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Atmosphere‐ocean dynamics in the tropics play a key role in global climate change. An abrupt cooling event that occurred between 4500 and 3900a BP, known as the 4.2 ka event, has long been believed to be linked to changes in sea surface temperatures (SSTs) associated with intensified El Niño and Southern Oscillation (ENSO) variability. However, the precise timing and amplitude of ENSO variability during the 4.2 ka event remain uncertain, largely due to the lack of resolution in records from the ENSO‐sensitive regions. Here we present a 104‐year‐long monthly resolved coral record from the South China Sea that spans from 4400 to 4300 years BP, corresponding to the onset of the 4.2 ka event. Using the Sr/Ca paleo‐thermometry, we show a significant decrease in ENSO variability during this time interval compared to the modern period. The ENSO events decrease in the frequency from one every 3.5 years during the modern era to one every 5.6 years during the onset of the 4.2 ka event, with weaker magnitude in the latter period. This reduction in ENSO variability is largely associated with an intensification of the Pacific Walker circulation. We therefore suggest that ENSO variability was a response to, not a driver of, the development of the 4.2 ka event. Plain Language Summary: Abrupt climate change occurred between 4,500 and 3,900 years ago, known as the 4.2 ka event, stands out as one of the most prominent climatic events in the Holocene and witnesses a northern hemisphere cooling and agricultural‐based civilization collapsed. A growing number of terrestrial records have shown that anomalous sea surface temperatures in the tropical oceans associated with intensified El Niño and Southern Oscillation (ENSO) variability triggered this abrupt climatic event. However, precise ENSO variability based on tropical oceanic records is still scarce, hindering our understanding of this event. Here, we evidence a significant reduction in ENSO variability during the onset of the 4.2 ka event using multiple monthly resolved coral records from the tropical oceans. This contrasts with previous studies that suggest increased ENSO variability triggers the 4.2 ka event, implying that anomalous ENSO variability may be a response to the 4.2 ka event. Key Points: Monthly resolved coral Sr/Ca record provides a century‐long evolution of El Niño and Southern Oscillation (ENSO) variability during the onset of the 4.2 ka eventThere is a significant reduction in ENSO variability during the onset of the 4.2 ka eventReduced ENSO variability during this time interval is associated with intensified Pacific Walker circulation [ABSTRACT FROM AUTHOR]

The trade winds cause strong upwelling in the eastern equatorial Pacific, and create the eastern Pacific Cold Tongue (EPCT) that has far‐reaching impacts on global climate. However, large discrepancies persist in quantifying 20th‐century EPCT sea surface temperature (SST) changes across different instrumental data sets. Here we synthesize four coral Sr/Ca‐SST records from the tropical central‐eastern Pacific to develop a Cold Tongue Index (CTI) reconstruction for 1887–1997. The coral CTI record shows a rapid 20th century warming of the EPCT, suggesting an underestimation of warming trends in instrumental CTI records. The decadal to multidecadal changes in reconstructed EPCT SST show an association with the Walker Circulation. Our reconstruction indicates that recent EPCT cooling during the global warming hiatus is not unusual in the context of the 20th century. Our results provide new evidence for 20th century EPCT SST changes and an observational constraint for predicting future tropical climate changes. Plain Language Summary: The tropical Pacific exhibits an asymmetric pattern of sea surface temperatures (SST) along the equator, with a warm pool in the west and a cold tongue in the east. The Eastern Pacific Cold Tongue (EPCT) plays a crucial role in influencing global climate. To date, however, considerable disagreement persists about changes and causes of the EPCT SST over the 20th century due to the sparse instrumental observations before ∼1960. Here we synthesize published coral Sr/Ca‐SST records from the tropical central‐eastern Pacific to develop a coral Cold Tongue Index (CTIcoral) for the period 1887–1997. The CTIcoral record correlates significantly with instrumental data since 1960, and thus allows reliable assessment of 20th century EPCT SST changes. Our record exhibits a rapid 20th century warming trend of the EPCT, indicating an underestimation of warming trends in instrumental CTI records. The decadal to multidecadal changes in CTIcoral also show an association with the Walker Circulation. Comparing the magnitude of the 1992–2011 trend in instrumental CTI with our reconstruction reveals that recent EPCT cooling during the global warming hiatus is not anomalous in the context of 20th century. Our findings have implications for predicting future EPCT SST changes by offering a constraint for model simulations. Key Points: Synthesizing coral Sr/Ca‐SST records yields robust estimates of 20th century warming trend in the eastern Pacific Cold Tongue (EPCT)A coral‐based reconstruction indicates an underestimation of 20th century EPCT warming trends in instrumental SST recordsRecent EPCT cooling during the global warming hiatus is not anomalous in the context of the 20th century [ABSTRACT FROM AUTHOR]

