Bayesian Age-depth Model Research Articles

Collapse wedges in periglacial eolian sands evidence Late Pleistocene paleoseismic activity of the Vienna Basin Transfer Fault (western Slovakia)

Studies of seismically triggered deformations in the depositional record contribute significantly to our knowledge of earthquake recurrence over geological time. This paper describes soft sediment deformation structures preserved in a periglacial Upper Pleistocene succession of eolian sand in the eastern Vienna Basin, Central Europe. The strata were formed dominantly by wind ripples migrating across a sand sheet, less frequently by deposition of fine-grained sand and silt from suspension during decreased wind speed and by small-scale eolian dunes. Most prominent deformations observed are collapse wedges, which are interpreted as being formed along dilatational fractures. Further detected deformations include chaotically disturbed strata, folded strata, and a pop-up structure caused by sliding. The tensile fractures are oriented systematically in a prevailing N–S direction, perpendicular to the W–E extension suggested for the transtensional Vienna Basin Transfer Fault, which underlies the study area. Repeated seismic shock is identified as the trigger of the deformations. The mechanism of deformation implies some degree of cohesion within the deformed strata, and this may be attributed to seasonal frost and the presence of vadose water within the upper levels of the sediment. These deformations appear periodically, in 25 distinct horizons of strata internally deformed by collapse wedges, and in 40 horizons if deformation both by collapse wedges and/or chaotically disturbed beds is considered. The vertical distribution of deformations allowed the calculation of the recurrence periods of earthquakes with the use of a Bayesian age-depth model; this, in turn, was based on seven OSL ages. The calculation yielded mean recurrence periods of ca. 100 years, though this figure is biased by a relatively high degree of uncertainty in the dating and in the age-depth model. Even with this caveat, the present study reveals the underexplored potential of periglacial eolian deposits to preserve paleoseismological signals.

Recalibrating the Devonian time scale: A new method for integrating radioisotopic and astrochronologic ages in a Bayesian framework

Abstract The numerous biotic, climatic, and tectonic events of the Devonian cannot be correlated and investigated without a well-calibrated time scale. Here, we updated the calibration of the Devonian time scale using a Bayesian age-depth model that incorporates radioisotopic ages and astrochronology durations. We used existing radioisotopic ages collected and harmonized in the last two geologic time scale compilations, as well as new U-Pb zircon ages from Emsian {Hercules I K-bentonite, Wetteldorf, Germany: 394.290 ± 0.097(0.21)[0.47] Ma} and Eifelian K-bentonites {Tioga B and Tioga F K-bentonites, Fayette, New York, USA: 390.82 ± 0.18(0.26)[0.48] Ma and 390.14 ± 0.14(0.23)[0.47] Ma, respectively}. We anchored floating astrochronology stage durations on radioisotopic ages and chained astrochronologic constraints and uncertainty together to extrapolate conditioning age likelihoods up or down the geologic time scale, which is a new method for integrating astrochronology into age-depth modeling. The modeling results in similar ages and durations for Devonian stages regardless of starting biostratigraphic scaling assumptions. We produced a set of rescaled biostratigraphic zonations, and a new numerical calibration of Devonian stage boundary ages with robust uncertainty estimates, which allow us to evaluate future targets for Devonian time scale research. These methods are broadly applicable for time scale work and provide a template for an integrated stratigraphic approach to time scale modeling.

Reef accumulation is decoupled from recent degradation in the central and southern Red Sea

Reefs are biogenic structures that result in three-dimensional accumulations of calcium carbonate. Over geological timescales, a positive balance between the production and accumulation of calcium carbonate versus erosional and off-reef transport processes maintains positive net accretion on reefs. Yet, how ecological processes occurring over decadal timescales translate to the accumulation of geological structures is poorly understood, in part due to a lack of studies with detailed time-constrained chronologies of reef accretion over decades to centuries. Here, we combined ecological surveys of living reefs with palaeoecological reconstructions and high-precision radiometric (U-Th) age-dating of fossil reefs represented in both reef sediment cores and surficial dead in situ corals, to reconstruct the history of community composition and carbonate accumulation across the central and southern Saudi Arabian Red Sea throughout the late Holocene. We found that reefs were primarily comprised of thermally tolerant massive Porites colonies, creating a consolidated coral framework, with unconsolidated branching coral rubble accumulating among massive corals on shallow (5–8 m depth) exposed (windward), and gently sloping reef slopes. These unconsolidated reef rubble fields were formed primarily from ex situ Acropora and Pocillopora coral fragments, infilled post deposition within a sedimentary matrix. Bayesian age-depth models revealed a process of punctuated deposition of post-mortem coral fragments transported from adjacent reef environments. That a large portion of Saudi Arabian Red Sea reef slopes is driven by allochthonous deposition (transportation) has important implications for modeling carbonate budgets and reef growth. In addition, a multi-decadal lag exists between the time of death for branching in situ coral and incorporation into the unconsolidated reef rubble. This indicates that recent climate related degradation in the 21st century has not had an immediately negative effect on reef building processes affecting a large portion of the reef area in the Saudi Arabian Red Sea.

