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Results for “Radiocarbon”

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

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At least 91 records · Page 5Linked to original sources
USGS1976

Bog stratigraphy, radiocarbon dates, and Pinedale to Holocene glacial history in the Front Range, Colorado

Radiocarbon dates and stratigraphic cores from bogs, kettle ponds, and former ice-marginal lakes on the east and west sides of the Front Range, Colo., between lat 40°00' and 40°24' N. suggest that (1) valley glaciers of Pinedale age began to recede from their terminal positions between about 14,600 and 13,000 yr ago, (2) revegetation of glaciated areas at altitudes of 2,600-2,900 m (8,600-9,500 ft) was complete by 11,000-10,000 yr ago, (3) at one site, 3,500±1,000 yr elapsed before peat began to form after deglaciation, (4) the formation of bogs within the glaciated areas kept pace with glacier recession in a general way, beginning at progressively later times as deglaciation proceeded upward, (5) Pinedale glaciers had disappeared or were reduced to small remnants by about 8,000 yr ago, (6) moraines that have been mapped as belonging to the early stade of Pinedale Glaciation are no younger than 13,000 yr B.P. and may be older than 14,600 yr, and those delimiting what has been mapped as late stade are no younger than about 7,600 yr B.P. and are probably older than 7,800 yr, and (7) most of the till mapped as Pinedale was deposited between about 14,600 and 8,000 yr ago.

Colorado↗
USGS1975

Radiocarbon dates indicate rates of graben downfaulting, San Jacinto Valley, California

Recent radiocarbon dates for wood samples collected from three depths in San Jacinto Valley graben indicate active tectonic downfaulting during the past 42,000 yr. The flood plain of graded San Jacinto River, entering the valley from the southeast and leaving toward the west, serves as a reference datum across the graben. Depositional rates suggest that downfaulting averaged about 0.007 ft (2.1 mm)/yr from 42,000 to 15,270 yr B.P. and since 15,270 yr B.P. has increased to about 0.019 ft (5.8 mm)/yr.

California↗
USGS2021

Bomb-produced radiocarbon across the South Pacific Gyre — A new record from American Samoa with utility for fisheries science

Coral skeletal structures can provide a robust record of nuclear bomb produced 14 C with valuable insight into air-sea exchange processes and water movement with applications to fisheries science. To expand these records in the South Pacific, a coral core from Tutuila Island, American Samoa was dated with density band counting covering a 59-yr period (1953–2012). Seasonal signals in elemental ratios (Sr/Ca and Ba/Ca) and stable carbon (δ 13 C) values across the coral core corroborated the well-defined annual band structure and highlighted an ocean climate shift from the 1997–1998 El Niño. The American Samoa coral 14 C measurements were consistent with other regional records but included some notable differences across the South Pacific Gyre (SPG) at Fiji, Rarotonga, and Easter Island that can be attributed to decadal ocean climate cycles, surface residence times and proximity to the South Equatorial Current. An analysis of the post-peak 14 C decline associated with each coral record indicated 14 C levels are beginning to merge for the SPG. This observation, coupled with otolith measurements from American Samoa, reinforces the perspective that bomb 14 C dating can be performed on fishes and other marine organisms of the region using the post-peak 14 C decline to properly inform fisheries management in the South Pacific.

Radiocarbon↗
USGS2023

Atmospheric radiocarbon for the period 1910 to 2021 recorded by annual plants

We present a timeseries of 14 CO 2 for the period 1910–2021 recorded by annual plants collected in the southwestern United States, centered near Flagstaff, Arizona. This timeseries is dominated by five commonly occurring annual plant species in the region, which is considered broadly representative of the southern Colorado Plateau. Most samples (1910–2015) were previously archived herbarium specimens, with additional samples harvested from field experiments in 2015–2021. We used this novel timeseries to develop a smoothed local record with uncertainties for “bomb spike” 14 C dating of recent terrestrial organic matter. Our results highlight the potential importance of local records, as we document a delayed arrival of the 1963–1964 bomb spike peak, lower values in the 1980s, and elevated values in the last decade in comparison to the most current Northern Hemisphere Zone 2 record. It is impossible to retroactively collect atmospheric samples, but archived annual plants serve as faithful scribes: samples from herbaria around the Earth may be an under-utilized resource to improve understanding of the modern carbon cycle.