Strontium isotopes (87Sr/86Sr) are commonly used to assess the age of Cenozoic dolostones found on many isolated oceanic islands. The possible influence of Rubidium (Rb) content on the 87Sr/86Sr ratios, however, has generally been ignored. This critical issue is assessed by examining the 87Sr/86Sr ratios and elements (Rb, Sr, Al, Ti and K) of seventy‐one samples that came from a ca 339 m thick cored interval of Cenozoic dolomite on the Xisha Islands, South China Sea. For the dolomite from the upper Pliocene Yinggehai Formation (180.5 to 308.5 m; average 23% dolomite) there is a positive correlation between the 87Sr/86Sr ratios and Rb concentrations, whereas for the dolomite from the lower late Miocene Huangliu Formation (308.5 to 519 m; average >98% dolomite) such a correlation is not evident. Combined with high‐temperature experiments, correlation between Al, Ti, K, Rb and 87Sr/86Sr indicates that Rb containing old 87Rb from terrestrial materials modified the 87Sr/86Sr ratios of the Yinggehai dolomite, but not those from the Huangliu dolomite. This is mainly attributed to the different sources of terrestrial dust carried by different East Asian monsoon systems between the late Miocene and Pliocene, and probably also influenced by the dolomitization process to some extent. To test for 87Rb contamination, extrapolation of results from the Yinggehai dolomite and Huangliu dolomite indicate that concentrations of Rb < 1 ppm, Al < 1000 ppm, Ti < 15 ppm and K < 250 ppm in the dolomite indicates that terrestrial inputs are minimal and that the measured 87Sr/86Sr ratios can be used for determining the dolomitization age. Potentially, these concentration thresholds may serve as a screening criterion for assessing the reliability of Sr isotope chronostratigraphy of other island dolostones throughout the world. [ABSTRACT FROM AUTHOR]

Understanding the history of sea‐level changes in the past and the associated mechanisms is crucial for predicting sea level in the future. The South China Sea is the largest semi‐enclosed marginal sea in the western tropical Pacific. Previous studies showed that this region experienced several sea‐level fluctuations up to half a meter on centennial timescales ∼7,000 cal yr BP. These fluctuations were greater in magnitude than the rise in global mean sea level induced by greenhouse effect over the 20th century. However, whether such sea‐level fluctuations occurred in a wider temporal context and their potential causes remain unclear. To answer these questions, we reconstructed the mid‐Holocene sea‐level fluctuations in the northern South China Sea using nine coral microatoll fossils collected from Hainan Island. The results suggest that sea‐level fluctuations with amplitude of about half a meter on centennial timescales were common during 6,143–4,384 cal yr BP. Based on recent studies of sea‐level driving mechanisms in the South China Sea and paleoclimate reconstructions, we infer that these sea‐level fluctuations were likely driven by changes of Walker circulation intensity. Due to the internal variability of the climate system, the possibility of similar sea‐level fluctuations in the foreseeable future cannot be ruled out. Key Points: Microatolls on Hainan Island revealed multiple half‐meter sea‐level fluctuations on centennial timescale during the mid‐HoloceneThese sea‐level fluctuations are similar to those occurred ∼7,000 cal yr BP in other regions of the South China SeaThe responsible mechanisms were likely related to the changes of the Pacific Walker circulation intensity [ABSTRACT FROM AUTHOR]

Tropical cyclones (TCs) lead to huge economic losses and casualties in coastal regions, but multi‐decadal variation of TC remains poorly understood due to the limited length of observational records and low‐resolution reconstructions. Porites corals provide continuous and monthly resolved records that allow for the determination of weather‐timescale (weekly to monthly) extreme events, including TC, to overcome the above limitations. Here we synthesize five coral Δδ18O records from the South China Sea (SCS) and develop a coral TC frequency index, in good agreement with post‐1980 instrumental records and sediment‐based TC reconstructions over the past century. Our results indicate that TC frequency in the SCS exhibits pronounced decadal variations, with active TCs corresponding to the negative phase of the Pacific Decadal Oscillation. Our findings here provide novel evidence for past TC changes in the SCS and a constraint for predicting future TC changes. Plain Language Summary: The impact of tropical cyclones (TCs) on coastal regions results in significant economic losses and casualties. Due to the relatively short observations or low‐resolution records, assessing long‐term changes of TCs has suffered from large uncertainties. Porites corals are sensitive to environmental changes and can provide century‐long, monthly resolved records, which could serve as a potential archive for reconstructing past TCs. Here we present five coral Δδ18O records from the South China Sea (SCS) and develop two TC frequency indexes. Both the TCs generated in the SCS and those entering the SCS from the Western North Pacific are reflected in our coral‐based TCs reconstruction. In addition, our reconstruction suggests that the TCs in the SCS were increased in 1878–1888, 1894–1904, 1934–1952, 1971–1978, and 1995–2004, which corresponded to the negative phase of the Pacific Decadal Oscillation over the past century. Our findings provide new evidence for historical changes in TCs in the SCS and a valuable constraint for predicting future TC variations. Key Points: The low residual oxygen isotope values of coral strongly respond to tropical cyclones in the South China SeaExcluding temperature, we reconstruct tropical cyclone activities in the South China Sea over the past century using coral oxygen isotopesPacific Decadal Oscillation modulates tropical cyclones in the South China Sea over the past century [ABSTRACT FROM AUTHOR]

