A total of 553 samples were collected from outcrops of the TWFB exposed along rivers in southwestern Taiwan, including 272 collected from the Kueichulin Formation by Dashtgard et al. (2021) and Hsieh et al. (2023b) along the Da'an River (Fig. 2A). This was combined with new data from the Chinshui Shale (n = 90; Tachia River) and the Cholan Formation (n = 191; Houlong River) (Fig. 2A). Data between 4.13 and 3.15 Ma are not available as no outcrop sections were accessible. Gamma-ray data were obtained from the HYS-1 borehole drilled through the TWFB. Age-equivalent material was also obtained from deep-sea sediment cores ODP Site 1146 (19°27.40^′ N, 116°16.37^′ E, 2092 m water depth, 179.8–343.1 m core depth; Holbourn et al., 2005, 2007) and Site 1148 (18°50.169^′ N, 116°33.939^′ E, 3294 m water depth, 118.9–206 m core depth; Tian et al., 2008; Cheng et al., 2004), archived in international core repositories. Sampling resolution averaged ∼ 1.4 m vertically through the TWFB stratigraphic sections, and ∼ 0.65 and ∼ 0.35 m through the ODP Sites 1146 and 1148 cores, respectively.

Samples from the Chinshui Shale and ODP sites were analysed for organic geochemistry and paleomagnetism. For the Chinshui Shale, total organic carbon (TOC) and total nitrogen (TN) concentrations were determined from pulverized rock samples in the Department of Geosciences at National Taiwan University (NTU) using an elemental analyser (Elementar TOC analyser soli TOC^® cube; Lin et al., 2025). Total carbon (TC) and TN abundances for ODP samples were determined with a CHNS Elemental Analyser (Thermo Finnigan Flash EA 1112) at the Institute of Earth Sciences (ISTE) at the University of Lausanne in Switzerland on oven-dried sieved and crushed sediment samples. The samples were heated to 900 °C, after which the combustion products were extracted into a chromatographic column where they were converted into simpler components: CO₂ and N₂. These components were then measured by a thermal conductivity detector, and the results were expressed as a weight percentage. Analytical precision and accuracy were determined by replicate analyses and by comparison with an organic analytical standard composed of purified L-cysteine, achieving a precision of better than 0.3 %. Organic matter (OM) analyses of ODP core samples were performed on whole-rock powdered samples using a Rock-Eval 6 at the ISTE following the method described by Espitalie et al. (1985) and Behar et al. (2001). Measurements were calibrated using the IFP 160000 standard. Rock-Eval pyrolysis provides parameters such as hydrogen index (HI, mg HC g⁻¹ TOC, HC = hydrocarbons), oxygen index (OI, mg CO₂ g⁻¹ TOC), Tmax⁡ (°C), and the TOC (wt %). HI, OI and Tmax⁡ values, which give an overall measure of the type and maturation of the organic matter (e.g., Espitalie et al., 1985), cannot be interpreted for TOC < 0.2 wt % and S2 values ≥ 0.2 mg HC g⁻¹. Total organic carbon accumulation rates (mg cm⁻² kyr⁻¹) for the ODP sites were calculated by multiplying mass-accumulation rates (MAR) derived from literature and TOC.

Organic carbon isotopic compositions (δ13C_org, ‰ relative to Vienna Pee Dee Belemnite) were measured by flash combustion on an elemental analyser (EA) coupled to an isotope-ratio mass spectrometer (IRMS) from pulverized, decarbonated (10 % HCl treatment) whole-rock samples. Samples from ODP sites were analysed at the Institute of Earth Surface Dynamics, University of Lausanne, using a Thermo EA IsoLink CN connected to a Delta V Plus isotope ratio mass spectrometer (Thermo Fisher Scientific, Bremen), both operated under continuous helium flow. The samples and standards are weighed in tin capsules and combusted at 1020 °C with oxygen pulse in a quartz reactor filled with chromium oxide (Cr₂O₃) and below with silvered cobaltous-cobaltic oxide. The combustion produced gases (CO₂, N₂, NO_x and H₂O) are carried by the He-flow to a second reactor filled with elemental copper and copper oxide wires kept at 640 °C to remove excess oxygen and reduce non-stoichiometric nitrous products to N₂. The gases are then carried through a water trap filled with magnesium perchlorate (Mg(ClO₄)). The dried N₂ and CO₂ gases are separated with a gas chromatograph column at 70 °C and then carried to the mass spectrometer. The measured δ13C values are calibrated and normalized using international reference materials and in-house standards (Spangenberg, 2016). Samples from the Chinshui Shale were analysed at the Stable Isotope Laboratory at National Taiwan University using a Flash EA (Thermo Fisher Scientific) coupled to a Delta V Advantage (Thermo Fisher Scientific). The δ13C values are calibrated using an international reference material, IAEA-CH-3. The reproducibility and accuracy are better than ± 0.1 ‰.

