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Seminar Details

Biological processes altering the preservation of flashflood deposits on the shallow shelf of the Gulf of Eilat, Red Sea

Date

29/06/2017

Lecturers

Alysse Mathalon - Dalhousie University, Canada; and The Interuniversity Institute of Eilat

Abstract

In the hyperarid environment of Eilat, Israel, ephemeral flashfloods transport large quantities of sediment into the Red Sea. The sedimentary record, therefore, may be useful for reconstructing a history of the frequency and magnitude of flashfloods in the region. This study focused on identifying the mechanisms for flashflood deposit preservation and alteration after settling on the shallow seafloor in the Gulf of Eilat- Aqaba (GOA). A suite of observations was made in 13 meters water depth offshore of the dominant drainage outlet of flashfloods to the the GOA from Eilat. (1) Changes in the grain size of fine-grained flood layers were tracked within the year after they settled on the seafloor through bi/tri-monthly sediment coring. (2) Effects of physical forcing and transport mechanisms on sediment resuspension and removal were explored by measuring water currents and water column properties in relation to suspended sediment concentrations. (3) Biological resuspension by fish was monitored by deploying GoPro cameras to photograph the near bottom environment. (4) Rates of bioturbation were studied with an in situ experiment that used fluorescent tracers as markers, which were emplaced into the seabed. The results revealed that flashflood deposits generally were not identifiable in the seafloor within a year after their deposition, due to a combination of sediment mixing, resuspension, and removal. Water currents at the study site (Avg.= 3.7 cm s-1) were generally not strong enough to resuspend sediments, while demersal fish were observed to resuspend sediments. Surface sediment mixing and removal was significantly stronger in the top 2 cm of the seabed compared to at deeper depths, indicating flood deposits thicker than 2 cm had higher preservation potential. Variations in seafloor relief, however, allowed for preservation of flashflood deposits within the seafloor in some locations through deposition within depressions in the seafloor, or subsequent burial by rapidly emplaced mounds (> 1 m-2) produced by burrowing organisms or by ensuing flashlood deposits. Flashflood deposits were not uniformly preserved within the seafloor, as they were altered at locally and temporally variable rates through biological mixing and resuspension. Therefore, to achieve ubiquitous flashflood deposit preservation, frequencies/magnitudes of flood events must be high enough to uniformly bury initially deposited flashflood sediment to depths below biological reworking.

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