Bayesian Age-depth Model Research Articles

An OSL-dated, stacked Holocene dust mass accumulation rate record on the Chinese Loess Plateau and its implications for Northern Hemisphere dust activity

Accurate reconstruction of geological dust activity is crucial for understanding past climate change and its interaction with dust cycle. Loess on the Chinese Loess Plateau (CLP) is an ideal and sensitive material for revisiting past dust activity. Previously, the changes in the dust mass accumulation rate (MAR) over various time scales were widely established on the CLP. However, few absolutely dated dust records have been obtained and synthesized for the Holocene. This study addresses this gap by compiling 23 optically stimulated luminescence-dated Holocene loess sections across the CLP. We developed a high-quality chronology via Bayesian age-depth modeling and derived a MAR record for each section. Then, we obtained a mean MAR record for the CLP by stacking individual records. We propose that, in contrast to site-specific MARs, stacked MARs represent the mean dust accumulation conditions for the entire CLP and can be used to track relatively large-scale dust activity and climate change. The stacked MAR record suggests a moderate weakening trend of dust accumulation from ∼11.5 to 7.5 thousand years ago (ka BP), followed by a pronounced strengthening trend from ∼7.5 to 3.0 ka BP. A comparison with other regional dust records reveals a shift in dust activity at ∼8–6 ka BP in northern China and western Mongolia. We argue that the East Asian winter and summer monsoons jointly contributed to Holocene mean dust MAR variations on the CLP by changing dust transport wind energy and dust source aridity, respectively. A comparison of Holocene dust records in Asia with those in the northwestern Pacific Ocean and Greenland suggests asynchronous variations in dust activity between proximal and distal Asia-sourced dust deposition. This is because, unlike proximal deposition, distal deposition can be controlled not only by the dust source conditions but also by the intensity and the position of the Westerlies.

Revised OSL chronology of the Kisiljevo loess-palaeosol sequence: New insight into the dust flux in the eastern Carpathian Basin during MIS 3 - MIS1

This study presents a detailed investigation of the Kisiljevo loess-palaeosol sequence in north-eastern Serbia, offering a refined understanding of its paleoenvironmental dynamics. Contrasting our updated OSL chronology with a previous study, reveals discrepancies, particularly at 400 cm depth, where a considerable age underestimation is evident. While variations in sampling depth and methodology may contribute to some differences, the substantial deviation raises concern about the reliability of the earlier chronology. Our robust age-depth model constructed using Bayesian modelling, and the consistent increase in ages with depth suggest a potential underestimation in the uppermost layer of the earlier study, possibly due to partial bleaching or post-depositional mixing. The Bayesian age-depth model portrays a continuous sedimentation history from the later stages of Marine Isotope Stage 3 (MIS 3) to the present day. The patterns in the calculated Mass Accumulation Rates reveal distinctive peaks during MIS 3 and the middle of MIS 2, deviating from typical dust deposition models. The MIS 3 peaks in dustiness could be attributed to regional factors such as increased transportation rates, enhanced trapping efficiency, or elevated palaeowind intensity. This research not only enhances our understanding of the Kisiljevo LPS but also provides valuable insights into regional paleoenvironmental dynamics. The study emphasizes the importance of considering local geological variations in reconstructing past climates from sediment archives and sets the stage for further investigations into the factors influencing dust deposition in north-eastern Serbia. The MAR trends established here serve as a crucial reference for broader paleoclimatic interpretations in the Carpathian Basin.

A formation record of modern Nitraria tangutorum nebkhas on the Ordos Plateau of northern China based on optically stimulated luminescence dating

Nebkhas, as special biologically mediated aeolian landforms, play an important role in indicating regional aeolian activities and aeolian environmental changes. The Ordos Plateau is located in arid and semiarid regions of northern China that have a wide distribution of Nitraria tangutorum nebkhas. There are few chronological records of nebkha formation, so the history of the formation and evolution of nebkhas in the plateau is still unclear. In addition, the optically stimulated luminescence (OSL) dating method, which has been widely used in the dating of Holocene aeolian sediments, has not yet been applied to the dating of nebkha sediments in the Ordos Plateau. In this study, OSL methods were used to discuss the formation age and relevant aeolian activities of Nitraria tangutorum nebkhas in the Ordos Plateau. The results demonstrated that (1) the coarse-grained quartz OSL signals of nebkha sediments were insensitive and thus were inapplicable by the single aliquot regenerative (SAR) protocol. (2) The luminescence characteristics of the K-feldspar fraction and internal checks of the post-infrared infrared-stimulated (pIRIR) dating indicated that the pIR50IR170 protocol with 200 °C of preheating was applicable for De determinations of nebkha sediments. (3) The residual doses of the pIRIR170 signals ranging from 0.02 to 0.14 Gy should be considered when calculating De values. (4) Bayesian age-depth models were used for the Nitraria tangutorum nebkha sedimentary sequences. The formation of nebkhas occurred during a relatively dry climate period accompanied by strong aeolian activity. That ∼0.38 ka was the main growth period of Nitraria tangutorum nebkhas. This study is the first report of the formation age of nebkhas of more than one hundred years in the region.

