Accelerator Mass Spectrometry Radiocarbon Ages Research Articles

Quaternary marine tephrochronology of Rock Garden accretionary ridge, Hikurangi Subduction Margin, New Zealand

The Taupō Volcanic Zone (TVZ) of the North Island, New Zealand (NZ), is one of the most active Quaternary rhyolitic systems globally, producing voluminous eruptions and associated tephra that punctuate the NZ stratigraphic record. Due to dominantly westerly winds tephras are well preserved in the marine sedimentary record off NZ's east coast and provide important isochrons for chronostratigraphic correlation of marine sediment cores. Two MeBo200 (Bremen seafloor drill rig) sediment cores (GeoB20824–4 and GeoB20846–1) were collected from Rock Garden (RG) offshore of Hawke Bay in water depths of 610–670 metres below sea level (mbsl) with 24 tephra beds (both primary and secondary) that provide a well-dated proximal marine record of TVZ volcanism. The tephra layers are dated by a combination of accelerator mass spectrometry (AMS) 14C of the underlying biogenic carbonates, secondary ion mass spectrometry (SIMS), and correlation of onshore deposits using geochemical fingerprinting. Here we present SIMS U-Pb dating of 189 zircons extracted from five tephra layers from core GeoB20824–4 and six from core GeoB20846–1. AMS radiocarbon ages were carried out on sediments at three depths in core GeoB20824–4 and two depths in core GeoB20846–1, all within the upper 4 metre below seafloor (mbsf). We present major element data for 11 tephra deposits from core GeoB20824–4 and 13 from core GeoB20846–1 analysed by electron microprobe (748 analyses). We identify 10 significant Quaternary tephra horizons including: Kawakawa/Oruanui, Onepuhi (or Rangitawa), Kaukatea, Kidnappers-B, Potaka/Kidnappers, Rewa, and Sub-Rewa. Three tephra layers are geochemically correlated to unnamed but previously recognized tephra from Ocean Drilling Program (ODP) cores further east of RG. In addition to these previously identified tephra, several tephra layers are present in both MeBo200 cores that have not been recognized in existing published on- or offshore records. We attribute the finding of these new tephra deposits to the proximal location of the cores to the source location (unlike existing ODP records), the expanded stratigraphic record at these proximal locations afforded by the new coring technique applied here, and the potentially poor preservation of equivalent deposits in the terrestrial record. The identification of Quaternary TVZ tephras within MeBo200 cores, and their correlation to on- and offshore records is a significant advance as it provides: (1) a tool for correlation of marine sediment cores at the tectonostratigraphically important RG site, (2) a more detailed history of the TVZ from a proximal marine tephra record, (3) a well-dated tephra bed record from marine sediment cores with radiocarbon ages for younger ( ≤45 kyr), and U-Pb zircon ages for older (up to 1.3 Myr) tephra beds, and (4) a first approximation to the regional tephra distribution of key tephra beds

Chronology of advance and recession dynamics of the southern Green Bay Lobe of the Laurentide Ice Sheet, south-central Wisconsin, USA

AbstractWe used a combination of accelerator mass spectrometry (AMS) radiocarbon dating, optically stimulated luminescence (OSL) age estimates, and stratigraphic data from cores collected along the southern margin of the Green Bay Lobe (GBL) of the Laurentide Ice Sheet to provide new information on the timing and dynamics of the end of advance of the GBL and the dynamics of the ice sheet while very near its maximum position. Coring at multiple sites along the margin of the GBL indicate that ice had reached a stable position near its maximum extent by 24.7 ka; that ice advanced several kilometers to the Marine Isotope Stage 2 maximum position sometime shortly after 21.2 ka; and that ice remained at or beyond that position through the time interval represented by an OSL age estimate of 19.2 ± 3.2 ka. The timeline developed from these chronological data is internally consistent with, and further refines, AMS radiocarbon ages and OSL age estimates previously published for the southern margin of the GBL. It also provides new chronological control on the expansion of the GBL from its late Marine Isotope Stage (MIS) 3 extent to its MIS 2 maximum.

