Accelerator Mass Spectrometry 14C Research Articles

Radiocarbon age discrepancies between the carbonate cement and the root relics of rhizoliths from the Badain Jaran and the Tengeri deserts, Northwest China

Carbonate rhizolith is one of mineral encrustations of plant roots. The radiocarbon (14C) dates and stable carbon isotopes of rhizoliths are widely used to study geochronology, paleoclimate, paleoecology and paleoenvironment changes. However, due to a few available comparisons of 14C dates of the carbonate cement and the plant root relics within the same rhizoliths, the carbon sources that form rhizolith cements are largely unknown. Here, we present accelerator mass spectrometry (AMS) 14C dates of carbonate cement and the plant root relics within the same rhizoliths from dune surface soils in the Badain Jaran and the Tengeri deserts, Northwestern China. The modern dead plant roots and bulk soil carbonates nearby were also dated. It is found that the root relics range from 1825 CE to 1983, while the carbonate cements of the rhizoliths have ages from ~6,900 to ~4,700 calibrated years before present (calBP). The ages of the bulk soil carbonates vary from 18,700 to 35,300 and 39,300 calBP, revealing complicated soil carbonate matrix. The results suggested that the carbon sources for the carbonate cements of the rhizoliths were derived mainly from ancient soil carbonates with a small contribution from dead plant roots and a minor contribution from lithogenic carbonates. Mass balance of stable carbon isotopes was used to calculate the proportional contribution of the carbon from ancient soil carbonates and plant roots relics. After subtracting the modern carbon contribution from the decomposed plant roots in fraction of modern carbon (F14C), the mass balance estimated that the ancient soil carbonates were formed within 7,750–5,000 calBP, about 850–300 years older than obtained rhizolith cement ages. It is proposed that dead roots decay triggered the rhizoliths formation by releasing CO2 to form acid media with free HCO31−/CO32– ions, which caused dissolution of the old soil and lithogenic carbonates in soil matrix. Carbonate recrystallization took place to form rhizoliths tubes around the dead roots. Detailed age discrepancies between rhizolith cements and dead root relics provide new insight for better understanding the dynamics of the rhizolith formation and associated implications for paleoenvironment reconstructions.

A 40,000-year record of aridity and dust activity at Lop Nur, Tarim Basin, northwestern China

The Taklamakan Desert is a major dust source area for northwestern China. The history of aridification and sandstorm activity of the desert is recorded in the lacustrine sediments of the Lop Nur paleolake, on the eastern margin of the desert. In this study, we used accelerator mass spectrometry 14C dating, the ln (Al/Ca) ratio, and the grain size of the non-biogenic fraction of sediment core KY01 taken from the center of Lop Nur to produce a ∼40 kyr record of local climate change. During mid–late Marine Isotope Stage (MIS) 3, increases in the ln (Al/Ca) ratio, alluvial mud content, and atmospheric dust content, and a decrease in the dust storm content indicate an overall wet climate. During MIS 2, marked decreases in the ln (Al/Ca) ratio and alluvial mud content, especially during the Last Glacial Maximum and Heinrich event 1 cold interval, indicate lowered atmospheric temperatures, as also recorded in Greenland ice cores. During MIS 2, fluvial input to the site was reduced and almost ceased, suggesting extreme aridity at Lop Nur, very strong low-altitude winds, and frequent dust storms. During early MIS 1, the contribution of dust storms to the lake sediments fell gradually and was then relatively constant, even during the Younger Dryas cold interval. This result might be ascribed to the increased supply of meltwater from snow and ice to Lop Nur, representing a response to the rise in atmospheric temperatures during the deglacial, accompanied by strong high-level winds. During 9–7 kyr BP there was an increase in the input of sediment from sandstorms, and hence an increase in low-level wind strength, possibly relating to the 8.2 kyr BP cold event. From 7.0 to ∼1.5 kyr BP there was a decrease in sandstorm input and thus a gradual weakening of the strength of low-level winds. However, after ∼1.5 kyr BP the sandstorm input increased. Overall, we conclude that dust storm activity in the region of Lop Nur since 40 kyr BP has been controlled mainly by Northern Hemisphere cold events, with atmospheric dust providing a background sedimentary component that was dominant during warm intervals. Our findings contribute to an improved understanding of the dynamics of dust transport and to the history of dust activity and the lacustrine sedimentary record in northwestern China.

