Pliocene-Pleistocene Stratigraphy and Sea-Level Estimates, Republic of South Africa With Implications for a 400 ppmv CO2 World

The Mid-Pliocene Warm Period (MPWP, 2.9 to 3.3 Ma), along with older Pliocene (3.2 to 5.3 Ma) records, offers potential past analogues for our 400-ppmv world. The coastal geology of western and southern coasts of the Republic of South Africa exposes an abundance of marine deposits of Pliocene and Pleistocene age. In this study, we report differential GPS elevations, detailed stratigraphic descriptions, standardized interpretations, and dating of relative sea-level indicators measured across similar to 700 km from the western and southern coasts of the Cape Provinces. Wave abrasion surfaces on bedrock, intertidal sedimentary structures, and in situ marine invertebrates including oysters and barnacles provide precise indicators of past sea levels. Multiple sea-level highstands imprinted at different elevations along South African coastlines were identified. Zone I sites average +32 +/- 5 m (six sites). A lower topographic Zone II of sea stands were measured at several sites around +17 +/- 5 m. Middle and late Pleistocene sites are included in Zone III. Shoreline chronologies using Sr-87/Sr-86 ages on shells from these zones yield ages from Zone I at 4.6 and 3.0 Ma and Zone II at 1.04 Ma. Our results show that polar ice sheets during the Plio-Pleistocene were dynamic and subject to significant melting under modestly warmer global temperatures. These processes occurred during a period when CO2 concentrations were comparable to our current and rapidly rising values above 400 ppmv. Plain Language Summary The ancient coastal deposits of the Republic of South Africa provide an opportunity to examine past climatic effects on ice sheets and sea levels during the Pliocene. This was the last time atmospheric CO2 was similar to our current and rising level of similar to 415 ppmv. Indicators of past sea levels were measured at a number of natural rock outcrops that date between about 1 and 5 million years old. Our studies have found that average sea levels ranged between 15 and 30 m higher than present. Postdepositional effects by crustal movements caused by plate tectonics or ice sheets are minor but have yet to be fully quantified. Regardless, our baseline field data have important implications for the magnitude of our present and future climate and sea-level changes.