Accelerator Mass Spectrometry 14C Research Articles

New data on tributary terraces and a reappraisal of the incision history of the Jinshan Gorge, middle Yellow River

River channel profiles and river terraces are two typical landscape features to investigate river incision history. However, incision history inferred from the two markers can be inconsistent. For example, along the Jinshan Gorge in the middle Yellow River, pseudo-terraces (a virtual metric derived from channel profile modelling) suggest a recent acceleration in incision rates only in the southern gorge, and the northern keeps steady, while river terraces in the trunk channel indicate recently increased incision rates all along the gorge. To mitigate these discrepancies, we investigate six tributaries along the Jinshan Gorge by mapping and dating several strath terraces at various distances from the confluence to the trunk channel. By comparing tributary terraces of similar age, we find generally increasing incision rates towards the confluence with the trunk river in the southern Jinshan Gorge. North to south increasing incision rates along the gorge are also observed from coeval tributary terraces. In addition, two different landscapes between the southern and northern tributaries near the confluence are recognized, one with a deeply incised gorge and another one with floodplains, suggesting incision rate in the northern gorge is steady through time. Integrating the new infrared stimulated luminescence (IRSL) and Accelerator Mass Spectrometry (AMS) 14C data from tributaries with the abundant literature data of terrace samples mostly in the Jinshan Gorge dated with various methods, we suggest published ages from optically stimulated luminescence (OSL) might be too young. The methodological limitation associated to the dating method can explain the observed mismatch of incision history in the northern gorge inferred from pseudo-terraces and from river terraces in the trunk channel, and reconcile some contradicting patterns in published trunk terrace data.

Magnetic Response to the Source‐To‐Sink Environmental Changes in the Bay of Bengal Since ∼60 ka

AbstractThe terrestrial magnetic minerals of marine sediments are utilized to track the climatic changes in the source area and the dynamic characteristics of sedimentation processes. However, due to the varied source‐to‐sink environments, the magnetic response to ambient climate cannot be generalized. Here, we conducted systematic environmental magnetic analyses on core CJ04‐50 from the Ninetyeast Ridge and investigated its magnetic response to source‐to‐sink environmental changes. Core CJ04‐50 covers the last 60 Kyr based on accelerator mass spectrometry (AMS) 14C dating and the relative paleointensity (RPI) record. Rare earth element (REE) results suggest that the terrestrial materials are fed by the Ganges‐Brahmaputra (G‐B) and Irrawaddy/Indo‐Burma Ranges. High/low magnetic mineral content corresponds to strong/weak terristrial input during the cold/warm period. This pattern differs from that in the East Asian marginal seas, which have a high magnetic mineral content in warm periods. It might be attributed to the heavier Indian summer monsoon (ISM) precipitation than that of East Asian summer monsoon. Excessive moisture (>1,500 mm/year) would not favor the formation and preservation of magnetic minerals in the source area during interglacials. By contrast, the enhanced physical weathering during glacials results in more magnetic contributions. A significant local magnetite dissolution occurred at the layer of Middle MIS 3, which may be caused by the non‐steady state diagenesis following deposition.

Late Holocene decoupling of lake and vegetation ecosystem in response to centennial-millennial climatic changes in arid Central Asia: A case study from Aibi Lake of western Junggar Basin

