Accelerator Mass Spectrometry Ages Research Articles

Neogene and Quaternary reef terraces of Mangaia, Rarotonga and Aitutaki (southern Cook Islands, South Pacific) revisited: gauges of sea-level change and island uplift?

New sedimentological data of facies and diagenesis as well as chronological data including strontium (87Sr/86Sr)-isotope ratios and uranium (U)-series dating, radiocarbon (14C) accelerator mass spectrometry (AMS) dating and biostratigraphy from elevated reef terraces (makatea) in the southern Cook Islands of Mangaia, Rarotonga and Aitutaki contribute to controversial discussions regarding age and sea-level relationships of these occurrences during the Neogene and Quaternary. The oldest limestones of the uplifted makatea island of Mangaia include reef-related facies which are mid-Miocene in age, based on new Sr-isotope and biostratigraphical data. In between these older deposits and the lowest coastal reef terrace of marine isotope stage (MIS) 5e, various older Pleistocene reef-related facies were identified. Based on Sr-isotope ratios, these were deposited during earlier Pleistocene highstands (as old as 2.28 Ma). Rare reef terraces on Rarotonga belong to the Plio-Pleistocene and the late Miocene, according to 87Sr/86Sr ratios. The late Miocene age is enigmatic as it exceeds the age of subaerially exposed volcanic rocks of Rarotonga island. The fossil reef could have formed on an older submarine volcanic high that was later displaced by younger volcanism to its present position, or the Sr-age could be too old due to diagenetic resetting. The Plio-Pleistocene Rarotonga reef terraces are overlain irregularly by Holocene reef deposits that are interpreted as storm rubble. Reef terraces on Aitutaki represent evidence of a higher-than-present (up to 1 m) sea-level during the late Holocene, based on 14C AMS age data. They are very similar to elevated late Holocene reefs of adjacent French Polynesia with regard to composition, elevation and age.

Distinguishing Late Holocene Storm Deposit From Shore-normal Beach Sediments From the Gulf of Thailand

Grain size, as one of sedimentological proxies, coupled with a detailed description of the sedimentary structures and luminescence dating were used to unveil the sediment sources and transport process of the Holocene ancient coastal storm events recorded in the beach ridge plain, wet swale and muddy environments at Prachuap Khiri Khan, in the Southern Peninsula of Thailand. In this study, a total of 141 sand samples were collected from the shore-normal ridge-swale topography and analyzed for layers of candidate storm deposits, revealing at least 21 candidate coastal storm events. The grain size distribution of beach sediments was, in general, unimodal, while the candidate storm sediments revealed a mixed combination of multimodal, bimodal and unimodal distributions. Plots of mean grain size against skewness and kurtosis and of skewness against kurtosis could differentiate storm deposits from shore-normal beach sediments. Sedimentary structures preserved in the ancient coastal storm deposits included parallel and inclined landward laminations, mud rip-up clasts, layers of shell fragments, a pebble grain, normal and reverse grading and sharp lower and upper contacts. Candidate storm layers overlain on a dry beach ridge intervened with mud in a swale showed a finer and thinner landward deposit. Marine shell fragments, smaller foraminifers, ostracod and scaphopod (tusk shell), were well preserved. Based on optically stimulated luminescence dating and a correlated accelerator mass spectrometry age, multiple layers of sand derived from different frequencies of coastal storms were deposited over the middle to late Holocene.

Chronological and geomorphological approach to the Holocene tephras from Tafí and Santa María valleys, NW Argentina

