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At least 1,063 records · Page 59Linked to original sources

USGS2011

Late Pleistocene dune activity in the central Great Plains, USA

Stabilized dunes of the central Great Plains, especially the megabarchans and large barchanoid ridges of the Nebraska Sand Hills, provide dramatic evidence of late Quaternary environmental change. Episodic Holocene dune activity in this region is now well-documented, but Late Pleistocene dune mobility has remained poorly documented, despite early interpretations of the Sand Hills dunes as Pleistocene relicts. New optically stimulated luminescence (OSL) ages from drill cores and outcrops provide evidence of Late Pleistocene dune activity at sites distributed across the central Great Plains. In addition, Late Pleistocene eolian sands deposited at 20-25 ka are interbedded with loess south of the Sand Hills. Several of the large dunes sampled in the Sand Hills clearly contain a substantial core of Late Pleistocene sand; thus, they had developed by the Late Pleistocene and were fully mobile at that time, although substantial sand deposition and extensive longitudinal dune construction occurred during the Holocene. Many of the Late Pleistocene OSL ages fall between 17 and 14 ka, but it is likely that these ages represent only the later part of a longer period of dune construction and migration. At several sites, significant Late Pleistocene or Holocene large-dune migration also probably occurred after the time represented by the Pleistocene OSL ages. Sedimentary structures in Late Pleistocene eolian sand and the forms of large dunes potentially constructed in the Late Pleistocene both indicate sand transport dominated by northerly to westerly winds, consistent with Late Pleistocene loess transport directions. Numerical modeling of the climate of the Last Glacial Maximum has often yielded mean monthly surface winds southwest of the Laurentide Ice Sheet that are consistent with this geologic evidence, despite strengthened anticyclonic circulation over the ice sheet. Mobility of large dunes during the Late Pleistocene on the central Great Plains may have been the result of cold, short growing seasons with relatively low precipitation and low atmospheric CO2 that increased plant moisture stress, limiting the ability of vegetation to stabilize active dune sand. The apparent coexistence of large mobile dunes with boreal forest taxa suggests a Late Pleistocene environment with few modern analogs. ?? 2011 Elsevier Ltd.

Quaternary Science Reviews↗

USGS2016

Water isotope systematics: Improving our palaeoclimate interpretations

The stable isotopes of oxygen and hydrogen, measured in a variety of archives, are widely used proxies in Quaternary Science. Understanding the processes that control δ18O change have long been a focus of research (e.g. Shackleton and Opdyke, 1973; Talbot, 1990 ; Leng, 2006). Both the dynamics of water isotope cycling and the appropriate interpretation of geological water-isotope proxy time series remain subjects of active research and debate. It is clear that achieving a complete understanding of the isotope systematics for any given archive type, and ideally each individual archive, is vital if these palaeo-data are to be used to their full potential, including comparison with climate model experiments of the past. Combining information from modern monitoring and process studies, climate models, and proxy data is crucial for improving our statistical constraints on reconstructions of past climate variability. As climate models increasingly incorporate stable water isotope physics, this common language should aid quantitative comparisons between proxy data and climate model output. Water-isotope palaeoclimate data provide crucial metrics for validating GCMs, whereas GCMs provide a tool for exploring the climate variability dominating signals in the proxy data. Several of the studies in this set of papers highlight how collaborations between palaeoclimate experimentalists and modelers may serve to expand the usefulness of palaeoclimate data for climate prediction in future work. This collection of papers follows the session on Water Isotope Systematics held at the 2013 AGU Fall Meeting in San Francisco. Papers in that session, the breadth of which are represented here, discussed such issues as; understanding sub-GNIP scale (Global Network for Isotopes in Precipitation, (IAEA/WMO, 2006)) variability in isotopes in precipitation from different regions, detailed examination of the transfer of isotope signals from precipitation to geological archives, and the implications of advances in understanding in these areas for the interpretation of palaeo records and proxy data – climate model comparison. Here, we briefly review these areas of research, and discuss challenges for the water isotope community in improving our ability to partition climate vs. auxiliary signals in palaeoclimate data.

