Late Tertiary Environments

Modern groups of flowering plants and mammals are thought to have evolved mainly during the Early Tertiary, from about 65 to 50 million years ago. This was a time of warm climate throughout most of the world, one of the warmest periods in the late Paleocene Epoch, about 57-55 million years ago. Evidence from marine fossils indicates that sea surface temperatures were as much as 10°C (18°F) warmer than they are today. Carbon dioxide concentrations in the atmosphere were as much as twice the level they are today. This high level of carbon dioxide created a greenhouse effect, trapping solar energy near the Earth's surface and warming global temperatures.

A study of fossils of late Paleocene age from the Arctic Ocean shows that the climate of that period was exceptionally mild at high latitudes, varying little throughout the year. Temperatures near the shores of the Arctic Ocean were as high as 11°C (52°F) at that time. Also, those sea water temperatures varied by only 6°C (11°F) through the year, so there was little difference between summer and winter temperatures.

The modern forest trees and shrubs of Beringia are descendants of species that existed in the region as far back as the Paleocene or Eocene Epochs (65-33 million years ago). The grasses and other herbaceous plants of the modern tundra and forest floors of Beringia have a shorter fossil record than the trees and shrubs. These probably evolved in the Miocene, Pliocene, and Pleistocene Epochs.

For much of the last 65 million years, land connections existed between Alaska and Northeast Asia. These connections allowed a free exchange of plant and animal species between the two continents. The Bering Strait region was flooded by sea water about 4-3 million years ago, severing the land connections between the two continents until the beginning of the Pleistocene Epoch, about 1.7-2 million years ago.

Beginning in the Miocene Epoch (24-5 million years ago), Arctic and Subarctic climates began to fluctuate dramatically. For instance, Miocene fossil beds from the Porcupine River region of northeastern Alaska contain the remains of a conifer forest with pine, redwood, and cypress. The pollen preserved in these deposits also includes such broad-leafed trees as oak, hickory, holly, and walnut, although these trees may have been growing farther south (i.e., their pollen may have been carried north by winds). This type of forest probably could not withstand severe winters, especially if it included some hardwood trees. It certainly required a much warmer climate than is found today in interior Alaska.

Progressively warmer climate is indicated by the fossil remains left by the younger forest assemblages at this site. Forest beds dating younger than about 16 million years contain hardwood tree pollen, including beech, oak, hickory, chestnut, walnut, sweetgum, basswood, and elm. Based on fossil plant research, the peak of the Middle Miocene warming in northeastern Alaska saw average summer temperatures greater than 20°C (68°F) and average winter temperatures of about -2°C (28°F). Today the average July temperature at Old Crow, Yukon Territory (the closest meteorological station to the fossil site), is 14.2°C (58°F) and the average January temperature is -33.1°C (-28°F), so summers were somewhat warmer than today in the Middle Miocene, but winters were quite a bit warmer. Warm Middle Miocene temperatures are also suggested by many other fossil plant assemblages from sites through the high northern latitudes.

There is some evidence that the Miocene warm period apparently came to an abrupt end, or at least that there was a major climatic reversal in this period. There is paleontological evidence for coastal glaciation in southern Alaska, beginning 15-16 million years ago. Global cooling during the Middle and Late Miocene is probably due in part to the uplifting of the Himalayas and the Tibetan Plateau. This mountainbuilding process brought about changes in atmospheric circulation patterns, as well as other environmental changes that may have had global impacts. Botanical evidence from the Arctic and Subarctic regions indicates a gradual cooling trend during the Late Miocene. Broadleaf trees were mostly replaced by conifers. Late Miocene forests of Alaska included species of pine, spruce, larch, fir, Douglas fir, hemlock, birch, alder, poplar, and willow. Spruce expanded its range in Alaska at the end of the Miocene. This is another piece of evidence indicating a climatic cooling at that time.

The Pliocene Epoch (5.3-1.8 million years ago) was also a time of large-scale climate change. Based on fossil records, Arctic climates during the Pliocene ranged from warmer than present to much colder than present. This was not just a high-latitude phenomenon. The global trend during the Pliocene was also one of general cooling. During the warm Pliocene intervals, fossil evidence from Greenland, Canada, and Alaska indicates that average summer temperatures were as much as 10°C warmer than today, and average winter temperatures were as much as 18°C warmer than today. Pine forests grew in northwestern Alaska 5.7 million years ago, where Arctic tundra vegetation thrives today. Conifer forests grew in northernmost Greenland as late as 2.5 million years ago. Warm Pliocene climates are also correlated with times of higher sea level. Marine transgressions (periods when sea water floods the land and marine sediments are deposited on top of terrestrial sediments) occurred three times in the Arctic during the Pliocene.

The Pliocene was not a uniformly warm period in Alaska or elsewhere in the northern high latitudes. Although some early Pliocene fossil beds point to extremely warm climates in the Arctic, after about 3 million years ago, climates had cooled somewhat, although they remained warmer than it is today. This reconstruction is based on the fossil evidence from the high Arctic islands of western Canada. Here, peat deposits of the Beaufort Formation reveal the development of coniferous woodland and tundra. Early Pliocene vegetation in Alaska included a greater abundance and diversity of herbaceous species than was seen in older fossil records. This has been taken to mean that climates were becoming cooler and drier, especially in the Arctic regions. The mountain-building process in southern and central Alaska started to exert strong regional climatic influences in the Late Tertiary. These tall mountains became barriers to the northward flow of warm, moist, Pacific air. New habitats for cold-adapted herbaceous vegetation developed on high mountain slopes. Arctic and alpine tundra veg etation communities were beginning to form, which would eventually become very important in Beringia.

By 2.8 million years ago, geologic evidence indicates that permafrost was developing in the Alaskan interior, and that multiple glaciations occurred in Alaskan mountain ranges between that time and the early Pleistocene (about 1.7 million years ago). The climatic cooling that developed between about 2.5 and 2.3 million years ago resulted not only in the development of glaciers but also in the development of lowland tundra and forest-tundra (mixtures of forest trees and open ground with tundra plant species). The stage was set for Pleistocene and its glacial climates. As the Tertiary gave way to the Quaternary, forests gave way to tundra in various forms.

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