Wenfeng Deng

Sea surface temperature records in the northern South China Sea from mid‐Holocene coral Sr/Ca ratios

[1] Three mid-Holocene sea surface temperature (SST) records spanning more than 30 years were reconstructed for the northern South China Sea using Sr/Ca ratios in Porites corals. The results indicate warmer than present climates between circa 6100 yr B.P. and circa 6500 yr B.P. with the mid-Holocene average minimum monthly winter SSTs, the average maximum monthly summer SSTs, and the average annual SSTs being about 0.5 degrees-1.4 degrees C, 0 degrees-2.0 degrees C, and 0.2 degrees-1.5 degrees C higher, respectively, than they were during 1970-1994. Summer SSTs decrease from circa 6500 yr B.P. to circa 6100 yr B.P. with a minimum centered at circa 6300 yr B.P. The higher average summer SSTs are consistent with a stronger summer monsoon during the mid-Holocene, and the decreasing trend indicates a secular decrease of summer monsoon strength, which reflects the change in summer insolation in the Northern Hemisphere. El Nino-Southern Oscillation (ENSO) cycles were apparent in both the mid-Holocene coral and modern instrumental records. However, the ENSO variability in the mid-Holocene SSTs was weaker than that in the modern record, and the SST record with the highest summer temperatures from circa 6460 yr B.P. to 6496 yr B.P. shows no robust ENSO cycle. This agrees with other studies that indicate that stronger summer monsoon circulation may have been associated with suppressed ENSO variability during the mid-Holocene.

Paleoprecipitation record from coral Sr/Ca and δ18O during the mid Holocene in the northern South China Sea.

Coupled high-resolution Sr/Ca and δ18O records of a modern and a mid-Holocene coral from Sanya in the southern Hainan Island, northern South China Sea (SCS), were reported and the residual δ18O (Δδ18O) were calculated to indicate precipitation change in this region. Unlike other paleoclimate studies, this study focused on changes of precipitation time rather than precipitation amount. As negative Δδ18O peaks in coral generally correspond to peak precipitations or rainy seasons in the surrounding region, the time offsets between negative Δδ18O peaks and other seasonal indicators, such as sea surface temperature (SST), can well indicate the time of rainy seasons, and the precise time offsets can be estimated by the method of cross spectral analysis. The results of the modern coral indicate that the variation of the coral Δδ18O lags that of the instrumental measured precipitation by about 2 months, and about 3 months to the SST derived from coral Sr/Ca ratios. This agrees well with the modern observation that the salinity change in the southern coastal regions generally lags that of the precipitation in Hianan Island by about 2 months, and the precipitation change lags about 1 month behind the SST in this region. Thus, coral Δδ 18O records can be a reliable proxy for the change of rainy seasons in this region. The results of the mid-Holocene coral show about 2.5 months’ leading of the Δδ18O variation ahead of the SST. By compensating the approximate 3 months’ lag of the Δδ18O variation behind the SST in modern time, the occurrence of rainy seasons during the mid Holocene may have advanced about 5—6 months. In detail, it may start around December and end around April to May with maximum occurring around February. Therefore, rainy seasons mainly occur in winter through early spring during the mid Holocene, compared with that from May through October in modern times. Such precipitation patterns appear to agree with the mid-Holocene pollen records in this region. Variations of large-scale circulation may possibly result in such a different precipitation pattern. Further studies, in particular climate model studies collaborated with meteorologists, are required for a better understanding of the mechanism.

Decadal variations in trace metal concentrations on a coral reef: Evidence from a 159 year record of Mn, Cu, and V in a Porites coral from the northern South China Sea

Geochemical cycles of trace metals are important influences on the composition and function of the marine ecosystem. Although spatial distributions of most trace metals have now been determined in at least some parts of the oceans, temporal variations have barely been studied on account of data limitations. In this paper, we report on a 159 year record of trace metal concentrations from a Porites coral from the northern South China Sea (SCS), and discuss how oceanic and climatic processes control variations in Mn, Cu, and V concentrations in this region. Our results show that trace metal concentrations in the coral skeleton demonstrate decadal to interdecadal fluctuations, and that their variations are controlled by different mechanisms. The input of Mn to reef water is partly controlled by the Pacific Decadal Oscillation (PDO), which controls precipitation and river runoff. Surface water concentrations of the nutrient-like element Cu are controlled by summer upwelling to the east of Hainan Island. The concentrations of V show complex interrelationships, and are linked to riverine input prior to the 1990 and to upwelling after the 1990. Our results imply that in the northern SCS, ocean-atmosphere climate fluctuations, such as the PDO and the East Asian Summer Monsoon (EASM), are important factors that influence long-term variability of Mn, Cu, and V concentrations in seawater, by controlling precipitation-related river runoff and the strength of upwelling systems.

