Biology 561 Barrier Island Ecology Global Distribution of Barrier Islands Biology 561 Barrier Island Ecology 28 August 2003

Biology 561 Barrier Island Ecology Global Distribution of Barrier Islands Historically, three factors considered as determining barrier island distribution –Low gradient of shelf –Abundant sediment supply –Low to moderate tidal range

Biology 561 Barrier Island Ecology Tectonic setting of barrier islands Is plate margin type correlated with barrier island distribution?

Biology 561 Barrier Island Ecology Tectonic Types Trailing plate margins – Amero- –Afro- –Neo- Collision plate margins Marginal sea coast

Biology 561 Barrier Island Ecology Percentage of coast occupied by barrier islands Trailing margins 12% Collision coasts 8% Marginal Seas 9.6%

Biology 561 Barrier Island Ecology Total Length of Barrier Islands (%) Trailing margins 49% Collision coast 24% Marginal seas 27%

Biology 561 Barrier Island Ecology Trailing Margin Coasts With decreasing shelf width and increasing gradient, the abundance of barrier islands decreases proportionally Sediment discharge toward trailing margin coastlines exceeds discharge toward collision coasts Amero-trailing coasts have drainage asymmetry creating large discharge

Biology 561 Barrier Island Ecology Collision Margins Continental margins (in collision with oceanic crust) –92% of collision BIs occur on this side Island arc side –8% of collision BIs occur on this side

Biology 561 Barrier Island Ecology Marginal Seas Continental margins –90% of collision BIs occur on this side Island Arc side –10% of collision BI occur on this side Phillipines

Biology 561 Barrier Island Ecology Shelf Width and Gradient Trailing Margins –156 km wide with gradient of 1.2 Collision Margins –58 km wide with gradient of 3.4 Marginal Seas –233 km wide with gradient of 0.89

Biology 561 Barrier Island Ecology Barrier Island Distribution on West Coast of North and South America Extensive BIs in Baja, California Extensive BIs in Mexico Guatemala, El Salvador, Nicaragua and Costa Rica Distribution correlates with – wider shelf –wider coastal plain –Absence of submarine canyons

Biology 561 Barrier Island Ecology Marginal Seas Shelf width is wider than trailing coasts and gradient is lower Shelf width and gradient are not sole controllers of barrier island distribution

Biology 561 Barrier Island Ecology Shelf Gradient and Barrier Island Abundance Glaeser provides several tests for shelf gradient and barrier island distribution –Low gradient does not guarantee the existence of barrier islands –No upper or lower value for gradient assures the existence of barrier islands Conclusion: gradient alone does not determine barrier island abundance along any continental coastlines

Biology 561 Barrier Island Ecology Influence of River Sediment Input on Distribution of Barrier Islands Data limitations –Sediment discharges not measured –Grain size of sediment load not measured High sediment loads of Huang Ho, Mekong and Yangtze rivers yield no barrier islands Eastern USA low sediment discharge (200X less than above) yield many barrier islands

Biology 561 Barrier Island Ecology Discussion Shelf gradient and sediment supply related to trailing margins only Barrier Island abundance related to –Abundance of sediment in shoreface –Response of coastal zone to shoreface retreat

Biology 561 Barrier Island Ecology Discussion Sands of shoreface and inner shelf are reworked Under optimum conditions, close association between barrier islands and broad, low relief coastal plains –Eastern USA--broad, low relief coastal plains ( km) –Asia mainland—high relief coastal plain (25 km)

Biology 561 Barrier Island Ecology Barrier Island Genesis Mainland beach detachment –Low (flat), wide coastal plain –Sediments are reworked as sea level rises –Long-lasting features (islands continually reform) Headland erosion/coastwise spit development –Steep coasts with little or no coastal plain –Short-lived features (not regenerated)