For years paleoclimatologists have held the general view that the last deglaciation began around 17,000 to 15,000 yr ago, that the shape of the globally integrated deglacial curve was smoothly sigmoidal with the fastest rate of change centered around 11,000 yr ago, and that the deglaciation ended around 7000 to 5000 yr ago. Recent studies have challenged several aspects of this consensus and have suggested that the mechanisms responsible for the deglaciation are significantly different from those previously proposed. As a result, an international workshop was held at Airlie House in Virginia during May 2–6 of 1983 to evaluate a wide range of evidence relevant to this controversy. The conference results suggested that (1) the decrease in global ice volume occurred in two steps, with the dating of the earlier step still in doubt, but the later step occurring at about 10,000–7000 yr ago and (2) the most likely feedback mechanisms for accelerating the initial forcing by orbital variations are delayed bedrock rebound, marine downdraw/calving, and CO2 heating.

Many of the problems that have been discussed with regard to the analog approach to paleoclimatic reconstruction may be illustrated by consideration of basic principles and definitions, and the logical consquences thereof. Specific attention is directed toward the definition of the regional macroclimate as the modal microclimate and to a series of axioms: (1) There are various microclimates which depart more or less from the macroclimate in each terrestrial region. (2) Environment and climate change on time scales from near instantaneous to millions of years. (3) Climate is multidimensional (a vector), not a single scalar datum. (4) Biotic assemblage changes in response to environmental (including climatic) changes are expressed primarily in terms of reproductive success. One may conclude from the corollaries to these axioms that perfect analogs cannot be found for any Holocene climate, but that “partial analogs” (“transfer equations”) may still be used if allowance is made for shared variance.

According to recent dating by several methods, the impressive cave sediments at Locality 1 at Zhoukoudian, about 40 m thick, have not only allowed the age of Peking Man to be established, but have calibrated the general course of cave development, permitted correlation with events both within China and farther afield, and placed the sequence in the global frameword of late Cenozoic climatic change. The cave deposits can be divided into 17 layers which are correlated with the great loess sequence (L9–L4) in China and with deep-sea-core oxygen-isotope stages 16-6. The 14th layer upward (730,000–230,000 yr B.P.) represents at least four glacial cycles.

Three time series based on precisely dated annual tree-ring widths have been used to reconstruct June plus July degree days for the central Alaska and northwestern Canada region. The time series are the longest recently developed chronologies for the area and represent 57 core samples from 27 trees. The degree-year-to-year variation and day reconstruction, extending back to A.D. 1524, exhibits much extended warming and cooling trends including a general warming trend from about 1840 to 1960. The reconstruction is in agreement with some subaretic glacial information and with data of percentage melting from arctic ice cores. This and similar reconstructions can provide quantitative information to compare with general circulation and energy budget models for longer time periods than are available in recorded meteorological data.

Dimensions of Holocene relict channels and sedimentological characteristics of point bars associated with these relict channels were used to reconstruct a Holocene history of long-term changes in magnitudes of 1.58-yr floods in Upper Mississippi Valley watersheds of southwestern Wisconsin. The reconstructed record of floods shows relatively large and persistent (nonrandom) departures from contemporary long-term average flood magnitudes. The flood history indicates climatic changes that are broadly similar to climatic changes indicated from fossil pollen in the same region. The Holocene floods ranged from about 10–15% larger to 20–30% smaller than contemporary floods of the same recurrence frequency. Large floods were characteristic between about 6000 – 4500 and 3000 – 2000 yr B.P., and during a brief interval after 1200 yr B.P. Small floods were common between about 8000 – 6500, 4500 – 3000, and 2000 – 1200 yr B.P. These fluvial responses were found to be closely associated with a long-term episodic mobility and storage of sediments in the Wisconsin watersheds. During periods of relatively large floods, relatively rapid lateral channel migration either reworked or removed extensive tracts of valley bottom alluvium. In contrast, during periods of relatively small floods, relatively slow lateral channel migration is apparent and the channel and floodplain system appear to have been relatively stable.

