15 From the Margins to the Deep: A Tribute to the Science and Art of A. Conrad Neumann

Friday, 20 March 2020
1:30 PM - 5:30 PM
Lake Fairfax

GENERATION AND TRANSPORT OF CARBONATE BOULDERS BY STORM WAVES ALONG ROCKY COASTLINES OF SAN SALVADOR ISLAND, BAHAMAS: CELEBRATING THE LEGACY OF A. CONRAD NEUMANN

Paper number: 15-10
Program: Theme Sessions
Time: 4:30 PM – 4:50 PM

Authors

Abstract

Conrad Neumann’s pioneering work on carbonate depositional and diagenetic processes in The Bahamas set the stage for ongoing research on the impact of sea-level and climate changes on this tropical island system. As part of this effort, we have been monitoring two storm-deposited boulder ridges on San Salvador Island since Jan. 2012. Initially, 27 large boulders were photographed, GPS-located, measured, and characterized by composition and morphology. Continuing monitoring from Jan. 2013, 2016, and 2017, revealed modifications to the boulder ridges by Hurricanes Sandy (Oct. 2012), Joaquin (Oct. 2015), and Matthew (Oct. 2016), respectively.

Significant changes were documented after Joaquin, which passed over the island as a category 3 hurricane with 120-130 mph sustained winds. Storm waves overtopped cliffs on the southern coast of the island, causing erosion at the leading edge and landward movement of boulders. This modified the formerly sharp-crested, narrow boulder ridge into a broad field, stripped of vegetation. New boulders, as large as 3 m in diameter, were generated, and blocks from prior storms, weighing 1-3 tons, moved up to 26 m inland. The edge of the boulder ridge moved landward 4-5 m, exposing a Pleistocene/Holocene boundary terra rossa paleosol, and marking the extent of storm erosion.

Documentation of boulder movement allowed calculation of flow velocity required to initiate their transport, but without tagging it was challenging to relocate individual boulders after major storms. This issue was addressed in June 2019 by tagging >100 boulders using RFID (radio frequency identification) technology. Drilling to insert small tags (23 and 32 mm long, <4 mm in diameter) is minimally invasive and also allows tagging of pebbles and cobbles. In conjunction with high-resolution drone imaging, use of tagging that can uniquely identify an object within a large population significantly increased our database and improved the monitoring efforts. These data can be used to enhance communication with stakeholders about vulnerability to hurricanes on San Salvador and elsewhere in The Bahamas, which is becoming increasingly important as consequences of changing climate and rising sea levels are amplified by powerful storms, such as Hurricane Dorian that devastated Grand Bahama and Abaco islands in Sep. 2019.

Geological Society of America Abstracts with Programs. Vol. 52, No. 2

doi: 10.1130/abs/2020SE-343422

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