Quaternary Science Reviews↗

USGS2025

An exploration of open-system uranium-series history of marine terrace corals, Perachora Peninsula, Greece and San Nicolas Island, California, USA

Emergent marine terraces record past periods of relatively high sea level and are common on uplifting coasts worldwide. Such landforms are extensive around the Gulf of Corinth area of Greece and along the coast of California, USA. In the Gulf of Corinth region, marine terraces record Quaternary uplift due to ongoing basin extension on the northern side of the Africa-Eurasia-Aegean Sea plate boundary. Although dating of terrace fossils has been extensive in the Perachora Peninsula part of this region, some apparent ages are much older (∼190 ka to ∼150 ka) and some are much younger (∼91 ka to ∼69 ka) than expected for a marine terrace that has long been hypothesized to be of last-interglacial (∼120 ka) age. New uranium-series analyses of terrace corals from two previously studied localities are reported here. As with previous work, the new results show that these corals have behaved as open systems, with back-calculated, initial 234 U/ 238 U values higher than those found in modern seawater. New and previously published U-series analyses of last-interglacial corals from San Nicolas Island, California (USA) illustrate the possible mechanisms of open-system behavior, which include both “early” alpha-recoil-derived additions of 230 Th and 234 U, and “continuing” alpha-recoil-derived additions of these nuclides. Many corals from the Perachora Peninsula appear to have experienced the “continuing” alpha-recoil-derived open-system history, which explains the apparent older ages, and permits an interpretation that all analyzed corals likely date to the last interglacial period, ∼120 ka. If the estimated age of ∼120 ka reported here is correct, this age, along with considerations of glacial isostatic adjustment (GIA) modeling of past sea level, indicates that the southern side of the Perachora Peninsula has experienced uplift rates ranging from ∼0.23 m/kyr to ∼0.57 m/kyr, increasing to the southeast, in agreement with previous work. Interpretation of the geomorphology of the northern side of the peninsula has been controversial for more than four decades. The “stepped” topography has been interpreted as either a series of successively higher and older marine terraces, as on the southern side, or a marine terrace of one age that has been displaced by a series of shore-parallel, northward-dipping normal faults. In the second interpretation, topographic steps in the landscape are interpreted to be fault scarps, not ancient sea cliffs. Interpretation of the open-system U-series ages indicates that marine deposits at elevations of ∼22 m and ∼44 m are both likely ∼120 ka, supporting the scenario that steps in the landscape are likely fault scarps, not sea cliffs. Using the new U-series ages of ∼120 ka, along with GIA-modeled paleo-sea level confirms another previous study that uplift rates on the northern side of the peninsula are at least as high as 0.39–0.46 m/kyr.

California↗

USGS2026

Cyanobacteria and aquatic ecosystem dynamics across 28,000 years of environmental changes in subtropical North America

Ecological pressures on aquatic ecosystems have increased over recent centuries due to human activities and climate change. However, contextualizing ecosystem deterioration is often challenging due to limited knowledge of environmental changes over millennial timescales. Subtropical Carolina bays in North Carolina, USA, have remained unglaciated, preserving paleolimnological records that extend back to the last glacial period. Here, we analyzed a sediment core from the ecologically rich Lake Waccamaw spanning more than 28,000 years for aquatic proxies of nutrients, photosynthetic pigments, cyanotoxins, carbon isotopes, and terrestrial proxies of pollen and charcoal. The study explored paleolimnological changes in the aquatic environment connected to land changes and climate during the late Quaternary in the southeastern Atlantic Coastal Plain. Results reveal that while current levels of colonial cyanobacteria are high, past levels of cyanobacteria, other primary producers, and cyanotoxins were higher under natural climate variability. Abrupt ecosystem responses to increasing trophic conditions during Interstadial 3 (27.8–26.4 cal ka BP) and the early Holocene (11.4–7 cal ka BP) were marked by increases in primary producer abundance, deciduous vegetation expansion, and fire activity. Cyanobacteria remained dominant throughout the record, with colonial forms prevailing during the Holocene. Increases in pigment concentrations aligned with Quercus and were primarily driven by hydroclimatic variability and nutrient stoichiometry. Transitions between Pinus and Quercus pollen matched stadials and interstadials in the δ 18 Ο record from the North Greenland Ice Core Project (NGRIP). This study highlights the value of multi-proxy millennial-scale paleolimnological records for understanding aquatic ecosystem responses to climate conditions during the late Pleistocene.

