Masters Theses

Date of Award

5-1999

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Paul A. Delcourt

Committee Members

Hazel R. Delcourt, Claudia I. Mora, Kenneth H. Orvis

Abstract

A 2.3 m sediment core from Nelson Lake, Mackinac Co., Michigan, provides multiple proxy evidence for mid- to late-Holocene climate fluctuations in the upper Great Lakes region of the American Midwest.

The sediment sequence from Nelson Lake includes a basal quartz sand layer (7000 to 5570 yr B.P.), an intermediate unit of biogenic marl (5570 to 2720 yr B.P.), and an upper layer of lacustrine gyttja (2720 to 0 yr B.P.). The marl-gyttja transition occurred gradually, starting at 2720 yr B.P., with the loss of marl by 2450 yr B.P. Variations in the ratios of ^18O/^16O and ^13C/^12C in calcite from biogenic marl provide strong evidence for an abrupt cooling The marl-gyttja transition is interpreted to have been caused by late-Holocene change in lake water chemistry, a local rise in water table and a regional climatic shift to cooler, wetter conditions, producing a lithologic change in sediments accumulating at the coring site. between 3000 and 2500 yr B.P. 8^18O values show a negative 4.0% shift (from -10.2% to -14% PDB) from 2850 to 2540 yr B.P., while 8^13C values shift from -0.1% at 3050 yr B.P. to -8.78% during the same period. Based on this proxy-climatic evidence, conditions warmer and drier than present are postulated at Nelson Lake until between 3000 and 2500 yr B.P., when a local water table rise signaled a climatic shift towards cooler, wetter conditions. Holocene circulation pattern of prevailing zonal (westerly) air flow was replaced by the late-Holocene onset of a meridional The mid- (north/south) circulation pattern, that has continued to today across Michigan’s Upper Peninsula.

Charcoal from naturally occurring forest fires was extracted from the core, influx rates were tabulated, and a plot of charcoal accumulation rate - cross sectional area (CHAR-CSA) was constructed. Relatively modest CHAR-CSA values from 7000 to 4,000 yr B.P. are interpreted to be the result of limited upland fires, bounded by high water level of nearby paleo-Lake Michigan. A significant decline in CHAR-CSA values after 2100 yr B.P is inferred to reflect diminished wildfires and the creation of a fire break formed by expanding Sphagnum marshes to the south and west of the study site. CHARCSA values decrease dramatically after 3000 yr B.P. as the climate became cooler and more moist.

Mid-Holocene fluctuations in the position of the Lake Michigan shoreline created an embayment 11 km southwest of Nelson Lake. Between 6900 and 4000 yr B.P., this embayment served as one natural firebreak for fires swept eastward through upland pine forests by prevailing summertime westerly winds. With continued isostatic rebound of uplands and coastal regression of the Lake Michigan shoreline, the loss of the western firebreak coincides with peak influx values for CHAR-CSA at approximately 3600, 3000 and 2200 yr B.P., indicating repetitive major fires impacting birch-dominated forests. From 2100 until 100 yr B.P., the abrupt reduction in local fires reflects expansion of a new firebreak, the Cranberry Lake Bog peatlands, bordering the site on the south and west, as fire intolerant, mesic forests of northern hardwoods established along Nelson Lake The historic peak in fire activity records Euro-American disturbance associated with logging.

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