Abstract:
Coral atolls are unique landforms in that they are the physical manifestations of the interplay between both biological and geological processes. Prominent amongst these processes is the ability of the reef organisms to produce CaCO3 and its subsequent erosion and dispersal as sediment. Overriding controls on this process are organic productivity, wave energy, and relative sea level. The development and stability of atolls are thus critically dependent on the balance between several processes which may change over time. Atolls are regarded as being particularly vulnerable to environmental change.
This study investigates the Holocene geological history of Palmyra atoll, at 5°52’N 162°04’W, in the northern Line Islands. Beachrock is used as an indicator of (a) paleo-sea level and (b) paleo-shoreline conditions from clasts trapped within the beachrock matrix. The study also models annual CaCO3 production and hydrodynamic conditions at the sea bed to provide an integrated assessment of the past and present sedimentary processes and reef island development at Palmyra Atoll. The atoll is currently the focus of intensive scientific study by the Palmyra Atoll Research Consortium and is particularly suited to this study because of the reduced human presence. This allows the examination of the relationship between beachrock, islet development and other processes, in an environment lacking ongoing anthropogenic development.
Beachrock was found at 10 locations at Palmyra Atoll and yielded 14C ages ranging from 1249 to 105 cal. yrs BP. Typically, the beachrock contains mostly coral and algal clasts and is thought to form in the intertidal zone. Continual wetting and drying throughout a tidal cycle results in the precipitation of marine phreatic cements, which thus, indicate paleo-shorelines and sea level elevation. The production of CaCO3 sediment at Palmyra was estimated using reef habitat zones from Hopley (1996) and suggests that the most productive areas are reef terraces and the reef edge. An estimate total of 91,500 tonnes of CaCO3 is produced annually on the reefs, although only approximately 9 % of this becomes sediment that remains on the reef islands.
Hydrodynamic processes were modelled using the SWAN model, a bathymetric grid from NOAA, and bottom conditions estimated from other studies. Input parameters were determined using a 13 year WAVEWATCHIII hindcast model of the wave climate for the central Pacific, as well as estimations of extreme wave events. Sediment transport was inferred from the modelled bed shear stress and these results show that to form most of the beachrock outcrops on Palmyra extremely strong wave action must be coupled with a higher sea level in order to allow the propagation of wave energy across the reef to some of the island shorelines.
Integration of all results suggests that growth of the reef islands at Palmyra Atoll was initiated as the sea level fell from the mid-Holocene Highstand, 1-2 m above present mean sea level. The islands subsequently grew progressively eastward, forming 3-4 island chains which strike north or northeast. The beachrock that formed on these island provided protection from later wave erosion. Despite limitations caused by lack of climatic and other environmental data due to the isolation of the study area, results are reliable and highlight the application of beachrock as a proxy for past climates and sea levels.