Radiocarbon↗
USGS2016

Comparison of manual and automated pretreatment methods for AMS radiocarbon dating of plant fossils

A new automated pretreatment system for the preparation of materials submitted for accelerator mass spectrometry (AMS) analysis is less time-consuming and results in a higher sample yield. The new procedure was tested using two groups of plant fossils: one group was pretreated using the traditional method, and the second, using the automated pretreatment apparatus. We compared the time it took to complete the procedure and the amount of sample material remaining. The automated pretreatment apparatus proved to be more than three times faster and, in most cases, produced a higher yield. We also observed a darker discoloration of the KOH solutions, indicating that the automated system is more thorough in removing humates from the specimen compared to the manual method.

Radiocarbon↗
USGS2016

Radiocarbon dates for lava flows from northeast rift zone of Mauna Loa Volcano, Hilo 7 1/2 minute quadrangle, Island of Hawaii

Twenty-eight 14 C analyses are reported for carbonized roots and other plant material collected from beneath 15 prehistoric lava flows erupted from the northeast rift zone (NERZ) of Mauna Loa Volcano (ML) utilizing the recovery techniques of Lockwood and Lipman (1980). Most samples were collected from the Hilo 7 1/2’ quadrangle during field work for a geologic map of that quadrangle (Buchanan-Banks, unpub data); a few sample sites are located in adjacent quadrangles: Piihonua to the west and Mountain View to the south. Altitudes are given in English units as well as metric to facilitate locating sites on USGS topographic maps.

Radiocarbon↗
USGS2002

Middle Palaeolithic refugium, or archaeological misconception? A new U-Series and radiocarbon chronology of Abric Agut (Capellades, Spain)

New U-Series and C14 (AMS) dates are provided for the Abric Agut (Capellades, Barcelona, Spain). This site was previously considered to be of Middle Palaeolithic age according to the characteristics of the lithic assemblage. In addition, human teeth were uncovered and attributed to neandertals. However, radiometric dating clearly indicates a Late Pleistocene-Early Holocene age. ?? 2002 Elsevier Science Ltd. All rights reserved.

Journal of Archaeological Science↗
USGS2017

Radiocarbon analyses from Cincinnati, Ohio, and their implications for glacial stratigraphic interpretations

Detailed analysis of a site near Cincinnati, Ohio, shows that 14C ages of samples in a single geologic unit can have a range of several thousand years and ages from different stratigraphic units can overlap. At the Sharonville site, four 14C samples from organic silt below glaciogenic deposits have an inverted chronologic sequence, suggesting contamination, but nevertheless they indicate the silt was deposited before 27,000 yr B.P. A stump cluster in growth position, wood fragments, and moss from the upper surface of the silt may differ by as much as 2300 14C yr. Five ages from the stump cluster constrain the timing of a glacier advance of the Laurentide ice sheet to its southern limit in the Cincinnati area at 19,670 ?? 68 yr B.P. Overlying glaciogenic sediments contain transported wood that may be as much as 3200 yr older than the advance. This range of ages points out that, for a given site, several age measurements are required to determine when a glacier advance occurred. Because the measured ages in this study span the entire interval suggested for a twofold sequence of advance, retreat, and readvance of the margin of the Miami sublobe, we suggest a single advance to its terminal position in the Cincinnati area as an alternate hypothesis for testing. ?? 1990.

Quaternary Research↗
USGS2014

Radiocarbon dating loess deposits in the Mississippi Valley using terrestrial gastropod shells (Polygyridae, Helicinidae, and Discidae)

Small terrestrial gastropod shells (mainly Succineidae) have been used successfully to date late Quaternary loess deposits in Alaska and the Great Plains. However, Succineidae shells are less common in loess deposits in the Mississippi Valley compared to those of the Polygyridae, Helicinidae, and Discidae families. In this study, we conducted several tests to determine whether shells of these gastropods could provide reliable ages for loess deposits in the Mississippi Valley. Our results show that most of the taxa that we investigated incorporate small amounts (1–5%) of old carbon from limestone in their shells, meaning that they should yield ages that are accurate to within a few hundred years. In contrast, shells of the genus Mesodon ( Mesodon elevatus and Mesodon zaletus ) contain significant and variable amounts of old carbon, yielding ages that are up to a couple thousand 14 C years too old. Although terrestrial gastropod shells have tremendous potential for 14 C dating loess deposits throughout North America, we acknowledge that accuracy to within a few hundred years may not be sufficient for those interested in developing high-resolution loess chronologies. Even with this limitation, however, 14 C dating of terrestrial gastropod shells present in Mississippi Valley loess deposits may prove useful for researchers interested in processes that took place over multi-millennial timescales or in differentiating stratigraphic units that have significantly different ages but similar physical and geochemical properties. The results presented here may also be useful to researchers studying loess deposits outside North America that contain similar gastropod taxa..

