Monsoon-regulated marine carbon reservoir effect in the northern South China Sea

Abstract

The ubiquitous marine radiocarbon reservoir effect (MRE) constrains the construction of reliable chronologies for marine sediments and the further comparison of paleoclimate records. Different reference values were suggested from various archives. However, it remains unclear how climate and MREs interact. Here we studied two pre-bomb corals from the Hainan Island and Xisha Island in the northern South China Sea (SCS), to examine the relationship between MRE and regional climate change. We find that the MRE from east of Hainan Island is mainly modulated by the Southern Asian Summer Monsoon-induced precipitation (with 11.4% contributed to seawater), rather than wind induced upwelling. In contrast, in the relatively open seawater of Xisha Island, the MRE is dominated by the East Asian Winter Monsoon, with relatively more negative (lower) ΔR values associated with high wind speeds, implying horizontal transport of seawater. The average SCS ΔR value relative to the Marine20 curve is –161±39 ¹⁴C years. Our finding highlights the essential role of monsoon in regulating the MRE in the northern SCS, in particularly the tight bond between east Asian winter monsoon and regional MRE.

Introduction

Marine sediments play an important role in interpreting past climate change which provides a crucial clue for future climate forecasting (Lisiecki and Raymo 2005; Tierney et al. 2020; Wang et al. 2014). Previous studies based on foraminifera, bivalve, and coral have made excellent progress in revealing the past climate change (Yan et al. 2017; Yu 2012; Zhang et al. 2021). However, the ambiguous nature of the marine radiocarbon reservoir effect (MRE) constrains the building of reliable chronology (Alves et al. 2018; Burr et al. 2009; Stuiver et al. 1986), thus hindering the reconstruction and comparison of these centennial to millennial scale climate changes. Hence, precisely evaluating temporal and spatial MRE variations is urgently needed in paleoclimate research.

MREs often reflect air-sea exchange, regional ocean circulation, and freshwater input (Stuiver and Ostlund 1983; Southon et al. 2002). A MRE can be expressed as the radiocarbon age difference between the atmosphere and the surface ocean. This difference is expressed as the value R (reservoir age) in radiocarbon years. Generally, MREs are cited as ΔR values, which are differences between measured R values and modeled R values (e.g. difference between IntCal13 and Marine13 calibration curves) for a particular region, based on calibration curves (e.g. Marine 13, Reimer et al. 2013).

Corals loyally document the ¹⁴C of seawater dissolved inorganic carbon (DIC) (Druffel 1997; Grumet et al. 2004) and can be accurately dated by U-series (Yu et al. 2006) or absolutely refined annual band, which provide the opportunity for evaluating ΔR. A series of studies have determined regional R and ΔR based on corals or known-age materials (Burr et al. 2009; Oliveira et al. 2019; Yoneda et al. 2007). However, R and ΔR are seen to vary with time in coral records (Hua et al. 2015; Yu et al. 2010) and model results (Butzin et al. 2017). Hence R and ΔR are a function of time and location. Previous studies have revealed a tight connection between seawater ¹⁴C and climate from pre- and post-bomb corals (Grottoli et al. 2003; Hirabayashi et al. 2017). Nevertheless, the temporal relationship between regional ΔR and climate change for most regions remains unknown.

As the largest marginal sea in the Pacific, with abundant riverine sediments and a high sedimentation rate, the monsoon-dominated South China Sea (SCS) has been a focus of paleoclimate research (e.g. Shen et al. 2022; Wang et al. 2014). A precise ΔR for the SCS and its temporal variability are of importance for elucidating these paleoclimate records. Previous studies have suggested different ΔR values for the SCS. For instance, ΔR at Hòn Tre Island, Vietnam (Bolton et al. 2016), was estimated to be 18±29 ¹⁴C years (relative to Marine 13, Reimer et al. 2013), larger than the ΔR at Con Dao Island (Dang et al. 2004) of –74±39 ¹⁴C years (relative to Marine 98, Stuiver et al. 1998) and the average ΔR (Southon et al. 2002) of –25±20 ¹⁴C years for the SCS (relative to Marine 98, Stuiver et al. 1998). Nevertheless, decadal ΔR variations related to climate have yet to be determined. Furthermore, the newly updated marine calibration curve (Marine20 curve) (Heaton et al. 2020), is almost 150 ¹⁴C years older than that implied by the Marine13 (Reimer et al. 2013) and previous marine curves. Hence, it is crucial to reconcile regional ΔR time series records to clarify the relationship between ΔR and climatic factors through time.