Central Asian lakes are crucial for sustaining fragile oasis ecosystems in arid regions. Understanding the response of ecosystems in these lake basins to climate change is essential for predicting future variations in these ecosystems. Aibi Lake is a notable terminal lake in arid Central Asia. In this study, a 95 cm lacustrine sediment core was drilled from Aibi Lake to investigate the response of lacustrine and terrestrial ecosystems in the lake basin to climate change over the past 3200 years. Accelerator mass spectrometry (AMS) 14C dating and multi-proxy analyses of diatoms, pollen, grain size, total organic content (TOC), total nitrogen (TN), and total hydrogen (TH) are applied to the sediment core. The grain size and geochemical results show that the lake experienced elevated levels and improved productivity and nutrition during the periods of 708–181 BCE, 0–740 CE, and 1428–1900 CE. The periods align with the Roman Warm Period, Dark Age Cold Period, and Little Ice Age, respectively. For the past 3200 years, benthic diatoms have consistently been the dominant species in the lake. During the climatic transition periods of the Dark Age Cold Period, Medieval Warm Period, and Little Ice Age, there were significant changes in the abundance and survival of certain species of diatoms. Specifically, the halophilic species (Nitzschia dissipata and Nitzschia hungarica), basophilic species (Synedra acus), and the nutrition level-relevant species (Navicula menisculus) experienced notable shifts. These changes ultimately resulted in a noticeable alteration in the structure and function of the diatom community. Over the past 3200 years, the lake ecosystem has undergone three noticeable regime shifts. In contrast, the vegetation in the lake catchment has remained relatively stable, primarily consisting of desert steppe. These results suggest that aquatic ecosystems in arid central Asia are more susceptible and exhibit more rapid responses to climate change compared to terrestrial vegetation.

The Establishment of the Agricultural Landscape of Central Sicily Between the Middle Neolithic and the Beginning of the Iron Age

The possible co-variation of human occupation and vegetation from the Middle Neolithic to the beginning of the Iron Age (7.5–2.8 ka BP) in Central Sicily in the context of the central Mediterranean between Middle and Late Holocene are analysed in this paper to provide new insights on Sicilian prehistoric demography. The demographic and economic trends during these millennia were reconstructed using archaeological, Accelerator Mass Spectrometry 14C dates, palynological, archaeobotanical, and zooarchaeological data from the northern, central, and southern sectors of Central Sicily through a diachronic comparison with variation in Arboreal Pollen, Anthropogenic Pollen Indicators, Olea-Juglans-Castanea pollen, microcharcoals, and Sporormiella from four pollen cores from sites in different ecosystems. A very significant spread of farming activities was found at the end of the Neolithic, together with an apparent demographic gap during the Middle Copper Age, and the emergence of agricultural landscapes at the end of the Copper Age associated with a striking increase in population. A combination of cultural and climatic changes during the late phase of the Bronze Age resulted in a subsequent overall decrease in population.

Development and carbon accumulation dynamics of a minerotrophic fen during the last millennium, northeast China

Peatlands are terrestrial ecosystems with high carbon sequestration efficiency, providing critical feedback on climate change. It is crucial to understand how peatland development and carbon accumulation respond to various factors to reveal the future direction of carbon reservoirs under changing climatic conditions. To assess the impact of climate and hydrological conditions on the development and carbon accumulation of peatlands, 15 peat core samples were analyzed to examine the processes of lateral expansion in the Dongfanghong (DFH) peatland, a minerotrophic fen located in the Changbai Mountains of northeast China. Accelerator Mass Spectrometry 14C, total organic carbon, dry bulk density, testate amoebae, δ13C of α-cellulose in Carex spp. and plant macrofossil analyses were conducted on a high-resolution peat core. The proxies were used to calculate carbon accumulation rate (CAR) and reconstruct water table depth as well as surface moisture and vegetation development of the DFH peatland. Our findings indicated that the DFH peatland was initiated at 1011 cal. CE and experienced paludification during the last millennium. The expansion of the DFH peatland was primarily determined by hydrological conditions rather than climatic factors such as total solar irradiance and regional temperature. The rapid expansion between 1500 and 1700 cal. CE was likely resulted from relatively low precipitation and a decrease in water table/surface moisture. CAR generally correlated with total solar irradiance, regional temperature, and precipitation, suggesting that climate may be crucial for CAR in DFH peatlands over multi-centennial timescales. However, the increased CAR under relatively dry conditions suggested that the CAR were regulated by hydrological conditions over a centennial timescale. This study highlighted the important roles of climate and hydrological conditions in the lateral expansion and CAR of the peatland in minerotrophic fen and that there may be a threshold for hydrology during the process of peatland development and carbon accumulation.