Thirty-three oriented palaeomagnetic core specimens (25 mm diameter) were collected at ∼ 3.5 m intervals from unweathered, mud-rich beds from the Chinshui Shale, then prepared and analysed at Academia Sinica in Taiwan following the methodology described in Horng (2014). Cores were cut into 2 cm samples, and bulk magnetic susceptibility measured using a Bartington Instruments MS2B magnetic susceptibility meter. Mass-specific magnetic susceptibility (χ) was then derived by normalising bulk magnetic susceptibility to sample mass.

Existing data for the ODP sites 1146 and 1148 were also compiled from literature, including clastic MAR (Site 1146 from Wan et al., 2010a, Site 1148 from Wang et al., 2000b), magnetic susceptibility (1146 from Wang et al., 2005b, 1148 from Wang et al., 2000b), hematite / goethite ratios (Hm / Gt) derived from spectral reflectance band ratios at 565 / 435 nm (1146 from Wang et al., 2000a, 1148 from Clift, 2006), continuous gamma-ray logs (1146 from Wang et al., 2000a, 1148 from Wang et al., 2000b), and titanium / calcium ratios (Ti / Ca; 1146 from Wan et al., 2010a, 1148 from Hoang et al., 2010). Magnetic susceptibility and Ti / Ca serve as proxies for physical erosion, recording variations in terrigenous sediment flux linked to summer monsoon precipitation. Intensified precipitation enhances basin sediment accumulation rates (Clift et al., 2014), and typically increases the magnetic susceptibility of marine sediment via enhanced runoff and terrestrial input (Clift et al., 2002; Kissel et al., 2017; Tian et al., 2005). In the SCS, magnetic susceptibility also serves as a sediment provenance indicator. Along the Taiwan Strait, sediment sourced from western Taiwan yields χ values that range from 0.9 ± 0.3 to 1.8 ± 0.5 × 10⁻⁷ m³ kg⁻¹, much lower than those sourced from the South China Block (4.0 ± 1.3 × 10⁻⁷ m³ kg⁻¹), indicating a relative depletion of magnetic minerals in Taiwan-sourced material (Horng and Huh, 2011). Titanium, associated with heavy mineral deposition, and calcium, linked to pelagic biogenic carbonate accumulation, yield Ti / Ca values that increase with enhanced monsoon-driven sediment export (Clift et al., 2014). Gamma-ray intensities broadly track changes in lithology (Green and Fearon, 1940; Schlumberger, 1989), where values < 75 American Petroleum Institute (API) typically mark sandstone-rich intervals, > 105 API mudstone-rich intervals, and intermediate values reflect mixed lithologies. Increased sediment export, particularly of coarser grains, may be expressed as lower API values.

Sedimentary TOC content provides a measure of organic carbon accumulation through time. Terrestrial and marine sources can also be differentiated by their δ13C_org values (Dashtgard et al., 2021; Hilton et al., 2010; Chmura and Aharon, 1995; Peterson and Fry, 1987; Martiny et al., 2013). Marine organic matter (e.g., plankton, particulate and dissolved organic matter) typically have more enriched values than terrestrial inputs (e.g., C₃ and C₄ plants, and soil and lithogenic organic carbon) (Table 1). Marine-derived organic matter mainly accumulates on the seafloor under fair-weather conditions, while terrestrial input increases under intervals of increased precipitation and erosion (Hsieh et al., 2023b; Dashtgard et al., 2021).