In this study, we present two coral radiocarbon records exceeding two decades from the east of Hainan Island and Xisha Island in the northern SCS. The relationship between regional ΔR and climatic factors (e.g. monsoon) in these corals were thoroughly examined and explored. Further, the newly average ΔR for the SCS base on the most recent marine radiocarbon Curve-Marine20 (Heaton et al. 2020) and its implication for paleoclimate research were specifically considered.

Materials and methods

Oceanographic setting of the SCS

As a semi-closed marginal sea, the SCS connects the surrounding sea through seven straits. The Luzon strait, contributes to the primary water exchange with the northwest Pacific Ocean (Qu et al. 2009), with a deep sill of 2400m (Qu et al. 2006). In addition, two large rivers, the Pearl River and Mekong River, supply a large amount of sediments into the north SCS and west SCS, with annual runoffs of 3.3 and 4.7×10¹¹ m³, respectively (Mckee et al. 2004). The surface circulation in the SCS seasonally shifts due to the reversing monsoon winds, with clockwise flows in summer and anti-clockwise flows in winter (Hu et al. 2000). Moreover, the summer monsoon influences coastal upwelling prevails around east of Hainan, off Vietnam and off Guangdong province, while the prevalent coastal upwelling west of Luzon is in winter under the effect of winter monsoon (Wu and Li 2003; Hu and Wang 2016).

The summer sea surface temperature (SST) in the SCS is roughly homogeneous above 28°C. In contrast, the winter SST displays conspicuous ascent from north to south (Figure 1 a–d). These seasonal differences in SST distribution patterns are mainly induced by seasonal reversing monsoon including the East Asian Winter Monsoon (EAWM) and the Southern Asian Summer Monsoon (SASM) (Liu et al. 2004). In this study, the two sample sites (shown in Figure 1 a and b) share distinct oceanography. At east of Hainan Island, prevailing in summer, the prominent upwelling dominant the regional circulation, which is mainly influenced by the SASM (Hu and Wang 2016; Jing et al. 2009). During summer, shallow regional mixed layer depth is observed (Figure 1 e) (Zeng et al. 2016), which favors upwelling of aged ¹⁴C to the surface. In contrast, at Xisha Island, an open ocean site, no seasonal upwelling is observed (deep mixed layer depth, Figure 1 f). The annual average wind speed is larger in Xisha Island than in Hainan Island, especially in winter (Figure 1 g and h). Besides, runoff from the Wanquan River and Wenjiao River at Hianan Island accounts for more than 6×10⁹ m³ yr^-1 (Zhang et al. 2013), while negligible runoff occurs at the Xisha Island.

Figure 1. Map and hydroclimate parameters of sampling site. (a) winter (December-January-February) and (b) summer (June-July-August) mean 850 hPa stream line from the ERA-Interim (https://climatedataguide.ucar.edu/climate-data/era-interim-derived-components), the red and black dots represent the sampling sites of coral core 15WC23 and 15XS28 from east of Hainan Island and Xisha Island, respectively. (c) Monthly mean sea surface temperature (SST), precipitation and (g) wind speed at east of Hainan Island (centered at 19°N, 110°E within 2° × 2° grid) from NCEP Reanalysis Dataset (https://psl.noaa.gov/cgi-bin/data/timeseries/timeseries1.pl). (d) Monthly mean sea surface temperature (SST), precipitation and (h) wind speed at Xisha Island (centered at 17°N, 112°E within 2° × 2° grid) from NCEP Reanalysis Dataset. All the monthly mean data are ranged from 1948 to 2021. (e) and (f) Monthly mean mixed layer (MLD) depth from SCSPOD14 (Zeng et al. 2016) ranging from 1971 to 2015 at east of Hainan Island (centered at 19.5°N, 111°E within 0.5° × 0.5° grid) and Xisha Island (centered at 17°N, 112.5°E within 0.5° × 0.5° grid), respectively.

