Controls on the provenance of late Eocene to Quaternary Mozambique Channel shales (DSDP 25 Site 242)

Abstract

Provenance determination of late Eocene to Quaternary sediments deposited at the DSDP 25 Site 242 may help to evaluate the mechanisms that have controlled the sediment deposition in the Mozambique Channel. To determine the provenance of the clay fraction, we measure major and trace element concentrations as well as strontium (87Sr/86Sr) and neodymium isotopic compositions (expressed as ƐNd(0)) of thirty-six sedimentary samples from DSDP 25 Site 242 on the eastern flank of the Davie Fracture Zone. Light Rare Earth Element (LREE) enrichment and Heavy REE (HREE) depletions against Post Archean Australian Shales (PAAS) associated with Ce anomalies higher than or equal to 0.90 for all the analyzed samples suggest very little influence of authigenic smectite incorporation. The absence of correlation of Al/Si and CIA with REE content, Eu/Eu*, Cr/Th, Th/Sc, and 87Sr/86Sr and ƐNd(0) suggests that chemical weathering has not modified these provenance proxies. Slight increase of Eu/Eu*, Cr/Th ratios, ƐNd(0) values associated with slight decrease in 87Sr/86Sr and Th/Sc ratios from the late Eocene to the Quaternary point to input of less differentiated and younger detritus to the DSDP 25 Site 242 with age. Based on variations in ƐNd(0) and Nd concentrations, we estimate an overall decrease of the Southeast African rivers (Congo/Zambezi) contribution from ~65.7% (Standard Deviation (S.D.) ~13.2) in the late Eocene to ~28.5% (S.D. ~10.0) in the early Pliocene. The causes for the low ƐNd(0) values and high Southeast African contribution between the late Eocene and the early Oligocene remain to be determined. It could record the onset of modern sedimentation in the Zambezi delta or the discharge of the Paleo Congo river in the Indian Ocean. The decrease of Zambezi contribution between the late Oligocene and early Pliocene is attributed to the cumulative effect of increasing tectonic activity of the Davie Fracture Zone and intensification of the Mozambique Current. The late Pliocene to Quaternary glacial-interglacial cycles promoted higher Zambezi contribution during glacial sea-level lowstands and in turn lower ƐNd(0) detrital values, whereas low Zambezi contributions are favored during interglacial highstand causing higher ƐNd(0) values. Hence, the modern dispersal pattern of fine-grained sediments within the Mozambique Channel is no younger than late Pliocene.