Hematite-to-goethite (Hm / Gt) ratios are widely applied as indicator of monsoon precipitation (Clift, 2006; Liu et al., 2007; Zhang et al., 2009). Hematite typically forms through iron oxidation under arid climates or seasonal climates with dry seasons, whereas goethite preferentially develops under humid climates (e.g., Kämpf and Schwertmann, 1983; Maher, 1986). In the northern SCS, however, Clift et al. (2014) documented a positive relationship between elevated Hm / Gt values and intensified East Asian Summer Monsoon (EASM) rainfall and seasonality. Beyond climate, hematite also reflects sediment provenance: sediment derived from Taiwan is notably depleted in hematite and enriched in pyrrhotite (Horng and Huh, 2011). Locally estimated scatterplot smoothing (LOESS) is applied to all data to reveal trends through the studied time interval (Cleveland et al., 1992).

The chronostratigraphic framework for the Kueichulin Formation, Chinshui Shale, and Cholan Formation of the TWFB was established by astronomically tuning the gamma-ray records to the δ18O record of Wilkens et al. (2017), Hsieh et al. (2023a), Vaucher et al. (2023b). However, the boundary between the top of the Kueichulin Formation and the base of the Chinshui Shale is not well-established. Therefore, a magnetobiostratigraphic age model was developed from nannofossil zones and magnetic reversals identified in oriented outcrop core samples from the Chinshui Shale outcrop using the methodology described in Horng (2014) to ground-truth the existing framework. The remanent magnetic intensity, and declination and inclination of oriented core samples were measured using a JR-6A spinner magnetometer (AGICO). To determine the stable remanent magnetization and polarity (i.e., normal or reversed) of each sample, unstable secondary magnetization was removed by thermally demagnetizing the samples stepwise from 25 to 600 °C. The characteristic remanent magnetization (ChRM) declination and inclination of thermally demagnetized samples were calculated using principal component analysis with a minimum of three demagnetization steps in the PuffinPlot software (Lurcock and Wilson, 2012) to determine the polarity of each sample. Thermal demagnetization diagrams for the Chinshui Shale samples showing the stable remanent magnetic declinations and inclinations after principal component analysis are presented in Fig. S1 in the Supplement.

Index nannofossils and corresponding biozonations identified by Shea and Huang (2003) for the Chinshui Shale were used to constrain paleomagnetic polarities. The resulting age model was then correlated to an orbitally tuned, benthic foraminiferal, stable oxygen isotope (δ18O) record from the equatorial Atlantic Ocean (Wilkens et al., 2017), which is tied to physical sedimentary properties independent of ice volume, and has a robust timescale. Variations in both parameters are assumed to be causally linked and temporally in-phase.

The age model for ODP Site 1146 (Wan et al., 2010a) was constructed by linear interpolation between magnetobiostratigraphic age control points established by Wang et al. (2000a). Stratal ages from ODP Site 1148 are constrained using biostratigraphic ages of benthic foraminifera (Wang et al., 2000b).

Provenance exerts a first-order control on sedimentary records in the SCS, owing to the region's complex geology and active tectonism, which channels sediment contributions from multiple major rivers (e.g., Clift et al., 2014, 2022; Horng and Huh, 2011; Kissel et al., 2016, 2017; Liu et al., 2009c, 2007, 2010b, 2016; Wan et al., 2010c; Milliman and Syvitski, 1992). During most of the Neogene, the Pearl River supplied the dominant sediment flux to the northern SCS (Clift et al., 2002; Li et al., 2003). The emergence of the Taiwan orogen in the early Pliocene fundamentally reorganised this system: by ∼ 5.4 Ma, and especially after ∼ 4.9 Ma, Taiwan had become a major sediment source to the adjacent TWFB and the wider SCS, as a result of rapid uplift and intense erosion and south-westward collision-zone migration (Fig. 5; Liu et al., 2010b; Hsieh et al., 2023b; Hu et al., 2022). This change in sediment provenance is tectonically driven and underscores the need to disentangle tectonic from climatic signals in SCS sedimentary archives (Clift et al., 2014; Hsieh et al., 2024).