Sampling

In 2015, two coral cores were obtained from living massive Porites lutea colonies from the northern SCS. The coral core 15WC23 (19.39°N, 110.76°E) was collected at a water depth of 5 m, from a fringing reef, off the east coast of Hainan Island (Figure 1 a and b, red dot). The other coral core 15XS28 (16.96°N, 112.25°E) was collected at a water depth of 4 m from Qilianyu Reef in the Xisha Islands (Figure 1 a and b, black dot). The chronology of the coral cores was well established by the annual density bands revealed by X-ray photograph (Figure S1, see Supplementary), with a growth period from 1900 to 2015 for 15WC23 (Chen et al. 2021) and from 1930 to 2015 for 15XS28 (Kang et al. 2021). The powder samples were carefully milled along the main growth axis by a digitally controlled milling machine (Deng et al. 2013). Limited by sample size, coral 15WC23 and 15XS28 were sampled with biennial and annual resolution, respectively.

Radiocarbon dating

All pre-1950 powder samples (45 samples in total) were employed for radiocarbon analysis. For each sample, about five to seven milligrams of coral powder were reacted with 85% phosphoric acid in a Y-shaped vessel under vacuum of less than 1×10^–1 torr. After complete reaction, the CO₂ gas was purified in a vacuum line and reduced to graphite using a zinc reduction method (Slota et al. 1987). Graphite samples were then measured for ¹⁴C analysis at the Xi’an Accelerator Mass Spectrometry (AMS) center (Zhou et al. 2016).

Result

The radiocarbon result (Table 1) of 45 coral samples is presented in Figure 2, expressed as ΔR. The ΔR data from east Hainan Island from 1900 to 1949 are cited with biennial resolution, while the Xisha Island results from 1930 to 1949 are cited with annual resolution. Recalculated previous-published average ΔR values from different sites around the SCS are summarized in Table 2, consisting of 725 ΔR values from eighteen sites. The R, ΔR and the associated Δ¹⁴C are calculated by the following equations (Stuiver et al. 1986; Stuiver and Polach 1977):

(1) $${\rm{Measured\ R}}\left( {\rm{t}} \right) = {\rm{Measure}}{{\rm{d}}\ ^{14}}{\rm{C\ age}}\left( {\rm{t}} \right)-{\rm{Terrestrial}}\ {^{14}}{\rm{C\ age}}\left( {\rm{t}} \right)$$

(2) $$\Delta {\rm{R}}\left( {\rm{t}} \right) = {\rm{Measured\ R}}\left( {\rm{t}} \right)-{\rm{modeled\ R}}\left( {\rm{t}} \right)$$

(3) $${\rm{Modeled\ R}}\left( {\rm{t}} \right) = {\rm{Modeled\ marin}}{{\rm{e}}\ ^{14}}{\rm{C\ age}}\left( {\rm{t}} \right)-{\rm{Terrestria}}{{\rm{l}}\ ^{14}}{\rm{C age}}\left( {\rm{t}} \right)$$

(4) $${\rm{Measure}}{{\rm{d}}\ ^{14}}{\rm{C\ age}}\left( {\rm{t}} \right) = -8033{\rm{ }} \times {\rm{ln}}\left( {{F}} \right)$$

(5) $${\Delta ^{14}}{\rm{C}} = {{F}}{{{e}}^{\lambda \left( {1950 - t} \right){\rm{ }} - 1}} \times 1000{\unicode{x2030}}$$

Table 1. ΔR result from east of Hainan Island and Xisha Island

Figure 2. The ΔR result from east of Hainan Island (15WC23, blue dotted line) and Xisha Island (15XS28, green dotted line).

Table 2. The detailed information of collected and recalculated ΔR data around the SCS

Where t is the true living age of the sample in cal BP (based on the counted annual density band), F represents the fraction of Modern carbon, λ=1/8267 yr^–1 is the radiocarbon decay constant associated with a 5730-yr half-life, and t is the calendar age of the sample (Stuiver and Polach 1977). The terrestrial and modeled ¹⁴C age were obtained from Intcal20 Curve and Marine20 Curve, respectively.

