SEARCH · Geology Reports

Results for “Holocene”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 757 records · Page 42Linked to original sources

USGS2005

Paleoceanographic history of the Guaymas Basin, Gulf of California, during the past 15,000 years based on diatoms, silicoflagellates, and biogenic sediments

High-resolution records of calcium carbonate, biogenic opal, diatoms, and silicoflagellates from western Guaymas Basin gravity core GGC55 and piston core JPC56 and eastern Guaymas Basin DSDP Site 480 reveal a complex paleoceanographic history of the central Gulf of California during the past 15,000 years. Prior to ∼ 6.2 ka, the eastern and western Guaymas Basin proxy records were remarkably similar. After conditions similar to those of today during the Bølling–Allerod, the Younger Dryas (YD) saw a major drop in diatom production, coincident with increased calcium carbonate and tropical microfossils suggestive of El Niño-like conditions. Biosiliceous productivity began increasing during the latter part of the YD, but it was only during the earliest Holocene (11.6 to 11.0 ka) that conditions similar to those of the Bølling–Allerod returned to the central Gulf. Between around 11.0 and 6.2 ka, tropical diatoms and silicoflagellates were virtually absent from the central Gulf, as relatively cooler and fresher surface waters resembling those of the modern northern Gulf were present in the central Gulf. Beginning at about 6.2 ka, tropical diatoms and silicoflagellates began increasing in the central Gulf, and coccoliths returned to western Gulf sediments. The onset of modern-day monsoon conditions in the American Southwest required the presence of warm SSTs in the northern Gulf, which probably did not occur until after about 5.4 ka, when tropical diatoms and silicoflagellates became relatively common in the central Gulf. Modern east–west contrasts, which arise from late winter–early spring coastal upwelling on the mainland side and lower diatom productivity on the western side of the Gulf, commenced between 6.2 and 5.4 ka, possibly due to a shift in the direction of late winter–early spring winds more towards the southeast, or down the axis of the Gulf. This proposed wind shift might have ultimately been due to a late Holocene strengthening of ENSO-like conditions in the eastern equatorial Pacific.

Gulf of California↗

USGS2014

Response of diatom and silicoflagellate assemblages in the central Gulf of California to regional climate change during the past 55 kyrs

High-resolution studies of diatoms and silicoflagellates of the past 55 kyrs in cores MD02-2517/2515 from the central Gulf of California (GoC) reveal profound changes in GoC surface waters. Roperia tesselata , a diatom proxy for late winter–early spring upwelling, and Dictyocha stapedia , a subtropical silicoflagellate indicative of GoC sea surface temperatures (SSTs) > 24 °C, are common during the Holocene but rare during Marine Isotope Stage (MIS) 2 and most of MIS 3, a relationship that likely reflects a more northerly position of the North Pacific High (NPH) during the Holocene. In contrast during most of MIS 2 (~ 27–15 ka), the persistent presence of Distephanus speculum , a silicoflagellate associated with SSTs < 16°, suggests that cold, low salinity waters penetrated into the GoC, consistent with southward displacement of the NPH. During MIS 3 (~ 55–27 ka), increased dominance of Azpeitia nodulifera (diatom) implies that stratified, tropical waters were present year round, whereas silicoflagellate assemblages suggest that stratified tropical conditions alternated with more productive, upwelling conditions on millennial timescales. Reduced biosiliceous productivity during Heinrich events likely reflected a reduction in both surface water nutrient levels and in the strength of northwest winds due to a weakened and more southerly NPH. Conversely, enhanced biosiliceous productivity during MIS 3 interstadials was probably linked to heightened nutrient levels and a strengthened NPH. Abrupt relative abundance increases of the silicoflagellate, Dictyocha aculeata , approximate the termination of MIS3 Heinrich events and may signal times when nutrient-rich deep waters associated with the resumption of enhanced Atlantic Meridional Overturning Circulation penetrated into the central Gulf.

