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We compare four emerging approaches to reconstructing Holocene vegetation and climate from south of the glacial border in northeastern Iowa, United States. Pollen, plant macrofossils, carbon isotopic ([Delta]13C) values from alluvial organic matter, and carbon isotopic values in stalagmites from a nearby cave all show similar paleovegetational and paleoclimatic trends during the Holocene. Pollen and plant macrofossils show a rapid change from forest to prairie about 6000 cal. yr B.P, followed by a return of oaks to a presumably savanna-like community about 3500 cal. yr B.P. The [Delta]13C values in alluvial organic matter and the percentage of [C.sub.4] plants both increase ca. 6300 cal. yr B.P., and then decrease in the last 3500 years. In the cave, [Delta]13C values rise beginning at 6000 cal. yr B.P. to a broad peak ca. 4500 to 3000 cal. yr B.P., and decrease thereafter. Pollen and plant macrofossils record the composition of the vegetation that produced the isotopic signals, and verify [C.sub.3]-[C.sub.4] interpretations based on the isotopic records. We demonstrate that these methods are complementary, but that any single method will provide an accurate reconstruction of past environments.

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 [C.sub.3] (predominantly forest) to [C.sub.4] (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 savanna is reflected in the gradual decline of Δ13C values in the last 3000 yr BP; O isotopic values drop sharply by ≅1 ° C ≅2800 yr BP and then were relatively stable. In contrast to the vegetational and isotopic records, the insect assemblages suggest little change in the local environments throughout most of the Holocene. All of the beetle taxa presently occur in eastern Iowa. The relative stability through the Holocene indicates that both open grassland and riparian woodland elements were present throughout. Settlement, land clearing, and land cultivation by EuroAmericans in the region caused rapid erosion of the upland landscape, the deposition of 1-2 m of sediment across the floodplain, a replacement of the native vegetation with ruderal species, a decimation of the native insect fauna, and a degradation of water quality in the stream. These changes in the landscape, vegetation, and insect faunas are as striking as those associated with glacial-interglacial transitions. The timing and direction of changes in the vegetation at Roberts Creek generally correlate well with the carbon and oxygen isotopic record in speleothems at nearby Coldwater Cave and indicate that climate was the main forcing function. However, the contrast between the vegetational change and the stability of the beetle population suggests that climatic changes were subtle. We hypothesize that the factors involved in the Holocene changes were seasonal changes in temperature and precipitation that may not have resulted in much mean annual change. Such changes may have affected the vegetation more than the insect fauna. Key words: alluvial history; carbon isotopes; Coleoptera; beetles; fire; Holocene; insects; oxygen isotopes; paleoclimatology; paleoecology; plant macrofossils; pollen; speleothems., INTRODUCTION This paper presents independent lines of evidence of Holocene paleoenvironments in northeastern Iowa from alluvial stratigraphy, soil development, pollen, vascular-plant and bryophyte macrofossils, beetles, and chronology of sediments exposed [...]

The lithostratigraphy, radiocarbon ages, and pollen and plant macrofossil assemblages in a low terrace on the Iowa River beyond the Wisconsinan glacial boundary reveal regional alluvial relationships and indicate that valleys were important corridors during vegetational change in the Midwest. Basal pebbly sands probably represent a valley train from a central Iowa glacial advance approximately 25,000 to approximately 21,000 yr B.P. Overlying organic silts indicate low energy conditions from approximately 21,000 to approximately 15,000 yr B.P. Pebbly sands of an upper valley train were deposited from approximately 14,000 yr B.P. until 10,000 yr B.P., about 1,500 yr after the Des Moines Lobe had retreated out of the Iowa River Basin. Pollen and plant macrofossils from the top of the upper pebbly sands indicate that the area was covered by mixed conifer-hardwood forest taxa whose pollen percentages do not match those predicted by models of vegetational change in eastern North America. Boreal elements with general distribution over 500 km to the north at 10,000 yr B.P. persisted in peatland habitats. The valley environment contained a variety of edaphic and microclimatic conditions that supported diverse plant communities. During glacial-interglacial transitions, medium to large valleys served as migration corridors for plant taxa and harbored riparian communities without modern analogs.

