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This paper develops a composite absolute paleointensity record for Holocene paleomagnetic secular variation (PSV) from central North America. Twelve full‐vector (inclination, declination, paleointensity) PSV records were assessed in order to build the composite record. Nine of the paleointensity records come from sediment paleomagnetic studies and are considered relative in intensity. Three of the paleointensity records come from absolute paleointensity measurements of archeological materials and lava flows. This paper develops a new method to normalize the sediment relative paleointensity records to the absolute intensity records. The final composite paleointensity record describes intensity variability over a region of Central North America delineated by 35°–48.6°N and 240.4–291.4°E (∼14° × 50°). This composite record shows a distinctive long‐duration (∼104 year) oscillation and a series of millennial‐scale intensity oscillations that are consistent over our study region. Plain Language Summary: This study develops a composite record of geomagnetic field intensity for the central North America for the last 8,000 years. This record is important for considering the total geomagnetic field variability I this region and its dynamo source(s). Key Points: We develop a new composite absolute paleointensity record for Holocene Central North AmericaWe develop a new renormalization technique to combine absolute paleointensity records and sediment relative paleointensity recordsThe final composite absolute paleointensity record has a distinctive long‐term trend and millennial‐scale variability [ABSTRACT FROM AUTHOR]

A reply is made to comments by Payette (2004) concerning the paper that attempted to determine the age of the Tsuga canadensis decline in eastern North America (Bennett and Fuller, 2002). It is pointed out that his criticisms of the methods used are all aspects covered in the original paper, and which make little, if any, difference to the result obtained. However, ecological aspects of the decline were outside the scope of the paper. [ABSTRACT FROM AUTHOR]

Proxies that use changes in the composition of ecological communities to reconstruct temporal changes in an environmental covariate are commonly used in paleoclimatology and paleolimnology. Existing methods, such as weighted averaging and modern analog technique, relate compositional data to the covariate in very simple ways, and different methods are seldom compared systematically. We present a new Bayesian model that better represents the underlying data and the complexity in the relationships between species' abundances and a paleoenvironmental covariate. Using testate amoeba–based reconstructions of water-table depth as a test case, we systematically compare new and existing models in a cross-validation experiment on a large training dataset from North America. We then apply the different models to a new 7500-year record of testate amoeba assemblages from Caribou Bog in Maine and compare the resulting water-table depth reconstructions. We find that Bayesian models represent an improvement over existing methods in three key ways: more complete use of the underlying compositional data, full and meaningful treatment of uncertainty, and clear paths toward methodological improvements. Furthermore, we highlight how developing and systematically comparing methods lead to an improved understanding of the proxy system. This paper focuses on testate amoebae and water-table depth, but the framework and ideas are widely applicable to other proxies based on compositional data. [ABSTRACT FROM AUTHOR]

Geochemically fingerprinted widespread tephra layers serve as excellent marker horizons which can directly link and synchronize disparate sedimentary archives and be used for dating various deposits related to climate shifts, faulting events, tsunami, and human occupation. In addition, tephras represent records of explosive volcanic activity and permit assessment of regional ashfall hazard. In this paper we report a detailed Holocene tephrochronological model developed for the Kamchatsky Peninsula region of eastern Kamchatka (NW Pacific) based on ∼2800 new electron microprobe analyses of single glass shards from tephra samples collected in the area as well as on previously published data. Tephra ages are modeled based on a compilation of 223 14 C dates, including published dates for Shiveluch proximal tephra sequence and regional marker tephras; new AMS 14 C dates; and modeled calibrated ages from the Krutoberegovo key site. The main source volcanoes for tephra in the region are Shiveluch and Kliuchevskoi located 60–100 km to the west. In addition, local tephra sequences contain two tephras from the Plosky volcanic massif and three regional marker tephras from Ksudach and Avachinsky volcanoes located in the Eastern volcanic front of Kamchatka. This tephrochronological framework contributes to the combined history of environmental change, tectonic events, and volcanic impact in the study area and farther afield. This study is another step in the construction of the Kamchatka-wide Holocene tephrochronological framework under the same methodological umbrella. Our dataset provides a research reference for tephra and cryptotephra studies in the northwest Pacific, the Bering Sea, and North America. [ABSTRACT FROM AUTHOR]