Temporal variation and pollution status of heavy metals in core sediments from Jiangsu intertidal area, China

Eight heavy metal concentrations were analyzed from a 60.35-m-long sediment core in the Jiangsu intertidal area, China. Based on the lithofacies characteristics, mean grain size, downcore distributions of elements, and accelerator mass spectrometry (AMS) 14C and optically stimulated luminescence (OSL) data, the sediments in the core were divided into three units that formed during marine isotope stages 4 (MIS 4), MIS 3, and MIS 1. Except for Cd, all the other heavy metals had the lowest average concentrations in U3, which formed during MIS 4 with the coarsest sediment, representing a fluvial deposit. Most of the heavy metals were positively correlated with Al, Fe, and the total organic carbon (TOC), indicating these metals had the same sources. Pearson's correlation coefficient, enrichment factor, geoaccumulation index, and principal component analysis suggested that there was no element enrichment or contamination in the core sediments and that all heavy metals were naturally sourced.

Mechanism and genesis of Sanmen Bay as a sink of Holocene Yangtze River sediment, Zhejiang Province, China

Introduction: The Holocene mud deposits that extend from the Yangtze River mouth to the Taiwan Strait along the Zhejiang–Fujian coast, East China Sea (ECS), have attracted considerable research attention. However, there is a lack of consensus regarding the redistribution flux of Yangtze River sediment after it enters the sea. There are many bays along the Zhejiang and Fujian coasts, but their role in the source–sink process of Yangtze River sediment is not well-documented owing to limited data.Methods: In this study, data from shallow seismic profiles, core samples, and surface sediment samples were analysed to assess the role of Sanmen Bay, the second largest bay in Zhejiang Province, in the source–sink process of Yangtze River sediments.Results: The results showed that Sanmen Bay is an important sink for Yangtze River sediments. The characteristics of the clay minerals indicate that the fine-grained sediments in Sanmen Bay are mainly derived from Yangtze River sediments. The thickness of Holocene deposits in Sanmen Bay ranges from 8.2 to 32.0 m, with an average of 23.4 m and a total sediment volume of 5.06 × 104 Mt, accounting for 1% of Yangtze River sediment in this time period. The Bayesian age-depth model based on borehole accelerator mass spectrometry (AMS) 14C dating shows that a high sedimentation rate occurred in the early and late Holocene but decreased significantly in the mid-Holocene, which is similar to that of the inner shelf of the East China Sea (ECS).Conclusion: The variation in the Holocene sedimentation rate in Sanmen Bay is related to the southward transport process and mechanism of Yangtze River sediments. The results of this study provide a basis for understanding the source–sink process of sediment in large rivers.

A prolonged dry Mid-Holocene recorded by Moon Lake in the Tengger Desert, arid and semiarid China

AbstractReconstructing the Mid-Holocene climate change in arid and semiarid areas can help predict regional moisture availability and resultant lake evolution and vegetation changes due to future warming. Here, we present a sediment core (YLH15A) from Moon Lake in the Tengger Desert, arid and semiarid China. Based on robust accelerator mass spectrometry 14C dating and multiproxy analyses (pollen, grain size, elements, and total organic carbon), we reconstructed regional climate changes since 7.6 cal ka BP. The climate was generally dry from 7.6 to 2.8 cal ka BP, as indicated by the dried-up lake, strong aeolian activities, and no vegetation, except for a short-term wet interval between 5.4 and 4.9 cal ka BP. The generally dry climate shifted after 2.8 cal ka BP, which is suggested by expanded steppe desert/steppe and increased vegetation cover; it was also accompanied by lake development, which was likely related to increased groundwater recharge originating from regional precipitation and temporary floods from adjacent mountain areas. Our results reveal a prolonged dry Mid-Holocene and relatively wet Late Holocene that are basically consistent with climatic records from the central–east Asian arid and hyperarid areas. The prolonged dry climate in the arid and hyperarid areas is likely to be related to high evaporation triggered by high temperatures during the Middle Holocene.