AbstractA comprehensive morphostratigraphic and chronological study of the complete set of Holocene tephras from Tafí and Santa María valleys (northwestern Argentina), including analyses of compositional characteristics, is presented. Five ash tephras are recognized: V0 (El Rincón), V1a (Carreras 1a ash), V1b (Carreras 1b ash), V2a (Carreras 2 ash), and V2b (El Paso 3 ash). Two of them (V1b and V2b) are described for the first time in the study area. The new 14C and accelerator mass spectrometry ages presented, along with the previously published information, allows for the establishment of a chronological framework. The V0 tephra was deposited in the Early Holocene (about 10,000 yr BP), V1a and V1b were deposited in the Middle Holocene (about 4200 and 3500 yr BP, respectively), and V2a and V2b were deposited in the Late Holocene (after about 800 yr BP). The mineralogical, textural, and geochemical characterizations of the five tephras suggest that their tephra provenance was mainly from the back-arc region. However, the determination of the exact source of each tephra requires more accurate high-resolution tephrochronological studies. At least five major eruptions affected the Tafí and Santa María valleys in the last 10,000 yr.

A multi-proxy study of changing environmental conditions in a Younger Dryas sequence in southwestern Manitoba, Canada, and evidence for an extraterrestrial event

AbstractMulti-proxy analyses of a sequence spanning the Younger Dryas (YD) in the Glacial Lake Hind basin of Manitoba provides insight into regional paleohydrology and paleovegetation of meltwater rivers and lakes spanning >4000 yr; the sequence is controlled by 25 new accelerator mass spectrometry ages. This lake, dammed by the Laurentide Ice Sheet, overflowed into Lake Agassiz. The pre-YD interval records rapid sedimentation from meltwaters that headed in proglacial lakes in the Canadian Prairies that are known to have been catastrophically released when ice or sediment barriers were breached. Pollen in this phase is dominated by pre-Quaternary forms eroded from Paleocene bedrock. At the onset of the YD at ~12.8 cal ka, the sudden appearance of concentrations of nanodiamonds, high-temperature magnetic spherules, platinum, and iridium provide evidence of an extraterrestrial (ET) event that others have identified at more than 40 sites in North America. Major changes in oceans and climate, and the catastrophic outflow of nearby Lake Agassiz at the onset of the YD, may be related. Lower water levels and a reduction of Souris River inflow to Lake Hind followed, which are reflected by more clayey and organic-rich sediments and a decrease in pre-Quaternary palynomorphs. This may have resulted from the deepening of river valleys caused by the release of meltwater triggered by the ET event. Wetlands then began to develop, leading to peat deposition from 12.3 to 11 cal ka. This was followed by a fluvial episode depositing sand and then by increased Holocene aridity that resulted in accumulation of a thick sequence of dune sands. A dry woodland environment with a mix of conifers (especially Picea and Larix) and deciduous trees (especially Populus and Quercus) covered the uplands from ~13 to 10 cal ka.

New ages from Boomplaas Cave, South Africa, provide increased resolution on late/terminal Pleistocene human behavioural variability

ABSTRACTBoomplaas Cave, South Africa, contains a rich archaeological record, with evidence of human occupation from >66,000 years ago until the protohistoric period. Notwithstanding a long history of research at the site, its existing chronology can benefit from revision. Many of the site’s members are currently delimited by only a single conventional radiocarbon date and some of the existing dates were measured on materials now known to be unsuitable for radiocarbon dating. Here we present the results of an ongoing effort to redate key late/terminal Pleistocene sequences in southern Africa. This paper presents a Bayesian-modelled radiocarbon chronology for the late/terminal Pleistocene horizons at Boomplaas. Our model incorporates previously published radiocarbon dates as well as new accelerator mass spectrometry ages. We also present archaeological evidence to examine in greater detail than was previously possible the nature of occupation patterning across the late/terminal Pleistocene and to assess technological change across two of the site’s Last Glacial Maximum (LGM) members. The new dates and archaeological data confirm that the site was occupied in a series of low intensity events in the early LGM and immediately thereafter. The site was occupied intensively in the terminal Pleistocene in line with major changes in palaeoenvironments and sea-level fluctuations. The lithic data show the use of variable technological strategies in contexts of shifting mobility and site occupation patterns. Our discussion informs upon hunter-gatherer behavioural variability that did not, and should not be expected to, reflect the strategies adopted and adapted by a handful of well-known arid-zone hunter-gatherers in the twentieth-century Kalahari.

