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.
Plain Language Summary
Around 4,200 years ago, a major climate event occurred, resulting in a global megadrought that might have caused the collapse of ancient civilizations. However, most evidence for this event comes from land‐based paleoclimate studies, leaving a gap in our understanding of how it impacted the ocean. In this study, we analyzed fossil coral samples from the South China Sea and found that the area was cooler and wetter during the event. While the cooling appears to have been consistent across East Asia, the hydrological changes exhibited a meridional pattern, with Central China also experiencing wetter conditions while North and South China were dry. By examining coral records from the Pacific and Indian Oceans, we concluded that the Central Pacific El Niño was likely occurring during this event, weakening the East Asian Summer Monsoon and causing the associated climate and hydrology changes in East Asia.
Key Points
Paired fossil coral Sr/Ca and δ18O records indicate a cold and wet climate in the northern South China Sea during the 4.2 ka BP event
Coral and terrestrial reconstructions reveal consistent cooling but meridionally‐varied hydrological changes in East Asia around 4.2 ka BP
Low‐latitude sea surface temperature anomalies in tropical Pacific significantly contribute to the shift in hydroclimate in East Asia
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.
Plain Language Summary
Zinc (Zn) is an essential micronutrient during the growth process of corals, and its contents in coral skeletons have been investigated for tracing the history of marine environmental pollution. However, the characteristics and potential applications of δ66Zn in coral skeletons have not been reported so far. In order to better understand the significance of δ66Zn in coral skeletons, we first investigated the monthly resolution δ66Zn values in the skeleton of a modern Porites coral from the Great Barrier Reef, and the bulk skeletal δ66Zn values of other shallow water coral species from the Hainan Island in the northern South China Sea. The results indicate that the changes of monthly δ66Zn values in coral skeletons may not be primarily controlled by external climate and environmental factors. The significant differences of skeletal δ66Zn values among different coral species imply that internal biological activities of coral holobiont play an important role in Zn isotopic fractionation in shallow‐water coral skeletons. In this regard, the application of δ66Zn in shallow water coral skeletons as a climate and environmental indicator is limited.
Key Points
The seasonal and bulk zinc (Zn) isotope compositions in shallow‐water coral skeletons were first reported
The correlations between seasonal Zn isotope composition in coral skeletons and environmental variables are poor
Interspecific differences in coral skeletal Zn isotope compositions suggest a biologically controlled fractionation process
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•The climate was cold and wet during 167–309 CE in the northern South China Sea.•The cold SSTs were caused by the weak solar irradiance.•The wet conditions were caused by the reduced ...northward shift of ITCZ/monsoon rainbelt.•The interannual and interdecadal variability may be associated with ENSO and PDO.
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 δ18O 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 δ18O (−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 δ18O recorded over the study period.
Climate change in the mid‐to‐late Holocene transition is very important for predicting future climate trends and understanding the relationship between abrupt climate change and the development of ...past human civilization. In this study, Sr/Ca ratios and δ18O records with the annual resolution extracted from four fossil corals that were growing during the Middle Bronze Age Cold Epoch (MBACE) were used to reconstruct sea surface temperature (SST) and seawater δ18O (δ18Osw) in the South China Sea (SCS) during the mid‐to‐late Holocene transition. The results indicate that the SCS experienced a rapid cooling and wetting event during the period of ∼3,500–3,800 years BP (before present year 1950). Specifically, the average SST and δ18Osw declined rapidly by ∼3°C and ∼0.65‰, respectively, over an interval of ∼100 years from ∼3,850 years BP to ∼3,750 years BP. This rapid climate change pattern recorded in coral archives broadly agrees with those in foraminiferal and stalagmite records from adjacent land and ocean areas. Consistent with other records from the North Atlantic, this cold event in the Asia‐Western Pacific region that occurred during the MBACE and was originally identified in the North Atlantic and European regions should have occurred at the global scale, which might be caused by changes in the Asian summer monsoon linked with solar irradiance and/or the North Atlantic climate. In addition, this rapid climate change might support the occurrence and timing of the outburst flood event during the Xia Dynasty and might have led to the fall of the Xia Dynasty.
Plain Language Summary
Rapid and drastic climate changes will have serious impacts on the ecological environment and the development of human society. Studies on such climate events enable to better understand the relationship between climate change and human civilization, and provide valuable materials for predicting future climate trends. There was an extremely cold event during the period of ∼3,800–3,500 years BP (before present year 1950) in the Middle Bronze Age in North Atlantic and Europe region, which had a serious impact on European civilization. We carry out high‐resolution paleoclimate reconstructions during this period by using geochemical tracers in fossil corals from the South China Sea. The results indicate that the South China Sea and even the entire Western Pacific have experienced a rapid cooling and wetting event during the period of ∼3,800–3,500 years BP and the extremely cold event during the Middle Bronze Age was global scale. In addition, the consistency between the coral records and the stalagmite records from the inner land of China suggest that a rapid cooling and drying event during the Middle Bronze Age led to the collapse of the Xia Dynasty and Emperor Yu's success in taming floods was attributed to this event.
