Differentiating between the d13C signature from environmental conditions and SOM cycling in eastern Australian peat sediments

The analysis of stable carbon isotopes is commonly used in Quaternary science to reconstruct the environmental conditions and vegetation contributions to sedimentary sequences. However, the measured d13C signature of the total organic matter (OM) pool can also reflect other complexities within depositional environments. The peats of the Thirlmere Lakes system in the southern section of the Blue Mountains World Heritage Area provides an excellent opportunity to closely scrutinise such d13C dynamics. These deposits are rich in TOC (20-40%) meaning analytical techniques such as 13C-NMR, used to characterise the OM pool, can be applied effectively. Furthermore, the identification of several peat units deposited over the last ~130 ka allows for temporal comparisons. d13C values determined for a 7 m sediment sequence from Lake Couridjah representing both the MIS 1 and MIS 5e interglacial periods vary by up to 4 to 6‰. These trends were subsequently identified in two other sediment sequences (Lake Baraba and Lake Werri Berri) proximal to Lake Couridjah. Initially we interpreted our results as reflecting a C3 dominated vegetation environment with MIS 1 wetter than MIS 5e, following the established relationship between water stress and d13C enrichment. However, spectral analysis of the OM pool indicates that d13C is driven by changing OM dynamics rather than large changes in environmental conditions. In these environments, the greater presence of carbohydrates (i.e. cellulose) in MIS 1 result in more depleted d13C values. In contrast, the MIS 5e peat is dominated by relative inert OM C fractions including charcoal and lipids (such as leaf waxes), which influences environmental proxies such as C/N. Thus, it is likely that the older MIS 5e peat is a more decomposed version of the active MIS 1 peat, and thus differentiating environmental conditions between the two using d13C alone is not particularly illuminating. To overcome this, we describe the d13C values for a coarse charcoal and high temperature hydrogen pyrolysis fractions, modern vegetation, catchment POC and DOC, and n-alkanes composition and generate catchment carbon models for both MIS 1 and MIS5e. Finally comparing the size of the OM pools of both interglacial deposits can provide useful information in estimating the carbon storage capacity of peat deposits in eastern Australia over these time scales. © The Authors.
Debashish Mazumder is miss spelt in the article author list which means the citation is sic. The name is shown as Mazumber.
Carbon 13, Environment, Australia, Peat, Sediments, Organic matter, New South Wales, Lakes
Forbes, M., Cohen, T., Marx, S., Sherborne-Higgins, B., Cadd, H., Francke, A., Cendon, D., Peterson, M., Mooney, S., Constantine, M., Boesl, F., Kobayashi, Y., & Mazumber, D. (2021). Differentiating between the d13C signature from environmental conditions and SOM cycling in eastern Australian peat sediments. Paper presented to the Australasian Quaternary Association/Friends of the Pleistocene Pop-up Conference 2021, 8-9th July 2021. Retrieved from: http://aqua.org.au/wp-content/uploads/2021/07/Abstract-book-2021-1.pdf