Styles of coastal evolution in response to Holocene changes in sea level and sediment supply
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This study employs a suite of geophysical, sedimentological, and chronostratigraphic tools to investigate the complex interactions among changes in sea level, climate, and sedimentation processes that have driven Holocene coastal evolution. These interrelationships were explored in investigations of three coastal sites with diverse sea-level and sedimentation histories: the Egyptian Red Sea (Wadi Gawasis), southern Brazil (Pinheira) and the Western Gulf of Maine (Plum Island). This study demonstrates the need to quantify the integrated impacts of spatially-diverse changes in global (sea level), regional (climate, sea level), and local (sedimentation) factors if we are to predict large-scale coastal evolution in response to the ongoing acceleration in sea-level rise. The mid-Holocene in both the Red Sea and southern Brazil was characterized by higher-than-present stands of sea level. Sedimentological, malachological, foraminiferal, and rheological studies at Wadi Gawasis reveal that this resulted in the formation of a shallow bay that reached its maximum extent prior to a 1.5-m highstand at 5 ka, demonstrating a dominance of sedimentation processes despite contrary sea-level change. Early bay closure was driven by sediment inputs enhanced by a wetter climate. Slowly falling sea level and coincidental climatic aridization allowed for the establishment of an Egyptian harbor 4 ka, followed by late-stage progradation dominated by sea-level fall. In southern Brazil, an abundant sediment supply and sea-level fall following the mid-Holocene highstand were responsible for the development of the 5-km wide Pinheira strandplain, composed of regular beach and dune ridges. Identification of anomalous barrier, lagoonal, and tidal fill deposits within this plain demonstrates the complex nature of the sedimentological response to a small-scale change in the rate of sea-level fall. By contrast, Plum Island formed in a regime of rapid sea-level rise that reworked shallow shelf and fluvial deposits. Geophysical and sedimentological studies reveal a complex barrier formation (aggradation, spit accretion and progradation), including evidence for inlet migration and closure. Time-transgressive backstripping of backbarrier facies shows that bay sedimentation in a regime of slowly rising sea level reduced tidal-prism and produced inlet closure. This is first study to demonstrate that the direct impact of backbarrier processes influencing barrier island development.
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