Post-glacial small delta process uncovered by luminescence and radiocarbon chronology of core sediments from coastal South China Sea

Reliable chronology is crucial for reconstructing delta processes. The past decades witnessed a boost of dating works on the most economically influential large deltas (i.e., subaerial area >1000 km2), but chronology remains lacking on many small deltas (i.e., subaerial area <1000 km2) that are also densely populated and economically active. Luohe River Delta (LRD) in the coastal South China Sea is such a small delta, whose evolution concerns hundreds of thousands of people, and literally no reliable dating results have been reported to support research of its processes. Herein, dating work combining radiocarbon (14C) on mollusk shells and quartz Optically Stimulated Luminescence (OSL) methods were performed on core LFZK06 from LRD, to test applicability of both methods by age comparison and to establish a chronological framework, using Bayesian age-depth models, of LRD for the first time. 14C ages are systematically c. 0.5–1 ka younger than OSL ages from the same depths. Such young bias of 14C ages in LRD contrasts with previously observed overestimated 14C results in the nearby Pearl River Delta (PRD), likely due to the distinction in bedrock types between Luohe River (granites and sandstones only) and Pearl River (limestone prevalence causing hardwater effect) drainages. Ages of core LFZK06 span from 57 ± 7 ka to 3.42 ± 0.05 cal ka BP and contain a hiatus between 57±7–11.9 ± 1.7 ka. Holocene deltaic sequence of the core shows three-stage sedimentary processes: (1) rapid deposition of prodelta/delta front sediments at 10.2‐0.4+0.5 ka–7.7‐0.3+0.4 ka related to rapid sea level rise, (2) hiatus during 7.7‐0.3+0.4 ka–5.7‐0.8+0.5 ka likely due to reduced sediments input or river channel migration, (3) rapid accumulation of delta plain sediments during 5.7‐0.8+0.5 ka–3.42‐0.85+0.42 ka reflecting depocenter shift toward core location. Moreover, changes of quartz OSL sensitivity were detected, indicative of sediment provenance transition. A change in sediment source from nearby granite weathering towards fluvial long-transported materials occurred at a depth of 24.7 m, with low quartz OSL sensitivity in the gravelly layer below (11.9 ± 1.7 ka) and one magnitude higher quartz OSL sensitivity in prodelta deposits above (10.2‐0.4+0.5 ka).

New constraints on the Middle-Late Pleistocene Campi Flegrei explosive activity and Mediterranean tephrostratigraphy (∼160 ka and 110–90 ka)

The Campi Flegrei (CF) caldera, in southern Italy, is the source of some of the most powerful Late Pleistocene eruptions of the European sub-continent (e.g., Campanian Ignimbrite, Neapolitan Yellow Tuff eruptions). Although the CF caldera has been continuously and intensively investigated for decades, relatively little is known regarding its earliest volcanic activity. In this work, integrating existing and new tephrostratigraphic data, we provide a comprehensive and updated framework for the CF volcanic activity which has occurred at ∼160 ka and between ∼110 ka and ∼90 ka. The new tephrostratigraphic, geochemical (EMPA + LA-ICP-MS), chronological (40Ar/39Ar dating) and grain-size distribution data relate to CF tephra deposits preserved in mid-proximal (Campanian Plain), distal (Tyrrhenian Sea) and ultra-distal (Lower Danube area) sedimentary archives. Our results allowed us to recognize the presence of at least 13 CF eruptions covering the investigated time frame, with 12 eruptions occurring between 110 and 90 ka. Our high-resolution stratigraphic and chronological investigation also allowed us to recognize that the Triflisco/C-22 tephra, previously considered as a single marker layer, can be actually separated into three different events, sourced from within the CF area in the short time interval of ∼93-90 ka, suggesting a more complex and intense volcanic history than previously thought. Moreover, a Bayesian age-depth model, constrained by previous and new high precision 40Ar/39Ar ages, has led to a reliable estimate of the ages of those undated CF eruptions. Overall, the updated framework on the stratigraphy, chronology, dispersion, and geochemistry of the CF tephra of ∼160 ka and between 110 ka and 90 ka consolidates the notion that the Middle-Late Pleistocene activity in theCF area represents a significant stage of its volcanic evolution, characterised by intense and frequent explosive eruptions.