Conflicting 14C scenarios in the Sopeña cave (northern Iberia): Dating the Middle-Upper Palaeolithic boundary by non-ultrafiltered versus ultrafiltered AMS 14C

Sopeña is a Palaeolithic rock shelter with a pristine, horizontal and deep stratigraphic sequence. It was used almost continuously by human beings during all of the last glacial cycle, from at least 60,000 years ago till some 20,000 years ago. Sopeña yielded well stratified deposits corresponding to the Mousterian, Early Upper Palaeolithic and Gravettian. Several accelerator mass spectrometry (AMS) 14C dates for Sopeña have been published in various papers and those dates were seen by some scholars as contradictory. Here, we aim to set straight the record on the known dates for the Neanderthal-Homo sapiens boundary in Sopeña, by showing how these dates are consistent both with sample vertical depth in the strata and cultural adscription. The Sopeña sequence of AMS 14C dates suggests a scenario of a long Neanderthal local survival followed by a very quick replacement by modern humans, all around 35.5 ky BP. Despite the stratigraphic and cultural consistency of these dates, all made by laboratories with long years of experience and carried out within the last 12 years, the suggested scenario is significantly more recent than what has been proposed as a consensus by recent publications (Maroto et al., 2012, Higham et al., 2014) that rely increasingly on ultrafiltration techniques for the extraction of collagen prior to its AMS 14C dating. To better delimit the chronology of the disappearance of the Mousterian in Sopeña, we obtained new dates by both ultrafiltered (UF) and non-ultrafiltered (non-UF) AMS 14C methods on fresh samples from the same levels, locations and depths. The interpretation of the new dates and their reliability is not without issues. Overall, both non-UF and UF dates support an earlier sequence of events by some millennia than that earlier proposed, with a survival of the Mousterian arguably up to c. 40 ky BP which is still a few millennia later than c. 47 ky BP currently proposed for the region (Higham et al., 2014), and the earliest arrival of modern humans around 38 ky BP, matching the proposal for this region of Iberia by Maroto et al. (2012). We do value as more parsimonious, the older chronological scenario for Neanderthal disappearance and the arrival of modern humans in Sopeña as presented here. Although, some issues put forward by the new dates, both non-UF and UF, seem difficult to interpret and do not put away the alternative scenario of a quick replacement as suggested by the earlier set of non-UF AMS dates.

Changes in the Near Eastern chronology between the 5th and the 3rd millennium BC: New AMS 14C dates from Arslantepe (Turkey)

In recent years the chronological framework provided by the AMS 14C dating has had a great impact on archaeological studies in the Near East, also affecting the previous synchronization of cultural events across the Mediterranean region. Here we present a consistent set of 14C dates from the site of Arslantepe (Turkey) between the 5th and the 3rd millennium BC. A total of 86 samples of charred wood remains were previously dated by the beta counting method, already providing a general framing to this exceptionally long sequence. However, the stratigraphy of building levels still needed a more accurate analysis, distinguishing sub-phases within the occupational periods. 13 new samples of mainly charred seeds and fruits from the Late Chalcolithic to the Early Bronze I levels have been dated by means of Accelerator Mass Spectrometry (AMS) A tendency towards older radiocarbon ages is recorded. The new site chronology dates the post-Ubaid levels during the mid-5th millennium BC, well corresponding to the Late Chalcolithic cultures of northern Mesopotamia. The last phases of the Late Chalcolithic were found in quick succession between 3600 and 3200 years BC, confirming the correlation with the Late Uruk culture in southern Mesopotamia. Lastly, the Early Bronze Age I levels appear to be earlier than the previous beta counting chronology. This indicates that the events following the collapse of the Late Chalcolithic Arslantepe society all belong to a very short period of time.