The growing threat of global warming on coral reefs underscores the urgency of identifying heat‐tolerant corals and discovering their adaptation mechanisms to high temperatures. Corals growing in intertidal rock pools that vary markedly in daily temperature may have improved heat tolerance. In this study, heat stress experiments were performed on scleractinian coral Porites lutea from subtidal habitat and intertidal rock pool of Weizhou Island in the northern South China Sea. Thermotolerance differences in corals from the two habitats and their mechanisms were explored through phenotype, physiological indicators, ITS2, 16S rRNA, and RNA sequencing. At the extremely high temperature of 34°C, rock pool P. lutea had a stronger heat tolerance than those in the subtidal habitat. The strong antioxidant capacity of the coral host and its microbial partners was important in the resistance of rock pool corals to high temperatures. The host of rock pool corals at 34°C had stronger immune and apoptotic regulation, downregulated host metabolism and disease‐infection‐related pathways compared to the subtidal habitat. P. lutea, in this habitat, upregulated Cladocopium C15 (Symbiodiniaceae) photosynthetic efficiency and photoprotection, and significantly increased bacterial diversity and coral probiotics, including ABY1, Ruegeria, and Alteromonas. These findings indicate that rock pool corals can tolerate high temperatures through the integrated response of coral holobionts. These corals may be 'touchstones' for future warming. Our research provides new insights into the complex mechanisms by which corals resist global warming and the theoretical basis for coral reef ecosystem restoration and selection of stress‐resistant coral populations. [ABSTRACT FROM AUTHOR]

Significant differences in environmental conditions between nearshore and offshore coral reefs lead to different symbiotic zooxanthellae density (ZD) of corals. From the perspective of energy supply, different ZD lead to different energy provided by the photosynthesis of zooxanthellae to the coral host. However, few studies have conducted a comparative analysis of coral trophic status (i.e., autotrophic photosynthesis of zooxanthellae and heterotrophic feeding of the host) between nearshore and offshore reefs. In this study, 70 coral samples of Favia palauensis were collected from the nearshore Sanya reefs, and offshore Xisha, Nansha reefs in the South China Sea (SCS). The ZD, δ15N of seawater particulate organic matter (δ15NPOM), host tissue (δ15Nh), zooxanthellae (δ15Nz) and the difference between δ15Nh and δ15Nz (i.e., △h−z 15N = δ15Nh−δ15Nz) were measured and statistically analyzed. The purpose is to elucidate the differences in the energy maintenance mechanisms of corals between nearshore and offshore reefs. Results show that ZD is significantly positively correlated with δ15Nz in all three reefs (p < 0.05). Corals in nearshore Sanya have higher autotrophic abilities than corals from offshore Xisha and Nansha reefs. By contrast, corals in offshore reefs are more dependent on heterotrophic feeding of the host to obtain energy. Our research demonstrate that corals can adjust their trophic status, especially improve their heterotrophic feeding intensity to meet energy needs under different environmental conditions. This dynamic adjustment of energy supply patterns is of great significance to improve coral's environmental adaptability. Plain Language Summary: Coral reefs are mainly divided into nearshore fringing reefs, barrier reefs and offshore atolls according to geographical distribution. Therein, ambient seawater in nearshore reefs are significantly affected by human activities and has relatively higher nutrient levels. In contrast, corals in offshore atolls are less affected by human activities as they are far away from the mainland, but they will be potentially affected by abnormal natural conditions. Different environmental conditions lead to significant differences in the physiological parameters of corals, such as symbiotic zooxanthellae density (ZD). From the perspective of energy supply, different ZD may lead to different energy provided by the photosynthesis of zooxanthellae to the coral host. However, few studies have conducted a comparative analysis of coral trophic status between nearshore and offshore reefs. Based on the δ15N labeling technology, we find that corals in nearshore fringing reefs have higher autotrophic abilities than corals from offshore reefs. By contrast, corals in the offshore reefs are more dependent on heterotrophic feeding of the host to obtain energy. This means that corals can adjust their trophic status to meet energy needs under different environmental conditions. This dynamic adjustment of energy supply patterns is of great significance to improve coral's environmental adaptability. Key Points: The δ15N can record coral trophic status changes between nearshore and offshore reefsCorals in nearshore reefs have higher autotrophic photosynthesis and heterotrophic energy than corals in offshore reefsCoral can independently adjust trophic status under different environmental conditions between nearshore and offshore [ABSTRACT FROM AUTHOR]