Optimal estimation of atmospheric 14C production over the Holocene: paleoclimate implications
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A tree-ring Delta(14)C record and a simple box model of the global C-14 cycle are combined using a method of optimal estimation theory (Rauch-Tung-Striebel smoother). The combination is used to infer information about the time evolution of C-14 production in the atmosphere (P) for the period 9400 year BC to AD 1900 year. Unlike previous attempts to infer P changes from the tree-ring record, the errors in both the Delta(14)C data and the model, which are assumed to be purely random ( not systematic), are formally considered. The optimal time evolution of P is compared to independent evidence of changes in cosmogenic nuclide production over the Holocene from a variety of records on their original chronology, e. g., a record of the virtual axial dipole moment (VADM) based on a compilation of archeomagnetic data, the record of Be-10 concentration from the GISP2 ice core ( Central Greenland), and the record of 10 Be concentration from the PS1 ice core ( South Pole). The rank correlations between P-VADM; P-Be-10(GISP2), and P-(10)Bed(PS1) are highly significant (p < 0.01), indicating that geomagnetic field intensity and Be-10 concentration in GISP2 and PS1 changed monotonically with C-14 production. The linear correlation coefficients between P-VADM; P-Be-10(GISP2), and P-Be-10(PS1) are also highly significant (p < 0.01) but relatively small (-0.76, 0.48, and 0.60, respectively). Thus, an important fraction (42-77%) of the variance in the geomagnetic and 10 Be data is not accounted for by linear regression on the 14 C productions implied by the tree-ring record. The P variance near the 1500 yr period, which previous authors interpreted as solar variability, represents a small fraction of the total variance in the P time series (<15% for the band 1200-1800 yr) and does not correspond to a spectral peak. Hence, the hypothesis of a direct solar forcing mechanism for the postulated millennial climate variability during the Holocene is not supported.