Biogeochemical evidence of the Oligocene and late Miocene–Pleistocene climatic variability from two deep sediment cores of the South China Sea

Abstract

Organic geochemical and stable isotope records of Oligocene and late Miocene–Pliocene sediments from IODP hole U1501C and Pliocene–Pleistocene sediments from U1499A of South China Sea (SCS) were studied to investigate clock sources of organic matter and carbonates and their spatiotemporal variations with East Asian climatic variability. Geochemical data was constrained using shipboard information. CaCO₃ and total organic carbon contents (wt%) varied between 1.32 to 56.52 and 0.12 to 1.13, respectively. δ¹³C_carb, δ¹⁸O_carb and δ¹³C_org ranged from −4.89 to 1.98‰, −5.54 to 1.96‰, and −24.66 to −28.13‰, respectively. Contributions from mixed sources of carbon were observed in the Oligocene, while the late Miocene–Pleistocene exhibited terrestrial dominance. Early Oligocene carbonate, low and higher TOC are attributed to the opening of SCS, increased terrigenous input, and prevalence of cooler climate. Stable isotopes suggest the expansion of the marine environment and the probable dawning of the East Asian Winter Monsoon (EAWM) during the Oligocene. The late Oligocene marked a transition to a warmer climate. The strengthening of EAWM since the late Miocene is indicated by moderate organic carbon and high carbonates with enriched isotopes. Glacial low sea levels and higher terrestrial inputs increased TOC, while dissolution affected late Plio–early Pleistocene carbonates. Higher carbonates and productivity since the mid-Pleistocene were influenced by alternate weakening and strengthening of EAWM.

Research highlights

• Organic matter, carbonates, C and O isotopes from Oligocene and late Miocene–Pleistocene sediments, northern SCS.

• Productivity in the South China Sea fluctuated, being low during the Oligocene, with a subsequent increase since the late Miocene.

• δ¹³C_org and δ¹³C_carb indicate cold climatic conditions and probable winter monsoon signatures since the early Oligocene, transitioning to warmer conditions during the late Oligocene.

• During the late Miocene (~8–5.6 Ma), deep-water circulation and intensified winter monsoons led to higher productivity.

• Pliocene sediments (since ~5.6 Ma) showed signs of climatic cooling, sea level fluctuations, and enhanced winter monsoons with carbonate dilution.

• The Plio-Pleistocene period witnessed glacial and interglacial cycles reflecting changing monsoon intensities.