Using multiple chronometers to establish a long, directly-dated lacustrine record: Constraining >600,000 years of environmental change at Chew Bahir, Ethiopia

Despite eastern Africa being a key location in the emergence of Homo sapiens and their subsequent dispersal out of Africa, there is a paucity of long, well-dated climate records in the region to contextualize this history. To address this issue, we dated a ∼293 m long composite sediment core from Chew Bahir, south Ethiopia, using three independent chronometers (radiocarbon, 40Ar/39Ar, and optically stimulated luminescence) combined with geochemical correlation to a known-age tephra. The site is located in a climatically sensitive region, and is close to Omo Kibish, the earliest documented Homo sapiens fossil site in eastern Africa, and to the proposed dispersal routes for H. sapiens out of Africa. The 30 ages generated by the various techniques are internally consistent, stratigraphically coherent, and span the full range of the core depth. A Bayesian age-depth model developed using these ages results in a chronology that forms one of the longest independently dated, high-resolution lacustrine sediment records from eastern Africa. The chronology illustrates that any record of environmental change preserved in the composite sediment core from Chew Bahir would span the entire timescale of modern human evolution and dispersal, encompassing the time period of the transition from Acheulean to Middle Stone Age (MSA), and subsequently to Later Stone Age (LSA) technology, making the core well-placed to address questions regarding environmental change and hominin evolutionary adaptation. The benefits to such studies of direct dating and the use of multiple independent chronometers are discussed.

Lateglacial and Holocene changes in vegetation and human subsistence around Lake Zhizhitskoye, East European midlatitudes, derived from radiocarbon-dated pollen and archaeological records

The numerous lake and peat sedimentary archives of Central European Russia are ideal for studying the Lateglacial–Holocene climate and vegetation history of the region. However, robustly dated palaeoenvironmental records have only become available in the last decade and are still few and far between. Together with the continuously growing archaeological record and increasing focus on absolute dating of cultural assemblages, such records, where available, offer an excellent opportunity to study human-environment interactions in the region. For this study we selected one of the most detailed regional pollen records, from a 4.5-m-long sediment section from Zmeinoe Mire (56°16′53″ N, 31°15′36″ E) at Zhizhitskoye Lake (165 m a.s.l.). We performed quantitative pollen-based biome reconstruction and constructed a Bayesian age-depth model to ensure maximum chronological control of the sediment succession. To link the reconstructed vegetation changes to human activities, we gathered and summarized available archaeological data from the study area. Our results show that humid conditions and taiga-like patchy forests (spruce, pine, birch) dominated the study region during the Lateglacial. The Early Holocene (ca. 11,650–8000 cal yr BP) was marked by a warming and drying trend indicated by the spread of cool-temperate summergreen broad-leaved trees and shrubs (elm, lime, hazel) and the retreat of spruce. Warmest conditions, expressed by the maximum contribution of temperate deciduous woody taxa to the pollen assemblages, were recorded during the Middle Holocene (ca. 8000–4000 cal yr BP) when Mesolithic and Neolithic hunter-fisher-gatherer groups flourished in the region. The pollen record suggests that the regional climate became cooler and wetter after 4000 cal yr BP, which may explain the decline of the Neolithic pile dwelling settlement Naumovo at Zhizhitskoye Lake around 3800 cal yr BP. Evidence for human impact on natural forest vegetation becomes visible from 5000 cal yr BP, although it remains at modest levels until the end of the Neolithic (3500–3000 cal yr BP). Decline in arboreal pollen percentages intensified after ca. 2800 cal yr BP, indicating the onset of increased deforestation for pasture, farmland, construction material and fuel by Iron Age populations. This is in line with the foundation of the fortified Iron Age Anashkino settlement north of Zhizhitskoye Lake dated to around 2750 cal yr BP. Forest clearance further increased during the second half of the Iron Age (5th century BCE–5th century CE) and again from ca. 1200 cal yr BP after the onset of the Middle Ages.