Linking the stone raw material of a lithic artifact to its original outcrop is an important endeavor that facilitates archaeologists' understanding of prehistoric mobility, trade, economics, and land use. Based on macroscopic identification, archaeologists' presume that prehistoric Holocene occupants of the Welling Site, Ohio, made use of the locally available Upper Mercer chert for the majority of their projectile points. Here we apply visible derivative spectroscopy (VDS) to quantitatively assess this claim by examining 55 projectile points from the site. VDS identified 85%, whereas macroscopic analysis found 73% of the projectile points examined derived from the Upper Mercer Chert. These proportions are not statistically different given the sample size. Although the VDS analysis supported the hypothesis that Welling's Holocene projectile points were predominately produced from Upper Mercer chert, discrepancies between the results of macroscopic versus VDS chert identification point to future avenues of research including an expansion of VDS analyzed hand samples and comparison of VDS to other sourcing methods. Nevertheless, we propose that VDS has much promise and should be researched further so that archaeologists' can add another tool to their lithic artifact raw material sourcing toolbox. [ABSTRACT FROM AUTHOR]

Parapatric distributions between ecologically similar species have the potential to reveal the effects of limiting similarity in large-scale patterns of species coexistence. We present a qualitative model that predicts three different types of parapatric boundaries between competing species pairs: gap parapatry, abrupt parapatry and narrow sympatry. Our model integrates both niche-based and neutral theory perspectives of coexistence in the context of climatic tolerances, species equivalence, contingencies of dispersal history, priority effects and a heterogeneous patchwork of optimal and suboptimal habitat. Using species distribution models, tests of niche overlap and phylogeographical data, we evaluated assumptions and conditions of this model for four species of mostly allopatric, rock-dwelling rattlesnakes (Viperidae: Crotalus) inhabiting arid mountain ranges of the Mohave–Sonoran desert system of western North America. Our model was adequate for predicting the different types of range boundaries observed across this guild of ecologically similar species. Where species distributions were separated by a steep climatic gradient without evidence of recent dispersal, we observed gap parapatry; where historical dispersal was asymmetric, we observed abrupt parapatry; and finally, where historical dispersal was symmetric, we observed narrow sympatry. [ABSTRACT FROM AUTHOR]

Stand-scale gap-phase dynamics is generally viewed as the main driver of development in mesic deciduous forests of the temperate biome. Soil charcoal of temperate forests in eastern North America are unnoticed in most surveys, thus explaining why fire is undervalued as a driver of forest succession. The extent to which gap-phase, fire, or other processes are responsible for the regeneration and maintenance of mesic deciduous forests is unknown because paleoecological evidence is lacking. We tested the fire-driven succession hypothesis on the development of this major forest type. Based on charcoal 14C dates of two sites, 44 and 55 fires occurred since early Holocene, with a mean interval of 170 to 215 years. The vegetation of both sites followed comparable post-glacial trajectories consisting of three distinct periods. Conifers dominated the two first periods during 5200–6000 years and were replaced by hardwoods–conifers over the last 3500 years. The first period was represented by boreal conifers, whereas the second period, dominated by white pine (Pinus strobus) forests, persisted during 3000–4300 years. The third period marked the development of hardwood (sugar maple, Acer saccharum) forests. Fires occurred continuously on the sites since early Holocene likely under dry conditions during the conifer periods and cooler and moister conditions during the hardwood–conifer period. Recurrent fires appear with climate as key drivers of the long-term dynamics of several temperate forests in eastern North America. Similar studies on other temperate forests should be pursued to test the hypothesis of climate–fire interactions influencing tree composition change. [ABSTRACT FROM AUTHOR]