New ages from Sehonghong rock shelter: Implications for the late Pleistocene occupation of highland Lesotho

Sehonghong rock shelter is situated in the eastern Lesotho highlands, a climatically extreme region of southern Africa. The site is one of a handful in southern Africa that preserves human occupations before, during, and after the Last Glacial Maximum (LGM). The site's long and well-preserved sequence makes it relevant to addressing questions of human mobility, subsistence, and technology in relation to broader environmental change. Here we present a Bayesian-modelled radiocarbon chronology for the LGM and terminal Pleistocene occupations at Sehonghong. Our model incorporates previously published radiocarbon dates and new accelerator mass spectrometry ages. We also present archaeological evidence to test the hypothesis that Sehonghong was occupied in a series of punctuated events, and that some of these occupations were more intensive than others. Previous chronological and archaeological data were insufficient for testing these hypotheses. The new dates and archaeological data confirm that the site was occupied intensively in the early LGM and immediately thereafter. The site was otherwise occupied sporadically. We find that greater site occupation density is not always correlated with intensified use of local resources as measured by increased bipolar reduction and fish consumption. The new dates further confirm that Sehonghong contains some of the oldest evidence for systematic freshwater fishing in southern Africa. The availability of fish, a high fat protein source, probably stimulated human occupation, however sporadic, of such montane environments during cooler and drier periods. These findings suggest behavioural variability in response to shifting mobility and subsistence strategies. Our brief discussion informs upon hunter-gatherer occupation of southern African montane environments more broadly and human behavioural variability during the LGM.

Ecosystem change in the South Pare Mountain bloc, Eastern Arc Mountains of Tanzania

Palaeoecological evidence is used to investigate climatic and anthropogenic drivers of vegetation and fire dynamics through the past ~1200 years in the Eastern Arc Mountains, Tanzania. Pollen and charcoal analyses are supported by a chronology derived from five accelerator mass spectrometry ages, a137Cs activity profile and marker horizons from two exotic pollen taxa. Pollen indicator taxa are used to develop a series of environmental indices, and compared with fluctuations in key timber species, and a fire history reconstruction. Prior to AD 1274, the ecosystem is characterised by moist montane forest that is somewhat anomalous with other East African records, providing evidence for the persistence of a mesic environment and ecosystem resilience of the Eastern Arc Mountains. An open drier forest type is recorded from AD 1275 to 1512, resulting from regional aridity; when the aforementioned buffering ability was exceeded and ecosystem resilience curtailed. Maize appears from ~AD 1737, possibly associated with the regional expansion of agriculture to supply the ivory trade. The peak of the caravan trade in the mid-19th century, and later colonial administration, coincides with intensified human impacts, specifically forest clearance suggested by substantial declines in the timber tree Ocotea. Following independence, there are tentative signals of montane forest recovery, which coincide with the establishment of forest reserves, and associated timber bans. Palaeoecological understanding of historical ecosystem change in the Eastern Arc Mountains biodiversity hotspot is vital to build informed conservation and forest management policies for a future characterised by growing human populations coupled with changing climate–ecosystem relationships.

A radiocarbon investigation of Moriori forest use on Rēkohu (Chatham Island), southwestern Polynesia

The first colonists of temperate climate, southwestern Polynesia settled an archipelago unsuited generally to the cultivation of several tropical homeland crops, including fruiting trees. On the Chatham Islands, these colonists achieved settlement permanency after the thirteenth century AD to become the indigenous Moriori in the absence of all Polynesian domesticates. We test the thesis that edible and storable seeds of introduced New Zealand karaka or (in Moriori) kōpi trees (Corynocarpus laevigatus J.R. Forst. & G. Forst.) replaced tropical crops in the Moriori economy. Archaeological midden sites associated with kōpi are dated by radiocarbon (14C) from Rēkohu (Chatham Island), the largest island of the Chathams group. We examine new accelerator mass spectrometry ages for individual kōpi seeds and twigs <10years old and review extant standard ages. A local marine 14C value is determined to improve confidence in mollusc dates. Our results include the earliest reliable 14C age on a kōpi seed from Rēkohu (calyrAD 1450–1620, 95% probability). Strategic tree plantings in sheltered locations and initial if not ongoing forest management are indicated. Archaeological evidence of comparable, later, leeward and windward island Corynocarpus use suggests that kōpi production supported the permanent Moriori settlement of Rēkohu.