Paleoecology and ice limits of the early Fraser glaciation (Marine Isotope Stage 2) on Haida Gwaii, British Columbia, Canada

AbstractOur study combines new geological and paleoecological information to reconstruct the glacial history and terrestrial paleoenvironments on Haida Gwaii during the advance phase of the Fraser glaciation (Marine Isotope Stage 2). At Cape Ball on eastern Graham Island, five accelerator mass spectrometry radiocarbon ages ranging from 23,200±280 to 26,650±390 14C yr BP (ca. 27,000–31,400 cal yr BP) record the earliest approach of mainland glaciers to Haida Gwaii. Abundant marine dinoflagellate cysts indicate isostatic depression by glacial ice in Hecate Strait to the east. At Mary Point on the north coast of Graham Island, similar outwash of a piedmont lobe advancing westward along Dixon Entrance preserves plant remains dated from 19,270±360 to 23,740±300 14C yr BP (22,500–28,600 cal yr BP). These sediments also contain marine indicators. Plant macrofossils, pollen, and invertebrates support the geological evidence of a proglacial environment under a colder-than-present macroclimate. Although some trees were likely present on Graham Island at this time, tundra-like plant communities dominated low-lying areas. A large area that appears to have been ice-free during this time is a portion of the continental shelf off the east coast of Moresby Island, referred to provisionally as the “Hecate Refugium.”

High-Resolution Age Model Based on AMS Radiocarbon Ages for Kettle Lake, North Dakota, USA

A high-resolution age model was developed for Kettle Lake, North Dakota, USA, from a series of 53 accelerator mass spectrometry (AMS) radiocarbon ages calibrated with Bayesian statistical methods, which provide a monotonically increasing series of calibrated ages with depth. Evident in the sediment are several slumps, debris flows, or landslides, which are confirmed by 14C dating. Removal of these facies produces a continuous sedimentary sequence for the past 13,000 yr with exception of one ≃260-yr hiatus associated with a 1.5-m-thick slump deposit. All ages except one are on terrestrial macrofossils and charcoal. A test age on aquatic organic detritus shows a hardwater effect of 600 yr at ≃2000 cal BP. Two ages from the same level on herbaceous charcoal and Chenopodium seeds are statistically the same, which further demonstrates the suitability of charcoal from grassland environments for AMS 14C age control. However, 2 specimens of wood charcoal are too old relative to bracketing ages and glacial geologic history. These ages confirm the sedimentary interpretation of redeposition and provide a caution about the longevity of wood charcoal in the environment and its suitability for age control in lacustrine sediments.

New Excavations of Middle Stone Age Deposits at Apollo 11 Rockshelter, Namibia: Stratigraphy, Archaeology, Chronology and Past Environments

This paper presents new information obtained from a recent excavation and reassessment of the stratigraphy, chronology, archaeological assemblages and environmental context of the Apollo 11 rockshelter, which contains the longest late Pleistocene and Holocene archaeological sequence in Namibia. The Middle Stone Age (MSA) industries represented at the site include an early MSA, Still Bay, Howieson’s Poort and late MSA. Optically stimulated luminescence (OSL) dating of individual quartz grains yielded numerical ages for the Still Bay and Howieson’s Poort, and indicated the presence of a post-Howieson’s Poort phase. OSL dating also verified conventional and accelerator mass spectrometry radiocarbon ages for a further two later MSA phases. The timing of the transition from the MSA to the early Later Stone Age was also investigated. Improved resolution of the excavation and a more detailed stratigraphy revealed the presence of near-sterile cultural layers, which in some cases assisted in subdividing the MSA cultural phases. Such information, in combination with the new radiocarbon and OSL chronologies, helps address questions about the duration and continuity of MSA occupation at the site. Analyses of the faunal and archaeobotanical remains show some differences between the occupation phases at the site that may be associated with changing environmental conditions.

