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 Holocene

Ten thousand years ago an ice advance in Canada blocked the northeastern drainage outlets of Lake Agassiz and the lake rose for the final time, flooding the southern part of the basin. Shortly afterwards, the ice retreated and Lake Agassiz drained for the last time. The lake may have persisted in North Dakota until about 8,200 years ago. Further west on the Missouri Coteau, buried ice had melted out 9000 years ago.

Isolated spruce populations persisted in North Dakota until the early Holocene. At Moon Lake in Barnes County (Figure 1), the spruce forest was replaced about 10,300 years ago by a parkland of mixed deciduous forest and prairie. This vegetational change is attributed to an increase in summer temperatures. A climate change to progressively drier summers is thought to have caused the demise of Ulmus (elm) and finally Quercus (oak). About 7000 years ago, the vegetation surrounding Moon Lake was a prairie. At Rice Lake in northwestern North Dakota (Figure 1), prairie replaced earlier parkland vegetation 9,400 years ago.

At Moon Lake, there is continuous prairie from about 7,000 years ago to the present. The pollen of Ambrosia and other weedy species is more abundant than that of grasses until 5,000 years ago. The increased representation of grass pollen during the last 5,000 years is also recorded in the pollen diagram for Rice Lake. The maximum drought conditions during the mid-Holocene occurred between 7,000 to 6,000 years ago. At Moon Lake, between 6,600 to 6,300 years ago, pollen of Iva annua (marsh elder), Ruppia (ditchgrass) and Picea, all show small increases. Ruppia is an aquatic and the presence of its pollen is believed to indicate a shallowing of the lake. The modern range of Iva annua does not extend north of Nebraska and its presence is thought to represent warmer conditions. The spruce pollen is believed to be reworked from older sediments eroded along the margins of the lake as the water-surface was lowered by intense evaporation. In one of the best dated records in the region, maximum drought conditions at Elk Lake, Itasca Park, Minnesota, occurred between 6200 to 6000 years ago.

Fossil diatom assemblages have been studied from several closed-basin lakes in North Dakota which provide one of the best indicators of salinity changes. The general assumption is that significant changes in lake levels are the result of climatic change. At Moon Lake, the water until 10,000 years ago was fresh. From 10,000 to 7,300 years ago it was moderately saline, and from 7,300 to about 2000 years ago it was highly saline. At Devils Lake, the change from fresh water to highly saline conditions occurred around 8,000 years ago, about 2000 years later than at Moon Lake. The very high salinity at Devils Lake persisted until about 5000 years ago, but during this time the lake was flushed periodically with freshwater. Five thousand to 2,000 years ago, the lake was moderately saline. Variations in the timing of salinity events in prairie lakes varies and has been attributed to a number of causes, including differences in hydrological sensitivity of lake basins to climatic change, poor chronological controls, and sensitivity differences between different proxy indicators. Even so, most lakes in North Dakota and the surrounding prairie region record intense episodes of drought during the mid-Holocene from 8000 to 4000 years ago. During the mid-Holocene, evaporation was so intense that shallower lakes completely dried up. At these times, wind erosion removed older sediments from many shallow lake basins. Basal ages of sediments in those basins date only to about 3,000 years. At Spiritwood Lake, near Jamestown, North Dakota, divers from Northwest Divers, Moorhead, Minnesota, found bison skulls and bones scattered across a shelf at about 6m depth. The bones could have been from individuals drowned as they broke through the ice during an early freeze or a late thaw. The horn core dimensions of a skull, however, are most similar to those of Bison bison occidentalis, the typical mid-Holocene form. This led me to conclude that the bones were more probably from bison that died on the margins of the mid-Holocene lake. Large bison skulls, initially reported as Bison crassicornis, but revised to B. bison occidentalis, were also described from the base of an alluvial fill at 6m depth on Spring Creek, near Zap, North Dakota. Beaver-gnawed wood associated with those specimens had an age of 5,400 years.

Holocene climate was the topic of symposia at the 1998-1999 meetings of the Geological Society of America, the American Quaternary Association, and the American Geophysical Union. The causes of mid-Holocene drought have been hotly debated. In the northern Great Plains, regional drought is generally associated with stronger westerly zonal air flow. One hypothesis suggests that stronger circulation was initiated by Milankovitch - driven insolation changes. A lag of 3,000 years following the insolation maximum 9000 years ago is attributed to a rapidly disintegrating ice sheet cooling the atmosphere and delaying the effects of heating. A second hypothesis, suggests that the increased strength in the westerlies was associated with increased solar-geomagnetic disturbances. Using spectral analyses, it was determined that cycles of 200, 100, 50, 22 and 20 years duration were represented in the varve record of Elk Lake. During the mid-Holocene it was further determined that there was an inverse relationship between varve thickness and the 200 year cycle in 14C production determined from tree rings. During this same period, the Earth's dipole moment was at its lowest during the Holocene, suggesting a link between an increase in solar-geomagnetic disturbance and the strength of the circulation.

The late Holocene pollen record for Moon Lake indicates an increase in grasses and a decrease in Ambrosia for the last 4,000 years. There were also several small increases in Ruppia pollen during this time that are believed to be associated with drawdowns of the water level. The diatom-inferred salinity at Moon Lake remained high until about 2,200 years ago, after which the frequency of droughts increased.

North Dakota does not have any long-lived trees but Pinus ponderosa and Juniperus scopulorum in the North Dakota badlands have records that extend back to about AD 1600. Instrumental records for climate change in North Dakota are about 100 years old. Comparison of the tree ring records with the instrumental climate records, indicates that the tree ring record is sensitive to drought. All the trees have thinner rings during the drought of the 1930's. Individual records show a lot of variation, but there appears to be a cyclicity to drought, with intense droughts occurring on a frequency of 40 - 60 years. What is particularly striking in the Moon Lake salinity record is the magnitude of a series of droughts prior to AD 1200: at AD 200-370, AD 700-850 and AD 1000 -1200. These droughts were all of a greater magnitude than the intense drought of the 1930's. They have been correlated with intense episodes of drought in western North America, suggesting that their cause lies in changes to atmospheric circulation over the Pacific Ocean.


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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