14C Reservoir Research Articles

Productivity of the eelgrass biotope and the resilience of fisher-hunter-gatherers in prehistoric Denmark

In the shallow coastal waters of the Danish archipelago, eelgrass (Zostera marina) forms extensive and extremely productive stands, which support food webs that are isotopically distinct from those based on the primary productivity of marine phytoplankton. The isotopic signatures of eelgrass and phytoplankton food webs explain much of the variation in δ13C, δ15N and δ34S values in marine vertebrate remains recovered from prehistoric coastal sites in Denmark. Isotope data from human and dog remains reflect the overwhelming importance of the eelgrass biotope to the fisher-hunter-gatherer subsistence during the Late Mesolithic Ertebølle epoch (∼7400-5800 cal BP). By recognising eelgrass and pelagic resources as isotopically distinct, we obtain more accurate diet reconstructions and dietary 14C reservoir effect corrections for Ertebølle individuals. Longstanding research issues, including technology, palaeodemography, and the resilience and sustainability of the Ertebølle economy need to be reassessed in view of the isotopic signature and productivity of the eelgrass biotope in Danish waters. Eelgrass productivity is also pertinent to comparisons between Ertebølle and Neolithic economies in Denmark, and between fisher-hunter-gatherer adaptations in Denmark and neighbouring regions, and may be key to understanding the much-discussed delay in the transition to farming.

Dietary 14C reservoir effects and the chronology of prehistoric burials at Sakhtysh, central European Russia.

We present a robust radiocarbon (14C) chronology for burials at Sakhtysh, in European Russia, where nearly 180 inhumations of Lyalovo and Volosovo pottery-using hunter-gatherer-fishers represent the largest known populations of both groups. Past dating attempts were restricted by poor understanding of dietary 14C reservoir effects (DREs). We developed a DRE correction approach that uses multiple linear regression of differences in 14C, δ13C, and δ15N between bones and teeth of the same individuals to predict DREs of up to approximately 900 years. Our chronological model dates Lyalovo burials to the early fifth millennium BCE, and Volosovo burials to the mid-fourth to early third millennium. It reveals a change in the subsistence economy at approximately 3300 BCE, coinciding with a reorientation of trade networks, and dates the final burial to the early Fatyanovo period, the regional expression of the Yamnaya/Corded Ware expansion. Our approach is applicable when freshwater 14C reservoir effects are poorly constrained and grave goods cannot be dated directly.

RADIOCARBON DATING AND FRESHWATER RESERVOIR EFFECTS OF AQUATIC MOLLUSKS WITHIN FLUVIAL CHANNEL DEPOSITS IN THE MIDWESTERN UNITED STATES

ABSTRACTLate Quaternary fluvial channel deposits are notoriously difficult to date. In the midwestern United States, shells of aquatic mollusks can be found within many fluvial channel sediments and therefore can be radiocarbon (14C) dated to determine the age of the deposits. However, carbonate platform rocks are abundant in this region, potentially causing freshwater 14C reservoir effects (FRE) in mollusk shells. We 14C dated 11 aquatic gastropod and bivalve shell samples from specimens collected live from a stream in southwestern Ohio during three different years to assess the modern 14C reservoir effect. Modern samples yielded an average 14C FREmodern of 518 ± 65 14C yrs for 2020 (n=5), 640 ± 34 14C yrs for 2021 (n=2), and 707 ± 76 14C yrs for 2022 (n=4). We also 14C dated matched pairs of organic wood or charcoal and aquatic mollusk shells from late Pleistocene and Holocene deposits in the Four Mile Creek floodplain to determine the FREfossil. These samples, free of any potential influence from nuclear bomb testing, yielded an overall weighted mean FREfossil of 1029 ± 345 14C yrs. We then assess the advantages and limitations of both the FREmodern and FREfossil methods for determining freshwater reservoir effects. Finally, we apply the FREfossil correction to a series of shell ages from fluvial terrace deposits as a case study. The results indicate that although there is a 14C FRE in streams from the midwestern United States, aquatic shells can provide robust age control on fluvial channel deposits. More research is needed to understand the spatial and temporal variability of FREs, as well as any species effects, among various watersheds across the midwestern United States.

