Isolation of the South China Sea from the North Pacific Subtropical Gyre since the latest Miocene due to formation of the Luzon Strait

The North Pacific subtropical gyre (NPSG) plays a major role in present global ocean circulation. At times, the gyre has coursed through the South China Sea, but its role in the evolutionary development of that Sea remains uncertain. This work systematically describes a major shift in NPSG paleo-circulation evident from sedimentary features observed in seismic and bathymetric data. These data outline two contourite depositional systems—a buried one formed in the late Miocene, and a latest Miocene to present-day system. The two are divided by a prominent regional discontinuity that represents a major shift in paleo-circulation during the latest Miocene (~ 6.5 Ma). The shift coincides with the further restriction of the South China Sea with respect to the North Pacific due to the formation of the Luzon Strait as a consequence of further northwest movement of the Philippine Sea plate. Before that restriction, data indicate vigorous NPSG circulation in the South China Sea. Semi-closure, however, established a new oceanographic circulation regime in the latest Miocene. This work demonstrates the significant role of recent plate tectonics, gateway development, and marginal seas in the establishment of modern global ocean circulation.

www.nature.com/scientificreports/ collision between the N-S trending Luzon volcanic arc of the Philippine Sea plate and the NE-SW trending northern South China margin of the Eurasian plate. Although the oblique collision initially occurred north of Taiwan, at 12-6.5 Ma, it gradually propagated southwestward to the present position 5,6 . Arc-continent collision resulted in uplift of Taiwan Island, an event referred to as the Taiwan orogeny 5,7 . As a consequence, the Luzon Strait formed, with Taiwan Island to the north and Luzon Island to the south. Our study area includes the Central Vietnam slope in the western South China Sea (Fig. 1), which hosts three main carbonate platforms: the Triton (Guangle), Zhongjian and Zhongjianbei (Fig. 2). These platforms began to develop in the early Miocene on the Eocene-Oligocene basement highs, then became less active in the late Miocene due to regional thermal subsidence and cooling 12,13 . Main sediment input in the study area is from west and north 14,15 . Found in the Central Vietnam shelf is a slip-strike fault system, the Red River Fault (Fig. 1), which is dextral at a low rate after its reversal from sinistral during the late Miocene (e.g. Zhu et al., 2009 16 ). Previous research has revealed contourite deposition (honeycomb structures) west of the Zhongjianbei platform 17 , and contourite channels around the Triton and Zhongjian platforms [18][19][20] .  Water (LCDW). The NPTW, driven by NPSG, circulates anticyclonically between 0 and 500 m water depth. The NPIW and PDW respectively flow southward between 500 and 1500 m water depth, and between 1500 and 4000 m water depth. The LCDW flows northward below 4000 m water depth. Water exchange between the Pacific Ocean and the South China Sea mainly occurs through the Luzon Strait ( Fig. 1), where the NPTW and PDW flow into the South China Sea while the SIW flows out of the South China Sea. Luzon Strait transport is estimated at 3 to 6.5 Sv (where 1 Sv = 1 × 10 6 m 3 /s) based on long-term observations (review in Cai et al., 2020 24 ).

Data and methods
This study relies on multibeam swath bathymetry and multichannel seismic reflection profiles.
Multibeam swath bathymetric surveys. The bathymetric survey, which covered the most of the study area ( Fig. 1b), was conducted using a Kongsberg EM122 multibeam system. The original multibeam sounding data were processed by means of CARIS HIPS and SIPS software (version 8.1.9). The final high-resolution seabed digital terrain model was built at a 100 m grid resolution using the swath angle surface method of CARIS HIPS and SIPS software (Fig. 2).
The bathymetric data served to identify modern contourite features together with seismic data following the morphological and seismic criteria defined by Seismic reflection data. Eight multichannel 2D seismic reflection profiles (Fig. 1b) were used in this study.
Seven of the eight profiles were collected by the Guangzhou Marine Geological Survey during 2013 and 2017, and the other one is adapted from Fyhn et al. (2013) 14 , which was acquired in 2003. The seismic data below the seabed were processed by means of a standard pre-stack time-migration procedure (see Supplementary Table S1 online). The major processing steps of the seismic sections above the seabed include direct wave attenuation, amplitude recovery, high-pass filtering, and constant velocity (seawater sound velocity) stack.
The seismic data were used to (a) execute a seismic stratigraphic analysis through a conceptual approach 30 , (b) identify large contourite features according to accepted criteria 29 , and (c) formulate an oceanographic interpretation of water mass structure following methods outlined in Holbrook et al. (2003) 31 . Age-control of seismic stratigraphic analysis is adopted from previous studies, especially Vu et al. (2017) 15 , with calibration of eight exploration wells in or near this study area. www.nature.com/scientificreports/

