Reef island dynamics on recent-past and geological timescales : New insights from the Indo-Pacific region

Abstract

Global mean sea level has been rising during the 20th century and will continue to rise in the future. Changes in sea level affect morphodynamic processes along the coastline, irrespective of whether these changes are climatically induced or due to vertical land movements. The sensitivity of coastal morphodynamic responses to changing boundary conditions depends on external controls and the rate of change. Yet it is clear that internal characteristics also influence the evolution of a coastline as a steep rocky cliff will react different to, say sea-level rise, than a sandy beach and intuitively, low-lying islands appear least persistent. However, the geomorphic stability of low-lying reef islands is essential for the continued existence of island nations along tropical and sub-tropical latitudes. Situated on reef platforms, reef islands are recent geomorphic landforms that consist of sand- to gravel-sized carbonate sediments derived from the surrounding coral reef environments. The sediment is transported over the reef flat and accumulates in areas of least current velocity where it becomes stabilized by early-diagenetic carbonate sedimentation and vegetation. Despite the fact that these islands provide valuable land area, our understanding of island formation over geological time scales and recent-past shoreline morphodynamics is still limited. The present thesis analyzes reef island dynamics on different time scales and based on a multi-proxy approach. First, a shoreline change analysis based on remote sensing data is presented for 15 reef islands on Takú Atoll, Papua New Guinea, over a seven-decade period. Over this period, local mean sea level has been rising with an average rate of 1.3 mm/year. Based on a new method that allows for the detection of long-term changes in beach width, this dissertation shows that almost half of the analyzed beaches disappeared since 1943 and it is argued that sand mining is the main reason for the observed beach erosion. Furthermore, changes in shoreline positions and island area are analyzed based on the edge of vegetation as shoreline proxy. A long-term increase of total land area on the reef flat since 1943 is detected that shifts towards shoreline instability for the last decade of analysis. However, this apparent shift can be explained by an individual storm event that masked persistent high-frequency shoreline oscillations in its aftermath. This thesis presents evidence that shoreline change rates depend on observation intervals and that elevated rates underline the inherent dynamics of these landforms. Consequently, the geomorphic concept of a time dependent reef island equilibrium is suggested where islands are in dynamic equilibrium over cyclic time and in steady-state equilibrium over graded time. Second, this thesis provides new insights into reef island dynamics on geological time scales as a potential analogue for future morphological response. A mid-Holocene sea-level curve is presented for the Spermonde Archipelago, southwest Sulawesi, Indonesia based on surveyed and radiocarbon-dated microatolls. A single highstand of 0.5 m above present mean sea level is detected at 5600 cal. yr BP and implies that the hitherto suggested meter-scale sea-level oscillations in this area can be excluded. Furthermore, this thesis presents a new technique that has the potential to considerably improve age-constraints of island depositional chronologies by dating selected segments of the calcifying macro-alga genus Halimeda. Based on sediment samples from an uninhabited reef island in the Spermonde Archipelago, it is demonstrated that Halimeda segments that are internally not secondarily cemented yield the youngest radiocarbon ages for the analyzed set of sediment samples. This approach allows to minimize the unknown time period between the death of the sediment-contributing organisms and its ultimate deposition on the island and characterizes Halimeda segments as reliable targets for the reconstruction of reef island evolution. Finally, the present dissertation also deals with the question how the local population on Takú Atoll perceives reef island dynamics. Although the focus of this aspect is on the ethnographic perspective and the work is still in progress, it emerges that an integration of ethnographic and geomorphic disciplines is required for pushing forward potential adaptation strategies to climate change and sea-level rise for inhabited reef islands.