Eccentricity modulation of weathering and accumulation rates: non-intuitive, empirical relationship suggests links between orbital pacing and pCO2

The high frequency oscillations between wet and dry conditions plus the warmer temperatures when the Earth comes closest to the sun, might suggest weathering and hence accumulation rates should be highest during times of maximum eccentricity and maximum precessional variability in the tropics. But time series analysis of 20 Myr of continuous cores of tropical, lacustrine Late Triassic-age strata of the Newark Rift Basin (202–222 Ma) surprisingly show that that is not the case because accumulation rates are highest during the times of lowest precessional variance at the modes of the Mars–Earth (g₄-g₃) orbital cycle, when eccentricity is at a minimum.

            Three different methods of analysis reveal an accumulation pattern at variance with this intuitive model. 1) Tuning the depth-domain depth rank, color, and natural gamma data series to the 405 kyr, Venus–Jupiter (g₂-g₅) eccentricity metronome reveals oscillations in accumulation rates of ~20m to ~100m/Myr/cycle (within a total range of 70m – 250m/Myr). Spectral analysis reveals these oscillations occur with the same period (~1.8Myr) as the Mars–Earth modulation of precession for that time, with highs in accumulation rate occurring during lows in eccentricity. A weaker signal of the Mars–Earth (s₄-s₃) inclination cycle is also present at about 1/2 the period of the eccentricity cycle. 2) Application of the eTimeOpt method of sedimentation rate analysis reveals the same pattern and magnitudes of sedimentation rate variations in depth rank and color. 3) Spectral analyses of gamma and XRF elemental data from intervals of low- vs high-precessional variance show that significantly lower accumulation rated occurred during extended times of high- vs low-precessional variation.

            Accumulation rate oscillations in the Newark Rift Basin should be tracking weathering rates to supply the immense volumes of sediment involved in the accumulation rate variations. Such volumes could not be somehow stored in the highlands for hundreds of thousands of years, otherwise potentially shifting weathering and accumulation rates out of phase.

            The implication of these empirical data is that because pCO₂ should be drawn down under higher weathering rates, and the phase of eccentricity modulation of precession is global, pCO₂ should be oscillating in phase with the Mars–Earth eccentricity cycle. On the short-term, low-pCO₂ should characterize times of low-precessional variability, evidently associated with high-accumulation rates, based on these empirical data, and not vice-versa as might be intuitively modeled. In turn, the oscillations in pCO₂ would be expected to cause global temperature oscillations at the g₄-g₃ frequency. These non-intuitive results, suggesting a hitherto unanticipated relationship between orbital pacing of climate and pCO₂, can be tested and further explored by continuous XRF elemental scanning of these cores, currently underway, and by collection of more densely sampled soil carbonate and leaf stomatal pCO₂ proxy data, from proposed new cores. The mechanisms driving the relationships between these reproducible empirical data are, however, not obvious, but would seem to be related to the precession-scale variability of climate, not just the magnitude of greenhouse gas concentrations or temperatures.