Constraints on early Cambrian carbon cycling from the duration of the Nemakit-Daldynian–Tommotian boundary δ13C shift, Morocco

Abstract

The Nemakit-Daldynian–Tommotian (ND-T) boundary marks the first appearance of metazoan reefs and calcite biomineralizers and is associated with the largest δ 13 C shift during the Phanerozoic Eon. Biological transitions in Earth history are often accompanied by excursions in the carbon isotopic composition (δ 13 C) of the ocean, where δ 13 C variability is interpreted to reflect changes in the global carbon cycle. The duration and thus rate of these δ 13 C anomalies are rarely known, making it difficult to constrain their possible causes and their relationship, if any, to biologic transitions. We report sedimentological and δ 13 C data from a new 2.5-km-thick section that spans the early Cambrian evolutionary “explosion” in the Moroccan Anti-Atlas Mountains. Three new zircon 206 Pb- 238 U ages from tuffs within the stratigraphy constrain the timing of the ND-T boundary to 524.84 ± 0.09 Ma. Two of the tuffs exactly bracket the ND-T transition and constrain the duration of the −8‰ δ 13 C shift to 506 ± 126 k.y. With a simple box model, we explore a range of geochemical processes that could account for such a rapid ND-T δ 13 C shift, and conclude that metamorphic and/or volcanic fluxes of carbon may have been sustained at levels 4–16 times higher than today for millions of years.