High-resolution sea-level fluctuations during the Mid-Holocene in the Ningshao Coastal Plain region, eastern China

Sea level changes during the Mid-Holocene directly influenced the Neolithic culture in the Yangtze River Delta region (YRD). However, the high-resolution sea level change characteristics for this period remain unclear. In this study, we performed a high-resolution palynological analysis, including pollen, Dinoflagellate cysts, and Foraminiferal organic linings, using a high-resolution sediment core from Shanglin Lake, in the North of Ningshao Plain (the south of Hangzhou Bay). 11 accelerator mass spectrometry 14C(AMS) datings indicate the age of the sediments range from 8 cal ka B.P. to 5.6 cal ka B.P. The results show that during the Mid-Holocene, Shanglin Lake evolved from an estuary – subtidal lagoon – semi-enclosed bay – semi-enclosed lagoon – semi-enclosed bay – enclosed lagoon to a modern freshwater lake. There was a period of no, or minimal, eustatic sea-level rise between 7733 and 7585 cal yr B.P. The Mid-Holocene high sea level comes in 7253–7082 cal yr BP. Between 7000 cal yr BP and 5502 cal yr BP, the sea level is close to modern value. The sea level change during this period had a significant impact on the local Neolithic human activity.

Identification of three stages of paleochannels and main source analysis beginning in the middle Pleistocene in the western Bohai sea in north China

Significant global sea transgressions–regressions have occurred under the influence of glacial–interglacial climatic change during the Quaternary period. Various types of paleochannels developed on the continental shelf of the western Bohai Sea as a result of sea level decline. Currently, the source of the paleochannels and the sedimentary environment and evolutionary pattern of the paleochannels are disputed. This study interprets high-resolution single-channel seismic data using a chronological framework based on accelerator mass spectrometry 14C (AMS14C) dating and optically stimulated luminescence (OSL) dating of core DZQ01. Furthermore, this research correlates this data with the cores of TJC-1 and H6 in the Western Bohai Sea and analyzes sedimentological characteristics based on grain size and clay mineral analyses. The results indicate that (1) three stages of paleochannels, namely Seismic Unit 6 (SU6), SU4, and SU2 are identified from bottom to top during Marine Isotope Stage 8 (MIS 8), MIS 6, and MIS 4, respectively; (2) from 300 to 243 cal. ka B.P., the SU6 paleochannel has four pathways, with the Yellow River serving as a primary source; from 191 to 130 cal ka B.P., the SU4 paleochannel has three pathways, and the main sources are the Yellow River, the Liao River, and the Luan River; from 74 to 14 cal. ka B.P., the SU2 paleochannels have two large tributaries, with the principal sources being the Yellow River and the Luan River. We, therefore, it can be inferred that the uplift of the Tibetan Plateau and glacial–interglacial climatic change constitute the formation mechanism for the paleochannels.

Radiocarbon (14C) accelerator mass spectrometry as a convenient tool for differentiation of flavor chemicals of synthetic origin from biobased sources and their in-vivo toxicity assessment

Plants are known to be the natural factory for the production of flavor chemicals. Essential oils comprised of aldehyde as a functional group are potent in deciphering flavor effects in beverages and fresh and prepared food products. In the majority, these are manufactured through synthetic routes, resulting in high product carbon footprints or CO2 equivalents in total greenhouse gas emission. FDA has banned some of the synthetic flavor chemicals due to the health hazards associated with them. However, consumer's preference for natural is at stake due to the absence of quantitative traceability tools. The accelerator mass spectrometer (AMS) analysis revealed a distinction between natural and fossil-derived citral and its blends in Cymbopogon essential oils. The plant-derived citral contained a percent modern carbon (pMC) value in the range of 99–100 %. In contrast, the fossil fuel-derived citral showed zero pMC. Similarly, blends of Cymbopogon oils with 30–50 % (w/w) of fossil origin citral contained pMC equivalent to the proportions of modern carbon. These results showed the usefulness of AMS in quantifying the amount of 14C associated with flavor ingredients. Besides, acute oral toxicity data revealed Cymbopogon oils as the safe flavoring substance at the highest 2000 mg/kg body weight dose in Swiss albino mice.