This diversity in sediment sources and mixing is reflected at ODP Sites 1146 and 1148, where the sediment records differ despite their spatial proximity. MAR, magnetic susceptibility, Hm / Gt and gamma-ray records diverge between the two sites until ∼ 3 Ma (Fig. 4). At ODP Site 1146, located on the continental slope, sediments are primarily derived from Eurasia (Fig. 5). At Site 1146, major element and clay mineral compositions point to a mixture of sources dominated by the Pearl River, with additional inputs from the Yangtze River, Taiwan, Luzon, and loess (Wan et al., 2007a; Liu et al., 2003; Hu et al., 2022). Pearl River sediment deposition is controlled by long-term sea-level changes and East Asian Monsoon variability (e.g., Liu et al., 2016; Clift, 2006), but its delivery to the open basin is strongly constrained by alongshore and surface- to intermediate-water currents that funnel most material along the continental shelf and slope (Liu et al., 2010b, 2016; Wan et al., 2007a; Liu et al., 2009a).

In contrast, ODP Site 1148, located on the continental rise, records a stronger Taiwanese imprint (i.e., less contribution from Eurasia; Fig. 5). Prior to ∼ 6.5 Ma, major element data suggest a mixture of Pearl River and Taiwan inputs, but since the onset of Taiwan orogenesis (∼ 6.5 Ma), Taiwanese material has increasingly dominated (Hu et al., 2022). Isotopic (⁸⁷Sr / ⁸⁶Sr, εNd) records, and clay mineralogy showing increasing illite with corresponding decreasing kaolinite near ∼ 5 Ma corroborate Taiwan as the dominant sediment contributor to the northern SCS since its emergence after ∼ 5.4 Ma (Bertaz et al., 2024; Boulay et al., 2005; Clift et al., 2014; Hsieh et al., 2023b). Erosion of modern and ancient Taiwan is primarily driven by tropical-cyclone precipitation (Dashtgard et al., 2021; Vaucher et al., 2021; Galewsky et al., 2006; Chen et al., 2010; Chien and Kuo, 2011; Janapati et al., 2019). Under warmer Pliocene climates (Fedorov et al., 2010; Yan et al., 2016) such storms were likely more frequent and intense (e.g., Yan et al., 2019), and especially if coinciding with EASM circulation, would have driven exceptionally high precipitation (Chen et al., 2010; Chien and Kuo, 2011; Kao and Milliman, 2008; Lee et al., 2015; Liu et al., 2008) and sediment export (Vaucher et al., 2023b). Sediment derived from Taiwan is subsequently redistributed into the northern SCS by downslope deep currents (Liu et al., 2010b, 2016; Hu et al., 2012; Liu et al., 2013). The emergence of Taiwan also reconfigured regional circulation, establishing a westward Kuroshio branch that delivered additional sediment from Taiwan and the Philippines (i.e., the Luzon Arc) into the northern basin (Liu et al., 2016).

The difference in sediment provenance and transport pathways between the continental slope and continental rise is reflected in the contrasting proxy trends observed at both ODP sites (Fig. 4). At ODP Site 1146, the long-term increase in magnetic minerals since ∼ 6.27 Ma reflects increased sediment input from Eurasia that is comparatively enriched in magnetic minerals than sediment from Taiwan (Horng and Huh, 2011; Hsieh et al., 2023b). Concurrently, low gamma-ray values and declining Ti / Ca until ∼ 3 Ma also reflect increased delivery of sand-rich, clastic detritus, while the decreasing Hm / Gt suggests a progressive weakening of the EASM rainfall and seasonality. Together, these proxy signals are consistent with global trends of long-term cooling and falling global mean sea level during this interval (Wan et al., 2007b; Haq et al., 1987; Miller et al., 2020; Westerhold et al., 2020; Holbourn et al., 2021; Berends et al., 2021; Rohling et al., 2014; Jakob et al., 2020; Haq and Ogg, 2024), as well as with evidence of diminished chemical weathering and progressive weakening of the EASM system (Clift et al., 2014; Wan et al., 2006, 2010a, b; Clift, 2025; Li et al., 2004; Wang et al., 2019). This interpretation is further supported by declining K / Al ratios observed at ODP Site 1146 between 5 and 3.8 Ma by Tian et al. (2011), which likewise indicate reduced chemical weathering and a shift towards long-term drying, which began ∼ 10 Ma in the region (Clift, 2025; Clift et al., 2014).

