Hailong Sun

Evaluating multiple competing biotic and abiotic processes and how they interact with carbon dioxide (CO ₂ ) in aquatic systems, is critical to understanding regional and global carbon cycles. Here, we used an aquatic continuum of the Bahe watershed, within a semi‐arid region, to establish a comprehensive conceptual framework for identifying CO ₂ drivers from measured hydrochemical properties, heavy metals (HMs), dissolved organic matter (DOM) characteristics, and phytoplankton properties. The results revealed that dissolved CO ₂ (cCO ₂ ) showed large spatiotemporal variability. Our results highlight that carbonate buffering coupled with aquatic photosynthesis, collectively regulated the CO ₂ dynamics. Additionally, DOM and HMs have biogeochemical pathways that are common to CO ₂ dynamics (i.e., the biological carbon pump). In particular, HMs were found to have a high potential for signaling CO ₂ dynamics. The findings contribute to a better understanding of the carbon dynamics in an aquatic continuum in a semi‐arid region, together with the underlying mechanisms.

Uranium‐series dates from coral boulders constrain the timing of a medieval tsunami from the Puerto Rico Trench. Previously reported evidence for this tsunami includes hundreds of coral boulders that came to rest hundreds of meters inland on Anegada, British Virgin Islands. New U‐series dates on these coral boulders provide limiting dates for the tsunami. The narrowest limits were by dating interior bands of a coral that retains the hemispherical form of a living coral colony, and which include adjustments for the number of annual density band couplets between the dated samples and the boulder exteriors. By those limits, the tsunami dates between 1381 and 1391 CE, and likely occurred during summer or fall. The tsunami is important as the only reported sign that the eastern Puerto Rico Trench has produced a great earthquake. The dating may aid in defining the earthquake source and in communicating tsunami hazards.

The response of mineral weathering and its related Weathering Sink for atmospheric CO₂ (WS_atm-CO2) to global vegetation greening are not well understood. After applying different biogeochemical models and a field experiment to investigate the influence of vegetation greening and warming on the variations of carbonate weathering and WS_atm-CO2 on regional and global scales, here we show a significant positive relationship between global carbonate weathering intensity ([HCO₃⁻] as a proxy) and vegetation greenness. During 1982–2018, under a warming climate, [HCO₃⁻] and WS_atm-CO2 increase by 5.8% and 6.1%, respectively, due to vegetation greening, in the carbonate areas of Southwest China. Meanwhile, the [HCO₃⁻] in global carbonate areas increases by +2.4% during the same period. By contrast, the [HCO₃⁻] in global carbonate areas decreases by −1.3% without a vegetation function due to the warming. Moreover, we estimated that the carbonate weathering enhancements due to vegetation restoration at the global scale could reach 43.8%. Our results demonstrate that future vegetation restoration is important for the carbon capture by mineral weathering.

The molecular composition of the lipid biomarkers in the surface water, water column, and surface sediments collected along the Pearl River was investigated to identify the mechanisms of the delivery and preservation of autochthonous organic carbon (AOC) and to estimate its contribution to the carbon sink. The spatial distribution of these lipid biomarkers showed that samples collected at high-DIC-concentration sites (DIC: dissolved inorganic carbon) had prominent aquatic autochthonous signatures, while samples collected at low-DIC-concentration sites showed greater terrestrial contributions, which were described as the DIC fertilization effects. In the summer, typically, intense precipitation and flood erosion diluted the biogeochemical composition and carried terrestrial plant detritus. Therefore, the percentage of AOC (auto%) was higher in the winter than in the summer. According to the calculation of the lipid biomarkers, the values of the auto% were 65% (winter) and 54% (summer) in the surface water, 55.9% (winter) and 44.6% (summer) in the below-surface water, and 52.1% (winter) and 43.9% (summer) in the surface sediment, which demonstrated that AOC accounted for a major portion of the TOC. Vertical variability was mainly present in sites with intense flood erosion, which resulted in the mixing and deposition of resuspended sediments. There was a positive correlation of the clay content with the auto% value and the biogeochemical composition, showing that clay adsorbed the organic carbon in the water, vertically deposited it into the sediment, and was the dominant mechanism of the vertical delivery of organic carbon (OC). According to the new karst carbon sink model, based on coupled carbonate weathering and aquatic photosynthesis, the karst carbon sink flux (CSF) in the Pearl River was 2.69 × 10⁶ t/a which was 1.7 times the original estimation (1.58 × 10⁶ t/a), and this did not consider the formation of AOC. This indicated that previously, the contribution of the riverine system to the global karst carbon sink may have been highly underestimated.

The coupled carbonate weathering represents a significant carbon sink and can be controlled by the riverine hydrochemical variations. However, magnitudes, variations, and mechanisms responsible for the carbon sink produced by coupled carbonate weathering are unclear. In view of this, temperature, pH, dissolved oxygen, turbidity, electrical conductivity, and discharge of the Xijiang River at Wuzhou Hydrologic Gauging Station was recorded during October 2013 to September 2015 to elucidate the temporal variations in riverine hydrochemistry and their controlling mechanisms. To obtain the complete carbon sink flux (CSF) produced by coupled carbonate weathering with terrestrial aquatic photosynthesis in the river basin, the fluxes of dissolved inorganic carbon (DIC), autochthonous organic carbon (AOC, sourced from the transformation of DIC via aquatic photosynthesis), and sedimentary AOC were all considered. The results show that seasonal hydrochemical variations in the Xijiang River were related not only to dilution effects but also aquatic primary production. These results demonstrate that the variations in discharge caused by rainfall played a dominant role in controlling the variations in the CSF due to the chemostatic behavior of DIC and dissolved organic carbon (DOC). The CSF of the Xijiang River produced by coupled carbonate weathering was calculated as 11.06 t C km⁻² a⁻¹, including DIC carbon sink flux of 6.56 t C km⁻² a⁻¹, AOC flux (F_AOC) of 2.25 t C km⁻² a⁻¹, and sedimentary AOC flux (F_SAOC) of 2.25 t C km⁻² a⁻¹. The F_AOC and F_SAOC together accounted for approximately 69% of DIC carbon sink flux, or approximately 41% of the CSF, indicating that the riverine AOC flux may be high and must be considered in the estimation of rock weathering-related carbon sinks.