THE POTENTIAL OF BIOGENIC FRACTION ANALYSIS BY RADIOCARBON IN FOOD, DRUG, AND COSMETIC PRODUCTS

ABSTRACTBiobased content analysis is a well-established, analytically independent, standardized method to determine the biobased content of fuels and plastics, based on differences of the specific radiocarbon (14C) activity of fossil and recent biogenic compounds. This biogenic content analysis can be useful for the producers as a quality assurance tool, for the customers as feedback about the truly biobased products and for the control organizations as an independent analytical tool to prove the biological origin. More than 100 commercially available foods, cosmetics, and drug samples have been used for biobased carbon content analysis by accelerator mass spectrometry (AMS) 14C measurement to demonstrate the potential of this technique. Our results show that this measurement technique is a unique tool for the determination of biocontent in foodstuff and medical products. Most of the tested materials were nearly or completely biobased (≥ 98 pMC), and no completely fossil-based final product was detected. The lowest biogenic compound was measured in a vanilla aroma flavor. In 45 of the 102 samples selected a wide range (2–98%) presented fossil-based carbon content. The method can be applied for monitoring raw materials and final products for biobased content in the industry and consumer protection as well.

Quantitative reconstruction of precipitation changes in the Mongolian Altai Mountains since 13.7 ka

Holocene climate changes in the Altai Mountains and the surrounding areas are well documented, but the timing of the Holocene precipitation maximum remains controversial. We obtained 17 accelerator mass spectrometry (AMS) 14C dates and 310 fossil pollen assemblages from sediments in Tolbo Lake, which we used to reconstruct the history of vegetation and climate change in the Mongolian Altai Mountains over the past 13,750 years. The results suggest that mean annual precipitation (Pann) was the most significant control on the fossil pollen record at the study site. Based on the fossil pollen record from Tolbo Lake and 469 surface pollen samples within the radius of 1300 km, the reconstructed Pann was lowest during 13.7–11.3 ka (1 ka = 1,000 cal yr BP); relatively low but increasing Pann occurred in the early Holocene (11.3–9.4 ka); and maximum Pann occurred during the middle to late Holocene (after 6.8 ka), possibly because of the high temperatures. A significant climatic and ecological transition occurred in the Altai Mountains at ∼7 ka: steppe and desert steppe vegetation with generally low Pann was present before ∼7 ka, while subsequently there was the expansion of forest vegetation and a marked increase in Pann, possibly in response to the minimum in Northern Hemisphere ice sheets and the increased radiative forcing caused by rising atmospheric greenhouse gases.

A 1500‐year record of North Atlantic storm flooding from lacustrine sediments, Shetland Islands (UK)

ABSTRACTSevere storm flooding poses a major hazard to the coasts of north‐western Europe. However, the long‐term recurrence patterns of extreme coastal flooding and their governing factors are poorly understood. Therefore, high‐resolution sedimentary records of past North Atlantic storm flooding are required. This multi‐proxy study reconstructs storm‐induced overwash processes from coastal lake sediments on the Shetland Islands using grain‐size and geochemical data, and the re‐analysis of historical data. The chronostratigraphy is based on Bayesian age–depth modelling using accelerator mass spectrometry 14C and 137Cs data. A high XRF‐based Si/Ti ratio and the unimodal grain‐size distribution link the sand layers to the beach and thus storm‐induced overwash events. Periods with more frequent storm flooding occurred 980–1050, 1150–1300, 1450–1550, 1820–1900 and 1950–2000 ce, which is largely consistent with a positive North Atlantic Oscillation mode. The Little Ice Age (1400–1850 ce) shows a gap of major sand layers suggesting a southward shift of storm tracks and a seasonal variance with more storm floods in spring and autumn. Warmer phases shifted winter storm tracks towards the north‐east Atlantic, indicating a possible trend for future storm‐track changes and increased storm flooding in the northern North Sea region.

