MODELING NEOPROTEROZOIC FREEBOARD

Changes in continental freeboard through time affect biogeochemical and climate cycles via weathering and erosion flux to the oceans. As freeboard is tied to mantle thermal evolution—where hotter mantle results in less freeboard and vice versa—many freeboard models simply mimic unidirectional mantle cooling through time; they increase rapidly early in Earth history and stay relatively constant for the rest of geologic time. Here, we consider a non-uniform mantle cooling model, controlled by the ratio of mantle convection cell length to height (CCR, or convection cell ratio) associated with supercontinent assembly and breakup, as a means by which continental freeboard may have varied through time. Following Grigné et al. (2005), we model mantle temperature through time based on a 500 million year sinusoidal supercontinent cycle, where supercontinent assembly corresponds to fewer mantle convection cells (CCR of 7), and dispersed continents correspond to more mantle convection cells (CCR of 3), creating a sinusoidal rather than unidirectional mantle cooling history. The resultant mantle temperature curve is coupled to models for oceanic crust thickness and continental freeboard. Next, we remove the sine function from the cooling history and allow the model to instantaneously switch between a CCR of 7 and 3 to approximate geologically sudden loss and initiation of subduction flux. Last, we consider times of passive margins (CCR=3) versus the style of supercontinent formed from Atlantic-type (CCR=5) or Pacific-type (CCR=7) ocean closure modified after Silver and Behn (2008) going from present to 1.8 Ga. The greatest freeboard increase results from the assembly of Rodinia in Neoproterozoic time, roughly doubling the percentage of land exposed to near the current value. Interestingly, the increase in freeboard (and concomitant weathering flux) is nearly coincident with the onset of extreme glaciations in Neoproterozoic time, perhaps providing a mechanism for the onset of Neoproterozoic glaciation via CO2 drawdown by silicate weathering of newly exposed material.