At ODP Site 1148, MAR increases near the onset of Taiwan's orogenesis (∼ 5.4 Ma), corresponding to enhanced sediment export from rapid erosion of the emerging orogen. An increase in magnetic susceptibility is also observed ∼ 5.4–4.3 Ma (Fig. 4), consistent with the erosion of passive-margin seafloor sediments enriched in magnetic minerals that was uplifted during the early stages of Taiwan's orogenesis (Hsieh et al., 2023b). After ∼ 4.3 Ma, magnetic susceptibility declines, coinciding with the deposition of the Yutengping Sandstone and increasing influx of sediment derived from the metasedimentary core of Taiwan, which is comparatively depleted in magnetic minerals (Hsieh et al., 2023b). Unlike Site 1146, the Hm / Gt record at Site 1148 does not appear to track long-term monsoon drying. Rather, the abrupt decrease in the Hm / Gt record at ∼ 5.4 Ma is attributed to the influx of hematite-depleted sediment from Taiwan as it emerged from the Pacific Ocean. The dispersal of Taiwan-sourced sediment into the northern SCS was facilitated by deep-water currents and by the westward-flowing Kuroshio Branch, both of which developed following the formation of the Taiwan and Luzon straits during orogenesis. Changes in ocean circulation during the early to middle Pliocene are also captured by K / Al records, which show contrasting trends between intermediate water depths (e.g., Site 1146) and deep water settings (e.g., Site 1148), which is interpreted as reflecting shifts in sediment dispersal pathways to the northern SCS (Tian et al., 2011). The subsequent rise in Hm / Gt near ∼ 3.2 Ma is attributed to the northward remobilization of Taiwan-sourced sediment following the formation of the Taiwan Warm Current (Fig. 3; Hsieh et al., 2024). The gamma-ray record also tracks the orogenic evolution of Taiwan at ODP Site 1148 (Fig. 4) and parallels observations from the TWFB (Fig. 3): values are elevated during the deposition of mudstone-rich Shihliufen Shale, decrease during formation of sand-dominated Yutengping Sandstone and rise again with the deposition of mudstone-rich Chinshui Shale and Cholan Formation. The increase in sediment export from Taiwan is also reflected in the Ti / Ca record, which increases after ∼ 5.4 Ma, in response to intensified physical erosion and elevated terrestrial flux linked to the onset of Taiwan orogenesis.

After ∼ 3 Ma, the onset of Northern Hemisphere Glaciation (NHG) resulted in enhanced seasonality and an intensification of the EASM (Fig. 5; Clift, 2025; Wan et al., 2006; Clift et al., 2014; Wan et al., 2007a, b). Although global cooling characterized the late Plio-Pleistocene (Lisiecki and Raymo, 2005), sea-surface temperatures in the northwest Pacific remained sufficiently high (26.5–27.0 °C) to sustain tropical cyclone activity (Tory and Frank, 2010). This combined influence of intensified EASM and frequent tropical-cyclone precipitation promoted elevated sediment production and large-scale export of fine-grained material enriched in TOC from river catchments into offshore depocenters. This is reflected in both sites by higher gamma-ray values, increased MAR, and rising Ti / Ca ratios (Fig. 4). Enhanced seasonality is further expressed in the greater amplitude observed in gamma-ray, Hm / Gt, and Ti / Ca records.

Organic carbon buried in the SCS can be broadly divided into two components: (1) terrestrial organic matter derived from rock, soil, and terrestrial vegetation exported from adjacent landmasses by precipitation-driven erosion, and (2) marine organic matter produced by primary productivity and exported to the seafloor.

At Site 1146, organic carbon accumulation, like bulk sediment accumulation, is primarily controlled by long-term global sea-level fall associated with the onset and intensification of NHG (Fig. 5). Total organic carbon values are closely coupled with MAR, with increases in sediment flux consistently accompanied by higher TOC concentrations (Fig. 4). Although δ13C_org values show a modest decline between ∼ 5.7 and 4.5 Ma, which is consistent with episodic dilution by terrestrial organic inputs, values remain within the marine range (Table 1). The gradual increase in terrestrial organic matter at ODP Site 1146 is interpreted to reflect increased Eurasian clastic influx under conditions of long-term sea-level fall. The cross-plot of δ13C_org and TOC also shows no distinct shift between organic matter delivered to Site 1146 before and after the emergence of Taiwan. As sediment transported eastward from the Eurasian margin would have longer residence times in the ocean, the dilution of land-derived organic material by marine organic material would increase, resulting in a more marine δ13C_org signature (Dashtgard et al., 2021) which supports the interpretation that organic material is derived mainly from Eurasia via the Pearl River (Fig. 6).

