Bayesian Age-depth Model Research Articles

High-Resolution Peat Core Chronology Covering the Last 12 KYR Applying an Improved Peat Bog Sampling

ABSTRACTThis work focuses on building a high-resolution age-depth model for quantitative palaeoclimate study from the Mohos peat bog, East Carpathian mountains. The investigated core presents a continuous peat profile for the last 12 kyr. The chronology was based on 36 accelerator mass spectrometry radiocarbon (AMS14C) analyses of the separatedSphagnumsamples from different depths of the profile. DrySphagnumsamples for AMS dating were prepared using the classic acid-base-acid (ABA) method followed with an oxidative bleaching step to get clean cellulose.Sphagnumcellulose samples were measured by AMS using the EnvironMICADAS at the ICER (Debrecen, Hungary). A high-resolution chronology was obtained with the use of Bayesian age-depth modeling. Peat accumulation rate has been calculated and the sections with variable accumulation rate values were observed along the profile.

Ultra-distal fine ash occurrences of the Icelandic Askja-S Plinian eruption deposits in Southern Carpathian lakes: New age constraints on a continental scale tephrostratigraphic marker

Here we present the results of the first cryptotephra investigation of two Late glacial-Holocene lake records from the Southern Carpathian Mountains in Romania, Lake Brazi and Lake Lia. The discovery of an important Icelandic tephrostratigraphic marker, the Askja-S, in the sedimentary records of both sites significantly extends the known ash dispersal from this Plinian eruption. Bayesian age-depth modelling of available radiocarbon (14C) data from both sedimentary records allows us to further refine the depositional age of this ultra-distal tephra. In combination with age constraints on the tephra from other well-dated European sites, we produce an updated age for this key tephrostratigraphic marker of 10,824 ± 97 cal yrs BP (95.4% range). The Askja-S tephra is stratigraphically positioned after the palaeoenvironmental proxy response to the Preboreal Oscillation at both sites. The widespread distribution of this tephra across Europe offers the potential to assess spatio-temporal variability of this climatic signal. The discovery of the Askja-S in lake records from the Southern Carpathians highlights the likelihood of finding other ultra-distal (Icelandic) cryptotephra marker layers within the region. Additionally, given the location of the Carpathian region, it offers the opportunity to further enhance and integrate tephrostratigraphic frameworks of north-western Europe with those of the Mediterranean and Anatolia regions, which will enable a more precise comparison of palaeoenvironmental archives across Europe.

Double the dates and go for Bayes — Impacts of model choice, dating density and quality on chronologies

Reliable chronologies are essential for most Quaternary studies, but little is known about how age-depth model choice, as well as dating density and quality, affect the precision and accuracy of chronologies. A meta-analysis suggests that most existing late-Quaternary studies contain fewer than one date per millennium, and provide millennial-scale precision at best. We use existing and simulated sediment cores to estimate what dating density and quality are required to obtain accurate chronologies at a desired precision. For many sites, a doubling in dating density would significantly improve chronologies and thus their value for reconstructing and interpreting past environmental changes. Commonly used classical age-depth models stop becoming more precise after a minimum dating density is reached, but the precision of Bayesian age-depth models which take advantage of chronological ordering continues to improve with more dates. Our simulations show that classical age-depth models severely underestimate uncertainty and are inaccurate at low dating densities, and also perform poorly at high dating densities. On the other hand, Bayesian age-depth models provide more realistic precision estimates, including at low to average dating densities, and are much more robust against dating scatter and outliers. Indeed, Bayesian age-depth models outperform classical ones at all tested dating densities, qualities and time-scales. We recommend that chronologies should be produced using Bayesian age-depth models taking into account chronological ordering and based on a minimum of 2 dates per millennium.