ΔR results from Hainan Island and Xisha Island

As shown in Figure 2, 25 ΔR results from east of Hainan Island (blue dotted line) fluctuate from –229±14 to –131±14 ¹⁴C years (average value –190±23 ¹⁴C years) from 1900 to 1949. Additionally, 20 ΔR data from Xisha Island (green dotted line) range from –230±17 to –150±18 ¹⁴C years (average value –201±16 ¹⁴C years) from 1930 to 1949. Generally, our sites have more negative average ΔR values than other SCS sites (Table 2, published ΔR values were all recalculated using Marine20), except for Con Dao Island, Vietnam (–226±18 ¹⁴C years) (Dang et al. 2004; Mitsuguchi et al. 2007) and Janao Bay, Philipines (–216±50 ¹⁴C years) (Southon et al. 2002). Additionally, the average Hainan Island ΔR value is lower than that off Hòn Tre Island, Vietnam (–135±12 ¹⁴C years) (Bolton et al. 2016; Goodkin et al. 2019), although upwelling occurs at both sites.

Averaged ΔR for the SCS

As summarized in Table 2, overall 725 ΔR values from 18 sites were compiled to recalculate the regional ΔR around the SCS ranging from 1900 to 1950. The majority of ΔR values were obtained from the coral skeletons. Twelve sites and 130 ΔR values in total were included within the SCS. The updated SCS ΔR values varied from –61±14 ¹⁴C years to –241±40 ¹⁴C years, with the maximum ΔR value from Houbihu, Nanwan Bay, Taiwan, and the minimum ΔR from Con Dao Island, Vietnam. The average SCS ΔR is –161±39 ¹⁴C years. Note that the average value and error were calculated by the formulas from http://calib.org/marine/AverageDeltaR.html.

Discussion

ΔR is mainly regulated by the regional upwelling, freshwater input, and the circulation (Stuiver and Ostlund 1983). Normally, areas with notable upwelling have high ΔR values, and areas with more freshwater have relatively low ΔR values. The effect of surface circulation on ΔR depends on the Δ¹⁴C value upstream of the site. Besides, the decreasing atmospheric Δ¹⁴C (Suess effect, Suess 1955) since the industrial revolution probably influence our ΔR calculation. However, given the ambiguous trend of ΔR at the Hainan Island and long equilibration times (∼10 yr) between atmosphere and ocean (Bolton et al. 2016), we believed negligible impact of the Suess effect on our coral Δ¹⁴C. Moreover, the Suess effect was ignored at Marine20 calibrated curve (Heaton et al. 2020), by its nonsignificant statistics. Therefore, we neglected potential influence from the Suess effect. As mentioned above, the ΔR difference between Xisha Island and Hainan Island reflects their different oceanographic setting (Hainan Island is a coastal site and Xisha is an open ocean site). Thereby the difference in sampling resolution would exert negligible effect on our ΔR variability. Considering the seasonal climate-dominant circulation in the SCS (Hu et al. 2000), we compare indicators of regional climate and corresponding ΔR values to explore possible linkages.

Monsoon rainfall influenced regional ΔR off east Hainan Island

As demonstrated by SST (Wu and Li 2003), nutrient (Hu et al. 2021), climatological Advanced Very High-Resolution Radiometer SST image and numerical model (Jing et al. 2009), wind-induced coastal upwelling at east of Hainan Island has been thoroughly investigated. Thus, ΔR values here are probably controlled by regional upwelling. However, the ΔR from Hainan Island showed poor correlation (Figure 3b, r=0.3, p=0.12) with reconstructed intensity of regional upwelling based on the summer sea surface temperature anomaly from a 121-yr coral record (Liu et al. 2013). In addition, despite our ΔR values displayed consistent variations with the Indian rainfall anomaly (viewed as intensity of SASM, Singh et al. 2019) in part, insignificant correlation (Figure 3c, r<0.2, p>0.5) between them did not support the dominant influence from SASM on ΔR off Hainan Island. Therefore, we excludeded the SASM induced wind as a driver for influencing ΔR off Hainan Island.

