[H-QR04] Deep time perspective on the geological response to climate change

[HQR04-04] Reconstructing the paleoenvironmental changes of lagoon along the Guangdong coastal region in China

*Zih-Wei Tang¹, Liang-Chi Wang², Huei-Fen Chen¹, Yu-Min Chou³ (1.Institute of Earth Sciences, National Taiwan Ocean University, Keelung 202, Taiwan, 2.Department of Earth and Environmental Sciences, National Chung Chen University, Chiayi 621, Taiwan, 3.Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou, China)

Keywords:Guangdong, Pinqing Lake, lagoon, carbon isotope analysis, TOC, grain-size analysis

This study aims to explore the paleo-environmental changes and reconstruction of Pinqing Lake in the southeastern coastal area of Guangdong, China. A sediment core, 410 cm long, was obtained from the southeastern shore of the lake, and a total of 12 dating materials were obtained. The environmental history record of the region from approximately 8300 years ago to the present was interpreted using the C-14 dating method. According to the analysis using the R package, variations in sedimentation rates during different periods were identified. It was observed that the overall sedimentation rate of Pinqing Lake ranged from 0.02 to 0.22 cm/year, with an average sedimentation rate of 0.05 cm/year. This study conducted a multi-proxy comparative analysis of Pinqing Lake, including sediment grain size, TOC content, C/N value, carbon isotope analysis, and SI variations. During the mid-Holocene (8200-4200 cal. BP), the C/N value reached its maximum value, while the TOC content and δ13C values gradually increased. Between 8000-6000 cal. BP, there was a period of high fluctuation, reflecting the peak influence of the summer monsoon. This period coincided with the highest point of sea level rise. During this period, the fine-grained proxy factor EM1 also exhibited strong oscillation signals, positively correlating with other indicators. The sedimentation rate during this time was approximately 0.1 cm/year. Subsequently, between 6000-1000 cal. BP, the sedimentation rate almost ceased, dropping to 0.02 cm/year. The reasons for such slow sedimentation could be attributed to the stabilization of the sedimentary environment due to the cessation of sea-level rise or a reduction in the extent of the lake resulting in sedimentary interruptions at the sampling area. The grain size indicators and proxies representing coarse grains, EM3 and EM4, all show significant coarse grain events during 3300 cal. BP, and from 2000-1000 cal. BP. Finally, from 1000 cal. BP to the present, it appears that the appearance of upstream river channels led to a peak in sedimentation rate, reaching a maximum of 0.22 cm/year. Other indicators also show active trends during this period, with both grain size and SI indicators reaching their highest values. Over the past 1000 years, there is a positive correlation between SI and grain size trends, while the peak periods of C/N value are related to the accumulated number of typhoons in Guangdong and Hainan Island. Overall, the precise interpretation of indicators may be heavily influenced by sedimentary environments.