Wet and cold climate conditions recorded by coral geochemical proxies during the beginning of the first millennium CE in the northern South China Sea

The past two millennia include some distinct climate intervals, such as the Medieval Warm Period (MWP) and the Little Ice Age (LIA), which were caused by natural forcing factors, as well as the Current Warm Period (CWP) that has been linked to anthropogenic factors. Therefore, this period has been of great interest to climate change researchers. However, most studies are based on terrestrial proxy records, historical documentary data, and simulation results, and the ocean and the tropical record are very limited. The Eastern Han, Three Kingdoms, and Western Jin periods (25–316 CE) cover the beginning first millennium CE in China, and were characterized by a cold climate and frequent wars and regime changes. This study used paired Sr/Ca and δO series recovered from a fossil coral to reconstruct the sea surface water conditions during the late Eastern Han to Western Jin periods (167–309 CE) at Wenchang, eastern Hainan Island in the northern South China Sea (SCS), to investigate climate change at this time. The long-term sea surface temperature (SST) during the study interval was 25.1 °C, which is about 1.5 °C lower than that of the CWP (26.6 °C). Compared with the average value of 0.40‰ during the CWP, the long-term average seawater δO (−0.06‰) was more negative. These results indicate that the climate conditions during the study period were cold and wet and comparable with those of the LIA. This colder climate may have been associated with the weaker summer solar irradiance. The wet conditions were caused by the reduced northward shift of the intertropical convergence zone/monsoon rainbelt associated with the retreat of the East Asian summer monsoon. Interannual and interdecadal climate variability may also have contributed to the variations in SST and seawater δO recorded over the study period

Metabolic significance of temperature-dependent changes in annual resolution coral δ13C from the South China Sea during the mid-Holocene

The complicated effects of climatic and environmental conditions on coral δC have hindered its use as a universal proxy for biological activity and climate change. Changes in annual resolution δC levels from the South China Sea (SCS) were studied to explore the biological and climatic significance of coral δC during the mid-Holocene. The growth-rate-related kinetic isotope effect on modern coral δC may be limited, site- and/or colony-specific. Furthermore, coral δC and sea surface temperatures (SST) are inversely related, but their correlation is weak. These results suggest the influence of complicated controlling factors on modern coral δC. As for the mid-Holocene corals, no obvious growth-rate-related kinetic isotope effect has been found in their coral δC series, and the effect of solar irradiation on coral δC is difficult to directly evaluate for the low resolution of reconstructed solar records and the dating errors. However, mid-Holocene coral δC series show temperature-dependent changes over the studied periods during 6100–6500 yr BP. The significant negative correlation between coral δC and SST has been attributed to the high SSTs during the mid-Holocene. This is based on the observation that coral δC and the photosynthesis to respiration ratio (P/R) are usually positively related, but P/R is significantly and negatively related to temperature under high temperature conditions in that photosynthetic activity of heat-stressed corals will drastically reduce due to the decreases of population density and photosynthetic rate of the zooxanthellae

Coral Paleoclimate Perspectives Support the Role of Low‐Latitude Forcing on the 4.2 ka BP Event

Abstract The 4.2 ka BP Event is an abrupt climate change that might have contributed to the collapse of ancient civilizations and marks the transition between the mid‐ and late‐Holocene. Despite considerable research on this event, our understanding remains primarily based on terrestrial paleoclimate reconstructions, leaving a significant gap in understanding the role of the ocean in this event. Here, we present paired sea surface temperature (SST) and seawater δ18O reconstructions based on four fossil corals from the South China Sea. Our results demonstrate that the climate during the event was cooler, and there were meridional dry‐wet patterns in East Asia, indicating a weakened summer monsoon. Furthermore, our examination of additional coral records from the Pacific and Indian Oceans suggests that low‐latitude forcing (i.e., SST anomalies in the tropical Pacific) plays a crucial role in driving hydrology shifts in East Asia over the 4.2 ka BP interval

A Pilot Study on Zinc Isotopic Compositions in Shallow‐Water Coral Skeletons

Abstract The trace metal element zinc (Zn) participates in coral metabolic processes and therefore accumulates in their skeletons. These metabolic processes are largely controlled by the changes of environment in which they live, so Zn isotopic compositions (δ66Zn) in coral skeletons may possibly serve as potential tracers for climate and environmental changes. In this study, we first reported the δ66Zn in shallow‐water coral skeletons by investigating with monthly resolution δ66Zn values in the skeleton of a modern Porites coral 10AR2 from the Great Barrier Reef of Australia, and the bulk skeletal δ66Zn values of several coral species from the Luhuitou Reef of Hainan Island in the northern South China Sea. Correlations between δ66Zn and other climate and environmental proxies (Sr/Ca, δ18O, and δ13C) and instrumental environmental variables (sea surface temperature, river runoff, and chlorophyll a) are poor, suggesting that the effects of external environmental changes on monthly variations in δ66Zn in coral skeletons are not significant. However, significant interspecific differences in the skeletal δ66Zn of corals growing under identical external environments may suggest the occurrence of biologically controlled δ66Zn fractionation during coral skeletons formation. In addition, the monthly δ66Zn in the 10AR2 coral skeleton roughly decreases with increasing temperature, which is in agreement with the recent finding that δ66Zn in coral tissues and zooxanthellae increases with increasing temperature and can serve as a proxy for thermal stress in corals. We thus suggest that the complicated coral internal biological processes hinder the use of skeletal δ66Zn as a climate and environmental proxy