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At least 19 recordsLinked to original sources
USGS2001

Estimativas de possiveis recursos de petroleo e gas na America Central e na America do Sul [Estimates of possible petroleum and gas resources in Central American and South America]

O U.S. Geological Survey recentemente completou estimativas de possíveis recursos de petróleo e gás em 130 áreas petrolíferas pré-determinadas no mundo (USGS, 2000). Vinte e três destas áreas ficam na América do Sul, na América Central, e no Caribe (fig. 1). Os resultados estão apresentados na tabela 1. Nas 23 áreas, estimamos um total de 105 BBO e um total de 487 TCFG. A região composta de América Central mais América do Sul ficou em terceiro lugar no mundo em termos de possíveis recursos de petróleo e gás. No primeiro lugar ficou o Oriente Médio e no segundo lugar ficou a antiga União Soviética (USGS, 2000). As áreas com maiores probabilidades de encontrar depósitos gigantes de petróleo e gás se localizam nas áreas do Oceano Atlântico começando com a Bacia de Santos no sul até a Bacia Guyana-Suriname no norte. As possibilidades de existirem depósitos gigantes são maiores nas áreas submersas do mar até profundidades de 3,600 m. Diversos depósitos gigantes de petróleo foram descobertos no mar na Bacia de Campos e ainda podem serem encontrados depósitos similares na Bacia de Campos e suas imediações.

Fact Sheet↗
USGS2007

Atlas of relations between climatic parameters and distributions of important trees and shrubs in North America: Ecoregions of North America

Climate is the primary factor controlling the continental-scale distribution of plant species, although the relations between climatic parameters and species' ranges are only now beginning to be quantified. This volume examines the relations between climate and the distributions of (1) Kuchler's 'potential natural vegetation' categories for the 48 contiguous States of the United States of America, (2) Bailey's ecoregions of North America, and (3) World Wildlife Fund's ecoregions of North America. For these analyses, we employed a 25-kilometer equal-area grid of modern climatic and bioclimatic parameters for North America, coupled with presence-absence data for the occurrence of each ecoregion under the three classification systems under consideration. The resulting relations between climate and ecoregion distributions are presented in graphical and tabular form. Presentation of ecoregion-climate relations here is intended to be useful for a greater understanding of ecosystem evolution, ecosystem dynamics, and potential effects of future climate change on ecoregions.

Professional Paper↗
USGS1984

Chronology of late Wisconsinan glaciation in middle North America

We propose a chronology of late Wisconsinan glacial fluctuations in middle North America, from Alberta to Wisconsin, based on radiocarbon dates derived solely from wood. Previous chronologies of the southwestern margin of the North American Continental Ice Sheet have depended to a considerable degree on radiocarbon dates from fine-grained organic sediment. This material is commonly contaminated with older carbon, resulting in chronologic confusion. By using only dates from wood, much of the confusion disappears. However, because of the scarcity of wood dates, only four of the sixteen identified fluctuations are accurately dated: an advance into Iowa about 14,000 to 13,500 BP, an advance into South Dakota and Iowa about 12,300 BP, an advance into the Lake Michigan basin about 11,700 BP, and an advance into the Lake Superior basin about 9900 BP. In addition, the beginning of late Wisconsinan glaciation, before 20,000 BP, is fairly well documented. None of the fluctuations in the western part of the region are accurately dated.