Human solid-phase gastric-emptying measurement by AMS using 14C tracer

Determining solid-phase gastric emptying (GE) in a human being is an important application in clinical practice as it can provide valuable information for the diagnosis of related digestive diseases. Currently, 99mTc gastric scintigraphy is considered as the gold standard for measuring GE. However, this method suffers from clinical drawback, such as radiation exposure. This study focuses on identifying alternative methods for GE measurement. Carbon-14 accelerator mass spectrometry (AMS) is a useful tool for biomedical projects due to its inherently high sensitivity and dynamic range, low dose and sample size requirements, and relatively short measurement time. However, it has not yet been implemented in GE tests. In this work, 14C AMS methodology was developed for breath samples. Measurements were conducted on a compact, single-stage AMS device at China Institute of Atomic Energy (CIAE). To investigate GE, micro-doses of 14C-octanoic acid, 13C-octanoic acid, and 99mTc were marked along with tracers in the same test meal and subsequently ingested by volunteers. The 14C-octanoic acid breath test (14C-OBT) in combination with AMS showed better counting statistics than those obtained for the 13C-OBT measured using infrared spectroscopy. The results also demonstrate a considerably lower radiation dose than that in 99mTc gastric scintigraphy. This work establishes an alternative method of 14C-AMS tracing to determine solid-phase GE in a human being, which could provide clinical technical support for a GE test and further expand the range of 14C-AMS applications in biomedicine.

Variations in chemical element compositions in different types of Holocene calcareous root tubes in the Tengger Desert, NW China, and their palaeoenvironmental significance

Calcareous root tubes (CRTs) contain abundant information about palaeoenvironmental conditions and have been used for palaeoenvironmental studies in the desert hinterlands of arid regions. However, as subclasses of CRTs, calcareous sheaths and rhizocretions are formed by different processes, and it remains unclear whether these differences produce variations in the chemical element composition. Furthermore, it remains uncertain whether variations in chemical element concentrations amongst different subclasses of the CRTs can affect palaeoenvironmental reconstructions. In this study, we collected 54 CRT samples from the Tengger Desert of northwestern China. All samples were dated by accelerator mass spectrometry (AMS) 14C dating, and the chemical element composition and concentration differences of the two CRT subclasses were determined using X‐ray fluorescence spectrometry. The CRT samples were dated to the Holocene. The calcareous sheath and rhizocretion samples contained varying concentrations of the same chemical elements. The rhizocretions had high concentrations of mobile elements (Ca, Mg and Sr) and P, whereas the calcareous sheaths had high concentrations of stable elements, including Al, Si, Ti, Zr, Rb and Ba. These differences were due to the different formation processes of the two subclasses of CRTs. Moreover, the Sr/Ca and Mg/Ca ratios in the calcareous sheaths were higher than those in the rhizocretions from the same period, but these ratio differences had little effect on palaeo‐effective moisture reconstructions at the millennial scale during the Holocene. The reconstructions were not influenced by the various CRT subclasses. The Holocene millennial‐scale moisture changes in the Tengger Desert revealed by the Sr/Ca and Mg/Ca ratios showed that there was an arid period during the Early Holocene, a humid period during the Middle Holocene and a humid to arid period during the Late Holocene.

Radiocarbon measurement using a gas/solid hybrid ion source and an automated sample preparation system at the University of Tsukuba

The first preliminary test of the 14C-accelerator mass spectrometry (AMS) system at the University of Tsukuba was successfully performed using a gas/solid hybrid ion source and an automated sample preparation system with an elemental analyzer (EA). The maximum 12C− current of ∼11 µA was achieved using the optimized CO2 flow rate to the hybrid ion source. The machine background of 14C/12C for the AMS system reached to ∼2.4 × 10−16 (∼0.02 percent modern carbon, pMC) using unprocessed mineral graphite. Moreover, the blank level of 14C/12C including the combustion process for an IAEA-C1 graphite sample was 4.6 × 10−16 (0.05 pMC), demonstrating the low blank level of the automatic sample preparation system. In contrast, the blank level of 14C/12C including the combustion process for an IAEA-C1 CO2 sample was 5.0 × 10−15 (0.49 pMC), an order of magnitude worse than that for the graphite sample. The measurement uncertainty for the gas ion source was 0.6%, 2–3 times worse than that for the solid ion source (0.2%–0.3%).

Sediment accumulation patterns on the slopes and abyssal plain of the southern Gulf of Mexico