California↗

USGS2008

Precipitation rates and atmospheric heat transport during the Cenomanian greenhouse warming in North America: Estimates from a stable isotope mass-balance model

Stable isotope mass-balance modeling results of meteoric ??18O values from the Cenomanian Stage of the Cretaceous Western Interior Basin (KWIB) suggest that precipitation and evaporation fluxes were greater than that of the present and significantly different from simulations of Albian KWIB paleohydrology. Sphaerosiderite meteoric ??18O values have been compiled from the Lower Tuscaloosa Formation of southwestern Mississippi (25??N paleolatitude), The Dakota Formation Rose Creek Pit, Fairbury Nebraska (35??N) and the Dunvegan Formation of eastern British Columbia (55??N paleolatitude). These paleosol siderite ??18O values define a paleolatitudinal gradient ranging from - 4.2??? VPDB at 25??N to - 12.5??? VPDB at 55??N. This trend is significantly steeper and more depleted than a modern theoretical siderite gradient (25??N: - 1.7???; 65??N: - 5.6??? VPDB ), and a Holocene meteoric calcite trend (27??N: - 3.6???; 67??N: - 7.4??? VPDB). The Cenomanian gradient is also comparatively steeper than the Albian trend determined for the KWIB in the mid- to high latitudes. The steep latitudinal trend in meteoric ??18O values may be the result of increased precipitation and evaporation fluxes (amount effects) under a more vigorous greenhouse-world hydrologic cycle. A stable-isotope mass-balance model has been used to generate estimates of precipitation and evaporation fluxes and precipitation rates. Estimates of Cenomanian precipitation rates based upon the mass-balance modeling of the KWIB range from 1400??mm/yr at 25??N paleolatitude to 3600??mm/yr at 45??N paleolatitude. The precipitation-evaporation (P-E) flux values were used to delineate zones of moisture surplus and moisture deficit. Comparisons between Cenomanian P-E and modern theoretical siderite, and Holocene calcite latitudinal trends shows an amplification of low-latitude moisture deficits between 5-25??N paleolatitude and moisture surpluses between 40-60??N paleolatitude. The low-latitude moisture deficits correlate with a mean annual average heat loss of 48??W/m2 at 10??N paleolatitude (present, 8??W/m2 at 15??N). The increased precipitation flux and moisture surplus in the mid-latitudes corresponds to a mean average annual heat gain of 180??W/m2 at 50??N paleolatitude (present, 17??W/m2 at 50??N). The Cenomanian low-latitude moisture deficit is similar to that of the Albian, however the mid-latitude (40-60??N) precipitation flux values and precipitation rates are significantly higher (Albian: 2200??mm/yr at 45??N; Cenomanian: 3600??mm/yr at 45??N). Furthermore, the heat transferred to the atmosphere via latent heat of condensation was approximately 10.6?? that of the present at 50??N. The intensified hydrologic cycle of the mid-Cretaceous greenhouse warming may have played a significant role in the poleward transfer of heat and more equable global conditions. Paleoclimatological reconstructions from multiple time periods during the mid-Cretaceous will aid in a better understanding of the dynamics of the hydrologic cycle and latent heat flux during greenhouse world conditions.

Palaeogeography, Palaeoclimatology, Palaeoecology↗

USGS2022

A new indicator approach to reconstruct agricultural land use in Europe from sedimentary pollen assemblages

