Northern Southern South China Sea Research Articles

Glacial sea level low-stands regulated the upper ocean hydrology in the southern South China Sea over the past ∼ 280 kyr

The southern South China Sea (SCS) is an ideal region for investigating the low-latitude climatic and oceanic dynamics in the context of local and global effects. However, records of upper-water hydrography beyond the last two glacial cycles are still limited from the southern SCS. In this study, we present shell δ18O, Mg/Ca and Nd/Ca records of three planktonic foraminiferal species (Globigerinoides ruber, Globigerinoides sacculifer and Pulleniatina obliquiloculata) from Core TX05 to explore changes in upper-water hydrography in the southern SCS over the past ∼280 kyr. The results show that shell δ18O of the three species exhibit more positive values in glacials than interglacials. The δ18O residual (i.e. δ18Osw-ice) calculated by shell δ18O subtracting the signals of ice volume and temperature suggests fresher upper seawaters (lower salinity) during glacials than interglacials. This may be caused by terrestrial freshwater discharge from the paleo-Sunda continent that exposed during glacial sea level low-stands. The freshwater discharge may further be ascribed to relatively abundant precipitation over the paleo-Sunda continent during glacials, possibly pointing to the collective effects of the southward shift of the Intertropical Convergence Zone and the enhanced Pacific Walker circulation. The vertical thermal gradients (ΔTrub-obl and ΔTsac-obl) at Site TX05 show lower values during glacials than interglacials. The SST differences of the southern SCS from the northern SCS (ΔSSTS-N) and from the western tropical Pacific (ΔSSTSCS-WTP) correlate well with indicators from the Chinese Loess section, indicating relatively strong East Asian winter monsoon during glacials. The good correlation of ΔT, ΔSSTS-N and ΔSSTSCS-WTP to the relative sea-level suggests a remarkable effect of sea level low-stands and the strengthened East Asian winter monsoon during glacials on the upper ocean hydrography in the southern SCS.

Formation and implication of cold-seep carbonates in the southern South China Sea

Southern South China Sea (SCS), rich in oil and gas resources, can provide a favorable condition for the formation and development of cold seeps. However, the geochemical effect of cold seep activities and the carbon cycle in the SCS are not well understood. Cold-seep carbonates as a window to exploration of gas hydrate are of great significance to investigate the material circulation in cold-seep system and to reconstruct the paleo-climate environment in the SCS. In this research, eight cold-seep carbonates collected from the Beikang Basin, southern SCS, are generally presented as yellow-black irregular column, crust and block, and mainly composed of high magnesium calcite (HMC), indicating they probably formed inside the deep fluid migration pathway. The middle rare earth element (MREE) enrichment with no or positive Ce anomaly in the carbonates from BH-CL37A suggests that they precipitated in anoxic pore water, while heavy rare earth element (HREE) enrichment in BH-CL32A carbonate shows great effect of seawater and they were formed in shallower depth. Different enrichments of redox sensitive trace elements and (Mo/U) EF indicate the strong anoxic environment. It is concluded that the difference of fluid environment could affect the enrichment degree and precipitation rate of trace elements by the relations between trace elements and Al. Extremely depleted 13C, slightly enriched 18O and the variation of Sr isotopes have archived the main biogenic methane source and the co-effect of seawater and seep fluid driven from a deep source (possibly containing the water from hydrate dissociation). The unique and complex geological conditions in the southern SCS will lead to the distinct regional specificity of the cold seep, which is different from the cold seep system in the northern SCS. This study plays an essential role in fulfilling the understanding of the activities of cold seeps in the SCS, revealing the element migration and transformation in cold seeps and their influencing factors.