Early, intensive marine resource exploitation by Middle Stone Age humans at Ysterfontein 1 rockshelter, South Africa

Modern human behavioral innovations from the Middle Stone Age (MSA) include the earliest indicators of full coastal adaptation evidenced by shell middens, yet many MSA middens remain poorly dated. We apply 230Th/U burial dating to ostrich eggshells (OES) from Ysterfontein 1 (YFT1, Western Cape, South Africa), a stratified MSA shell midden. 230Th/U burial ages of YFT1 OES are relatively precise (median ± 2.7%), consistent with other age constraints, and preserve stratigraphic principles. Bayesian age-depth modeling indicates YFT1 was deposited between 119.9 to 113.1 thousand years ago (ka) (95% CI of model ages), and the entire 3.8 m thick midden may have accumulated within ∼2,300 y. Stable carbon, nitrogen, and oxygen isotopes of OES indicate that during occupation the local environment was dominated by C3 vegetation and was initially significantly wetter than at present but became drier and cooler with time. Integrating archaeological evidence with OES 230Th/U ages and stable isotopes shows the following: 1) YFT1 is the oldest shell midden known, providing minimum constraints on full coastal adaptation by ∼120 ka; 2) despite rapid sea-level drop and other climatic changes during occupation, relative shellfish proportions and sizes remain similar, suggesting adaptive foraging along a changing coastline; 3) the YFT1 lithic technocomplex is similar to other west coast assemblages but distinct from potentially synchronous industries along the southern African coast, suggesting human populations were fragmented between seasonal rainfall zones; and 4) accumulation rates (up to 1.8 m/ka) are much higher than previously observed for dated, stratified MSA middens, implying more intense site occupation akin to Later Stone Age middens.

An uncertainty-focused database approach to extract spatiotemporal trends from qualitative and discontinuous lake-status histories

Changes in lake status are often interpreted as palaeoclimate indicators due to their dependence on precipitation and evaporation. The Global Lake Status Database (GLSDB) has since long provided a standardised synopsis of qualitative lake status over the last 30,000 14C years. Potential sources of uncertainty however are not recorded in the GLSDB. Here we present an updated and improved relational-database framework that incorporates uncertainty in both chronology and the interpretation of palaeoenvironmental data. The database uses peer-reviewed palaeolimnological studies to produce a consensus on qualitative lake-status histories, whose chronologies are revised and standardized through the recalibration of radiocarbon dates and the application of Bayesian age-depth modelling for stratigraphic archives. Quantitative information on absolute water-level elevation is preserved if available from geomorphological sources. We also propose a new probabilistic analytical framework that accounts for these uncertainties to reconstruct synoptic, integrated environmental signals. The process is based on a Monte Carlo algorithm that iteratively samples individual lake-status histories within the limits of their uncertainties to produce many possible scenarios. We then use Recursively-Subtracted Empirical Orthogonal Function analysis to extract dominant patterns of lake-status variability from these scenarios.As a proof of concept, we apply this framework to 67 sites in eastern and southern Africa whose lake-status histories cover part of the late Pleistocene and/or Holocene. We show that, despite the sometimes large temporal and interpretation uncertainties, and the inclusion of highly discontinuous lake-status time series, identifying the major known millennial-scale climatic phases during the last 20,000 years is possible. Our framework was also able to identify an antiphased response between the lake basins in eastern and interior southern Africa to these changes. We propose that our new database and methodology framework serves as a template for efficient lake-status data synthesis, encourages the incorporation of lake-status data in palaeoclimate syntheses, and expands the possibilities for the use of such data in the evaluation of climate models.

