A comparative study on the effect of acid preparation methodologies on bulk organic materials, and a long-core geochemical palaeoenvironmental reconstruction from South China.

Abstract

This thesis has two aims: i) an investigation of the effect of acid treatment on C/N, δ13C and δ15N of organic materials and; ii) the reconstruction of palaeoenvironmental change from a lake sediment sequence from south China, using geochemical proxies. To address these aims, a systematic comparative study was developed to examine the three most common acid treatment methods (capsule method; rinse method; fumigation method) and a range of acid reagents (HCl; H2SO3; H3PO4), on a range of terrestrial and aquatic, modern and geological sample materials. Acid treatment is a necessary step in the analysis of organic matter (OM) due to the distinct δ13C of inorganic carbon (IC) relative to the δ13C of OM; however, there is no consensus on “best practice”.

I find that C/N, δ13C and δ15N values of OM are not just dependent upon environmental processes but also on acid treatment method, which adds significant non-linear biasing to the OM signal several orders of magnitude above instrument precision. This biasing is caused by the inefficient removal of IC from sample materials and the alteration of OM by the acid treatment process. Consequently, this can significantly alter the environmental interpretation of these proxies, for example, in determining OM provenance, indicating that the assumption that the effect of acid treatment on sample OM is either negligible or systematic, is flawed, with biasing in C/N ~ 1 – 100; δ13C ~ 0.2 – 7.2 ‰ and; δ15N ~ 0.2 – 1.7 ‰ in my range of samples.

In addition, a long lake sediment sequence was cored from Lake Tianyang, Leizhou Peninsula, south China (20o31’1.11” N, 110o18’43.02” E) to reconstruct palaeoenvironmental variability using a suite of geochemical proxies (δ13C of bulk OM; XRF elemental ratios, magnetic susceptibility). The Tianyang δ13C record and elemental ratios (MIS 9 through MIS 6) show a strong glacial – interglacial imprint, providing evidence for periods of aridity during interglacial/interstadial periods in south China. The elemental ratios lose this imprint during MIS 5, likely due to an increase in catchment erosion (captured in the La, Nb, Ni, Th and Au records). Additionally, I tested the hypothesis that Ti concentrations reflect winter AM variability and showed that, whilst this may be valid, a robust interpretation is not possible due to the dilution of sedimentary Ti concentrations by local sources. A regional comparison of the Tianyang records appears to suggest a change in the strength of the land-sea thermal contrast in south China, implying a shift in mosiutre source region.