The reconstruction of human impact is pivotal in palaeoecological studies, as humans are among the most important drivers of Holocene vegetation and ecosystem change. Nevertheless, separating the anthropogenic footprint on vegetation dynamics from the impact of climate and other environmental factors (disturbances such as fire, erosion, floods, landslides, avalanches, volcanic eruptions) is a challenging and still largely open issue. For this purpose, palynologists mostly rely on cultural indicator pollen types and related indices that consist of sums or ratios of these pollen types. However, the high environmental and biogeographical specificity of cultural indicator plants hinders the application of the currently available indices to wide geographical settings. Furthermore, the achievable taxonomic resolution of cultural indicator pollen types may hamper their indicative capacity. In this study, we propose the agricultural land use probability (LUP) index, a novel approach to quantify human impact intensity on European ecosystems based on cultural indicator pollen types. From the ‘classic’ cultural indicators, we construct the LUP index by selecting those with the best indicator capacity based on bioindication criteria. We first train the LUP index using twenty palynological sequences along a broad environmental gradient, spanning from treeless alpine to subtropical mediterranean evergreen plant communities. We then validate the LUP index using independent pollen datasets and archaeological proxies. Finally, we discuss the suitability of the selected pollen types and the potential of the LUP index for quantifying Holocene human impact in Europe, concluding that careful application of the LUP index may significantly contribute to refining pollen-based land-use reconstructions.

Palaeogeography, Palaeoclimatology, Palaeoecology↗

USGS2020

2,200-Year tree-ring and lake-sediment based snowpack reconstruction for the northern Rocky Mountains highlights the historic magnitude of recent snow drought

In recent decades, Rocky Mountain accumulated snowpack levels have experienced rapid declines, yet long-term records of snowpack prior to the installation of snowpack observation stations in the early and mid 20th century are limited. To date, a small number of tree-ring based reconstructions of April 1 Snow Water Equivalent (SWE) in the northern Rocky Mountains have extended modern records of snowpack variability to ∼1200 C.E. Carbonate isotope lake sediment records, provide an opportunity to further extend tree-ring based reconstructions through the Holocene, providing a millennial-scale temporal record that allows for an evaluation of multi-scale drivers of snowpack variability, from internal climate dynamics to orbital-scale forcings. Here we present a ∼2200 year preliminary reconstruction of northern Rockies snowpack based on δ 18 O measurements of sediment carbonates collected from Foy Lake, Montana. We explore the statistical calibration of lake sediment δ 18 O to an annually resolved snowpack reconstruction from tree rings, and develop an approach to assess and quantify potential sources of error in this reconstruction approach. The sediment-based snowpack reconstruction shows strong low-frequency variability in snowpack over the last two millennia with few snow droughts approaching the magnitude of recent snowpack declines. Given the growing availability of high-resolution, carbonate-rich lake sediment records, such reconstructions could help improve our understanding of how snowpack conditions varied under previous climatic events (mid-Holocene climate optimum ca. 9−6 ka), providing critical insights for anticipating future snowpack conditions.

Alberta, British Columbia, Idaho, Montana, Nevada,↗

USGS2006

Landscape response to deglaciation in a high relief, monsoon-influenced alpine environment, Langtang Himal, Nepal

Significant glacial fluctuations and rapid paraglacial reworking of glacigenic sediments characterize the Middle and Late Holocene of the Langtang Khola Valley, Central Nepal Himalaya. Geomorphic mapping and beryllium-10 cosmogenic radionuclide (CRN) dating of moraines and paraglacial fans were undertaken to test the existing paraglacial fan, terrace and moraine chronologies. The new dating compares favorably with prior studies that utilized radiocarbon, adding additional support to the assumption that fan and terrace formation are strongly linked to deglaciation. Fan and terrace denudation rates are so rapid in this region, averaging ∼33 mm/yr, that no depositional landforms older than 5 ka are preserved within 250 m of the valley floor. In this region, high rates of denudation during the Late Quaternary are driven by a combination of rapid tectonic uplift, numerous glacial fluctuations and intense weathering driven by an active monsoon climate. Extensive reworking of glacigenic sediments in Langtang during the latter half of the Holocene is consistent with studies completed in other areas of the Himalaya that are strongly influenced by the monsoon.