Seismic analysis
Within the water column, two major bodies can be discerned according to their reflectivity characteristics (Fig. 3). The first is a shallower body (between 0 and 750 m water depth) that appears in seismic data as highamplitude reflections interbedded with low-amplitude reflections. This body corresponds to the vigorous South China Sea surface water (SSW). The second, deeper body (> 750 m water depth) exhibits moderate-amplitude reflections and corresponds to sluggish South China Sea intermediate water (SIW). The sedimentary record shows three main seismic units (SUs) (Fig. 3). The lowermost SU represents a sedimentary package without contourite features (pre-drift). The two succeeding seismic units (SU1 and SU2, from bottom to top) consist mostly of large drift deposits. They are bound by two regional discontinuities (H1 and H2) and the modern seafloor. The H1 discontinuity defines the base of seismic unit SU1. This surface exhibits  in (a,b) were generated using GeoFrame software (version 4.5, http://slb-sis.com.cn/produ cts-servi ces/GeoFr ame.aspx). The maps in (d,e) were generated using Surfer software (version 13, https ://www.golde nsoft ware.com/produ cts/surfe r). All maps were designed using CorelDraw Graphics Suite X8 (https ://www.corel draw.com/cn/). www.nature.com/scientificreports/ continuous, high-amplitude reflections and partial erosional truncation over the underlying pre-drift package around the Triton platform; the reflections of SU1 are of moderate amplitude, and moderate to continuous length (Fig. 3). Locally, the unit contains contorted, chaotic or free reflections below depressions and mounds. The H2 discontinuity defines the base of seismic unit SU2. This surface is marked by continuous, highamplitude reflections and partial erosional truncation over the underlying SU1 and around the east flank of the Triton platform. The SU2 reflections show onlap and downlap with respect to H2. The moderate-amplitude, relatively continuous reflections of SU2 are sometimes interrupted by discontinuous, contorted reflections within depressions.

Contourite features
In both SU1 and SU2, a number of large erosional, depositional and erosion-depositional contourite features appear along the western South China Sea margin (Figs. 2 and 3).
Erosional features. Three types of erosional features were identified.
(a) Contourite channels. A single contourite channel (larger valley) extends southward along the upper slope. This channel splits into four distributaries after traversing the passage between the Triton and Zhongjian platforms. It occupies a 700 to 900 m water depth, has a total length of 63 km, a width of 0.5-2.1 km, and incises to 130-220 m depth ( Table 1). The channel starts at the H1 discontinuity and records cycles of erosion and infill within SU1.  Table 1). In SU1, they extend along the western flank of the Triton platform (Fig. 3c) and migrate towards the east, where they reach 1.6-13.1 km in width and 50-140 m in depth.
(c) Furrows. These smaller erosional valleys run generally oblique to isobaths. At least 70 furrows could be identified between 450 and 1000 m water depth on the upper slope. They are 4-40 km in length, 0.5-1.5 km in width, and reach depths of 30-120 m ( Table 1).
Depositional features. Two types of sediment drifts were identified (Figs. 2 and 3).
(a) Plastered drifts. A plastered drift exhibits a broad, very slightly mounded geometry and thins both onshore and offshore within SU2 (Fig. 3). This feature extends along the upper slope between 600 and 1100 m water depth to cover the eastern and northern flank of the Triton platform. The drift is 170 km long, up to 85 km wide, and reaches 425-680 m in sedimentary thickness (Table 1). It exhibits an aggradational configuration, and is incised by the contourite channel and furrows described above.
(b) Mounded drifts. Two mounded drifts (MD-1 and MD-2) were identified in SU1 mainly on the basis of their external shape (Fig. 3). The MD-1 drift, 300-500 m thick and 15-26 km wide (Table 1)  www.nature.com/scientificreports/ ning along the western flank of the Triton platform between 1.2 and 2.2 s (twt). This feature has an aggradational and progradational (eastward) configuration. Moats are found along the eastern side of the drift.