Postglacial vegetation and climate history and traces of early human impact and agriculture in the present-day cool mixed forest zone of European Russia

This study is based on the reassessment and correlation of five pollen records from the upper Western Dvina River region. For the most complete record (Korovinskoe Mire; 56°16′N, 31°20′E) we constructed a Bayesian age-depth model to provide robust chronological control for the reconstructed changes in the regional environments and to facilitate interregional comparisons. The results show that the Holocene Thermal Maximum in the study region (ca. 8600–6900 cal BP) started and ended some centuries earlier than suggested for the neighboring Baltic region (ca. 8100–5600 cal BP). The spruce (Picea) pollen records corroborate other palaeobotanical records from the wider region indicating a relatively dry early and middle Holocene and cooler/wetter conditions after ca. 5400 cal BP. Local initial opening of forests by Neolithic populations is indicated by 7600 cal BP and a stepwise intensification of forest clearance is registered from ca. 4200/3700 and ca. 1400–1000 cal BP. There is evidence for an asynchronous pattern of human impact during the early period of occupation prior to ca. 4000 cal BP, which can be explained by a low density and uneven distribution of prehistoric hunter-fisher-gatherer populations. Although single Cerealia-type pollen grains are registered as early as ca. 6700 cal BP, intensive cultivation of cereals in the study region probably did not occur before ca. 1400 cal BP, i.e. much later than suggested for the Baltic states, Poland, and Belarus.

The identification of late-Holocene bog bursts at Littleton Bog, Ireland: Ecohydrological changes display complex climatic and non-climatic drivers

In order to clearly understand the response of raised mires to past climate change, it is important to consider the full range of drivers and responses of these ecohydrological archives. To this end, a high-resolution ecohydrological record from Littleton Bog, Co. Tipperary, Ireland, was generated utilizing a combination of plant macrofossils, testate amoebae and humification analysis. Chronological control for this record was provided by a Bayesian age–depth model based on AMS radiocarbon dates. Testate amoebae–derived reconstructed peatland water tables indicate a series of sudden shifts to dry bog surface conditions at c. 3140, c. 2510 and c. 1540 cal. BP. These events display a distinctive palaeoecological signal and chronological tempo that is best explained as a result of a series of bog burst events and which seem inconsistent with other explanations. The chronological correspondence between the bog bursts at Littleton and a set of similar events at Derryville Bog, c. 5 km to the north, is noted, as is the broad correspondence of these events with wet shifts indicated in regional peatland water table compilations from Britain and Ireland. A range of possible driving mechanisms for these events is proposed, including anthropogenic disturbance of the bog surface, non-linear response to climate forcing, internal bog dynamics, vegetation succession or a combination of factors. We illustrate the need for further multi-proxy investigations to fully understand these phenomena.

Component age estimates for the Hell Gap Paleoindian site and methods for chronological modeling of stratified open sites

AbstractThe Hell Gap National Historic Landmark, located on the northwestern plains of Wyoming, is one of the most important Paleoindian archaeological sites in North America because it contains a stratified sequence of occupations spanning nearly the entirety of the Paleoindian period. Although Hell Gap is central to archaeological knowledge concerning North American Paleoindian chronology, consistently assigning component ages has been problematic due to conflicting radiocarbon determinations from individual strata, stratigraphic age reversals in age-depth relationships, and other issues related to the stratified open campsite. Toward resolving the Hell Gap chronology, we devised a procedure for correcting age-depth relationships for incorporation in chronostratigraphic models and then used the Bayesian age-depth modeling qprocedures in Bchron to estimate the ages of 11 stratified components present at Hell Gap Locality 1. We present these age estimates and discuss their significance to Paleoindian chronology. Notable aspects of our chronology include a revised age estimate for the Goshen complex, the identification of three Folsom components spanning the entirety of the Folsom temporal range, and relatively young age estimates for the Late Paleoindian Frederick/Lusk component(s) at Locality 1. More broadly, our study demonstrates a procedure for creating chronometric models of stratigraphically complicated open stratified sites of any type.