Collection of seismic reflection data from continental margins and ocean basins surrounding North America makes it possible to estimate the amount of material eroded from the area formerly covered by Laurentide ice sheets since major glaciation began in North America. A minimum estimate is made of 1.62 × 106 km3, or an average 120 m of rock physically eroded from the Laurentide region. This figure is an order of magnitude higher than earlier estimates based on the volume of glacial drift, Cenozoic marine sediments, and modern sediment loads of rivers. Most of the sediment produced during Laurentide glaciation has already been transported to the oceans. The importance of continental glaciation as a geomorphic agency in North America may have to be reevaluated. Evidence from sedimentation rates in ocean basins surrounding Greenland and Antarctica suggests that sediment production, sediment transport, and possibly denudation by permanent ice caps may be substantially lower than by periodic ice caps, such as the Laurentide. Low rates of sediment survival from the time of the Permo-Carboniferous and Precambrian glaciations suggest that predominance of marine deposition during some glacial epochs results in shorter lived sediment because of preferential tectonism and cycling of oceanic crust versus continental crust.

Paleomagnetic records of declination and inclination from sediments recovered from the bed of Lake Kinneret (32.4°N, 35.7°E) have been dated by radiocarbon techniques. The sediments span the last 5000 yr. The changes in inclination down the sediment cores are more pronounced than the declination fluctuations and are repeatable between the three coring sites, which are several kilometers apart. Magnetic susceptibility logs display 13 maxima in the 5-m-long sequences, with a pronounced susceptibility minimum about 1000 yr B.P. Many of the susceptibility maxima and minima can be easily correlated between coring sites and are shown to be dominantly related to changes in sediment carbonate content. The natural remanent magnetization intensity follows a similar pattern to that of susceptibility, and the natural remanence of the Kinneret sediments is presumed to reside in detrital magnetite grains carried into the lake by the river Jordan from the basalt-rich bedrock of the rift floor and the Golan Heights. The 14C chronology is strongly supported by a pollen study in which pronounced changes in the proportion of olive pollen were interpreted as being due to extensive cultivation of olives around Galilee in the Hellenistic and Byzantine periods. The Kinneret paleosecular variation records, if accurately dated, point to a complex spatial pattern of Holocene secular variation with significant variations over distances as small as 1000–2000 km.

Pollen and charcoal analysis of radiocarbon-dated sediment cores from Duck Pond in the Cape Cod National Seashore provide a continuous 12,000-yr vegetation and climate history of outer Cape Cod. A Picea-Hudsonia parkland and then a Picea-Pinus banksiana-Alnus crispa boreal forest association grew near the site between 12,000 and 10,000 yr B.P. This vegetation was replaced by a northern conifer forest of Pinus strobus-P. banksiana, and, subsequently, by a more mesophytic forest (Pinus strobus, Tsuga, Quercus, Fagus, Acer, Ulmus, Fraxinus, Ostrya) as the climate became warmer and wetter by 9500 yr B.P. By 9000 yr B.P. a Pinus rigida-Quercus association dominated the landscape. High charcoal frequencies from this and subsequent levels suggest that the pine barrens association developed during a warmer and drier climate that lasted from 9000 to about 5000 yr B.P. Increased percentages of Pinus strobus pollen indicate a return to moister and cooler conditions by about 3500 yr B.P. A doubled sedimentation rate, increased charcoal, and increased herb pollen suggest land disturbance near the pond before European settlement. These results suggest a rapid warming in the northeast in the early Holocene and support a hypothesis of a rapid sea level rise at that time. Comparison of the pollen results from Duck Pond with those from Rogers Lake, Connecticut, illustrates the importance of edaphic factors in determining the disturbance frequency and vegetation history of an area.