North Carolina↗

USGS2017

Peak discharge of a Pleistocene lava-dam outburst flood in Grand Canyon, Arizona, USA

The failure of a lava dam 165,000 yr ago produced the largest known flood on the Colorado River in Grand Canyon. The Hyaloclastite Dam was up to 366 m high, and geochemical evidence linked this structure to outburst-flood deposits that occurred for 32 km downstream. Using the Hyaloclastite outburst-flood deposits as paleostage indicators, we used dam-failure and unsteady flow modeling to estimate a peak discharge and flow hydrograph. Failure of the Hyaloclastite Dam released a maximum 11 ?? 109 m3 of water in 31 h. Peak discharges, estimated from uncertainty in channel geometry, dam height, and hydraulic characteristics, ranged from 2.3 to 5.3 ?? 105 m3 s-1 for the Hyaloclastite outburst flood. This discharge is an order of magnitude greater than the largest known discharge on the Colorado River (1.4 ?? 104 m3 s-1) and the largest peak discharge resulting from failure of a constructed dam in the USA (6.5 ?? 104 m3 s-1). Moreover, the Hyaloclastite outburst flood is the oldest documented Quaternary flood and one of the largest to have occurred in the continental USA. The peak discharge for this flood ranks in the top 30 floods (>105 m3 s-1) known worldwide and in the top ten largest floods in North America. ?? 2005 University of Washington. All rights reserved.

Quaternary Research↗

USGS2017

New optically stimulated luminescence ages provide evidence of MIS3 and MIS2 eolian activity on Black Mesa, northeastern Arizona, USA

Eolian deposition on the semiarid southern Colorado Plateau has been attributed to episodic aridity during the Quaternary Period. However, OSL ages from three topographically controlled (e.g. falling) dunes on Black Mesa in northeastern Arizona indicate that eolian sediments there were deposited in deep tributary valleys as early as 35–30 ka, with most sand deposited before 20 ka. In contrast, the oldest OSL ages for sand sheets fall within the Pleistocene-Holocene climatic transition (~ 12–8 ka). Thus most eolian sediment accumulated on Black Mesa under climatic conditions that were in general cooler, moister, and more variable than today, not more arid, pointing to a considerable increase in sediment supply.

Arizona↗

USGS2017

Radiocarbon ages of terrestrial gastropods extend duration of ice-free conditions at the Two Creeks forest bed, Wisconsin, USA

Analysis of terrestrial gastropods that underlie the late Pleistocene Two Creeks forest bed (~ 13,800–13,500 cal yr BP) in eastern Wisconsin, USA provides evidence for a mixed tundra-taiga environment prior to formation of the taiga forest bed. Ten new AMS 14 C analyses on terrestrial gastropod shells indicate the mixed tundra-taiga environment persisted from ~ 14,500 to 13,900 cal yr BP. The Twocreekan climatic substage, representing ice-free conditions on the shore of Lake Michigan, therefore began near the onset of peak warming conditions during the Bølling–Allerød interstadial and lasted ~ 1000 yr, nearly 600 yr longer than previously thought. These results provide important data for understanding the response of continental ice sheets to global climate forcing and demonstrate the potential of using terrestrial gastropod fossils for both environmental reconstruction and age control in late Quaternary sediments.

