Eric C. Grimm

A 50,000-Year Record of Climate Oscillations from Florida and Its Temporal Correlation with the Heinrich Events

Oscillations of Pinus (pine) pollen in a 50,000-year sequence from Lake Tulane, Florida, indicate that there were major vegetation shifts during the last glacial cycle. Episodes of abundant Pinus populations indicate a climate that was more wet than intervening phases dominated by Quercus (oak) and Ambrosia-type (ragweed and marsh-elder). The Pinus episodes seem to be temporally correlated with the North Atlantic Heinrich events, which were massive, periodic advances of ice streams from the eastern margin of the Laurentide Ice Sheet. Possible links between the Tulane Pinus and Heinrich events include hemispheric cooling, the influences of Mississippi meltwater on sea-surface temperatures in the Gulf of Mexico, and the effects of North Atlantic thermohaline circulation on currents in the Gulf.

DROUGHT CYCLES AND LANDSCAPE RESPONSES TO PAST ARIDITY ON PRAIRIES OF THE NORTHERN GREAT PLAINS, USA

Widespread drought is among the most likely and devastating consequences of future global change. Assessment of drought impacts forecast by atmospheric models requires an understanding of natural drought variability, especially under conditions more arid than today. Using high-resolution lake-sediment records from the northern Great Plains, we show pronounced 100- to 130-yr drought cycles during the arid middle Holocene (8000 calendar yr BP). During drought phases, grass productivity declined, erosion and forbs increased, and fuel limitation reduced fire importance. Intervening humid decades saw grass production rise, with stabilization of soils and renewed fire as fuels became abundant. Although both C3 and C4 grasses declined during droughts, a lasting shift to C3 dominance occurred during a single drought -8200 calendar yr BP. During the more humid Late Holocene (2800 calendar yr BP), climate was less variable and without evident drought cyclicity. Consequently, drought severity during past, and possibly future, arid phases cannot be anticipated from the attenuated climate variability evident during contemporary humid phases. Our study demonstrates that agriculturally important grassland ecosystems respond sensitively to drought variability, uncertainty in which has profound implications for the future of these ecosystems.

A NUMERICAL ANALYSIS OF HOLOCENE FOREST AND PRAIRIE VEGETATION IN CENTRAL MINNESOTA

Fossil-pollen samples from Billys Lake in central Minnesota are compared with 105 presettlement pollen samples from Minnesota and adjacent states by ordinating both sets of data with detrended correspondence analysis. The pollen record from Billys Lake reveals that the vegetation changed from pine forest (10 000-8020 BP), to prairie (8020-3400 BP), to deciduous forest (3400- 1000 BP), and finally back to pine forest (1000 BP-present). The numerical comparison indicates that most of the fossil samples have analogs in the presettlement pollen assemblages from Minnesota. Fossil samples from the early Holocene pine forest/prairie transition have no analogs because a belt of deciduous forest presently occurs between pine forests and prairie. The early prairie also has no analogs in Minnesota because of abundant Artemisia, which today is characteristic of prairie farther west. This Artemisia-rich prairie may indicate that the climatic gradient across the region was steeper in the early Holocene than at present. The rate of palynological change is assessed by smoothing the Billys Lake pollen curve through the ordination. Change is continual throughout the last 10 000 yr, but is most rapid in the early and late Holocene and least rapid 7000-6000 BP, when prairie occurred in the region. Inasmuch as pollen-assemblage change reveals vegetational change, these results show that, in central Minnesota, vegetational constancy has been low for at least 9000 of the past 10 000 yr.