A new flow cytometry method enabling rapid purification of fossil pollen from terrestrial sediments for AMS radiocarbon dating

ABSTRACTRadiocarbon (14C) accelerator mass spectrometry (AMS) dating has played a significant role towards improving our understanding of the timing of events and rates of change in archaeological and environmental records over the last 50 000 years. Although it is not always possible to find suitable macrofossils for 14C dating, microfossils, notably plant pollen, are a viable alternative. Obtaining preserved pollen samples of known provenance and of sufficient quantity for dating by 14C AMS is, however, challenging because of time‐consuming methods of extraction and purification and possible contamination from other organic material. Here we report a new, rapid and straightforward method using flow cytometry (FCM) to distinguish, sort and collect sufficient quantities of fossil pollen with minimal contamination from lake sediments for radiocarbon dating. Using this approach, we demonstrate 14C AMS ages that date back to at least 40 ka BP. While future work may be required to refine purification methodologies for different sample types and to precisely quantify the dating limit of this approach, FCM dating of microfossils shows considerable promise in generating robust geochronological frameworks for sequences that have previously proved problematic. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Dating Charred Soil Organic Matter: Comparison of Radiocarbon Ages from Macrocharcoals and Chemically Separated Charcoal Carbon

Radiocarbon dating of charcoal in soils is commonly used to reconstruct past environmental processes. Also microcharcoal that is chemically isolated from soil organic matter by high-energy UV photo-oxidation can be dated with 14C accelerator mass spectrometry (AMS). We compared the 14C AMS ages of 13 pairs of hand-picked macrocharcoals and microcharcoal samples separated via the UV oxidation method; both charcoal fractions were taken from the same soil samples (prehistoric pit fillings). We found that in most cases, the microcharcoal fraction yielded older ages than the single macrocharcoal pieces, and that the differences between the ages are not systematic. A reason for these age differences might be that the microcharcoal fraction consists of more stable components than macrocharcoals and thus yields older ages. Dating of microcharcoal would give a mean age of charred organic matter in soil material and the ages of the more stable compounds. Thus, 14C data obtained from the microcharcoal fraction in soils is not comparable to macrocharcoal ages and should not be used to complement existing macrocharcoal data sets.

Chronology of the shell bar section and a discussion on the ages of the Late Pleistocene lacustrine deposits in the paleolake Qarhan, Qaidam basin

Shell bar, composed of abundant fossil shells of Corbicula fluminea muller and Corbicula largillierti philippi and located at the southeastern end (36°30′N, 96°12′E) of the paleolake Quarhan, is one of the most prominent features in the Qaidam basin. It is the highest site where such species of fossil shells have been found in the Late Pleistocene age. A 2.6-m-thick fresh profile was manually excavated to determine the formation ages and the scope of the high paleolake levels. Accelerator mass spectrometry (AMS), conventional radiocarbon dating, and sector inductively coupled plasma-mass spectroscopy 230Th methods were used to investigate the reliability and accuracy of dating results in different laboratories. Ages of various components (e.g. acid residual and acid soluble fraction of the organic matter) from the same sample were determined. Age differences of a variety of materials (e.g. organic matter, fossil shells, and salt crystals), and age differences at the same sampling position were evaluated. Dating shows that the AMS ages given by alkali residue and acid soluble fractions vary greatly, e.g., from 124.5 cm upwards. The dates given by the acid soluble fraction were normal. Down the column, the dates show a reverse pattern and those of alkali residues, especially the lower part of the section, show an unstable pattern. These imply that organic matter had been influenced by two separate processes. One possible explanation is that the alkali residual fraction most likely had been contaminated by dead carbon-bearing reworked material because the study section is located near the edge of the paleolake and could have been easily influenced by old eroded deposits. Another is that the acid soluble fraction could have been contaminated by upward-flowing groundwater containing soluble organic matter. The amount of this soluble organic material should be very small because the strongest age reversal is in the lower part, where the TOC content remains low, meaning that the change of soluble organic fraction alters neither TOC content nor the δ13C dramatically. It is concluded that a uniform mega-paleolake developed in the Qaidam basin in the northeastern Tibetan plateau between 39.7 and 17.5 14C kaB.P. During the period when the high paleolake level of Qarhan was formed, the huge paleolake covered a vast area with dramatic lake level fluctuations. It is found that the ages determined may be influenced by either radioactive 14C variations of repeated deposition or content variations in the atmosphere. The ages given by fossil shells are 15–18 ka older than those given by organic matter. The large differences between the ages of fossil shells and organic matter might have resulted from the large water areas and huge water volume and the special location of the high-elevation Qaidam basin. This study also shows the reliability and accuracy of the 230Th dating method on the salt crystals but further study is needed to determine whether this method could be applied to the study area.