SYSTEMATIC ANALYSES OF RADIOCARBON AGES OF COEXISTING PLANKTONIC FORAMINIFERA

ABSTRACTWe compare radiocarbon (14C) ages of coexisting planktonic foraminifera species from sediment cores VM12-107 and KNR166-2-26JPC from the Equatorial Atlantic Ocean for three time periods (Holocene, Heinrich Stadial 1, last glacial maximum). We find a maximum inter-species difference of 120014C yr. On average, the14C ages deviate by ∼300 yr betweenGlobigerinoides ruberand other species. In most cases, this exceeds the analytical uncertainty range of the measurements and thus renders the choice of species for generating age models as important as sample weight. While modern stratified water-column profiles imply an increase in14C ages with water depth, we observe an expected parallel increase of14C ages and δ18O only at VM12-107. The mismatch between14C ages and δ18O at KNR166-2-26JPC likely results from the effects of bioturbation and the hydrographic setting. The largest difference in14C ages between mixed-layer versus thermocline-calcifying planktonic foraminifera are observed during Heinrich Stadial 1 despite a decrease in upper-ocean stratification at that time. This difference is likely the result of inconsistent increases in14C reservoir ages during times of reduced overturning circulation masking the potential of14C ages of coexisting planktonic foraminifera to reflect the density stratification of the water column.

CENTENNIAL TO MILLENNIAL-SCALE FLUCTUATIONS OF THE LAKE SUIGETSU ATMOSPHERIC 14C RECORD REPRESENT AUTHENTIC 14C FEATURES OVER LAST GLACIAL-TO-DEGLACIAL TIMES

ABSTRACT Short-term fluctuations in atmospheric radiocarbon (14C) concentration mark the tree-ring record for the last ∼15 kyr. Terrestrial macrofossils from sediment cores of Lake Suigetsu, Japan, extend this record of fluctuations back to >35 cal ka BP. Their significance, however, is under debate since the signal-to-noise ratio of the Suigetsu record is low and progressively decreases with increasing age. Coherent semi-millennial-scale structures of the Suigetsu 14C record have nevertheless been identified by three different techniques, namely visual inspection, analyses of the first derivative of 14C vs. calendar age, and Bayesian spline inflections of 14C concentration vs. calendar age, and hence appear objectively real. These 14C fluctuations correlate closely with those of the tree-ring-based 14C master record ∼10–14 cal ka. Thus, Suigetsu fine structures attain global significance and may properly reflect atmospheric 14C variability back to ∼35 cal ka. Carbonate-based 14C records from speleothems and ocean sediments are far smoother and form, together with Suigetsu and other data, the backbone of the IntCal20 record >14 cal ka that largely lacks the Suigetsu fine structure. 14C decay reduces 14C-signal amplitudes over time, so Holocene-style 14C signals of solar modulation disappear in the noise beyond ∼10 cal ka. The remaining older 14C fine structures had larger forcings, most likely linked to climate and carbon cycle, especially ocean-atmosphere CO2 exchange, and thus contain valuable information about these factors. They may also provide global stratigraphic tie points to correlate 14C records of oceanic plankton sediments and climate signals independent of problems with local 14C reservoir effects.

