INVESTIGATION OF GLOBAL GLACIAL THRESHOLDS IN THE CAMBRIAN USING AN EARTH SYSTEM MODEL OF INTERMEDIATE COMPLEXITY

Following the Neoproterozoic glaciations, the Cambrian is generally regarded as a fairly warm period in which multicellular life emerges and dominates the oceans. However, recent work tentatively pointing to evidence of ice in the early and late Cambrian, as well as work showing changes in carbon isotopes in the mid-Cambrian suggests there may have been episodic environmental changes associated with cooling. In light of this work, an earth system model of intermediate complexity is utilized to evaluate the roles of Cambrian continental configuration and atmospheric pCO₂ concentration on glacial thresholds, and ascertain the feasibility of low and high latitude Cambrian glaciation. The Planetary Simulator (PLASIM), developed at the Meteorological Institute of the University of Hamburg, is utilized at T21 spectral resolution (5.6° latitude x 5.6º longitude) with a 50 m deep slab ocean in six experiments. Four experiments employ a Late Cambrian continental configuration with pCO₂ ranging from 2800-7000 ppm. This is in the range estimated for the Cambrian by carbon cycle modeling studies. All Cambrian experiments are integrated without any land surface vegetation, and with solar luminosity reduced by 6%. The experiments also utilize a uniform land surface boundary condition consisting of sand with an albedo of 0.37. Two additional experiments are identical to the Cambrian experiments, but use modern continental configuration. Due to the low solar luminosity and high land surface albedo, ice is present in all model experiments. In the lower CO₂ Cambrian scenario (2800 ppm), the Northern Hemisphere perennial sea ice margin extends as far south as 30°N. The Southern Hemisphere sea ice margin extends northward to ~8°S. A comparable experiment, but with modern continental configuration, produced a snowball earth scenario. Overall, model output suggests that the Cambrian continental configuration, in which much of the land surface area is distributed in the Southern Hemisphere, plays a key role in keeping global average temperature warmer than it would be with a modern configuration, and that while Cambrian low-latitude glaciation was unlikely, the presence of high-latitude sea ice during this time is plausible, based on comparison of model output with climate inferences from proxies.