A 121‐ka record of Western Andean fluvial response to suborbital climate cycles recorded by rhythmic grain size variations of the Lima fluvial fan

AbstractA complete, fluvial stratigraphic record for the last glacial period of the Western Andes in Peru is not available due to preservation issues and spatial variability in sedimentation. Deposits are typically restricted to incomplete records of fluvial terraces or localised occurrences of alluvial fans and landslides. These landforms are thought to have formed under a regime of climate cyclicity controlling increases in precipitation. Because of the fragmented preservation of these deposits, as well as dating uncertainties, it remains unclear if orbital climate cycles, such as the precession cycle, or suborbital cycles, such as the wet Heinrich events, are driving Andean sedimentation. In this paper, we try to answer this question through a sedimentological–stratigraphical analysis of a much more complete sedimentary sequence than usually found in the region. We present the results of a grain size analysis of 5000 clasts and 13 new luminescence ages of a 52‐m‐long, stratigraphic section of the Lima fluvial fan in Peru. Bayesian age–depth modelling resulted in a robust chronostratigraphic framework and derived sedimentation rates. The stratigraphic record registered sedimentation from 121.7 ± 4 to ka. Three major sedimentation periods occurred between 121.7 to , 87 ± 1 to , and to ka. These periods registered various unconformities and coarsening–fining upward sequences which chronologically correlate to suborbital pluvial periods, recognised from speleothems and lake records, that drove fluvial deposition. They also correlate with the timing of other recognised sedimentation events throughout the Western Andes. Marine regression resulted in fan progradation and not in incision. The Lima fan stratigraphy represents therefore the most complete, last glacial fluvial record for the Peruvian Western Andes to date and it highlights the potential of fluvial fans as recorders of suborbital climate variability.

A Bayesian astrochronology for the Cambrian first occurrence of trilobites in West Gondwana (Morocco)

Abstract The first occurrence of trilobites at ca. 520 Ma is an iconic feature of the Cambrian Explosion. Developing a robust evolutionary view on early Cambrian life is generally hindered by large uncertainties in the ages of fossil finds and their global stratigraphic correlation. We developed an astrochronological interpretation for the Tiout section in Morocco that features some of the oldest trilobite fossils. Our novel approach to incorporating individual astronomical cycle durations in an integrated radioisotopic and astrochronological Bayesian age-depth model results in an age estimate of 519.62 Ma (519.70–519.54 Ma 95% highest posterior distribution) for the first occurrence of trilobites in West Gondwana. This level of precise age estimation is exceptional for biological events in deep time and demonstrates the power of our novel approach.

Kettle-hole peatlands as carbon hot spots: Unveiling controls of carbon accumulation rates during the last two millennia

Understanding carbon sequestration patterns in time and space is crucial for models and future projections of carbon uptake. However, their past provides an insight to this understanding, and peatlands play an essential role in the carbon cycle. Hence, peat carbon accumulation rates (PCAR) were reconstructed for the last ±1500 years in three Polish kettle-hole peatlands (Jaczno, Głęboczek and Pawski Ług bogs). Absolute chronologies, retrieved from the Bayesian age-depth models based on high-resolution 14C AMS dating, provided good time control, whereas the selected time interval spanned a broad spectrum of environmental changes and human activity. An additional variable was connected to climate, as the chosen sites are situated along the west-east gradient of northern Poland.The aim was to find factors responsible for changes in carbon accumulation, so the results of PCAR were combined with pollen, testate amoebae, plant macrofossil, and charcoal data. The research showed that the investigated peatlands varied regarding local environmental conditions (e.g. water level), peatland development, vegetation changes, and fire activity. Generally, the PCAR values were higher in the Sphagnum-dominated sections of the profiles with high shares of mixotrophic testate amoebae unless fire hampered the carbon sequestration. However, our research showed that the changes in carbon uptake resulted from various overlapping factors rather than just one. Nevertheless, high values of PCAR in Jaczno and Pawski Ług (mean 74.6 and 79.64 g C/m2/yr, respectively) point to kettle-hole peatlands being exceptionally efficient in carbon sequestration.