The Younger Dryas phase of Great Salt Lake, Utah, USA

Field investigations at the Public Shooting Grounds (a wildlife-management area on the northeastern shore of Great Salt Lake) and radiocarbon dating show that the Great Salt Lake rose to the Gilbert shoreline sometime between 12.9 and 11.2 cal ka. We interpret a ripple-laminated sand unit exposed at the Public Shooting Grounds, and dated to this time interval, as the nearshore sediments of Great Salt Lake deposited during the formation of the Gilbert shoreline. The ripple-laminated sand is overlain by channel-fill deposits that overlap in age (11.9–11.2 cal ka) with the sand, and by wetland deposits (11.1 to 10.5 cal ka). Consistent accelerator mass spectrometry radiocarbon ages were obtained from samples of plant fragments, including those of emergent aquatic plants, but mollusk shells from spring and marsh deposits yielded anomalously old ages, probably because of a variable radiocarbon reservoir effect. The Bonneville basin was effectively wet during at least part of the Younger Dryas global-cooling interval, however, conflicting results from some Great Basin locations and proxy records indicate that the regional effects of Younger Dryas cooling are still not well understood.

Floodplain Chronology and Vertical Sedimentation Rates Along the Red River, Southern Manitoba*

This paper reports the accelerator mass spectrometry radiocarbon ages and geomorphic context of one bone and 38 charcoal samples collected from 12 floodplain exposures along the Red River, southern Manitoba. The stratigraphically oldest dates range from 3760 to 6710 cal BP and indicate that floodplain deposits of at least mid-Holocene age are preserved along the valley bottom. The average rate of lateral channel migration at the meander adjacent to section Ju30-1-98 is up to 0.08 m a-1 over the past 3850 years. In sections Se04-1-97, Ju30-1-98, Ju23-1-98 and Ju21-2-98, the rates of vertical sedimentation above and below 1 to 3 m depth, range from 0.3 to 0.8 mm a-1 and 1.7 to 3.7 mm a-1, respectively. The decease in sedimentation rates is interpreted to represent a shift from oblique accretion/proximal overbank sedimentation to distal overbank sedimentation. In sections Ju28-1-98, Au05-1-00, Au03-1-98 and Ju21-1-98, which are late Holocene in age, the average rates of vertical sedimentation range from 1.8 to 3.9 mm a-1. A toy plastic pail and two radiocarbon ages from section Au05-1-00 reveal an increase in modern sedimentation rates from 1.6 to 14.3 mm a-1. This increase may have been caused by greater fluvial erosion from the large-scale introduction of European agricultural practices in the 19th century.

Holocene loess deposition and soil formation as competing processes, Matanuska Valley, southern Alaska

Although loess–paleosol sequences are among the most important records of Quaternary climate change and past dust deposition cycles, few modern examples of such sedimentation systems have been studied. Stratigraphic studies and 22 new accelerator mass spectrometry radiocarbon ages from the Matanuska Valley in southern Alaska show that loess deposition there began sometime after ∼6500 14C yr B.P. and has continued to the present. The silts are produced through grinding by the Matanuska and Knik glaciers, deposited as outwash, entrained by strong winds, and redeposited as loess. Over a downwind distance of ∼40 km, loess thickness, sand content, and sand-plus-coarse-silt content decrease, whereas fine-silt content increases. Loess deposition was episodic, as shown by the presence of paleosols, at distances >10 km from the outwash plain loess source. Stratigraphic complexity is at a maximum (i.e., the greatest number of loesses and paleosols) at intermediate (10–25 km) distances from the loess source. Surface soils increase in degree of development with distance downwind from the source, where sedimentation rates are lower. Proximal soils are Entisols or Inceptisols, whereas distal soils are Spodosols. Ratios of mobile CaO, K 2O, and Fe 2O 3 to immobile TiO 2 show decreases in surface horizons with distance from the source. Thus, as in China, where loess deposition also takes place today, eolian sedimentation and soil formation are competing processes. Study of loess and paleosols in southern Alaska shows that particle size can vary over short distances, loess deposition can be episodic over limited time intervals, and soils developed in stabilized loess can show considerable variability under the same vegetation.