EFFECTS OF HOLOCENE CLIMATE CHANGE ON THE C4 GRASSLAND/WOODLAND BOUNDARY IN THE NORTHERN PLAINS, USA

To determine how grassland, woodland, and bordering forests respond to increased aridity, we used paleoecological methods to examine past responses along a transect of three sites at the eastern boundary of the Northern Plains of North America. Our study region corresponds to the confluence of three air streams that control central North American climates and, hence, should be sensitive to climate change. Sediment cores were analyzed for evidence of Holocene vegetation and fire from tall-grass prairie in eastern North Dakota (Moon Lake), from mixed forest near the prairie border in northwestern Minnesota (Deming Lake), and from mixed forest more remote from prairie in western Wisconsin (Dark Lake). Together with pollen and charcoal analysis, we present a new method for determining δ13C of terrestrial (charred) organic matter and, thus, the relative importance of C3 and C4 photosynthetic pathways in past vegetation. Paleorecords were supplemented with surface charcoal accumulation and δ13C from 21 North American lakes that span boreal, deciduous, pine, and mixed forest to tall- and mixed-grass prairie. Surface charcoal and δ13C follow vegetation and climate gradients, with high charcoal accumulation and δ13C (−20‰) in the Plains (Dakotas, Nebraska, and southwest Minnesota) and decreases to the east, west, and north. The δ13C pattern is consistent with observed patterns of C3:C4 dominance across the region. Sediment, pollen, charcoal, and terrestrial δ13C show that vegetation response to climate change varied substantially among tall-grass prairie, bordering woodland, and forest. During maximum aridity (8000;nd4000 yr BP) prairie vegetation in eastern North Dakota showed a demise of woody vegetation followed by a fluctuating dominance of grasses (40% C4) and forbs. Meanwhile, prairie expanded eastward into northwestern Minnesota, where it produced a shifting dominance between mostly C4 grasses and woody vegetation until more humid conditions and mixed forest developed after 4000 yr BP. Mixed forest in southwestern Wisconsin showed little response to mid-Holocene aridity. Elevated δ13C values from 5000 to 3000 yr BP suggest that composition of grasses changed (to increased C4), although pollen data indicate that the total abundance of grasses remained constant. The increase in C4 grasses at this time is consistent with previous studies suggesting a delayed dry interval in eastern Iowa. Reduced aridity of the last 2000 yr brought increased fire to tall-grass prairie as higher primary productivity led to increased fuel load. Meanwhile, forest expanded in northwestern Minnesota, leading to decreased ignition and fine fuels, in turn resulting in decreased fire at the woodland margin.

Holocene Vegetation and Climate Variability in the Americas

Publisher Summary The feature of Holocene climate that changes along the Pole-Equator-Pole: Americas transect is both hemispheres were generally warmer and drier in the early, and middle Holocene. In North America, the northwestern and southeastern parts of the continent were warmer, and drier because of increased insolation, and strengthening of both the Bermuda High, and the Eastern Pacific Subtropical High. However, the heat low in the midcontinent strengthened the monsoon in the southwestern United States. In the the basin of Mexico, and southwestern United States, seasonality of precipitation was enhanced. Wetter conditions also existed in a band from Texas to the eastern Midwest. Northeast Canada, which was still covered by the Laurentide ice sheet, was the exception to an early Holocene thermal maximum. During the early, and middle Holocene, westerly storm tracks were more tightly focused than they are today between 45° and 50 ° S. Holocene climate change in the Amazon tropical rain forest was relatively minor. A reduced north south migration of the Intertropical Convergence Zone (ITCZ) during the early to middle Holocene is a possible explanation for the synchrony of maximum aridity in the two hemispheres. The mean position of the ITCZ must also have remained more northerly year-round in the early to middle Holocene, reducing easterly moisture throughout South America. Throughout the Americas, the late Holocene was generally wetter, and cooler.

Fossil-pollen evidence for abrupt climate changes during the past 18,000 years in eastern North America

A quantitative measure of the rate at which fossil-pollen abundances changed over the last 18 000 years at 18 sites spread across eastern North America distinguishes local from regionally synchronous changes. Abrupt regional changes occurred at most sites in late-glacial time (at ≈13700, ≈ 12 300, and ≈ 10000 radiocarbon yr BP) and during the last 1000 years. The record of abrupt late-glacial vegetation changes in eastern North America correlates well with abrupt global changes in ice-sheet volume, mountain snow-lines, North Atlantic deep-water production, atmospheric CO2, and atmospheric dust, although the palynological signal varies from site to site. Changes in vegetation during most of the Holocene, although locally significant, were not regionally synchronous. The analysis reveals non-alpine evidence for Neoglacial/Little Ice Age climate change during the last 1000 years, which was the only time during the Holocene when climate change was of sufficient magnitude to cause a synchronous vegetational response throughout the subcontinent. During the two millennia preceding this widespread synchronous change, the rate of change at all sites was low and the average rate of change was the lowest of the Holocene.

Late-Quaternary vegetation history of the eastern United States

Publisher Summary This chapter describes the late Quaternary vegetational and climatic history of eastern North America and focuses on a number of outstanding issues and problems. Vegetation is an opportunistic assemblage of species with similar tolerances and adaptations to climatic and other environmental variables. Because every species has a different range of tolerance, assemblages of species change as climate changes. Climate reconstructions based on modern pollen distributions are limited to some extent by the tremendous human influence on modern vegetation. Because of modern human influence, some dominant native types diagnostic of certain biomes, for example, Ambrosia, are left out of the analyses. The application of polymerase chain reaction (PCR) techniques to fossils offers an exciting possibility for understanding the genetic shifts associated with climatic change and migration, and these studies are in their infancy. Even without anthropogenic forcing, climate will change in the future and that greenhouse gases trap heat is indisputable. Paleoecological and paleoclimatic insights may help clarify the future.

