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

Trachsel, Mathias; Telford, Richard J.

doi:10.1177/0959683616675939

Cited by 97 publications

(99 citation statements)

References 29 publications

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“…They also found that the impact of different values for thickness was dependent on acc.shape, the accumulation shape prior. As Blaauw and Christen (2011) did not explicitly make any recommendations for how to choose an appropriate value for thickness, Trachsel and Telford (2017) suggested that the length be shorter than the mean distance between dated intervals and to choose a value that allowed for faster model convergence.…”

Section: S5 Parameters and Priors Used For Bacon Age-depth Modelmentioning

confidence: 99%

U-Th dating of lake sediments: Lessons from the 700 kyr sediment record of Lake Junín, Peru

Chen¹,

McGee²,

Woods³

et al. 2019

Preprint

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*----- NOTE: This is a non-peer reviewed preprint. The manuscript has been submitted as an invited research article to Quaternary Science Reviews and is under peer review. -----* Deep sediment cores from long-lived lake basins are fundamental records of paleoenvironmental history, but the power of these reconstructions has often been limited by poor age control. Uranium-thorium (U-Th) dating has the potential to fill a gap in current geochronological tools available for such sediment archives. We present our systematic approach to U-Th date carbonate-rich sediments from the ~100-m-long drill core from Lake Junín, Peru. The results form the foundation of an age-depth model spanning ~700 kyrs. High uranium concentrations (0.3--4 ppm) of these sediments allow us to date smaller quantities of material, giving us the opportunity to improve sample selection by avoiding detrital contamination, the greatest limiting factor to the success of previous U-Th dating efforts in other lake basins. The dates from 174 analyses on 55 bulk carbonate samples revealed significant scatter that could not be resolved with traditional isochrons, suggesting that at least some of the sediments have not remained closed systems. To understand the source of noise in the geochronological data, we first apply threshold criteria that screen samples by their U/Th ratio, reproducibility, and d234U initial value. We then compare these results with facies types, trace element concentrations, carbonate and total organic carbon content, color reflectance, mineralogy, and ostracode shell color to investigate the causes of open system behavior. We find that the greatest impediment to U-Th dating of these sediments is not detrital contamination, but rather post-depositional remobilization of uranium. After examining U-Th data in these contexts, we identify samples that have likely experienced the least amount of alteration, and use dates from those samples as constraints for the age-depth model. Our work has several lessons for future attempts to U-Th date lake sediments, namely that the sample's geologic context is equally important as the accuracy and precision of analytical measurements when determining the age of sample materials. In addition, we caution that significant geologic scatter may remain undetected if not for labor intensive tests of reproducibility achieved through replication. As a result of this work, the deep sediment core from Lake Junín is the only continuous record in the tropical Andes spanning multiple glacial cycles that is constrained entirely by independent radiometric dates. As such, this record is uniquely poised to offer new insights on past climate and environmental changes in the tropical Andes, complementing and testing the long but tuned sediment records from Sabana de Bogotá to the north (~5°S; Groot et al., 2011) and Lake Titicaca to the south (~16°S; Fritz et al., 2004, 2007).

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Section: S5 Parameters and Priors Used For Bacon Age-depth Modelmentioning

confidence: 99%

U-Th dating of lake sediments: Lessons from the 700 kyr sediment record of Lake Junín, Peru

Chen¹,

McGee²,

Woods³

et al. 2019

Preprint

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“…Despite the benefits of low-resolution archives, these records often contain relatively large errors associated with radiometric dating techniques (Anchukaitis and Tierney, 2012;Moberg et al, 2005) and uncertainty associated with using age-depth models to predict ages of undated layers (Trachsel and Telford, 2017;Telford et al, 2004). Age ambiguity leads to difficulty in cross-correlating records.…”