Paleoseismic studies of the San Francisco Peninsula segment of the San Andreas Fault zone near Woodside, California

A late Holocene slip rate is proposed for the San Francisco Peninsula segment of the San Andreas fault zone at the Filoli Center, located between the town of Woodside and Upper Crystal Springs Reservoir, California. At this site the primary trace of the San Andreas fault zone traverses N35°W across the head of a large alluvial fan, offsetting buried stream channel deposits. One deposit, with a minimum calibrated accelerator mass spectrometry (AMS) radiocarbon age of 2070±120 years B.P., is offset 30±2 m, yielding an average geologic slip rate of 17±4 mm yr−1 for the late Holocene. A stratigraphically lower channel sequence, with a minimum calibrated AMS radiocarbon age of 330±200 years B.P., is offset 4.1±0.5 m. We interpret that this older sequence records both the penultimate earthquake on the peninsula segment, accompanied by 1.6±0.7 m of dextral slip, and the ∼2.5 m of slip that occurred in 1906. On the basis of radiocarbon analyses and historical descriptions we propose that the pre‐1906 earthquake occurred in June 1838. Our interpretations suggest that the San Francisco Peninsula segment of the San Andreas fault zone may rupture in earthquakes of at least two magnitudes: (1) a large‐magnitude (≈Mw 8) event with a recurrence of ∼250–320 years that involves the North Coast, San Francisco Peninsula, and Santa Cruz Mountains segments, as occurred in 1906 and (2) a Mw 7.0–7.4, 1838‐type event that ruptures only the San Francisco Peninsula segment. The apparent difference in slip rate of 7 ± 7 mm ‐yr−1 between the North Coast segment (24±3 mm yr−1) and the peninsula segment (17±4 mm yr−1) suggests a possible slip rate gradient on the northern California section of the San Andreas fault.

Late Holocene Eolian Activity in the Mineralogically Mature Nebraska Sand Hills

The age of sand dunes in the Nebraska Sand Hills has been controversial, with some investigators suggesting a full-glacial age and others suggesting that they were last active in the late Holocene. New accelerator mass spectrometry radiocarbon ages of unaltered bison bones and organic-rich sediments suggest that eolian sand deposition occurred at least twice in the past 300014C yr B.P. in three widely separated localities and as many as three times in the past 80014C yr at three other localities. These late Holocene episodes of eolian activity are probably the result of droughts more intense than the 1930s “Dust Bowl” period, based on independent Great Plains climate records from lake sediments and tree rings. However, new geochemical data indicate that the Nebraska Sand Hills are mineralogically mature. Eolian sands in Nebraska have lower K-feldspar (and K2O, Rb, and Ba) contents than most possible source sediments and lower K-feldspar contents than dunes of similar age in Colorado. The most likely explanation for mineralogical maturity is reduction of sand-sized K-feldspar to silt-sized particles via ballistic impacts due to strong winds over many cycles of eolian activity. Therefore, dunes of the Nebraska Sand Hills must have had a long history, probably extending over more than one glacial–interglacial cycle, and the potential for reactivation is high, with or without a future greenhouse warming.

RADIOCARBON DATES FOR BEESWAX FIGURES IN THE PREHISTORIC ROCK ART OF NORTHERN AUSTRALIA

Accelerator mass spectrometry radiocarbon ages have been taken for a test suite of small samples of material removed from some of the ‘beeswax’art figures found in rock shelters in northern Australia. The results indicate that we can reliably date this unique form of rock art with no noticeable damage. We had not expected to find figures of any great antiquity, and so we were surprised to find that the ages obtained spanned the time period from the recent past to about 4000 BP.