Mississippi Valley↗
USGS2014

Crustal migration of CO2-rich magmatic fluids recorded by tree-ring radiocarbon and seismicity at Mammoth Mountain, CA, USA

Unrest at Mammoth Mountain over the past several decades, manifest by seismicity, ground deformation, diffuse CO 2 emissions, and elevated 3 He/ 4 He ratios in fumarolic gases has been driven by the release of CO 2 -rich fluids from basaltic intrusions in the middle to lower crust. Recent unrest included the occurrence of three lower-crustal (32–19 km depth) seismic swarms beneath Mammoth Mountain in 2006, 2008 and 2009 that were consistently followed by peaks in the occurrence rate of shallow (≤10 km depth) earthquakes. We measured 14 C in the growth rings (1998–2012) of a tree growing in the largest (∼0.3 km 2 ) area of diffuse CO 2 emissions on Mammoth Mountain (the Horseshoe Lake tree kill; HLTK) and applied atmospheric CO 2 concentration source area modeling to confirm that the tree was a reliable integrator of magmatic CO 2 emissions over most of this area. The tree-ring 14 C record implied that magmatic CO 2 emissions from the HLTK were relatively stable from 1998 to 2009, nearly doubled from 2009 to 2011, and then declined by the 2012 growing season. The initial increase in CO 2 emissions was detected during the growing season that immediately followed the largest (February 2010) peak in the occurrence rate of shallow earthquakes. Migration of CO 2 -rich magmatic fluids may have driven observed patterns of elevated deep, then shallow seismicity, while the relationship between pore fluid pressures within a shallow (upper 3 km of crust) fluid reservoir and permeability structure of the reservoir cap rock may have controlled the temporal pattern of surface CO 2 emissions.

California↗
USGS2020

Evaluation of soil zone processes and a novel radiocarbon correction approach for groundwater with mixed sources

Estimates of groundwater age based on 14C is often limited by the uncertainty in geochemical processes that alter the 14C concentration measured in water and the composition (δ13C and 14C) of carbon sources needed to appropriately parametrize 14C adjustment models. Estimated ages for samples that contain a mixture of young and old groundwater will be particularly sensitive to model parametrization as relatively small additions of modern 14C from recent recharge can mask the presence and amount of old groundwater. A novel multi-model approach based on inverse geochemical modeling and lumped parameter modeling of age tracers (3H, 3Hetrit, and SF6) was used to better constrain 14C dilution caused by dissolution of carbonates in the unsaturated zone or shallow parts of the Glacial aquifer, which extends over 2000 miles across the northern contiguous United States. Calibration of 14C inverse geochemical models to LPM computed 14C concentrations in modern water indicated that 14C of soil zone and shallow aquifer carbonates were not 14C-dead (0 pmC), as is typically assumed for 14C correction models. 14C of such carbonates was on average about 53 pmC (ranged 0-110 pmC, n = 72). This information was used to correct 14C concentrations for water recharged entirely before 1950 and water that is a mixture of pre- and post-1950 water. The multi-model approach developed here was compared to an analytical 14C-adjustment model (Revised Fontes and Garnier) that assumed solid carbonates were 14C-dead. 14C corrections using the analytical adjustment model tended to over-correct final 14C concentrations by 21 pmC and underestimates mean ages by 40% for groundwater mixtures. In fact, 14C corrections based on analytical model yielded negative ages (14C > 120 pmC) in nearly 36% of mixed samples. This work presents a new approach to constraining 14C corrections and age estimates of mixtures of young and old groundwater. The new method is applied to three well networks distributed across the spatially expansive Glacial aquifer.

Glacier aquifer system↗