Here we present new results on the spatial distribution patterns of lithogenic and biogenic components and sedimentation processes in the deep-water region (966–3741 m) of the southern Gulf of Mexico (sGM). A collection of 44 cores retrieved from the continental slopes and the abyssal plain were dated with the aid of accelerator mass spectrometry (AMS) 14C determinations on planktic foraminifera. We applied a mixing box model (MBM) to estimate the mixed layer age (TML), the mean sediment accumulation rate (Sm), and the mixing depth (XML) for each core. Observed ranges for these variables were Sm (3.4–17.7 cm/kyr), TML (393–6021 yr) and XML (2.4–24.3 cm). Deep-sea sediments in the study region fall into three main groups, which reflect the importance of the main sediment components, lithogenic and biogenic, and sedimentation processes at depth in the sGM. Cores clustered in Group 1 include most of the stations in the abyssal plain, show the oldest TML, relatively low Sm, and a relatively higher ratio between the carbonate and terrigenous components (mostly silt and clay). Group 2 cores were collected from stations in the transitional region, between the abyssal plain and the continental slopes, show relatively higher Sm, a younger TML and a relatively lower ratio between the carbonate and the terrigenous content than cores from G1. Group 3 cores were collected from different water depths on the continental slopes, show the highest Sm, younger TML, and the highest terrigenous content. These distinct groupings enabled us to recognize anomalous properties in certain cores, with implications on the local processes that alter their sedimentation processes at those sites. Our results highlight the importance of the terrigenous input to this basin, as one of the main controls of the burial rates with important implications on the possible fate of anthropogenic inputs and its sequestration in the deep-sea sediments of the sGM.

Late Quaternary vegetation, climate, and fire history of the Southeast Atlantic Coastal Plain based on a 30,000-yr multi-proxy record from White Pond, South Carolina, USA

AbstractThe patterns and drivers of late Quaternary vegetation dynamics in the southeastern United States are poorly understood due to low site density, problematic chronologies, and a paucity of independent paleoclimate proxy records. We present a well-dated (15 accelerator mass spectrometry14C dates) 30,000-yr record from White Pond, South Carolina that consists of high-resolution analyses of fossil pollen, macroscopic charcoal, andSporormiellaspores, and an independent paleotemperature reconstruction based on branched glycerol dialkyl tetraethers. Between 30,000 and 20,000 cal yr BP, openPinus-Piceaforest grew under cold and dry conditions; elevatedQuercusbefore 26,000 cal yr BP, however, suggest warmer conditions in the Southeast before the last glacial maximum, possibly corresponding to regionally warmer conditions associated with Heinrich event H2. Warming between 19,700 and 10,400 cal yr BP was accompanied by a transition from conifer-dominated to mesic hardwood forest.Sporormiellaspores were not detected and charcoal was low during the late glacial period, suggesting megaherbivore grazers and fire were not locally important agents of vegetation change.Pinusreturned to dominance during the Holocene, with step-like increases inPinusat 10,400 and 6400 cal yr BP, while charcoal abundance increased tenfold, likely due to increased biomass burning associated with warmer conditions. Low-intensity surface fires increased after 1200 cal yr BP, possibly related to the establishment of the Mississippian culture in the Southeast.

Mid and Late Holocene evolution of Brateș Lake region (Danube floodplain) based on the multiproxy analysis

This study proposes a local paleo–landscape reconstruction of the Danube floodplain based on a stratigraphic sequence retrieved from Brateș Lake which, by its emplacement near the confluence of Danube – Prut rivers, was fully receptive to changes associated to hydrological, geomorphological or anthropogenic driven events. Due to its intermediate position within the Lower Danube valley Brateș Lake is a proxy for the evolution of Cotul Dunării area (the region of Danube valley turning from S–N to W–E direction) and provide valuable information about the timing of Danube river advancement to the Black Sea after its reconnection to World Ocean. The sediments were analysed to get the history of their deposition by means of accelerator mass spectrometry (AMS) 14C dating, grain–size parameters, organic matter and carbonate content, magnetic susceptibility together with paleo–fauna and pollen content which altogether led to the identification of main stages: i) delta front advance into Danube estuary (before 8000 BP), ii) shoreline foreshore deposits which describe shoreline position (8000–7900 yrs BP), iii) river floodplain development (7900–5300/5000 yrs BP), iv) lake formation (5300/5000 yrs BP – present).