Early-Holocene limnological and climatic variability in the Northern Great Plains

Information on the timing and direction of climatic and environmental change on a millennial scale exists for many regions of North America, whereas little is known about decadal-to centennial-scale variability. Here we present a high-resolution analysis of diatom-inferred salinity from a site in the Northern Great Plains to reconstruct multidecadal-and centennial-scale climatic patterns during the early Holocene. The diatominferred salinity indicates a transition from fresh to highly saline conditions between c. 13 400 and 7700 cal. yr BP, which suggests a major shift in climate from wet to dry conditions. The overall trend toward increasing salinity is interrupted by several freshwater intervals between c. 9800 and 7950 cal. yr BP, which may be the result of an increase in the frequency of monsoonal flow from the Gulf of Mexico. The early Holocene is considered to be a time of rapid change in climate and vegetation within the Holocene. Although rates of change in the Moon Lake diatom assemblages were high during parts of the early Holocene, in general the rate of change was as great or greater during the last two millennia. This finding may be the result of a generally directional change in climate in the early Holocene, in contrast to shorter-term fluctuations and little directionality in the late Holocene.

THE INFLUENCE OF ARIDITY AND FIRE ON HOLOCENE PRAIRIE COMMUNITIES IN THE EASTERN PRAIRIE PENINSULA

The role of climate and fire in the development, maintenance, and species composition of prairie in the eastern axis of the tallgrass Prairie Peninsula intrigued early North American ecologists. However, evaluation of the long-standing hypotheses about the regions environmental history has been hampered by the scarcity of paleorecords. We conducted multiproxy analyses on early and middle Holocene sediments from two Illinois, USA, lakes to assess long-term climatic, vegetational, and fire variability in the region. Sediment mineral composition, carbonate delta18O, ostracode assemblages, and diatom assemblages were integrated to infer fluctuations in moisture availability. Pollen and charcoal delta13C were used to reconstruct vegetation composition, and charcoal influx was used to reconstruct fire. Results indicate that fire-sensitive trees (e.g., Ulmus, Ostrya, Fraxinus, and Acer saccharum) declined and prairie taxa expanded with increased aridity from 10,000 yr BP to 8500 yr BP. Between approximately 8500 yr BP and approximately 6200 yr BP, aridity declined, and prairie coexisted with fire-sensitive and fire-tolerant (e.g., Quercus and Carya) trees. After approximately 6200 yr BP, prairie taxa became dominant, although aridity was not more severe than it was around 8500 yr BP. Along with aridity, fire appears to have played an important role in the establishment and maintenance of prairie communities in the eastern Prairie Peninsula, consistent with the speculations of the early ecologists. Comparison of our data with results from elsewhere in the North American midcontinent indicates that spatial heterogeneity is a characteristic feature of climatic and vegetational variations on millennial time scales.

Palynology of the Peace Creek site, Polk County, Florida

The Peace Creek site in the central Florida Peninsula is a sinkhole completely filled with sediment. The basal ;50 m of an ;70m-long core contains well-preserved pollen. The regular occurrence of Pterocarya and other regionally extinct taxa indicates a late Neogene age. Geological data place the age in the Pliocene, and high sea levels during the middle Pliocene argue for a late Pliocene age. The most probable time of formation and infilling is in the late Pliocene, after 2.8 Ma, corresponding with the onset of significant Eurasian and North American glaciation and lower sea levels. In addition to Pterocarya and Sciadopitys, wellknown indicators of late Neogene pollen assemblages from the Atlantic coastal region, we identify cf. Ginkgo and cf. Dacrydium for the first time from Neogene sediments in eastern North America (cf. here indicates a type that is similar in morphology to an extant family, genus, or species). However, these regionally extinct types are rare, and the overall pollen assemblage is quantitatively similar to late Holocene assemblages from Florida and quite distinct from late Pleistocene glacial stage assemblages. A rich and diverse aquatic pollen assemblage indicates that a shallow, freshwater wetland occupied the sinkhole throughout the time of deposition. Three cycles having a long Pinus phase and a short Quercus phase characterize the upland pollen assemblage.