Environmental and vegetation changes in southeastern Amazonia during the late Pleistocene and Holocene

New insights about climate conditions and their effect on vegetation and lakes of southeastern Amazonia were achieved over the last 50 cal ka using recent data obtained from a filled lake located in the Serra Sul de Carajás. Our analysis was done in a sediment core (R2) comprising of faciological, palynological, isotopic and radio carbon measurement supported by Bayesian age-depth modeling, as well species distribution modeling. Wet and cool climate conditions are proposed for the mid-Pleniglacial, which is favorable to intense weathering of the lake's catchment area and the expansion of high altitude taxa (cool-adapted) into forest formations during this period. From the late Pleniglacial to the Last Glacial Maximum, a change from relatively wet to dry climate possibly converted the studied lake from oxidizing to reducing conditions, which leads to the formation of diagenetic siderites at the end of this time interval. Warming during the Pleistocene to Holocene transition caused the disappearance of cool-adapted taxa in the study site. Concomitantly, increasing precipitation allowed the expansion of forest and high lake levels. A drier early-mid Holocene produced extensive open vegetation and desiccated lakes. Current climate and environmental conditions were likely attained in the region from the mid to late Holocene.

Lost in dating – Problems with the absolute chronologies and sedimentation rates of Late Glacial and Early Holocene oxbow lake deposits in Central Europe

Abstract This article presents potential errors related to the construction of absolute chronologies for Late Glacial and Early Holocene sediments accumulated in oxbow lakes in Central Europe and their influence on palaeoecological reconstruction. Our study is based on two sediment cores: (i) from the San River (Stubno-Naklo site in south-eastern Poland; S-N) and (ii) from the Dniester River (Luka site in western Ukraine) valleys. Accelerator mass spectrometry (AMS) 14C dating was performed on terrestrial plant macrofossils from each site to produce Bayesian age–depth models and to calculate sedimentation rates (SRs). The SR in both sites was characterised by high variability reflecting the developmental stage of oxbow lake. The highest SR values (1.18–1.23 cm yr-1) were identified at the S-N site at the beginning of the Holocene (ca. 11,550–11,470 cal. BP) and related to enhanced overbank deposition. The lowest SR values (0.01–0.02 cm yr-1) were identified in the transitional layer between the Younger Dryas and the Preboreal chronozone (ca. 11,810–11,160 cal. BP) in the Luka profile; this may have been related to the sedimentary gap(s) caused by falls in the water table and/or temporary desiccation of the water body. Moreover, our study revealed the presence of three distinctly aged AMS 14C dates in the Younger Dryas section of the Luka profile. These radiocarbon inversions were completely undetected by pollen analysis, which indicated the domination of Pinus and Betula without any significant fluctuations throughout the entire subsection. The presence of such ‘cryptic’ insertions of older material could lead to erroneous palaeoenvironmental reconstruction. Similarly the S-N site revealed anomalously ‘aged’ dates which were also difficult to detect by pollen analysis.

Precise age for the Permian–Triassic boundary in South China from high-precision U-Pb geochronology and Bayesian age–depth modeling

Abstract. This study is based on zircon U-Pb ages of 12 volcanic ash layers and volcanogenic sandstones from two deep water sections with conformable and continuous formational Permian–Triassic boundaries (PTBs) in the Nanpanjiang Basin (South China). Our dates of single, thermally annealed and chemically abraded zircons bracket the PTB in Dongpan and Penglaitan and provide the basis for a first proof-of-concept study utilizing a Bayesian chronology model comparing the three sections of Dongpan, Penglaitan and the Global Stratotype Section and Point (GSSP) at Meishan. Our Bayesian modeling demonstrates that the formational boundaries in Dongpan (251.939 ± 0.030 Ma), Penglaitan (251.984 ± 0.031 Ma) and Meishan (251.956 ± 0.035 Ma) are synchronous within analytical uncertainty of ∼ 40 ka. It also provides quantitative evidence that the ages of the paleontologically defined boundaries, based on conodont unitary association zones in Meishan and on macrofaunas in Dongpan, are identical and coincide with the age of the formational boundaries. The age model also confirms the extreme condensation around the PTB in Meishan, which distorts the projection of any stratigraphic points or intervals onto other more expanded sections by means of Bayesian age–depth models. Dongpan and Penglaitan possess significantly higher sediment accumulation rates and thus offer a greater potential for high-resolution studies of environmental proxies and correlations around the PTB than Meishan. This study highlights the power of high-resolution radio-isotopic ages that allow a robust intercalibration of patterns of biotic changes and fluctuating environmental proxies and will help recognizing their global, regional or local significance.

