Little Ambergris

Exploring Microbial Mat Ecosystems on Little Ambergris Cay, B.W.I.

Little Ambergris Cay is a ~6 km long, 1.6 km wide uninhabited island on the Caicos platform in the Turks and Caicos. Much of the island is covered by microbial mats, vertically stratified collections of microorganisms that respond to vertical gradients in sunlight and chemistry. Little Ambergris provides an analog for the study of microbial mat development in the sedimentary record. Recent field mapping during July of 2016 used UAV- and satellite-based images, differential GPS (DGPS), and total station theodolite (TST) measurements to characterize sedimentology and biofacies across the entirety of Little Ambergris Cay. Nine facies were identified in-situ during DGPS island transects including oolitic grainstone bedrock, sand flats, cutbank and mat-filled channels, hardground-lined bays with EPS-rich mat particles, mangroves, EPS mats, polygonal mats, and mats with blistered surface texture. These facies were mapped onto a ~15 cm/pixel visible light orthomosaic of the island generated from more than 1500 nadir images taken by a UAV at ~350 m standoff distance. A corresponding stereogrammetric digital elevation map was generated from drone images and 910 DGPS measurements acquired during several island transects. More than 1000 TST measurements provide additional facies elevation constraints, control points for satellite-based water depth calculations, and means to cross-calibrate and reconstruct the topographic profile of bedrock exposed at the beach. Additionally, the thickness of the underlying Holocene sediment fill was estimated over several island transects using a depth probe. Sub-cm resolution drone-based orthophotos of microbial mats were used to quantify polygonal mat size and textures. The mapping results highlight that sedimentary and bio-facies (including mat morphology and fabrics) correlate strongly with elevation. Notably, mat morphology was observed to be highly sensitive to cm-scale variations in topography and water depth. Because of the sensitivity of the microbial mats to water depth, Little Ambergris is also an excellent site to track how the mats respond to short-scale changes brought on by drought, sea-level rise, and stochastic events like hurricanes. In September of 2017, Little Ambergris was directly hit by (in the eye of) the category 5 hurricane Irma. We will monitor how the mats respond to the influx of sediment caused by Irma.

Fieldwork was sponsored by the Agouron Institute.

See also:
Grotzinger, J.P. et al. (incl. N. Stein). 2016. Context, biogeochemistry, and morphology of diverse and
spatially extensive microbial mats, Little Ambergris Cay, Turks and Caicos Islands, B.W.I. AGU. B244B-07.

Gomes, M.L. et al. (incl. N. Stein). 2016. Linking the modern to the ancient with a comprehensive
geobiological understanding of biosignature preservation in microbial mats. AGU. B21E-0467.

Orzechowski et al. (incl. N. Stein). 2016. Age and construction of Little Ambergris Cay bedrock rim,
southeastern Caicos platform, British West Indies. AGU. EP11A-0990.

Trower, L. et al. (incl. N. Stein). 2016. Evidence of active ooid growth from Little Ambergris Cay, Turks
and Caicos, B.W.I. AGU. B24B-04.

Stein, N., Quinn, D.P., Grotzinger, J.P., Fischer, W.W., Knoll, A.H., Cantine, M., Metcalfe, K., Gomes, M.L.,
Grotzinger, H.M., Lingappa, U., O'Reilly, S.S., Orzechowski, E.A., Riedman, L.A., Strauss, J.V., and Trower, L.
2016. UAV, DGPS, and Laser Transit Mapping of Microbial Mat Ecosystems on Little Ambergris Cay, B.W.I.:
American Geophysical Union Annual Meeting, Paper No. EP21D-0921.