A sequence of loess-mantled sediments exposed along Clanton Creek provides the first evidence of fluvial, pedologic, and biotic environments before the last glacial maximum in south-central Iowa. Two fining-upward fluvial sequences, one inset into the other, are exposed. Radiocarbon ages indicate that the alluvial fills are about 34,000 yr old. Basal gravel in the oldest fill contains well-preserved mammoth (Mammuthus) bones. Sparse seeds from this horizon suggest weedy flood-plain conditions. The younger alluvial fill contains well-preserved pollen, plant macrofossils, and insects. The pollen is dominated by nonarboreal taxa and Pinus, suggesting a prairie border or savanna environment. Vascular-plant and bryophyte macrofossils indicate a variety of aquatic and marsh environments on the flood plain. Insects are mostly sympatric in mixed conifer and hardwood forest that extends along latitude 47[degrees] to 49[degrees] between eastern North Dakota and New England, but forest beetles are rare, and the fauna is dominated by open-ground forms. The site is interpreted as an open flood plain dotted with marshes and oxbow lakes; it was surrounded by open woodland or savanna similar to that in northeastern North Dakota at present. July temperatures at the St. Charles site were probably 3 to 5 C[degrees] cooler than those at present. The pre-loess stratigraphy of the site differs markedly from that of upland sites and demonstrates that correlation between upland and lowland sequences cannot be done without adequate dating.

Midwestern Holocene Paleoenvironments Revealed by Floodplain Deposits in Northeastern Iowa SITES FOR LATE-GLACIAL AND HOLO-cene paleoecological studies are rare beyond the Wisconsinan glacial margin in the upper Midwest. Lakes and [...]

Peats and silts near Biggsville, western Illinois, have yielded the most detailed paleoecological record available in the midcontinent for the time when Late-Wisconsinan glaciers began to advance. These deposits were analyzed for pollen, vascular-plant macro-fossils, bryophytes, insects, and stratigraphy, and date from [is nearly equal] 28 000 to 22 000 BP. Upland vegetation was a Picea-Pinus banksiana forest from 27 900 to 26 000 BP. From 26 600 to [is nearly equal] 22 700 BP, Picea and Larix laricina were the dominant trees, and Pinus had all but disappeared. This change probably reflects a climatic cooling when glaciers began to advance into the Midwest. A decrease in Picea pollen percentages and total pollen accumulation rate, an increase in Cyperaceae and other non-arboreal pollen, and the appearance of Selaginella selaginoides after 22 700 BP suggest a further cooling, and indicate an open environment as Late-Wisconsinan glaciers advanced into Illinois. Locally, a small pond, which accumulated organic silts and supported a rich aquatic flora, was present from 27 900 to 27 000 BP. Peatlands surrounding the pond were dominated by rich fen bryophytes and a diverse wetland herbaceous flora. A Picea mariana muskeg replaced the pond and fen between 27 000 and 23 500 BP, the peat became increasingly woody, and the wetland became dominated by P. mariana, Larix laricina, and Betula glandulosa. Peat deposition ceased by 23 500 BP when loess accumulation began; small ponds again were present in the lowland, but they supported only a sparse aquatic flora. By 21 400 BP, the basal loess and paleosol had capped the peats, and subsequently rapid loess deposition buried all previous deposits.

Plant macrofossils from sediments exposed in cutbanks of small streams on the Great Plains yield details of the history of the vegetation and climate, and potentially of the hydrologic behaviour of the streams during the past 9000 radiocarbon years. Five zones are recognized by visual inspection. Macrofossils in zone 1 (c. 9000 to c. 8500 BP) indicate that upland deciduous forest was sparsely present, but that prairie, weedy and wetland habitats were also well established. Zone 2 (c. 8500 to c. 5800 BP) is sparsely represented, but upland trees disappeared, riparian trees and wetland plants were sparse, and weeds and prairie taxa fluctuated from low to high values. This interval was probably the warmest, driest part of the Holocene. Zone 3 (c. 5800 to c. 3100 BP) had a rich riparian forest, along with well-developed prairies, disturbed habitats and wetlands. This period was more moist but had intermittent droughts. Zone 4 (c. 3100 to c. 2700 BP) has peaks in many prairie and weedy elements, whereas riparian forest elements nearly disappeared, suggesting another warm, dry interval. Zone 5 represents a return to somewhat less arid conditions and records a strong signature of introduced weeds in post-settlement time. [ABSTRACT FROM AUTHOR]

Carbon and oxygen isotopic trends from seven Midwestern speleothems record significant offsets in the timing of middle-Holocene vegetation change. Interactions of dry Pacific and moist Gulf of Mexico air masses maintained a sharp moisture gradient across Iowa, Minnesota, and Wisconsin such that the arrival of prairie was offset by 2000 years between caves and pollen sites located only 50 km apart. Oxygen isotopes shift concomitantly with carbon in most cases, although these changes are believed to represent increased evaporative enrichment of [sup 18]O prior to infiltration during the prairie period. [ABSTRACT FROM AUTHOR]