Palaeoecological records of coral community development on a turbid, nearshore reef complex: baselines for assessing ecological change

Understanding past coral community development and reef growth is crucial for placing contemporary ecological and environmental change within appropriate reef-building timescales. On Australia’s Great Barrier Reef (GBR), coral reefs situated within coastal inner-shelf zones are a particular priority. This is due to their close proximity to river point sources, and therefore susceptibility to reduced water quality discharged from coastal catchments, many of which have been modified following European settlement (ca. 1850 AD). However, the extent of water-quality decline and its impacts on the GBR’s inner-shelf reefs remain contentious. In this study, palaeoecological coral assemblage records were developed for five proximal coral reefs situated within a nearshore turbid-zone reef complex on the central GBR. A total of 29 genera of Scleractinia were identified from the palaeoecological inventory of the reef complex, with key contributions to reef-building made by Acropora, Montipora, and Turbinaria. Discrete intervals pre- and post-dating European settlement, but associated with equivalent water depths, were identified using Bayesian age–depth modelling, enabling investigation of competing ideas of the main drivers of nearshore coral assemblage change. Specifically, we tested the hypotheses that changes in the composition of nearshore coral assemblages are: (1) intrinsically driven and linked to vertical reef development towards sea level, and (2) the result of changes in water quality associated with coastal river catchment modification. Our records found no discernible evidence of change in the generic composition of coral assemblages relative to European settlement. Instead, two distinctive depth-stratified assemblages were identified. This study demonstrates the robust nature of nearshore coral communities under reported water-quality decline and provides a useful context for the monitoring and assessment of ecological change on reefs located within the most nearshore turbid-zone environments of the central GBR.

High-precision 40Ar/39Ar dating of pleistocene tuffs and temporal anchoring of the Matuyama-Brunhes boundary

Abstract High-precision 40 Ar/ 39 Ar ages for a series of proximal tuffs from the Toba super-volcano in Indonesia, and the Bishop Tuff and Lava Creek Tuff B in North America have been obtained. Core from Ocean Drilling Project Site 758 in the eastern equatorial Indian Ocean contains discrete tephra layers that we have geochemically correlated to the Young Toba Tuff (73.7 ± 0.3 ka), Middle Toba Tuff (502 ± 0.7 ka) and two eruptions (OTTA and OTTB) related to the Old Toba Tuff (792.4 ± 0.5 and 785.6 ± 0.7 ka, respectively) ( 40 Ar/ 39 Ar data reported as full external precision, 1 sigma). Within ODP 758 Termination IX is coincident with OTTB and hence this age tightly constrains the transition from Marine Isotope Stage 19–20 for the Indian Ocean. The core also preserves the location of the Australasian tektites, and the Matuyama-Brunhes boundary with Bayesian age-depth models used to determine the ages of these events, c. 786 and c. 784 ka, respectively. In North America, the Bishop Tuff (766.6 ± 0.4 ka) and Lava Creek Tuff B (627.0 ± 1.5 ka) have quantifiable stratigraphic relationships to the Matuyama-Brunhes boundary. Linear age-depth extrapolation, allowing for uncertainties associated with potential hiatuses in five different terrestrial sections, defines a geomagnetic reversal age of 789 ± 6 ka. Considering our data with respect to the previously published age data for the Matuyama-Brunhes boundary of Sagnotti et al. (2014), we suggest at the level of temporal resolution currently attainable using radioisotopic dating the last reversal of Earths geomagnetic field was isochronous. An overall Matuyama-Brunhes reversal age of 783.4 ± 0.6 ka is calculated, which allowing for inherent uncertainties in the astronomical dating approach, is indistinguishable from the LR04 stack age (780 ± 5 ka) for the geomagnetic boundary. Our high-precision age is 10 ± 2 ka older than the Matuyama-Brunhes boundary age of 773 ± 1 ka, as reported previously by Channell et al. (2010) for Atlantic Ocean records. As ODP 758 features in the LR04 marine stack, the high-precision 40 Ar/ 39 Ar ages determined here, as well as the Matuyama-Brunhes boundary age, can be used as temporally accurate and precise anchors for the Pleistocene time scale.

