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The Ross Sea is located at the southwestern boundary of the Antarctic continent within the Pacific sector of the Southern Ocean, and is the second largest bay in this region, covering an area of approximately 750,000 km² (Smith et al., 2012). It is bordered by Marie Byrd Land to the east, the Transantarctic Mountains and Victoria Land to the west, and to the south, it adjoins the Ross Ice Shelf, which is the largest ice shelf in the world, spaning about 500,000 km². The Ross Sea is a vital source region for Antarctic Bottom Water (Whitworth and Orsi, 2006). Sea ice distribution in the Ross Sea displays significant seasonal fluctuations, with polynyas like the Ross Sea Polynya (RSP), McMurdo Sound Polynya (MSP), and Terra Nova Bay Polynya (TNBP) forming earlier in the summer along the front of the Ross Ice Shelf and the coast of Victoria Land, primarily due to katabatic wind (Parish et al., 2006; Tamura et al., 2008). The Ross Sea boasts high primary productivity, contributing approximately one-third of the Southern Ocean’s primary productivity (Arrigo et al., 2008a, 2008b), and hosts a thriving ecosystem. Notably, the Adélie penguin population in the Ross Sea represents one-third of the global population (Xu et al., 2021).

The Ross Sea continental shelf is geographically divided into Eastern and Western regions by approximately 180° longitude, with water depths ranging from less than 500 m to over 1000 m. The eastern shelf is predominantly characterized by a broad basin and gently undulating shoals, while the western shelf features narrow basins and more pronounced undulating shoals (Halberstadt et al., 2016; Gales et al., 2021; Ha et al., 2022). TOC in the surface sediments of the Ross Sea primarily originates from upper ocean primary production (DeMaster et al., 1996; Langone et al., 1998) based on biomarker compounds (Song et al., 2019), OC/TN ratio and δ¹³C (Xiu et al., 2017). The southwestern Ross Sea, including RSP and MSP, exhibits high content of biogenic components (OC, TN, and opal) and represents a distinct environment setting compared to the western Ross Sea (Zhou et al., 2022). Based on the differences in the geographic location and biogenic components, our sediment samples were categorized into three distinct groups: Southwest, Western and Eastern ( Figure 1 , Supplementary Table S1 ).

Eighteen surface sediments were collected during the 31^st and 32^nd Chinese National Antarctic Research Expeditions (CHINARE-31 and -32) ( Figure 1 ). They were collected by a box sampler, and the 0 – 1 cm sediment layer was taken as the surface sample. A gravity core sediment sample at station JB03 (170.698°E, 75.804°S, water depth = 615 m) within the southern Joides Trough were collected during CHINARE-31. The total length of the sediment core was 130 cm, and was divided at 2 cm intervals. All samples were frozen at –20°C, brought back to the laboratory and kept frozen at –20°C until analysis.

The particle size of sediment samples was determined using a laser particle size analyzer (range 0.02 to 4000 μm, Mastersizer 3000, UK, the precision is better than 1%). Ca. 1.0 to 2.0 g untreated wet sediment samples was placed into beakers with a small amount of deionized water to soak the samples. A small amount of 30% hydrogen peroxide (H₂O₂) was added until no bubbles were produced to remove sedimentary organic matter. 0.25 mol/L of dilute HCl was added to remove carbonates in the sample, followed by 20 mL of a 1 mol/L NaCO₃ solution, and the beaker was placed into a constant temperature water bath at 85°C for 4 h. Deionized water was added and the solution was stirred with a glass rod, left to stand for 24 h, and the supernatant collected. This step was repeated for 3 times, until the washing produced a neutral pH solution. 1 mL of 0.5 mol/L sodium hexametaphosphate was added to the combined supernatant as a dispersive substance, and a suspension was formed by ultrasonic oscillation for 120 s. The suspension was collected and particle sizes were measured on the instrument.

Sediment samples were separated into 3 standard size fractions: sand (> 63 μm), silt (4 – 63 μm), and clay (< 4 μm). Based on the φ values ( φ 5 ,  φ 16 , φ 25 , φ 50 , φ 75 , φ 84 , φ 95 ) corresponding to the 5%, 16%, 25%, 50%, 75%, 84%, and 95% points on the cumulative probability curve, the mean particle size (M_Z), sorting coefficient (σ), skewness (SK) and kurtosis (K_G) were calculated to characterize the sedimentary environment (Folk and Ward, 1957). Generally, Mz is constrained by a value of 4 φ, with values greater than 4 φ indicating a low-energy sedimentary environment, while values less than 4 φ suggesting a high-energy environment. A smaller σ value denotes better sediment sorting. SK is used to measure the symmetry of particle size frequency distribution. Furthermore, a smaller K_G value reflects a broader distribution in the sharpness of the grain size frequency curve (see Supplementary Text S1 for indicative meanings of the above parameters).

