Neodymium isotopic composition in foraminifera and authigenic phases of the South China Sea sediments: Implications for the hydrology of the North Pacific Ocean over the past 25 kyr

Abstract

International audienceepsilon Nd and normalized Rare Earth Elements (REE) patterns of benthic and planktonic foraminifera and Fe-Mn coatings precipitated on sediments have been investigated for the South China Sea (SCS) to (1) assess the reliability of the extraction of past seawater epsilon Nd in the SCS and to (2) reconstruct past hydrological changes during the last 25 kyr. Reductively cleaned mono-specific planktonic foraminifera (Globigerinoides ruber) and mixed benthic foraminifera in core-top sediments from 1500 to 2400 m display similar epsilon Nd values to those of the modern Pacific Deep Water (PDW) (epsilon Nd of -3.9 to -4.4). Furthermore, the epsilon Nd of the reductive cleaning solutions shows similar epsilon Nd values to ones obtained on cleaned foraminifera. Combined with PAAS-normalized REE patterns, these results confirm that the oxidative and reductive cleaning procedure applied to foraminifera does not totally remove all of the Fe-Mn coatings and that epsilon Nd values yielded by cleaned planktonic foraminifera retain the epsilon Nd imprint of the bottom and/or pore water. epsilon Nd values obtained from a leaching procedure carried out on the bulk non-decarbonated sediments are comparable to the epsilon Nd values of the modern PDW, whereas a similar leaching procedure applied to decarbonated sediments reveals a bias due to contamination with Nd deriving from lithogenic particles. In core MD052904, seawater epsilon Nd, reconstructed from planktonic foraminifera, indicates that the last glacial period is characterized by lower epsilon Nd (-5.2 +/- 0.2 to -6.4 +/- 0.3) than the late Holocene (-4.1 +/- 0.2). Assuming that Nd input from river does not change strongly the epsilon Nd of the PDW of the northern SCS, these epsilon Nd variations suggest a higher relative proportions of southern-sourced water in the deep water of the western subtropical Pacific Ocean during the last glacial period

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

“…The recently compiled C. wuellerstorfi δ 13 C profiles also support no changes in deep water sources in the SCS during the LGM, with the evidence of a decreasing δ 13 C trend from the Southern Ocean to the SCS and sub‐Arctic Pacific (Wan et al, ; this study). The SCSDW circulation records in deep SCS (Andree et al, ; Wan & Jian, ; Wan et al, ; Wu et al, ; this study) match with its oxygenation from redox‐sensitive elemental data in this study. During the LGM, the poorly ventilated PDW intruded into the deep SCS and conditioned the accumulation of authigenic U in sea‐bottom sediments.…”

“…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 ).…”

Deepwater circulation variation in the South China Sea since the Last Glacial Maximum

“…However, MD02‐2489, in the Gulf of Alaska, displays high ventilation rate at 3600 m water depth during H1 [ Rae et al ., ], consistent with our paleomagnetic proxies. Nd isotope and ventilation age evidences in cores MD05‐2903/2904 and SO17940‐2 from the NE SCS also confirm the penetration of deep water into the SCS during the early deglaciation period [ Wan and Jian , ; Wu et al ., ]. The rapid deep current strengthening occurring during H1 in the SCS could thus be associated with the abrupt increase in NPDW on the premise of the terminology of Talley et al .…”

Hydrological variations of the intermediate water masses of the western Mediterranean Sea during the past 20 ka inferred from neodymium isotopic composition in foraminifera and cold-water corals

“…The cleaned samples were dissolved in 1N acetic acid and finally centrifuged to ensure that all residual particles were removed, following the procedure described in Roberts et al (2010). Nd was separated following the analytical procedure reported in Wu et al (2015). For details on the measurement of Nd isotopes see the section above.…”

“…Differently from the CWC, the εNd composition of fossil planktonic foraminifera is not related to the ambient seawater at calcification depth but reflects the bottom and/or pore water εNd due to the presence of authigenic Fe-Mn coatings precipitated on their carbonate shell after deposition onto the sediment (Roberts et al, 2010;Elmore et al, 2011;Piotrowski et al, 2012;Tachikawa et al, 2014;Wu et al, 2015). Therefore, the εNd composition of planktonic foraminiferal tests can be used as a useful tracer of deep-water circulation changes in the past, although the effect of pore water on foraminiferal εNd values could potentially complicate the interpretation (Tachikawa et al, 2014).…”