Abstract: The prairie-forest transition in midcontinental North America is a major physiognomic boundary, and its shifts during the Holocene are a classic example of climate-driven ecotonal dynamics. Recent work suggests asymmetrical Holocene behavior, with a relatively rapid early Holocene deforestation and more gradual reforestation later in the Holocene. This paper presents a new synthesis of the Holocene history of the Great Plains prairie-forest ecotone in the north-central US and central Canada that updates prior mapping efforts and systematically assesses rates of change. Changes in percent woody cover (%WC) are inferred from fossil pollen records, using the modern analog technique and surface-sediment pollen samples cross-referenced against remotely sensed observations. For contemporary pollen samples from the Great Plains, %WC linearly correlates to percent arboreal pollen (%AP), but regression parameters vary interregionally. At present, %AP is consistently higher than %WC, because of high background levels of arboreal pollen. Holocene maps of the eastern prairie-forest ecotone agree with prior maps, showing a rapid decrease in %WC and eastward prairie advance between 10,000 and 8000 ka (1 ka=1000 calibrated years before present), a maximum eastward position of the ecotone from 7 to 6 ka, and increased %WC and westward prairie retreat after 6 ka. Ecotone position is ambiguous in Iowa and southeastern Minnesota, due to a scarcity of modern analogs for early-Holocene samples with high Ulmus abundances and for samples from alluvial sediments. The northern prairie-forest ecotone was positioned in central Saskatchewan between 12 and 10 ka, stabilized from 10 to 6 ka despite decreases in %WC at some sites, then moved south after 6 ka. In both east and north, ecotonal movements are consistent with a dry early Holocene and increasing moisture availability after 6 ka. Sites near the ecotone consistently show an asymmetric pattern of abrupt early Holocene deforestation (<300 years) and gradual reforestation after 6 ka. Early Holocene decreases in %WC are faster than the corresponding drops in %AP, because the analog-based %WC reconstructions correct for the high background levels of arboreal pollen types that blur temporal variations in %AP. For example, at Elk Lake, the %AP decline lasts 1000 years, whereas the %WC decline occurs between adjacent pollen samples, approximately 300 years apart. Thus, early Holocene deforestation may have been even more abrupt than previously recognized. Rapid deforestation likely was promoted both by rapid climate changes around 8.2 ka and positive fire-vegetation feedbacks. Non-linear vegetational responses to hydrological variability are consistent with 1) other paleorecords showing rapid die-offs of some eastern tree species in response to aridity and 2) observations of threshold-type ecological responses to recent climate events. The 21st-century trajectory for the Great Plains prairie-forest ecotone is uncertain, because climate models differ over the direction of regional precipitation trends, but future drying would be more likely to trigger threshold-type shifts in ecotone position. [Copyright &y& Elsevier]

The new date provided by Bennett and Fuller (2002) for the mid-Holocene hemlock (Tsuga canadensis) decline in eastern North America is questioned in the light of recent advances in the palaeoecology of the decline. It is concluded that the 'new' date gives a false impression of the dating accuracy of a complex ecological event of relatively long duration. [ABSTRACT FROM AUTHOR]

Recently-developed Holocene lake-level reconstructions from the Rocky Mountains offer a quantitative target for testing the skill of state-of-the-art climate system models in simulating hydroclimate change. Here, we use a combination of hydrologic models of catchment streamflow, lake energy balance, and lake water balance to simulate lake level at Little Windy Hill Pond (LWH) in the Medicine Bow Range of Wyoming for a period of severe drought during the mid-Holocene (MH; approximately 6000 years ago). Using Coupled Model Intercomparison Project (CMIP5) output to drive our hydrologic models, we find that none of our simulations reproduce the significantly lowered lake levels at LWH during the MH. Rather, simulated hydroclimate changes for the MH are modest (< 10% reductions in precipitation and streamflow and generally 10–30% increases in lake evaporation), and LWH lake-level changes are buffered by the large volume of snowmelt runoff that the lake receives. Only when winter precipitation is approximately halved in sensitivity experiments do water inputs to the lake become small enough that lake level can be significantly drawn down by year-over-year negative water balances. Possible explanations for the model-data mismatch could lie in the realism of our hydrological modeling framework or in the accuracy of the CMIP5 output, the latter having important implications for projections of future drying in western North America. [ABSTRACT FROM AUTHOR]