All age–depth models are wrong, but are getting better

The construction of accurate age–depth relationships and a realistic assessment of their uncertainties is one of the fundamental prerequisites for comparing and correlating late Quaternary stratigraphical proxy records. Four widely used age–depth modelling routines – CLAM, OxCal, Bacon and Bchron – were tested using radiocarbon dates simulated from varved sediment stratigraphies. All methods produce mean age–depth models that are close to the true varve age, but the uncertainty estimation differs considerably among models. Age uncertainties are usually underestimated by CLAM, whereas age uncertainties produced by Bchron are often too large. With OxCal and Bacon, the setting of model-specific parameters influences the estimated uncertainties, which vary from too large to too small. The variability of sediment accumulation rates is underestimated by CLAM but overestimated by Bacon and Bchron. Bayesian age–depth models mainly improve the assessment of uncertainties of age–depth models.

Changes in New Zealand forest plant communities following the prehistoric extinction of avian megaherbivores

AbstractQuestionsFollowing the mid 15th century AD extinction of moa (Aves: Dinornithiformes) there was a ~400‐yr period when large herbivores were absent from New Zealand's terrestrial ecosystems. This ended with the introduction of mammalian herbivores by European settlers in the 19th century. For at least 40 yr researchers have speculated about how New Zealand's vegetation communities might have responded to the ‘herbivore gap’. Did plant taxa once consumed by moa flourish in forest understoreys without large herbivores to browse them? Did forest understoreys become denser, preventing regeneration of long‐lived conifers and altering forest composition? Such questions remain to be tested using palaeoecological records.LocationThree locations on South Island, New Zealand (from <20 to 750 m a.s.l) where forest has not been burned since human settlement, and where there has been no obvious geological or climatic disturbance events over the past 1000 yr that have influenced vegetation dynamics.MethodsWe identified and quantified pollen grains and fern spores from soil/peat cores taken beneath, or adjacent to, tall forest canopies. Soil/peat samples from the cores were radiocarbon dated and Bayesian age–depth models were constructed for each core, from which depths corresponding to the herbivore gap period were identified. Changes in pollen/spore abundances occurring within the herbivore gap were identified and critically examined for consistency between cores and likelihood of being caused by loss of browsing pressure from large herbivores (through comparison with plant taxa known to have been consumed by moa).ResultsGround fern spores and Coprosma pollen exhibited increases in relative abundance at all three sites during the herbivore gap. Both occur frequently in moa coprolites and gizzard content, and so can be interpreted as possible responses to moa extinction. However, other changes in pollen assemblages at this time were inconsistent between sites, or could be better explained as being related to anthropogenic burning in the local region.ConclusionsNew Zealand forest plant communities appear to have experienced only relatively minor changes during the herbivore gap. Our results contradict long‐standing hypotheses that shrubs and ferns had flourished in New Zealand forest understoreys at this time, preventing podocarp regeneration. However, due to the relatively recent extinction of moa, and long life span of many forest trees, there may be an extinction debt where the long‐term consequences of moa extinction are still to be fully realized.

Quantifying system disturbance and recovery from historical mining-derived metal contamination at Brotherswater, northwest England

