Richard G. Baker

A high-resolution record of holocene climate change in speleothem calcite from cold water cave, northeast iowa.

High-precision uranium-thorium mass spectrometric chronology and 18O-13C isotopic analysis of speleothem calcite from Cold Water Cave in northeast Iowa have been used to chart mid-Holocene climate change. Significant shifts in †18O and †13C isotopic values coincide with well-documented Holocene vegetation changes. Temperature estimates based on 18O/16O ratios suggest that the climate warmed rapidly by about 3�C at 5900 years before present and then cooled by 4�C at 3600 years before present. Initiation of a gradual increase in †13C at 5900 years before present suggests that turnover of the forest soil biomass was slow and that equilibrium with prairie vegetation was not attained by 3600 years before present.

Patterns of Holocene environmental change in the midwestern United States

Abstract Four pollen sequences along a transect from north-central Iowa to southeast Wisconsin reveal the distribution of prairie and forest during the Holocene and test the use of pollen isopolls in locating the Holocene prairie-forest border. Prairie was dominant in central Iowa and climate was drier than present from about 8000 to 3000 yr B.P. During the driest part of this period in central Iowa (6500-5500 yr B.P.), mesic forest prevailed in eastern Iowa and Wisconsin, suggesting conditions wetter than at present. Prairie replaced the mesic forest about 5400 yr B.P. in eastern Iowa but did not extend much farther east; mesic forests were replaced in southern Wisconsin and northern Illinois about 5400 yr B.P. by xeric oak forests. This change from mesic to xeric conditions at 5400 yr B.P. was widespread and suggests that the intrusion of drier Pacific air was blocked by maritime tropical air from the Gulf of Mexico until the late Holocene in this area.

Evidence for increased cool season moisture during the middle Holocene

Isotopic analyses of three stalagmites from Cold Water Cave, northeast Iowa, United States, reveal nearly identical δ 13 C trends from ca. 7 to 2 ka. However, δ 18 O patterns differ by as much as 3‰ from 5.7 to 3.2 ka. These disparate δ 18 O values reflect evaporative 18 O enrichment in meteoric water prior to infiltration, suggesting that previously calculated temperatures based on a single Cold Water Cave stalagmite overestimated middle Holocene warming. The coincidence of elevated middle Holocene growth rates in the stalagmites with the lowest oxygen isotopic compositions indicates that the middle Holocene was marked by a predominance of coolweather precipitation during a period of increased overall aridity.

Holocene environments of the central Great Plains: multi-proxy evidence from alluvial sequences, southeastern Nebraska

Abstract Pollen, plant macrofossils, phytoliths, carbon isotopes, and alluvial history from sediments exposed along the South Fork of the Big Nemaha River, southeastern Nebraska, USA, provide an integrated reconstruction of changes in Holocene vegetation, climate, and fluvial activity. From 9000 to 8500 uncalibrated 14 C yr BP, climate became more arid and the floodplain and alluvial fans in the main valley aggraded rapidly, upland deciduous forest declined, and prairie attained its Holocene dominance. From 8500 to 5800 yr BP. upland forest elements disappeared, and even riparian trees were sparse under dry climatic conditions. Alluvial fans continued to aggrade but aggradation in the main valley was interrupted by a stable episode 7000 yr BP. From 5800 to 3100 yr BP, riparian forests returned to prominence, and droughts were intermittent. Alluviation was slower and punctuated by two major episodes of channel incision and terrace formation in the main valley. Aggradation on alluvial fans slowed and finally ceased near the end of this period. During a short dry interval from 3100 to 2700 yr BP riparian trees (except elm) disappeared, and prairie and weedy species became more abundant. This interval is represented by the organic Roberts Creek Member, and the alluvial setting was a slightly incised meandering channel belt. Habitats became similar to presettlement conditions during the last 2700 yr BP. Weedy taxa dominate modern samples, reflecting widespread disturbance. Alluvial fans and terrace surfaces were stable during the last 2500 years, but episodes of floodplain aggradation were punctuated by incision of the main channel.

Comparison of multiple proxy records of Holocene environments in the midwestern United States

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 (δ 13 C) 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 δ 13 C values in alluvial organic matter and the percentage of C 4 plants both increase ca. 6300 cal. yr B.P., and then decrease in the last 3500 years. In the cave, δ 13 C 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 3 -C 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.

