Sea Level Change and Nearshore Biodiversity: The Role of Transgressive-Regressive Transitions

The role of global eustatic changes in the evolution of the nearshore marine biota occurs along a continuum of temporal scales ranging from millions to tens of thousands of years and may be global, regional or local in their affects. During the late Neogene, a succession of transgressive-regressive (T-R) cycles occurred that both lowered mean sea level over this period and showed an increasing amplitude between the low and high stand levels. While it has long been recognized that T-R cycles have substantial impacts on coastal geomorphology through erosion and deposition, the potential impact on benthos availability related to the erosive transitions between T-R phases has not been previously addressed. Examination of benthos area within 75 m of current sea level along the west coast of North America revealed shoals that appear to be correlated with the depth of the most recent T-R transitions over the last 0.5 Mya. During the last 3 million years, the depth of T-R transitions has become progressively deeper, moving from about 20-40 m between 2.5 and 3.0 Mya to 75 -100 m during the last 0.5 Mya. The presence or absence of these shoals within the habitat range of marine invertebrate and vertebrate species is further mediated by subsequent sea level changes that provide or deny access to these shoals. These combined effects of sea level change and T-R transition depths may have had substantial affects on population sizes of marine animals, especially those of diving marine mammals and sea birds, during the Pleistocene. This interaction may have also been important in the evolution of the North Pacific nearshore marine fauna during the late Neogene.