Dilemma of Dating on Lacustrine Deposits in an Hyperarid Inland Basin of NW China

Conventional and accelerator mass spectrometry (AMS) radiocarbon, TL, OSL, and IRSL dating results on samples from the cores D100 and I70 from Ejina Basin, one of the most important inland basins in arid-hyperarid NW China, show that it is difficult to determine the ages of sediments at different depths. AMS ages of core D100 samples demonstrate that the sediments at depths from 10 to 90 m were formed between 14 to 30 kyr BP. The inverted ages from both the D100 and I70 cores imply that there was a strong reworking of the sediments during and after deposition processes. The inverted ages also indicate drastic fluctuations of groundwater bearing soluble organic matters, which might be related to neotectonic activities and climate changes during the period. Consequently, it is impossible to establish an accurate and reliable chronology for the cores based only on these dates. All AMS ages, if they are reliable and acceptable, indicate a high deposition rate (5∼8 mm/yr), and since all TL, OSL, and IRSL ages are much older than those given by AMS, it makes these methods questionable for determining the ages of lacustrine-fluvial-alluvial deposits.

Establishment and extinction of a population of South Georgian diving petrel (Pelecanoides georgicus) at Mason Bay, Stewart Island, New Zealand, during the late Holocene

A population of South Georgian diving petrels (Pelecanoides georgicus) (c. 130 g) became extinct at Mason Bay, on the west coast of Stewart Island, before European settlement. Pacific rat (Rattus exulans) bones with the diving petrel fossils provided an opportunity to determine whether the rats arrived before the petrels went extinct. Fifteen 14C accelerator mass spectrometry (AMS) ages on purified diving petrel bone gelatin from various parts of Mason Bay clustered unexpectedly in the 14th and 15th centuries ad, and none was older. Bayesian statistical analysis, using a Markov Chain Monte Carlo procedure, gave a 95% probability that the diving petrel colony was founded between 1338 and 1440 ad, lasted 40–310 years, and became extinct between 1475 and 1650 ad. Possible reasons for the late colonisation of Mason Bay by South Georgian diving petrels burrow are discussed. Bayesian analysis of five 14C AMS determinations on Pacific rat bone gelatin did not exclude the possibility that the Pacific rat arrived before the diving petrel colony was established. However, the enriched 513C of their bone gelatin suggests that the rats had a partially marine diet, and a terrestrial calibration procedure for their AMS ages was probably not appropriate. The Pacific rat is likely to have arrived after the diving petrel colony became established and probably caused the bird's extinction after a short period of coexistence.

