Nature and origin of variations in late-glacial and Holocene atmospheric 14C as revealed by global carbon cycle modeling

Abstract

Simulations with a global box-diffusion $\sp$C model indicate that the millennium- and century-scale atmospheric $\Delta\sp$C variations during the Holocene are more likely explained by fluctuations in $\sp$C production rate (Q) than by changes in air-sea CO$\sb2$ exchange rate (F) or internal ocean mixing (parameterized as an "eddy diffusivity" K$\sb{\rm z}$. The $\sp$C reservoir model deconvolves histories for each of these three processes that are compatible with a 96-yr bi-decadal atmospheric (tree-ring)$\Delta\sp$C record assuming alternative pre-Holocene $\sp$C conditions. Holocene microparticle concentrations in ice cores and dust grain sizes in marine sediment cores disagree with the model-derived global wind speeds necessary to explain (through F variations) the millennium-scale trends in atmospheric $\Delta\sp$C. Alternately, foram $\sp$C data do not support the history in the oceanic ventilation index generated by millennium-scale K$\sb{\rm z}$ variations. Coral $\sp$C data for recent centuries conflict with the marine $\Delta\sp$C history associated with century-scale variations in F or K$\sb{\rm z}$ but are consistent with changes in $\sp$C production rate.The $\sp$C production rates derived theoretically from an 11,000-yr record of averaged global dipole moments strongly correlate with the Q history required to explain tree-ring $\Delta\sp$C. Several pre-Holocene Q histories were calculated from limited dipole moment data available for the past 30,000 yrs and do not contradict $\sp{234}$U/$\sp{230}$Th-calibrated coral $\sp$C measurements. Relative variations in Greenland ice-core $\sp$Be concentrations (reflecting changes in $\sp$Be production) over the past 9000 yrs also correlate strongly with tree-ring Q fluctuations except for a 4500-3500 BC discrepancy.Simulations of transient variations in Q, F, and K$\sb{\rm z}$ supplement previous studies of alternative steady-state $\sp$C situations. The modeling of combined climate and production rate scenarios (i.e. F + K$\sb{\rm z}$, F + Q, K$\sb{\rm z}$ + Q) incorporates "feedback" effects which depend on the instantaneous marine $\sp$C profile and atmosphere/surface ocean $\sp$C disparity.Spectral analyses of the 9600-yr tree-ring Q history generally produce power at or near harmonics of a 420-yr cycle. Century-scale Q periodicities may reflect harmonic or nonsinusoidal solar processes. Residual $\sp{14}$C production and bi-decadal sunspot numbers from AD 1700 to 1840 define a preliminary history of relative sunspot numbers back to 7730 BC. "Fine-tuned" Q periodicities are used to forecast future solar behavior.