Erosion-depositional features. Two types of erosion-depositional (mixed) features were identified (Figs. 2 and 3).
(a) Contourite terraces. A contourite terrace appears along the upper slope at 600-1100 m water depth, exhibiting both erosional (contourite channel and furrows) and depositional (plastered drift) phases. It has a mean seafloor gradient of 0.2° (Table 1). This terrace represents the proximal sector of a plastered drift located along the middle slope.  (Table 1). Depressions are filled by contorted and/or sub-parallel reflections, and appear in SU2.

Decoding circulation from the late Miocene to the present
In the present-day seafloor, the contourite terrace is basically located at an interface depth range between the modern SSW and SIW, which define the upper and lower sections of the water column (Fig. 3a, b). In winter, the interface of the two shifts to a shallower position relative to that of summertime (Fig. 1c). Above the interface the SSW circulates faster (up to 100 cm/s) than the SIW (5-15 cm/s 22 ). The terrace´s erosional features (contourite channel and furrows) record sweeping of the seafloor by cores, branches and filaments of faster currents within the water mass. A plastered drift-muddy, as inferred from exploring wells 15 -appears in areas with slower currents. Terrace formation thus implies vertical fluctuations of that interface in time, evoking the mechanism proposed in previous studies of terraces 32,33 and plastered drifts 34 in other marine basins. When the interface is deeper (i.e., below terrace depth), the terrace is eroded by the fastflowing SSW (Fig. 4a); when the interface is shallower, the terrace comes under the weaker influence of the SIW (Fig. 4a), which favors depositional processes.
The oblique intersection of the furrows on isobaths along the terrace (Fig. 2) indicates the definite central effect of water mass circulation, although other processes beside water mass circulation-e.g. turbidity currents, internal waves, and cascading dense water-possibly make a contribution meriting further study. In addition, the furrows usually cross trains of depressions (Fig. 2), indicating the contribution of depression to the development of the furrows.
Based on the criteria of Faugères et al. (1999) 25 , the present position of the contourite channel with respect to the adjacent drift along the eastern flank of the Triton platform would indicate a southward-flowing SSW. Current data confirm the hypothesized Coriolis-induced imbalance in the geostrophic flow along the eastern side of the platform 22 . Both the contourite channel and the plastered drift appear within the present-day seafloor, but also throughout SU2 (Fig. 3). We can therefore infer that present-day SSW circulation was already established by the time discontinuity H2 occurred at the base of SU2. These features represent the onset of the modern contourite depositional system. The H2 boundary corresponds to the base of Sequence 5 from Vu et al. (2017) 15 , who dated eight wells as upper Upper-Miocene based on biostratigraphy (see Supplementary Table S2 and Fig. S1 online), to the base of Pliocene from Fyhn et al. (2013) 14 , and to T30 (base of Pliocene) from Lu et al. (2017Lu et al. ( , 2018 35,36 . Growth of the muddy plastered drift during SU2 indicates the influence of a weak water mass (SIW) at this depth 25,34 . The seismic reflection variations in SU2 (Fig. 3) may be related to changes of the sluggish SIW intensity variation, sediment source change and other sedimentary processes.
A more deeply buried contourite depositional system appears within SU1, which is muddy, inferred from drilling results 15 Supplementary Table S2 and Fig. S1 online).
Assuming an average interval velocity of 1700 m/s for SU1 and SU2 deposition, the buried late Miocene contourite depositional system in SU1 lies 1175 to 1650 m below the modern sea surface level. The estimated paleo-water depth of the seabed was 20-50 m at the beginning of the late Miocene and about 500-700 m by the end of the late Miocene 37 , the deepening over time being a result of rapid post-rift subsidence in the late Miocene 12,37 . A buried contourite depositional system therefore formed under the influence of a surface water mass flowing northward along the western South China Sea (Fig. 4b).
The modern East Asian summer monsoon could have facilitated clockwise surface water circulation south of the study area, but not reaching it-the modern SSW in the study area is always southward 22,38 . During the late Miocene, the intensity of the East Asian summer monsoon would have been slightly weaker than in the Pliocene to present 39,40 . In other words, the late Miocene East Asian summer monsoon could not have driven the northward circulation of the SSW in the study area. Yet throughout the late Miocene, the South China Sea had a greater degree of exchange with the Pacific Ocean than today, owing to the much more southern position of Luzon Island and the non-appearance of Taiwan Island 7,9