Provenance of deep-sea sediments in the eastern South China Sea since Marine Isotope Stage 5 (MIS 5) and implications for climate change and volcanic activity

The South China Sea deep-sea basin receives large amounts of marine sediments and volcanic materials from the surrounding continents and islands, and it is an ideal region to study the effects of climate change and volcanic activity on sediment provenance. Core K2 was collected in a water depth of 4078 m in the eastern South China Sea and is mainly composed of diatoms, tephra, volcanic glass and slag. The results of a total of 102 analysed grains show that the depositional processes were dominantly suspension, uniform suspension and overbank-pool facies suspension, with no rolling. Six accelerator mass spectrometry (AMS) 14C ages from planktonic foraminifers and fifty-four δ18O data points of G. ruber suggest that the age of Core K2 is 125 kyr before present (BP). Forty-four clay mineralogical data points show that the clay minerals in the sediments are mainly illite+chlorite (24.3%–49.7%), with a small amount of smectite (2.6%–11%), suggesting a sediment source mainly from Taiwan with a small amount of sediment from Luzon. Forty-five samples yield 143Nd/144Nd and 87Sr/86Sr ratios ranging from 0.512250 to 0.512769 and from 0.704383 to 0.710690, respectively, indicating that sediments in K2 represent a mixture of volcanic inputs from in or around Luzon and sediments from Taiwan and Luzon. Based on calculations of clay minerals and SrNd isotopic data, the relative proportions of sediments from Taiwan and Luzon and volcanic input vary from 8% to 88%, from 1% to 23% and from 0% to 91%, respectively, and the average proportions of sediments from Taiwan and Luzon and volcanic input are 55%, 12% and 33%, respectively. Significant temporal changes in SrNd isotopes are attributed to variations in the volcanic eruption intensity or distance to the sampling location. Moreover, high εNd values and low 87Sr/86Sr ratios of the sediments in Core K2 during interglacial periods could represent volcanic eruptions.

Relict and Modern Sediments on the Continental Shelf of the Northern South China Sea: A Reconsideration

AbstractUnderstanding the temporal‐spatial patterns of modern and relict sediments is of importance for assessing changes in the Quaternary environment and sea‐level. Sedimentological and geochemical data is presented, along with in situ shell‐based accelerator mass spectrometry 14C ages of 30 samples from the surface sediments on the northern shelf of the South China Sea (NSSCS). The authors’ data show that the NSSCS surface sediments exhibit considerable diversity in composition. Modern sediments are primarily constrained to the NW inner shelf, which is fed by fluvial sands sourced from Coastal South China river systems and dominated by the Pearl River Estuary delivery. The transport and discharge of the terrestrial sediments to the NSSCS is highly influenced by the Guangdong Longshore Current and its secondary cyclonic eddies. Relict sediments dominated by well‐sorted, medium‐ and coarse‐grained sands were identified in the NW Shenhu and NW Dongsha areas of the outer NSSCS. The sedimentology and geochemistry of the relict sediments imply that they were deposited in a dry and cold environment either during the low sea levels of the late Pleistocene (∼40 ka) or the early Holocene (∼10 ka). To the east, the Taiwan Shoal and vicinity are dominated by a sand mixture, at which the relict sediments were reworked by terrigenous supply and modern hydrodynamic environment due to the compound action of the Guangdong Longshore Current, seasonal cyclones, and Kuroshio Intrusion. The present isobaths of ∼−90 m in the NSSCS might be the reflective of the early Holocene coastal delta or the last glacial maximum shoreline.

