High precision C-14 dating of Holocene peat deposits: A comparison of Bayesian calibration and wiggle-matching approaches

The chronologies of five northern European ombrotrophic peat bogs subjected to a large ANIS C-14 dating effort (32-44 dates/site) are presented here. The results of Bayesian calibration (BCal) of dates with a prior assumption of chronological ordering were compared with a Bayesian wiggle-match approach (Bpeat) which assumes constant linear accumulation over sections of the peat profile. Interpolation of BCal age estimates of dense sequences of C-14 dates showed variable patterns of peat accumulation with time, with changes in accumulation occurring at intervals ranging from 20 to 50 cm. Within these intervals, peat accumulation appeared to be relatively linear. Close analysis suggests that some of the inferred variations in accumulation rate were related to the plant macrofossil composition of the peat. The wiggle-matched age-depth models had relatively high chronological uncertainty within intervals of closely spaced 14 C dates, suggesting that the premise of constant linear accumulation over large sections of the peat profile is unrealistic. Age models based on the assumption of linear accumulation over large parts of a peat core (and therefore only effective over millennial timescales), are not compatible with studies examining environmental change during the Holocene, where variability often occurs at decadal to centennial time-scales. Ideally, future wiggle-match age models should be constrained, with boundaries between sections based on the plant macrofossil composition of the peat and physical-chemical parameters such as the degree of decomposition. Strategies for the selection of material for dating should be designed so that there should be enough C-14 dates to accurately reconstruct the peat accumulation rate of each homogeneous stratigraphic unit. (c) 2006 Elsevier Ltd. All rights reserved.

IntCal09 and Marine09 radiocarbon age calibration curves, 0 - 50,000 years cal BP

The IntCal04 and Marine04 radiocarbon calibration curves have been updated from 12 cal kBP (cal kBP is here defined as thousands of calibrated years before AD 1950), and extended to 50 cal kBP, utilizing newly available data sets that meet the IntCal Working Group criteria for pristine corals and other carbonates and for quantification of uncertainty in both the ¹⁴C and calendar timescales as established in 2002. No change was made to the curves from 0-12 cal kBP. The curves were constructed using a Markov chain Monte Carlo (MCMC) implementation of the random walk model used for IntCal04 and Marine04. The new curves were ratified at the 20th International Radiocarbon Conference in June 2009 and are available in the Supplemental Material at www.radiocarbon.org.

Methods and code for 'classical' age-modelling of radiocarbon sequences.

Age–depth models form the backbone of most palaeoenvironmental studies. However, procedures for constructing chronologies vary between studies, they are usually not explained sufficiently, and some are inadequate for handling calibrated radiocarbon dates. An alternative method based on importance sampling through calibrated dates is proposed. Dedicated R code is presented which works with calibrated radiocarbon as well as other dates, and provides a simple, systematic, transparent, documented and customizable alternative. The code automatically produces age–depth models, enabling exploration of the impacts of different assumptions (e.g., model type, hiatuses, age offsets, outliers, and extrapolation).

Dating peat profiles using tephra: stratigraphy, geochemistry and chronology

In this article we provide a brief overview of the protocols for dating peat profiles using tephrochronology. A standardised methodology for the detection, extraction and analysis of tephras is presented and the relevant problems and limitations are discussed.

High-resolution 14C dating of a 25,000-year lake-sediment record from equatorial East Africa