Isolation of the South China Sea from the North Pacific Subtropical Gyre
Buried and modern contourite depositional systems generated by water masses flowing in opposite directions provide evidence of major paleo-oceanographic shifts in the latest Miocene (Fig. 4c,d). Such changes indicate isolation of the South China Sea from the NPSG due to formation of the Luzon Strait as a consequence of the further northwest (N307°) movement of the Philippine Sea plate since the latest Miocene (~ 6.5 Ma) 5,7 . This event entails northward movement of Luzon Island, rapid uplift of Taiwan Island, and remarkable volcanism within the Luzon arc 7 -features forming a barrier for the westward extension of the NPSG and isolating the South China Sea. Deflection by the barrier caused a significant eastward shift in Pacific surface circulation 41 . The isolation of the South China Sea exemplifies that restriction of a small marginal sea by plate tectonics can influence both regional circulation in marginal seas and open ocean circulation. It is noteworthy that all the small-scale depressions occur in SU2, most likely formed by erosion and deposition involving sub-mesoscale currents triggered by mesoscale eddies 17,42 , although fluid flow activity may also have contributed to the depression formation 35 . Accordingly, isolation might have significantly enhanced the activity of mesoscale eddies in the sea. Such oceanographic shifts can occur prior to the complete closure of gateways. A similar scenario affected the Sea of Japan, another semi-enclosed marginal sea isolated from the North Pacific during the late Miocene to middle Pliocene 43 .
The isolation and further restriction of the South China Sea roughly coincide with (at least) three major paleo-oceanographic events linked to global ocean circulation. First, the Central American Seaway became shallow enough to block intermediate water circulation between the Atlantic and Pacific Oceans 44 . Second, the narrowing of the Indonesian Seaway reduced equatorial water mass exchange between the Indian and Pacific Oceans 45 . Third, the Mediterranean/Atlantic connection closed, and then opened (Strait of Gibraltar) by the  (a,b) were generated using GeoFrame software (version 4.5, http://slb-sis.com.cn/produ cts-servi ces/GeoFr ame. aspx). All maps were designed using CorelDraw Graphics Suite X8 (https ://www.corel draw.com/cn/). www.nature.com/scientificreports/ beginning of the Pliocene 46,47 . All these global changes, within the same time frame, contributed to the development of modern global ocean circulation.

Conclusion
The South China Sea has been isolated from the NPSG since the latest Miocene due to formation of the Luzon Strait, in turn related to further northwest movement of the Philippine Sea plate. Isolation because of recent plate tectonic activity (including the closure and opening of other gateways) caused major coeval paleo-oceanographic and sedimentary changes within the South China Sea. Finally, the latest Miocene paleo-oceanographic change promoted the present-day circulation in both the Pacific Ocean and the South China Sea.