by Alexei HERMAN, Dr. Sc. (Geol. & Mineral.), Geological Institute, Russian Academy of Sciences

Our planet's climate has seen continuous changes in the course of geological history, with the most striking changes occurring in the arctic latitudes taking up a major part of Russia's territory. Back in the latter half of the Cretaceous (99.6 to 65.5 mln years ago) the arctic climate was starkly different from what we are having today. The plant and animal kingdoms were quite different, too.

PLANE TREES JUST 1,000 KM FROM THE POLE

The summertime Arctic is dazzling in its vestal beauty. This is the sun up in the clear cerulean skies for weeks on end, the pellucid streams flowing from firn basins, and the blue-green ice mounds and hillocks on rivers that never melt away in short cool summers... A long polar night descends on the Arctic in winter with snowdrifts and ice-bound seas. The subpolar regions of the Far North call up the image of numerous herds of reindeer*, swarms of biting mosquitos and stunted trees here and there**. In spring the arctic tundra plains are out in blossom producing seed during the short growing season so as to give birth to new life next year. Yet things were different in the dim and distant past.

The global climate of the earth of the Late Cretaceous was much warmer than it is today. There were small, if any, ice caps on the poles-at any rate, not so large as today. Thermophilic, heat-loving plants and animals traveled far into high latitudes of the Northern and Southern Hemispheres, and woods spread as far as the latitude of 85° N. That is the Arctic had what looks like a humid, warm temperature climate: the mean warm month temperatures (as calculated by the architecture of leaves of the then extant dicot plants) were in the +17 to +22°C range, and those of the coldest month-between -2 to +9°C, while the mean annual temperatures ranged from +7 to +14°C; the level of monthly precipitation during the growing season used to be within 100 - 160 mm.*** But most surprisingly, all that went hand in hand with the sharp seasonal fluctuations of solar insolation: even 100 mln years ago the inclination of the axis of

* See: A. Herman, "Paleobotany and Paleoclimates of the Earth: Reminiscing about the Future". Science in Russia, No. 1, 2006. - Ed.

** See: Ye. Syroyechkovsky, "The Problem of the Reindeer", Science in the USSR, No. 2, 1990. - Ed.

*** See: V. Bolshakov, "Green Mound on the Arctic Circle", Science in Russia. No. 5. 2004. - Ed.

Articles in this rubric reflect the opinion of the authors. - Ed.

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Solar radiation incident on horizontal surface at different latitudes during winter and summer half-years (not counting in light absorption and dissipation by the atmosphere).

Profile of the Western Interior Seaway in North America in the Coniacian Age of the Cretaceous (87 mln years ago).

the globe must have differed but little from the present one, which means that in arctic winter the sun was below the horizon as long as today.

At present the aggregate amount of solar radiation at subpolar latitudes is only half as much as in circume-quatorial regions; besides, there is an obvious shortage of sunlight in the wintertime. Yet in summer its amount is about the same as at lower latitudes on account of the long polar day persisting for 3 to 5 months. Furthermore, the share of diffuse light is higher in polar and subpolar regions than farther south; such light is absorbed more readily in photosynthesis than direct incident light. The abundance of diffuse light is due to the low position of the sun above the horizon so that sunrays have to pierce a thicker layer of air with its enhanced concentration of water vapor, frequent cloud conditions and fogs. Thus the two factors - a long continuous summerday, and the abundance of diffuse light-contributed to the favorable conditions for plant growth in the then warm and humid Arctic.

All through the north of Siberia, in Russia's northeast and in northern Alaska we find numerous deposits of ancient plants of the same age; the northernmost deposit on the island of Novaya Sibir lie at paleolatitude 82° N conforming to the reconstructed position of the North Pole in the Cretaceous. The new Siberian flora (age, 89 to 93 mln years) was remarkable for the abundance and diversity of species, in particular, of the ancestors of the present plane trees with very large leaves; such trees predominated in sheer numbers. The paleowoods were similar to the present forests in Russia's south, but-mind you! - they extended too far northwards, to less than a thousand kilometers short of the North Pole!