Chronological Assessment of the Balta Alba Kurgan Loess-Paleosol Section (Romania) – A Comparative Study on Different Dating Methods for a Robust and Precise Age Model

Loess-paleosol sequences (LPSs) are important terrestrial archives of paleoenvironmental and paleoclimatic information. One of the main obstacles for the investigation and interpretation of these archives is the uncertainty of their age-depth relationship. In this study, four different dating techniques were applied to the Late Pleistocene to Holocene LPS Balta Alba Kurgan (Romania) in order to achieve a robust chronology. Luminescence dating includes analysis of different grain-size fractions of both quartz and potassium feldspar and the best results are obtained using fine-grained quartz blue‐stimulated and polymineral post-infrared infrared-stimulated luminescence measurements. Radiocarbon (14C) dating is based on the analysis of bulk organic carbon (OC) and compound-specific radiocarbon analysis (CSRA). Bulk OC and leaf wax-derived n-alkane 14C ages provide reliable age constraints for the past c. 25–27 kyr. CSRA reveals post-depositional incorporation of roots and microbial OC into the LPS limiting the applicability of 14C dating in older parts of the sequence. Magnetic stratigraphy data reveal good correlation of magnetic susceptibility and the relative paleointensity of the Earth’s magnetic field with one another as well as reference records and regional data. In contrast, the application of paleomagnetic secular variation stratigraphy is limited by a lack of regional reference data. The identification of the Campanian Ignimbrite/Y-5 tephra layer in the outcrop provides an independent time marker against which results from the other dating methods have been tested. The most accurate age constraints from each method are used for two Bayesian age-depth modeling approaches. The systematic comparison of the individual results exemplifies the advantages and disadvantages of the respective methods. Taken as a whole, the two age-depth models agree very well, our study also demonstrates that the multi-method approach can improve the accuracy and precision of dating loess sequences.

Multi-profile fine-resolution palynology of Late Mesolithic to Bronze Age peat at Cat Stones, Rishworth Moor, Central Pennines, UK

Palynological data from three radiocarbon dated peat profiles at Rishworth Moor in the Pennine hills of northern England provide a record of vegetation change and human impacts in the Late Mesolithic, Neolithic and Bronze Age periods. All three cultures have archaeological sites present in the vicinity, with Late Mesolithic sites by far the most abundant, comprising mainly very late assemblages dominated by ‘rod’-shaped microliths. Pollen evidence of vegetation disturbance occurs during all three archaeological periods, and is attributed to the results of human activity. Microscopic charcoal and non-pollen palynomorph analyses support the pollen evidence. Sites at Cat Stones 2 and Cat Stones 3 record evidence of Neolithic and Bronze Age date only. Cat Stones 1 extends into the Late Mesolithic period, and fine-resolution pollen analyses have been applied in this profile to the Mesolithic–Neolithic transition leading up to the mid-Holocene Elm ( Ulmus) Decline, which starts here at c. 494014C BP (5670 cal. BP), a typical date for it in the central Pennines. Bayesian age-depth modelling provides precise ages for the vegetation changes and their spatial correlation. The plateau was dominated by scrub and grassy Calluna heath from the Late Mesolithic onwards. Elevated microscopic charcoal levels and ruderal herb pollen record phases of fire disturbance below the Elm Decline, including a grain of Hordeum cereal-type. Whether this grain represents early cultivation requires further research. Bronze Age impacts are of greater intensity, but disturbances of all three cultural periods are low scale, agreeing with the results of previous research in the Pennines.

Chronostratigraphy of a 1.5±0.1 Ma composite sedimentary record from Colônia basin (SE Brazil): Bayesian modeling based on paleomagnetic, authigenic 10Be/9Be, radiocarbon and luminescence dating

We present a new 52 m composite record (COL17c) composed of five sedimentary sequences retrieved from the Colonia basin (Southeastern Brazil). The COL17c record is composed of two main sub-units deposited under different regimes: a peatland above ∼14 m and lacustrine sediments below. Sedimentary description and core scanning (MSCL and XRF) are first used to align individual cores in order to produce a nearly continuous composite record. We then established an original chronological framework based on radiocarbon, paleomagnetic, optically stimulated luminescence (OSL), post-infrared infrared stimulated luminescence at elevated temperature (pIRIR290°C) and authigenic 10Be/9Be dating. We describe the protocols used for each technique and discuss the positive and negative results, as well as their implications for proxy interpretation. In spite of negative results of some techniques, mainly due to the absence of adequate sample material, the multi-proxy approach used is essential to maximize the chances of obtaining a satisfactory age model for this unique continental sedimentary sequence. All valid chronological proxies, comprising three AMS 14C ages, one pIRIR290°C age, two paleomagnetic constraints and eight authigenic 10Be/9Be datum, are integrated within a Bayesian age-depth modeling to produce a 1.5 ± 0.1 Ma chronology for the whole COL17c sequence. This age model represents the first long-term chronological estimate of sediments deposition in the Colonia basin. The record allows to refine the age of the Colonia basin formation between 5.3 and 11.2 Ma, much older than previously estimated from short sedimentary cores.