Metal ore extraction in historical times has left a legacy of severe contamination in aquatic ecosystems around the world. In the UK, nationwide surveys of present-day pollution discharged from abandoned mines are ongoing but few assessments of the magnitude of contamination and impacts that arose during historical metal mining have been made. We report one of the first multi-centennial records of lead (Pb), zinc (Zn) and copper (Cu) fluxes into a lake (Brotherswater, northwest England) from point-sources in its catchment (Hartsop Hall Mine and Hogget Gill processing plant) and calculate basin-scale inventories of those metals. The pre-mining baseline for metal contamination has been established using sediment cores spanning the past 1500 years and contemporary material obtained through sediment trapping. These data enabled the impact of 250 years of local, small-scale mining (1696–1942) to be quantified and an assessment of the trajectory towards system recovery to be made. The geochemical stratigraphy displayed in twelve sediment cores show strong correspondence to the documented history of metal mining and processing in the catchment. The initial onset in 1696 was detected, peak Pb concentrations (>10,000 µg g−1) and flux (39.4 g m−2 year−1) corresponded to the most intensive mining episode (1863–1871) and twentieth century technological enhancements were reflected as a more muted sedimentary imprint. After careful evaluation, we used these markers to augment a Bayesian age-depth model of the independent geochronology obtained using radioisotope dating (14C, 210Pb, 137Cs and 241Am). Total inventories of Pb, Zn and Cu for the lake basin during the period of active mining were 15,415, 5897 and 363 kg, respectively. The post-mining trajectories for Pb and Zn project a return to pre-mining levels within 54–128 years for Pb and 75–187 years for Zn, although future remobilisation of metal-enriched catchment soils and floodplain sediments could perturb this recovery. We present a transferable paleolimnological approach that highlights flux-based assessments are vital to accurately establish the baseline, impact and trajectory of mining-derived contamination for a lake catchment.

Charcoal and mollusc shell 14 C-dating of the Dunaszekcső loess record, Hungary

Abstract Accelerator mass spectrometry (AMS) radiocarbon dating of charcoals and 10 mollusc species (+2 identified at the family level) yield 64 new ages from the Dunaszekcső loess record, Hungary. Charcoal 14 C ages are found to be protocol-dependent, with ages obtained from the high temperature (800 °C) fraction of the two-step combustion (TSC) protocol being always the oldest and likely most reliable. One step combustion (OSC) at 1000 °C produces comparable ages with those obtained from the low temperature (400 °C) fraction of the TSC protocol. Discrepancies between the ABA-TSC 400 and TSC 800 ages become larger for older, and less well-preserved charcoal fragments. Testing of shell ages against those of charcoals reveals that Succineidae sp. , Chondrula tridens , Trochulus hispidus and members of the family Clausiliidae yield 14 C ages that are statistically indistinguishable from charcoals recovered from the same stratigraphic interval, or with the ABA-TSC 800 charcoal age. Thus, these species show great potential for creating accurate and precise chronologies for loess records within the useful range ( 14 C-dating of land snails. Conservative estimates of 95% uncertainties of the resulting Bayesian age-depth models are in the range of 500–800 yr. However, these uncertainties are strongly dependent on the number of dates available and the resolution of sampling, but they are usually well below those of luminescence chronologies.

Sedimentary environment, lithostratigraphy and dating of sediment sequences from Arctic lakes Revvatnet and Svartvatnet in Hornsund, Svalbard

Abstract The sedimentary environment, sediment characteristics and age-depth models of sediment sequences from Arctic lakes Revvatnet and Svartvatnet, located near the Polish Polar Station in Hornsund, southern Svalbard (77°N), were studied with a view to establishing a basis for paleolimnological climate and environmental reconstructions. The results indicate that catchment-to-lake hydroclimatic processes probably affect the transportation, distribution and accumulation of sediments in different parts of lakes Revvatnet and Svartvatnet. Locations with continuous and essentially stable sedimentary environments were found in both lakes between water depths of 9 and 26 m. We used several different dating techniques, including 137Cs, 210Pb, AMS 14C, and paleomagnetic dating, to provide accurate and secured sediment chronologies. Arecovered sequence from the northern basin of Revvatnet spans more than one thousand years long with laminated stratigraphy in the upper part of the sediment. Based on AMS 14C dates, it is possible to suppose that Revvatnet basin was not occupied by a valley glacier during the Little Ice Age. The dates were supported by 137Cs chronologies, but not confirmed with other independent dating methods that extent beyond the last 50 years. A sedimentary sequence from the northern basin of Svartvatnet provides a potential archive for the study of climate and environmental change for the last ca. 5000 years. Based on the stratigraphy and a Bayesian age-depth model of AMS14C and paleosecular variation (PSV) dates, the recovered sediment sections represent a continuous and stable sedimentation for the latter half of the Holocene.

