400 KA ECCENTRICITY CYCLE MODULATION OF THE SPICE CARBON ISOTOPE EXCURSION ON BOTH SIDES OF THE CAMBRIAN EARTH

The Steptoean Positive Carbon Isotope Excursion (SPICE) uniquely defines a 2-3 million-year interval of Cambrian time that can be recognized globally (Saltzman et al., JSR 74, 366-377, 2004). Well-sampled sections in Nevada (Egan Range) and Australia (DDH Mt Whelan 1) record both the +5 permil excursion in d¹³C and higher frequency modulations that have an amplitude of 1 permil and a wavelength of about 20 meters. As the duration of the SPICE event is constrained by interpolation among available U-Pb ages, the higher frequency modulations can be plausibly associated with the 400 ka cycle in the eccentricity of the Earth’s orbit. Long (600 Ma) numerical integrations of solar system dynamics, together with theoretical considerations, make it clear that the 400 ka cyclicity is metronomic, not chaotic. Therefore, the 400 ka signal in the Cambrian carbon isotope records may be used to: (1) provide an accurate duration for the Spice excursion and for biological and geological events within it; (2) demonstrate the existence of long-term climatic variability during the Cambrian that was recorded by the oceanic carbon cycle; and (3) assist with understanding how trivial changes in Earth’s eccentricity can be responsible for amplified responses in the behavior of Earth’s climate and its biogeochemical proxies.