TOC and TN contents of the samples were determined by an elemental analyzer (Elementar Vario MICRO cube, Germany). TOC determination requires acid removal of inorganic carbon, while TN does not (Schubert and Nielsen, 2000). Briefly, 0.5 g of lyophilized and ground sediment samples were weighed into 15 mL glass test tubes, 10 mL of a 1 M HCl solution was added and stirred, and the tubes were placed in a constant temperature water bath at 50°C for 48 h (Faust et al., 2021). The samples were then centrifuged (2500 r/min, 5 min) and the supernatant poured into a new glass container. The samples were rinsed with distilled water until a neutral pH was achieved, and finally the samples were lyophilized in a freeze-dryer and weighed. The decarbonated samples were accurately weighed (30.0 ± 0.2 mg) into tin capsules for analysis. The sediment standard sample GB07314 (offshore marine sediments, The State Bureau of Quality and Technical Supervision of China) was analyzed in parallel as a quality control standard, with replicate measurements varying by less than 1%.

Δ¹⁴C of TOC from 11 sub-samples (i.e., 0 – 2, 2 – 4, 18 – 20, 54 – 56, 68 – 70, 72 – 74, 78 – 80, 102 – 104, 110 – 112, 112 – 114, and 128 – 130 cm) of the core sediment were analyzed using a 250 KeV NEC single stage particle accelerator at Beta Analysis Laboratory in the USA. Results are ISO/IEC-17025:2017 accredited. Briefly, the samples were subjected to ultrasonic mixing and screening to remove impurities. Inorganic carbon was eliminated through pickling, and graphite targets were prepared for testing in an accelerator mass spectrometer. The standard was NIST SRM-4990C (oxalic acid). The analytical precision for Δ¹⁴C measurements is typically < 5‰.

Ca. 3.0 to 6.0 g of lyophilized and ground sediment samples were accurately weighed and loaded into the extraction cell of a rapid solvent extractor (ASE-350, USA), and hexadecane deuterium (nC₂₄D₅₀) was added as an internal standard. Organic matter extracts were obtained by introducing dichloromethane and methanol (9:1, v:v) to the cell and heating at 100°C for 5 min, followed by extraction for 10 min. The heating and extraction were repeated 3 times. The extracts were initially concentrated by rotary evaporation, followed by evaporation under a steady stream of N₂. When the volumes were < 10 mL, the extracts were transferred to 10 mL glass bottles and evaporated with N₂ until dry. Next, a 6% potassium hydroxide in methanol solution was added and ultrasonicated for 10 min. The hydrolysate was removed by adding 4 mL of hexane and waiting for the polar and non-polar layers to separate. The non-polar layer was removed and transferred to a 20 mL glass bottle. The extraction was repeated 4 times, with all extracts combined in the new container. 0.2 mL of the combined organic matter extract was sub-sampled and dried under N₂, re-constituted in a known quantity of hexane, and then separated on an activated silica gel column. The non-polar components were obtained by leaching with n-hexane.

An Agilent gas chromatograph (Agilent 6890N, USA) with a flame ionization detector (GC-FID) was used for n-alkane analysis. The GC was equipped with an HP-1 (dimethylpolysiloxane) column (50 m × 0.32 mm × 0.17 μm). Analytical conditions were as follows: inlet temperature 310°C, FID detector temperature 320°C, carrier gas (N₂) flow rate 1.2 mL/min. The GC oven initial temperature was 60°C, which was held for 1 min before increasing to 200°C at a rate of 10°C/min. The temperature was then increased to 300°C at a rate of 5°C/min, and then to 310°C at a rate of 5°C/min, where it was held for 15 min. An example n-alkane chromatogram with relative abundance (station RB08B) was shown in Supplementary Figure S1 . The retention time of target compounds was determined by comparing the retention time of 34 n-alkanes in a mixed standard (nC₇ to nC₄₀). The relative response values of each component peak of the mixed standard and the nC₂₄D₅₀ internal standard peak were applied to the peak areas of the target compounds to quantify their abundance. Yields of n-alkanes were normalized to the mass of sediment extracted, and values are expressed as ng/g.