The Late Cretaceous climate of the Arctic was thus characterized by mild, warm temperatures, heavy precipitation and significant seasonal fluctuations of sunshine, something we do not see nowadays. So we can justly call that climate an "extinct" one. It must have been propitious for the lush growth of grass and trees in summer to provide rich food for large herbivorous dinosaurs whose remains are found in the deposits of that age in Chukotka and northern Alaska. The high productivity of vegetation ought to have contributed to large-scale coal accumulations in the subpolar regions of Asia and North America. For one, the coal deposits formed at that time on the North slope of Alaska are estimated at 2.5 * 10¹² tons of high-grade coal, or a third of the total coal reserves of the United States. Such deposits likewise occur in northern Siberia and in the Russian northeast, and huge coal deposits dating to the Cretaceous are found in the Lena River basin, too.

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WARM CURRENTS

How come? What did sustain the relatively high, positive temperatures during long polar nights in the Arctic? We might suppose that this effect came largely from the warm Arctic basin where only a small amount of ice happened to be formed in wintertime. The Arctic of that age was much warmer than the present Arctic Ocean - the eloquent proof thereof is the absence of glacial deposits of the Late Cretaceous in northern Eurasia. Apparently the Arctic basin evened out the seasonal fluctuations of temperatures off the ocean coastline keeping them high enough above freezing.

But what about the very fact of the unusually warm Arctic basin which in those days was separated from the proto Pacific Ocean by the Bering "bridge", or neck of land, that used to link Eurasia with North America? Prof. Robert A. Spicer, a British paleobotanist and pale-oclimatologist, and yours truly, the author of the present article, suggest: this basin was "heated up" by waters moving from equatorial latitudes along the Western Interior Seaway on the North American continent (poleward heat transport). This seaway was relatively shallow all through; it was about 4,800 km long, and 1,600 to 1,800 km wide during transgressions*, and 800 km in regressions. Dmitry Naidin, a Russian geologist, has proposed to name this major gate a sea-strait. The relics of certain sea mollusks found in northern Eurasia and Amerasia argue in favor of the marine link between the Western Interior Seaway and the Arctic basin, and the West Siberian Sea and further south in the Asian continent.

Now what kept that warm current moving? As we see it, acting in the role of the driving force was the North Passaat** Current which crossed the equatorial ocean Tethys and the water area of what is now the central Atlantic from east to west. Getting to the land barrier at North America's tip and festoon islands and shoals between North America and the northern part of South America, the stream turned clockwise. This caused a "swelling" in the Caribbean basin and a powerful runoff along the eastern edge of the Western Interior Seaway. There might have been a weaker counterstream moving from north to south along the western edge of the sea-

* Transgression - the advance of sea on dry land in consequence of its subsidence, seamount or due to an increase in the oceanic water mass. The reverse process is known as regression, or fall-back, caused by a decrease in the water mass. - Ed.

** Passaat - viento de passada, or trade wind. - Tr.

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Hypothetical reconstruction of the main sea currents (arrows) in the Northern Hemisphere during the Late Cretaceous. АО-Arctic Ocean, HS-Hudson Strait, NPC- North Passaat Current, RS-Russian Strait, TS - Teichert Strait, TuS- Turgai Strait, WIS-Western Interior Seaway, WSB-West Siberian Basin. Yellow indicates land, pale blue-shallow sea basins, blue-deep sea basins.

way, as seen in the graphic example of the large nonflying toothed seabirds named Hesperornis. According to Dr. Lev Nesov's data published at the close of the 20th century, these seabirds would multiply at high latitudes: the remains of young birds were located only in the Arctic, whereas bones of adult ones were found at the latitude of Kansas, USA. The younglings must have migrated southwards along the western margin of the seaway, and only grown-up, adult birds could make it to the southern boundaries of their propagation area in a long course of their sea navigation. Their reverse migration northwards prior to the reproduction period appears to have passed along the eastern edge of the Western Interior Seaway.

The back flow of water from the Arctic basin is thought to have occurred through a narrow Atlantic strait between Fennoscandia and Greenland (the Atlantic Ocean was non-existent then) and through the seaways connecting the West Siberian basin with the Tethys Ocean in the south, as evidenced by the related-ness of the late Cretaceous faunas of the West-Siberian and East European (otherwise known as the Russian Strait) basins. These were linked by the Turgai Strait, with the exchange of water and faunas in it directed both north and south. The Hesperornis fledglings might have migrated southwards from their reproduction regions in the Arctic basin in the north and in the northeast of Fennoscandia. Traveling along the eastern boundary of the seaway between Scandinavia and Britain (where a northward current could have been then), these birds, now growing adult, could get back into the Arctic basin.