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Continuous sediment, charcoal, and pollen records were developed from a ~7-m sediment core from Prater Canyon in Mesa Verde National Park (MEVE), Colorado, USA. Sediment input into the canyon is episodic and is linked to precipitation runoff and vegetation cover. Pollen recovered from the Prater Canyon sediment core reflect the vegetation changes within the MEVE region. During the period recorded, the vegetation of the region surrounding Prater Canyon transitioned from xeric adapted species in an open environment to a more mesic, Pinus edulis–Juniperus osteosperma (piñon–juniper) woodland over the last 1500 years. Two distinct changes in fire frequency occurred. Before 4080 cal. yr BP, fires occurred at a much more frequent rate (2.5–12 fires/200 years) than from 4060 cal. yr BP to present (0–2 fires/200 years). Most importantly, the variations occurring in the charcoal record for the past 2500 years coincide with both shifts in human occupation and climate fluctuations within the region, with burning increasing during Ancestral Puebloan occupation and moist but increasingly dry conditions, and declines in both at the end of the ‘Medieval Climate Anomaly’ (MCA). The record from Prater Canyon demonstrates the importance of the Ancestral Puebloans in landscape modification during their occupation from ad1 to 1300. Charcoal deposition also increased during the 20th- to 21st-century transition with the highest deposition rates of the core recorded then. [ABSTRACT FROM PUBLISHER]

A palynological reconstruction (n = 25 profiles) suggests that the northern extent of lodgepole pine (Pinus contorta var. latifolia Engelmann ex S. Watson) occurred between 59° and 60°N latitude in northwest North America from 10,000 to 7000 calendar years before present (cal. yr BP) prior to entering Yukon. Although specific migration pathways could not be resolved with the available palynological data, mountains along the southern edge of Yukon appear to have diverted lodgepole pine migration through the Carcross and Frances Lake areas in southwest and southeast Yukon, respectively. Migration in the southwest (70 m/yr) was likely confined to lower elevations of the Yukon and Teslin river valleys, with lodgepole pine reaching 61°N ~2000 cal. yr BP. Along the eastern route, migration was channeled through a 15–20 km wide pass in a 200 km mountainous front. After breaching the Liard drainage divide north of Frances Lake ~4000 cal. yr BP, migration progressed northwest (160–220 m/yr) along the Tintina Trench. Lodgepole pine was estimated to have reached its near present-day northern limit (~63°N) ~1790 cal. yr BP, which is ~1290 years earlier than previously thought. This difference in arrival dates is due the use of a >5% rather than a >15% pine pollen content threshold, which appears to correspond with >1% pine tree cover in the landscape. Climatic cooling after 1000 cal. yr BP that caused a population decline at higher elevations is hypothesized to explain the present-day sparse and disjunct distribution of pine across Yukon north of 61°N. [ABSTRACT FROM AUTHOR]