middle North America↗
USGS1999

Fire and aquatic ecosystems in forested biomes of North America

Synthesis of the literature suggests that physical, chemical, and biological elements of a watershed interact with long‐term climate to influence fire regime, and that these factors, in concordance with the postfire vegetation mosaic, combine with local‐scale weather to govern the trajectory and magnitude of change following a fire event. Perturbation associated with hydrological processes is probably the primary factor influencing postfire persistence of fishes, benthic macroinvertebrates, and diatoms in fluvial systems. It is apparent that salmonids have evolved strategies to survive perturbations occurring at the frequency of wildland fires (10°–10 2 years), but local populations of a species may be more ephemeral. Habitat alteration probably has the greatest impact on individual organisms and local populations that are the least mobile, and reinvasion will be most rapid by aquatic organisms with high mobility. It is becoming increasingly apparent that during the past century fire suppression has altered fire regimes in some vegetation types, and consequently, the probability of large stand‐replacing fires has increased in those areas. Current evidence suggests, however, that even in the case of extensive high‐severity fires, local extirpation of fishes is patchy, and recolonization is rapid. Lasting detrimental effects on fish populations have been limited to areas where native populations have declined and become increasingly isolated because of anthropogenic activities. A strategy of protecting robust aquatic communities and restoring aquatic habitat structure and life history complexity in degraded areas may be the most effective means for insuring the persistence of native biota where the probability of large‐scale fires has increased.

Transactions of the American Fisheries Society↗
USGS2003

Mid-Holocene climates of the Americas: a dynamical response to changed seasonality

Simulations of the climatic response to mid-Holocene (6 ka BP) orbital forcing with two coupled ocean–atmosphere models (FOAM and CSM) show enhancement of monsoonal precipitation in parts of the American Southwest, Central America and northernmost South America during Northern Hemisphere summer. The enhanced onshore flow that brings precipitation into Central America is caused by a northward displacement of the inter-tropical convergence zone, driven by cooling of the equatorial and warming of the northern subtropical and mid-latitude ocean. Ocean feedbacks also enhance precipitation over the American Southwest, although the increase in monsoon precipitation there is largely driven by increases in land-surface temperature. The northward shift in the equatorial precipitation band that causes enhanced precipitation in Central America and the American Southwest has a negative feedback effect on monsoonal precipitation in northern South America. The simulations demonstrate that mid-Holocene aridity in the mid-continent of North America is dynamically linked to the orbitally induced enhancement of the summer monsoon in the American Southwest, with a spatial structure (wet in the Southwest and dry in the mid-continent) similar to that found in strong monsoon years today. Changes in winter precipitation along the west coast of North America, in Central America and along the Gulf Coast, caused by southward-displacement of the westerly storm tracks, indicate that changes in the Northern Hemisphere winter monsoon also play a role in regional climate changes during the mid-Holocene. Although the simulations with FOAM and CSM differ in detail, the general mechanisms and patterns are common to both. The model results thus provide a coherent dynamical explanation for regional patterns of increased or decreased aridity shown by vegetation, lake status and aeolian data from the Americas.

Climate Dynamics↗
USGS1981

Geographic distribution and dispersal of normapolles genera in North America

Normapolles pollen have been found in North America in Cretaceous and Lower Tertiary rocks from the eastern Atlantic Seaboard, the Mississippi embayment region and from the states and provinces from western North America as far north as the District of Mackenzie, Northwest Territories. Previous postulates relating to the Normapolles floral province (western Europe-eastern North America) were re-examined in the light of new finds of Normapolles genera in rocks from west of the Cretaceous epeiric seaway which separated the Normapolles province from the western North American Aquilapollenites province. A study of published occurrences of Normapolles genera and U.S. Geological Survey Denver Laboratory Normapolles records revealed that of the approximately 60 Normapolles genera recognized from western Europe, only 26 of these have been recognized from eastern North America. These data suggest that Normapolles-producing plants originated in western Europe and migrated to eastern North America prior to the opening of the north Atlantic seaway. Ten of these 26 genera also have been found in rocks from west of the Cretaceous epeiric seaway, suggesting that these genera were the only ones able to cross this barrier. At least six genera having Normapolles characteristics occur in eastern North America but have not yet been recorded from Europe. Two additional genera with Normapolles characteristics have been reported only from the Aquilapollenites province of western North America. Several discrepancies in the record need resolution, such as the latitudinal restriction of Thomsonipollis and Nudopollis to areas south 40??N latitude, the absence of records of Thomsonipollis east and north of central Georgia, and the absence of records of Kyandopollenites and Choanopollenites west of eastern Texas. These data show that the known boundaries of the Normapolles province are somewhat hazy and that firm conclusions regarding the geographic distribution and history of dispersal of Normapolles genera are premature. ?? 1981.