Key Points
A rapid cooling event over the western Pacific region during the Middle Bronze Age was identified
The cooling event recorded in coral archives broadly agrees with those in foraminiferal and stalagmite records
The rapid cooling and drying process in inland China might have led to the collapse of the Xia Dynasty
In the face of diverse challenges like global warming, ocean acidification, and human activities, the world's coral reefs are confronting a severe ecological crisis. Understanding the historical ...coevolution of corals with their environment and their resilience to current climate change is crucial for protecting these ecosystems and predicting their future. In this context, metal stable isotopes in corals present a novel and alternative methodology. Their significant fractionation during coral biological processes, persistent presence in coral skeletons, and relatively straightforward sources make them a valuable tool. However, the complexity of coral biology necessitates a deeper investigation into the fundamental mechanisms behind the isotopic fractionation of these biologically utilized metal elements. A comprehensive and systematic study of the roles of metal stable elements in coral biological processes is essential. This includes examining the fractionation of metal isotopes across different parts of the corals, such as tissues, zooxanthellae, and skeletons. To achieve these goals, multidisciplinary collaborations are essential. They should focus on several key areas: interpreting metal stable isotopes data in the context of coral physiology and ecology; conducting controlled laboratory experiments on coral cultivation; engaging in comparative studies with inorganically precipitated aragonites; and developing a holistic understanding within the framework of coral biomineralization models.
Plain Language Summary
Coral reefs around the world are facing serious threats from global warming, ocean acidification, and human activities. Understanding how corals have historically adapted to the environment and climate changes can provide useful information for protecting coral reefs and predicting their future. An innovative approach to this is examining stable isotopes of specific metal elements found in corals. These isotopes act as long‐lasting markers that can trace coral life cycles and reveal how corals adjust to changing climates from a geochemical standpoint. However, understanding how these metal stable isotopes vary in different parts of the coral, such as in tissues, symbiotic algae, and skeletons, is complex and needs further research. This calls for a united effort from both geochemists and coral biologists. Together, they can study these metal stable isotopes in relation to coral biology and ecology, perform detailed experiments on coral growth in the lab, compare coral skeletons to similar non‐living materials, and develop comprehensive models to explain how corals form their skeletons. This collaborative approach is vital for a deeper insight into coral resilience and for effective coral reef conservation.
Key Points
Deciphering coral adaptability to climate and environmental change is pivotal for predicting reef evolution
Bio‐utilized metal stable isotopes offer insight into coral adaptation to climate and environmental changes
Probing metal stable isotopes in corals requires interdisciplinary collaboration and integrated methods
The complicated effects of climatic and environmental conditions on coral δ13C have hindered its use as a universal proxy for biological activity and climate change. Changes in annual resolution δ13C ...levels from the South China Sea (SCS) were studied to explore the biological and climatic significance of coral δ13C during the mid-Holocene. The growth-rate-related kinetic isotope effect on modern coral δ13C may be limited, site- and/or colony-specific. Furthermore, coral δ13C 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 δ13C. As for the mid-Holocene corals, no obvious growth-rate-related kinetic isotope effect has been found in their coral δ13C series, and the effect of solar irradiation on coral δ13C is difficult to directly evaluate for the low resolution of reconstructed solar records and the dating errors. However, mid-Holocene coral δ13C series show temperature-dependent changes over the studied periods during 6100–6500 yr BP. The significant negative correlation between coral δ13C and SST has been attributed to the high SSTs during the mid-Holocene. This is based on the observation that coral δ13C 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.
•Suess effect and global warming lead to negative correlated modern coral δ13C-SST.•Mid-Holocene coral δ13C shows temperature-dependent changes.•Negative correlated fossil coral δ13C-SST is a response of coral to thermal stress.
► Dynamic recrystallization of powder metallurgy molybdenum occurs in the temperature region (1200–1450
°C). ► The value of strain hardening index
n decreases along with the temperature rising. ► The ...value of strain-rate sensitivity exponent
m increases slowly at first and achieves a peak value at 1350
°C. ► Deformation strengthening is the main strengthening mechanism at low temperature. ► Rheological strengthening becomes the primary strengthening mechanism at high temperature.
The high-temperature deformation behavior of powder metallurgy molybdenum has been investigated based on a series of isothermal hot compression tests, which were carried out on a Gleeble-1500 thermal mechanical simulator in a wide range of temperatures (900–1450
°C) and strain rates (0.01–10
s
−1). Through the research on the experimental stress–strain curves, it reveals that dynamic recrystallization softening effect of powder metallurgy molybdenum occurs in the temperature range from 1200
°C to 1450
°C, in which the flow stress is significantly sensitive to temperature. In comparison with the value of strain hardening index
n which decreases along with the temperature rising, the value of strain-rate sensitivity exponent
m does not change obviously; however, it increases slowly with the increasing of temperature at first and achieves a peak value at 1350
°C. Furthermore, relying on the comparison of mean value of
n and
m, it is suggested that deformation strengthening is the main strengthening mechanism at low temperature while the rheological strengthening changes into the primary strengthening mechanism at high temperature.
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