Wisconsin↗

USGS2012

Sea-level history of past interglacial periods: New evidence from uranium-series dating of corals from Curaçao, Leeward Antilles islands

Curaçao has reef terraces with the potential to provide sea-level histories of interglacial periods. Ages of the Hato (upper) unit of the “Lower Terrace” indicate that this reef dates to the last interglacial period, Marine Isotope Stage (MIS) 5.5. On Curaçao, this high sea stand lasted at least 8000 yr (~ 126 to ~ 118 ka). Elevations and age of this reef show that late Quaternary uplift rates on Curaçao are low, 0.026–0.054 m/ka, consistent with its tectonic setting. Ages of ~ 200 ka for corals from the older Cortalein unit of the Lower Terrace correlate this reef to MIS 7, with paleo-sea level estimates ranging from − 3.3 m to + 2.3 m. The estimates are in agreement with those for MIS 7 made from other localities and indicate that the penultimate interglacial period was a time of significant warmth, on a par with the present interglacial period. The ~ 400 ka (MIS 11) Middle Terrace I on Curaçao, dated by others, may have formed from a paleo-sea level of + 8.3 to + 10.0 m, or (less likely) + 17 m to + 20 m. The lower estimates are conservative compared to previous studies, but still require major ice sheet loss from Greenland and Antarctica.

Leeward Antilles↗

USGS2012

Lacustrine records of Holocene flood pulse dynamics in the Upper Paraguay River watershed (Pantanal wetlands, Brazil)

The Pantanal is the world's largest tropical wetland and a biodiversity hotspot, yet its response to Quaternary environmental change is unclear. To address this problem, sediment cores from shallow lakes connected to the UpperParaguayRiver (PR) were analyzed and radiocarbon dated to track changes in sedimentary environments. Stratal relations, detrital particle size, multiple biogeochemical indicators, and sponge spicules suggest fluctuating lake-level lowstand conditions between ~ 11,000 and 5300 cal yr BP, punctuated by sporadic and in some cases erosive flood flows. A hiatus has been recorded from ~ 5300 to 2600 cal yr BP, spurred by confinement of the PR within its channel during an episode of profound regional drought. Sustained PR flooding caused a transgression after ~ 2600 cal yr BP, with lake-level highstand conditions appearing during the Little Ice Age. Holocene PR floodpulsedynamics are best explained by variability in effective precipitation, likely driven by insolation and tropical sea-surface temperature gradients. Our results provide novel support for hypotheses on: (1) stratigraphic discontinuity of floodplain sedimentary archives; (2) late Holocene methane flux from Southern Hemisphere wetlands; and (3) pre-colonial indigenous ceramics traditions in western Brazil.

Pantanal Wetlands;Paraguay River↗

USGS2017

The Snowmastodon Project

Studies of terrestrial biotic and environmental dynamics of the last interglacial period, Marine Oxygen Isotope Stage (MIS) 5, provide insight into the effects of long-term climate change on Pleistocene ecosystems. In North America, however, there are relatively few fossil sites that definitively date to MIS 5. Even fewer contain multiple ecosystem components (vertebrates, invertebrates, plants) that have been studied in detail, and none are located at high elevation. Thus, our view of North American ecosystems during MIS 5 is, at best, an incomplete composite view, and alpine ecosystems are entirely undocumented. The Ziegler Reservoir fossil site allows us to begin filling these gaps. Discovered on October 14, 2010 by a construction crew while enlarging a small reservoir near Snowmass Village, Colorado (USA), the site is situated high in the Rocky Mountains at an elevation of ~ 2705 m. Initial excavations exposed a series of stacked fossil ecosystems, including abundant faunal and floral components. Subsequent work revealed an exceptionally diverse biota composed of nearly 6000 large bones of late Pleistocene megafauna, including mastodons, mammoths, ground sloths, horses, camels, deer, bison, black bear, coyotes, and big horn sheep. This collection is complemented by more than 200 bones from at least 40 species of small animals including otters, muskrats, minks, rabbits, beavers, frogs, lizards, snakes, fish, and birds, and more than 20,000 salamander bones. Geomorphic evidence and multiple dating techniques show that the small, ridge-top basin containing the site was formed by a lateral lobe of a valley glacier that filled, and ultimately overtopped, the Snowmass Creek drainage during the Bull Lake glaciation (MIS 6). When the glacier receded, a lake that occupied the basin began to fill slowly with what is hypothesized to be eolian sediment and occasional input from slope failures of the impounding moraine. As sedimentation continued, the lake transformed first into a shallow pond, then a marsh, and later an alpine meadow. In all, the Ziegler Reservoir sedimentary record spans the time encompassed by the end of MIS 6, all of MIS 5 and MIS 4, and the earliest part of MIS 3. MIS 5e, which is considered to be the peak warmth of the last interglacial period, is especially well represented. Preservation of organic material at the Ziegler Reservoir fossil site is exceptional. Even after more than 100,000 years of burial, sedge and willow leaves were still green, mollusks and gastropods showed color, beetle parts remained iridescent, fossil conifer cones were intact, and an entire beach of 20-meter-long driftwood logs was found intact. The exquisite preservation allowed for detailed analysis of pollen, plant macrofossils, conifer cones, and fossil wood, as well as insects, chironomids, mollusks, ostracodes, and other invertebrates — all in addition to the spectacular faunal remains. This special volume of Quaternary Research represents a comprehensive scientific report of “The Snowmastodon Project,” as the investigations at the Ziegler Reservoir fossil site came to be known. Summarizing the work of scientists from more than 20 institutions, the volume details the environments in which the animals lived, provides insight into how they died, and forms the foundation of our understanding of alpine ecosystem dynamics during the last interglacial period in the Rocky Mountains.