Origin of large-scale variegated reef limestones in the southern South China Sea: Implications for Miocene regional and global geological evolution

Marine sediments featuring special colors have always played an important role in disclosing regional or global environmental transitions and geological evolution. A deep well (Nanke-1), drilled on Meiji Atoll in 2017, penetrated Cenozoic reef strata in the central Nansha Block (also referred to as Dangerous Grounds) in the southern South China Sea (SCS) and revealed two pronounced variegated (containing colors of white, reddish and red brown) reef limestone layers in the Miocene. Similar large-scale variegated reef limestones have not been found in other reef drilling cores to date, so its coloration origin, formation mechanism and geological implications are unclear. Here, synthesized mineralogical, petrographic, and geochemical analyses demonstrated that the presence of trace amounts of amorphous microsized hematite with varying crystallinity imparted reddish or red brown colors to reef limestones. Hematite formed during meteoric water-induced karstification after the carbonate platform was exposed subaerially. Long-term exposures were mainly controlled by regional tectonic uplifts closely associated with the subduction of the proto-SCS, the seafloor spreading of the Southwest Sub-basin and the collision between blocks (i.e., the Sabah collision and the Palawan collision). The different tectonic settings have led to the different developments of carbonate platforms in the northern and southern SCS during most of the Early and Middle Miocene, but the major expansion of the high-latitude Antarctic ice sheets and the resultant significant drop in global sea level may have contributed to wide exposure of carbonate platforms both in the northern and southern SCS in the late Middle Miocene. Our studies demonstrated that the occurrences of variegated reef limestones in isolated oceanic reef carbonate platforms have great potential in indicating subaerial exposure, regional tectonic uplift or sea level fall.

Mesoscale Eddy Effects on Nitrogen Cycles in the Northern South China Sea Since the Last Glacial

Archaeal ammonia oxidation is the most important intermediate pathway in regulating the oceanic nitrogen cycle; however, the study of its specific role on a geological time scale is restricted to a specific part of marginal seas; thus far, only in the southern South China Sea (SCS). To explore the spatial pattern of the role of archaeal ammonia oxidation in the SCS, the GDGT-[2]/[3] ratio (Glycerol Dialkyl Glycerol Tetraether), an indicator of the archaeal ammonia oxidation rate, was analyzed and examined from the collected data profiles since the last glacial period in the northern SCS. The results showed that the GDGT-[2]/[3] ratio in the northern SCS was opposite to that in the southern SCS, with higher GDGT-[2]/[3] values during the Holocene compared to the last glacial period. Based on existing published depths of thermocline (DOT) data in the northern SCS since 30 ka, we believe that hydrological structural variations induced by mesoscale eddies caused this difference. Therefore, physical processes are very important factors that control the nitrogen cycle over a long-time scale. This study may provide new insights into the understanding of the role of archaeal ammonia oxidation within the marine nitrogen cycle over geological time scale.

Sedimentary Responses to Climate Changes and Human Activities Over the Past 7400 Years in the Western Sunda Shelf

High-resolution records of grain size, major and trace elements, and Sr-Nd isotopes of Core K17 from the western Sunda Shelf were investigated to evaluate the response of weathering and terrigenous input to climatic changes and human activities over the past 7400 years. Sr-Nd isotopic results indicate that the Kelantan River is the main source of sedimentary material in the study core since the mid-Holocene. Chemical weathering levels are represented by the chemical index of alteration (CIA), αAlNa, and K2O/Al2O3 ratios; and geochemical and grain size proxies (including TiO2/CaO, Rb/Sr ratios, and grain size end-member) were used to establish variations of terrigenous input into the study core since 7400 cal yr BP. Based on these records, the evolution of weathering and terrigenous input processes in the western Sunda Shelf can be divided into four stages. During stage 1 (7400–3700 cal yr BP), increasing precipitation and decreasing temperature jointly balanced the relatively stable weathering and terrigenous sediment supply. Dramatically decreasing weathering rates were consistent with less rainfall and lower temperatures during stage 2 (3700–2600 cal yr BP). Heavy rainfall played a more important role than low temperature in controlling weathering and erosion, leading to increasing terrigenous input in stage 3 (2700–1600 cal yr BP). Because of the decoupling between weathering, erosion, and climate in the late Holocene (stage 4, since 1600 cal yr BP), increasing agriculture and related human activities likely dominated weathering and erosion relative to climate changes. Furthermore, the initial time at which human activity overwhelmed natural processes in the southern South China Sea (SCS) is similar to that in the northern SCS. Our results highlight that human activities during the past 1600 years have gradually overwhelmed natural climatic controls on weathering and erosion processes in the western Sunda Shelf.