L/H, TAR (Meyers, 1997), CPI_H and CPI_L (Eglinton and Calvin, 1967; Cranwell et al., 1987) were calculated based on the content of n-alkanes of different carbon numbers (see Supplementary Text S2 for indicative meanings of the above parameters):

A Pearson correlation analysis and a two-tailed test of significance were performed using the statistical software SPSS (Version 25) to determine relationships between the measured parameters. Statistically, significant differences were identified using one-way analysis of variance with a 95% confidence interval (p < 0.05).

A reliable chronostratigraphic framework based on TOC should consider marine reservoir effects (Andrews et al., 1999; Pudsey et al., 2006) and fossil carbon contamination from glacial erosion on the Antarctic continent (Hillenbrand et al., 2010). A marine reservoir age of 825 a was referenced from Anderson et al. (2014) and Huang et al. (2016). A fossil carbon contamination age of 3045 a was referenced from station ATN31-JB06 (173.907°E, 74.473°S, water depth = 567 m) (Huang et al., 2016; Fan et al., 2021) due to the absent of foraminifera in JB03. Calendar ages were determined using the 7.10 software and Marine 13 program (http://calib.org/calib/calib.html). Linear interpolation and extrapolation were employed to establish a chronological framework, resulting in an age of 27.3 ka BP at the bottom of core JB03. The different depth intervals (0 – 72, 72 – 78 and 78 – 130 cm) corresponded to the Holocene (11.7 – 0 ka BP), the last deglaciation (21.0 – 11.7 ka BP) and the LGM (27.3 – 21.0 ka BP), respectively ( Table 1 ; Figure 2 ). Sedimentation rates for each layer were calculated using 11 age control points and the interpolation method. The sedimentation rate for the interval 0 – 72 cm ranged from 6.31 to 7.35 cm/ka. The age difference between 78 – 80 cm and 72 – 74 cm layers was as high as 13.2 ka, and the lowest sedimentation rate for the whole core was calculated to be 0.45 cm/ka. Layers between 102 – 130 cm exhibited extremely high sedimentation rate, up to > 100 cm/ka ( Table 1 ; Figure 2 ).

The composition of sand, silt and clay in surface sediments of the Ross Sea varied from 4.03% to 56.7%, 37.1% to 78.3% and 6.16% to 35.4%, respectively ( Supplementary Table S1 ; Figures 3A–C ). Spatially, the proportions of sand and silt did not exhibit significant (p > 0.05) differences among the three regions, while clay content in the Eastern sediment (27.0% ± 4.74%) was significantly (p < 0.01) higher than that in the Southwest and Western sediments (11.8% ± 0.25% and 13.0% ± 5.34%). Following Folk (1980) sediment classification method, the predominant sediment type in the study area was clayey silt, mainly located in Cape Adare and Glomar Challenger Trough. This was followed by sandy silt, distributed in Joides Trough and Drygalski Trough. Silty sand sediment was present at stations R02 and R08 near Terra Nova Bay, while silt sediment was at stations JB01 and JB03 in McMurdo Sound and Joides Trough ( Supplementary Figure S2 ). Hydrodynamic indicators, including M_Z, σ, SK, and K_G, ranged from 4.17 to 7.00 φ, 1.63 to 2.44, –0.17 to 0.37 and 0.82 to 1.04, respectively ( Supplementary Table S1 ; Figures 3D–G ). The M_Z in the Eastern sediment (6.75 ± 0.21 φ) was significantly (p < 0.01) higher than that in the Southwest and Western sediments (5.70 ± 0.24 φ and 5.42 ± 0.68 φ), while σ, SK and K_G did not show significant (p > 0.05) differences among the three regions.

