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Motu Formation and Evolution with Sea-Level Rise: Investigating Atolls in French Polynesia

This project aims to investigate the formation of motu or sandy cays and their response to sea-level rise. Motu are small islets found around the edge of atolls atop the reef platform. It is hypothesized that these islets may form due to tropical cyclone activity throwing coral rubble on top of the reef platform. However, motu are found on atolls that experience few to no large storm events (such as at the equator) so this cannot be the only mechanism of formation. These complex systems may contain vegetation and finer grained sand. We want to understand motu formation by using GPR to image the underlying geology and high-resolution GPS for precise elevation data on different Atolls in French Polynesia. We will collect surface sediment samples to understand the sedimentology of the islets. We are interested in two different sea level questions. First, accurately constraining the sea-level curve for the late Holocene and second understanding how these systems will respond over the next century to rising sea levels. Coral samples from relict high-stand coral reefs will be cored for radiocarbon and gas-bench dating to constrain a past sea-level curve for the area. Given the predicted rates of sea-level rise at least 1 meter by the end of the century (Vermeer and Rahmstorf, 2009), for island nations where the highest point of land maybe less than 5 meters, it is important to predict the response of these formations by first understanding their formation (Woodroffe, 2008). Moreover, motu may be the only habitable land on the atoll for humans. Given that sea level rise will produce more complex morphologic response than just inundation, we will gather evidence about how these islets form and thus predict their geomorphic response to changing sea level. The project will leverage a cruise with SEA that visits 5 different islands in French Polynesia with two weeks of extensive fieldwork on Maupiti and Rangiroa. This work on atoll and motu formation and evolution with sea-level rise will be a significant portion of my thesis and provide field data for modeling motu response to varying wave climate and sea levels.

Motu are geomorphic landforms composed of coral reef sediment, rubble, and relict reefs. These islets are capable of sustaining vegetation and maybe found in high-energy wave and storm environments (McLean and Woodroffe, 1994). They often have seaward shingle ridge and a leeward sand deposits (Murphy, 2009). As suggested in Figure 1 b), the coarse-grained rubble maybe deposited on the reef rim during large storm events (e.g. tropical cyclones) (Bourrouilh-Le Jan and Talandier, 1985; Harmelin-Vivien and Laboute, 1986). These same high-energy events may easily erode the fine-grained sand inwards towards the lagoon (Stoddart et al., 1971). The mean-wave climate, on the other hand, may tend to deposit the sand on the motu. Motu hold the majority of freshwater available in an atoll. Rainwater infiltrates through the partially lithified rubble and sand of the motu and forms an unconfined aquifer on top of the underlying salt water as seen in Figure 1 a (Terry and Chui, 2012). This freshwater lens is important for our work because it enables GPR for imaging the underlying lithology. Motu and atolls are morphologically dynamic landforms that respond to external forcing like sea-level change or a change in wave climate.

The current model for motu formation relies on extreme events like hurricanes generating coral rubble from the outer reef and depositing that rubble on top of the reef platform (Figure 1b). However, this hypothesis is incomplete as there are motu found on atolls along the equator, where there are no hurricanes. We would like to refine this model of motu formation by our findings from the fieldwork in French Polynesia. A variety of different atolls and islands will be visited including large atolls with tens of motu and other islands with only 2-3 motu (Figure 2) to understand their formation. We will visit different sites of the same atoll to comparatively look at the influence of environmental (wave climate) forcing on the motu development. We will utilize our findings as preliminary results for further grant proposals.

Sea level is important in controlling atoll and coral formation and growth. Corals can be used to estimate an approximate sea-level height by knowing the depth to which the coral will grow. Intact coral atop atolls serve as relict sea-level highstand markers and can be dated using radiocarbon or gas-bench. Several Pacific atolls experienced a sea-level highstand about 1 meter higher than modern sea level in the late Holocene (Mitrovica and Milne, 2002; Nunn, 1990; Pirazzoli and Montaggioni, 1986). We will constrain the Holocene sea level curve for French Polynesia and the late Holocene high-stand that some Pacific atolls may have experienced.

I have received partial funding from WHOI’s Ocean Venture Fund for the SEA cruise. We have partnered with SEA’s Sustainability in Polynesian Island Cultures and Ecosystems (SPICE) cruise, which travels to 5 different islands in French Polynesia over a course of 6 weeks. By participating on this cruise, I will get broad exposure to the landscape and culture of French Polynesia, assistant undergraduate researchers, increased equipment portability, and valuable collaboration. We would like to couple this cruise with two weeks of in depth fieldwork on Maupiti and Rangiroa. I would like to request funds from COI for the per diem expenses of the field research on Maupiti and Rangiroa. In addition, I am applying to the Exxon Mobil Geosciences grant for additional funds to cover laboratory analyses costs.

Bibliography

Bourrouilh-Le Jan, F., and Talandier, J., 1985, Sedimentation et fracturation de haute energie en milieu recifal: tsunamis, ouragans et cyclones, leurs effets sur la sedimentation et la geometrie d'un atoll: Marine Geology, v. 67.

Harmelin-Vivien, M. L., and Laboute, P., 1986, Catastrophic impact of hurricanes on atoll outer reef slopes in the Tuamotu (French Polynesia): Coral Reefs, v. 5. McLean, R. F., and Woodroffe, C. D., 1994, Coastal Evolution: Late Quaternary shoreline morphodynamics, Cambridge, Cambridge University Press.

Mitrovica, J. X., and Milne, G. A., 2002, On the origin of the late Holocene sea-level highstands within the equatorial basins: QuaternaryScience Reviews, v. 21. Murphy, F. J., 2009, Motu, in Gillespie, R. G., and Clague, D. A., eds., Encyclopedia of islands: Berkeley, University of California Press.

Nunn, P. D., 1990, Coastal Processes and Landforms of Fiji: Their Bearing on Holocene Sea-Level Changes in the South and West Pacific: Journal of Coastal Research, v. 6, no. 2.

Pirazzoli, P. A., and Montaggioni, L. F., 1986, Late Holocene Sea-Level Changes in the Northwest Tuamotu Islands, French Polynesia: Quaternary Research, v. 25.

Stoddart, D. R., Taylor, J. D., Fosberg, F. R., and Farrow, G. E., 1971, Geomorphology of Aldabra Atoll: Philosophical Transactions of the Royal Society of London, no. Series B, Biological Sciences.

Terry, J. P., and Chui, T. F. M., 2012, Evaluating the fate of freshwater lenses on atoll islands after eustatic sea-level rise and cyclone-driven inundation: A modelling approach: Global and Planetary Change, v. 88-89.

Vermeer, M., and Rahmstorf, S., 2009, Global sea level linked to global temperature: Proceedings of the National Academy of Sciences of the United States of America, v. 106, no. 51, p. 21527-21532.

Woodroffe, C. D., 2008, Reef-island topography and vulnerability of atolls to sea-level rise: Global and Planetary Change, v. 62.

Last updated: December 3, 2013