Coastal wetlands reveal a non-synchronous island response to sea-level change and a palaeostorm record from 5.5 kyr to present

Abstract

Here, we describe environmental change on Lord Howe Island (LHI; 31°30′S, 159°05′E) over the last 5500 cal. BP, derived from the analysis of the accumulating sediments in four coastal wetlands. Grain-size analyses, loss on ignition (LOI) and micrometre-resolution x-ray fluorescence (XRF) geochemical data were combined with 10 accelerator mass spectrometry (AMS) ¹⁴C dates to determine a chronology of environmental change. Sedimentary coastal features were initiated on the drowned LHI basalt coastline after ~4500 cal. BP, which was followed by gradual development of wetland environments from 4200 cal. BP to present. Within this period, a series of high-intensity storm events are recorded, perhaps related to low-pressure systems, including cyclones and East Coast Low (ECL) events. A lack of synchronicity between the studied sites resulted from their position in the landscape relative to the coast, features within the lagoon and greater sediment availability after 2800 cal. BP as sediment filled sinks in the lagoon floor. Increasing westerly wind strength from 600 cal. BP also combined with late-Holocene falling sea levels and lagoon infilling to facilitate the rapid growth of the coastal plain at ~500 cal. BP. The coastal wetlands of LHI preserve a record of rapid changes superimposed over more gradual environmental change since the mid-Holocene. The findings of this study further the understanding of the development of this ‘World Heritage Area’ as well as provide an increased understanding of how small oceanic islands respond to rising sea levels.