Review of Palaeobotany and Palynology↗
USGS2018

Lessons learned from research and surveillance directed at highly pathogenic influenza A viruses in wild birds inhabiting North America

Following detections of highly pathogenic (HP) influenza A viruses (IAVs) in wild birds inhabiting East Asia after the turn of the millennium, the intensity of sampling of wild birds for IAVs increased throughout much of North America. The objectives for many research and surveillance efforts were directed towards detecting Eurasian origin HP IAVs and understanding the potential of such viruses to be maintained and dispersed by wild birds. In this review, we highlight five important lessons learned from research and surveillance directed at HP IAVs in wild birds inhabiting North America: (1) Wild birds may disperse IAVs between North America and adjacent regions via migration, (2) HP IAVs can be introduced to wild birds in North America, (3) HP IAVs may cross the wild bird-poultry interface in North America, (4) The probability of encountering and detecting a specific virus may be low, and (5) Population immunity of wild birds may influence HP IAV outbreaks in North America. We review empirical support derived from research and surveillance efforts for each lesson learned and, furthermore, identify implications for future surveillance efforts, biosecurity, and population health. We conclude our review by identifying five additional areas in which we think future mechanistic research relative to IAVs in wild birds in North America are likely to lead to other important lessons learned in the years ahead.

Virology↗
USGS2009

Database of the geologic map of North America— Adapted from the map by J.C. Reed, Jr. and others (2005)

Introduction The Geological Society of America's (GSA) Geologic Map of North America (Reed and others, 2005a; 1:5,000,000) shows the geology of a significantly large area of the Earth, centered on North and Central America and including the submarine geology of parts of the Atlantic and Pacific Oceans. This map is now converted to a Geographic Information System (GIS) database that contains all geologic and base-map information shown on the two printed map sheets and the accompanying explanation sheet. We anticipate this map database will be revised at some unspecified time in the future, likely through the actions of a steering committee managed by the GSA and staffed by scientists from agencies including those responsible for the original map compilation. Regarding the use of this product, as noted by the map's compilers: “The Geologic Map of North America is an essential educational tool for teaching the geology of North America to university students and for the continuing education of professional geologists in North America and elsewhere. In addition, simplified maps derived from the Geologic Map of North America are useful for enlightening younger students and the general public about the geology of the continent.” With publication of this database, the preparation of any type of simplified map is made significantly easier. More important perhaps, the database provides a more accessible means to explore the map information and to compare and analyze it in conjunction with other types of information (for example, land use, soils, biology) to better understand the complex interrelations among factors that affect Earth resources, hazards, ecosystems, and climate.