Diet and Radiocarbon Dating of Tollund Man: New Analyses of an Iron Age Bog Body from Denmark

ABSTRACTTollund Man is one of the most famous Iron Age bog bodies due to his well-preserved head. Since he was unearthed in 1950 in Bjældskovdal, Denmark, he has been subjected to several scientific investigations, but until now no attempts to reconstruct his general diet through isotope analyses have been conducted. Furthermore, previous radiocarbon (14C) analyses have only been able to date him broadly to the 3rd–4th century BC. In this study, stable isotope measurements (δ13C, δ15N) on bone collagen from Tollund Man’s femur and rib showed that the diet of Tollund Man was terrestrial-based and that the crops he ate probably were grown on manured fields. Accelerator mass spectrometry (AMS) 14C dates were obtained on both the <30kDa and >30kDa fractions of ultrafiltered collagen. Results showed that the ultrafiltration removed contamination from older substances from the burial environment. The femur was dated to 2330±23 BP, the rib to 2322±30 BP. These dates statistically agree with a previously published AMS 14C age on skin. By combining the new dates with the previous date of his skin it was possible to narrow down the age of Tollund Man to the period 405–380 cal BC (95.4% confidence interval).

Age of a late Quaternary (13.6 cal kyr BP) eruption and associated marker tephra of the Tengchong volcanoes, Yunnan Province, southwestern China

The Tengchong volcanoes in western Yunnan Province are some of the youngest volcanoes in China, and determining their eruption ages would further our understanding of their genesis and historical activity. In this study, the age of a late Quaternary eruption of Dayingshan volcano was obtained by dating organic-rich sediments that envelope a 5-cm-thick visible tephra layer found in the Yubi peat section. Compositionally, the purified glass shards extracted from the layer by density separation are calc-alkaline trachyandesite and trachyte/trachydacite, which is consistent with the composition of the Dayingshan lava. An age-depth model of the Yubi section was constructed from accelerator mass spectrometry (AMS) 14C dating of peat samples and Poisson models of deposition (P_sequence) functions, using OxCal with the IntCal 13 database. This yielded an age for the tephra layer of 13,642 ± 84 cal yr BP. A nearly identical tephra layer occurs in the Qinghai Crater Lake sediments. The Yubi tephra is now a reliable age and stratigraphic marker layer, useful for regional paleoclimate and tephrochronology research.

Radiocarbon dating legacy collections: A Bayesian analysis of high-precision AMS 14C dates from the Par-Tee site, Oregon

Archaeological materials in museum collections provide an excellent opportunity for researchers to investigate social, cultural, and environmental change. However, the precision of the archaeological analysis and interpretation is dependent on a firm understanding of the site chronology. The Par-Tee site (35CLT20), located on the northern Oregon Coast, produced a large archaeological collection including artifacts and faunal remains excavated in the 1960s and 1970s. Radiocarbon dates have been obtained on materials from the Par-Tee collections by several different researchers since the 1970s, but these data have not been adequately assessed for chronometric hygiene. To establish a reliable chronology for the Par-Tee site, we obtained new high-resolution accelerator mass spectrometry (AMS) radiocarbon dates and collagen peptide mass fingerprinting of cervid bones from throughout the site. We evaluate these new radiocarbon dates along with previous radiocarbon dates from the site, using chronometric hygiene assessments and Bayesian statistics to build a refined chronology for the Par-Tee site and museum collection. Previous research suggests site habitation occurred between 350 cal BC to cal AD 1150. Our reassessment of the site chronology suggests the primary site habitation occurred from cal AD ~100–800. We also identified evidence of subsequent site occupation around cal AD ~1490–1635 supporting previous interpretations of site habitation after the primary shell midden forming occupation. The latter occupation may be associated with a change in site use from a semi-sedentary village to a cemetery.

An improved method for the sealed-tube zinc graphitization of microgram carbon samples and 14C AMS measurement

Advances in accelerator mass spectrometry (AMS) and environmental chemistry techniques have increased demand for the natural abundance 14C analysis of ultra-small (<25 µgC) and compound-specific samples. The sealed-tube zinc (Zn) method is used for the reduction of sample CO2 to graphite on an iron catalyst. This method provides reliable, low AMS 14C backgrounds and high measurement precision over a wide range of sample sizes (1000 µgC to <15 µgC). Recently, Rinyu and co-workers demonstrated improved ion beam currents and 14C backgrounds when performing sealed-tube Zn graphitization by applying a thermal gradient (from room temperature to 550 °C) on 25–100 µgC samples [1]. However, many compound-specific radiocarbon analysis (CSRA) measurements are limited to smaller sample sizes – often <10 µgC per analyte with repeated collection by gas or liquid chromatography. Using a similar thermal gradient setup, we present an improved sealed-tube Zn method for the graphitization of samples ranging from ultra-small mass (2 µgC) and up to 100 µgC in the Keck Carbon Cycle AMS facility (KCCAMS) at the University of California, Irvine. Our improved method produces high AMS currents (he12C+ ∼1 µA µgC−1), a low extraneous C blank (0.5–0.7 µgC), high measurement precision (2–10% as relative % error) and accuracy (95–97%, as an absolute % difference of measured vs. consensus fraction modern values) for modern 2–5 µgC samples. These improvements have resulted in a sealed-tube Zn method which equals the performance of the ultra-small mass H2 reduction method for 2–10 µgC samples [2].