11 000 years of fire history and climate in the mountain hemlock rain forests of southwestern British Columbia based on sedimentary charcoal

Little is known about the role of fire in the mountain hemlock (Tsuga mertensiana (Bong.) Carrière) rain forests of southern British Columbia. High-resolution analysis of macroscopic charcoal from lake sediment cores, along with 102 accelerator mass spectrometry (AMS) ages on soil charcoal, was used to reconstruct the long-term fire history around two subalpine lakes in the southern Coast and North Cascade Mountains. AMS ages on soil charcoal provide independent evidence of local fire around a lake and support the interpretation of peaks in lake sediment charcoal as distinct fire events during the Holocene. Local fires are rare, with intervals ranging from centuries to several millennia at some sites. Overall fire frequency varied continuously throughout the Holocene, suggesting that fire regimes are linked to climate via large-scale atmospheric circulation patterns. Fires were frequent between 11 000 and 8800 calendar years BP during the warm and dry early Holocene. The onset of humid conditions in the mid-Holocene, as rain forest taxa established in the region, produced a variable fire period until 3500 calendar years BP. A synchronous decrease in fire frequency from 3500 to 2400 calendar years BP corresponds to Neoglacial advances in the region and cool humid climate. A return of frequent fire between 2400 and 1300 calendar years BP suggests that prolonged summer drought occurred more often during this interval, which we name the Fraser Valley Fire Period. The present-day fire regime was established after 1300 calendar years BP.

The earliest archaeological maize (Zea mays L.) from highland Mexico: new accelerator mass spectrometry dates and their implications.

Accelerator mass spectrometry age determinations of maize cobs (Zea mays L.) from Guilá Naquitz Cave in Oaxaca, Mexico, produced dates of 5,400 carbon-14 years before the present (about 6,250 calendar years ago), making those cobs the oldest in the Americas. Macrofossils and phytoliths characteristic of wild and domesticated Zea fruits are absent from older strata from the site, although Zea pollen has previously been identified from those levels. These results, together with the modern geographical distribution of wild Zea mays, suggest that the cultural practices that led to Zea domestication probably occurred elsewhere in Mexico. Guilá Naquitz Cave has now yielded the earliest macrofossil evidence for the domestication of two major American crop plants, squash (Cucurbita pepo) and maize.

Estimating the Time Since Final Stabilization of a Perched Dune Field Along Lake Superior

The Nodaway dune field is perched along Lake Superior in Upper Michigan. This study uses absolute and relative-age dating methods to test the hypothesis that the dune field finally stabilized after the Nipissing high stand, about 4,000 years ago. Surface soils on snouts of all dunes are moderately developed Spodosols, indicating that dunes stabilized within a few hundred years of each other. One thermoluminescence date provided an age of 8 ka from soil parent material, but is probably overestimated due to residual thermoluminescence. Subsequent optical stimulated luminescence and accelerator mass spectrometry age estimates indicate that the most recent accumulation of sand occurred between ˜3.7 and 3.0 ka. This interval suggests one of three possibilities: 1) that the dune field was reactivated during the Algoma high stand and then stabilized; 2) that the dune field stabilized gradually, probably as sand supply diminished after the Nipissing high stand; and 3) that a combination of these two processes occurred.

New accelerator mass spectrometry radiocarbon ages for the Mazama tephra layer from Kootenay National Park, British Columbia, Canada

Charcoal fragments recovered from the Mazama air-fall tephra layer in cores from Dog and Cobb lakes, Kootenay National Park, British Columbia, yielded accelerator mass spectrometry ages of 6720 ± 70 and 6760 ± 70 14C years BP, respectively. These two new ages, together with other previously published radiocarbon ages on charcoal and twig fragments from Mazama air-fall deposits, indicate that the climactic eruption of Mount Mazama occurred 6730 ± 40 14C years BP.