Data Series↗
USGS2022

MIS 5e sea-level history along the Pacific coast of North America

The primary last interglacial, marine isotope substage (MIS) 5e records on the Pacific coast of North America, from Washington (USA) to Baja California Sur (Mexico), are found in the deposits of erosional marine terraces. Warmer coasts along the southern Golfo de California host both erosional marine terraces and constructional coral reef terraces. Because the northern part of the region is tectonically active, MIS 5e terrace elevations vary considerably, from a few meters above sea level to as much as 70 m above sea level. The primary paleo-sea-level indicator is the shoreline angle, the junction of the wave-cut platform with the former sea cliff, which forms very close to mean sea level. Most areas on the Pacific coast of North America have experienced uplift since MIS 5e time, but the rate of uplift varies substantially as a function of tectonic setting. Chronology in most places is based on uranium-series ages of the solitary coral Balanophyllia elegans (erosional terraces) or the colonial corals Porites and Pocillopora (constructional reefs). In areas lacking corals, correlation to MIS 5e often can be accomplished using amino acid ratios of fossil mollusks, compared to similar ratios in mollusks that also host dated corals. Uranium-series (U-series) analyses of corals that have experienced largely closed-system histories range from ∼124 to ∼118 ka, in good agreement with ages from MIS 5e reef terraces elsewhere in the world. There is no geomorphic, stratigraphic, or geochronological evidence for more than one high-sea stand during MIS 5e on the Pacific coast of North America. However, in areas of low uplift rate, the outer parts of MIS 5e terraces apparently were re-occupied by the high-sea stand at ∼100 ka (MIS 5c), evident from mixes of coral ages and mixes of molluscan faunas with differing thermal aspects. This sequence of events took place because glacial isostatic adjustment processes acting on North America resulted in regional high-sea stands at ∼100 and ∼80 ka that were higher than is the case in far-field regions, distant from large continental ice sheets. During MIS 5e time, sea surface temperatures (SSTs) off the Pacific coast of North America were higher than is the case at present, evident from extralimital southern species of mollusks found in dated deposits. Apparently, no wholesale shifts in faunal provinces took place, but in MIS 5e time, some species of bivalves and gastropods lived hundreds of kilometers north of their present northern limits, in good agreement with SST estimates derived from foraminiferal records and alkenone-based reconstructions in deep-sea cores. Because many areas of the Pacific coast of North America have been active tectonically for much or all of the Quaternary, many earlier interglacial periods are recorded as uplifted, higher-elevation terraces. In addition, from southern Oregon to northern Baja California, there are U-series-dated corals from marine terraces that formed at ∼80 ka, during MIS 5a. In contrast to MIS 5e, these terrace deposits host molluscan faunas that contain extralimital northern species, indicating cooler SST at the end of MIS 5. Here I present a review and standardized database of MIS 5e sea-level indicators along the Pacific coast of North America and the corresponding dated samples. The database is available in Muhs et al. (2021b; https://doi.org/10.5281/zenodo.5903285 ).

Earth System Science Data↗
USGS1998

North-South precipitation patterns in western North America on interannual-to-decadal timescales

The overall amount of precipitation deposited along the West Coast and western cordillera of North America from 25??to 55??N varies from year to year, and superimposed on this domain-average variability are varying north-south contrasts on timescales from at least interannual to interdecadal. In order to better understand the north-south precipitation contrasts, their interannual and decadal variations are studied in terms of how much they affect overall precipitation amounts and how they are related to large-scale climatic patterns. Spatial empirical orthogonal functions (EOFs) and spatial moments (domain average, central latitude, and latitudinal spread) of zonally averaged precipitation anomalies along the westernmost parts of North America are analyzed, and each is correlated with global sea level pressure (SLP) and sea surface temperature series, on interannual (defined here as 3-7 yr) and decadal (>7 yr) timescales. The interannual band considered here corresponds to timescales that are particularly strong in tropical climate variations and thus is expected to contain much precipitation variability that is related to El Nino-Southern Oscillation; the decadal scale is defined so as to capture the whole range of long-term climatic variations affecting western North America. Zonal EOFs of the interannual and decadal filtered versions of the zonal-precipitation series are remarkably similar. At both timescales, two leading EOFs describe 1) a north-south seesaw of precipitation pivoting near 40??N and 2) variations in precipitation near 40??N, respectively. The amount of overall precipitation variability is only about 10% of the mean and is largely determined by precipitation variations around 40??-45??N and most consistently influenced by nearby circulation patterns; in this sense, domain-average precipitation is closely related to the second EOF. The central latitude and latitudinal spread of precipitation distributions are strongly influenced by precipitation variations in the southern parts of western North America and are closely related to the first EOF. Central latitude of precipitation moves south (north) with tropical warming (cooling) in association with midlatitude western Pacific SLP variations, on both interannual and decadal timescales. Regional patterns and zonal averages of precipitation-sensitive tree-ring series are used to corroborate these patterns and to extend them into the past and appear to share much long- and short-term information with the instrumentally based zonal precipitation EOFs and moments.The overall amount of precipitation deposited along the West Coast and western cordillera of North America from 25?? to 55 ??N varies from year to year, and superimposed on this domain-average variability are varying north-south contrasts on timescales from at least interannual to interdecadal. In order to better understand the north-south precipitation contrasts, their interannual and decadal variations are studied in terms of how much they affect overall precipitation amounts and how they are related to large-scale climatic patterns. Spatial empirical orthogonal functions (EOFs) and spatial moments (domain average, central latitude, and latitudinal spread) of zonally averaged precipitation anomalies along the westernmost parts of North America are analyzed, and each is correlated with global sea level pressure (SLP) and sea surface temperature series, on interannual (defined here as 3-7 yr) and decadal (>7 yr) timescales. The interannual band considered here corresponds to timescales that are particularly strong in tropical climate variations and thus is expected to contain much precipitation variability that is related to El Nino-Southern Oscillation; the decadal scale is defined so as to capture the whole range of long-term climatic variations affecting western North America. Zonal EOFs of the interannual and decadal filtered versions of the zonal-precipitation series are remarkably similar. At both tim