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

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

Sedimentary evidence of the Late Holocene tsunami in the Shetland Islands (UK) at Loch Flugarth, northern Mainland

Tsunami deposits around the North Sea basin are needed to assess the long‐term hazard of tsunamis. Here, we present sedimentary evidence of the youngest tsunami on the Shetland Islands from Loch Flugarth, a coastal lake on northern Mainland. Three gravity cores show organic‐rich background sedimentation with many sub‐centimetre‐scale sand layers, reflecting recurring storm overwash and a sediment source limited to the active beach and uppermost subtidal zone. A basal 13‐cm‐thick sand layer, dated to 426–787 cal. a CE based on 14C, 137Cs and Bayesian age–depth modelling, was found in all cores. High‐resolution grain‐size analysis identified four normally graded or massive sublayers with inversely graded traction carpets at the base of two sublayers. A thin organic‐rich ‘mud’ drape and a ‘mud’ cap cover the two uppermost sublayers, which also contain small rip‐up clasts. Grain‐size distributions show a difference between the basal sand layer and the coarser and better sorted storm layers above. Multivariate statistical analysis of X‐ray fluorescence core scanning data also distinguishes both sand units: Zr, Fe and Ti dominate the thick basal sand, while the thin storm layers are high in K and Si. Enriched Zr and Ti in the basal sand layer, in combination with increased magnetic susceptibility, may be related to higher heavy mineral content reflecting an additional marine sediment source below the storm‐wave base that is activated by a tsunami. Based on reinterpretation of chronological data from two different published sites and the chronostratigraphy of the present study, the tsunami seems to date to c. 1400 cal. a BP. Although the source of the tsunami remains unclear, the lack of evidence for this event outside of the Shetland Islands suggests that it had a local source and was smaller than the older Storegga tsunami (8.15 cal. ka BP), which affected most of the North Sea basin.

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

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

Marine inundation history during the last 3000 years at Lake Kogare-ike, a coastal lake on the Pacific coast of central Japan

Sediment cores collected at Lake Kogare-ike, a coastal lake on the Pacific coast of central Japan, record the marine inundation history during the last 3000 years. The sediments consist mainly of organic mud, sand, gravel, inorganic mud, and volcanic ash, and inundation events were recognized as 19 event deposits (E1–E19, from top to bottom) interbedded with the organic mud. Visual observation by naked eyes and X-ray computed tomography (CT) images identified 16 event deposits based on quantitative and qualitative changes in sand contents and changes in the textures and colors of the sediment samples (E1–E3, E5, E6, E8, E9, and E11–E19). The other three event deposits (E4, E7, and E10) were identified only on the CT images as layers with higher radiodensity than the underlying and overlying organic mud layers. The sedimentary features, the spatial bias of the event deposits toward seaward areas, the diatom assemblages, and the frequency of inundation events suggest that 13 (E1–E10 and E12–E14) of the 19 event deposits were formed by tsunamis or extraordinary storms. To constrain the depositional ages of the event deposits, Bayesian age–depth models were constructed based on radiocarbon dating of plant macrofossils and concentrated fossil pollen and the 137Cs profile. The depositional ages of the event deposits indicate that five or possibly six event deposits can be correlated with historical tsunamis along the Nankai Trough: E2, either of E3 or E4, E5, E7, and E9 correspond to the 1707 CE Hoei, the 1605 CE Keicho, the 1498 CE Meio, the 1096 CE Eicho, and the 684 CE Hakuho tsunamis, respectively. E1 was possibly formed by the 1944 CE Showa-Tonankai tsunami, the 1854 Ansei–Tokai tsunami, the 1959 Isewan typhoon, or a combination of two or all three events.