Langtang Himal↗

USGS2009

Evidence for prolonged El Nino-like conditions in the Pacific during the Late Pleistocene: a 43 ka noble gas record from California groundwaters

Information on the ocean/atmosphere state over the period spanning the Last Glacial Maximum - from the Late Pleistocene to the Holocene - provides crucial constraints on the relationship between orbital forcing and global climate change. The Pacific Ocean is particularly important in this respect because of its dominant role in exporting heat and moisture from the tropics to higher latitudes. Through targeting groundwaters in the Mojave Desert, California, we show that noble gas derived temperatures in California averaged 4.2 ?? 1.1 ??C cooler in the Late Pleistocene (from ???43 to ???12 ka) compared to the Holocene (from ???10 to ???5 ka). Furthermore, the older groundwaters contain higher concentrations of excess air (entrained air bubbles) and have elevated oxygen-18/oxygen-16 ratios (?? 18 O) - indicators of vigorous aquifer recharge, and greater rainfall amounts and/or more intense precipitation events, respectively. Together, these paleoclimate indicators reveal that cooler and wetter conditions prevailed in the Mojave Desert from ???43 to ???12 ka. We suggest that during the Late Pleistocene, the Pacific ocean/atmosphere state was similar to present-day El Nino-like patterns, and was characterized by prolonged periods of weak trade winds, weak upwelling along the eastern Pacific margin, and increased precipitation in the southwestern U.S.

Quaternary Science Reviews↗

USGS2010

Quaternary Sea-ice history in the Arctic Ocean based on a new Ostracode sea-ice proxy

Paleo-sea-ice history in the Arctic Ocean was reconstructed using the sea-ice dwelling ostracode Acetabulastoma arcticum from late Quaternary sediments from the Mendeleyev, Lomonosov, and Gakkel Ridges, the Morris Jesup Rise and the Yermak Plateau. Results suggest intermittently high levels of perennial sea ice in the central Arctic Ocean during Marine Isotope Stage (MIS) 3 (25-45 ka), minimal sea ice during the last deglacial (16-11 ka) and early Holocene thermal maximum (11-5 ka) and increasing sea ice during the mid-to-late Holocene (5-0 ka). Sediment core records from the Iceland and Rockall Plateaus show that perennial sea ice existed in these regions only during glacial intervals MIS 2, 4, and 6. These results show that sea ice exhibits complex temporal and spatial variability during different climatic regimes and that the development of modern perennial sea ice may be a relatively recent phenomenon. ?? 2010.

Quaternary Science Reviews↗

USGS2013

Radiocarbon dating late Quaternary loess deposits using small terrestrial gastropod shells

Constraining the ages and mass accumulation rates of late Quaternary loess deposits is often difficult because of the paucity of organic material typically available for 14 C dating and the inherent limitations of luminescence techniques. Radiocarbon dating of small terrestrial gastropod shells may provide an alternative to these methods as fossil shells are common in loess and contain ∼12% carbon by weight. Terrestrial gastropod assemblages in loess have been used extensively to reconstruct past environmental conditions but have been largely ignored for dating purposes. Here, we present the results of a multi-faceted approach to understanding the potential for using small terrestrial gastropod shells to date loess deposits in North America. First, we compare highly resolved 14 C ages of well-preserved wood and gastropod shells (Succineidae) recovered from a Holocene loess section in Alaska. Radiocarbon ages derived from the shells are nearly identical to wood and plant macrofossil ages throughout the section, which suggests that the shells behaved as closed systems with respect to carbon for at least the last 10 ka (thousands of calibrated 14 C years before present). Second, we apply 14 C dating of gastropod shells to late Pleistocene loess deposits in the Great Plains using stratigraphy and independent chronologies for comparison. The new shell ages require less interpretation than humic acid radiocarbon ages that are commonly used in loess studies, provide additional stratigraphic coverage to previous dating efforts, and are in correct stratigraphic order more often than their luminescence counterparts. Third, we show that Succineidae shells recovered from historic loess in the Matanuska River Valley, Alaska captured the 20th century 14 C bomb spike, which suggests that the shells can be used to date late Holocene and historic-aged loess. Finally, results from Nebraska and western Iowa suggest that, similar to other materials, shell ages approaching ∼40 ka should be viewed with caution as they may reflect trace amounts of contamination. In sum, our results show that small terrestrial gastropod shells, especially from the Succineidae family, provide reliable ages for late Quaternary loess deposits in North America.

