Coastal response to late-stage transgression and sea-level highstand

Abstract

Coastal morphologic features associated with past shoreline transgressions and sea-level highstands can provide insight into the rates and processes associated with coastal response to the modern global rise in sea level. Along the eastern and southern Brazilian coasts of South America, 6000 years of sea-level fall have preserved late-stage transgressive and sea-level highstand features 1–4 m above present mean sea level and several kilometers landward of modern shorelines. GPS with real-time kinematics data, ground-penetrating radar, stratigraphy, and radiocarbon dating within a 2–3‐km-wide river-associated strandplain in central Santa Catarina (southern Brazil) uncovered a diverse set of late-stage transgressive and highstand deposits. Here, the highstand took the forms of (1) an exposed bedrock coast in areas of high wave energy and low sediment supply; (2) a 3.8-m-high transgressive barrier ridge where landward barrier migration was prohibited by the presence of shallow bedrock; and (3) a complete barrier-island complex containing a 5.2-m-high barrier ridge, washover deposits, a paleo-inlet, and a backbarrier lowland, formed in a protected cove with ample sediment supply from small local streams and the erosion of upland sediments. Similar signatures of the mid-Holocene highstand can be traced across all coastal Brazilian states. This study presents the first complete compilation of the diversity of these sedimentary sequences. They are broadly classified here as exposed bedrock coasts (type A), backbarrier deposits (type B), transgressive barrier ridges (type C), and barrier-island complexes (type D), according to localized conditions of upland migration potential, wave exposure, and sediment supply. These Brazilian systems present a paradigm for understanding future coastal response to climate change and accelerated sea-level rise: the recognition of a minimum threshold sea-level-rise rate of ∼2 mm yr–1 above which transgression proceeded too rapidly for the formation of these stable accretionary shoreline features demonstrates the nonlinearity of coastal response to sea-level change, and the site specificity of conditions associated with the formation of each highstand deposit type, even within a single small embayment, demonstrates the non-uniformity of that response.