Redox conditions of Early Cambrian Ocean as deciphered from multi-proxy geochemical and isotopic studies of Proto-Tethys carbonaceous sediments from Outer Lesser Himalaya, India.

Ediacaran to Early Cambrian sequences are well preserved in several synclines within the Lesser Himalaya. We have conducted multi-proxy geochemical and stable isotopic studies on carbonaceous sediments of the basal Tal Group to decipher redox conditions of the Earliest Cambrian Ocean. Organic matter is immature, predominantly aliphatic and likely derived from marine phytoplankton. The S/C(organic) ratios above 0.4; strong enrichment of redox-sensitive trace elements V, Mo, U and Cd; higher DOPT, U/Th, V/Sc, Mo/Sc and V/(V+Ni) ratios; V/Al–U/Al–Mo/Al systematics and U–Mo enrichment factors indicate anoxic to euxinic conditions of deposition. Large variations (5–7‰) in δ13C likely reflect spatial heterogeneity or vertical gradient in DIC compositions of the Early Cambrian Ocean, and possibly capture the part of Basal Cambrian negative Carbon isotope Excursion (BACE). Finely disseminated texture of pyrite framboids with sizes mostly below 10 µm suggest sulfide reduction within the water column in an open system. Isotopically heavy δ34S values (+4.5 to +22.8‰) overlap with the δ34Ssulfate of contemporary Fortunian ocean (+10 to +40‰), suggesting sulfidic conditions and small size of the sulfate reservoir. Whereas the REE+Y proxies of the same set of samples show seawater-like patterns, with strong HREE enrichment, positive Y and La anomalies and negative Ce anomaly (avg. Ce/Ce* = 0.78), indicating REE uptake by phytoplankton from an oxidized-euphotic surface zone. The combined geochemical and isotopic data suggest a strongly stratified ocean with sulfidic bottom water and oxidized surface water. Episodic shoaling of anoxic deep water onto the surface possibly caused the extinction of benthic Ediacaran fauna, and the surface oxidized zone likely provided the niche for further metazoan radiation. [ABSTRACT FROM AUTHOR]