Quaternary Science Reviews↗

USGS2013

Sea-level change during the last 2500 years in New Jersey, USA

Relative sea-level changes during the last ∼2500 years in New Jersey, USA were reconstructed to test if late Holocene sea level was stable or included persistent and distinctive phases of variability. Foraminifera and bulk-sediment δ 13 C values were combined to reconstruct paleomarsh elevation with decimeter precision from sequences of salt-marsh sediment at two sites using a multi-proxy approach. The additional paleoenvironmental information provided by bulk-sediment δ 13 C values reduced vertical uncertainty in the sea-level reconstruction by about one third of that estimated from foraminifera alone using a transfer function. The history of sediment deposition was constrained by a composite chronology. An age–depth model developed for each core enabled reconstruction of sea level with multi-decadal resolution. Following correction for land-level change (1.4 mm/yr), four successive and sustained (multi-centennial) sea-level trends were objectively identified and quantified (95% confidence interval) using error-in-variables change point analysis to account for age and sea-level uncertainties. From at least 500 BC to 250 AD, sea-level fell at 0.11 mm/yr. The second period saw sea-level rise at 0.62 mm/yr from 250 AD to 733 AD. Between 733 AD and 1850 AD, sea level fell at 0.12 mm/yr. The reconstructed rate of sea-level rise since ∼1850 AD was 3.1 mm/yr and represents the most rapid period of change for at least 2500 years. This trend began between 1830 AD and 1873 AD. Since this change point, reconstructed sea-level rise is in agreement with regional tide-gauge records and exceeds the global average estimate for the 20th century. These positive and negative departures from background rates demonstrate that the late Holocene sea level was not stable in New Jersey.

New Jersey↗

USGS2014

An 8700 year paleoclimate reconstruction from the southern Maya lowlands

Analysis of a sediment core from Lago Puerto Arturo, a closed basin lake in northern Peten, Guatemala, has provided an &sim;8700 cal year record of climate change and human activity in the southern Maya lowlands. Stable isotope, magnetic susceptibility, and pollen analyses were used to reconstruct environmental change in the region. Results indicate a relatively wet early to middle Holocene followed by a drier late Holocene, which we interpret as reflecting long-term changes in insolation (precession). Higher frequency variability is more likely attributable to changes in ocean/atmosphere circulation in both the North Atlantic and the Pacific Oceans. Pollen and isotope data show that most of the period of prehispanic agricultural settlement, i.e. &sim;5000–1000 cal yr BP, was characterized by drier conditions than previous or subsequent periods. The presence of Zea (corn) pollen through peak aridity during the Terminal Classic period (&sim;1250–1130 cal yr BP) suggests that drought may not have had as negative an impact as previously proposed. A dramatic negative shift in isotope values indicates an increase in precipitation after &sim;950 cal yr BP (hereafter BP).

Quaternary Science Reviews↗

USGS2015

A sea-level database for the Pacific coast of central North America

A database of published and new relative sea-level (RSL) data for the past 16 ka constrains the sea-level histories of the Pacific coast of central North America (southern British Columbia to central California). Our reevaluation of the stratigraphic context and radiocarbon age of sea-level indicators from geological and archaeological investigations yields 600 sea-level index points and 241 sea-level limiting points. We subdivided the database into 12 regions based on the availability of data, tectonic setting, and distance from the former Cordilleran ice sheet. Most index (95%) and limiting points (54%) are <7 ka; older data come mainly from British Columbia and San Francisco Bay. The stratigraphic position of points was used as a first-order assessment of compaction. Formerly glaciated areas show variable RSL change; where data are present, highstands of RSL occur immediately post-deglaciation and in the mid to late Holocene. Sites at the periphery and distant to formerly glaciated areas demonstrate a continuous rise in RSL with a decreasing rate through time due to the collapse of the peripheral forebulge and the reduction in meltwater input during deglaciation. Late Holocene RSL change varies spatially from falling at 0.7 ± 0.8 mm a −1 in southern British Columbia to rising at 1.5 ± 0.3 mm a −1 in California. The different sea-level histories are an ongoing isostatic response to deglaciation of the Cordilleran and Laurentide Ice Sheets.