Midwestern Holocene paleoenvironments revealed by floodplain deposits in northeastern Iowa

Pollen analysis of pond deposits in the upper reaches of a stream from northeastern Iowa, an area beyond the last glacial margin, provides a nearly complete record of vegetational changes during the last 12.5 thousand years. Sixty-one radiocarbon dates provide good chronological control. Spruce forest was replaced by deciduous forest before 9.1 thousand years ago, followed by prairie from 5.4 to 3.5 thousand years ago, and oak savanna from 3.5 thousand years ago until presettlement times. The prairie invasion was nearly 3 thousand years later here than at other sites in Iowa and Minnesota, documenting a late Holocene, rather than an early-middle Holocene, period of maximum warmth and dryness for the southern part of the upper Midwest.

Speleothem evidence for Holocene fluctuations of the prairie-forest ecotone, north-central USA

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 18O prior to infiltration during the prairie period.

Vegetational changes in Western Illinois during the onset of Late Wisconsinan glaciation

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 macrofossils, bryophytes, insects, and stratigraphy, and date from 28 000 to 22 000 BP. Upland vegetation was a Picea-Pinus banksiana forest from 27 900 to 26 600 BP. From 26 600 to 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.

Palaeohydrology, vegetation, and climate since the late Illinois Episode (∼130 ka) in south-central Illinois☆

Abstract Our interpretation of pollen and ostracode successions from four basins in south-central Illinois provides a new synthesis of palaeovegetation, palaeohydrology, and palaeoclimate for the period from the late Illinois Episode (about 130,000 years ago) to near the end of the Wisconsin Episode (about 25,000 years ago). Correlations of pollen biozones between Raymond, Pittsburg, and Bald Knob basins are the basis for identifying the late Illinois glacial, Sangamon interglacial, Wisconsin glacial, and Hudson interglacial episodes. Glacial episodes were identified primarily by the presence of Picea pollen and the ostracode Limnocythere friabilis, whereas interglacial episodes were identified by Quercus pollen and by the ostracodes Candona caudata and Heterocypris punctata. Within interglacial and glacial episodes, pollen and ostracode assemblages varied with changes in moisture balance. Local palaeohydrology was assessed primarily on the basis of environmental tolerance indices of ostracodes and the stable isotope (C,O) stratigraphy of ostracodal calcite. Regional moisture balance was assessed from multivariate analyses of the pollen successions. Three climatic regimes occurred during the Sangamon Episode. (1) One regime was characterised by precipitation exceeding evaporation that promoted basin overflow. This climate was inferred from the high percentages (generally >80%) of deciduous-forest pollen. Peaks in the abundance of Liquidambar and Fagus pollen indicate that winters may have been slightly warmer, and effective moisture slightly greater, than at present. (2) The second climatic regime was continental, similar to the present climate of Illinois in which precipitation is equal to or just less than evaporation. This climate is inferred from abundant Ambrosia pollen (40 to 60%) and abundant nektic (swimming) ostracode valves which suggest a shallow lake. These conditions probably developed in association with a ‘heat-low’ over the interior of North America during marine oxygen isotope stages 5e and 5c. Associated with the transition between the first two climates are fossils of the subtropical ostracode Heterocypris punctata and the giant tortoise Geochelone crassiscutata that suggest short periods in winter when polar low-pressure systems did not extend into Illinois as they do today. (3) The third climatic regime occurred during the transition from the Sangamon interglacial episode to the Wisconsin glacial episode. A severely continental climate is indicated by the heat-tolerant ostracode Pelocypris tuberculatum, variable δ18O values of ostracode valves, and high environmental tolerance index values for the ostracode assemblages. The weedy Chenopodiaceae and Amaranthaceae families grew on exposed mudflats. The tree pollen associated with this type of climate included low percentages of Picea and Liquidambar, an assemblage that has no modern analogue. We suggest that this transitional climatic regime was associated with the large-scale changes in the climate system during marine oxygen isotope stage 4.

A late Quaternary pollen sequence from Blacktail Pond, Yellowstone National Park, Wyoming, U.S.A.

Abstract The pollen record at Blacktail Pond provides evidence for the vegetational development of the lowest and driest site yet reported in Yellowstone National Park (elev. 2,018 m). The record begins shortly after deglaciation, over 14,000 yr B.P. (before present). Tundra‐like vegetation initially covered the area but was succeeded by Picea parkland. A Picea‐Pinus albicaulis‐Pinus contorta forest developed at about 11,800 yr B.P. Gradually, as the climate became warmer and drier, the forest opened and P. contorta became more prevalent. From 6,700 to 1600 yr B.P. Pseudotsuga parkland grew in the vicinity of the site, indicating a maximum of warmth and dryness. After 1,600 yr B.P., the climate again became somewhat cooler and moister, and although the Blacktail Pond valley has remained a Pseudotsuga parkland, P. contorta now also occurs in scattered groves, and it probably grows at lower elevations than it did prior to 1,600 yr B.P. A volcanic ash at 4.6 m correlates stratigraphically with the basal ash ...