Quaternary Research↗

USGS2000

Geochemical and mineralogical evidence from eolian sediments for northwesterly mid-Holocene paleowinds, central Kansas, USA

A prominent (4500 km2) dune field in the Great Plains is the Great Bend Sand Prairie of south-central Kansas. Dunes here overlie late Quaternary alluvium and were reactivated extensively in the late Holocene. Geomorphic and soil evidence suggests that the most likely eolian sand source is the Arkansas River valley to the northwest. Nevertheless, orientations of stabilized dunes indicate that the most recent dune-forming winds came from the south or southwest, in agreement with modern wind data. Mineralogy and trace element concentrations in eolian sands of the Great Bend Sand Prairie are similar to those to the Arkansas River, which permits the Arkansas River as a sediment source. Ca and Sr abundances, which reflect small amounts of carbonate minerals, are higher in Arkansas River sand compared to eolian sands and show a systematic depletion away from the Arkansas River to the southeast. These trends are likely due to carbonate mineral depletion downwind from abrasion and size reduction. Thus, paleowinds probably were northwesterly during initial deposition. Northwesterly winds occur today when dry, Pacific-derived air is dominant. We hypothesize that the residence time of this air mass was much greater while dunes initially formed, possibly during a warmer and drier mid-Holocene period.

Quaternary International↗

USGS2001

Sea-level and environmental changes since the last interglacial in the Gulf of Carpentaria, Australia: an overview

The Gulf of Carpentaria is an epicontinental sea (maximum depth 70 m) between Australia and New Guinea, bordered to the east by Torres Strait (currently 12 m deep) and to the west by the Arafura Sill (53 m below present sea level). Throughout the Quaternary, during times of low sea-level, the Gulf was separated from the open waters of the Indian and Pacific Oceans, forming Lake Carpentaria, an isolation basin, perched above contemporaneous sea-level with outlet channels to the Arafura Sea. A preliminary interpretation is presented of the palaeoenvironments recorded in six sediment cores collected by the IMAGES program in the Gulf of Carpentaria. The longest core (approx. 15 m) spans the past 130 ka and includes a record of sea-level/lake-level changes, with particular complexity between 80 and 40 ka when sea-level repeatedly breached and withdrew from Gulf/Lake Carpentaria. Evidence from biotic remains (foraminifers, ostracods, pollen), sedimentology and geochemistry clearly identifies a final marine transgression at about 9.7 ka (radiocarbon years). Before this transgression, Lake Carpentaria was surrounded by grassland, was near full, and may have had a surface area approaching 600 km×300 km and a depth of about 15 m. The earlier rise in sea-level which accompanied the Marine Isotopic Stage 6/5 transgression at about 130 ka is constrained by sedimentological and biotic evidence and dated by optical- and thermoluminescence and amino acid racemisation methods.