Consequently, the above-cited paleontological data show the following pattern: throughout the Late Cretaceous (barring its very end) the Arctic basin was relatively warm, which must have been the result of a more intensive-than today-transfer of heat by sea currents from the equatorial belt through the Western Interior Seaway and, at the close of the Cretaceous, through the Turgai Strait.

HOW TO SURVIVE IN THE WARM POLAR WINTER?

The contemporary earth knows no proxy of the Late Cretaceous flora of the Arctic. The warmer global climate told much on the distribution of vegetation types. The deciduous coniferous-broadleaved woods of the Arctic were a feature of the biosphere of that period as

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a token of the humid or warm temperate climate at high northern latitudes. Subpolar plants did not suffer for lack of heat and moisture in the long growing season of 5 to 8 months; in addition, owing to the long polar day and enhanced presence of diffuse light in solar radiation, they used to get a sufficient amount of energy- quite compatible to what plants growing farther south, in the low and middle latitudes, were getting-for effective photosynthesis. Thus, contrary to the orthodox opinion, it is lack of heat, not of light in summer-that keeps woodlands away from the present-day Arctic.

The Late Cretaceous plant kingdom flourished there under an extra seasonal climate according to solar radiation: today at 75° N the "dead" polar night lasts for about six weeks followed by something like three weeks of a twilight dimout in the daytime when the sun does not rise above the horizon. The mean cold month temperature reconfigured from the morphology of angiosperm leaves seldom went down below zero (0°C); for the most part, however, it lay within positive values, and amounted to plus 4 to 7 degrees centigrade, occasionally even to 9°C. The rate of metabolism and of all chemical reactions, as we know, is temperature-dependent. Consequently, the absence of sunlight in winter combined with positive winter temperatures taxed the arctic flora which found itself under conditions when photosynthesis was outright impossible, while the ambient air temperature was not low enough for arresting metabolic processes in plant

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Cycadophyte NILSSONIOCLADUS, the remains of which were found in Russia's northeast and on the North Slope of Alaska (age, 99.6 years): A - the physiognomy of a long shoot; B - a short shoot with leaves attached to the top; C - reconstruction of the plant.

leaves. As a result, the arctic plants had to overexpend their vital resources, a factor having an adverse effect on shoots and saplings. Wintertime quiescence at relatively high temperatures is a much more difficult arrangement for plant life than at lower ones. How did the arctic vegetation cope?

Under unfavorable conditions most of the trees shed leaves, branches and even short leaved shoots. This is true of wood plants, such as caytonias, ginkgo trees, czekanowskialeans and many conifers. It looks like cycadophytes, a class of dedritic plants growing in subpolar regions, were not evergreen either, contrary to the common opinion.

In the Arctic of the Late Cretaceous the change of the summer radiation regime (long light-day) to the winter one (twilight and polar night) must have occurred within a few weeks, i.e. like it is now. Therefore deciduous (leaf-shedding) plants had developed an adaptive mechanism enabling them to get rid of the burdensome foliage before the winter season and thus achieve a drastic slow-down in the metabolic rate throughout the long polar night. The arctic cycadophytes also shed short shoots together with leaves at the end of the growing season. The same happened to many coniferous plants, too.

The angiosperms of that period have not left reliable representatives of present-day taxa and related forms, something to make us assume those were deciduous, leaf-shedding plants. The taphonomic* characteristics of their burials confirm this conclusion: there are frequent interlayers occurring in deposits with leaf impressions tightly superposed one upon another. The Russian paleobotanist Vsevolod Vakhrameyev (elected to the USSR Academy of Sciences as Corresponding Member in 1979) supposed those were the buried "fall of leaves", or remains of the leaves shed at the end of the growing season.

The leaf-shedding of the Late Cretaceous arctic plants was one of the strategies whereby they overcame the stresses of protracted warm polar nights. It let them economize on vital resources in the wintertime by arresting metabolism in leaves and expend the thus stored energy for foliar growth in spring.