Ultra-distal tephra deposits and Bayesian modelling constrain a variable marine radiocarbon offset in Placentia Bay, Newfoundland

Abstract. Radiocarbon dating marine sediments is complicated by the strongly heterogeneous age of ocean waters. Tephrochronology provides a well-established method to constrain the age of local radiocarbon reservoirs and more accurately calibrate dates. Numerous ultra-distal cryptotephra deposits (non-visible volcanic ash more than 3000 km from source) have been identified in peatlands and lake sediments across north-eastern North America and correlated with volcanic arcs in the Pacific north-west. Previously, however, these isochrons have not been identified in sediments from the north-west Atlantic Ocean. In this study, we report the presence of two ultra-distal cryptotephra deposits; Mazama Ash and White River Ash eastern lobe (WRAe), in Placentia Bay, North Atlantic Ocean. We use these well-dated isochrons to constrain the local marine radiocarbon reservoir offset (ΔR) and develop a robust Bayesian age–depth model with a ΔR that varies through time. Our results indicate that the marine radiocarbon offset in Placentia Bay was -126±151 years (relative to the Marine20 calibration curve) at the time of Mazama Ash deposition (7572 ± 18 yr BP) and −396 ± 144 years at the time of WRAe deposition (1098–1097 yr BP). Changes in ΔR appear to coincide with inferred shifts in relative influences of the inner Labrador Current and the Slopewater Current in the bay. An important conclusion is that single-offset models of ΔR are easiest to apply and often hard to disprove. However, such models may oversimplify reservoir effects in a core, even over relatively short timescales. Acknowledging potentially varying offsets is critical when ocean circulation and ventilation characteristics have differed over time. The addition of tephra isochrons permits the calculation of semi-independent reservoir corrections and verification of the single ΔR model.

High-precision U-Pb geochronology and Bayesian age-depth modeling of the glacial-postglacial transition of the southern Paraná Basin: Detailing the terminal phase of the Late Paleozoic Ice Age on Gondwana

The timing of the terminal deglaciation of the Late Paleozoic Ice Age – LPIA in the Paraná Basin has been under investigation. However, the low availability of high-precision radioisotopic ages prevents the construction of a detailed chronostratigraphic framework necessary to constrain the demise of the glaciation and perform correlation with contemporaneous records across neighboring basins and within the extensive Paraná Basin. This study aims to contribute to the chronostratigraphic detailing of the Carboniferous-Permian succession of the southern Paraná Basin, interval preserving LPIA glacial-postglacial transition. Therefore, we document new high-precision U-Pb radioisotopic ages and reexamine previously published samples through single crystal chemical abrasion, isotope dilution-thermal ionization mass spectrometry (CA-IDTIMS). We integrate all high-precision radioisotopic ages into the first Bayesian age-depth models for the late Carboniferous-early Permian succession recorded in the southern Paraná Basin. Age-depth models were performed on DU-08-RS and HN-120-RS drill cores, located in the Capané and Candiota paleovalleys, respectively. The weighted mean 206Pb/238U date for the Rio Bonito Formation are 297.914 ± 0.10 (T6), 297.495 ± 0.11 (HN), 296.796 ± 0.114 (T1), 296.293 ± 0.098 (T1.5), 296.252 ± 0.117 (T2), and 296.148 ± 0.136 Ma (RB). For the Itararé Group sample, the weighted mean date is 301.809 ± 0.224 Ma (ITA). T1 and T2 high-precision ages turned out to be older than LA-ICP-MS ages published before (Cagliari et al., 2014), and RB and ITA ages younger (Cagliari et al., 2016). Age-depth models constraints the Rio Bonito Formation between Gzhelian and Sakmarian stages, encompassing ~6.9 and ~1.6 Myr in cores DU-08-RS and HN-120-RS, respectively. The results demonstrate that the successions of the Rio Bonito Formation recorded in Capané and Candiota paleovalleys are contemporaneous. They also corroborate to assign the final glaciation to the early Gzhelian (Late Pennsylvanian) and to place the Carboniferous-Permian boundary within the Rio Bonito Formation, in the study area.