Premise of the study: Predicting species responses to climate change has become a dynamic field in global change research. A crucial question in this debate is whether-or-not species have been and will be able to respond quickly enough to keep up with changing climatic conditions. Methods: Focusing on fossil pollen records and paleoclimatic simulations, this work assesses the change in realized climatic niches (climatic temporal trajectories) of 20 plant taxa over the last 16000 yr, and whether this tracking has been the same for different climatic niche dimensions. Key results: Climatic factors showed a consistent trend toward warmer temperatures and higher precipitation. Although the response types varied across taxa, species' realized climatic niches lagged in response to changes in climatic conditions. Temperature niches responded to late Pleistocene (16000-11 000 yr ago) climate change, but did so at slower rates than changes in climatic conditions during the same period. In contrast, precipitation niches were relatively stable from 16000 to 11 000 yr ago, but still lagged behind changes in climatic conditions. Changes in temperature and precipitation niches eventually stabilized during the Holocene (1 I 000-1000 yr ago). Conclusions: These results underscore how the climatic niche realized at any one moment represents a subset of the climate conditions in which a taxon can persist, particularly during times of fast climatic change. Variability in the rates of temporal trajectories across evaluated climatic variables showed taxa specific responses to changes in climatic conditions over time and emphasizes the need to incorporate variation, intensity, and duration of lag effects in assessments of the possible effects of climatic change. [ABSTRACT FROM AUTHOR]

The mid-continent of North America has experienced dramatic and abrupt climate change during the Holocene, but the response of grassland vegetation to past climate change has been difficult to quantify. To improve interpretation of tallgrass prairie vegetation from pollen assemblages, we acquired and analysed a surface sample set collected from 25 small ponds (less than 10 ha surface area) in the largest contiguous remnant of tallgrass prairie in the USA. We compared these tallgrass prairie assemblages to 476 modern pollen samples classified as 'prairie' in the North American Surface Sample database. We then compared the surface pollen assemblages with fossil pollen assemblages from sediment cores at two sites in Kansas-Cheyenne Bottoms and Muscotah Marsh-using the modern analog technique. Pollen assemblages in the Flint Hills surface samples were very similar to each other, with an average squared chord distance of 0.19. They were different than other modern grassland pollen assemblages mainly due to higher percentages of pollen from six woody taxa: Carya, Cornus, Juniperus, Juglans, Maclura, and Platanus. Arboreal pollen percentages ranged from 17 to 62 % and did not correlate with woody cover among sites. Cheyenne Bottoms was open grassland for the past 25,000 years, but it did not have many tallgrass prairie analogs. Muscotah Marsh did not have many grassland analogs over the past 30,000 years, possibly due to its position on the prairie-forest border or its surrounding wetland vegetation. [ABSTRACT FROM AUTHOR]

The mid-Holocene decline of Tsuga canadensis (hereafter Tsuga) populations across eastern North America is widely perceived as a synchronous event, driven by pests/pathogens, rapid climate change, or both. Pattern identification and causal attribution are hampered by low stratigraphic density of pollen-sampling and radiometric dates at most sites, and by absence of highly resolved, paired pollen and paleoclimate records from single sediment cores, where chronological order of climatic and vegetational changes can be assessed. We present an intensely sampled (contiguous 1-cm intervals) record of pollen and water table depth (inferred from testate amoebae) from a single core spanning the Tsuga decline at Irwin Smith Bog in Lower Michigan, with high-precision chronology. We also present an intensively sampled pollen record from Tower Lake in Upper Michigan. Both sites show high-magnitude fluctuations in Tsuga pollen percentages during the pre-decline maximum. The terminal decline is dated at both sites ca. 5000 cal yr BP, some 400 years later than estimates from other sites and data compilations. The terminal Tsuga decline was evidently heterochronous across its range. A transient decline ca. 5350 cal yr BP at both sites may correspond to the terminal decline at other sites in eastern North America. At Irwin Smith Bog, the terminal Tsuga decline preceded an abrupt and persistent decline in water table depths by ∼200 years, suggesting the decline was not directly driven by abrupt climate change. The Tsuga decline may best be viewed as comprising at least three phases: a long-duration pre-decline maximum with high-magnitude and high-frequency fluctuations, followed by a terminal decline at individual sites, followed in turn by two millennia of persistently low Tsuga populations. These phases may not be causally linked, and may represent dynamics taking place at multiple temporal and spatial scales. Further progress toward understanding the phenomenon requires an expanded network of high-resolution pollen and paleoclimate chronologies. [ABSTRACT FROM AUTHOR]