Characteristics and Formation Mechanisms of Spring SST Anomalies in the South China Sea and Its Adjacent Regions

Characteristics of the springtime sea surface temperature anomalies (SSTAs) in the South China Sea and its adjacent regions (SCSAR), as well as their possible impacts on the Asian and Indo-Pacific climate, were investigated by using multiple datasets. According to the result from an empirical orthogonal function (EOF) analysis on the spring SSTAs in the SCSAR, the dominant pattern is a uniformly warming pattern in the whole SCSAR region. While the second mode is a sandwich pattern with cold SSTA over the central SCSAR centered near 10° N, flanked by warm SSTA over the northern oceans near 25° N and in the subtropics near 10° S. The uniformly warming pattern is associated with the anomalous warming in the Indian Ocean from the preceding autumn to the spring, and the sandwich pattern is mainly caused by the El Niño-Southern Oscillation. In the uniformly warming pattern, rainfall increases in the Meiyu region and decreases over the southern South China Sea (SCS). In the sandwich pattern, the anomalous anticyclone at 850-hPa causes less rainfall in the Philippine Sea, the Marine Continent, and the SCS. The positive rainfall anomalies could be found in the northern SCS and adjacent regions. Associated with the second EOF mode, there is a wave train emitted from the SCSAR to East Asia, northwest Pacific, and North America. The wave train spreads the energy from mid-latitudes to higher latitudes through atmospheric teleconnection, which can even influence the North American atmospheric circulation in spring.

Subsidence of the northern South China Sea and formation of the Bashi Strait in the latest Miocene: Paleoceanographic evidences from 9-Myr high resolution benthic foraminiferal δ18O and δ13C records

We present astronomically tuned high time resolution (3–4kyr) benthic foraminiferal δ18O and δ13C records between 9Ma and 5Ma from Ocean Drilling Program (ODP) Site 1143 in the southern South China Sea (SCS). We found that the late Miocene ocean carbon shift (LMOCS) which was linked with the late Miocene/early Pliocene “Biogenic Bloom” also occurred in the northern and southern SCS. A marked deep-water δ13C gradient as much as ~1.0‰ of the northern and southern SCS was observed between 6.5Ma and 5.9Ma, which was probably caused by the significant difference in paleoproductivity between the northern and southern SCS. The deep δ13C gradient was likely produced by rapid tectonic subsidence in the northern SCS at ~6.0Ma and the final formation of the Bashi Strait at ~6.5Ma. A reversed benthic δ13C gradient of the northern and southern SCS was also observed between 1.2Ma and 0.35Ma, which was probably caused by the consecutive closures of the interarc water gates within the Bashi strait at ~3.0Ma and ~1.2Ma. Closures of the interarc water gates within the Bashi strait at ~3.0Ma and ~1.2Ma further semi-enclosed the SCS basin and made the paleoproductivity the major factor influencing the deep water δ13C, leading to the reversed deep water δ13C gradient of the northern and southern SCS between 1.2Ma and 0.35Ma.

Characteristics of pollen in surface sediments from the southern South China Sea and its paleoclimatic significance

We investigated pollen distributions in surface sediments of the southern South China Sea (SCS) and their relation to seabed topography and the environment of the terrigenous sources. In this study, we analyzed 62 surface sediment samples collected from the SCS through modern pollen analysis methods. The results of our study can be summarized as follows. (1) trilete spores dominate the pollen and spore spectrum, which reflects the dominance of ferns in the Nansha Island vegetation. Similar pollen characteristics were found for different periods of high sea level and global warming after 12Ma in ODP site 1143 and site 18287 in the southern South China Sea. Most pollen and spores on the Kalimantan Island coast are produced by herbaceous plants and trees, while few trilete spores produced by ferns are present. Furthermore, similar pollen characteristics were found for different periods of low sea level and global cooling in ODP site 1143 and site 18287. (2) Through an analysis of pollen samples from a transect line extending from the northwest to the southeast, the complex terrain of the Nansha Islands was observed to be unsuitable for pollen migration through ocean currents. Pollen counts in samples collected near the southern Nansha Islands decreased gradually from the east and west continental shelf areas to the central basin area. However, the same analysis confirmed that the Nansha Trough facilitates the entry, dispersal, and storage of pollen from external sources. (3) Principal component analysis (PCA) of samples collected from two different water depths in the southern SCS demonstrates that trilete spores and monolete spores in 4 samples near the Mekong River with water depths of <200m are transported primarily by river flow. Conversely, pollen and spores mainly transported by rivers near Kalimantan Island were found at water depths of >200m. However, analysis of samples from the northern SCS in a previous study obtained different results.

