Long-term cycles in the carbon reservoir of the Quaternary ocean: a perspective from the South China Sea

In recent years, long-term, high-resolution records from the deep sea and ice-cores have offered new research opportunities for Quaternary science. Paleoclimate studies are no longer restricted to individual glacial cycles, but extend to long-term (≥105 yr) processes across those cycles. Ocean Drilling Program Leg 184 of the South China Sea in 1999 uncovered well-preserved sediment sections, in which three long-term cycles in Pleistocene carbon isotope (δ13C) sequence have been found and demonstrated to be common in the global ocean. Subsequent discoveries confirm the existence of long-term processes of 105 yr in both the hydrologic (ice-sheet changes) and carbon (biogeochemical changes) cycles, posing the question whether the two processes are related. The present review shows that the long-eccentricity cycles prevail throughout the δ13C and other biogeochemical records in geologic history, and 400-kyr cycles in the oceanic δ13C sequence before the Quaternary can be hypothetically explained by changes in ratio between particulate and dissolved organic carbon (POC/DOC) in the ocean, depending on the monsoon-controlled nutrient supply. This is a ‘DOC hypothesis’. However, ocean restructuring at 1.6 Ma marked by the isolation of a sluggish abyss under the Southern Ocean has obscured the long-eccentricity 400-kyr signal in oceanic δ13C. The last million-year period has experienced two major changes in the climate regime, namely the mid-Pleistocene transition (MPT) centered at 0.9 Ma and the mid-Brunhes event (MBE) around 0.4 Ma. The MPT and MBE were preluded by δ13C maxima-III (δ13Cmax-III) ∼ 1.0 Ma and δ13Cmax-II ∼ 0.5 Ma, respectively. Together with similar hydroclimatic phenomena over corresponding glacial cycles, the two groups of hydrologic and biogeochemical events appear to have been driven largely by oceanographic changes in the Southern Ocean. Therefore, we interpret that the long-term biogeochemical processes originating from the Southern Ocean must have played a crucial role in Quaternary ice-sheet waxing and waning.