Evidence of temporal changes in the marine 14C reservoir in the South Pacific and implications for improved dating resolution across a radiocarbon plateau (2600-2350 cal BP)

The settlement of the Pacific is an important chapter in human dispersal for which radiocarbon (14C) dates provide chronological control. At some point during the last 3000 years, Polynesian culture developed within Tonga and Samoa out of earlier ancestral populations. Unfortunately, between ∼2650 and 2350 cal BP, the terrestrial 14C calibration curve used to convert 14C ages into calendar dates is relatively flat, resulting in low precision date interpretations. At this time, the South Pacific marine 14C reservoir appears to have diverged from the terrestrial reservoir for reasons that are not yet understood. This divergence may be key to refining the chronology of coastal sites dating to this broad 14C plateau. We investigate this at the site of Talasiu in Tongatapu – a site with evidence of both late Lapita and early Polynesian ceramics. Moreover, we show how careful evaluation of δ13C and δ18O, in combination with shell taxa diet and habitat, enables shell 14C dates impacted by hard water to contribute and refine chronologies blurred by this 2650-2350 cal BP plateau. Our finding has significant global implications for understanding the relationship between the marine and terrestrial 14C reservoirs and is of interest to those interested in refining dates from this time.

SOIL EROSION CAUSED THE INCREASING HOLOCENE RADIOCARBON RESERVOIR EFFECT OF LAKE KANAS IN THE ALTAI MOUNTAINS

ABSTRACTRadiocarbon (14C) dating of the total organic carbon (TOC) content of lacustrine sediments is always affected by a 14C reservoir effect and the 14C dates are often systematically older than the true ages. However, there are few studies of the temporal changes of the 14C reservoir effect, with reference to the specific influencing factors. We collected TOC samples from the Holocene sediments of Lake Kanas, in the southern Altai Mountains, for AMS 14C dating and compared the results with AMS 14C ages based on terrestrial plant macrofossils from the same depths. The results show that the reservoir ages progressively increased from ∼0 to ∼2800 yr between ∼9700 cal BP and ∼530 cal BP. As the lake catchment was glaciated prior to the Holocene, and Holocene soils and peats are the main sources of the TOC in the lake sediments, we argue that soil erosion is the major factor contributing to the progressive increase in the reservoir age. Based on previously reported evidence for increasing moisture in central Asia and glacier advances in the mid-to-late Holocene, we suggest that the intensified soil erosion on the hillslopes was caused by increased precipitation during the mid-to-late Holocene and by anthropogenic forest clearance after 1500 cal BP.

A chronologically reliable record of 17,000 years of biomass burning in the Lake Victoria area

Fire regimes differ across tropical and subtropical biomes depending on multiple parameters whose interactions and levels of importance are poorly understood, particularly at multidecadal and longer time scales. In the catchment of Lake Victoria, savanna, rainforest, and Afromontane vegetation have interspersed over the last 17,000 years, which may have influenced the fire regime and vice versa. However, climate and humans are most often the primary drivers of fire regime changes, and analysing their respective roles is critical for understanding current and future fire regimes. Besides a handful of radiocarbon dates on grassy charcoal, the timescales of published studies of Lake Victoria sediment chronologies rely mostly on dates of bulk sediment, and chronological disagreements persist, mainly due to variation between estimations of the 14C reservoir effect. Here, we provide independent 14C chronologies for three Late Glacial and Holocene lacustrine sediment cores from various water depths and compare them with the biostratigraphy to establish a new chronological framework. We present the first continuous sedimentary charcoal records from Lake Victoria; these suggest that fire activity varied substantially during the past 17,000 years. Our new pollen records reveal the long-term vegetation dynamics. The available evidence suggests that before human impact increased during the Iron Age (ca. 2400 yr BP), biomass burning was linked to climate and vegetation reorganizations, such as warming, drying, and the expansion of rainforests and savannas. Our results imply that climate can trigger substantial fire regime changes and that vegetation responses to climate change can co-determine the fire regime. For instance, biomass burning decreased significantly when the rainforest expanded in response to increasing temperatures and moisture availability. Such insights into the long-term linkages between climate, vegetation, and the fire regime may help to refine ecosystem management and conservation strategies in a changing global climate.