Aim Using a new approach to analyse fossil pollen data, we investigate temporal and spatial patterns in Populus (poplar, cottonwood, aspen) from the Late Glacial to the present at regional to continental scales. Location North America. Methods We extracted data on the timing and magnitude of the maximum value of Populus pollen from each pollen diagram in the North American Pollen Database (NAPD). The information was plotted in histograms of 150-year bins to identify times when Populus was abundant on the landscape. We also mapped the maximum values to identify spatial patterns and their causes. Results Our analyses show that there have been several periods since the Late Glacial when Populus was abundant on the landscape: (1) from 12.35 to 12.65 kyr bp, in eastern North America, largely in response to the opening of the forest following the onset of the Younger Dryas; (2) from 10.85 to 11.75 kyr bp, following the termination of the Younger Dryas; and (3) during the last 150 years, as land was cleared for agricultural use, especially in the midwestern United States. Main conclusion Since the Late Glacial, changes in the abundance of Populus were caused more by the effects of abrupt climate change on its major competitors, rather than the direct effects of climate on Populus itself. [ABSTRACT FROM AUTHOR]

The Global Pollen Database is an example of a successful data synthesis effort that has uses for biogeographical and climate change studies. Results are of interest in many fields of physical geography. Continental-scale maps of past conditions have been used in data-model comparison studies. Time series, developed by averaging quantitative reconstructions from many sites, have indicated that millennial-scale climate variability has affected the vegetation of Europe and North America during the Holocene. Major transitions in the vegetation of Europe and North America occurred at the same time, suggesting the overriding climate effect on the vegetation of both continents. The database can also be used to test biogeographical hypotheses, as several examples illustrate, without the need for collecting new data. Hundreds of studies over the past 50 years show that pollen analysis is more precise than frequently acknowledged: vegetation responds rapidly to climate variations, changes in vegetation are spatially coherent and the taxonomic resolution available in the database is greater than frequently acknowledged. The availability of a public, freely available database enables different analyses to be performed on the same data, thereby ensuring that results are not dependent on methodology. [ABSTRACT FROM AUTHOR]

Contemporary evolution of Pacific salmon ( Oncorhynchus spp.) is best viewed in the context of the evolutionary history of the species and the dynamic ecosystems they inhabit. Speciation was complete by the late Miocene, leaving c. six million years for intraspecific diversification. Following the most recent glacial maximum, large areas became available for recolonization. Current intraspecific diversity is thus the product of recent evolution overlaid onto divergent historical lineages forged during recurrent episodes of Pleistocene glaciation. In northwestern North America, dominant habitat features have been relatively stable for the past 5000 years, but salmon ecosystems remain dynamic because of disturbance regimes (volcanic eruptions, landslides, wildfires, floods, variations in marine and freshwater productivity) that occur on a variety of temporal and spatial scales. These disturbances both create selective pressures for adaptive responses by salmon and inhibit long-term divergence by periodically extirpating local populations and creating episodic dispersal events that erode emerging differences. Recent anthropogenic changes are replicated pervasively across the landscape and interrupt processes that allow natural habitat recovery. If anthropogenic changes can be shaped to produce disturbance regimes that more closely mimic (in both space and time) those under which the species evolved, Pacific salmon should be well-equipped to deal with future challenges, just as they have throughout their evolutionary history. [ABSTRACT FROM AUTHOR]

