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.

Low-resolution Australasian palaeoclimate records of the last 2000 years

“…Some records are presented as having annual or sub-annual resolution (e.g. Martin et al, 2014); however, many of these records are not constrained by annually resolved markers. These types of records are regarded as non-annually resolved records in this study.…”

Temporal approaches of historical extreme storm events based on sedimentological archives

“…Sedimentological high-resolution elemental analyses are evaluated using an Avaatech© X-ray fluorescence (XRF) core scanner. Element intensities are normalized by the total intensity (Bouchard et al, 2011;Martin et al, 2014). Strontium (Sr) and Calcium (Ca) are the two elements which are mostly cited as marine (e.g.…”

“…To increase precision of the OM estimation, a LECO © carbon analyzer estimates the CO 2 percentage after a 1400°C dioxygen burning and a mineral decarbonizing with sulfuric acid solution (Andrews et al, 2008;Michaelson G. J. et al, 2011). Grain size and elemental analyses are also measured with a Malvern Mastersizer 2000 © particle size analyzer, and a Avaatech© X-ray fluorescence (XRF) core scanner (Bouchard et al, 2011;Gee and Or, 2002;Martin et al, 2014). Marine geochemical ratios, extracted from a statistical study, are used (Pouzet et al, 2019).…”

Response to Reviewer 1

“…For peat bogs, the mineral content is determined as the non-combustible fraction of dry peat mass (Marx et al, 2011(Marx et al, , 2009. Mapped and modelled variations in trace element signatures and ratios of "far-travelled" sediment can elucidate shifts in dust sources and net transport (Marx and Kamber, 2010;Marx et al, 2011;Petherick et al, 2009). Grain size and morphology of quartz grains have also been analysed as proxies for wind speed and aeolian transport pathways (Stanley and De Deckker, 2002).…”

“…Frozen sediment cores were sampled at multi-decadal resolution. Cores from both sites were examined for trace element concentration, which allowed for the reconstruction of overall dust deposition as a proxy for continental aridity, as well as contribution of dust from specific source areas (Marx and Kamber, 2010;Marx et al, 2005).…”