In contrast, carbon burial at Site 1148 is primarily linked to the uplift and erosion of Taiwan and associated increase in sediment and nutrient delivery to the marine environment (Fig. 5). The onset of Taiwan's emergence at ∼ 5.4 Ma coincides with a marked rise in MAR, followed by an increase in TOC beginning near ∼ 4.9 Ma (Fig. 4). This pattern reflects significant export of terrestrial sediment from the rapidly uplifting Taiwan orogen, a process further amplified by the coupling between tropical cyclone and monsoon precipitation (Vaucher et al., 2023b). Notably, TOC increases proportionally with MAR, implying that carbon burial was not diluted by high sediment flux but rather enhanced by intensified sediment export and/or better preserved by rapid burial, highlighting the role of Taiwan as a contributor of organic carbon in the northern SCS. The influence of sedimentation from Taiwan on organic matter buried at Site 1148 is also evident from the cross-plot between δ13C_org and TOC, which shows a distinct increase in TOC prior to and after the emergence of Taiwan (Fig. 6).

Taiwan's steep topography and active tectonics generate exceptionally high sediment yields to adjacent marine systems (Liu et al., 2013; Dadson et al., 2003, 2004). Turbidity currents, especially via submarine canyon systems (e.g., the Gaoping Submarine Canyon in southern Taiwan), efficiently transport organic-rich sediment eroded from Taiwan to deep-sea environments approximately 260 km offshore into the northeastern Manila Trench (Liu et al., 2009b, 2016; Zheng et al., 2017; Yu et al., 2009; Nagel et al., 2018). Within the TWFB, this process is manifested as an abrupt increase in terrestrial organic matter and sand-rich deposition near ∼ 4.9 Ma with the emplacement of the Yutengping Sandstone (Fig. 4). At Site 1148, TOC increases markedly in association with the emergence of Taiwan, and δ13C_org values remains stable above -25 ‰. While C₄ plants are characterized by high δ13C_org values (Table 1), and an expansion of C₄ plants in the South China region has been documented since 35 Ma (Li et al., 2023; Xue et al., 2024), the organic carbon at Site 1148 are interpreted to be of marine in origin as C₃ plants remain the dominant vegetation type in the study area (Luo et al., 2024; Still et al., 2003; Wang and Ma, 2016). Furthermore, sediment provenance markers (Sect. 5.1) indicate an influx of Taiwan-sourced material to Site 1148 after the emergence of Taiwan, and δ13C_org values in the TWFB reflect an increase in terrestrial organic matter. The presence of Taiwan-sourced material combined with high proportions of marine organic carbon at Site 1148 suggests that terrestrial organic matter from Taiwan was largely confined to proximal coastal environments, and that enhanced carbon burial in deeper settings reflects processes beyond direct terrigenous input. Likewise, terrestrial organic matter contribution from the Pearl River into deeper-water depocenters is limited, as sediment is dispersed along the continental shelf by alongshore and shallow- to intermediate-water currents (Liu et al., 2010b, 2016; Wan et al., 2007a). During transport and sedimentation, degradation does not appear to significantly alter the isotopic composition of organic matter, since there is little fractionation between reactants and products. If post-depositional alteration were a dominant control, δ13C_org values should become progressively less negative with depth, as lighter isotopes are preferentially removed. However, the δ13C_org records from the two sites show distinct trends, suggesting that the influence of post-depositional isotopic fractionation is insignificant.

Taiwan's rapid denudation delivers large quantities of sediment and nutrients to the northern SCS, profoundly shaping basin productivity and carbon cycling. The export of bioessential nutrients stimulates intense coastal primary production, as reflected by modern chlorophyll-a and nitrogen distributions that peak along Taiwan's coast before rapidly declining offshore due to swift uptake (Kao et al., 2006; Ge et al., 2020; Huang et al., 2020). Episodic inputs from tropical cyclones, which contribute up to 80 % of summer particulate organic carbon, further amplify productivity and promote lateral dispersal of sediments (Liu et al., 2013). Marine organic matter produced through enhanced coastal productivity could be redistributed by deep-water contour currents and mesoscale eddies (Lüdmann et al., 2005; Zhang et al., 2014; Zhao et al., 2015; Hsieh et al., 2024), enabling its bypass into the deeper water depths and resulting in the marine signature of the δ13C_org records from the northern SCS.