We dated a continuous, ~22-m long sediment sequence from Lake Challa (Mt. Kilimanjaro area, Kenya/Tanzania) to produce a solid chronological framework for multi-proxy reconstructions of climate and environmental change in equatorial East Africa over the past 25,000 years. The age model is based on a total of 168 AMS 14C dates on bulk-organic matter, combined with a 210Pb chronology for recent sediments and corrected for a variable old-carbon age offset. This offset was estimated by i) pairing bulk-organic 14C dates with either 210Pb-derived time markers or 14C dates on grass charcoal, and ii) wiggle-matching high-density series of bulk-organic 14C dates. Variation in the old-carbon age offset through time is relatively modest, ranging from ~450 yr during glacial and late glacial time to ~200 yr during the early and mid-Holocene, and increasing again to ~250 yr today. The screened and corrected 14C dates were calibrated sequentially, statistically constrained by their stratigraphical order. As a result their constrained calendar-age distributions are much narrower, and the calibrated dates more precise, than if each 14C date had been calibrated on its own. The smooth-spline age-depth model has 95% age uncertainty ranges of ~50–230 yr during the Holocene and ~250–550 yr in the glacial section of the record. The d13C values of paired bulk-organic and grass-charcoal samples, and additional 14C dating on selected turbidite horizons, indicates that the old-carbon age offset in Lake Challa is caused by a variable contribution of old terrestrial organic matter eroded from soils, and controlled mainly by changes in vegetation cover within the crater basin.

Interhemispheric gradient of atmospheric radiocarbon reveals natural variability of Southern Ocean winds

Tree ring Delta C-14 data (Reimer et al., 2004; McCormac et al., 2004) indicate that atmospheric Delta C-14 varied on multi-decadal to centennial timescales, in both hemispheres, over the period between AD 950 and 1830. The Northern and Southern Hemispheric Delta C-14 records display similar variability, but from the data alone is it not clear whether these variations are driven by the production of C-14 in the stratosphere (Stuiver and Quay, 1980) or by perturbations to exchanges between carbon reservoirs (Siegenthaler et al., 1980). As the sea-air flux of (CO2)-C-14 has a clear maximum in the open ocean regions of the Southern Ocean, relatively modest perturbations to the winds over this region drive significant perturbations to the interhemispheric gradient. In this study, model simulations are used to show that Southern Ocean winds are likely a main driver of the observed variability in the interhemispheric gradient over AD 950-1830, and further, that this variability may be larger than the Southern Ocean wind trends that have been reported for recent decades (notably 1980-2004). This interpretation also implies that there may have been a significant weakening of the winds over the Southern Ocean within a few decades of AD 1375, associated with the transition between the Medieval Climate Anomaly and the Little Ice Age. The driving forces that could have produced such a shift in the winds at the Medieval Climate Anomaly to Little Ice Age transition remain unknown. Our process-focused suite of perturbation experiments with models raises the possibility that the current generation of coupled climate and earth system models may underestimate the natural background multi-decadal- to centennial-timescale variations in the winds over the Southern Ocean.

Understanding the variability in freshwater radiocarbon reservoir offsets: a cautionary tale

Freshwater resources in past diets can lead to inaccuracies when attempts are made to ascertain their radiocarbon ages or those of the consumers. Radiocarbon reservoir effects may lead to significant age offsets when the bones or other tissues of these consumers are radiocarbon dated. A number of recent studies have investigated freshwater reservoir offsets. However no study thus far has satisfactorily obtained a ubiquitous freshwater reservoir correction due to variability in the ecosystems analysed. This study tests the possibility of predicting freshwater reservoir effects from the carbonate alkalinity of the water with measurements on modern fish bone and water samples. A predictive capability would be especially valuable in the absence of well-preserved archaeological fish bone. We surveyed samples from lakes and rivers in varying geological settings in Britain and Ireland. Modern fish bone and water samples were analysed to investigate modern radiocarbon offsets from the atmosphere. Archaeological fish bone was also analysed to examine past reservoir offsets at selected sites. Stable carbon and nitrogen isotope values were measured to aid in interpretation of any variability in the offsets. Large freshwater reservoir offsets were measured in some modern and archaeological samples (maximum offset = 1638 14C years). The freshwater reservoir offsets in the fish bone were highly correlated with alkalinity of water in modern lake sites analysed. However, a high amount of variation within and between fish species was also evident in the results, precluding the possibility of providing regional corrections for freshwater reservoir offsets from alkalinity although this still may provide a general guideline. The variability is thought to be due to differences in the diet of individual fish.