Gulf Of Carpentaria↗

USGS1998

Paleopedology plus TL, 10Be, and14C dating as tools in stratigraphic and paleoclimatic investigations, Mississippi River Valley, U.S.A.

Thick ( ??? 35 m) loess deposits are present on ridges and high bluffs in the northern-half of the Lower Mississippi Valley (LMV), U.S.A. Detailed descriptions of the loess sections and pedologic, physiochemical, and mineralogic analyses and TL, 14C, and 10Be age determinations, allow preliminary paleoclimatic reconstructions for the late Quaternary of central North America. No age data are available for the oldest (Fifth) loess. 10Be and TL age data suggest a 250-200 ka age for the Fourth or Crowleys Ridge(?) Loess, and indicate that the Loveland or Third Loess is time equivalent to oxygen isotope stage 6, ??? 190-120 ka. A weakly developed paleosol is present in the basal-half of the Loveland. The Sangamon Geosol is present in the upper 5 m and represents all of oxygen isotope stage 5, ??? 130-60 ka. It formed in a climate as warm as, but drier and (or) with greater variation in precipitation, than the present. The Roxana Silt (second loess) was deposited during oxygen isotope stages 4 and 3, ??? 65-26 ka. The early Wisconsinan interglacial-glacial transition, represented by the Sangamon Geosol and the unnamed paleosol in the basal Roxana Silt, was slow. The paleoclimate during the 35 k yr of Roxana deposition was cool to cold and wet. Age and pedologic data indicate that deposition of the Peoria Loess (the youngest) began around 25 ka when the area's climate changed abruptly from cool or cold and wet to cold and dry, with periods of sustained high winds.

Quaternary International↗

USGS2004

Limits of mountain and continental glaciations east of the Continental Divide in northern Montana and north-western North Dakota, U.S.A.

This chapter provides an overview of the limits of glaciations and glacial history in, and east and south-east of, Glacier National Park, Montana, and on the Northern Plains further east in Montana and north-western North Dakota. The term “Laurentide glacier” was applied to a continental ice sheet east of the Rocky Mountains in North America. It describes Laurentide Ice Sheet as any Quaternary continental ice sheet east of the Rocky Mountains in the United States and Canada. Laurentide till refers to till deposited by a Laurentide Ice Sheet. A Laurentide continental ice sheet is distinguished from a Cordilleran continental ice sheet in the Cordilleran region in parts of Washington, Idaho, and Montana in the United States and in adjacent Canada. Clague indicated that Cordilleran Ice Sheets formed several times during the Pleistocene. The chapter also reviews that the base for the digital map is simplified. Selected hydrographic features, selected towns and cities, selected physiographic features, and a grid of 1° × 2° topographic quadrangles are included to aid the reader in location of the glacial limits and other features depicted here on other maps at different scales.

Montana, North Dakota↗

USGS2018

Desert wetlands record hydrologic variability within the Younger Dryas chronozone, Mojave Desert, USA

One of the enduring questions in the field of paleohydrology is how quickly desert wetland ecosystems responded to past episodes of abrupt climate change. Recent investigations in the Las Vegas Valley of southern Nevada have revealed that wetlands expanded and contracted on millennial and sub-millennial timescales in response to changes in climate during the late Quaternary. Here, we evaluate geologic evidence from multiple localities in the Mojave Desert and southern Great Basin that suggests the response of wetland systems to climate change is even faster, occurring at centennial, and possibly decadal, timescales. Paleowetland deposits at Dove Springs Wash, Mesquite Springs, and Little Dixie Wash, California, contain evidence of multiple wet and dry cycles in the form of organic-rich black mats, representing periods of past groundwater discharge and wet conditions, interbedded with colluvial, alluvial, and aeolian sediments, each representing dry conditions. Many of these wet-dry cycles date to within the Younger Dryas (YD) chronozone (12.9–11.7 ka), marking the first time intra -YD hydrologic variability has been documented in paleowetland deposits. Our results illustrate that desert wetland ecosystems are exceptionally sensitive to climate change and respond to climatic perturbations on timescales that are relevant to human society.

