Holocene Paleoenvironments of Northeast Iowa

Abstract

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 ≈ 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 ≈ 8000 to 5100 yr BP, when the δ13C values indicate a change in vegetation dominated by C3 (predominantly forest) to C4 (predominantly prairie) plants. About 4900 yr BP, the rise in 18O values indicates a temperature increase of ≈ 1.5°C. The fact that the vegetational change suggested by the δ13C 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 18O values remain constant from ≈ 5100 to ≈ 3000 yr BP, but the δ13C 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 sav...

The importance of plant macrofossils in the reconstruction of Lateglacial vegetation and climate: examples from Scotland, western Norway, and Minnesota, USA

“…9). Evidence supporting the reconstruction of boreal conditions is provided by the plentiful macrofossil records of forest shrubs and herbs, aquatic and wetland taxa including bryophytes, and insect remains (Baker et al, 1996). Macrofossils of U. americana (bud scales), Juglans cinerea, T. americana, Ostrya virginica (leaves), F. nigra, Carpinus caroliniana, and Quercus, together with understorey taxa associated with deciduous forest, were recovered from the early Holocene sediments in association with much higher pollen percentages (Fig.…”

“…Other dates for the Picea/Pinus transition, generally taken as the start of the Holocene in the Minnesota area, are all on whole sediment and cover a range 9700-ca 10,500 14 C yr BP. (The dates on wood at Roberts Creek do not date the start of the Holocene (Baker et al, 1996)). The chronological problems are compounded by the existence of radiocarbon plateaux during this time period.…”

“…Such a favourable site is Roberts Creek, N.E. Iowa (Baker et al, 1996). A series of organic deposits were analysed from the river floodplain and provided a detailed multiproxy record for the last 12,500 years.…”

Reconstructing Vegetation Past: Pre-Euro-American Vegetation for the Midwest Driftless Area, USA

“…While browsing by elk and deer, and potentially bison, may have also had an impact on vegetation dynamics (Anderson 2006), fire with a frequent return interval (1-15 years) would have been required to maintain open vegetation types over such large extents (Dickmann andCleland 2002, Nowacki andAbrams 2008). Fire likely was a major determinant of the Driftless Area's HRV for several thousand years; indeed, paleoecological investigations at locations within or near the Driftless Area revealed that oak ecosystems were the dominant during the late Holocene leading up to Euro-American settlement (Davis 1977, Winkler et al 1986, Baker et al 1996, Bogen and Hotchkiss 2007.…”

Grass response to seasonal burns in experimental plantings

“…Storm patterns suggest that summer lightning fires were once common throughout central North America (Komarek 1968), and macrofossils of flowering individual plants indicate that tallgrass prairies were repeatedly burned during the growing season at least millenia ago (Baker et al 1996). Emphasis on promotion and study of dominance of C 4 grasses by prescribed dormant-season fire may lead us to overlook the quite different attributes of grasslands left unburned or burned by summer lightning fires (Howe 1994a).…”