Climate Change in North Dakota Since the Last Glaciation
By Allan Ashworth
Department of Geosciences, North Dakota State University


Introduction

The Last Glaciation

The Late Glacial

The Holocene

Conclusions

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The Late Glacial

Twelve thousand years ago the margin of the Laurentide ice sheet stretched across central North Dakota from the northwest to the southeast corners of the state. Ice from the Des Moines lobe still occupied the Red River Valley. However, the margin of the ice sheet was north of Winnipeg 11,000 years ago. Large segments of the ice sheet stagnated along the margin of the Missouri Escarpment and proglacial lakes formed along the actively retreating ice margin. Meltwater flowed southeastwards across the state and accumulated in low areas to form large lakes, including Lake Dakota, Lake Agassiz and Lake Souris. The pollen from Moon Lake, Barnes County, North Dakota indicates that the initial vegetation was Picea (spruce)- dominated forest. This forest persisted from about 11,800 to 10,300 years ago. Other regional pollen sites have a basal zone that is dominated by spruce pollen.

Lake Agassiz formed on the eastern margin of the state as the ice margin retreated northward. Around 11,000 years ago M. primigenius (woolly mammoth) inhabited the strandlines. Grasses were probably the main diet of M. primigenius and a grassland, not a spruce forest, was probably the preferred habitat. Wind and cold surface waters of the lake may have favored a narrow zone of open vegetation around the shorelines. Further, the mammoths, isolated by their ecology into a narrow zone between spruce forest to the west and the lake to the east, may have been preyed on by paleo-indian hunters who entered North Dakota about this time. Mammoth became extinct in North America about 11,000 years ago.

The southern basin of Lake Agassiz was colonized by plants when the waters drained to the Atlantic Ocean during the Moorhead Phase, between 10,900 to about 9,900 years ago. Deposits from a cut bank on the Red River, Fargo, with an age of 10,300 years contain pollen, macroscopic plant remains, fossil beetles, gastropods and bivalves. The fossil assemblages are dominated by aquatic organisms. Most of the wood preserved at the site is Populus, probably P. tremuloides (aspen), but there is also a cone of Alnus (alder) and a few, poorly preserved leaves of Picea (spruce). The macroscopic fossils indicate eutrophic conditions in a shallow, lagoonal environment. The climate was probably similar to that in northern Minnesota at the present day. The pollen assemblage from the Seminary site, about 1 km to the south, at a similar stratigraphic position and with an age of 10,000 years, was dominated by spruce. This pollen, however, could have been reworked and redeposited from older sediments.

Further to the west, two late-glacial and early Holocene fossil assemblages have been examined from the area of the stagnant moraines of the Missouri Coteau. The sediments of Johns Lake, with an age of 10,800 years contain abundant cones and leaves of Picea. This site also has a rich fossil beetle fauna, including several species of Scolytidae (bark beetles) associated with Picea. A few of the beetle species are those typically found in prairie habitats, suggesting that the forest was open and possibly more like the southern margins of the boreal forest today. Based on macrofossils from the Seibold site, spruce persisted until about 9000 years ago on the Missouri Coteau. In addition to spruce the Seibold sediments contain exceptionally well-preserved fossils, including complete leaves of Populus (cottonwood), complete skeletons of fish and frogs, bones of muskrat, coprolites of beaver, exoskeletons of amphipods, insect larvae, aquatic bugs, and beetles. The beetles, like those of the Johns Lake site, represent some species associated with spruce forest and others associated with prairie.


Dr. Allan Ashworth, Chairman
Geology Department
130 Stevens Hall
North Dakota State University
Fargo, ND 58105
Phone: 701-231-7919
E-mail: ashworth@plains.nodak.edu


NDSU Central Grasslands Research Extension Center

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