Journal of Climate↗
USGS1996

Uranium provinces of North America — Their definition, distribution, and models

Uranium resources in North America are principally in unconformity-related, quartz-pebble conglomerate, sandstone, volcanic, and phosphorite types of uranium deposits. Most are concentrated in separate, well-defined metallogenic provinces. Proterozoic quartz-pebble conglomerate and unconformity-related deposits are, respectively, in the Blind River–Elliot Lake (BRELUP) and the Athabasca Basin (ABUP) Uranium Provinces in Canada. Sandstone uranium deposits are of two principal subtypes, tabular and roll-front. Tabular sandstone uranium deposits are mainly in upper Paleozoic and Mesozoic rocks in the Colorado Plateau Uranium Province (CPUP). Roll-front sandstone uranium deposits are in Tertiary rocks of the Rocky Mountain and Intermontane Basins Uranium Province (RMIBUP), and in a narrow belt of Tertiary rocks that form the Gulf Coastal Uranium Province (GCUP) in south Texas and adjacent Mexico. Volcanic uranium deposits are concentrated in the Basin and Range Uranium Province (BRUP) stretching from the McDermitt caldera at the Oregon-Nevada border through the Marysvale district of Utah and Date Creek Basin in Arizona and south into the Sierra de Peña Blanca District, Chihuahua, Mexico. Uraniferous phosphorite occurs in Tertiary sediments in Florida, Georgia, and North and South Carolina and in the Lower Permian Phosphoria Formation in Idaho and adjacent States, but only in Florida has economic recovery been successful. The Florida Phosphorite Uranium Province (FPUP) has yielded large quantities of uranium as a byproduct of the production of phosphoric acid fertilizer. Economically recoverable quantities of copper, gold, molybdenum, nickel, silver, thorium, and vanadium occur with the uranium deposits in some provinces. Many major epochs of uranium mineralization occurred in North America. In the BRELUP, uranium minerals were concentrated in placers during the Early Proterozoic (2,500–2,250 Ma). In the ABUP, the unconformity-related deposits were most likely formed initially by hot saline formational water related to diagenesis (»1,400 to 1,330 Ma) and later reconcentrated by hydrothermal events at »1,280–»1,000, »575, and »225 Ma. Subsequently in North America, only minor uranium mineralization occurred until after continental collision in Permian time (255 Ma). Three principal epochs of uranium mineralization occurred in the CPUP: (1) » 210–200 Ma, shortly after Late Triassic sedimentation; (2) »155–150 Ma, in Late Jurassic time; and (3) » 135 Ma, after sedimentation of the Upper Jurassic Morrison Formation. The most likely source of the uranium was silicic volcaniclastics for the three epochs derived from a volcanic island arc at the west edge of the North American continent. Uranium mineralization occurred during Eocene, Miocene, and Pliocene times in the RMIBUP, GCUP, and BRUP. Volcanic activity took place near the west edge of the continent during and shortly after sedimentation of the host rocks in these three provinces. Some volcanic centers in the Sierra de Peña Blanca district within the BRUP may have provided uranium-rich ash to host rocks in the GCUP. Most of the uranium provinces in North America appear to have a common theme of close associations to volcanic activity related to the development of the western margin of the North American plate. The south and west margin of the Canadian Shield formed the leading edge of the progress of uranium source development and mineralization from the Proterozoic to the present. The development of favorable hosts and sources of uranium is related to various tectonic elements developed over time. Periods of major uranium mineralization in North America were Early Proterozoic, Middle Proterozoic, Late Triassic–Early Jurassic, Early Cretaceous, Oligocene, and Miocene. Tertiary mineralization was the most pervasive, covering most of Western and Southern North America.