Quaternary Science Reviews↗

USGS2015

Isotopes in North American Rocky Mountain snowpack 1993–2014

We present &sim;1300 new isotopic measurements (δ 18 O and δ 2 H) from a network of snowpack sites in the Rocky Mountains that have been sampled since 1993. The network includes 177 locations where depth-integrated snow samples are collected each spring near peak accumulation. At 57 of these locations snowpack samples were obtained for 10–21 years and their isotopic measurements provide unprecedented spatial and temporal documentation of snowpack isotope values at mid-latitudes. For environments where snowfall accounts for the majority of annual precipitation, snowmelt is likely to have the strongest influence on isotope values retained in proxy archives. In this first presentation of the dataset we (1) describe the basic features of the isotope values in relation to the Global Meteoric Water Line (GMWL), (2) evaluate space for time substitutions traditionally used to establish δ 18 O-temperature relations, (3) evaluate site-to-site similarities across the network and identify those that are the most regionally representative, (4) examine atmospheric circulation patterns for several years with spatially coherent isotope patterns, and (5) provide examples of the implications this new dataset has for interpreting paleoclimate records (Bison Lake, Colorado and Minnetonka Cave, Idaho). Results indicate that snowpack δ 18 O is rarely a simple proxy of temperature. Instead, it exhibits a high degree of spatial heterogeneity and temporal variance that reflect additional processes such as vapor transport and post-depositional modification. Despite these complexities we identify consistent climate-isotope patterns and regionally representative locations that serve to better define Holocene hydroclimate estimates and their uncertainty. Climate change has and will affect western U.S. snowpack and we suggest these changes can be better understood and anticipated by oxygen and hydrogen isotope-based reconstructions of Holocene hydroclimate using a process-based understanding of the controls on snowpack isotope ratios.

Colorado, Idaho, Montana, New Mexico, Wyoming↗

USGS2017

Vegetation history since the last glacial maximum in the Ozark highlands (USA): A new record from Cupola Pond, Missouri

The timing and drivers of vegetation dynamics and formation of no-analog plant communities during the last deglaciation in the unglaciated southeastern US are poorly understood. We present a multi-proxy record spanning the past 19,800 years from Cupola Pond in the Ozarks Mountains, consisting of replicate high-resolution pollen records, 25 AMS radiocarbon dates, and macrofossil, charcoal, and coprophilous spore analyses. Full-glacial Pinus and Picea forests gave way to no-analog vegetation after 17,400 yr BP, followed by development of Quercus -dominated Holocene forests, with late Holocene rises in Pinus and Nyssa . Vegetation transitions, replicated in different cores, are closely linked to hemispheric climate events. Rising Quercus abundances coincide with increasing Northern Hemisphere temperatures and CO 2 at 17,500 yr BP, declining Pinus and Picea at 14,500 yr BP are near the Bølling-Allerød onset, and rapid decline of Fraxinus and rise of Ostrya/Carpinus occur 12,700 yr BP during the Younger Dryas. The Cupola no-analog vegetation record is unusual for its early initiation (17,000 yr BP) and for its three vegetation zones, representing distinct rises of Fraxinus and Ostrya/Carpinus . Sporormiella was absent and sedimentary charcoal abundances were low throughout, suggesting that fire and megaherbivores were not locally important agents of disturbance and turnover. The Cupola record thus highlights the complexity of the late-glacial no-analog communities and suggests direct climatic regulation of their formation and disassembly.