High-precision analysis of Sr/Ca and Mg/Ca ratios in corals by laser ablation inductively coupled plasma optical emission spectrometry

A method has been developed to determine high-precision Sr/Ca and Mg/Ca ratios in corals by laser ablation inductively coupled plasma optical emission spectrometry (LA-ICP-OES) using aqueous solution standard calibration. Simultaneous determination of the signals of the entire analytical wavelengths by ICP-OES and the high performance of the new type of LA system (Resonetics 193 nm ArF excimer laser-ablation system, RESOlution M-50) improve the precision for elemental ratios. Repeated measurements on a coral base synthesized working standard, BH-7, provide precisions of about 0.4% and 0.8% for the Sr/Ca and Mg/Ca ratios, respectively, which are better than the formerly reported precision by the LA-ICP-MS method, about 1%. Such precision is comparable to those obtained by the solution nebulization-ICP-OES (SN-ICP-OES) method, and is adequate for paleoclimate reconstruction. In addition, the LA-ICP-OES can provide results with much higher spatial/time resolution. Comparisons between the LA and SN methods were handled by measuring along the same track of a coral. The Sr/Ca results by these two methods agree quite well with each other. The LA-ICP-OES method is very promising for the analysis of element/Ca ratios in coral and other carbonates used in paleoclimate studies such as stalagmite. Systematic discrepancy, however, was observed in the Mg/Ca ratios, likely due to the existing state of magnesium in the coral skeleton.

Decadal variability in seawater pH in the West Pacific: Evidence from coral δ11B records

Long-term seawater pH records are essential for evaluating the rates of ocean acidification (OA) driven by anthropogenic emissions. Widespread, natural decadal variability in seawater pH superimposes on the long-term anthropogenic variations, likely influencing the OA rates estimated from the pH records. Here, we report a record of annual seawater pH estimated using the δB proxy over the past 159 years reconstructed from a Porites coral collected to the east of Hainan Island in the northern South China Sea (SCS). By coupling this time series with previously reported long-term seawater pH records in the West Pacific, the decadal variability in seawater pH records and its possible driving mechanisms were investigated. The results indicate that large decadal variability in seawater pH has occurred off eastern Hainan Island over the past 159 years, in agreement with previous records. The Qiongdong upwelling system, which controls nutrient supplies, regulates surface water productivity, and is driven by the East Asian summer monsoon, is the primary control of this decadal variability, while terrestrial inputs appear not influence significantly. Meanwhile the impacts of the Pacific Decadal Oscillation (PDO) and the El Nino and Southern Oscillation (ENSO) systems on seawater pH off eastern Hainan Island is likely limited. In contrast, the PDO is the main factor to influence the decadal seawater pH variability offshore the East Australia, while the mechanism controlling the decadal seawater pH variability in Guam is not clear yet. Meanwhile, The rate of decrease in seawater pH estimated from coral records are significantly different in different regions and over different time spans, which may reflect a combination of natural decadal variability in seawater pH and long-term variations. Therefore, understanding the mechanisms driving natural variability in seawater pH is important for improving estimates of ocean acidification rates driven by anthropogenic emissions.

A comparison of the climates of the Medieval Climate Anomaly, Little Ice Age, and Current Warm Period reconstructed using coral records from the northern South China Sea

For the global oceans, the characteristics of high-resolution climate changes during the last millennium remain uncertain because of the limited availability of proxy data. This study reconstructs climate conditions using annually resolved coral records from the South China Sea (SCS) to provide new insights into climate change over the last millennium. The results indicate that the climate of the Medieval Climate Anomaly (MCA, AD 900–1300) was similar to that of the Current Warm Period (CWP, AD 1850-present), which contradicts previous studies. The similar warmth levels for the MCA and CWP have also been recorded in the Makassar Strait of Indonesia, which suggests that the MCA was not warmer than the CWP in the western Pacific and that this may not have been a globally uniform change. Hydrological conditions were drier/saltier during the MCA and similar to those of the CWP. The drier/saltier MCA and CWP in the western Pacific may be associated with the reduced precipitation caused by variations in the Pacific Walker Circulation. As for the Little Ice Age (LIA, AD 1550–1850), the results from this study, together with previous data from the Makassar Strait, indicate a cold and wet period compared with the CWP and the MCA in the western Pacific. The cold LIA period agrees with the timing of the Maunder sunspot minimum and is therefore associated with low solar activity. The fresher/wetter LIA in the western Pacific may have been caused by the synchronized retreat of both the East Asian Summer Monsoon and the Australian Monsoon.

Variations in the timing of the rainy season in the northern South China Sea during the middle to late Holocene

The amount and timing of precipitation in East Asia are important aspects of the East Asian monsoon. Many paleoclimate records that act as proxies for wet or dry climatic conditions have been linked to changes in precipitation amounts and are thus used to indicate changes in the East Asian monsoon system. However, few studies have examined changes in the timing of the rainy season. Here, we consider the timing of the seasonal precipitation cycle during the middle to late Holocene, using records derived from coupled high-resolution Sr/Ca and O-18 records preserved in Porites corals from the northern South China Sea. These records indicate that the timing of the rainy season in this region changed during the middle to late Holocene. The present-day rainy season generally occurs over the summer and autumn (June-October), which has also been recorded in Porites coral, whereas the rainy season at around 1500 and 6800years B.P. occurred through the autumn and winter (August-December), and the rainy season at around 2540 and 5000years B.P. occurred in the winter and spring (January-April or December-March). During the period around 5900years B.P., the seasonal distribution of precipitation varied from year to year. These shifts in the timing of the rainy season require changes in temperature and humidity patterns and appear to agree with changes in the magnetic susceptibility of lake sediments in this region, which may reflect movements of the Intertropical Convergence Zone location and changes in atmospheric circulation during the middle to late Holocene. Key Points Timing of rainy season changed during the middle to late Holocene over northern SCS Variation patterns of rainy season agreed with magnetic susceptibility records Shift of rainy season may be associated with ITCZ and atmospheric circulation

