Radiocarbon (14C) dating is widely used to determine the age of organic material in palaeoenvironmental research. Here we compare 14C dates (n=17) resulting from macro-charcoal (>250 μm), short-lived plant macrofossils and pollen-rich residues isolated from two mire environments in eastern Australia. In most samples we found that short-lived plant macrofossils were the youngest organic component, the charcoal samples most often fell into the middle and the pollen-rich residues consistently returned older dates than the other samples. Although pollen-rich residues have been widely used for 14C dating in Australasia we suggest some caution in their use, perhaps because in our fire-prone environments these samples often also contain fine charcoal and other oxidative resistant organic matter that is older than the surrounding sediment matrix. The macro-charcoal samples also often returned older calibrated ages compared to short-lived plant macrofossils from the same depth, although this difference was relatively small (<245 years). Our results demonstrate that 14C dating of short-lived plant macrofossils are likely to yield more accurate chronologies and we advocate their routine use in palaeoenvironmental research when they are available.
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.
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