to within narrow limits, annual temperatures(and, in the case of vegetational particularly palaeotemperature estimates. A key component physiognomy, mean annual range of temperature), even in in assessing air temperatures is the signal provided by land highly seasonalregimes, they provide responses to winter vegetation. conditions thatare integrated with thoseto rest of the It might be argued that the use of a summary climatic thermalregime. Furthermore, winter temperatures can be statistic such as mean annual temperature has little estimated indirectly by considering the effects of freezing application to such a highly seasonal regime such asthat both on the plants themselves and on the sediments. experienced at high latitudes.This objection is easily North Slope Cretaceous sediments contain no unequivo- countered because while it is truethat the meanannual cal evidence for nearshore seaice, or ice in the Arctic temperature might be only experienced for a short period Ocean generally (Clark 1988; cf. Frakes & Francis 1988). during the spring and autumn, such astatistic provides a Such 'drop stones'as there might be (only found in the datum for comparison with other climatic conditions Early Cretaceous Pebble Shale Unit) exhibit no ice striation experienced in less seasonal situations at lower latitudes or andalthough rafting by near shore or river ice may have changing conditions throughtime. To obtaina more been possible, equally likely is transport by tree root rafting rounded summary of thethermal regime of any environ- (Molenaar et al. 1987). Ice-wedge and solifluction structures ment, mean annual temperatures should be used in are lacking evenin the Kogosukrukpalaeosols, and the conjunction with mean annual range, warm monthmeans thermal domain of all the North Slope Cretaceous and early and cold month means. Tertiaryvegetation suggests cold monthmeans no colder A more seriousargument against the validity of the than -11 "C. This figure is the lower boundary of the mean annual temperature asused here might be that the thermal domain occupied by mixed coniferous forests (sensu plants are possibly reflecting only the conditions of growth Wolfe 1979) of anytype (Parrish et al. 1987) and is and not those experienced during dormancy. Thus the mean consistent with the cold monthmean implied by a annual temperature obtained by plantstudies might be combination of theCretaceous minimum mean annual biased towards the summerregime. If, however, we temperatureand maximum meanannual range of examine either modern leaf margin ratios, or vegetational temperature associated with such forests. Wood cell physiognomic data, with respect to present climate, we find (tracheid) cross-sectional areas tend tobe large(Spicer 1989a, that the correlations still hold up even in seasonal temperate 1990) which is consistent with a lack of water stress, even climates where winter dormancy is normal for most, if not that which might be induced by frozen soil during the spring all, the plantsconcerned. Clearly the plants are not only flush of growth. It seems unlikely that soil freezing extended responding tothe temperatures experiencedduring the throughout all or even most of the tree rooting depth even growing season, but also those through which they have to during the latest Cretaceous thermal minimum. Therefore, survive while dormant. In part at least, this may be due to while shortduration frost may havebeen a frequent the fact that the growing season itself is not isolated from occurrence, a continuous hard freeze throughout the winter the winter conditions, because low winter temperatures will darkperiod seems unlikely. Althougha lack of insolation be reflected in the degree of spring warming and autumnal would rapidly depress air temperatures, the probable higher cooling. The lack of detectable changes in the relationship frequency of penetration of extratropical storm systems to between physiognomy and palaeolatitude associated with high latitudes (dueto aweaker polar front) may have the Cretaceous Arctic Circle suggests that there is no reason