Bulletin↗
USGS2002

Waterbird conservation for the Americas: The North American waterbird conservation plan, version 1

The North American Waterbird Conservation Plan (the Plan) is the product of an independent partnership of individuals and institutions having interest and responsibility for conservation of waterbirds and their habitats in the Americas. This partnership - Waterbird Conservation for the Americas - was created to support a vision in which the distribution, diversity, and abundance of populations and habitats of breeding, migratory, and nonbreeding waterbirds are sustained or restored throughout the lands and waters of North America, Central America, and the Caribbean. The Plan provides a continental-scale framework for the conservation and management of 210 species of waterbirds, including seabirds, coastal waterbirds, wading birds, and marshbirds utilizing aquatic habitats in 29 nations throughout North America, Central America, the islands and pelagic waters of the Caribbean Sea and western Atlantic, the U.S.-associated Pacific Islands and pelagic inland and pelagic waters of the Pacific. Birds as familiar as herons, loons, pelicans, and gulls, as well as the lesser known albatrosses, petrels, auks, and rails are among the species considered in the Plan. These birds' dependence on aquatic habitats such as wooded swamps, stream corridors, salt marshes, barrier islands, continental shelf waters and open pelagic waters make them especially vulnerable to the myriad threats facing water and wetland resources globally. In addition, the congregatory behavior of many waterbirds increases population risks by concentrating populations in limited areas.

Book↗
USGS2006

General circulation model simulations of winter and summer sea-level pressures over North America

Temporal variability of synoptic-scale circulation is a dominant factor in regional temporal climate variations. In the midlatitudes, temporal variability of synoptic-scale circulation has been found to be related to temporal variations in regional air temperature and precipitation. General circulation model (GCM) simulations of long-term changes in synoptic-scale circulation may be useful in assessing the effects of climate change on regional air temperature and precipitation. Atmospheric pressure is an important component of synoptic-scale circulation. In this paper, observed sea-level pressures were used to evaluate winter and summer sea-level pressures over North America simulated by the Goddard Institute for Space Studies (GISS) and the Geophysical Fluid Dynamics Laboratory (GFDL) general circulation models. The objective of the study is to determine how similar the spatial and temporal distributions of GCM-simulated daily sea-level pressures over North America are to observed distributions. Overall, both models are better at reproducing observed within-season variance of winter and summer sea-level pressures than they are at simulating the magnitude of mean winter and summer sea-level pressures. Results indicate that winter sea-level pressures are better simulated by the GISS model than by the GFDL model. The GFDL model simulates anomalously high winter sea-level pressures over north-eastern North America. In summer, the GISS model underestimates sea-level pressures, whereas the GFDL model overestimates sea-level pressures over most of North America. Kolmogorov-Smirnov tests indicate that much of the error in model simulations can be attributed to differences between simulated and observed means. Comparisons of spatial correlation matrices of observed and simulated sea-level pressures indicate that GCMs simulate spatial variability of sea-level pressures that is similar to observed. These results suggest that GCMs are able to simulate major components of synoptic-scale circulation over North America, especially for winter.

