QUANTITATIVELY ASSESSING THE EXTENT OF EPEIRIC SEAS THROUGH THE PHANEROZOIC

The Paleozoic sedimentary record is fundamentally different from the later Mesozoic and Cenozoic records because essentially all extant Paleozoic marine sediments accumulated on continental crust, and well-preserved sediments nearly all derive from shallow inland epeiric seas. There are good reasons to believe that environments in these seas were different from normal open-marine conditions with respect to the mean values of, and variability in, temperature, salinity, oxygenation, and/or nutrients. As such, paleoclimate proxy data and biodiversity patterns derived from such settings may not be representative of average zonal or global conditions and indeed could be systematically biased in comparison to them. Recent progress has been made in assessing geochemical and biotic heterogeneity within epeiric seas and in modeling their circulation and chemistry, but analyzing the extent to which they might influence Phanerozoic patterns relies on a quantitative and geographically explicit understanding of their extent and distribution through time.

Measuring where and how expansive epeiric seas were is non-trivial; most studies have used qualitative or semi-quantitative methods that are open to interpretation, not easily repeatable, and not consistently applicable through geologic time. We propose a novel metric with which to unambiguously quantify the degree of “restrictedness” across all marine locations in any given time interval using paleogeographic maps and a surface diffusion model written in R. Essentially, we record how often a randomly moving particle originating from every spot in the ocean encounters a shoreline within a set time limit, providing a continuous measure of “restrictedness” for each spot, which in this case refers to the centroids of equal-area hexagon grids with areas of ~23,000 square km. Preliminary versions of the method use 72 particles that spread out evenly from each spot in 5 degree increments, however the number and directions of particles can be changed to accommodate computer processing and resolution needs. Because the only required input data are shoreline polygons, this methodology can be used across multiple paleogeographic interpretations, allowing restrictedness measurements to be easily updated alongside paleogeographic maps and across multiple studies.