Values of the gradient of the former ice surface can be inferred at points along a flow line from deviations of esker paths or transitions in esker type and numerically integrated to give the profile. A profile calculated in this way shows that during formation of the Katahdin esker system about 12,700 yr ago the ice thickness at distances of 10, 20, 50, 100, and 140 km from the terminus, which is about two thirds of the distance to the ice divide, was approximately 200, 300, 600, 750, and 900 m. The terminal reach was computed by assuming an unknown uniform basal drag and matching the profile to its known elevation at the terminus and known gradient 10 km upglacier. Correction for isostatic rebound based on the elevations of contemporaneous deltas and of the marine limit proved unnecessary, because the tilt due to the difference in uplift at the two ends of the profile is only 0.1 m km−1. With other plausible assumptions as to sea levels in the past, elevations of the marine limit, or exact location of the terminus the profile could be as much as roughly 100 m higher. It hits Mount Katahdin about 500 m below its summit, which is at 1600 m, in agreement with the geological evidence farther west. The steepening of the upper part of the profile suggests that the mountain dammed and diverted the ice. Basal drag computed from the profile varies from about 20 kPa (0.2 bar) near the terminus to 30 kPa (0.3 bar) at 100 km to 70 kPa (0.7 bar) at 140 km. The relatively low values away from the influence of Mount Katahdin agree with independent evidence from deep-sea cores of substantial late Wisconsin ice-sheet thinning without comparable areal reduction. The method has potential for application over wide areas that were occupied by the Laurentide and Scandinavian ice sheets.

Studies of the pedogenic iron oxyhydroxides in suites of latest Holocene to middle Pleistocene soils formed on fluvial deposits of the transverse ranges, southern California, indicate that the content and composition of iron oxyhydroxide change in a systematic manner. Analysis of total secondary free iron oxides (dithionite extractable, Fe2O3d) and ferrihydrite (oxalate extractable, Fe2O3o) shows that (1) a single-logarithmic model (Y = a + b log X) or double logarithmic model (log Y = a + b log X), where Y is the total mass of pedogenic Fe oxides (g/cm2-soil column) and X is soil age, describes the rate of increase in Fe2O3d with time; (2) the Fe2O3d content correlates linearly with soil reddening and clay content; (3) the Fe2O3oFe2O3d ratio, which indicates the degree of Fe oxide crystallinity, is moderately high to very high (0.22–0.58) in middle Holocene to latest Pleistocene soils and progressively decreases to less than 0.10 in older soils; (4) the value of the Fe2O3oFe2O3d ratio also appears to be infuenced by climate; and (5) temporal changes in Fe oxide content and mineralogy are accompanied by related, systematic changes in clay mineralogy and organic matter content. These relationships are attributed to a soil environment that must initially favor ferrihydrite precipitation and/or organic matter-Fe complexation. Subsequent transformation to hematite causes increasingly intense reddening and a concomitant decrease in the Fe2O3oFe2O3d ratio. The results demonstrate that iron oxide analysis is useful for numerical age studies of noncalcic soils and shows potential as an indicator of paleoclimates.

A nitric acid digestion and ignition technique developed to measure relative frequencies of charcoal in lake and bog sediments measures the weight of charcoal relative to the dry weight of the sample (percentage charcoal) and gives results comparable to counting of microscopic charcoal. Differences in the two techniques caused by changes in sediment type were found especially in postsettlement and late-glacial parts of cores. Charcoal values from sediments less than 100-yr old cannot be compared to older charcoal values unless wood charcoal and charcoal from fossil fuel burning are separated microscopically. Tests of this chemical assay on mixtures of known weights of charcoal, leaves, and sand give correlations between observed and expected values of 0.99 to 1.00.

Near Summer Lake in southern Oregon, 54 tephra beds of late Quaternary age are exposed in pluvial lake sediments of Lake Chewaucan. Seven of the tephra beds near the top can be correlated with tephra deposits younger than 117,000 yr at Mount St. Helens, Washington, at Crater Lake, Oregon, and in northwestern Nevada in the deposits of pluvial Lake Lahontan. However, most of the section at Summer Lake lies below the correlated units, and contains 39 tephra beds older than 117,000 yr.

Major-element chemistry of tephra glasses was determined by electron microprobe analysis; petrography supports the correlations made from chemical evidence. Compositions correlated range from 70 to 76% SiO2; the least silicic Summer Lake glass contained 57%.

Extrapolation of depositional rate suggests that most of the sediments at Summer Lake are younger than about 335,000 yr, but older lake beds containing tephra layers occur at one place. The long lacustrine record suggests that Lake Chewaucan persisted through the last interpluvial stage, and that the lake may have dried up at the end of the Pleistocene due to diversion of the Chewaucan River by relict shore features.

