Provenance, paleo-weathering and -redox signatures of estuarine sediments from Ghana, Gulf of Guinea

摘要

This paper discriminates the source of sediment supply (provenance), degree of chemical weathering, and redox conditions of Holocene sediments cored from the Ankobra and Pra estuaries of Ghana in the Gulf of Guinea using their geochemical characteristics. Considerable stratigraphical variations were observed in major element compositions in the sediments from both the Ankobra (Al2O3: 5.3-14.2%, SiO2: 75.9-88.2%, CaO: 0.3-1.2%, K2O: 0.5-1.2%) and the Pra (Al2O3: 7.8-16.9%, SiO2: 72.8-86.5%, CaO: 0.3-0.4%, K2O: 0.7-1.2%) estuaries. Elemental compositions were similar in both estuaries depicting similarities in their source rock characteristics. Some of the major and trace elements correlated significantly (r(2) 0.8) with Al2O3 in the Pra estuary confirming their possible hydraulic fractionation. Geochemical classification showed the sediments from both estuaries were predominantly composed of sub-arkoses with only a few litharenites. The chemical index of alteration (CIA*), Chemical index of weathering (CIW*), plagioclase index of alteration (PIA*), the Index of compositional variability (ICV), the SiO2Al2O3 ratios and SiO2 vs. (Al2O3 K2O + NaO), suggest high degrees of chemical weathering coinciding with warm climatic conditions in the source regions as well as sub-matured to mature estuarine sediments deposited under semi-arid to semi-humid conditions. The sediment geochemistry and discrimination diagrams denote similar continental signature derivatives of the sediments from both estuaries. Both sediments are likely derived from intermediate to felsic granitoid source rocks as well as from a quartzose sedimentary provenance formed in a passive continental margin. The Ni/Co and V/Cr ratios depict sediment deposition under oxic conditions through time in both estuaries.