Astronomical and tectonic influences on climate and deposition revealed through radioisotopic geochronology and Bayesian age-depth modeling of the early Eocene Green River Formation, Wyoming, USA

The Wilkins Peak Member (WPM) of the Green River Formation in Wyoming, USA, comprises alternating lacustrine and alluvial strata that preserve a record of terrestrial climate during the early Eocene climatic optimum. We use a Bayesian framework to develop age-depth models for three sites, based on new 40Ar/39Ar sanidine and 206Pb/238U zircon ages from seven tuffs. The new models provide two- to ten-fold increases in temporal resolution compared to previous radioisotopic age models, confirming eccentricity-scale pacing of WPM facies, and permitting their direct comparison to astronomical solutions. Starting at ca. 51 Ma, the median ages for basin-wide flooding surfaces atop six successive alluvial marker beds coincide with short eccentricity maxima in the astronomical solutions. These eccentricity maxima have been associated with hyperthermal events recorded in marine strata during the early Eocene. WPM strata older than ca. 51 Ma do not exhibit a clear relationship to the eccentricity solutions, but accumulated 31%−35% more rapidly, suggesting that the influence of astronomical forcing on sedimentation was modulated by basin tectonics. Additional high-precision radioisotopic ages are needed to reduce the uncertainty of the Bayesian model, but this approach shows promise for unambiguous evaluation of the phase relationship between alluvial marker beds and theoretical eccentricity solutions.

Bayesian age–depth modelling applied to varve and radiometric dating to optimize the transfer of an existing high-resolution chronology to a new composite sediment profile from Holzmaar (West Eifel Volcanic Field, Germany)

Abstract. This study gives an overview of different methods to integrate information from a varve chronology and radiometric measurements in the Bayesian tool Bacon. These techniques will become important for the future as technologies evolve with more sites being revisited for the application of new and high-resolution scanning methods. Thus, the transfer of existing chronologies will become necessary because the recounting of varves will be too time consuming and expensive to be funded. We introduce new sediment cores from Holzmaar (West Eifel Volcanic Field, Germany), a volcanic maar lake with a well-studied varve record. Four different age–depth models have been calculated for the new composite sediment profile (HZM19) using Bayesian modelling with Bacon. All models incorporate new Pb-210 and Cs-137 dates for the top of the record, the latest calibration curve (IntCal20) for radiocarbon ages as well as the new age estimation for the Laacher See Tephra. Model A is based on previously published radiocarbon measurements only, while Models B–D integrate the previously published varve chronology (VT-99) with different approaches. Model B rests upon radiocarbon data, while parameter settings are obtained from sedimentation rates derived from VT-99. Model C is based on radiocarbon dates and on VT-99 as several normal distributed tie points, while Model D is segmented into four sections: sections 1 and 3 are based on VT-99 only, whereas sections 2 and 4 rely on Bacon age–depth models including additional information from VT-99. In terms of accuracy, the parameter-based integration Model B shows little improvement over the non-integrated approach, whereas the tie-point-based integration Model C reflects the complex accumulation history of Holzmaar much better. Only the segmented and parameter-based age integration approach of Model D adapts and improves VT-99 by replacing sections of higher counting errors with Bayesian modelling of radiocarbon ages and thus efficiently makes available the best possible and most precise age–depth model for HZM19. This approach will value all ongoing high-resolution investigations for a better understanding of decadal-scale Holocene environmental and climatic variations.

Radiocarbon sampling efforts for high-precision lake sediment chronologies

High-resolution chronologies with the best time control are key for comparing palaeoenvironmental studies with independent high-precision historical, archaeological or climatic data. Precise chronologies are also essential for inter-site comparisons of palaeo records at decadal to centennial time scales. We present an updated sediment chronology from Burgäschisee, a small and well-studied lake in the Swiss lowlands. The new age-depth relationship was generated using a large number of new radiocarbon samples of terrestrial plant remains extracted from the Burgäschisee sediments and Bayesian age-depth modelling. The results reveal 2σ uncertainties of only ±19 years for the entire record covering the Early Bronze Age (3800 cal. BP) to the Early Middle Ages (1150 cal. BP). The differences between four age-depth modelling techniques (Bayesian and non-Bayesian) are minor (around 25 years) and remain stable with lower radiocarbon date availability. The maximum age offset between the preliminary previously published and the refined chronology from Burgäschisee is 225 years. Our results demonstrate the importance of a rigorous subsampling strategy that includes a careful selection of the best terrestrial plant material and avoiding radiocarbon calibration plateaus whenever possible. The new chronology from Burgäschisee now allows a more accurate site-to-site comparison with archaeological, historical and other palaeoecological evidence from the region.

