Redai dili (Oct 2023)
Growth Rate of Porites Corals from Tanmen, Hainan Island: Climatic Significance during Mid-Holocene
Abstract
Coral growth rate is a physical index that is particularly closely associated with climatic factors such as Sea Surface Temperature (SST). The growth rate is minimally affected by environmental changes and serves as a high-resolution indicator in the study of climatic and environmental variation, making it a useful tool in paleoclimatic coral research. However, previous research on coral growth rate has primarily focused on the modern period, with limited investigation of the Mid-Holocene, which shares similar climatic characteristics with the current warm period. It is especially important to understand the climate and environmental conditions during the Mid-Holocene to gain insight into contemporary climate change processes and mechanisms. To address this gap, we first collected a modern Porites coral core and 23 Mid-Holocene subfossil coral cores from Tanmen, located on the eastern coast of Hainan Island. Using X-ray radiography and image processing methods, we measured and analyzed the coral growth rates of all cores, resulting in the identification of growth rate patterns during the Mid-Holocene. Additionally, by analyzing the environmental factors that impact modern coral (2005-2021 AD), we established a linear relationship between coral growth rate (L) and SST in Tanmen, expressed as SST=2.945±0.237×L+22.481±0.301(1 s.e.). By applying this equation, we reconstructed an annual average SST sequence of 406 years during the Mid-Holocene (6,143-4,356 a BP). The results indicate that the average coral growth rate during the Mid-Holocene was 1.079 cm/a, ranging from 0.607 to 1.670 cm/a, with noticeable fluctuations. The coral growth rate sequence also revealed three consecutive periods of low growth rate, accompanied by a significant increase in interannual variability after 4,515 a BP, resulting in more complex fluctuations. Moreover, the reconstructed SST sequence based on coral growth rate data shows that the mean SST of the Mid-Holocene was 25.7±0.54 ℃, which is comparable to the modern SST in the context of global warming. The average annual SST varied from 24.7 to 26.8℃ in the Mid-Holocene, exhibiting considerable fluctuations between warm and cold periods, and with three distinct periods of low SST at 5,860, 5,660, and 5,160 a BP. This provides detailed insights into the temperature variations during the Mid-Holocene. In addition, a comparison of the spectral cycles of coral growth rates between the modern and Mid-Holocene periods (5,427-5,394, 5,243-5,209, 4,515-4,456, and 4,404-4,356 a BP) revealed significant ENSO cycles of three to seven years, observed in both modern and Mid-Holocene corals. During the Mid-Holocene, the primary ENSO cycle changed: its frequency decreased considerably, indicating that ENSO activity was weaker than that observed in the modern period. However, further statistical analysis utilizing the Probability Density Function (PDF) demonstrated a gradual increase in ENSO variability during this period. The results of this study offer novel insights into tropical climate characteristics of the Mid-Holocene.
Keywords
