Mangrove wetlands are superior sites for the deposition of various pollutants, but they are also more susceptible to environmental contamination. To investigate the health threats of phosphorus to mangrove ecosystems, this study presents the distribution, chemical speciation and factors influencing phosphorus (P) forms in 38 surface sediments from the mangrove wetlands of Dongzhai Harbor on Hainan Island, China. P forms were measured using the sequential extraction (SEDEX) procedures. The results showed that the total phosphorus content in the sediment was at a high level, and there was a significant spatial variability of phosphorus in each form. Among them, inorganic phosphorus accounted for 74.64% of the total phosphorus, and organic phosphorus accounted for 25.35% of the total phosphorus. Among the inorganic phosphorus, detrital limestone phosphorus and Ca-bound phosphorus were dominant, while the content of exchangeable phosphorus and Fe-bound phosphorus had a smaller percentage. The concentration of biologically available phosphorus, ranged from 1.76 umol/g to 18.57 umol/g, and the Fe-bound and organic forms of phosphorus had a controlling effect on it. Based on C_org/OP, N/P and land use surveys, it was concluded that phosphorus was mainly an exogenous input. The correlation analysis concluded that the main sources of phosphorus in the study area are agriculture and aquaculture. The development of agriculture and aquaculture led to moderate levels of phosphorus pollution in the study area as a whole, with more serious a phosphorus pollution at the wetland park in the southeast corner of Dongzhai Harbor and in the western areas, with lush vegetation and developed river systems, mainly influenced by human activities, topography, vegetation and water system conditions.

Hainan Island is the only large island located on the northern margin of the South China Sea and is surrounded by Cenozoic graben basins, including the Qiongdongnan, Yinggehai, and Beibuwan basins. The uplift and denudation history of the Jianfeng pluton on southwestern Hainan Island is significant for understanding the formation of the regional geomorphology and adjacent basin evolution. This paper presents apatite and zircon fission-track (FT) analyses conducted on the Jianfeng pluton. The zircon FT (ZFT) ages of the pluton range are from 63 ± 4 to 108 ± 8 Ma, and the apatite FT (AFT) ages are from 19.4 ± 1.8 to 43.9 ± 4.4 Ma. The average confined track lengths in apatite are relatively short (11.9–12.8 μm). An age–elevation plot indicates that two rapid cooling events occurred during 73–63 and 44–40 Ma. Thermal modeling revealed four stages of 73–63 Ma, 44–40 Ma, 40–11 Ma, and 11–0 Ma. From the Late Cretaceous to the middle Eocene (73–40 Ma), the Jianfeng area underwent episodic rapid uplift and denudation. At the end of the Late Cretaceous (73–63 Ma), the area was affected by mid-ocean ridge spreading in the Proto-South China Sea. During the middle Eocene (44–40 Ma), the Yinggehai Basin underwent abrupt expansion and subsidence, which increased the elevation difference between the Jianfeng area and the Yinggehai Basin. From the middle Eocene to the middle Miocene (40–11 Ma), the Jianfeng area underwent slow denudation, and the Yinggehai Basin was rapidly infilled, which eliminated the original elevation difference between the two areas. From the middle Miocene to the present (11–0 Ma), the Jianfeng area has undergone reactivated rapid uplift and denudation, which was driven by the remote effects of the India–Eurasia collision.

The Nanping geothermal field in Hainan is situated within the Wuzhi Mountain fold belt of the South China fold system based on its geotectonic units. Although there is abundant surface heat detected and widespread distribution of Late Mesozoic granite in the area, the geological background of geothermal resources remains unclear. In this article, we collected core samples from boreholes within the Nanping geothermal field to conduct testing and analysis on rock thermal conductivity and heat-production rate. By combining these results with temperature logging data, we discuss a method for diterming the heat flow background of convective geothermal system. Furthermore, the study analyzed the geothermal flux and deep thermal structure of the research area. The results demonstrate that the average radioactive heat production rate of the Baocheng rock mass in the study area is 3.16 μW/m³, primarily attributed to the decay heat of Th and U, while the heat contribution of K is negligible. The thermal conductivity values of the rocks are relatively high, ranging from 2.29 to 3.75 W/(mK), slightly exceeding the average thermal conductivity of the upper crust. The study area represents a typical convective geothermal field influenced by groundwater convection, exhibiting a high geothermal temperature gradient. Using the groundwater-correction method, the geothermal flux in the study area is calculated to be 89–108.27 mW/m², of which the thermal conduction component is 73.17 mW/m² and the convective component is 15.83–35.1 mW/m². Among these components, heat generated from radioactive decay of crustal radioactive elements contributes 35.44 mW/m² to thermal conduction, while deep mantle conduction accounts for a heat flux is 37.73 mW/m², with a ratio of 1:1.07 between them. The difference between crustal and mantle heat fluxes is minimal in this region, indicating an approximation towards a “crust-mantle heat source balance zone”. Furthermore, the thickness of the “hot” lithosphere in the study area ranges from 42 to 46 km, indicating significant characteristics of extension-thinning.

Hainan Island is the only large island located on the northern margin of the South China Sea and is surrounded by Cenozoic graben basins, including the Qiongdongnan, Yinggehai, and Beibuwan basins. The uplift and denudation history of the Jianfeng pluton on southwestern Hainan Island is significant for understanding the formation of the regional geomorphology and adjacent basin evolution. This paper presents apatite and zircon fission-track (FT) analyses conducted on the Jianfeng pluton. The zircon FT (ZFT) ages of the pluton range from 63 ± 4 to 117 ± 8 Ma, and the apatite FT (AFT) ages from 19.4 ± 1.8 to 43.9 ± 4.4 Ma. The average confined track lengths in apatite are rela-tively short (11.9–12.8 μm). An age–elevation plot indicates that two rapid cooling events occurred during 73–63 and 44–40 Ma. Thermal modeling revealed a three-stage history of uplift and denudation. From the Late Cretaceous to middle Eocene (73–40 Ma), the Jianfeng area underwent episodic rapid uplift and denudation. At the end of the Late Cretaceous (73–63 Ma), the area was affected by mid-ocean ridge spreading in the Pro-to-South China Sea. During the middle Eocene (44–40 Ma), the Yinggehai Basin un-derwent abrupt expansion and subsidence, which increased the elevation difference between the Jianfeng area and the Yinggehai Basin. From the middle Eocene to middle Miocene (40–11 Ma), the Jianfeng area underwent slow denudation and the Yinggehai Basin was rapidly infilled, which eliminated the original elevation difference between the two areas. From the middle Miocene to present (11–0 Ma), the Jianfeng area has undergone reactivated rapid uplift and denudation, driven by the remote effects of In-dia–Eurasia collision.