Figure 3. Comparison of ΔR from Hainan Island with regional climate. (a) comparison of ΔR (black dotted line) with Δδ¹⁸ O (orange line) from Hainan Island corals (Deng et al. 2017). (b) comparison of ΔR (black dotted line) with SST anomaly (SSTA, blue line) from Hainan Island corals (Liu et al. 2013). (c) comparison of ΔR (black dotted line) with Indian monsoon index (purple line) derived from Indian rainfall anomaly (Singh et al. 2019). (d) comparison of ΔR (red dotted line) off Hòn Tre Island, Vietnam (Bolton et al. 2016) with ΔR (black dotted line) at east Hainan Island.

Significantly, reconstructed regional SASM precipitation index (Δδ¹⁸O) from Hainan Island correlated well (r=0.45, p<0.05) with our ΔR (Figure 3 a, Deng et al. 2017), which probably indicated that monsoon rainfall dominated regional ΔR. The minor difference between average ΔR value in Xisha Island (–201±16 ¹⁴C years) and Hainan Island (–190±23 ¹⁴C years) suggested the underlying effect of runoff/rainfall from Hainan Island, considering that aged seawater vigorously upwelled off Hainan Island. The influence from freshwater, lowering the ΔR values, likely explained the lower average ΔR values than the average ΔR off the coastal site at Hòn Tre Island, Vietnam (Bolton et al. 2016), areas with distinct upwelling activity (Figure 3 d). Furthermore, previous research indicated that both the river plume and coastal upwelling contributed to the carbonate system at east of Hainan according to observation data from 2014 to 2015 (Dong et al. 2017). Lin et al. (2016) suggested that freshwater impacted the northeastern coast of Hainan Island more than the eastern coast of Hainan Island, based on cruise observations and reanalysis data from 2009. Therefore, we proposed that runoff/rainfall dominated the ΔR values off Hainan Island. Further, as part of SASM, wind and precipitation from SASM both have an impact on Hainan Island, the former dominates the upwelling off Hainan Island (Hu and Wang 2016; Li et al. 2012), while the latter regulates the regional ΔR.

As mentioned above, upwelling off Hainan Island held less contribution to variable ΔR, despite the seawater ages still older than the atmospheric. To quantify how the freshwater input contributed to ΔR off Hainan Island, a two end-members model was used as follows.

For convenience, all measured F value were employed for the model. We supposed the F value of the freshwater runoff was determined from atmospheric F value of the same year. The F value of the upwelled deepwater was equated with the F value of off Hòn Tre Island, Vietnam (Bolton et al. 2016), considering its characteristics with significant upwelling activity. The contribution of the two end-members were computed using Equations (6–7):

(6) $${{\rm{f}}_{\rm{r}}} + {\rm{ }}{{\rm{f}}_{\rm{w}}} = 1$$

(7) $${{\rm{F}}_{\rm{r}}}{{\rm{f}}_{\rm{r}}} + {{\rm{F}}_{\rm{w}}}{{\rm{f}}_{\rm{w}}} = {{\rm{F}}_{{\rm{obs}}}}$$

where f indicates the fractions of runoff or freshwater (f_r), upwelled deepwater (f_w). F_r, F_w, and F_obs denote the F value of the runoff, the deepwater, and the measured F value east of Hainan Island, respectively. Given the negligible difference of DIC between runoff (1805 mol kg^–1) and the offshore seawater (1937 mol kg^–1, observational data from 2014) (Dong et al. 2017), f_r and f_w are assumed to represent the runoff and deepwater fractions.

The two end-members model results revealed that the f_r varied from 0% to 19.2%, with an average of 11.4%. The calculated runoff proportion correlated well (r=-0.53, p<0.01) with the local precipitation reconstructed from the coral (Figure S2, see Supplementary) (Deng et al. 2017). This correlation reinforced our two end-members model, despite previous study suggested nearly 50% runoff contribution, as observed from measured pH value (Dong et al. 2017). Hence, ΔR off Hainan Island was mainly dominated by the SASM induced precipitation rather than wind, namely the upwelling activity.

