Abstract
Facies-belt dynamics are poorly understood at timescales of decades to centuries because of the lack of quantitative data spanning these time periods. A wealth of vintage-military aerial photography acquired in the years surrounding the Second World War exists, however, and can be paired with modern high-resolution satellite imagery to quantify temporal change over sufficiently long periods to allow meaningful extrapolation to geological timescales. This project focuses on quantifying the dynamics of inter- and supratidal carbonates at two ends of the energy spectrum. For the high energy, I follow Purkis et al. (2016) and consider the 40-yr. migration of the coastlines of 20 atoll islands situated atop isolated carbonate platforms in the Chagos Archipelago (Indian Ocean). My data set indicates the these fundamental characteristics of coastline behaviors: (i) coastlines facing the prevailing trade winds retreat through time, while those in leeward positions tend to expand; (ii) coastline expansion and retreat are in balance such that total land area of all the considered islands are virtually static over the last 50 years through using commonly accepted +/-3% threshold; (iii) small islands (<20 ha) are substantially more dynamic than large ones. It is worth noting that the stretches of the Diego Garcia coastline in close vicinity to human modification are more likely to suffer erosion than those situated far from human activity, and more likely to erode than those on the uninhabited islands of Peros Banhos. A comparison between the behavior of a broad portfolio of islands spanning the Indian and Pacific Oceans emphasizes that local factors to be better determinants of coastline dynamics than the rate of eustatic sea-level rise. In aggregate, our data suggest that atoll islands are likely to persist in the face of accelerating sea-level rise, but their high rate of coastline dynamics might plausibly challenge habitability. For low-energy, I examine the tidal flats of Andros Island (Bahamas) – one of the “classic” venues for comparative carbonate sedimentology. The time-separated data assembled for Andros spans 75 years. My results suggest that: (i) the seaward margin of the intertidal deposit remains static in the face of rising sea level, but the deposit’s landward margin retrogrades markedly, broadening the intertidal channelized zone; (ii) in unison to the broadening of the channelized zone, the abundance of laminated (Scytonema) cyanobacterial fabrics within it decreases, accompanied by the lengthening and avulsion of the network of tidal channels that traverse this zone; (iii) the prominence of these changes to the architecture of the flats are surprising, given that the amplitude of sea level rise during the 30 yr. period of observation was only 10 cm. Through the concept of comparative sedimentology, it is anticipated that my observations from the modern might be applied to better recognize the onset of the transgression in ancient carbonate deposits, even for low-amplitude, sub-orbital sea-level cycles.