The geochemical proxy of ocean pH is crucial for understanding the dynamics of ancient ocean carbon reservoirs. This study employs the cyclization index of branched glycerol dialkyl glycerol tetraether compounds (brGDGTs), widely used in terrestrial environments for paleo-pH reconstruction, to investigate the intermediate water pH in the South China Sea. The South China Sea, a semi-enclosed marginal sea in the western Pacific, serves as the study area. By collecting data including global soil datasets, subtropical soil data, samples with water depths less than 50 meters in the northern South China Sea, surface sediment samples in the South China Sea, and brGDGTs data from rock cores in the northern South China Sea, significant differences were found in the composition of brGDGTs in marine sediments compared to those in soils. This proves the in-situ self generation viewpoint of brGDGT in the ocean, mainly generated in the middle water column, providing a basis for reconstructing ocean acidification. The research materials for establishing a global formula include deep-sea sediment samples from the South China Sea, Western Pacific, Southeast Pacific, Northeast Atlantic, and Southwest Atlantic, all of which were collected at depths of over 300 meters to avoid terrestrial influence. Lipids were extracted using a modified Bligh-Dyer method and analyzed using high-performance liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry(HPLC-APCI-MS). A positive correlation between brGDGTs indices (CBT’ and #Ringstetra)and seawater pH was established, allowing for the development of empirical formulas for reconstructing ancient midwater seawater pH. Utilizing published brGDGTs data from Holocene sediments in the northern South China Sea, the reconstructed pH values indicate a maximum of approximately 7.89 around 6.5 ka and a minimum of about 7.72 around 1.5 ka. The study validates that brGDGTs in deep-sea sediments are of marine origin. This research demonstrates that brGDGTs in marine sediments are autochthonous and can be employed to reconstruct intermediate water pH, providing significant insights into ocean acidification history.
Yang Q., Hu L., Jia G. & Wang M., 2025. Quantitative reconstruction of intermediate water pH in the South China Sea based on branched tetraether lipids. Quaternary Sciences 45(2): 535-545. Article.