Insights into subtropical Australian aridity from Welsby Lagoon, north Stradbroke Island, over the past 80,000 years

Terrestrial sedimentary archives that record environmental responses to climate over the last glacial cycle are underrepresented in subtropical Australia. Limited spatial and temporal palaeoenvironmental record coverage across large parts of eastern Australia contribute to uncertainty regarding the relationship between long-term climate change and palaeoecological turnover; including the extinction of Australian megafauna during the late Pleistocene. This study presents a new, high-resolution, calibrated geochemical record and numerical dating framework from Welsby Lagoon, a wetland from North Stradbroke Island that records key periods of late Pleistocene environmental change. Single-grain optically stimulated luminescence and radiocarbon dating are integrated into a Bayesian age-depth model for the sedimentary sequence spanning Marine Isotope Stage (MIS) 5 to the present. Scanning micro X-ray fluorescence (XRF) and bulk sediment XRF assays are used to infer past dust dynamics, with changes in the abundance of silica and potassium interpreted as proxies for aridity across local and regional sources. Variations in dust flux were contemporaneous with hydrological change, concordant with changes in vegetation cover on the island and, relate to deflation events at major dust source regions on the Australian continent. The Welsby Lagoon record supports the notion of a variable MIS4 within which an increased dust flux (71–67 ka), may be indicative of drier climate. Additionally, the record also shows a lower dust flux through the Last Glacial Maximum (LGM) than is evident in other Australian aeolian records. However, this low LGM flux is attributed to the wetland’s evolution, rather than a reduction in total dust flux.

A taxonomically harmonized and temporally standardized fossil pollen dataset from Siberia covering the last 40 kyr

Abstract. Pollen records from Siberia are mostly absent in global or Northern Hemisphere synthesis works. Here we present a taxonomically harmonized and temporally standardized pollen dataset that was synthesized using 173 palynological records from Siberia and adjacent areas (northeastern Asia, 42–75∘ N, 50–180∘ E). Pollen data were taxonomically harmonized, i.e. the original 437 taxa were assigned to 106 combined pollen taxa. Age–depth models for all records were revised by applying a constant Bayesian age–depth modelling routine. The pollen dataset is available as count data and percentage data in a table format (taxa vs. samples), with age information for each sample. The dataset has relatively few sites covering the last glacial period between 40 and 11.5 ka (calibrated thousands of years before 1950 CE) particularly from the central and western part of the study area. In the Holocene period, the dataset has many sites from most of the area, with the exception of the central part of Siberia. Of the 173 pollen records, 81 % of pollen counts were downloaded from open databases (GPD, EPD, PANGAEA) and 10 % were contributions by the original data gatherers, while a few were digitized from publications. Most of the pollen records originate from peatlands (48 %) and lake sediments (33 %). Most of the records (83 %) have ≥3 dates, allowing the establishment of reliable chronologies. The dataset can be used for various purposes, including pollen data mapping (example maps for Larix at selected time slices are shown) as well as quantitative climate and vegetation reconstructions. The datasets for pollen counts and pollen percentages are available at https://doi.org/10.1594/PANGAEA.898616 (Cao et al., 2019a), also including the site information, data source, original publication, dating data, and the plant functional type for each pollen taxa.

