Deep Hydrography of the South China Sea and Deep Water Circulation in the Pacific Since the Last Glacial Maximum

Abstract

The oxygen (δ18O) and carbon (δ13C) isotopic compositions of benthic foraminifer Cibicidoides wuellerstorfi as reliable proxies of deep water properties were compiled from 50 core‐top and down‐core sites in the South China Sea (SCS) for two time slices of the late Holocene and Last Glacial Maximum (LGM) to reconstruct the glacial deep hydrographic structure. The bathymetric profiles of both δ18O and δ13C in the SCS show similar trends between the LGM and the late Holocene, but the δ18O gradients between intermediate (∼500–1,000 m) and deep (>1,500 m) waters obviously increased during the LGM, indicating an intensified bathyal stratification and weakened vertical mixing between intermediate and deep waters in the SCS during the glacial period. A spatial comparison of bathymetric δ18O and δ13C profiles was also made for the Southern Ocean (Indo‐Pacific sector) and southwest Pacific, western equatorial Pacific (i.e., Ontong Java Plateau), off‐Japan margin as well as the SCS. The meridional δ13C profiles indicate a decreasing trend of δ13C (“aging effect”) in the deep layer (∼1,200–2,600 m) from south to north, reflecting a northward flow pathway of deep waters from the southwest Pacific (upstream) to the low‐latitude northwest Pacific and the SCS (downstream) during the LGM. The gradients of deep water δ18O (∼1,200–2,600 m) between these upstream and downstream regions were higher during the LGM relative to the Holocene, implying that a greater contribution of the northern‐sourced North Pacific Intermediate Water (NPIW) with a relatively low δ18O signature to the deep waters in the low‐latitude Pacific during the LGM.

Changes in Deep Water Oxygenation of the South China Sea Since the Last Glacial Period

“…The inference on the source of the SCSDW based on redox‐sensitive elements in the study is supported by recent circulation records from ventilation ages (Wan & Jian, ), benthic foraminiferal δ 13 C (Wan et al, ) and deep water ε Nd (Wu et al, ) in the SCS. The 14 C age difference between planktonic and benthic foraminifers (B‐P) from sediment cores in southern deep SCS (MD05–2896 and VM 35–6) mostly falls within the range of the age‐difference from two western Tropic Pacific records (MD98‐2181 and MD01‐2386) at 2,100–2,800 m water depths (Broecker et al, ; Stott et al, ) (Figure ).…”

“…The synchronous change in oxygenation of entire deep waters cannot be simply explained by the shift of the low-oxygen SCSIW (Löwemark et al, 2009). In the SCS, the bathymetric profile of C. wuellerstorfi δ 13 C from the late Holocene has a similar trend with the distribution of δ 13 C of seawater ∑CO 2 (Lin, 2003;Wan et al, 2018). It generally supports the point that the C. wuellerstorfi δ 13 C serves as a reliable proxy for dissolved inorganic carbon (DIC) 13 C/ 12 C (Duplessy et al, 1988).…”

“…In the SCS, the bathymetric profile of C . wuellerstorfi δ 13 C from the late Holocene has a similar trend with the distribution of δ 13 C of seawater ∑CO 2 (Lin, ; Wan et al, ). It generally supports the point that the C .…”

“…wuellerstorfi δ 13 C serves as a reliable proxy for dissolved inorganic carbon (DIC) 13 C/ 12 C (Duplessy et al, ). Recently compiled benthic foraminiferal δ 13 C profiles in the SCS by Wan et al () show similar hydrographic structures between the LGM and late Holocene and a small vertical expansion of intermediate waters during the last glacial period. The C .…”

Nitrogen Fixation Changes Regulated by Upper Water Structure in the South China Sea During the Last Two Glacial Cycles

“…Enhanced mixing across the SCS probably further relates to deeper circulation. The SCS deep waters ventilate faster (Wan & Jian, ) with reduced vertical density stratification (Wan et al, ) in the Holocene in comparison with the last glacial period. Accordingly, the glacial‐cyclic consistent variations in the δ 15 N records and the vertical temperature gradient over the last 250 ka can be explained by the physical‐biological coupling between N fixation rates and upper water structure in the SCS.…”