Isobar suppression for 36Cl accelerator mass spectrometry at the University of Tsukuba

A 6 MV tandem accelerator at the University of Tsukuba, Japan, has been working on multi-nuclide (10Be, 14C, 26Al, 36Cl, 41Ca, and 129I) accelerator mass spectrometry (AMS) since 2016. This paper reports improvement of isobar suppression techniques for 36Cl AMS. Previous studies have shown that a Cu sample holder with AgBr backing is the most suitable for 36Cl AMS to decrease isobaric 36S interference. Therefore, the method of AgBr preparation was considered, and we found that preparation under acidic conditions was most appropriate to reduce 36S in 36Cl AMS. Cl charge states and their analysis method were considered to improve peak separation. 36Cl8+ was chosen for detection because it allows clear separation of 36S from the 36Cl spectrum. Furthermore, 36S spectrum tails could be removed using a third anode (dE3) signal. As a result, a 36Cl/Cl background of ∼3×10-15 was achieved.

OSL and AMS 14C Age of the Most Complete Mammoth Fossil Skeleton from Northeastern China and its Paleoclimate Significance

ABSTRACTApplying radiocarbon (14C) dating using accelerator mass spectrometry (AMS) to the skeleton of a mammoth and the associated plant remains have been dated. The fossil of Zhalai Nur mammoth was dated to 43,500 +1000/–900 14C yr BP. The results of optically stimulated luminescence (OSL) dating, which show that a fluvially deposited gravel layer, from that the mammoth fossils were excavated, formed between 51,300±2100 and 26,600±1200 yr BP, place the new AMS 14C dates in a well-developed chronological framework. Through this study, it can be summarized that, firstly, using suitable sample material, it is possible to obtain reliable AMS 14C results, even when the ages of the target materials approach the upper limits of the method. Second, it reveals that a depositional hiatus exists during the Late Pleistocene, between ca. 26,000 yr BP and ca. 13,000 yr BP. Finally, large rivers and widely distributed areas of alluvial-fluvial deposits existed in this present-day desert area between ca. 51,000 and 26,000 yr BP. These results may shed new light on the study of the Mammuthus-Colelodonta-Bubalus fauna, the most important and fully developed fauna during the Late Pleistocene in northeastern China. They also deepen our understanding about the eco-environments of the region.

A new find of macrofossils of Picea crassifolia Kom. in early–middle Holocene sediments of the Qinghai Lake basin and its paleoenvironmental significance

AbstractHere we report a new find of abundant woody debris and cones in stratum of two sections located to the east of the Qinghai Lake basin in China. Analysis of the anatomical structure of the wood and cones confirmed that they are Picea crassifolia Kom. The results of accelerator mass spectrometry 14C dating indicate that the buried Qinghai spruce grew during 9.7–4.2 ka, and the ages of the large trunks or branches are mainly concentrated within the interval 7.5–6.5 ka. This finding gives direct evidence at the species level about the presence of coniferous forest in the early–middle Holocene in Qinghai lake basin. In addition, the buried cones suggest that the early-middle Holocene environment was suitable for the propagation of Picea crassifolia Kom. The variations in the occurrence of Qinghai spruce forest in the Holocene probably reflect changes in humidity/moisture. The humid early-middle Holocene was suitable for the growth and reproduction of Qinghai spruce forest, while a shift toward an increasingly arid climate during the late Holocene resulted in the disappearance of Picea crassifolia Kom. from the Qinghai Lake basin, although human activities may also have contributed to the environmental change.