Development of a Middle–Late Holocene subaqueous clinoform in the northern Jiangsu coastal zone, western South Yellow Sea

The northern Jiangsu coastal zone (NJCZ) is part of the western coast of the South Yellow Sea and is located between the mouths of the Yellow and Yangtze rivers. The two rivers and the Huaihe River have discharged into the NJCZ at various times during the Holocene. As a result, multi-stage deltas and clinoforms have developed in the NJCZ, including the old Yellow River delta (OYRD) (1128–1855 CE). An old clinoform has recently been identified just below the OYRD, but its age and origin are uncertain because of limited data. In this study, integrated analyses of a new borehole core SYS-1601 penetrating the clinoform were conducted to document the evolution of the NJCZ and to provide a study case for other coastal regions influenced by major river systems and complex hydrodynamic conditions. Analyses included accelerator mass spectrometry (AMS) 14C and optically stimulated luminescence (OSL) dating, measurements of grain size and Sr-Nd isotopes, and identification of benthic foraminifera and ostracoda. Results show that the evolution of the subaqueous clinoform prior to the development of the OYRD in the western South Yellow Sea was concurrent with and genetically linked to the formation of the Huaihe delta on the NJCZ. The clinoform started to form at the same time as the Middle Holocene maximum flooding of the NJCZ at ∼7.0 ka. Cross-shelf progradation of the clinoform was coeval with progradation of the Huaihe delta and terminated at 1128 CE, when the Yellow River changed course and started to discharge off the Jiangsu coast to form the OYRD. The clinoform exhibits an overall coarsening-upward succession, comprising clayey silt and silt in the lower part grading upwards into silt and sandy silt, and its sediment was sourced largely from the Huaihe and Yangtze rivers. During the development of the clinoform from ∼7.0 ka to 1128 CE, the Yellow River did not flow into the South Yellow Sea through the Jiangsu coast for a sufficiently long time to contribute a substantial amount of sediment to the formation of the clinoform. The initiation and evolution of the clinoform were controlled mainly by complex interactions of dynamic processes, geomorphological conditions, and sediment supply into the western South Yellow Sea during the Middle–Late Holocene.

Holocene evolution of the Banni Plain at the north‐east margin of the Arabian Sea: Constraints from a ca 50 m long sediment core

AbstractHolocene evolutionary history of the Banni Plain in the Great Rann of the Kachchh Basin is reconstructed from a subsurface sediment core of ca 50 m. Detailed data on textural and lithofacies variations, grain‐size analysis, environmental magnetism and accelerator mass spectrometry 14C dates on seven samples were generated on the sediment core retrieved from the Banni Plain near Berada. A high‐resolution record extending back to 10 ka has been reconstructed from the top ca 40 m of the core section comprising shallow marine sediments. The core is divisible into five depositional units. The basal part is a fluvial depositional unit followed upward by estuarine, sub‐tidal, intertidal and supra‐tidal environments. The sediment accumulation rate is highest in the sub‐tidal to intertidal facies (1.9 cm year−1) and decreases towards the supra‐tidal facies to 0.09 cm year−1. Environmental magnetic analysis, χlf coupled with the S‐ratio, indicates high magnetic mineral concentrations during the Early Holocene, suggesting a wet period accompanied by high monsoon precipitation. This is followed by the onset of semi‐arid conditions in the Great Rann of the Kachchh Basin as indicated by the low values of the χlf and S‐ratios. A westward and northward shift in the shoreline towards the deeper part of the basin is suggested during the Late Holocene, which is coupled with aridity and reduced monsoonal conditions. The change in depositional pattern from the retrogradational deposit of fluvial (Unit 1) to estuarine sediment (Unit 2), progressing to sub‐tidal (Unit 3), is attributed to sea‐level transgression followed by regressive intertidal (Unit 4) to supra‐tidal deposition (Unit 5), culminating in complete withdrawal of the sea, aided by tectonic uplift, during the Late Holocene. The results reveal that the sediment accumulation rates and depositional environments changed over time in response to changes in sea level from minima to maxima and then eventually to the present level.