SEA SURFACE TEMPERATURE CHANGE IN SOUTH CHINA SEA SINCE PLEISTOCENE AND ITS PALEOCLIMATIC IMPLICATIONS

The thermal history of the southern South China Sea(SCS) exhibited a reducing tendency since Pleistocene.The temperature profile is characterized by a cooling trend during glaciations but remains steady during interglaciations.Meridional thermal gradient between southern and northern SCS reflects the Eastern Asian Winter Monsoon(EAWM) change.Thermal gradient history during the past 2 Ma indicated that EAWM was strengthened before mid-Pleistocene and weakened since then,but restrengthened in the later Quaternary.This long term trend is superimposed by orbital glacial cycles with the EAWM stronger during interglaciations and weaker during glaciations.Tectonic uplifting of the Tibetan Plateau may have important influence on the strengthening of EAWM in the mid-Pleistocene.Global ice volume and ENSO changes may be the main reasons for the overall trend and the orbital cycle of EAWM.

Sea-level rises at Heinrich stadials of early Marine Isotope Stage 3: Evidence of terrigenous n-alkane input in the southern South China Sea

The timing of suborbital-scale sea-level fluctuations within Marine Isotope Stage 3 (MIS 3) remains a matter of debate. We use a set of co-registered signals of the sediment core MD05-2897 from the southern South China Sea (SCS) to evaluate the phase relationship between sea-level fluctuations and climatic events during early MIS 3. During Heinrich Stadials 6, 5a and 5 (HS6, HS5a, HS5), planktonic foraminiferal δ18O values of core MD05-2897 increased by 0.8–1.0‰ due to an enrichment of δ18O in precipitation and/or a reduction of monsoon precipitation. The alkenone-derived sea surface temperature (U37K′-SST) recorded 1.0–2.0°C cooling signatures during wintertime of HS6, HS5a and HS5, which were attributed to a stronger influx of cold surface waters from the northern SCS driven by the strengthened winter monsoon. Therefore, a combination of foraminiferal δ18O and U37K′-SST results well documents these Heinrich stadials of the East Asian monsoon climate. Moreover, we find that the terrigenous n-alkane abundance of core MD05-2897, which is independent of changes in vegetation types around the southern SCS throughout MIS 3, is potentially an indicator for relative sea-level changes on suborbital timescale. The n-alkane abundance was relatively higher during the first half of HS6, HS5a and HS5 and afterward significantly decreased during the second half of these Heinrich stadials. This may suggest relatively lower sea level at the beginning of Heinrich stadials and sea-level rises afterward. Our new results add to the growing body of evidence that the timing of millennial-scale sea-level rises during early MIS 3 was simultaneous with Heinrich stadials.

Coral reef ecosystems in the South China Sea as a source of atmospheric CO2 in summer

Field measurements of air-sea CO2 exchange in three coral reef areas of the South China Sea (i.e. the Yongshu Reef atoll of the Nansha Islands, southern South China Sea (SCS); Yongxing Island of Xisha Islands, north-central SCS; and Luhuitou Fringing Reef in Sanya of Hainan Island, northern SCS) during the summers of 2008 and 2009 revealed that both air and surface seawater partial pressures of CO2 (pCO2) showed regular diurnal cycles. Minimum values occurred in the evening and maximum values in the morning. Air pCO2 in each of the three study areas showed small diurnal variations, while large diurnal variations were observed in seawater pCO2. The diurnal variation amplitude of seawater pCO2 was ∼70 μmol mol−1 at the Yongshu Reef lagoon, 420–619 μmol mol−1 on the Yongxing Island reef flat, and 264–579 μmol mol−1 on the reef flat of the Luhuitou Fringing Reef, and 324–492 μmol mol−1 in an adjacent area just outside of this fringing reef. With respect to spatial relations, there were large differences in air-sea CO2 flux across the South China Sea (e.g. ∼0.4 mmol CO2 m−2 d−1 at Yongshu Reef, ∼4.7 mmol CO2 m−2 d−1 at Yongxing Island, and ∼9.8 mmol CO2 m−2 d−1 at Luhuitou Fringing Reef). However, these positive values suggest that coral reef ecosystems of the SCS may be a net source of CO2 to the atmosphere. Additional analyses indicated that diurnal variations of surface seawater pCO2 in the shallow water reef flat are controlled mainly by biological metabolic processes, while those of deeper water lagoons and outer reef areas are regulated by both biological metabolism and hydrodynamic factors. Unlike the open ocean, inorganic metabolism plays a significant role in influencing seawater pCO2 variations in coral reef ecosystems.