Clarifying the distal to proximal tephrochronology of the Millennium (B–Tm) eruption, Changbaishan Volcano, northeast China

Tephra dispersed during the Millennium eruption (ME), Changbaishan Volcano, NE China provides one of the key stratigraphic links between Asia and Greenland for the synchronization of palaeoenvironmental records. However, controversy surrounds proximal-distal tephra correlations because (a) the proposed proximal correlatives of the distal ME tephra (i.e. B–Tm) lack an unequivocal chronostratigraphic context, and (b) the ME tephra deposits have not been chemically characterized for a full spectrum of element using grain-specific techniques. Here we present grain-specific glass chemistry, including for the first time, single grain trace element data, for a composite proximal sequence and a distal tephra from Lake Kushu, northern Japan (ca. 1100 km away from Changbaishan). We demonstrate a robust proximal-distal correlation and that the Kushu tephra is chemically associated with the ME/B–Tm. We propose that three of the proximal pyroclastic fall units were erupted as part of the ME. The radiocarbon chronology of the Kushu sedimentary record has been utilised to generate a Bayesian age-depth model, providing an age for the Kushu tephra which is consistent with high resolution ages determined for the eruption and therefore supports our geochemical correlation. Two further Bayesian age-depth models were independently constructed each incorporating one of two ice-core derived ages for the B–Tm tephra, providing Bayesian modelled ages of 933–949 and 944–947 cal AD (95.4%) for the Kushu tephra. The high resolution ice-core tephra ages imported into the deposition models help test and ultimately constrain the radiocarbon chronology in this interval of the Lake Kushu sedimentary record. The observed geochemical diversity between proximal and distal ME tephra deposits clearly evidences the interaction of two compositionally distinct magma batches during this caldera forming eruption.

Geologic records of Holocene typhoon strikes on the Gulf of Thailand coast

Washover sedimentation resulting from modern typhoon strikes on the Gulf of Thailand coast forms anomalous sand layers in low-energy coastal environments including marshes, ponds and swales. The primary diagnostic for recognizing prehistoric typhoon-deposited sand layers in the geologic record are sharp upper and lower contacts between coarser-grained transported sand layers and finer-grained in-situ sediments. In this first paleotempestology study in Thailand, cores from two low-energy settings on the Gulf of Thailand coast – a coastal marsh near Cha-am and beach ridge plain swales near Kui Buri – reveal geologic evidence of up to 19 typhoon strikes within the last 8000years. The sand layers have sharp upper and lower contacts with enclosing finer sediments. Some sand layers also contain other evidence of a sudden powerful landward-directed surge of ocean water, including gravel-sized clasts, offshore foraminifera, abundant shell fragments, plant debris and mud rip-up clasts. Sand layers record eleven typhoon strikes at Cha-am, ranging in age from AD 1952 to 7575cal.yr. BP, and eight typhoon strikes at Kui Buri, ranging in age from 4075 to 7740cal.yr. BP. Bayesian age–depth models, derived from eight AMS radiocarbon dates, suggest that the frequency of typhoon strikes was 2–5 times greater from 3900 to 7800cal.yr. BP compared to 0–3900cal.yr. BP. Possible explanations for this variability in the typhoon record are that typhoons were more frequent and/or more intense in Southeast Asia in the mid-Holocene because of climatic changes associated with the Mid-Holocene Warm Period or that the record reflects site sensitivity changes resulting from a mid-Holocene sea-level highstand. The preliminary finding of a possible link between warmer conditions and a greater frequency of intense typhoon strikes could have important societal implications, given possible consequences of ongoing global warming.