A 12 000 year radiocarbon date of deglaciation from the Continental Divide of northwestern Montana

During the Pinedale (Late Wisconsinan) glaciation, an outlet glacier from a mountain ice field flowed eastward across the Continental Divide through Marias Pass in northwestern Montana. This outlet glacier was the major source of the Two Medicine glacier, a large piedmont glacier that extended from the mountain front east about 55 km onto the plains. An accelerator mass spectrometry radiocarbon age of 12 194 ± 145 BP (AA-9530) was obtained from a wood fragment, underlying a Glacier Peak tephra and a Mount Saint Helens set J tephra in a section of lake sediments, near Marias Pass. This radiocarbon age provides a minimum date of deglaciation for the Marias Pass area that is about 800 years older than a previous estimate. Furthermore, the radiocarbon age indicates that the Two Medicine glacier was no longer being supplied by its major source and if it still existed was only as a dying, stagnant ice mass.

Varve calibrated records of carbonate and organic carbon accumulation over the last 2000 years in the Black Sea

Sedimentologic and geochemical studies of box and gravity cores recovered from the Black Sea during the first leg of a multileg international Black Sea expedition in 1988 allow reconstruction of the basinwide Holocene environmental history of the Black Sea. In the deeper parts of the basin, box cores typically recovered a flocculent surface layer (“fluff”), laminated coccolith marls of Unit I (25–45 cm thick), and the upper 5–10 cm of finely laminated, dark‐colored sapropels of Unit II. Fine‐grained, homogeneous mud turbidites are interbedded with Units I and II over much of the basin, but the stratigraphie position of these turbidites differs, from site to site. The deposition of individual turbidites up to 15 cm thick does not appear to have significantly disturbed underlying laminae. Sediment trap deployments in the Black Sea suggest that light and dark laminae couplets represent annual increments of sedimentation (i.e., varves); we have therefore constructed a varve chronology for the sequence in order to correlate and date distinctive sedimentation and paleoenvironmental events. Distinctive groups of laminae in Unit I can be correlated across the entire deeper basin (a distance of more than 1000 km). This implies a remarkable homogeneity in production, accumulation, and preservation of biogenic material over much of the Black Sea during deposition of Unit I. The change from deposition of finely laminated, organic carbon‐rich sapropels (Unit II) to laminated, more calcareous, coccolith‐rich marls (Unit I) is thought to represent the crossing of a salinity threshold for Emiliania huxleyi. The varve chronology sets this change at about 1.63 ka (1633±100 yr B.P.), but the record of magnetic secular variation measured in several cores produces an age estimate of about 2.0 ka for the base of Unit I, or about 1.2 times the varve age. The average of six calibrated accelerator mass spectrometry radiocarbon ages for the base of Unit I is 2.7 ka, or about 1.7 times the varve age. Following the initial change to coccolith‐dominated sedimentation, deposition of sapropel resumed for at least one significant period, 1.56–1.25 ka. Since 1.25 ka, cycles of carbonate deposition with quasi‐decadal periodicities have produced characteristic darker and lighter assemblages of laminae. These cycles may have been climatically driven. Geochemical analyses coupled with the varve ages adopted herein indicate that accumulation rates of carbonate are nearly an order of magnitude higher in Unit I (averaging 35–45 g m−2 yr−1) than in sapropelic Unit II. which contains primarily detrital carbonate. The accumulation of lithogenic components in parts of Unit I is only 1.5 times the rate in Unit II. Deepwater organic carbon accumulation rates are somewhat higher in Unit I (3.5–4.5 g m−2 yr−1) than in the upper part of Unit II. Organic carbon accumulation rates in Unit I are somewhat antithetic to those of carbonate, and on the basis of this and additional constraints placed by pyrolysis and carbon isotopic analyses of organic material, it appears that terrestrial organic matter is an important component (perhaps >25%) of total organic carbon burial in the basin. Unit I in the western part of the Black Sea has a higher terrestrial organic component and higher accumulation rates of terrigenous clastic material than Unit I in the eastern part. This difference between eastern and western Black Sea is to be expected because of the major rivers that empty into the western Black Sea from eastern Europe, Ukraine, and Russia. Shallow slope sites, but still within euxinic bottom waters, have lower organic carbon accumulation rates and lower pyrolysis hydrogen indices than deepwater basinal sites, suggesting selective resuspension and oxidation of organic matter at basin margins and focusing of organic matter deposition toward the basin center. A comparison of the Black Sea data with those from several open ocean sites with similar water depths showed no significant difference between organic carbon accumulation rates under oxic and anoxic conditions. For a given bulk accumulation rate the organic carbon accumulation rates, normalized to primary productivity, are about the same in both settings.

