Holocene paleoenvironments of northeast Iowa

R G Baker; E A Bettis III; D P Schwert; D G Horton; C A Chumbley; L A Gonzalez; M K Reagan

This paper presents the biotic, sedimentary, geomorphic, and climatic history of the upper part of the Roberts Creek Basin, northeastern Iowa for the late-glacial and Holocene, and compares these records with a C-O isotopic sequence from Coldwater Cave, 60 km northwest of Roberts Creek. The biotic record (pollen, vascular plant and bryophyte macrofossils, and insects) is preserved in floodplain alluvium that underlies three constructional surfaces separated by low scarps. Each surface is underlain by a lithologically and temporally distinct alluvial fill. The highest surface is underlain by the Gunder Member of the Deforest Formation, dating from 11 000 to 4000 yr BP; beneath the intermediate level is the Roberts Creek Member, dating from 4000 to 400 yr BP; and the lowest level is underlain by the Camp Creek Member, deposited during the last 380 yr. Pollen and plant macrofossils in the alluvial fill show that a typical late-glacial spruce forest was replaced by Quercus and Ulmus in the early Holocene. This early-to-middle Holocene forest became dominated by mesic elements such as Acer saccharum, Tilia americana, Ostrya virginiana, and Carpinus caroliniana as late as 5500 yr BP; in contrast, the closest sites to the west and north were at their warmest and driest and were covered by prairie vegetation between 6500 and 5500 yr BP. After 5500 yr BP, the forest in the Roberts Creek area was replaced by prairie, as indicated by a rich assemblage of plant macrofossils, although only Ambrosia and Poaceae became abundant in the pollen record. The return of Quercus approximately 3000 BP (while nonarboreal pollen percentages remained relatively high) indicates that oak savanna prevailed with little change until settlement time. The bryophyte assemblages strongly support the vascular plant record. Rich fen species characteristic of boreal habitats occur only in the late-glacial. They are replaced by a number of deciduous-forest elements when early-to-middle Holocene forests were present, but mosses of forest habitats completely disappear when prairie became dominant. A few deciduous-forest taxa return during the late-Holocene, when oak savanna prevailed. The C-O isotopic record from stalagmite "s" in Coldwater Cave indicates a relatively stable environment from approximately 8000 to 5100 yr BP, when the delta super(13)C values indicate a change in vegetation dominated by C sub(3) (predominantly forest) to C sub(4) (predominantly prairie) plants. About 4900 yr BP, the rise in super(18)O values indicates a temperature increase of approximately 1.5 degree C. The fact that the vegetational change suggested by the delta super(13)C values preceded the temperature increase suggests that fire may have been an important factor in converting forest to prairie. Abundant charred seeds and other plant material at Roberts Creek 4830 yr BP support this hypothesis. The super(18)O values remain constant from approximately 5100 to approximately 3000 yr BP, but the delta super(13)C values gradually rise, indicating that soil formed under forest takes at least 2000 yr for its carbon to reach equilibrium after replacement by prairie vegetation. The return of oak to form savanna is reflected in the gradual decline of delta super(13)C values in the last 3000 yr BP; O isotopic values drop sharply by approximately 1 degree C approximately 2800 yr BP and then were relatively stable.

Holocene paleoenvironments of northeast Iowa

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