Reef island evolution in a turbid-water coral reef province of the Indo-Pacific

Abstract

Coral reef islands are vulnerable landforms to environmental change. Constructed of largely unconsolidated reef-derived sediments, they are highly sensitive to variations in metocean boundary conditions, raising global concern about their future resilience and stability in the face of increased natural hazards, sea-level rise and anthropogenic climate change. This study examines the evolution of an inshore turbid reef island from the southern Pilbara region of Western Australia (Indo-Pacific) using detailed analyses of island chronostratigraphy (composition, texture) and geochronology (21 in-situ radiometric dates) from Eva Island. Downcore, composition of island-grade (reef-derived) sediments were homogenous, dominated by molluscan (37%–42%) and coral (32%–37%) constituents. The 14C radiometric dating of island sediments, beachrock and coral microatolls identified five stages of island formation across changing sea-level regimes over the mid to late Holocene: (1) limestone platform accretion at ca 6,000 cal yr BP, coinciding with reef decline or ‘give-up’ on neighbouring Exmouth Gulf reefs; (2) sand cay (i.e. core) initiation and vertical aggregation at ca 5,000 cal yr BP during the point of sea-level regression to current levels; (3) major accretion and lateral progradation of the island between 3,500 cal yr BP and 2,500 cal yr BP including the modification of island shorelines through alongshore reworking of sediment; (4) lateral accretion and minor expansion to the north and formation of beachrock pavement between 2,500 and 650 cal yr BP; and (5) planform adjustment (erosion of the north-west island) and backstepping under stabilised sea levels over the past 650 years. While this model is comparative to studies on island formation following incremental sea-level fall following the mid-Holocene highstand, it demonstrates active landform readjustment under stabilised sea levels over the past 2,000 years, probably the influence of local-scale metocean boundary conditions within climate windows across the mid to late Holocene period (i.e. independent of sea-level fluctuations). Importantly, while sediment production rates are predicted to be lower in turbid-water reef systems than clear water, Eva Island shows no change in carbonate producers (i.e. proportion of mollusc and coral) over the course of island building, indicating the carbonate factory has not experienced significant adjustments in reef ecology, but has remained stable despite low water quality.