The 14C reservoir effects and paleoclimatic reconstructions in Lake Qinghai, northeastern Tibet

Lake Qinghai located in the northeastern Tibetan Plateau provides ideal Holocene paleoclimatic records to explore the climatic and environmental changes controlled by the Asian summer monsoon and the westerlies. Radiocarbon (14C) is commonly used to constrain the sediment chronology, however, the 14C reservoir effects in Lake Qinghai are still under debate. Here, we firstly carry out multiple 14C dating (plant remains, authigenic carbonate and bulk organic matter) and multi-proxy analyses (mineralogy, major elements, total organic carbon (TOC), total nitrogen (TN), total inorganic carbon (TIC), carbon isotopic composition of bulk organic matter (δ13Corg) and inorganic matter (δ13Cinorg), and oxygen isotopic composition of inorganic matter (δ18Oinorg)) on a 3.5 m-long core (QH1) in Lake Qinghai to illustrate the 14C reservoir effects and paleoclimate changes during the last 6000 yr. The 14C age-depth linear regressions of plant remains, authigenic carbonate and bulk organic matter present similar sediment accumulation rates of 17.59 ± 0.21, 17.74 ± 0.17 and 17.07 ± 0.22 yr cm−1 but different reservoir effects of 910 ± 41, 1661 ± 35 and 1031 ± 49 yr, respectively. This evidence suggests that the new 14C age-depth model used in this study is valid to estimate sediment accumulation rate and to distinguish different reservoir effects between multiple dating materials. Combing with the geochemical proxies, the paleoclimate fluctuated with a light drying trend from ∼6,000 to ∼4,000 cal yr BP. After the abrupt shift at ∼4,000 cal yr BP, the paleoclimate became colder and drier. Since the Mid-Holocene millennial-timescale warm and humid periods occurred at ∼5,900, ∼4,800, ∼4,000, ∼1,800, ∼1,100, ∼600 and ∼100 cal yr BP and relative droughts occurred at ∼5,200, ∼4,600, 4,000–2,000, ∼1,500, ∼900 and ∼300 cal yr BP. These features can be attributed to the rapid strengthening and weakening of the summer monsoon circulation in the northeastern Tibetan Plateau.

Linking sedimentary and speleothem precipitation isotope proxy records to improve lacustrine and marine 14C chronologies

The chronology of geological records in lacustrine, peatland and marine sediments for the late glacial depends mainly on 14C dating technology, which provides the basic database for global paleoclimate research. However, 14C reservoir correction always challenges the accuracy of the 14C chronology of terrestrial and marine sediments and the uncertainty of the 14C chronology associated with the carbon reservoir effect becomes critical for high resolution paleoclimate studies. Here, based on the hypothesis of synchronization of precipitation isotopes, we verify and identify a series of in-phase points of precipitation isotopes (IPPIs) between sediment leaf-wax hydrogen and stalagmite-calcite oxygen isotopes. Because stalagmite oxygen isotope records are accurately dated by U–Th dating technology, the ages of stalagmite IPPIs could be used to improve 14C reservoir correction of lacustrine IPPIs. We found that reservoir-corrected 14C ages of lacustrine, peat, and marine IPPIs are scattered with an average uncertainty of 1 ka when compared with the corresponding IPPIs on U–Th age scales. We suggest that the reservoir age correction at different time intervals could be further adjusted by using the U–Th age-backed IPPIs, which largely reduces the uncertainty of the 14C chronology and thus provides more accurate paleoclimate records.

Influences of environmental factors on the modern 14C reservoir effects in Qinghai-Tibet Plateau lakes

Influences of environmental factors on the modern 14C reservoir effects in Qinghai-Tibet Plateau lakes