Central Mediterranean tephrochronology between 313 and 366 ka: New insights from the Fucino palaeolake sediment succession

Thirty‐two tephra layers were identified in the time‐interval 313–366 ka (Marine Isotope Stages 9–10) of the Quaternary lacustrine succession of the Fucino Basin, central Italy. Twenty‐seven of these tephra layers yielded suitable geochemical material to explore their volcanic origins. Investigations also included the acquisition of geochemical data of some relevant, chronologically compatible proximal units from Italian volcanoes. The record contains tephra from some well‐known eruptions and eruptive sequences of Roman and Roccamonfina volcanoes, such as the Magliano Romano Plinian Fall, the Orvieto–Bagnoregio Ignimbrite, the Lower White Trachytic Tuff and the Brown Leucitic Tuff. In addition, the record documents eruptions currently undescribed in proximal (i.e. near‐vent) sections, suggesting a more complex history of the major eruptions of the Colli Albani, Sabatini, Vulsini and Roccamonfina volcanoes between 313 and 366 ka. Six of the investigated tephra layers were directly dated by single‐crystal‐fusion 40Ar/39Ar dating, providing the basis for a Bayesian age–depth model and a reassessment of the chronologies for both already known and dated eruptive units and for so far undated eruptions. The results provide a significant contribution for improving knowledge on the peri‐Tyrrhenian explosive activity as well as for extending the Mediterranean tephrostratigraphical framework, which was previously based on limited proximal and distal archives for that time interval.

Effective Moisture Evolution since the Last Glacial Maximum Revealed by a Loess Record from the Westerlies-Dominated Ili Basin, NW China

Moisture variation is extremely relevant for the stability of ecosystems in Central Asia (CA). Therefore, moisture evolution and its potential driving mechanism over the region are always a hot research topic. Although much effort has been devoted to understanding the processes of moisture evolutions in CA during the Quaternary, particularly the Holocene, the associated underlying mechanisms remain in a state of persistent debate. In this study, the granulometry, clay mineral and chroma properties of a loess section (named ZSP section) in the westerlies-dominated Ili Basin, NW China are investigated. With the accelerator mass spectrometry radiocarbon dating (AMS 14C)-based Bayesian age–depth model, we provide a sensitive record of effective moisture evolution since the last glacial maximum (LGM) in the basin, and the results help enhance understanding of the possible driving mechanisms for westerly climate change. Comparisons of clay mineralogy indices shows that the study area is involved in the Northern Hemisphere dust cycle processes as a dust source, and the content of &lt;2 μm grain size fraction in the ZSP section can thereby be used to reflect the westerlies’ intensity. After deducting the complicated influencing factors for lightness changes throughout the section, the calibrated lightness is adopted to indicate the regional effective moisture. Our findings show that effective moisture is relatively abundant during the LGM and the middle–late Holocene, with dry climate conditions during the last deglaciation and early Holocene. We argue that westerlies’ intensity was the main factor for driving the effective moisture evolution in the Ili Basin since the LGM. Local and source evaporation intensity and effective intra-annual control time of the westerlies over the study area exerted a minor influence on the moisture changes.

THE APPLICATION OF POLLEN RADIOCARBON DATING AND BAYESIAN AGE-DEPTH MODELING FOR DEVELOPING ROBUST GEOCHRONOLOGICAL FRAMEWORKS OF WETLAND ARCHIVES

ABSTRACTWetland sediments are valuable archives of environmental change but can be challenging to date. Terrestrial macrofossils are often sparse, resulting in radiocarbon (14C) dating of less desirable organic fractions. An alternative approach for capturing changes in atmospheric 14C is the use of terrestrial microfossils. We 14C date pollen microfossils from two Australian wetland sediment sequences and compare these to ages from other sediment fractions (n = 56). For the Holocene Lake Werri Berri record, pollen 14C ages are consistent with 14C ages on bulk sediment and humic acids (n = 14), whilst Stable Polycyclic Aromatic Carbon (SPAC) 14C ages (n = 4) are significantly younger. For Welsby Lagoon, pollen concentrate 14C ages (n = 21) provide a stratigraphically coherent sequence back to 50 ka BP. 14C ages from humic acid and &gt;100 µm fractions (n = 13) are inconsistent, and often substantially younger than pollen ages. Our comparison of Bayesian age-depth models, developed in Oxcal, Bacon and Undatable, highlight the strengths and weaknesses of the different programs for straightforward and more complex chrono-stratigraphic records. All models display broad similarities but differences in modeled age-uncertainty, particularly when age constraints are sparse. Intensive dating of wetland sequences improves the identification of outliers and generation of robust age models, regardless of program used.