A plant microfossil record of Late Quaternary environments and human activity from Rano Aroi and surroundings, Easter Island

To investigate past vegetation change and human activity at the highest elevations on Easter Island, this study examines pollen, phytoliths, diatoms, starch and arthropods preserved in sediment/soil profiles from Rano Aroi crater, and another, newly described wetland in the adjacent, much smaller Rano Aroi Iti depression. A Bayesian age–depth model for Rano Aroi provides adequate certainty for the Late Pleistocene and last ~1000 years, but is poorly constrained from 12,000 to 1000 cal BP. The occurrence of cf. Potamogeton and Lycopodium pollen types at this highland site (~425 m altitude), while absent or rare at lowland sites, could well be related to the cooler higher altitude conditions. Smaller quantities of Arecaceae (palm) pollen than at lowland sites indicate that this relatively high-altitude part of the island was near the altitudinal forest limit in the Late Pleistocene, with extensive Asteraceae-dominated shrubland. Arecaceae forest dominated the Holocene, for which there is evidence for a prolonged dry phase. The subsequent disappearance of charcoal and reappearance of diatoms, particularly Eunotia cf. pectinalis, suggest that the dry phase ended prior to human settlement. Polynesian activity is best constrained by abundant microscopic charcoal fragments beginning in a layer at 710 (2σ: 645–797) cal BP, and suggesting a period of forest clearance and burning, culminating at 339 (2σ: 177–428) cal BP. Thinner soils compared with lowland horticultural sites and 14C dates of macroscopic Sophora charcoal suggest that the site was occupied after 1670 CE. Newly described terraces, and pollen, phytoliths and starch of cf. Broussonetia papyrifera (paper mulberry), cf. Colocasia esculenta (taro) and Musa (banana) sp. identified in this study show the value of a combined microfossil approach and provide evidence for extension of cultivation of these Polynesian-introduced cultigens to this least accessible part of the island. Rano Aroi Iti yielded a Bayesian 14C age–depth chronology to a basal age of 1530–1314 cal BP, giving an unexpectedly old age for presumably introduced New World pollen of Sisyrinchium, which occurs throughout the core.

Holocene floodplain palaeoecology of the Humberhead Levels; implications for regional wetland development

The acquisition of high quality, well-dated local site records is essential for progressing regional environmental reconstructions. As part of a wider study designed to examine intra- and extra- site ecosystem responses to environmental change, this paper presents new palaeoecological data from the floodplain of the River Torne in the Humberhead Levels, South Yorkshire. The sampling site lies adjacent to the lowland raised mire of Hatfield Moors, a location with a long history of palaeoecological investigations. The potential of using floodplain records to reconstruct local variations in ecosystem response to environmental change is also considered. Coleoptera and pollen are used to reconstruct floodplain ecosystem dynamics, whilst chronologies are established using Bayesian age–depth modelling. Between 10,200 cal BP and 2300 cal BP, the floodplain experienced multiple phases of ecological change. At 10,200–9910 cal BP, a cut-off channel began to infill with peat, while the surrounding floodplain remained relatively dry with Pinus forest growing nearby. Between 9630–9500 cal BP and 7270–7020 cal BP, a depositional hiatus occurred in the sedimentary record. By the end of this period, the local woodland had diversified and expanded to mixed deciduous tree cover. A wet shift identified at 6870–6160 cal BP was shortly followed by a rise in Alnus and Tilia from 6410–6160 cal BP. At this time, widespread floodplain paludification had occurred in the Humberhead Levels, which was largely controlled by relative sea-level (RSL) rise and the associated rise in regional water tables. Floodplain expansion also resulted in the widespread occurrence of Alnus dominated fen woodland. The local Torne floodplain experienced varying levels of wetness that influenced the decline and subsequent regeneration of the woodland from 5870–5640 cal BP. At this time, the Ulmus decline is identified in the pollen stratigraphic record. Floodplain hydrology appears to have been controlled by a combination of water table fluctuations and changes in channel pattern/flow, both of which can be linked to RSL variations recorded in the Humber Estuary. Floodplain alluviation, also linked to rising water tables, is dated to 4360–4160 cal BP. Anthropogenic woodland clearance further upstream may have further compounded this event.