CHEMOSTRATIGRAPHIC AND SEDIMENTOLOGICAL FRAMEWORK OF THE LARGEST NEGATIVE CARBON ISOTOPIC EXCURSION IN EARTH HISTORY: THE NEOPROTEROZOIC SHURAM FORMATION (NAFUN GROUP, OMAN)

Abstract

Oman records in its Neoproterozoic Nafun Group (Huqf Supergroup) an essentially complete, carbonate-rich Ediacaran succession. The Nafun Group overlies the presumed Marinoan rift-related Fiq Member (ca. 635 Ma) and ends just below the Precambrian-Cambrian boundary (542 Ma). Tectonic subsidence caused by thermal contraction following Fiq-aged rifting allowed the preservation of about 1 km of postrift stratigraphy, with no major stratigraphic breaks. The Nafun Group above the Marinoan cap carbonate (Hadash Formation) represents two siliciclastic to carbonate 'grand cycles', both initiated by significant transgressions: these cycles comprise the Masirah Bay/Khufai formations and the Shuram/Buah formations. The Khufai-Shuram boundary is associated with the start of a major carbon isotope perturbation. The uppermost ramp carbonates of the Khufai Formation record a smooth decrease in δ13C from about +4‰ to values around 0, followed by two descending steps across which values plunge to a nadir of -12‰ in the overlying red siltstones and shales interbedded with thin limestones of the Shuram Formation. This fall in isotopic values is temporally rapid and coincident in both shallow and deep-water sections in the time span of a single parasequence. The δ13C nadir is then followed by 50 million years of monotonic recovery. The 'Shuram shift' represents the largest δ13C inorganic carbon negative excursion in Earth history. Although the snowball Earth theory links periods of depleted carbon isotopic ratios with periods of global glaciation, the non-glaciated context of the Shuram Formation suggests that the causal relationship between global glaciation and negative carbon isotopic excursions is non-unique. Although the precise mechanism driving this major perturbation of the carbon cycle remains enigmatic, the long-term remineralization of an isotopically depleted organic carbon reservoir in ocean water is a promising candidate. © 2006 Elsevier B.V. All rights reserved.