Holocene climate and environmental changes inferred from sediment characteristics and diatom assemblages in a core from Hwajinpo Lagoon, Korea

Hwajinpo is the largest lagoon in Korea and is located along the east coast of the country. It possesses Holocene sediments that provide an important record of past climate change. We studied the evolution of Hwajinpo Lagoon using grain size data and diatom assemblages in an 11.0-m core (HJ02), which was obtained at the mouth of a small river that drains into the lagoon. Core chronology was established with accelerator mass spectrometry 14C dates and optically stimulated luminescence dates. Diatom assemblages and grain size analysis revealed that estuarine conditions in the inner lagoon area transitioned to an open embayment ca. 8 ka as a result of sea-level rise. Around 7.8 ka, the open bay became a semi-closed bay as a consequence of development of a sand barrier. After the bay was semi-closed, marine water inflow was increasingly restricted as the sand barrier developed, and the semi-closed bay became a completely enclosed, low-salinity, brackish lagoon around 6 ka. There was an erosional hiatus between 5.5 and 1.7 ka (7.0 m depth), likely caused by river flooding and a switch in the location of drainage along the delta. The lagoon became oligohaline around 1.6 ka, likely because of increasing precipitation associated with an intensified Asian summer monsoon. This increase in precipitation resulted in expansion of the sand bar by sediment inflow, driven by agricultural development in the area. About 1000 years ago, the diatom assemblage was similar to the modern assemblage, suggesting the lagoon’s current geomorphic conditions had been established.

Radiocarbon dating of aquatic gastropod shells and its significance in reconstructing Quaternary environmental changes in the Alashan Plateau of northwestern China

Using landform evolution and lake sediment records to reconstruct Quaternary environmental changes requires reliable chronology. However, there is an ongoing debate regarding the fossil shells of aquatic gastropods in carbonate rock area to be used for radiocarbon dating. The focus of this debate is whether living gastropods incorporate the dead carbon, which would affect the accuracy of the dating results. Living aquatic gastropods were selected from habitats on carbonate terrain in arid region. In this study, 64 shell samples from living gastropods, including Cipangopaludina chinensis, Gyraulus convexiusculus, and Radix auricularia, collected from the Alashan Plateau of northwestern China, were dated by accelerator mass spectrometry (AMS) 14C dating. The results revealed that only five samples (~8%) from the same site contained no dead carbon. The other 59 samples (~92%) contained dead carbon in various amounts: ~59% samples contained 0–10% dead carbon, and ~33% samples contained more than 10% dead carbon. Of the samples, 56 were Radix auricularia and their 14C ages varied from present to 3253 cal yr BP. Two of the samples were Gyraulus convexiusculus samples dated at 379 and 290 cal yr BP which are highly consistent with 14C ages of Radix auricularia samples from the same site. The other six Cipangopaludina chinensis samples were dated at 407–863 cal yr BP. Thus, most of living aquatic gastropods from habitats on carbonate terrain contained dead carbon. The amount of dead carbon in the gastropod shells was nonselective among the three species. Radiocarbon dating of aquatic gastropod shells may not be useful for studying processes that occurred over multi-millennial timescales at different localities in areas of carbonate rock. It is difficult to establish high-resolution chronologies using 14C ages of gastropod shells in arid regions with carbonate rocks unless the limestone effect has been corrected.

Luminescence Chronology and Radiocarbon Reservoir Age Determination of Lacustrine Sediments from the Heihai Lake, NE Qinghai-Tibetan Plateau and Its Paleoclimate Implications

The accurately determining the lake 14C reservoir age has a crucial significance for climatic reconstruction. In this study, the optically stimulated luminescence (OSL) dating method is employed to date samples from highstand lacustrine sediments, palaeoshoreline, fluvial terrace, and the alluvial fan of the Heihai Lake catchment. Accelerator mass spectrometry (AMS) 14C dating was also used to date fossil plants from highstand lacustrine sediments. Based on the calculations of linear regression with OSL against radiocarbon ages for same layers of two sections, the quantitative 14C reservoir ages were estimated to lie between 3 353 and 3 464 yr during the 1.8 to 2.4 ka, which showed temporal variation. The sources of old carbon are the dissolution of carbonate bedrocks distributed along the Kunlun Mountain. The OSL ages of the different members of the hydatogen sedimentary system at Heihai Lake catchment indicate that a stronger hydrologic condition occurred from 3.0±0.2 to 1.8±0.2 ka, with a maximum lake level of 9 m higher than present. This humid stage was widely recorded in different sediments on the QTP and Chinese Loess Plateau (CLP), indicating its broad synchronicity across the Asian Summer Monsoon region. The enhanced East Asian Summer Monsoon (EASM) and the Indian Summer Monsoon (ISM) resulted in the increase of moisture availability for the Heihai Lake area during this stage.