| Popis: |
The field of paleotempestology has gained more awareness in the past decade as a result of increased hurricane intensity, quantity, and duration within the late Holocene. One of the best localities to find records of hurricane overwash deposits is in the Bahamian islands. San Salvador Island, Bahamas (SSI) is a small (150km2), isolated carbonate platform, that contains shallow (0.5-3m) saline lakes that occur in between interdune areas of arcuate dune ridges. Due to San Salvador Island’s location within the Bahamian archipelago and the Atlantic Ocean, the island is poised to record hurricane strikes and the record of these events can be found in coastal lakes. Clear Pond is a shallow (~1m), variably saline (16-30 ppt) lake in the southwestern edge of the island, that is separated from the Atlantic Ocean by Holocene dunes. It was a previously uncharacterized pond, but like many other depositional environments on San Salvador, it is constantly recording climatic and anthropogenic changes on the island. This study addresses the following questions: 1) Is there a seasonal salinity variation within the pond and what is the general limnology of the pond, 2) Can we identify large storm events in the sedimentary record of Clear Pond, and 3) What is the depositional history for Clear Pond for the past 4000 years? Nine sediment cores, ranging in length from 50 to 150 cm, were recovered from Clear Pond. The cores were analyzed for organic and carbonate content, dry bulk density, grain size, sediment fabric, and mollusk and ostracode composition. Additionally, x-ray fluorescence, spectrophotometry, and x-radiography were carried out on sediment cores. Salinity varied from brackish conditions in the summer and fall to more marine during the winter and spring seasons. It was also influenced by a previously uncharacterized karst conduit. Large storm events were identified through an increase in grain size and dry bulk density, and additionally by x-radiographs. A catastrophic hurricane recurrence interval of 478 years and an annual landfall probability of 0.21% were found as well. The record of Clear Pond indicates an average sedimentation rate of 2.3 cm/ 100 years which suggests the record recovered is at least 5200 years old.These cores record a major climatic shift occurring at 1460 ybp which correlates with the end of a documented Caribbean dryness period. The facies shift is from a sand and mud laminated facies to a recent, massive bioturbated facies. The record documents multiple hurricane strikes from 1600-3400 ybp which supports a hurricane "hyperactivity period" from 1000- 3000 ybp from other areas in the Atlantic and Gulf of Mexico. These large storm events are linked to two large-scale climate mechanisms, the location of the Bermuda high and the strength of the North Atlantic Oscillation. The ability to characterize and identify past overwash deposits in the Bahamas may be useful in climate reconstruction and landfall probability assessments. These results are the first proxy records for hurricane strikes on the west coast of San Salvador Island. |