East Asian winter monsoon variability over the last glacial cycle: Insights from a latitudinal sea-surface temperature gradient across the South China Sea

High-resolution planktonic foraminifer Mg/Ca sea surface temperature (SST) and δ 18O records of IMAGES core MD052896 from the southern South China Sea (SCS) provide a history of East Asian winter monsoon variability over the past 23 kyr. Specifically, we find that the latitudinal SST gradient of the north–south SCS shows promise as a useful proxy of East Asian winter monsoon changes. The ΔSST record of core MD052896 from the southern SCS and ODP site 1145 from the northern SCS documents several positive anomalies indicating East Asia winter monsoon maxima during cold periods of the past 23 kyr, including the H1 and Younger Dryas events. The ΔSST record also indicates that after ∼ 8.5 ka the East Asian winter monsoon strengthened relative to the deglaciation, reaching levels of the last glacial period. Comparison of the SCS ΔSST record with stalagmite δ 18O records suggests that both the East Asian summer and winter monsoons strengthened from the last glacial to the Holocene, and that summer and winter monsoon strength probably are not anti-correlated, at least over the last glacial–interglacial cycle.

Planktonic foraminiferal records of East Asia monsoon changes in the southern South China Sea during the last 40,000 years

Planktonic foraminiferal assemblages and benthic foraminiferal oxygen isotopes were obtained in a gravity core NS07-25 (6°39.945′ N, 113°32.936′ E, water depth 2006 m) collected from the southern South China Sea (SCS) to investigate East Asia monsoon changes during the last 40,000 years. Four cold events identified from four peaks of the relative abundance of Neogloboqua d rina pachyderma (dex.) and Globigerina quinqueloba correlate well with those in the Hulu monsoon record within age uncertainty, and can be linked to Heinrich events in the Northern Atlantic high latitudes. We suggest that the relative abundance of N. pachyderma (dex.) and G. quinqueloba in the southern SCS can be used as a direct proxy for East Asia winter monsoon (EAWM) changes during glacial periods. In contrast to its low abundance in the northern SCS and Okinawa Trough, the relative abundance of Pulleniatina obliquiloculata shows high values during the last glacial in the southern SCS, caused likely by the combined impact of optimum winter sea-surface temperature (SST) and stronger mixing/enhanced upwelling at that time. A P. obliquiloculata minimum event (PME) found in the Okinawa Trough and the northern and southern SCS during the H1 period was possibly related to a large drop in winter SST due to the intensified EAWM. We propose that this PME can be also used as a reliable regional stratigraphic marker as the Holocene one. A 9-cm sandy silt layer deposit in core NS07-25 during the H1 period probably related to the enhanced current that was caused by the especially strengthened EAWM at that time. The last deglacial warming recorded in the relative abundance of warm and cold species of planktonic foraminifer reveals that changes between the EASM and EAWM were not in phase but alternating between a winter monsoon dominated northern hemisphere influence and a summer monsoon dominated influence of the tropical dynamics and/or southern hemisphere climate changes.

High-resolution benthic foraminifer δ 13C records in the South China Sea during the last 150 ka

High-resolution δ 13C ratios of benthic foraminifer, Cibicidoides wuellerstorfi, in Core MD97-2151 from the southern South China Sea (SCS) and Core SO50-31KL from the northern SCS indicate deep-water variations in the SCS during the last 150 ka. The δ 13C records of both show a general variation pattern with higher δ 13C during interglacial periods than glacial periods, and rapid increases during terminations. This variation pattern resembles those of the West Pacific intermediate/deep water, the sources of the SCS deep water and the general variation of global deep water, indicating that the variation of the SCS deep water is largely controlled by global factors, such as carbon cycles and global deep-water circulation. Spectra analysis exhibits robust Milankovitch cycles in the SCS deep-water δ 13C, again supporting global controls. Significant semi-precession cycles can also be seen in the spectra, implying that changes of local surface productivity also influence the SCS deep-water δ 13C.

