Replicated small-sample radiocarbon measurements as a corrective lens for sedimentary proxy records.

andrew.dolman [ at ]


Due to mixing processes, sediment samples taken from a single depth can contain particles (e.g. foraminifera) with a wide range of ages. When radiocarbon measurements are made on samples containing many of these individual particles, the resulting values can hide a lot of internal age variation. Furthermore, if the measured samples contain a material from only a small number of individual particles, the resulting 14C ages will be noisy estimates of the true mean age of material from that depth. Similarly, for proxies such as Mg/Ca, or d18O, the range of ages contained in a single sample results in measurements that represent average values for an extended time period. Again, these values will be noisy if the number of particles per sample is small, but even with large samples, the resulting proxy records are “smoothed out” and the reconstructed amplitude of climate transitions is reduced. The advent of ultra-small-sample 14C dating means that samples consisting of very small numbers of foraminiferal shells now can be dated. This poses both a problem, as individual 14C ages will be less representative of their layer, but also an opportunity as it allows for a direct estimate of the heterogeneity in the age of material at a given depth. We used 14C measurements on samples of 3-30 foraminifera to estimate the underlying standard deviation in the age of individuals picked from the same depth. We repeated this for cores with sedimentation rates ranging from 3-30 cm/kyr and found age-variances consistent with simple sediment mixing models and typical bioturbation depths. These direct estimates of age-variance allow for more realistic estimates of age uncertainty and have already proven useful to use in reconciling apparently inconsistent age-depth profiles from adjacent sediment cores. They also allow for a better-informed interpretation of proxy records, both in terms of the relative timing of events and in terms of the amount of amplitude reduction of the climate signal to be expected at different timescales. Knowing the length of time represented in a single sediment sample also allows us to more clearly interpret changes in the statistics of individual foraminifera variation, whether they can be interpreted as changes to the amplitude of the seasonal cycle, the strength of ENSO variations, or multidecadal climate variation.

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Conference (Talk)
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20th Congress of the International Union for Quaternary Research (INQUA), 25 Jul 2019 - 31 Jul 2019, Dublin, Ireland.
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Dolman, A. M. , Groeneveld, J. , Ho, S. L. and Laepple, T. (2019): Replicated small-sample radiocarbon measurements as a corrective lens for sedimentary proxy records. , 20th Congress of the International Union for Quaternary Research (INQUA), Dublin, Ireland, 25 July 2019 - 31 July 2019 .

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