Arizona, California, Nevada↗

USGS2018

Overview of the oxygen isotope systematics of land snails from North America

Continental paleoclimate proxies with near-global coverage are rare. Land snail δ 18 O is one of the few proxies abundant in Quaternary sediments ranging from the tropics to the high Arctic tundra. However, its application in paleoclimatology remains difficult, attributable in part to limitations in published calibration studies. Here we present shell δ 18 O of modern small (<10 mm) snails across North America, from Florida (30°N) to Manitoba (58°N), to examine the main climatic controls on shell δ 18 O at a coarse scale. This transect is augmented by published δ 18 O values, which expand our coverage from Jamaica (18°N) to Alaska (64°N). Results indicate that shell δ 18 O primarily tracks the average annual precipitation δ 18 O. Shell δ 18 O increases 0.5–0.7‰ for every 1‰ increase in precipitation δ 18 O, and 0.3–0.7‰ for every 1°C increase in temperature. These relationships hold true when all taxa are included regardless of body size (ranging from ~1.6 to ~58 mm), ecology (herbivores, omnivores, and carnivores), or behavior (variable seasonal active periods and mobility habits). Future isotopic investigations should include calibration studies in tropical and high-latitude settings, arid environments, and along altitudinal gradients to test if the near linear relationship between shell and meteoric precipitation δ 18 O observed on a continental scale remains significant.

Quaternary Research↗

USGS2022

Assessing reproducibility in sedimentary macroscopic charcoal count data

Current understanding of global late Quaternary fire history is largely drawn from sedimentary charcoal data. Since publication, CharAnalysis increasingly has been relied upon as a robust method for analyzing these data. However, several underlying assumptions of the algorithm have not been tested. This study uses replicated charcoal count data to examine the assumption of Poisson distribution and reproducibility of peak detection. Results show <10% of the replicate counts are Poisson distributed, a maximum peak replication rate of 60%, and, for >90% of the data, intra-level count differences were larger than the threshold used to identify significance in inter-level differences. A pronounced “edge effect” was observed at the beginning and end of the records, cautioning against validation of results based on sections corresponding to the historical period. The proximal cause for low reproducibility is likely a lack of spatial randomness of charcoal particles at the scale of a core diameter. Until and unless decomposition methods can be developed that accommodate the observed limitations inherent in particle count data, best practices for interpreting charcoal records may be to rely on qualitative interpretations based on smoothed influx values and minimum particle count values in the hundreds.

Quaternary Research↗

USGS2023

The role of lithology and climate on bedrock river incision and terrace development along the Buffalo National River, Arkansas

The Buffalo National River in northwest Arkansas preserves an extensive Quaternary record of fluvial bedrock incision and aggradation across lithologies of variable resistance. In this work, we apply optically stimulated luminescence (OSL) dating to strath and fill terraces along the Buffalo River to elucidate the role of lithology and climate on the development of the two youngest terrace units (Qtm and Qty). Our OSL ages suggest a minimum strath planation age of ~250 ka for the Qtm terraces followed by a ~200 ka record of aggradation. Qtm incision likely occurred near the LGM, prior to the onset of Qty fill terrace aggradation ~14 ka. Our terrace ages are broadly consistent with other regional fluvial terrace records, and comparison with available paleoclimatic archives suggests that terrace aggradation and incision occurred during drier and wetter hydrological conditions, respectively. Vertical bedrock incision rates were also calculated using OSL-derived estimates of Qtm strath planation and displayed statistically significant spatial variability with bedrock lithology, ranging from 0.04 mm/yr in the higher resistance reaches and 0.02 mm/yr in the lower resistance reaches. In combination with observations of valley width and terrace distribution, these results suggest that vertical processes outpace lateral ones in lithologic reaches with higher resistance.

Arkansas↗