The Gage Street site in Kitchener, Ontario, is a peat/marl sequence representing continuous lacustrine sedimentation from the time of deglaciation (ca. 13,000 yr B.P.) through 6900 yr B.P. Insect, pollen, and plant macrofossil remains isolated from the sediments indicate that from ca. 13,000 to 12,500 yr B.P. the region was characterized by parkland-tundra vegetation existing within thermal conditions more analogous to those today of the midboreal forest. The transition from parkland to coniferous forest at ca. 12,500 yr B.P. occurred within a climate that was only gradually warming. By the time of the spruce/pine transition at 10,500 yr B.P., an insect fauna had become established that is typical of southwestern Ontario today. The replacement of this fauna at ca. 8400 yr B.P. by one characteristic of the lowlands of the east-central United States represents the beginning of Hypsithermal conditions in southern Ontario. Vegetation and insects indicate that the climate continued to gradually warm through the mid-Holocene.

Drill cores of Enewetak Atoll, Marshall Islands, reveal six stratigraphic intervals, numbered in downward sequence, which represent vertical coral growth during Quaternary interglaciations. Radiocarbon dates indicate that the Holocene sea transgressed the emergent reef platform by about 8000 yr B.P. The reef grew rapidly upward (about 5 to 10 mm/yr) until about 6500 yr B.P. Afterward vertical growth slowed to about 0.5 mm/yr, then lateral development became dominant during the last several thousand years. The second interval is dated at 131,000 ± 3000 yr B.P. by uranium series. This unit correlates with oxygen-isotope substage 5e and with terrace VIIa of Huon Peninsula, New Guinea, and of Main Reef-2 terrace at Atauro Island. The third interval is not dated because corals were recrystallized and it is tentatively correlated with either oxygen-isotope stages 7 or 9. The age of the fourth interval is estimated at 454,000 ± 100,000 yr B.P. from measured 234U238U activity ratios. This unit is correlated with either oxygen-isotope stage 9, 11, or 13.

Simulations of cool-temperate forest growth in response to climatic change using the JABOWA computer model show that a decrease of 600 growing degree-days (equivalent to a 2°C decrease in mean annual temperature) causes red spruce (Picea rubens) to replace sugar maple (Acer saccharum) as the dominant tree. These changes are delayed 100–200 yr after the climatic cooling, producing gradual forest changes in response to abrupt temperature changes, and reducing the amplitude of response to brief climatic events. Soils and disturbances affect the speed and magnitude of forest response. The delayed responses are caused by the difference in sensitivity of adult trees and younger stages. The length of the delay depends on the life history characteristics of the dominant species. Delayed responses imply that fossil pollen deposits, even if they faithfully record the abundances of trees in forests, may not be able to resolve climatic changes within 100–200 yr, or to record very brief climatic events. This explains why pollen deposits do not as yet show responses to climatic changes during the past 100 yr. Only the Little Ice Age, which lasted several centuries, caused sufficient forest change to be recorded in fossil pollen, and only at certain sites.

Small-scale late Holocene vegetation changes were determined from a series of 13 modern and fossil packrat middens collected from a site in the Greenwater Valley, northern Mojave Desert, California. Although the site is above the modern lower limit of Coleogyne ramosissima (black-brush), macrofossils of this shrub are only present in samples younger than 270 yr B.P. In order to measure changes more subtle than presence vs absence, macrofossil concentrations were quantified, and principal components and factor analyses were used to distinguish midden plant assemblages. Both the presence/absence data and the statistical analyses suggest a downward shift of 50 to 100 m for Coleogyne (blackbrush) communities between 1435 and 1795 A.D.