EAWM influenced regional ΔR in Xisha Island

Different from Hainan Island, the oceanographic setting around Xisha Island is not dominated by upwelling. Due to negligible runoff, freshwater input is expected to exert a minor impact on seawater Δ¹⁴C. Hence, ΔR values at Xisha Island were dominated by the regional circulation. Lacking observational data, we correlated the Xisha Island ΔR with the reconstructed SST (Sun et al. 2004), precipitation (Han et al. 2019), and wind speed (intensity of EAWM, Song et al. 2012) from Xisha Island corals (Figure 4 a–c). We observed a negative correlation between EAWM and ΔR (with one year lag, r=−0.37, p<0.1), and an insignificant relationship between ΔR and reconstructed SST (r<0.1, p>0.5) and precipitation (r=−0.15, p>0.5). Similarly, the SASM displayed a negligible correlation (r<0.1, p>0.5) with the ΔR. Therefore, the ΔR in Xisha Island was mainly modulated by the EAWM, the same as the surface circulation in winter.

Figure 4. Comparison of ΔR from Xisha Island with regional climate. (a) comparison of ΔR (black dotted line) in Xisha Island with reconstructed SST (red line) from coral in Xisha Island (Sun et al. 2004). (b) comparison of ΔR (black dotted line) with Δδ¹⁸O (orange line) from coral at Yongxing Island (Han et al. 2019). (c) comparison of ΔR (black dotted line) with reconstructed winter wind velocity anomaly (blue line) from coral in Xisha Island (Song et al. 2012). Note that ΔR lagged the wind velocity anomaly by one year.

However, given the Ekman transport, the stronger the wind, the more overturning of the deep water, thus the more negative seawater DIC Δ¹⁴C and higher ΔR values (Druffel and Griffin 1993; Grumet et al. 2004). The abnormal negative correlation here probably reflects horizontal water mass movement. Not unique, a seasonal Δ¹⁴C signal from 1968 to 1995, in Currimao, the Philippines, was found to be positively correlated with winter wind speed, implying horizontal transport from the Luzon Strait (Hirabayashi et al. 2017). Moreover, the winter Δ¹⁴C variations from coral at Houbihu (Ramos et al. 2019) in the 1970s displayed a distinct positive correlation between Δ¹⁴C and EAWM. Despite post-bomb rather than pre-bomb relationship between coral Δ¹⁴C and EAWM was revealed, the positive correlation between coral Δ¹⁴C and EAWM did support the horizontal transport of sea water from the Luzon Strait. This horizontal transport of sea water was consistent with our proposal mentioned above. Furthermore, from the Luzon Strait to the Xisha Island, the ΔR should gradually decrease as a result of keeping exchanging between the seawater and air, as can be seen the ΔR values along the path from the Palaui island (–99±105 ¹⁴C years) (Ramos et al. 2019) to Currimao (–142±38 ¹⁴C years) (Hirabayashi et al. 2019), and Xisha Island (–201±16 ¹⁴C years). Consequently, the strong EAWM would facilitate the transport of surface water from Luzon Strait to Xisha Island, leading to a distinct negative correlation with the ΔR. Likewise, in spite of the long time, the one-year lag of ΔR probably represents the transporting time.