Section: Caveats On Resolving Changes From Low-resolution Sedimentarymentioning

confidence: 99%

Low-resolution Australasian palaeoclimate records of the last 2000 years

et al. 2017

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Abstract. Non-annually resolved palaeoclimate records in the Australasian region were compiled to facilitate investigations of decadal to centennial climate variability over the past 2000 years. A total of 675 lake and wetland, geomorphic, marine, and speleothem records were identified. The majority of records are located near population centres in southeast Australia, in New Zealand, and across the maritime continent, and there are few records from the arid regions of central and western Australia. Each record was assessed against a set of a priori criteria based on temporal resolution, record length, dating methods, and confidence in the proxy-climate relationship over the Common Era. A subset of 22 records met the criteria and were endorsed for subsequent analyses. Chronological uncertainty was the primary reason why records did not meet the selection criteria. New chronologies based on Bayesian techniques were constructed for the high-quality subset to ensure a consistent approach to age modelling and quantification of age uncertainties. The primary reasons for differences between published and reconstructed age-depth models were the consideration of the non-singular distribution of ages in calibrated 14 C dates and the use of estimated autocorrelation between sampled depths as a constraint for changes in accumulation rate. Existing proxies and reconstruction techniques that successfully capture climate variability in the region show potential to address spatial gaps and expand the range of climate variables covering the last 2000 years in the Australasian region. Future palaeoclimate research and records in Australasia could be greatly improved through three main actions: (i) greater data availability through the public archiving of published records; (ii) thorough characterisation of proxy-climate relationships through site monitoring and climate sensitivity tests; and (iii) improvement of chronologies through coretop dating, inclusion of tephra layers where possible, and increased date density during the Common Era.

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“…Although classical age-depth modeling may underestimate the uncertainties inherent to radiocarbon dating (Blaauw et al 2018), we decided to continue using this approach because gaining realistic influx values (charcoal and dung fungal spores) is crucial for this study. Indeed, the Bayesian routines currently available provide unrealistic deposition time estimates because they are based on piecewise linear models that create large jumps around the radiocarbon-dated levels (Trachsel and Telford 2017). However, we considered the 95% confidence intervals of the estimated ages obtained with Bacon 2.3.6 (Brezoso, default settings; Viñuelas, thick = 2, acc.mean = 50, acc.shape = 1.3), a Bayesian approach to age-depth modeling that takes into account chronological ordering and sedimentation by sampling from gamma and beta distributions (Blaauw and Christen 2011), instead of those provided by clam.…”

Section: Coring Radiocarbon Dating and Age-depth Modelingmentioning

confidence: 99%

Unprecedented herbivory threatens rear‐edge populations of Betula in southwestern Eurasia

Morales‐Molino

Tinner

Perea

et al. 2019

Ecology

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Mediterranean rear‐edge populations of Betula, located at the southwestern Eurasian margin of the distribution range, represent unique reservoirs of genetic diversity. However, increasing densities of wild ungulates, enhanced dryness, and wildfires threaten their future persistence. A historical perspective on the past responses of these relict populations to changing herbivory, fire occurrence and climatic conditions may contribute to assessing their future responses under comparable scenarios. We have reconstructed vegetation and disturbance (grazing, fire) history in the Cabañeros National Park (central‐southern Spain) using the paleoecological records of two small mires. We particularly focused on the historical range of variation in disturbance regimes, and the dynamics of rear‐edge Betula populations and herbivore densities. Changes in water availability, probably related to the North Atlantic Oscillation (NAO) index, and land‐use history have played a crucial role in vegetation shifts. Our data suggest that heathlands (mainly Erica arborea and E. scoparia) and Quercus woodlands dominated during dry phases while Sphagnum bogs and Betula stands expanded during wet periods. Betula populations survived past moderately dry periods but were unable to cope with enhanced land use, particularly increasing livestock raising since ~1,100–900 cal. yr BP (850–1,050 CE), and eventually underwent local extinction. High herbivore densities not only contributed to the Betula demise but also caused the retreat of Sphagnum bogs. Ungulate densities further rose at ~200–100 cal. yr BP (1750–1850 CE) associated with the historically documented intensification of land use around the Ecclesiastical Confiscation. However, herbivory reached truly unprecedented values only during the last decades, following rural depopulation and subsequent promotion of big game hunting. For the first time in temperate and Mediterranean Europe, we have used the abundances of fossil dung fungal spores to assess quantitatively that current high herbivore densities exceed the historical range of variation. In contrast, present fire activity lies within the range of variation of the last millennia, with fires (mainly human‐set) mostly occurring during dry periods. Our paleodata highlight the need of controlling the densities of wild ungulates to preserve ecosystem composition and functioning. We also urge to restore Betula populations in suitable habitats where they mostly disappeared because of excessive human activities.

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

Cited by 97 publications

References 29 publications

U-Th dating of lake sediments: Lessons from the 700 kyr sediment record of Lake Junín, Peru

U-Th dating of lake sediments: Lessons from the 700 kyr sediment record of Lake Junín, Peru

Low-resolution Australasian palaeoclimate records of the last 2000 years

Unprecedented herbivory threatens rear‐edge populations of Betula in southwestern Eurasia

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