Evidence for Cirque Glaciation in the Colorado Front Range during the Younger Dryas Chronozone

Late Pleistocene glacial chronologies developed for the Front Range Mountains of Colorado include two or more cirque glacier advances, locally known as the Satanta Peak Advances. Sediment cores were recovered from Sky Pond, an alpine lake (3320 m) located less than 100 m downvalley from a moraine that exhibits late Pleistocene to early Holocene relative age features and appears to correlate to the Satanta Peak deposits. One of the cores penetrated 0.5 m of basal diamict and recovered 3.3 m of overlying sediments that are predominantly gyttja. An accelerator mass spectrometry (AMS) age of 12,040 ± 60 yr14C B.P. was obtained from directly above the basal diamict and is similar to other reported ages for cirque deglaciation in the Front Range Mountains. The lower portion of the gyttja contains an interval of clastic sediments that show characteristics consistent with glacial activity in alpine catchments. Radiocarbon ages obtained from below and near the upper contact of this clastic interval are 11,070 ± 50 and 9970 ± 8014C yr B.P., respectively. An additional AMS age of 10,410 ± 9014C yr B.P. was obtained from within the clastic interval in a second core. The most likely source for this interval of clastic sediments is a moraine situated directly upvalley from Sky Pond, and consequently, it appears that the deposition of this moraine was coeval with the European Younger Dryas event (11,000–10,00014C yr B.P.). Similarities in soil development, weathering features, and altitude between this moraine and the type Satanta Peak moraines suggest that the moraines are correlative. These findings are in agreement with a growing body of evidence that suggests a relatively minor advance of alpine glaciers occurred in the North American Rockies during the Younger Dryas Chron.

Chronology of Holocene Cheniers and Oyster Reefs on the Coast of Bohai Bay, China

Cheniers and oyster reefs are two essential components of Holocene strata on the coast of Bohai Bay, China. The existing nonconventional14C dates, often with unsuitable sample positions, less-tested samples, and unreasonable data comparisons, limit the refined analysis of the local chronostratigraphy. On the basis of a number of pretreatment routines, including geological investigations, X-ray diffraction analysis (XRD), δ13C measurement of shells, selection of appropriate shell species (Umboniumsp. and Terebridae) for14C dating, and determination of the local mean δ13C value (−2.68‰ PDB) for the common shells, a set of samples was radiocarbon-dated by Accelerator Mass Spectrometry (AMS). These new ages, obtained from the lower part of cheniers, enable us to estimate the initiation of the cheniers, and confirm that the existing nonconventional dates are often questionable due to unsuitable sample positions. Another two AMS ages, dated for two different microgrowth layers, precipitated in a varying water body, of the sameCrassostrea gigasshell are statistically identical within 2σ error. This implies that the different water masses in the coastal environment would be rapidly in balance with the contemporaneous atmospheric CO2. Both MARINE93 and INTERCAL93 were used for calibration of radiocarbon dates. These amended the time frame of the local Holocene history.

Abrupt early Holocene (9.9-9.6 ka) ice-stream advance at the mouth of Hudson Strait, Arctic Canada

Radiocarbon-dated glacial-geologic evidence documents an abrupt advance of the northern margin of the Labrador sector of the Laurentide Ice Sheet during the last deglaciation. Ice-flow directional indicators, together with ice-marginal features found onshore and offshore, delimit an ice stream that advanced north-northeast >300 km, crossed the mouth of Hudson Strait and outer Frobisher Bay, and overran summits ∼400 m above sea level on outer Hall Peninsula, southeast Baffin Island. Fifty-eight 14 C ages, including 27 accelerator mass spectrometry ages on single shells, indicate that the advance was extremely rapid. The entire advance-retreat cycle took place in an ∼300 yr ( 14 C) interval, 9.9-9.6 ka. At its maximum extent, the ice stream supported a calving margin >200 km long terminating in open water ∼500 m deep, implying a massive iceberg release. Marine evidence for the outflow is preserved along the Labrador Sea shelf as thick carbonate-rich glacial-marine drift but has not been recognized farther east in the North Atlantic. Either the discharge of icebergs was insufficient to produce a trans-North Atlantic, carbonate-rich (Heinrich) layer, or the icebergs tracked southward where they encountered warming sea-surface temperatures. Although the advance seems to have been coincident with a brief interval of minor regional cooling, it failed to trigger a significant climate perturbation, suggesting that the ocean-atmosphere system had attained its stable interglacial condition by that time.