The landscapes of northern New England and adjacent areas of Canada changed greatly between 14,000 and 9000 yr B.P.: deglaciation occurred, sea levels and shorelines shifted, and a vegetational transition from tundra to closed forest took place. Data from 51 14C-dated sites from a range of elevations were used to map ice and sea positions, physiognomic vegetational zones, and the spread of individual tree taxa in the region. A continuum of tundra-woodland-forest passed northeastward and northward without major hesitation or reversal. An increased rate of progression from 11,000 to 10,000 yr B.P. suggests a more rapid warming than in the prior 2000–3000 yr. Elevational gradients controlled the patterns of deglaciation and vegetational change. The earliest spread of tree taxa was via the lowlands of southern Vermont and New Hampshire, and along a coastal corridor in Maine. Only after 12,000 yr B.P. did the taxa spread northward through the rest of the area. Different tree species entered the southern part of the area at different times and continued their spread at different rates. The approximate order of arrival follows: poplars (13,000–12,000 yr B.P. in the south), spruces, paper birch, and jack pine, followed by balsam fir and larch, and possibly ironwood, ash, and elm, and somewhat later by oak, maple, white pine, and finally hemlock (10,000–9000 yr B.P. in the south).

Paleosols that formed primarily before the last glaciation cannot be adequately explained by a model of Holocene-like interglacial periods of about 10,000 yr in length which has been inferred from the marine 18O16O record. Geological and palynological evidence from terrestrial, midlatitude sites suggests that temperate conditions conducive to soil formation existed for periods greatly in excess of 10,000 yr during the middle and late Pleistocene. Available data on soils formed during this interval from Western Europe and midwestern United States are best explained by relatively long interglaciations or the development of composite soils over a number of temperate periods. Climatic contrasts between the interglaciations appear to have been overemphasized: widespread assumptions regarding greater warmth in interglaciations prior to the present one are generally unproven. In the absence of precise dating, progress in relating paleosols to fragmentary local records of Quaternary climatic changes will be achieved by detailed analytical techniques such as soil micromorphology.

A vegetation map reconstructed for the Japanese Archipelago (based upon pollen data from 28 sites and plant macrofossil data from 33 sites) at the time of last glacial maximum shows that coniferous forests covered extensive areas of the land. Boreal conifer forests (dominated by the Picea jezoensis complex, P. glehnii, Abies sachalinensis, A. mariesii, Tsuga diversifolia, and Pinus with Larix gmelinii, though the latter species was confined only to the northern part of northeastern Honshu and Hokkaido) occupied the modern cool-temperature deciduous broadleaf and mid-temperate conifer forest zones, and temperate coniferous forests (mainly Picea maximowiczii, P. polita, P. bicolor, P. koyamai, Abies firma, A. homolepis, Tsuga sieboldii, and Pinus), the present warm-temperate evergreen (laurilignosa) forest zone. Small populations of various broadleaf forest species were scattered in the full-glacial temperate conifer forest mainly along the coastal belt, and the true laurilignosa forest was limited in distribution, occurring only in the paleo-Yaku Peninsula.

Outcrops of an ash bed at several localities in northern California and western Nevada belong to a single air-fall ash layer, the informally named Rockland ash bed, dated at about 400,000 yr B.P. The informal Rockland pumice tuff breccia, a thick, coarse, compound tephra deposit southwest of Lassen Peak in northeastern California, is the near-source equivalent of the Rockland ash bed. Relations between initial thickness of the Rockland ash bed and distances to eruptive source suggest that the eruption was at least as great as that of the Mazama ash from Crater Lake, Oregon. Identification of the Rockland tephra allows temporal correlation of associated middle Pleistocene strata of diverse facies in separate depositional basins. Specifically, marine, littoral, estuarine, and fluvial strata of the Hookton and type Merced formations correlate with fluvial strata of the Santa Clara Formation and unnamed alluvium of Willits Valley and the Hollister area, in northwestern and west-central California, and with lacustrine beds of Mohawk Valley, fluvial deposits of the Red Bluff Formation of the eastern Sacramento Valley, and fluvial and glaciofluvial deposits of Fales Hot Spring, Carson City, and Washoe Valley areas in northeastern California and western Nevada. Stratigraphic relations of the Rockland ash bed and older tephra layers in the Great Valley and near San Francisco suggest that the southern Great Valley emerged above sea level about 2 my ago, that its southerly outlet to the ocean was closed sometime after about 2 my ago, and that drainage from the Great Valley to the ocean was established near the present, northerly outlet in the vicinity of San Francisco Bay about 0.6 my ago.