Figure 4
Conceptual depositional model for manganiferous sediments at Morro da Mina. Left side – Mn shuttle in a redox-stratified ocean: Mo (with δ98Mo<0) and Ce were preferentially adsorbed onto Mn-oxide particles in oxidized seawater above the redoxcline. Across the redoxcline Mn-oxide particles underwent reductive dissolution, leading to the formation of Mn-carbonate particles. Molybdenum (with δ98Mo<0), released by the reductive dissolution, was scavenged by organic C in euxinic seawater and transferred to Mn-rich carbonaceous sediments. The reductive dissolution of Mn-oxide particles across the redoxcline also caused Ce enrichment (Ce/Ce* >1). Right side: in a deeper part of the basin, where bottom waters were depleted in Mn due to Mn upwelling to swallow waters, seawater Mo was directly scavenged by organic C and deposited as carbonaceous muds that captured the Mo-isotope signal of Ce-depleted seawater (δ98Mo>0, Ce/Ce* <1). The Mn-rich carbonaceous sediments were metamorphosed at high temperature (>600 °C) to queluzite, and the carbonaceous sediments to graphitic schist. The former is a Mn-carbonate rock containing Mn silicates, Mn sulfide and graphite; the latter is poorer in Mn and marks ductile shear zones. Despite metamorphic and tectonic overprints, palaeoenvironmental information – i.e., primary differences in Mo-isotope signals – is still preserved and can be retrieved.
