Abstract
ABSTRACTRadiocarbon (14C) dating is often carried out upon multi-specimen samples sourced from bioturbated sediment archives, such as deep-sea sediment. These samples are inherently heterogeneous in age, but existing 14C calibration techniques were originally developed for age homogeneous material, such as archaeological artifacts or individual tree rings. A lack of information about age heterogeneity leads to a systematic underestimation of a sample’s true age range, as well as the possible generation of significant age-depth artifacts during periods of the Earth’s history coinciding with highly dynamic atmospheric Δ14C. Here, a new calibration protocol is described that allows for the application of sedimentological priors describing sediment accumulation rate, bioturbation depth and temporally dynamic species abundance. This Bayesian approach produces a credible calibrated age distribution associated with a particular laboratory 14C determination and its associated sedimentological priors, resulting in an improved calibration, especially in the case of low sediment accumulation rates typical of deep-sea sediment. A time-optimized computer script (biocal) for the new calibration protocol is also presented, thus allowing for rapid and automated application of the new calibration protocol. This new calibration protocol could be applied within existing age-depth modeling software packages to produce more accurate geochronologies for bioturbated sediment archives.
Highlights
Radiocarbon (14C) analysis is routinely used to determine the age of marine sediment archives up to ∼50 ka in age, and has been fundamental in increasing our understanding of the spatiotemporal development of palaeoclimate during the last glacial and the Holocene
This age distribution is mainly governed by the sediment accumulation rate (SAR) and bioturbation depth (BD), the latter of which is typically around 10 cm (Trauth et al 1997; Boudreau 1998)
In order to calibrate 14C activity measurements carried out upon heterogeneous samples retrieved from bioturbated sediment, the following sedimentological priors are defined: s = estimated sediment accumulation rate (SAR), in cm yr–1 m = bioturbation depth (BD), in cm k = the fraction of the analyzed microfossils that are fragmented a = time series of abundance of the analyzed species relative to itself
Summary
Radiocarbon (14C) analysis is routinely used to determine the age of marine sediment archives up to ∼50 ka in age, and has been fundamental in increasing our understanding of the spatiotemporal development of palaeoclimate during the last glacial and the Holocene. E.g., deep-sea sediment archives, many tens of single microfossil specimens from a discrete core depth are often pooled into a single sample for measurement. Systematic bioturbation of deep-sea sediment causes discrete downcore intervals of deep-sea sediment to have an age distribution that is characterized by an exponential probability density function with a long tail towards older ages (Berger and Heath 1968). This age distribution is mainly governed by the sediment accumulation rate (SAR) and bioturbation depth (BD), the latter of which is typically around 10 cm (Trauth et al 1997; Boudreau 1998).
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