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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.

The Foram Index (FI) is used to evaluate the ecological health of coral reef areas based on the content of benthic foraminifer functional groups (symbiont-bearing, opportunistic, heterotrophic) in the sediments of coral reef areas. Thus far, the application of the FI in China remains limited. In this study, an LYJ2 sediment core, 287 cm in length, which is estimated to represent approximately 2,665 years before present (a BP), was obtained from the lagoon of the Lingyang Reef on the Xisha Islands. Samples were taken at intervals of 0.5 cm to identify the foraminifera under a microscope, and the FI values were calculated. The key results are as follows. 1) the FI in the last 2,600 years ranges from 4.1 to 7.9, mean value of 5.9. 2) FI values show a fluctuating pattern, which can be divided into three periods of increased abundance: 2,380‒1,628, 1212‒572, and 252‒92 a BP; three periods of rapid decline: 1,628‒1,212, 572‒252, and 92 a BP; and one period of moderate decline: 2,665‒2,380 a BP. 3) FI exhibits interdecadal fluctuations at different scales, with cycle of 66.7 and 54.4 a . Based on the relationship between the FI values and the health status of coral reefs, we speculate that the ecological environment of the Xisha Lingyang Reef has been generally healthy for the past 2,600 years. Based on a comparative analysis of the FI and sea surface temperature, it appears that FI is mainly affected by sea surface temperature, which , in turn, is related to climate change. A high FI roughly corresponds to the Medieval Warm Period and Roman Warm Period, and a low FI roughly corresponds to the Little Ice Age (LIA) and Dark Age Cold Period (DACP). The FI values exhibit three stages of rapid decline. The first two (1,628‒1,404 and 572‒252 a BP) correspond to LIA and DACP, respectively, which may be due to the increase in atmospheric dust, rainfall, and nutrients in the waters of Lingyang Reef caused by the enhancement of the winter monsoon in the cold period. The later period (92 a BP to date) corresponds to the rapid degradation of coral reef ecosystems in the Xisha Islands in recent decades, possibly due to increased nutrient concentrations in the Antelope Reef Sea area caused by increased human activity and atmospheric nitrogen deposition fluxes. Studies have shown that FI can generally record the health status of coral reefs in the South China Sea and can be used to evaluate the health status of coral reefs in geological history.