Effects of spatially variable accommodation rate on channel belt distribution in an alluvial sequence: Authigenic 10Be/9Be-based Bayesian age-depth models applied to the upper Miocene Volkovce Fm. (northern Pannonian Basin System, Slovakia)

Relationships between spatially variable accommodation rate and the distribution of channel belt deposits are important issues in alluvial stratigraphy. This segment of sedimentology still records important progress using experimental models and field examples. This study focuses on the alluvial stratigraphy of the Volkovce Fm. which was deposited between ~9.0–6.0 Ma in the northern Pannonian Basin System, and it provides an excellent natural laboratory to study this topic. The spatial variability of channel belt distribution was determined in 809 borehole logs drilled to 200–300 m depths while the facies associations were defined on the outcrops. A significant variance in channel belt content was observed, since it reaches 10–40% of the stratigraphic column in the southwestern part of the study area and only 0–15% in the northeastern portion. Moreover, the transition between the areas of different channel belt distribution is oriented perpendicular to the paleo-drainage network axis. Geochronology was then established by applying the authigenic 10Be/9Be dating method to three borehole cores. The Bayesian age-depth models used herein proved a decrease in average accommodation rates from ~200 m/Myr to ~50 m/Myr towards the area with <15% of channel belts. The results indicate that the four times higher subsidence caused dominant deposition of channel belts in the area of maximum subsidence and avoided the basin-scale avulsion to relocate the channels in the area of lower accommodation rate. This was most likely caused by topographic confinement. The deposition of poorly-drained floodplain and swamp facies appeared in the area of lower accommodation rate, where absence of channels caused low sediment supply. The study highlights that authigenic 10Be/9Be dating Bayesian age-depth models can provide a strong geochronological framework for the late Cenozoic alluvial depositional record necessary for interpretation of the influence of allogenic factors on stratigraphic architecture.

A Multi-Proxy Reconstruction of Environmental Change in the Vicinity of the North Bay Outlet of Pro-Glacial Lake Algonquin

We present a multi-proxy study of environmental conditions during and after the recessional phases of pro-glacial Lake Algonquin in the vicinity of the North Bay outlet, Great Lakes Basin. Data presented comes from a new sedimentary profile obtained from the Balsam Creek kettle lake c. 34 km north-east of the city of North Bay. This site lies close to the north-east margin of the maximum extent of the post-Algonquin lake sequence, which drained through the Ottawa-Mattawa valley system. Our data are presented against a Bayesian age-depth model, supporting and extending regional understanding of vegetation succession in this part of north-east Ontario. The core profile provides a minimum age for the formation of the glacial outwash delta in which the kettle is set, as well as tentative timing for the Payette (post-Algonquin) lake phase. We highlight two discrete intervals during the Early Holocene, with modelled mean ages of: 8475–8040 cal. BP (332–316 cm) and 7645 cal. BP (286 cm), when climatic aridity affected the growth of vegetation within the kettle vicinity. Association with volcanic activity is posited. Cryptotephra dating to 7660–7430 cal. BP (mean age: 7580 cal. BP) is chronologically and geochemically assigned to the Mazama climactic eruption, while an earlier ash accumulation 8710–7865 cal. BP is tentatively sourced to an unknown eruption also in the Cascades region of Oregon. Outside of these periods, the Balsam Creek sequence shows considerable habitat stability and a character akin to that seen at more southerly latitudes. On this evidence we propose that access to reliable resources within kettle features could have aided the initial colonisation of northern Ontario’s environmentally dynamic early post-glacial landscape.

Sediment Cores from White Pond, South Carolina, contain a Platinum Anomaly, Pyrogenic Carbon Peak, and Coprophilous Spore Decline at 12.8 ka

A widespread platinum (Pt) anomaly was recently documented in Greenland ice and 11 North American sedimentary sequences at the onset of the Younger Dryas (YD) event (~12,800 cal yr BP), consistent with the YD Impact Hypothesis. We report high-resolution analyses of a 1-meter section of a lake core from White Pond, South Carolina, USA. After developing a Bayesian age-depth model that brackets the late Pleistocene through early Holocene, we analyzed and quantified the following: (1) Pt and palladium (Pd) abundance, (2) geochemistry of 58 elements, (3) coprophilous spores, (4) sedimentary organic matter (OC and sedaDNA), (5) stable isotopes of C (δ13C) and N (δ15N), (6) soot, (7) aciniform carbon, (8) cryptotephra, (9) mercury (Hg), and (10) magnetic susceptibility. We identified large Pt and Pt/Pd anomalies within a 2-cm section dated to the YD onset (12,785 ± 58 cal yr BP). These anomalies precede a decline in coprophilous spores and correlate with an abrupt peak in soot and C/OC ratios, indicative of large-scale regional biomass burning. We also observed a relatively large excursion in δ15N values, indicating rapid climatic and environmental/hydrological changes at the YD onset. Our results are consistent with the YD Impact Hypothesis and impact-related environmental and ecological changes.

