Development of a southern hemisphere subtropical wetland (Welsby Lagoon, south-east Queensland, Australia) through the last glacial cycle
Introduction
The last glacial cycle (from ca. 123,000 years before present; 123 ka) is a period of substantial environmental and cultural importance in Australia, encompassing the arrival of humans and the mass extinction of the Australian megafauna (Clarkson et al., 2017; Hamm et al., 2016; Roberts et al., 2001). The main terrestrial environmental and climate information for this period originate from the interpretation of proxies preserved in wetland sediments. However, the scarcity of continuous wetland sedimentary sequences extending beyond the Last Glacial Maximum (LGM; previously defined as 22–18 ka, Reeves et al., 2013a), through MIS3 (57–29 ka) and MIS4 (71–57 ka), hampers efforts to robustly investigate environmental and climatic changes. The majority of Australian sedimentary records of substantial antiquity suffer from depositional hiatuses or sedimentary deflation, commonly during the period of cold and arid climates characteristic of the LGM (Black et al., 2006; D'Costa et al., 1993; Harle et al., 2002; Longmore, 1997; Singh et al., 1981). Thus, spatially diverse interpretations of continental climate and Australian landscape change are lacking during the important time periods of MIS3 and MIS4 (Turney et al., 2006).
Welsby Lagoon provides a regionally important sedimentary record of changes in subtropical, eastern Australia during MIS3 and MIS4. Optically stimulated luminescence (OSL) ages from the basal sands of Welsby Lagoon indicate that the wetland formed during MIS5, with a basal age of ca. 130 ka (Tibby et al., 2017). The two other continuous wetland sedimentary sequences that extend through MIS3 and MIS4 (Fig. 1), Caledonia Fen in the cool-temperate south (Kershaw et al., 2007b) and Lynch's Crater in the wet tropics (Kershaw et al., 2007a), have become the focus of a substantial number of studies (e.g. Johnson et al., 2016; Kershaw et al., 2007a, 2007b; Rule et al., 2012; Turney et al., 2004). These spatially isolated records provide the basis for many interpretations of terrestrial climate and environmental change for continental Australia. However, as highlighted by Reeves et al. (2013a), Australian climate variability during the last glacial period was spatially complex. The wide diversity of topography, large latitudinal range, extensive low-relief interior and fringing mountainous areas, has led to asynchronous, and often contradictory, responses from wetland, vegetation and ocean records. Welsby Lagoon is a lowland, subtropical site that fills a large spatial gap between the previously documented continuous terrestrial MIS4 and MIS3 records from tropical and temperate eastern Australia (Fig. 1).
Understanding the depositional nature of wetland sedimentary sequences is a fundamental requirement for their use in robust reconstructions of external environmental and climatic conditions (Birks and Birks, 2006). Furthermore, knowledge of the origin and fate of organic matter is essential for robust palaeoclimate interpretations based on organic proxies. Sedimentary organic matter in wetlands may be derived from terrestrial production within their catchments or be produced by autotrophs within wetlands. Autochthonous primary producers in wetlands typically consist of algae and cyanobacteria, as phytoplankton, periphyton or metaphyton and submerged, floating or emergent vascular macrophytes (Finlay and Kendall, 2008). In macrophyte-dominated wetlands, phytoplankton production is typically low, and the dominant source of sedimented organic matter is macrophytes (Blindow, 1992). The importance of emergent macrophytes diminishes with increasing wetland water depth, such that in deep lakes, primary production and hence organic matter deposition, tends to be dominated by unicellular photoautotrophs (Schlesinger and Bernhardt, 2013). These varying sources of organic matter are charactered by different concentrations and isotopic ratios of carbon and nitrogen (Meyers and Teranes, 2001).
In order to understand the history of wetland formation and variability in the sources of organic matter at Welsby Lagoon, we analysed carbon and nitrogen isotope ratios (δ13C and δ15N) of organic matter and total organic carbon (TOC), total nitrogen (TN), macrofossil remains, autotroph pigments, sediment lignin content and pollen composition categorised into plant habitat from a 12.7 m sediment core. This multi-proxy approach provides a reconstruction of the depositional environment of Welsby Lagoon from which we infer its evolving nature, particularly its progression from a lacustrine to palustrine wetland. This foundation study provides vital information to support future research into the climate and environmental history of this region.
Section snippets
Regional setting
North Stradbroke Island (NSI; 27°27′S, 153°28′E), south-eastern Queensland, is situated in the subtropical climate zone of south-eastern Australia (Fig. 1A). The ocean proximity moderates temperatures, with the island experiencing warm, humid summers (mean 26 °C) and mild, dry winters (mean 19 °C) (BOM, 2018). The majority of annual precipitation (1500 mm) occurs during Austral summer and autumn months, with only 15% of rainfall occurring from July to October (BOM, 2018).
The key synoptic
Sediment coring
The thickest sedimentary sequence, and hence the location of core collection, was identified by a systematic sediment probing survey (Fig. 1C and D). Coring was undertaken in approximately 0.5 m of water depth from a Kawhaw coring platform. Sediments were extracted as two, 0.5 m offset, parallel cores (WL15-1 and WL15-2) extending to 12.78 m and 12.72 m depth respectively. In this study only WL15-2 is discussed. The coring process minimised sediment exposure to light by using black PVC tubing
Results
The modern plant and algae samples all have δ13C values consistent with those of C3 plants (−24 to −34‰, Fig. 2, Supplementary Table 1; Meyers and Teranes, 2001). Terrestrial plants have the lowest δ13C values of all samples, from −30.4 to −31.6‰ (Fig. 2A and C). Algae exhibited the largest range of δ13C, from −27.2 to −31.7‰ (mean −29.2‰) (Fig. 2A and C). Emergent aquatic plants have the highest δ13C, from −23.1 to −29.3‰, however these still fall within the global C3 plant range (Fig. 2A).
Discussion
The modern terrestrial, algae and emergent aquatic plant samples indicate that organic material with C:N > 20 may be indicative of either lacustrine algae, emergent aquatic or terrestrial plant sources. In general, terrestrial and aquatic vascular plants contain large proportions of nitrogen poor structural carbon, such as lignin, leading to high C:N ratios. Non-vascular, unicellular algae contain minimal structural carbon and are relatively nitrogen rich due to their high protein content,
Conclusions
We have inferred the evolution of Welsby Lagoon and its interactions with broad scale prevailing climate for the last ca. 130,000 years by considering patterns in the variability of multiple proxies preserved in the 12.7 m sediment sequence of Welsby Lagoon. The formation of a perching layer in the basin that now supports Welsby Lagoon occurred at ca. 130 ka during MIS5e. Continuous sediment deposition began sometime after this and has persisted until present.
The Welsby Lagoon sediments from
Acknowledgments
We acknowledge Minjerribah (NSI) and the surrounding waters as Quandamooka Country. The study was funded by Australian Research Council project DP150103875. We thank Jie Chang, Jacinta Greer, Matthew Jones, Patrick Moss and Cameron Schulz for assistance in the field. We sincerely thank Fred Oudyn for analysis and data processing of pigment samples with such efficiency and Janine McGowan (CSIRO Agriculture and Food) for assistance with NMR sample analysis and data processing. We thank Adriana
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