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to suspect that the thermal signals obtained from Cretaceous was greater in the warm-poleCretaceous experiments of high latitudevegetation are significantly compromised by Barron & Washington (1984, 1985), and increased latent the light regime. Thusthe mean annual temperature heattransport can result in higher precipitation at high obtained from plant data has as much validity in respect of latitudes (Manabe & Wetherald 1980; Barron & Washington theLate Cretacous polar environment as it has to any 1985), it would be risky to assume that this predicted higher modem seasonal temperature regime. precipitation would have occurred onthe North Slope in In spite of potential problems of calibrating a thermal particular during theLate Cretaceous.Nevertheless, the signal possibly biased by plant physiognomic evolution, slight drying towards the end of the Cretaceous suggested by independent palaeobotanical approaches yield qualitative the increase in charcoal, and the decrease in temperature (tree ring data, diversity comparisons) and quantitative (leaf suggested by the vegetation, are not complementary climatic margin analysis and vegetational physiognomic analysis) changes. Thus, we might speculate that if the polar regions data that are in close agreement and show the same trends were coolerduring Kogosukruk deposition than during throughtime (Fig. 9). Althoughcomplementary high Nanushuk deposition, poleward latent heat transport might latitude data based on isotopicstudies aresparse, where have been less important, resulting in lower precipitation. If presentthey are in broadagreement with thosedata the decreaseinprecipitation exceeds the decreasein obtained from theplants (Pirrie & Marshall 1988; Rich et al. evaporation dueto cooling, the climate would appearto 1988; Rich & Rich 1989). become drier. The cool temperate climates that existed above Although the continental interiors were too cold in the palaeolatitude 75”N throughoutthe Late Cretaceous and experiments of Barron & Washington (1985) and Schneider early Tertiary are cooler than some previous estimates (e.g. et al. (1985), the association of the coldest regions with high Smiley 1967) or valuesused in modelling exercises (e.g. topography at least partly reflects the altitudinal dependence Barron 1983b). The coolness of the climate also goes some of temperature. First-order estimates of temperature in the way towards explaining the apparent wetness on the North palaeo-Brooks Range indicatepermanent snow fields or Slope and hence the development of peats and major coal montane glaciation above1700m throughout theLate deposits. Precipitation and evaporation combined determine Cretaceous and possibly as low as 1000 m during the coldest whether the climate of a particular region is wet or dry, and interval. The presence of montane ice has important evaporation is largely controlled by temperature. Low consequences for climate modelling. First it might be argued temperatures(and therefore low evaporation) combined that summer melting would supply water to the coastal plain with low rainfall can produce a climate as ‘wet’ as high and, therefore, provide a spuriously wet vegetational signal. temperatures(and therefore high evaporation) combined However the abundance of thick, low sulphur, low ash, with high rainfall. This iswhy the rainfall predictions of coals lacking significant clastic contaminationin the Parrish et al. (1982), for example, tell only half the story Nanushuk Group are best explained as resulting from raised when it comes to interpretingthe distribution of peat mires that only form under conditions of high humidity-dependent climatic indicators discussed in Parrish atmospheric humidity (Spicer et al. 1990). Secondly, & Barron (1986). Although poleward latent heat transport significant year-round montane ice and seasonal ice at sea level would greatlyraise the albedo at these critical high 14 latitudes. The palaeo-Brooks Range did not appear on the I l palaeogeographic map used in Barron & Washington’s (1985) experiments,probably because it was below the resolution of the geographic grid. An increase in grid 12 - resolution plus the inclusion of better defined vegetation and ice-related albedo,and better constrained temperature parameters, is likely to yield more realistic model results. 10 - The emphasis by North et al. (1983), North & Crowley 9 (1985), and Kutzbach & Guetter (1986) onthe seasonal cycle ratherthan mean annual temperature for ice-cap l- U formation principally referred to changesin the Milanko- 8- vitch cycles that operated on a much shorter time scale than those resolved in this discussion. Nevertheless,their work may have some relevance. Inasmuch as the Milankovitch 6- cycles affect the polar regions most strongly,and cooling through the Late Cretaceouswould have brought the system closer to a critical pointas defined by North & Crowley (1985), the cooler summer temperatures during Kogosukruk CenomanianConiacian Campanian Maastrichtian Paleocene time might, at theleast, have increased theextent of palaeo-Brooks Range glaciation. It is possible that during Age Kogosukruk time the polar regions may well have come Fig. 9. Estimated changes in North Slope sea level mean annual close tothat critical point.Had it beenreached and full temperatures throughout the Late Cretaceous and early Tertiary scale glaciation ensued, the history and present state of the based on Parrish & Spicer (1988a) and this paper.Error bars Earth’s climate and biota would havebeen very different represent uncertainty in estimating mean annual temperature indeed. because of small samplesize, but are constrained by comparison The effect of the Cretaceous/Tertiaryboundary event, with thermal domains and signals derived from vegetational and leaf reviewed by Spicer (1989b), is only very weakly expressed physiognomic analyses of adjacent assemblages. at high latitudes although this may in part be due to thefact

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Received 23 February 1989; revised typescript accepted 1 June 1989.

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