International Journal of Climatology↗
USGS2011

Genetic characterisation of Toxoplasma gondii in wildlife from North America revealed widespread and high prevalence of the fourth clonal type

Little is known of the genetic diversity of Toxoplasma gondii circulating in wildlife. In the present study wild animals, from the USA were examined for T. gondii infection. Tissues of naturally exposed animals were bioassayed in mice for isolation of viable parasites. Viable T. gondii was isolated from 31 animals including, to our knowledge for the first time, from a bald eagle ( Haliaeetus leucocephalus ), five gray wolves ( Canis lupus ), a woodrat ( Neotoma micropus) , and five Arctic foxes (Alopex lagopus) . Additionally, 66 T. gondii isolates obtained previously, but not genetically characterised, were revived in mice. Toxoplasma gondii DNA isolated from these 97 samples (31 + 66) was characterised using 11 PCR-restriction fragment length polymorphism (RFLP) markers (SAG1, 5′- and 3′-SAG2, alt.SAG2, SAG3, BTUB, GRA6, c22–8, c29–2, L358, PK1 and Apico). A total of 95 isolates were successfully genotyped. In addition to clonal Types II, and III, 12 different genotypes were found. These genotype data were combined with 74 T. gondii isolates previously characterised from wildlife from North America and a composite data set of 169 isolates comprised 22 genotypes, including clonal Types II, III and 20 atypical genotypes. Phylogenetic network analysis showed limited diversity with dominance of a recently designated fourth clonal type (Type 12) in North America, followed by the Type II and III lineages. These three major lineages together accounted for 85% of strains in North America. The Type 12 lineage includes previously identified Type A and X strains from sea otters. This study revealed that the Type 12 lineage accounts for 46.7% (79/169) of isolates and is dominant in wildlife of North America. No clonal Type I strain was identified among these wildlife isolates. These results suggest that T. gondii strains in wildlife from North America have limited diversity, with the occurrence of only a few major clonal types.

International Journal for Parasitology↗
USGS2001

Biomes of western North America at 18,000, 6000 and 0 14C yr BP reconstructed from pollen and packrat midden data

A new compilation of pollen and packrat midden data from western North America provides a refined reconstruction of the composition and distribution of biomes in western North America for today and for 6000 and 18,000 radiocarbon years before present (14C yr BP). Modern biomes in western North America are adequately portrayed by pollen assemblages from lakes and bogs. Forest biomes in western North America share many taxa in their pollen spectra and it can be difficult to discriminate among these biomes. Plant macrofossils from packrat middens provide reliable identification of modern biomes from arid and semiarid regions, and this may also be true in similar environments in other parts of the world. However, a weighting factor for trees and shrubs must be used to reliably reconstruct modern biomes from plant macrofossils. A new biome, open conifer woodland, which includes eurythermic conifers and steppe plants, was defined to categorize much of the current and past vegetation of the semiarid interior of western North America. At 6000 14C yr BP, the forest biomes of the coastal Pacific North-west and the desert biomes of the South-west were in near-modern positions. Biomes in the interior Pacific North-west differed from those of today in that taiga prevailed in modern cool/cold mixed forests. Steppe was present in areas occupied today by open conifer woodland in the northern Great Basin, while in the central and southern Rocky Mountains forests grew where steppe grows today. During the mid-Holocene, cool conifer forests were expanded in the Rocky Mountains (relative to today) but contracted in the Sierra Nevada. These differences from the forests of today imply different climatic histories in these two regions between 6000 14C yr BP and today. At 18,000 14C yr BP, deserts were absent from the South-west and the coverage of open conifer woodland was greatly expanded relative to today. Steppe and tundra were present in much of the region now covered by forests in the Pacific North-west.

Journal of Biogeography↗