Thick Holocene Bignell Loess sections provide new evidence for the timing of early to middle Holocene aridity in the central Great Plains. The immediate source of loess in these sections was dune fields just upwind, based on thickness trends and grain size data. Thus, periods of rapid loess accumulation indicate episodes of extensive dune activity under drier-than-present climate. In typical Bignell Loess sections, a thick zone of coarse-textured loess with minimal pedogenic alteration is interpreted as a record of the most rapid, sustained Holocene loess accumulation. Optical ages indicate that loess in this zone was deposited beginning about 9000–10000 years ago at five study sites. Accumulation ended shortly after 6500 years ago at three sites, but possibly earlier at the other two. In two well-dated sections, the average rate of loess accumulation between 9000–10 000 and 6500 years ago was about 1.6–3.5 times greater than in the late Holocene. Thus, we infer that the most extensive, sustained Holocene dune field activity, reflecting sustained aridity, occurred from around 9000–10 000 years ago to just after 6500 years ago. Comparison of the Bignell Loess record with other proxy data from the Great Plains indicates general agreement on an early–middle Holocene dry period, but also provides new evidence for spatial variation of early to middle Holocene climatic change, with possible time transgression, both from west to east and from north to south. [ABSTRACT FROM AUTHOR]

Biotic causes of changes in palaeorecords are discussed as a counterbalance to the (over) emphasis on climatic drivers in the interpretation of palaeoecological change. Biotic and climatic drivers are considered as competing hypotheses with particular reference to the importance of early-Holocene drought in the northern central grassland region of North America. It is concluded that the observed patterns are explained better by grazers than by the direct effects of climatic variation. The elevation of biotic factors as competing hypotheses with climate will require new conceptual and methodological approaches to palaeoecology in North America. [ABSTRACT FROM AUTHOR]

Dunefields of the Great Plains contain stratigraphic records of episodic Holocene aeolian activity, potentially providing a valuable record of climatic change. It has been difficult to establish unambiguous links between activity at specific sites and regional palaeoclimate, however. Here we demonstrate that widespread late-Holocene aeolian activity in the Nebraska Sand Hills, the largest dunefield on the Great Plains of North America, occurred during a period of hydrological drought. Aeolian sand sheets are interbedded with peat that accumulated in interdunes throughout much of the Holocene. The youngest of these sand sheets was deposited between 950 and 650 cal. BP, about the same time as the most recent major episode of aeolian activity at upland sites up to 120 km apart. Sand sheets could only have advanced across the interdunes if the local groundwater flow systems that maintain interdune wetlands were temporarily reduced or eliminated by a reduction in recharge. Both reduced recharge and upland aeolian activity are best explained by regional megadrought. [ABSTRACT FROM AUTHOR]

Quantitative climate reconstructions based on marine dinocysts and terrestrial pollen sequences are consistent through the Holocene in northeastern North America. Principal components analysis (PCA) indicates a large-scale climate signal in the dinocysts and pollen. The combined and separate analyses of marine and nearby terrestrial pollen sequences from Hudson Bay, Labrador and the St Lawrence estuary differentiate tundra, boreal forest and deciduous forest assemblages in time and space. These analyses indicate that the marine pollen record reflects vegetation changes of the regional terrestrial environment and allows direct correlations between the marine and terrestrial stratigraphies. Sea-surface temperatures estimated from dinocysts and terrestrial air temperatures from pollen using the method of modern analogs show that the three regions had differing climate histories associated with their location with respect to deglaciation and air mass boundaries. High frequency climatic changes reconstructed for the St Lawrence estuary and Gulf, and a cooling reconstructed for the period prior to 8000 yr BP, are less reliable due to the larger values of the dissimilarity coefficients. Prior to 6000 yr BP, cool temperatures reconstructed along the Labrador margins, both in the marine and terrestrial environments, are in agreement with climate simulations indicating the persistence of an anticyclone over the Québec-Labrador ice sheet. In both Labrador and northwestern Québec, a late-Holocene cooling affects sites in the forest-tundra, but is not evident in Boreal forest sites, suggesting movements in the mean position of the polar front. [ABSTRACT FROM AUTHOR]

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 diatom-inferred 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. [ABSTRACT FROM AUTHOR]