Missouri↗

USGS2019

Late Quaternary paleohydrology of desert wetlands and pluvial lakes in the Soda Lake basin, central Mojave Desert, California (USA)

Sediment cores taken near extant springs along the western margin of Soda Lake playa, as well as from the playa center, reveal dramatic hydrologic changes that occurred in the central Mojave Desert during the late Quaternary. Results of stratigraphic, chronologic, physical, chemical, and microfossil analyses of seven cores, ranging in length from 5 to 23 m, help refine the timing and character of the final stages of pluvial Lake Mojave during the late Pleistocene and define distinct periods of wetland development in the early and late Holocene. Evidence shows that an incipient lake occupied the central Soda Lake basin by at least 25.0 ka (ka = thousands of calibrated 14 C years before present), and a fully developed Lake Mojave was present between 20.5 and 12.8 ka, before receding and ultimately yielding to playa conditions by 11.0 ka. Organic-rich “black mats” appear in several cores along the playa margin between 10.7 and 9.0 ka, suggesting that spring-fed wetlands persisted in this area long after the lake had regressed. The basin remained relatively dry throughout most of the Holocene until wetland ecosystems expanded along the margins between 0.73 and 0.18 ka, coincident with part of the Medieval Climate Anomaly and Little Ice Age. Overall, our results demonstrate that buried sediments surrounding extant and extinct springs can be used to reconstruct past hydrologic conditions in desert environments on a variety of spatial and temporal scales, and provide important baseline information for effective management of limited desert resources.

California↗

USGS2020

Late Quaternary evolution and stratigraphic framework influence on coastal systems along the north-central Gulf of Mexico, USA

Coastal systems in the Gulf of Mexico are threatened by reduced sediment supply, storm impacts and relative sea-level rise (RSLR). The geologic record provides insight into geomorphic evolution thresholds to these forcing mechanisms to help predict future barrier evolution in response to climate change. This study synthesizes ∼2100 km of geophysical data, 700 + sediment cores, and 62 radiocarbon dates to regionally map two lowstand sequence boundaries, multiple ravinement surfaces and fourteen depositional facies demonstrating stratigraphic and antecedent topographic influences on coastal evolution. The Mississippi-Alabama (MSAL) barriers are anchored by a marine isotope stage (MIS) 5e section of Dauphin Island coupled with an MIS 2 surface gradient change. Sand for the modern MSAL barriers were largely sourced through Holocene transgressive ravinement of relict valley fill deposits, providing up to 300 × 106 m3 of sand. Mud-filled MIS 2 tributaries correspond to areas of repeated storm breaching or tidal inlets. A Holocene geomorphic evolutionary model was created for Petit Bois and Dauphin Islands, highlighting RSLR rates, changes in sediment supply and the antecedent geologic framework. As the MIS 2 surface was flooded, tidal/wave scour supplied sand to migrating marine shoals. These transgressing shoals converted drowned paleovalleys to estuaries ∼9ka BP. Islands formed in their modern positions ∼6ka BP, when sediment supply was high and RSLR rates were 2 mm/yr. Between ∼4ka-1750 CE, islands prograded from reduced RSLR rates of 1-0.4 mm/yr and sufficient sand supply from alongshore/inner shelf sources. Currently, the islands experience 3.74 mm/yr of RSLR and reduced sediment supply, resulting in barrier degradation.

Alabama, Mississippi↗

USGS2023

Paleohydrologic history of pluvial lake San Agustin, New Mexico: Tracking changing effective moisture in southwest North America through the last glacial transition