Late Pleistocene boreal molluscs in the Gulf of Cadiz: Past and current oceanographic implications

Remains of molluscs were collected from the seafloor on the north-eastern margin of the Gulf of Cadiz, between 300 and 1000 m water depth, using different sampling methods (e.g. dredging, trawling and box-coring), during several deep-sea expeditions. Samples contained a suite of species which nowadays mostly occur northwards of the English Channel, together with other widespread species. Species now locally extinct in the Gulf of Cadiz and restricted to northern latitudes, which unequivocally indicate a faunal shift, include the gastropods Buccinum undatum, Colus gracilis, Liomesus ovum and Neptunea antiqua, the bivalves Arctica islandica, Chlamys islandica, Modiolus modiolus, Mya truncata and Nuculana pernula and the scaphopod Antalis entalis. These species represent “Boreal Guests” of marked palaeoclimatic significance, some of which are reported for the first time in the Gulf of Cadiz. The boreal species collected were mostly large (>5 cm) whereas smaller boreal species were extremely scarce, probably winnowed away by strong bottom currents. The pteropod Limacina retroversa, at present restricted to water masses northwards of the Iberian Peninsula but widespread in Mediterranean sediments of the last glaciation, was also recorded. Accelerator mass spectrometry (AMS) 14C dates obtained from nine specimens of molluscs ranged between 26.1 and 14.6 kyr B.P., thus confirming their attribution to a last glacial assemblage. The abundance of these molluscan remains in the present Mediterranean Outflow Water pathway could be explained if this outflow was reduced in intensity or more likely shifted to a deeper level, leaving the upper slope in contact with suitable Atlantic intermediate waters. The findings of Boreal Guests in the Gulf of Cadiz document the continuity of the faunal shift which is well-known in the Mediterranean basin. Species still living in the Gulf of Cadiz and the Alboran Sea nevertheless account for 84.6% of specimens among the larger species.

Rapid retreat of the East Asian summer monsoon on the western Chinese loess Plateau during the middle to late Holocene and its environmental and societal implications

Spatiotemporal changes in the Holocene climate of the arid region of China were principally caused by shifts in the intensity and location of the East Asian summer monsoon (EASM). However, our knowledge of these EASM variations is limited, especially since the middle Holocene. We investigated two aeolian sedimentary sections (the Linxia/LX and Gulang/GL sections) on the western Chinese Loess Plateau (CLP), to explore the variations in the intensity and location of the EASM and their impact on the regional environment and human cultural development during the middle to late Holocene. The longitudes of these two loess sections are similar, but the GL section is located on the northern edge of the CLP, in the eastern Hexi Corridor, and it is situated further north than the LX section. The chronologies of both sections are based on accelerator mass spectrometry 14C dating, and environmental magnetic parameters and grain size end-members were used to reconstruct their moisture histories. The results suggest that the climate at the LX section was relatively dry before 3.5 ka, after which it became wetter. In contrast, at the GL section, the climate was also relatively dry before 3.5 ka, with the wettest conditions occurring during 3.5–2 ka, but subsequently there was an abrupt shift to a dry climate. We suggest that these spatiotemporal patterns reflect the westward movement and rapid southward retreat of the EASM and the linked monsoon rainfall belt. TraCE-21 ka climate simulation results support our inference. We also found that, compared with other ancient cultures in the eastern Hexi Corrido, sites of the Shajing culture, dated to ∼2.4–2.8 ka, are predominantly located in areas that are arid at the present-day, and relatively distant from rivers. We suggest that this distribution pattern reflected the occurrence of a humid climate during the period of the Shajing culture, thus demonstrating the impact of the EASM on cultural development in this region.