Deconvolution of the atmospheric radiocarbon record in the last 50,000 years

Abstract

Here we identify the principal causes of changes in the atmospheric Δ¹⁴C record by removing the influence of two discrete large changes in the geomagnetic field between 45 and 30 kyr. The marked transitions in theΔ¹⁴C record during this period can be interpreted as being the result of either "zero magnetic field" for periods of the order of 6500 y and 600 y at ~40 kyr and 32 kyr B.P., respectively, or for longer durations if the field was non-zero for appreciable time during the event. Once the effect of these events has been removed, the residual Δ¹⁴C record shows a plateau with nearly constant value at ~300‰ during 28-17 kyr B.P, followed by a sharp decrease in Δ¹⁴C between 17 kyr B.P. and the present, to a Δ¹⁴C value of ~0‰. Estimated global paleomagnetic fields derived from paleointensity measurements in deep sea sediments (SINT-800, NAPIS-75 and Sumatra Basin) can only explain a maximum of 50% of this residual Δ¹⁴C record during 30-0 kyr B.P. We propose that the remainder must have resulted from changes in oceanic circulation leading to isolation of ≥ 20% of oceanic radiocarbon inventory from large scale mixing with the atmosphere during 28-17 kyr B.P. Subsequent inclusion of this carbon may be responsible in part for the decrease between 17 kyr B.P. to the present, to a Δ¹⁴C value of ~0‰. We also note that the deconvoluted Δ¹⁴C record during the past 15,000 y B.P. seems to be primarily influenced by the changes in the atmospheric CO₂ concentrations. These possibilities have been discussed in previous works, but our results here point to a seemingly cyclical change in carbon exchange that was initiated prior to the Last Glacial Maximum.