The TOC content of surface sediment samples ranged from 0.40% to 1.34% (wt%), which is consistent with the ranges of previous studies (0.2% to 2%) (Langone et al., 1998; Andrews et al., 1999; Chen et al., 2019b). Spatially, TOC content in the Southwest sediment (1.08% ± 0.23%) was significantly (p < 0.05) higher than that in the Eastern sediments (0.62% ± 0.21%), and intermediate in the Western sediments (0.82% ± 0.24%) ( Supplementary Table S1 ; Figure 3H ). TN content ranged from 0.07% to 0.23%, showing a similar spatial distribution pattern to TOC content ( Figure 3I ). A significant positive correlation was observed between TOC and TN content (r = 0.99, p < 0.001, Figure 4A ). The TOC/TN ratio varied from 5.57 to 7.00, with no significant differences observed among the three regions (6.62 ± 0.19, 6.35 ± 0.28 and 6.28 ± 0.49 in the Southwest, Western and Eastern Ross Sea sediments, respectively) ( Supplementary Table S1 ; Figure 3J ).

In the JB03 core sediments, TOC content ranged from 0.43% to 1.85%, while TN content ranged from 0.03% to 0.23%. Similar to the surface sediments, a significant positive correlation between TOC and TN was noted in the core sediments (r = 0.97, p < 0.001, Figure 4B ). The TOC/TN ratio ranged from 6.91 to 16.81 ( Supplementary Table S2 ). In the downcore profiles, TOC decreased from 1.56% ± 0.30% during the period of 0.6 – 8.2 ka BP to 0.74% ± 0.12% during 21.0 – 27.3 ka BP, while the TOC/TN ratio increased from 8.18 ± 0.51 to 13.63 ± 1.29 ( Figure 5 ).

The concentration of n-alkanes in the surface sediments of the Ross Sea varied from 482 to 2221 ng/g, with higher values found in the Glomar Challenger Trough and Joides Trough (stations RB02B and JB05), and the lowest concentration found near Cape Adare (station R19) ( Figure 3K ). The carbon chain lengths of n-alkanes ranged from nC₁₂ to nC₃₅, characterized by a unimodal distribution with main carbon peak at nC₁₇ or nC₁₉ in Western Ross Sea ( Figure 6A ), a bimodal distribution with main carbon peaks at nC₁₉ and nC₂₅ in the Southwest Ross Sea ( Figure 6B ), and a combination of unimodal and bimodal distributions with main carbon peaks at nC₁₇ or nC₁₉ and nC₂₇ in the Eastern Ross Sea ( Figure 6C ). The L/H ranged from 0.38 to 6.72, being significantly (p < 0.05) higher of Western sediments (3.60 ± 2.18) than those in Southwest and Eastern sediments (0.62 ± 0.21 and 1.60 ± 0.58) ( Figure 3L ). The TAR varied from 0.07 to 2.03, significantly (p < 0.01) higher in the Southwest sediments (1.26 ± 0.66) than in the Western and Eastern sediments (0.22 ± 0.14 and 0.61 ± 0.30) ( Figure 3M ). N-alkanes exhibited odd carbon dominance (CPI_L = 1.26 to 1.68), except for station R02, which displayed an even carbon distribution (CPI_L = 0.76) ( Figure 3N ). The CPI_H ranged from 1.01 to 2.43, being highest in the Eastern sediments (2.04 ± 0.28), lowest in the Southwest sediments (1.18 ± 0.16), and intermediate in the Western sediments (1.52 ± 0.19) ( Figure 3O ; Supplementary Table S1 ).

In core JB03, the n-alkane content ranged from 416 to 1577 ng/g, exhibiting a slight increasing trend downcore, which contrasts with TOC profile ( Figure 5 ). The carbon chain lengths extended from nC₁₃ to nC₃₃, displaying a unimodal distribution with nC₁₇, nC₁₈ or nC₁₉ as main carbon peaks during 0.6 – 8.2 ka BP ( Figure 6D ), bimodal distribution with main carbon peaks as nC₁₈/nC₂₅ and nC₁₉/nC₂₅ during 8.2 – 21.0 ka BP ( Figure 6E ), and bimodal distribution with main carbon peaks of nC₁₇ or nC₁₉ and nC₂₇ during 21.0 – 27.3 ka BP ( Figure 6F ). The source indices, including the L/H, TAR, CPI_L and CPI_H, ranged from 0.53 to 5.50, 0.14 to 1.50, 0.92 to 1.14 and 1.29 to 2.07, respectively ( Supplementary Table S2 ). Downcore analysis revealed that the L/H decreased from 2.27 ± 1.07 during 0.6 – 8.2 ka BP to 1.17 ± 0.37 during 21.0 – 27.3 ka BP. Conversely, TAR increased from 0.41 ± 0.32 to 0.83 ± 0.31, while CPI_L and CPI_H did not show significant downcore trends ( Figure 5 ).