14C GEOCHRONOLOGY AND RADIOCARBON RESERVOIR EFFECT OF REVIEWED LAKES STUDY IN CHINA

ABSTRACTLacustrine sediments are important archives for paleoclimate research, but there are evident carbon reservoir effects. Radiocarbon (14C) ages of lake sediments must be corrected for these effects before applying them to paleoclimate research. The authors review the lacustrine research from the last 20 years from different climatic regions in China, and systematically investigate the 14C age and correction methods used in the studies of 81 lakes. It is found that the climate-vegetation cover and distribution of carbonate around lakes are dominant factor controlling radiocarbon reservoir effects. In eastern China, the average 14C reservoir age is about 500 14C years and is associated with relatively dense vegetation. However, in northwest China and Qinghai-Tibet Plateau, widespread carbonate bedrock may markedly increase the radiocarbon reservoir age which frequently is about 1500 and 2500 14C years. A piecewise linear regression model provides more reliable 14C reservoir age correction that accounts for sedimentary facies and sedimentation rate changes. It is worth mentioning that when analyzing 14C ages deviated greatly from time sequence, the age anomalies may indicate important effects relevant to the study of climate and environmental changes.

Plateaus and jumps in the atmospheric radiocarbon record – potential origin and value as global age markers for glacial-to-deglacial paleoceanography, a synthesis

Abstract. Changes in the geometry of ocean meridional overturning circulation (MOC) are crucial in controlling past changes of climate and the carbon inventory of the atmosphere. However, the accurate timing and global correlation of short-term glacial-to-deglacial changes of MOC in different ocean basins still present a major challenge. The fine structure of jumps and plateaus in atmospheric and planktic radiocarbon (14C) concentration reflects changes in atmospheric 14C production, ocean–atmosphere 14C exchange, and ocean mixing. Plateau boundaries in the atmospheric 14C record of Lake Suigetsu, now tied to Hulu Cave U∕Th model ages instead of optical varve counts, provide a stratigraphic “rung ladder” of up to 30 age tie points from 29 to 10 cal ka for accurate dating of planktic oceanic 14C records. The age differences between contemporary planktic and atmospheric 14C plateaus record the global distribution of 14C reservoir ages for surface waters of the Last Glacial Maximum (LGM) and deglacial Heinrich Stadial 1 (HS-1), as documented in 19 and 20 planktic 14C records, respectively. Elevated and variable reservoir ages mark both upwelling regions and high-latitude sites covered by sea ice and/or meltwater. 14C ventilation ages of LGM deep waters reveal opposed geometries of Atlantic and Pacific MOC. Like today, Atlantic deep-water formation went along with an estuarine inflow of old abyssal waters from the Southern Ocean up to the northern North Pacific and an outflow of upper deep waters. During early HS-1, 14C ventilation ages suggest a reversed MOC and ∼1500-year flushing of the deep North Pacific up to the South China Sea, when estuarine circulation geometry marked the North Atlantic, gradually starting near 19 ka. High 14C ventilation ages of LGM deep waters reflect a major drawdown of carbon from the atmosphere. The subsequent major deglacial age drop reflects changes in MOC accompanied by massive carbon releases to the atmosphere as recorded in Antarctic ice cores. These new features of MOC and the carbon cycle provide detailed evidence in space and time to test and refine ocean models that, in part because of insufficient spatial model resolution and reference data, still poorly reproduce our data sets.

Impact of hydrological conditions on the radiocarbon reservoir effect in lake sediment 14C dating: the case of Kusai Lake on the northern Qinghai-Tibet Plateau

Abstract The hydrological conditions of a lake system strongly influence the reservoir effect in the radiocarbon dating of endogenic organic and inorganic carbon in lake sediments. In this paper, we compare 14C dating with varve counting results for cores from Kusai Lake to determine the temporal changes in 14C reservoir ages during the last 2000 years. Based on the 14C budget equation for a lake system, the characteristics of geomorphology, topography, and river systems in the catchment, and the physical and chemical properties of modern water in Kusai Lake, the hydrological conditions and the reservoir effect age are discussed. The reservoir age of the sediment core from northwestern Kusai Lake (i.e., deep water area) fluctuates slightly between 2980 and 3310 years, while that from southeastern Kusai Lake (i.e., shallow water area) fluctuates greatly between 3130 and 4010 years. Under the relatively steady conditions of atmospheric 14C levels for the past 2000 years, the 14C level in the dissolved inorganic carbon (DIC) of Kusai Lake is mainly related to carbon influxes from river and glacial meltwater and carbon outfluxes in lake water. The water in the shallow water area is more affected by the Kusai River, glacier meltwater, and lake outflow than the water in the deep water area, which has caused larger temporal changes in the radiocarbon reservoir ages in the shallow water area than in the deep water area during the last 2000 years.