Age of the Crowfoot advance in the Canadian Rocky Mountains: A glacial event coeval with the Younger Dryas oscillation

Research Article| May 01, 1994 Age of the Crowfoot advance in the Canadian Rocky Mountains: A glacial event coeval with the Younger Dryas oscillation Mel A. Reasoner; Mel A. Reasoner 1Department of Geology, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Gerald Osborn; Gerald Osborn 2Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 4N1, Canada Search for other works by this author on: GSW Google Scholar N. W. Rutter N. W. Rutter 1Department of Geology, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Geology (1994) 22 (5): 439–442. https://doi.org/10.1130/0091-7613(1994)022<0439:AOTCAI>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Mel A. Reasoner, Gerald Osborn, N. W. Rutter; Age of the Crowfoot advance in the Canadian Rocky Mountains: A glacial event coeval with the Younger Dryas oscillation. Geology 1994;; 22 (5): 439–442. doi: https://doi.org/10.1130/0091-7613(1994)022<0439:AOTCAI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract A suite of sediment core samples was recovered from two lakes, Crowfoot and Bow lakes, that are adjacent to the Crowfoot moraine type locality, to identify and radiocarbon date sediments related to the Crowfoot advance. The Crowfoot moraine system, widely recognized throughout northwestern North America, represents a glacial advance that is post-Wisconsin and pre-Mazama tephra in age. An interval of inorganic sediments bracketed by accelerator mass spectrometry radiocarbon ages of ca. 11,330 and 10,10014C yr B.P. is associated with the Crowfoot moraine. The Crowfoot advance is therefore approximately synchronous with the European Younger Dryas cold event (ca. 11,000-10,000 14C yr B.P.). Furthermore, the termination of the Crowfoot advance also appears to have been abrupt. These findings illustrate that the climatic change responsible for the European Younger Dryas event extended beyond the northern Atlantic basin and western Europe. Equilibrium-line altitude (ELA) depressions associated with the Crowfoot advance are similar to those determined for the Little Ice Age advance, whereas Younger Dryas ELA depressions in Europe significantly exceed Little Ice Age ELA depressions. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Accelerator mass spectrometry radiocarbon dating of rock varnish

Accelerator mass spectrometry (AMS) 14C dating off rock varnish provides minimum-limiting ages for landforms and archaeological artifacts in arid and semiarid lands that are undatable by conventional radiocarbon methods. Experiments on sample collection, the effect of different individuals preparing samples, the influence of different chemical-extraction procedures, the incorporation of carbonate detritus, possible contamination from rock underlying varnish, the role of bio-geochemical erosion of varnish, and other influences reveal procedures that yield reproducible results. Conventional radiocarbon dates from charcoal beneath lava flows of Hualalai Volcano, Hawaii, and from arid sites in western North America provide controls to test varnish radiocarbon dating. AMS radio-carbon ages on the very bottom layer of varnishes are typically ≤10% younger than the 14C ages of these controls. Applications are illustrated here for fluvial, aeolian, peri-glacial, hillslope, lacustrine, and glacial geo-morphology, as well as rock-art research in archaeology.

Aspartic Acid Racemization Ages of California Paleoindian Skeletons

The redating of the Laguna and Los Angeles Man skeletons by the accelerator mass spectrometry (AMS) radiocarbon method has shown that the Upper Pleistocene dates for these skeletons determined previously by the conventional radiocarbon method are probably incorrect. Using the revised ages for these skeletons to calibrate the aspartic acid racemization rate yields racemization ages for the Del Mar and Sunnyvale skeletons that are compatible with their AMS radiocarbon ages, a point that Taylor et al. (1985) fail to discuss.