Certain conifers and cycadophytes were evergreen in all likelihood: here and there we find thick and leaved perennial shoots in deposits, but no annual ones. Remarkably those plants carried small scaly or hard hooked leaves, the characters proper to denizens of arid places. However, such xeromorphy (drought endurance) does not seem to have been related to dry climate-rather, the real cause was in a different hibernation strategy, without shedding leaves or leaved-

* With reference to taphonomy, a division of paleontology involved with processes of natural burial of organisms and formation of their fossilized remains. The founder of this discipline was the paleontologist and writer Ivan Yefremov (1907 - 1972), who coined the very term, taphonomy (from the Greek tdphos, or grave). - Ed.

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shoots in a period unfavorable to growth. In that case the plants spent most of their vital resources through persisting metabolism in winter, but economized them in spring for photosynthesis, -that is such plants did not have to form photosynthetic organs again for the growing season. Consequently the wintertime metabolic rate had to be reduced as much as possible. The acquisition of xeromorphic characters prevented excessive losses of moisture in winter, and slowed down the rate in the performance of the root and transport systems.

Notably the present-day evergreen dwarf shrubs (like rhododendron, red whortleberry, heather)-widespread in the zones of northern taiga greenwoods and coniferous forests and hibernating under snow with leaves on-have a different cause for the xeromorphy of their leaves, namely, the inaccessibility of water because of its freezing.

Consequently, the Late Cretaceous flora of the Arctic exhibited these two wintering strategies-with and without leaves on. Each had advantages and disadvantages of its own, and we cannot tell which one happened to be more resource-saving. If we take paleo high-latitude floras spared the wintertime rigors capable of killing the foliage (say, frost in the absence of snow cover), leaf plants were better off than evergreens wherever winters were relatively warm, and thus the leaf-shedding plants felt no shortage of vital resources or heat needed for the formation of a new crown. The deciduous-to-evergreen plants ratio in such kingdoms must be reflecting accessibility of resources at the onset of the growing season;

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if leaf-shedding plants were predominant in real terms, this goes to show that conditions for their intensive growth in spring were favorable. On the other hand, hibernation with foliage on was preferable where wintertime temperatures dropped to values inhibiting the metabolic rate.

WHY LARGE LEAVES ARE NEEDED

Yet another surprising feature proper to Late Cretaceous plants was their overly large and broad leaves 40 cm (16 inches) long and more. The propagation of large-leaved angiosperms in the subpolar regions during the Late Cretaceous must have been related to the specifics of the high-latitude climate of that period. Diffuse solar light is actually devoid of infrared radiation which, in real terms, is not absorbed by chlorophyll, but which can overheat and destroy large leaves. As noted long ago, many present-day plant species of the Northern Hemisphere have larger leaves if they grow father north; botanists explain this phenomenon by persistent low-intensity insolation. Intensive direct sunlight suppresses shoot growth. The leaves of most plants growing on shaded sites are larger and broader than those exposed to incident light; the most large-leaved forms grow under the canopy of tropical humid

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forests with weak diffuse light and in the virtual absence of direct solar insolation. Relevant studies of broad-leaved trees show that a photosynthetic maximum is attained at relative illumination of 30 percent and less.

Humidity is likewise important in addition to the amount and qualitative composition of light: large leaves can develop only at high humidity, something that inhibits evaporation of moisture from leaf surface; besides, abundant water supply stimulates intensive growth. Arctic plants were never short of moisture: let's recall that the monthly mean of precipitation in the growing season ran to 100 - 160 mm.

Thus, the wide propagation of angiosperms with overly large leaves in the Arctic of the Late Cretaceous may be put down to the peculiar features of the contemporary high-latitude climate: it was humid, warm and with predominantly diffuse insolation in the summer growing season; such radiation was relatively weak and persistent all day long.

Clearly the climate, landscapes and biota at high latitudes of the Northern Hemisphere were essentially different from those of the present Arctic. Wherein is the significance of this conclusion? The point is that our planet's climatic system is dynamic and in a constant flux. In the past hundred years human activities have evolved into a powerful factor for its future. The technogenic global warming and the action of this process on the global biosphere and economic activity of man are a matter of the ever growing concern not to academic minds alone.*

In this context datasets on paleoclimates, especially at high latitudes, are very important for a better understanding of what humankind is riding for. This is not to mean that plane trees are going to come up at Tixie or thereabouts in the Far North within the predictable time framework; however, a knowledge of the arctic biota in the geologic epochs of global warming may help us evaluate the nature, scope and possible aftereffects of imminent climatic changes.

* See: Yu. Izrael, "Threat of Climate Catastrophe", Science in Russia, No. 4, 2004, -Ed.