RE-EVALUATING THE BRONZE AND EARLIEST IRON AGE IN LATVIA: CHANGES IN BURIAL TRADITIONS IN THE LIGHT OF 14C DATES

ABSTRACTUntil recently, there was a lack of radiocarbon (14C) dates from the Bronze and Earliest Iron Age (1800–500–1 BC) burial sites in Latvia. The chronology of the sites was assessed on the basis of archaeological analogies with neighboring regions and typological studies of the rather meagre grave inventory. In order to establish a firm foundation for an absolute chronology of burial sites and to better understand changes in mortuary practices during the period, sequences of samples from various burial sites have been dated. In this paper we report 48 14C dates from 12 different sites and discuss them in the context of previously established archaeological chronologies. 14C reservoir effects are addressed: regarding FRE, stable isotope analysis is helpful; however, more data should be gathered in future research. In some cases, the new dates are in accord with previous chronologies, while in other cases some widely accepted assumptions may need to be revised. The new dates have proved false several previous assumptions about both the dates of individual graves and whole sites. Based on the 14C dates, we model the chronological spread of burial barrows in Latvia along waterways, the earliest examples appearing in coastal western Latvia.

MARINE RESERVOIR AGE CORRECTION FOR THE ANDAMAN BASIN

ABSTRACTMarine reservoir age is an important component for correction in radiocarbon (14C) dating of marine and coastal samples. 14C concentration in pre-bomb marine samples of known age are used to derive marine reservoir age of a region. Annually banded coral from Landfall island in the northern Andaman has been analyzed for its 14C concentration during the pre-bomb period 1948–1951. 14C age and reservoir effect (∆R) are reported for these pre-bomb coral samples from the northern Andaman region. The mean 14C age of 331 ± 61 yr BP was obtained for the period 1948–1951 with an average reservoir age correction of –138 ± 61 yr. This reservoir age correction is lowest reported from the northern Indian Ocean. ∆R value of the northern Andaman and the Bay of Bengal appears lower than that of southern Andaman. The ∆R values obtained using mollusk shells and coral from the Andaman region shows large variability. The lower reservoir age correction for the Landfall Island situated in the northern part of the Andaman archipelago, could result due to freshwater flux and reduced upwelling in the region.

Vital evidence: Change in the marine 14C reservoir around New Zealand (Aotearoa) and implications for the timing of Polynesian settlement

Precise and accurate radiocarbon chronologies are essential to achieve tight chronological control for the ~ 750-years since Polynesian settlement of New Zealand. This goal has, however, been elusive. While radiocarbon datasets in the region are typically dominated by marine and estuarine shell dates, such chronological information has been ignored by those interpreting the timing of key events because a detailed regional calibration methodology for marine shell, comparable to the highly precise Southern Hemisphere calibration curve, is lacking. In this paper, we present the first temporal 14C marine offset (ΔR) model for New Zealand based on paired estuarine/marine and terrestrial radiocarbon dates from 52 archaeological contexts. Our dataset displays significant offsets between the measured New Zealand data and the modelled global marine radiocarbon curve. These shifts are associated with oceanographic fluctuation at the onset of the Little Ice Age ~ AD 1350–1450 (650–500 BP). The application of a regional and temporal correction to archaeological shell dates provides complimentary information to terrestrial radiocarbon production and has the potential to add structure to the blurred chronology that has plagued archaeological theories about the colonization of New Zealand, and other Pacific islands, for decades.