Implication for paleoclimate

As shown in Figure 5, the average ΔR inside the SCS (–161±39 ¹⁴C years) is similar to the ΔR on Guam Island (–153±38 ¹⁴C years) (Andrews et al. 2016) and Palau Island (–154±21 ¹⁴C years) (Glynn et al. 2013). This similarity implies that the water in the SCS originated from the western Pacific (Qu et al. 2009), despite that the Kuroshio Current (KC) furnished the SCS to a large extent and the ΔR along the KC path progressively decreased (Nan et al. 2015; Yoneda et al. 2007). Nevertheless, the ΔR inside the SCS is not uniform. The distribution of regional average ΔR was nearly paralleled with the circulation. From the Luzon Strait to the south SCS, the gradually decreased ΔR probably indicated that the seawater kept exchanging with the air along the current path. In addition, the upwelling area held the larger ΔR due to the upwelled depleted Δ¹⁴C water, like the Hon Tre Island (Bolton et al. 2016). In contrast, supplied by plentiful runoff, the ΔR values from estuary was extremely low, like the minimum ΔR in Con Dao Island (Dang et al. 2004). Additionally, near the Luzon Strait, the regional ΔR ranged from –76±98 to –99±105 ¹⁴C years, higher than the ΔR from other sites. Considering the regional bedrock (without limestone) and the special deep circulation across the Luzon Strait, the higher ΔR may be contributed to the overturned deepwater in the Bashi Strait (Qu et al. 2006). Moreover, there seems homogeneous ΔR in the south SCS and northwest SCS, with ΔR of nearly –161±9 ¹⁴C years (average of ΔR in Singapore, Saigon and Ko Ang Trang in Thailand) and –194±22 ¹⁴C years (average of ΔR in Xisha and Hainan Island), respectively. In summary, the average ΔR of –161±39 ¹⁴C years in the SCS was similar to ΔR in the western Pacific. The northwest SCS and estuarine area (such as the Mekong River estuary) were characterized with low ΔR, the Luzon Strait area and the upwelling zone processed high ΔR.

Figure 5. The distribution of ΔR around the SCS. The ΔR was based on Marine 20 Curve, expressed as regional average ΔR with errors (black symbol). Information of the sites (blue dot) is given in Table 2. The average ΔR for Xisha Island of –200±19 ¹⁴C years shown in this figure was calculated using data from this study and from Southon et al. (2002). The red circles denote the coastal upwelling area (Hu and Wang 2016). The thick gray line represents circulation outside the SCS, while the thin gray solid and dashed line represents winter and summer circulation inside the SCS, respectively (Fang et al. 1998). NEC = North Equatorial Current, KC = Kuroshio Current, MC = Mindoro Current.

Previous studies indicated that the El Niño Southern Oscillation (ENSO) and the East Asian summer monsoon affected the SCS ΔR values through time, using paired ¹⁴C and U-Th dates on pristine corals from the past 8000 years (Hua et al. 2020; Yu et al. 2010). However, in spite of the short duration of our ΔR time series data, our results indicated that the monsoon (like SASM and EAWM) mainly modulated the ΔR in the north SCS. Normally, large-scale ΔR variability will be dominated by ocean circulation (Stuiver and Ostlund 1983), with minor effect near estuarine area. These tight relations have been verified from coral Δ¹⁴C (Druffel et al. 2014; Fallon and Guilderson 2008; Rafter et al. 2017). In addition, based on the interannual coral Δ¹⁴C records from the Luzon Strait, Ramos et al. (2019) highlighted that the monsoon primarily control inside the SCS, while ENSO dominated outside the SCS. Therefore, given the negative correlation between ΔR from Xisha Island and EAWM, the glacial SCS ΔR was probably lower than interglacial period, rather than expected from previous studies (Siani et al. 2001).

Conclusion

Here two corals from the north SCS were studied to reveal temporal ΔR variation and its relationship to climate change. The coral ΔR at east of Hainan Island was modulated by the Southern Asian Summer Monsoon (SASM), which affected the regional runoff, rather than wind induced upwelling. The freshwater from runoff contributed average 11.4% based on the two end-members model. Different from Hainan Island, the coral ΔR at Xisha Island was mainly regulated by the East Asian Winter Monsoon (EAWM), which blew the surface water to Xisha Island along the circulation. Hence the stronger the wind, the lower the ΔR at Xisha Island, especially during glacial, when the EAWM became stronger. The average ΔR in the SCS was –161±39 ¹⁴C years, by compiling the radiocarbon data from 1900 to 1950, based on the new calibration curve, Marine20. In spite the limited data, our result from Xisha Island and Hainan Island together implied that the ΔR was mainly regulated by the monsoon. The newly average ΔR for the SCS would contribute to paleoclimate research in the SCS. Further research is needed for the long-term and seasonal ΔR variety.