Decoupling of coral skeletal δ13C and solar irradiance over the past millennium caused by the oceanic Suess effect

Many factors influence the seasonal changes in δC levels in coral skeletons; consequently, the climatic and environmental significance of such changes is complicated and controversial. However, it is widely accepted that the secular declining trend of coral δC over the past 200 years reflects the changes in the additional flux of anthropogenic CO from the atmosphere into the surface oceans. Even so, the centennial-scale variations, and their significance, of coral δC before the Industrial Revolution remain unclear. Based on an annually resolved coral δC record from the northern South China Sea, the centennial-scale variations of coral δC over the past millennium were studied. The coral δC and total solar irradiance (TSI) have a significant positive Pearson correlation and coupled variation during the Medieval Warm Period and Little Ice Age, when natural forcing controlled the climate and environment. This covariation suggests that TSI controls coral δC by affecting the photosynthetic activity of the endosymbiotic zooxanthellae over centennial timescales. However, there was a decoupling of the coral skeletal δC and TSI during the Current Warm Period, the period in which the climate and environment became linked to anthropogenic factors. Instead, coral δC levels have a significant Pearson correlation with both the atmospheric CO concentration and δC levels in atmospheric CO. The correlation between coral δC and atmospheric CO suggests that the oceanic C Suess effect, caused by the addition of increasing amounts of anthropogenic CO to the surface ocean, has led to the decoupling of coral δC and TSI at the centennial scale.

Saltier sea surface water conditions recorded by multiple mid-Holocene corals in the northern South China Sea

The typical features of the mid-Holocene can be used to better understand present-day climate conditions and the potential trends of future climate change. The surface conditions, including sea surface temperature (SST) and sea surface salinity (SSS), of the South China Sea (SCS) are largely controlled by the East Asian monsoon system. Surface water conditions reconstructed from coral proxies can be used to study the evolution of the East Asian monsoon during the mid-Holocene. However, there are some discrepancies among existing coral-based studies regarding whether the mid-Holocene sea surface water was much saltier than the present day surface waters. Based on paired Sr/Ca and δ18O of modern and three fossil corals, this paper reconstructs the patterns of seasonal variation in SSS during the mid-Holocene in the northern SCS. The Δδ18O records (a proxy for SSS) derived from the three fossil corals were all heavier than that from the modern coral, which suggests the presence of more saline surface waters during the mid-Holocene in the northern SCS. These results are consistent with previous studies based on records reconstructed from coral and foraminifera, as well as from numerical simulations. Reduced rainfall caused by the strengthened Asian Monsoon and/or the northward shift of the intertropical convergence zone during the mid-Holocene would explain the increased salinity of the surface waters of the northern SCS. The findings presented here clarify the discrepancies among previous studies and confirm the existence of saltier surface waters in the northern SCS during the mid-Holocene. This article is protected by copyright. All rights reserved.

Super instrumental El Niño events recorded by a Porites coral from the South China Sea

The 2–7-year periodicities recorded in fossil coral records have been widely used to identify paleo-El Niño events. However, the reliability of this approach in the South China Sea (SCS) has not been assessed in detail. Therefore, this paper presents monthly resolution geochemical records covering the period 1978–2015 obtained from a Porites coral recovered from the SCS to test the reliability of this method. The results suggest that the SCS coral reliably recorded local seawater conditions and the super El Niño events that occurred over the past 3 decades, but does not appear to have been sensitive enough to record all the other El Niños. In detail, the Sr/Ca series distinctly documents only the two super El Niños of 1997–1998 and 2014–2016 as obvious low values, but does not match the Oceanic Niño Index well. The super El Niño of 1982–1983 was identified by the growth hiatus caused by the coral bleaching and subsequent death of the coral. Three distinct stepwise variations occur in the δ13C series that are coincident with the three super El Niños, which may be related to a substantial decline in endosymbiotic zooxanthellae density caused by the increase in temperature during an El Niño or the selective utilization of different zooxanthellaes that was required to survive in the extreme environment. The increase in rainfall and temperatures over the SCS during El Niños counteracts the effects on seawater δ18O (δ18Osw) and salinity; consequently, coral Δδ18O series can be used as a proxy for δ18Osw and salinity, but are not appropriate for identifying El Niño activity. The findings presented here suggest that the method to identify paleo-El Niño activity based on the 2–7-year periodicities preserved in the SCS coral records might not be reliable, because the SCS is on the edge of El Niño anomalies due to its great distance from the central equatorial Pacific and the imprints of weak and medium strength El Niño events may not be recorded by the corals there.