Roles of changes in land weathering intensity in the Nd cycle of the South China Sea during the past 30 kyr as inferred from neodymium isotopic composition in foraminifera

Abstract

Neodymium isotopic compositions (εNd) in the marginal sea is influenced by the ocean circulations as well as the lithogenic input from continents, but their relative contributions and the impact of detrital sediments are not well constrained. In this study, Nd isotopic compositions of mixed planktonic foraminifera species and carbonate-free fraction of sediment from two cores collected at 1250–1350 m water depth in the southwestern (core MD01–2393) and northeastern (core MD18–3569) South China Sea (SCS) have been investigated in order to constrain the relative contributions of lithogenic Nd inputs from large Asian rivers and hydrological variations at intermediate depth in the SCS. The foraminiferal εNd values of both cores (−7.3 ± 0.2 to −5.8 ± 0.2 for core MD18–3569 and − 8.1 ± 0.2 to −7.2 ± 0.2 for core MD01–2393) indicate strong modifications of the initial εNd of the North Pacific Intermediate Water (NPIW) (εNd of −4) flowing into the SCS by unradiogenic sediments (−13 to −11) from large Asian rivers. The foraminiferal εNd record of core MD01–2393 displays significantly unradiogenic values from 18 to 8 cal kyr BP which are associated with intensification of monsoon rainfall and river input of detrital material characterized by strongly altered minerals (high illite chemical index and kaolinite/illite ratio) deriving from tropical plain soils of the Mekong River basin. We suggest here that pedogenetic minerals from tropical plain soils are more efficient in terms of Nd exchange with the seawater than primary minerals produced by intense physical erosion during the glacial period. In contrast, the foraminiferal εNd record for the northern core MD18–3569 is characterized by radiogenic εNd values from 18 to 12 cal kyr BP; these values have been linked to an increased intrusion of radiogenic glacial NPIW to the northern SCS and greater ventilation of water masses (deduced from Δ14C) in the northern deep basin of the SCS compared to the southern one. This time interval is associated with newly formed NPIW in the subarctic Pacific Ocean beginning at about 18 cal kyr BP. Overall, our results indicate that the state of chemical weathering of sediment delivered by rivers is the main factor controlling the detrital modification of past seawater εNd distributions in the marginal sea of the SCS.