Re-evaluating <sup>14</sup>C dating accuracy in deep-sea sediment archives

Abstract

<p><strong>Abstract.</strong> The current geochronological state-of-the-art for applying the radiocarbon (<sup>14</sup>C) method to deep-sea sediment archives lacks key information on sediment bioturbation. Here, we apply a sediment accumulation model that simulates the sedimentation and bioturbation of millions of foraminifera, whereby realistic <sup>14</sup>C activities (i.e. from a <sup>14</sup>C calibration curve) are assigned to each single foraminifera based on its simulation timestep. We find that the normal distribution of <sup>14</sup>C age typically used to represent discrete-depth sediment intervals (based on the reported laboratory <sup>14</sup>C age and measurement error) is unlikely to be a faithful reflection of the actual <sup>14</sup>C age distribution for a specific depth interval. We also find that this deviation from the actual <sup>14</sup>C age distribution is greatly amplified during the calibration process. We find a systematic underestimation of total geochronological error in many cases (by up to thousands of years), as well as the generation of age-depth artefacts in downcore calibrated median age. Specifically, we find that even in the case of <q>perfect</q> simulated sediment archive scenarios, whereby sediment accumulation rate (SAR), bioturbation depth, reservoir age and species abundance are all kept constant, the <sup>14</sup>C dating and calibration process generates temporally dynamic median age-depth artefacts, on the order of hundreds of years – even in the case of high SAR scenarios of 40 cm ka<sup>−1</sup> and 60 cm ka<sup>−1</sup>. Such age-depth artefacts can be especially pronounced during periods corresponding to dynamic changes in the Earth's Δ<sup>14</sup>C, where single foraminifera of varying <sup>14</sup>C activity can be incorporated into single discrete-depth sediment intervals. In certain SAR scenarios, a discrete depth’s true median age can consistently fall outside the 95.45 % calibrated age range predicted by the <sup>14</sup>C dating and calibration process. Our findings suggest the possibility of <sup>14</sup>C-derived age-depth artefacts in the literature: since age-depth artefacts are likely to coincide with large-scale changes in global Δ<sup>14</sup>C, which themselves can coincide with large-scale changes in global climate (such as the last deglaciation), <sup>14</sup>C-derived age-depth artefacts may have been previously been (partially) misinterpreted as due to changes in global climate. Our study highlights the need for the development of improved deep-sea sediment <sup>14</sup>C calibration techniques that include an a priori representation of bioturbation for multi-specimen samples.</p>