Paleoclimate records across the Intermountain West region of North America show significant regional variation in timing and magnitude of wet conditions that accompanied the last glacial-interglacial transition. To understand the climate controls on paleohydrologic change, well-dated records are needed across the region. The Plains of San Agustin (New Mexico, USA) is a closed-basin watershed of the American Southwest influenced by both winter westerly and summer North American Monsoon precipitation. The flat valley floors of the Plains contain lake and groundwater discharge deposits that record multiple periods of past wet climate. We present a record of hydroclimate for the past 26,000 years based on radiocarbon, U–Th series, and OSL dating of these deposits and stratigraphic correlation across the three sub-basins of the lake system. We find that two major lake oscillations occurred, coincident with the global Last Glacial Maximum (∼23–18 ka) and with Heinrich Stadial 1 (∼17–14 ka). The LGM lake cycle created a deep lake in the lowermost sub-basin, fed by marsh/lake overflow in the upper sub-basins. The Heinrich Stadial 1 wet interval attained the highest recorded lake level between ∼17.0 and 15.3 ka, merging the lower two sub-basins into one lake. Both lake cycles agree well in timing and magnitude with other lake-based records from the southwestern U.S., supporting previous interpretations that a south-shifted cool season storm track brought a dipole-like pattern of enhanced moisture to the southwest at the expense of drier conditions in the northwest during the LGM and deglaciation. A transition from lake to groundwater discharge conditions followed during the Bølling-Allerød (14.7–12.9 ka) across the study area. Wet meadows prevailed in the lower sub-basin during the Younger Dryas (12.9–11.7 ka), with marsh and open lake conditions in the upper and middle sub-basins, respectively. During the early Holocene, discrete wet intervals are recorded by ages for wet meadow deposits in all sub-basins with centennial-millennial frequency at 9.9, 8.8, 8.2 ka. These events agree well with other Intermountain West records showing wetter-than-present conditions prior to 8 ka. Two additional wet periods, centered at 6.8 and 5.4 ka, occurred during the driest known interval of the middle Holocene, and likely were partly supported in the Plains of San Agustin by summer moisture associated with the peak strength of the North American Monsoon. Our record highlights that both winter and summer moisture support water resources in New Mexico watersheds.

New Mexico↗

USGS2026

Millennial-scale climatic and cultural impacts on vegetation and fire at the southern edge of the Rocky Mountains, USA

Mountain forests and woodlands in semiarid regions of the world are threatened by climatic change and other human impacts. In the southwestern USA, climate and culturally driven changes to the structure and fire regimes of dry coniferous forests over recent centuries are well documented by tree-ring archives. However, the roles of climate and people as drivers of millennial-scale changes are less understood. We present a new record of vegetation dynamics and regional fire activity inferred from pollen, microscopic charcoal, and sediment geochemistry from Santa Fe Lake, NM (3532 m a.s.l.), the southernmost natural lake in the Rocky Mountains. Declining elemental intensities (e.g., Ti, Fe, Si) record local deglaciation after 14,000 cal yr BP followed by upslope expansion of trees indicated by increasing Pinus and Picea pollen percentages as the climate warmed during the Late Glacial. Cool, dry growing seasons probably limited the establishment of dense forests until after 12,200 cal yr BP when we hypothesize that Pinus ponderosa (ponderosa pine) expanded regionally (i.e., within 10–100 km) due to continued warming and a possible increase in monsoonal precipitation. Subalpine forests established near Santa Fe Lake by 10,300 cal yr BP as indicated by abundant conifer stomates and increasing Picea and Pinus aristata pollen percentages, which are highest after 5100 cal yr BP. Increasing Juniperus -type, Quercus , and Pinus edulis -type pollen record changes in vegetation belts at lower elevations after 10,300 cal yr BP. Maize pollen, an unambiguous indicator of Puebloan agriculture, first appears at 800 cal yr BP (1150 cal yr CE) and is present through 1600 cal yr CE when Spanish colonization brought cultural upheaval and population loss to local indigenous communities. Regional fire activity, inferred from microscopic charcoal influx, remained relatively constant for most of our record. However, average microscopic charcoal influx more than doubled relative to the rest of the Holocene during the last 2000 years, with the highest influx after 1550 cal yr BP (400 cal yr CE). The increased influx of microscopic charcoal coincides with archaeological evidence for dramatic regional population growth in the northern Rio Grande region and probably reflects extensive cultural burning associated with the development of an agricultural economy. Although climate was an important driver of vegetation change and fire regimes in the mountains of the southwestern USA during the Holocene, our results suggest that human impacts on regional burning during the last 2000 years far exceeded earlier climate-driven variation.

Rocky Mountains↗