Fluvial input from Taiwan, especially via submarine canyon systems, makes the northern SCS a depocenter for organic carbon burial, with important implications for the basin's sedimentary architecture, long-term carbon budget, and even hydrocarbon source rock potential (Kao et al., 2006). Paleoceanographic records indicate that productivity and organic carbon burial increased during glacial periods (Thunell et al., 1992), likely driven by nutrient delivery from Taiwan's sediments that enhanced the biological pump and contributed to regional carbon drawdown. In the modern setting, episodic sediment fluxes during typhoons sustain unusually high chlorophyll-a concentrations in deep SCS waters relative to the global ocean (Shih et al., 2019). Moreover, northeast monsoon-driven mixing between the China Coastal Current and Taiwan Strait Current, reinforced by sediment and nutrient inputs dominantly from Taiwan and the Yangtze River, sustains elevated productivity in the northern SCS (Huang et al., 2020). Collectively, these processes highlight Taiwan's sediment flux as a key linkage between monsoon forcing, nutrient cycling, and primary production across both modern and in the past.

In the TWFB, carbon geochemistry and gamma-ray data largely reflect the evolution of the foreland basin synchronously with the shifts in the regional climate regime (Fig. 3). During the deposition of the Chinshui Shale in the late Pliocene (∼ 3.2 to 2.5 Ma), reconstructions for the northwest Pacific show relatively high global sea levels and stable sea-surface temperatures (Li et al., 2011; Berends et al., 2021). Such conditions favoured the accumulation of fine-grained sediment, while elevated sea levels deepened the TWFB and promoted offshore depositional environments-both of which are expressed in the Chinshui Shale (e.g., Nagel et al., 2013; Vaucher et al., 2023b). Greater water depths and increased distance from the terrestrial sediment sources also enhanced the relative contribution of marine organic matter. The gamma-ray record of the TWFB strata further reveals depositional cycles related to interactions between EASM and tropical cyclone precipitation after ∼ 4.92 Ma, with variability expressed at both short-eccentricity and precession frequency bands (Vaucher et al., 2023b; Hsieh et al., 2023a).

During the early Pleistocene, with deposition of the Cholan Formation (∼ 2.5–1.95 Ma), global sea level and regional sea-surface temperatures became markedly more variable (Li et al., 2011; Berends et al., 2021). The continued uplift and southwest migration of Taiwan promoted the development of shallow-marine depositional environments recorded in the Cholan Formation (e.g., Pan et al., 2015; Vaucher et al., 2021, 2023a, b). This is expressed in the gamma-ray and carbon records as an increase in terrestrially sourced, sandstone-rich intervals with high variability (Fig. 3). The enhanced export of coarser-grained sediment from land to sea is likely related to the onset of NHG, when repeated sea-level minima promoted clastic delivery to the basin (Vaucher et al., 2021, 2023b).

In addition, global climate deterioration related to NHG intensified and destabilised the EASM (Wan et al., 2006, 2007a). Paleoclimate reconstructions from East Asia document a strengthening of the EASM during the late Pliocene, generally near ∼ 3.5 Ma (Zhang et al., 2009; Yang et al., 2018; Yan et al., 2018; Xin et al., 2020; Nie et al., 2014; Hoang et al., 2010). While the causal relationship between monsoon intensity and NHG remains debated (Zhang et al., 2009; Xin et al., 2020; Wan et al., 2010b; Nie et al., 2014), long-term global cooling and sea-level fall coupled with monsoon and tropical cyclone precipitation likely acted together to amplify sediment export from land to sea (Vaucher et al., 2023b). In the northern SCS, MAR and TOC values and amplitudes at both ODP sites increased after ∼ 3 Ma, consistent with increased sediment export (Figs. 4, 5). At the same time, δ13C_org values at ODP Site 1148 increases after ∼ 3 Ma, suggesting increasing marine contribution to organic carbon. This trend is attributed to enhanced marine primary production driven by nutrient enrichment. Independent evidence for increased marine primary productivity in this interval comes from elevated abundances of planktonic foraminifera Neogloboquadrina dutertrei and higher biogenic silica production (Wang et al., 2005a).