Reef shallowing is a critical control on benthic foraminiferal assemblage composition on nearshore turbid coral reefs

Declining water quality represents a major threat to the coral reefs of Australia's Great Barrier Reef (GBR). Due to their close proximity to river point sources, reefs situated on the inner-shelf of the GBR are widely perceived to be most susceptible to the increased discharge of nutrients and sediments from coastal catchments, many of which have been modified since European settlement. However, the impact and magnitude of water quality change on the GBR's inner-shelf reefs remains unclear. Much of the uncertainty can be attributed to a paucity of long-term records with which to assess ecological and environmental change over appropriate reef-building timescales. Here, we present benthic foraminiferal palaeo-records from three proximal nearshore turbid-zone reefs located within the central region of the GBR. Bayesian age-depth modelling was used to identify the core intervals corresponding to the timing of European settlement in the region (c. 1850 CE), enabling the investigation of: (1) the composition and variability of benthic foraminiferal assemblages during reef shallowing towards sea level; and (2) whether any signal of increased nutrient and sediment inputs, as the result of historical land-use change, can be discerned on the most nearshore reefs of the central GBR. Multivariate analyses identified two assemblage groups, delineated by a significant increase in the relative abundance of Pararotalia spp. up-core. Our results suggest that post-European settlement associated increases in nutrient and sediment inputs are unlikely to have driven the observed shifts in benthic foraminiferal assemblage composition. Rather, we interpret changes in the composition of benthic foraminiferal assemblages as being linked to changes in hydrodynamic energy, light availability and the carbonate content of reef-matrix sediments during reef shallowing towards sea level. Our findings support the hypothesis that nearshore turbid-zone reefs have a higher resistance to increased nutrient and sediment inputs than those located further offshore, towards the inner/mid-shelf boundary of the GBR.

An improved approach to age-modeling in deep time: Implications for the Santa Cruz Formation, Argentina

AbstractAccurate age-depth models for proxy records are crucial for inferring changes to the environment through space and time, yet traditional methods of constructing these models assume unrealistically small age uncertainties and do not account for many geologic complexities. Here we modify an existing Bayesian age-depth model to foster its application for deep time U-Pb and 40Ar/39Ar geochronology. More flexible input likelihood functions and use of an adaptive proposal algorithm in the Markov Chain Monte Carlo engine better account for the age variability often observed in magmatic crystal populations, whose dispersion can reflect inheritance, crystal residence times and daughter isotope loss. We illustrate this approach by calculating an age-depth model with a contiguous and realistic uncertainty envelope for the Miocene Santa Cruz Formation (early Miocene; Burdigalian), Argentina. The model is calibrated using new, high-precision isotope dilution U-Pb zircon ages for stratigraphically located interbedded tuffs, whose weighted mean ages range from ca. 16.78 ± 0.03 Ma to 17.62 ± 0.03 Ma. We document how the Bayesian age-depth model objectively reallocates probability across the posterior ages of dated horizons, and thus produces better estimates of relative ages among strata and variations in sedimentation rate. We also present a simple method to propagate age-depth model uncertainties onto stratigraphic proxy data using a Monte Carlo technique. This approach allows us to estimate robust uncertainties on isotope composition through time, important for comparisons of terrestrial systems to other proxy records.

New bracketing luminescence ages constrain the Sima de los Huesos hominin fossils (Atapuerca, Spain) to MIS 12

