Organic proxy data of the Sandaohezi peat, Altai Mountains

1 Samples collection and ageing Sandaohaizi is located at the watershed of the Altai Mountains in Qinghe County, Xinjiang covering a total area of 596.16 km2. Field surveys indicate that extensive peatlands are developed in the Zhonghaizi within Sandaohaizi Peat. The peat mounds in this area are sparse, scattered, low and irregular in shape, with an average height of 2.0 m and diameters ranging from 5 to 7 m, exhibiting parallel strip-like patterns (Zhang et al., 2018). The in situ vegetation is dominated by Carex and Sphagnum, while the surrounding vegetation includes Festuca, Poaceae, Stellaria and Myosotis. The groundwater depth is ~5 cm, with atmospheric precipitation and snowmelt serving as the primary recharge sources. A peat core (46°53′N, 90°50′E, 2342 m a.s.l.) with a length of 83 cm was retrieved and sectioned in the field at 1-cm interval in 2013. Plant residues from seven horizons were selected for AMS 14C dating. The chronological data were calibrated using the IntCal13 database, and a depth-age model for the Sandaohaizi core was established using the Bayesian age-depth modeling via the Bacon software.2 n-Alkane proxy analysis  Sample aliquots were freeze-dried and homogenized to <120 mesh particle size. Sequential extraction was performed using an Accelerated Solvent Extraction (ASE) system with chloroform (5×1 g aliquots). The combined extracts were concentrated via rotary evaporation and subsequently dried under nitrogen purge. The residue was reconstituted in n-hexane prior to fractionation. Saturated hydrocarbon fractions were isolated through silica gel column chromatography (60-100 mesh) using 20 mL n-hexane as eluent. Purified fractions were concentrated under N2 stream and transferred to GC vials for subsequent analysis. Compound identification and quantification were conducted using a Shimadzu GCMS-QP5050A system equipped with a DB-5MS capillary column (30 m×0.25 mm i.d.×0.25 μm film thickness). Operational parameters included: 250 °C ion source temperature, 70 eV electron impact ionization, and helium carrier gas at 1.2 mL/min constant flow. The temperature program initiated at 80 °C (held for 1 min), ramped to 175°C at 3 °C/min, followed by a secondary ramp to 300 °C at 4°C/min (final hold 20 min) to establish protocols (Zhang et al., 2024, 2026). Select n-alkane individual standards (Sigma-Aldrich) with a purity of ≥ 99% were used to ensure the calibration interval includes the expected concentration of the target component in the samples. Verification was performed using a mid-concentration calibration point (e.g., 10 μg/mL) every 8 hours or before testing each batch of samples. 3. GDGTs proxy analysis GDGTs were analyzed using a high-performance liquid chromatography atmospheric pressure chemical ionization-mass spectrometer (HPLC–MS; Agilent 1200 series HPLC, 6460A triple quadrupole MS). An aliquot of a sample (10–30 μl) was injected, and separation was achieved with an Alltech Prevail Cyano column (150 mm×2.1 mm, 3 μm; Grace, Deerfield, IL, USA). The elution gradient suggested by Schouten et al. (2007) was used. GDGTs were detected with selected ion monitoring (SIM) mode, scanning from m/z 950–1450 on the Agilent 1200 HPLC–MS at m/z 1302, 1300, 1298, 1296, 1292, 1050, 1048, 1046, 1036, 1034, 1032, 1022, 1020, 1018, targeting m/z 744 for the internal standard. Relative abundances were determined from the peak area integration of [M+H]+ in the extracted ion chromatograms. Except where otherwise noted, the fractional abundance of an individual GDGT is its proportion within total isoGDGTs or brGDGTs. The final concentration can only be considered as semi-quantitative because the response factors for GDGTs vs. the C46 GTGT standard were not determined.