Variable Late Holocene 14C Reservoir Ages in Lake Bosten, Northwestern China

Radiocarbon (14C) dating has been widely used in paleoclimate reconstruction. However, the reliability of the 14C age in lake sediments is sensitive to the 14C reservoir effect, especially for a lake in arid regions. In this study, we evaluated the 14C reservoir ages under different hydroclimatic conditions over the past ~2300 years in Lake Bosten, northwestern China, by comparing with different dating results and with multi-proxy indices of a vertical-down sediment core collected in this lake. The 14C reservoir ages during ~ 1800 to 650 BP (a dry interval) are estimated to be approximately 1170 years older than those during ~ 650 -100 BP and those during 2200-1800 BP (wet intervals). We proposed that variation in 14C exchange rate between the dissolved CO2 in lake water and CO2 in the atmosphere, as well as the changing proportion of organic matter in the lake and the catchment, could have contributed to the variable 14C reservoir ages in Lake Bosten. The result of this study suggests that the 14C reservoir effect may be larger in dry periods than in wet periods in the arid/semi-arid area, which should be considered when establishing age models of lake sediment cores from these regions.

Tracing Carbon Isotope Variations in Lake Sediments Caused by Environmental Factors During the Past Century: A Case Study of Lake Tapeliai, Lithuania

ABSTRACTIn this study, we examined how land use and urbanization changes in adjacent areas affected biological productivity and carbon cycling in a lake ecosystem over 100 years and how these changes are reflected in carbon isotope variations. We performed radiocarbon (14C) activity and stable carbon isotope ratio analysis in two organic fractions: humin and humic acids of lake sediment. Additionally, we performed pigment and diatom analysis and determined the carbonate and organic matter (OM) content in sediments. Over the last century, the estimated 14C reservoir age in both sediment organic fractions varied from 1136 ± 112 yr to 5733 ± 122 yr. The increase in the reservoir age by 1175 ± 111 yr was related with higher inputs of pre-aged organic carbon and 14C depleted hard water due to the opening of the channel connecting two lakes. Nuclear weapons tests caused an increase in the reservoir age of up to 5421 ± 135 yr and 5733 ± 122 yr in humin and humic acids, respectively. 13C values in the humic acid fraction showed a tendency to decrease, depending on the content of autochthonous versus allochthonous OM in sediments, while changes in the sources of OM had a minor impact on the stable carbon isotope composition in the humin fraction.

Temporal variability of 14C reservoir effects and sedimentological chronology analysis in lake sediments from Chibuzhang Co, North Tibet (China)

Abstract Chronological analysis is the basic premise for paleoenvironmental research on lake sediment cores. It mainly employs bulk organic matter as a mixed dating material, although terrestrial plant remains are believed to provide reliable radiocarbon (14C) ages. 14C reservoir effects (REs) are generally problematic in establishing a final chronology and are usually recorded in aquatic macrophytes by the incorporation of dissolved inorganic carbon in high-altitude lakes of the Tibetan Plateau. We present a typical case of a 5.48 m-long core sampled from Chibuzhang Co, a lake in north Tibet, China, which contained two obvious obstacles: high REs and age reversals. The 14C RE was recently as much as ∼3500 years in the upper part according to total organic carbon (TOC) 14C ages and related dating from 210Pbex activity. It notably changed during the climatic transition to the Holocene due to carbonate input from catchment erosion and glacial meltwater, resulting in reversed 14C ages in the lower section of the core. 14C-dead carbon (or old carbon) dissolved in lake water was incorporated in the dating materials, as inferred from the 14C ages of plant samples. By combining data on grain size and carbonate content changes, two sedimentary units were identified with a boundary at 390 cm depth. These were used to estimate individual sediment accumulation rates (SARs) for the final age-depth models, instead of using a constant RE-corrected chronology. Our results provide an environmental history of ∼12.7 kyr in a long sediment core, based on an optimized SAR model of conventional 14C ages, which is supported by related lacustrine records and Bacon models. Use of separate 14C SAR analyses can solve the problem of age inversion in sediment cores, as sedimentological chronology analysis.