Contrasting Characteristics of Convection over the Northern and Southern South China Sea during SCSMEX

Abstract Observations from two enhanced sounding arrays during the May–June 1998 South China Sea Monsoon Experiment (SCSMEX) are used to determine and contrast the properties of convection over the northern and southern South China Sea (SCS). A regression analysis between SST data and monthly rainfall indicates that the ENSO signal exerted a strong influence on the rainfall distribution over the SCS during SCSMEX. This resulted in wetter-than-normal conditions along the south China coast and northern SCS, and generally drier-than-average conditions elsewhere, particularly over the Philippine Islands. The monsoon onset as determined by a shift in the low-level winds from easterly to southwesterly over the SCS occurred around mid-May. Over the southern enhanced sounding array (SESA), the onset was characterized by a rainy period associated with the passage of a convectively coupled Kelvin wave. This was followed by a weeklong break and then several episodic rain events with increasingly higher rain rates. Rainfall over the northern enhanced sounding array (NESA), which was largely out of phase with SESA rainfall events, occurred primarily during two 10-day periods separated by a weeklong break. Convective characteristics over the SESA, deduced primarily from heat and moisture budget profiles, indicate a high stratiform rain fraction consisting of alternating periods with decaying mesoscale systems that organized near the western Borneo coastline and shallower convective clouds. In contrast, NESA-averaged profiles were indicative of deep convection with a relatively small stratiform rain fraction, which was confirmed with radar analyses during the onset convective period. The diurnal cycle of convection is a dominant feature throughout much of the SCS. Over both budget regions, early morning (0500–0800 LT) convective systems were frequently initiated near the coasts, then gradually dissipated during the course of the day as the midlevel steering currents moved the systems away from the coastline. These decaying convective systems resulted in an early afternoon (1400 LT) rainfall peak over both sonde arrays.

Response of planktonic foraminifera to glacial cycles: Mid-Pleistocene change in the southern South China Sea

A high-resolution study (∼2 ka) of planktonic foraminifers in 997 samples from ODP Site 1143 (9°21.72′N, 113°17.11′E, water depth 2772 m) in the southern South China Sea (SCS) reveals faunal response to glacial cycles in the last 2.1 Ma. Most distinctive is the contrasting variations in the abundance of two tropical–subtropical species that exhibit similar resistance to dissolution in modern oceans. The relative and absolute abundances of Globorotalia menardii are high in interglacials and low in glacials and covary with δ 18O fluctuations over the studied interval. Pulleniatina obliquiloculata, however, follow oxygen isotopes only before the Mid-Pleistocene Revolution (MPR), showing high abundances during glacials and low during interglacials after ∼850 ka. Three abundance maxima of P. obliquiloculata are observed, corresponding to three major glacial episodes, Marine Isotope Stages (MIS) 2, 12, and 16, respectively. This change in abundance patterns of P. obliquiloculata appears to be unique to the southern SCS, as in late Pleistocene records from other western Pacific regions, including the Okinawa Trough and the northern SCS, all show co-varying patterns with δ 18O as G. menardii does. Cross-spectral analyses performed between percentages of Globorotalia menardii and δ 18O records show high coherency at 41 ka obliquity and then at 100 ka eccentricity bands. We suggest that the high abundances of G. menardii during interglacials in the past 2.1 Ma reflect warmer temperatures. Percentages of Pulleniatina obliquiloculata vs. oxygen isotope records show quite large phase difference at 41 ka obliquity band before the MPR. As a contrast, there are negligible phase differences in the vicinity of 100 and 41 ka after the MPR, which means reverse trend between percentages of P. obliquiloculata and δ 18O fluctuations. The differences of P. obliquiloculata from G. menardii in abundance and orbital forcings are interpreted as resulting from a profound change in the regional upper ocean structure during the MPR time, probably relating to a final glacial cut-off of the southern SCS from the Indian water at ∼850 ka and to higher salinity during glacial times after 850 ka. We speculate that this change is related to the decreased connection between the Pacific and Indian Ocean.