Using N-Alkane As A Proxy To Reconstruct Paleoenvironment In Thale Noi, Phatthalung Province

Abstract

Normal alkane (n-alkane) is the widely used hydrocarbon proxy for the paleoenvironmental study because of their specific characteristic and resistance to microbial degradation. The n-alkanes from the sediment cores TLN-CP5 and TLN-CP7 from Thale Noi, Phatthalung Province, were analyzed and subsequently compared with those derived from the recent plant samples. The plant samples were taken from mangrove forests, i.e. true mangrove, back mangrove, and mangrove associate, based on the previous study on pollen analysis. The n-alkane distributions, the average chain length (ACL), the carbon preference index (CPI) and the proportion of aquatic components (Paq) were further analyzed in order to reconstruct past environmental changes. For the modern plant samples, the n-alkane distributions, ACL and CPI values in each plant group are diverse and high uncertainty. These ambiguity results are possibly caused by growing environment influent on n-alkane synthesis and too small sample size to represent the entire plant groups. In contrast, the n-alkanes distribution and their parameters in lake sediment have obviously corresponded to change of paleoenvironment. Our results suggest that the change of C21, C27, C29, and C31 abundances of n-alkane indicate the dynamic of vegetations and water level in the lake. C21 relate to the high-water level that opposing to C27, C29, and C31. The ACL, CPI, and Paq can be used to indicate a sea-level fluctuation. During about 8,300 to 8,000 cal yr BP, the ACL and CPI gradually increase that suggest a sea level regression. And then the ACL and CPI gradually decrease about 7,600 to 7,200 cal yr BP that correspond to a transgression. A high Paq and increasing of ACL and CPI presence in the surface sediment demonstrate that freshwater has more influence than seawater after 7,200 cal yr BP to recent.