FETCH LIMITED BARRIER ISLANDS: INDICATORS OF A SEA LEVEL HIGHSTAND IN THE SOUTHERN HEMISPHERE

Evidence of a mid-Holocene sea level highstand in the southern hemisphere is both obvious and ubiquitous. Adding to this accumulation of evidence are fetch limited barrier islands, which are quite sensitive to and important indicators of sea level changes, yet have been little largely overlooked. Globally there are approximately 7000 active barrier islands in bays, lagoons, sounds, and other sheltered marine waters, 2500 of which are located in the southern hemisphere. We observe in the southern hemisphere several occurrences of multiple sequences of discrete, shore-parallel, fetch limited barrier islands. As many as eight narrow (5-20m), closely spaced (<10-50m) islands occur in a sequence, where each island in a lagoonward direction is at a slightly lower elevation. Locations where such islands have been observed include (1) Spencer Gulf, Australia (2) Shark Bay, Australia (3) Baie Naquero, Mozambique (4) Fenambosy, Madagascar (5) Golfo San Jose, Argentina (6) Rio de la Plata, Argentina and (7) Lagoa dos Patos, Brazil. There is a glaring absence of geomorphological evidence for a sea level high stand among fetch limited barriers along the Atlantic Coast of the United States. There are no apparent occurrences of closely spaced, multiple sequences of fetch limited barrier islands in low energy environments in the southeastern U.S. exhibiting similar characteristics of those in the southern hemisphere. However, further examination of both active fetch limited barrier islands in the southeast and inactive marsh surrounded barrier islands may prove telling of previous local sea level conditions. There is a growing body of evidence of mid-Holocene sea level high in the Gulf of Mexico, so the question remains: why are there differences in fetch limited barrier island morphology between the northern and southern hemispheres? Why, in general, is the evidence for a mid-Holocene sea level high so much more widespread in the southern hemisphere?