Recent chronological studies of the Sima de los Huesos (SH) hominin fossil site, Atapuerca, Spain, have established a close minimum age of at least 430 ka for sedimentary material immediately overlying the human remains. However, a firm maximum age limit still needs to be established for the SH fossils in order to better constrain the timing for the onset of Neandertal speciation. In the present study, we address this important chronological gap at SH by providing direct ages for the sediment deposits that host, and immediately underlie, the hominin fossils. Depositional ages were obtained using single-grain thermally-transferred optically stimulated luminescence (TT-OSL), a technique that has yielded reliable ‘extended-range’ luminescence chronologies at several independently dated Atapuerca sites. Four single-grain TT-OSL depositional ages of 453 ± 56 ka, 437 ± 38 ka, 457 ± 41 ka and 460 ± 39 ka were obtained for the red clay lithostratigraphic units (LU-5 and LU-6) found underlying and encasing the SH hominin bones. A Bayesian age-depth model was constructed using previously published chronologies, as well as the new single-grain TT-OSL ages for LU-5 and LU-6, in order to derive combined age estimates for individual lithostratigraphic units preserved at SH. The combined modeled ranges reveal that the hominin-bearing layer (LU-6) was deposited between 455 ± 17 ka and 440 ± 15 ka (mean lower and upper boundary 68.2% probability range ± 1σ uncertainty, respectively), with a mean age of 448 ± 15 ka. These new bracketing ages suggest that the hominin fossils at SH were most likely deposited within Marine Isotope Stage (MIS) 12, enabling more precise temporal constraint on the early evolution of the Neandertal lineage. The SH fossils represent the oldest reliably dated hominin remains displaying Neandertal features across Eurasia. These Neandertal features are first observed in the facial skeleton, including the mandible and teeth, as well as the temporomandibular joint, and appear consistently across the SH collection. Our chronological findings suggest that the appearance of these Neandertal traits may have been associated with the climatic demise of MIS 12 and the ecological changes that occurred in Iberia during this period. Other Middle Pleistocene hominin fossils from Europe dated to MIS 12–11, or later, show different morphological trends, with some lacking Neandertal specializations. The latest SH dating results enable improved temporal correlations with these contrasting hominin records from Europe, and suggest a complex picture for hominin evolution during the Middle Pleistocene.

Methodological approach for dating harbor sediments by using luminescence dating—a case study in Ephesus, Western Turkey

For this study, a 15-m sediment core from the Roman harbor of Ephesus has been dated with the luminescence technique. 14C age estimates from the same and a neighboring core were used for comparing and validating the luminescence dating results via a single aliquot regenerative dose (SAR) protocol: infrared stimulation of polymineral and blue stimulation of quartz. The results reveal new insights into the deposition of the sediments of the Roman harbor from 2000 BC to AD 1500 approximately. Some samples showed incomplete bleaching. The difference between the polymineral and the quartz techniques described the bleaching condition before burial. When both approaches give consistent ages, complete bleaching can be assumed. Consistent ages were further applied to the Bayesian age-depth model and discussed within the archeological context. Based on the Bayesian age-depth model, a high sedimentation rate of ca. 65 mm/year was calculated for the period of ~ 700–400 BC suggesting a fast advance of the delta front. However, other age estimates suggest a fast advance for the period 200 BC to ca. AD 100. This is probably due to incomplete bleaching of the luminescence samples and the use of bulk samples for 14C for this new study. Comparing the results with nearby cores suggests that dredging was conducted in the southern part of the Roman harbor at a depth of ca. 5 m b.s.l.

Millennial- and centennial-scale droughts at the northern margin of the East Asian summer monsoon during the last deglaciation: Sedimentological evidence from Dali Lake

The semi-arid areas at the northern margin of the East Asian summer monsoon (EASM) have experienced an increased frequency of drought in recent decades. However, it is unclear whether past monsoonal precipitation in the region were determined mainly by El Niño-Southern Oscillation (ENSO) variability or high-latitude cooling. In this study, we use a high-resolution grain-size dataset combined with a new Bayesian age–depth model of a sediment core from Dali Lake to analyze the occurrence of millennial- and centennial-scale droughts at the EASM margin during the interval of climatic warming during the last deglaciation. The polymodal grain-size distributions are partitioned using a log-normal distribution function fitting method. High percentages of the coarse components (C4+C5+C6) are recognized as proxy indicators of strong aeolian activities and low lake levels, and thus droughts in the region. The drought events indicated thereby are generally accompanied by decreases in regional temperature, catchment surface runoff and bio-productivity, on both millennial and centennial timescales. These results imply that the droughts at the EASM margin were caused by a significantly weakened EASM intensity. In addition, the droughts may be linked, within the age uncertainties, to the Heinrich 1 (H1) and Younger Dryas (YD) events on millennial timescales, and to the Older Dryas (OD) cold event on centennial timescales, implying strong high-latitude forcing. Given that these cold reversals over northern high latitudes were induced by the rapid input of glacial melt-water to the North Atlantic during the last deglaciation, it is possible that ongoing climatic warming, and the resulting high northern-latitude ice-sheet melting, may cause an increased incidence of drought in northern China.