A history of the δ13C composition of oceanic plankton

Carbonates of western Newfoundland span the Cambro-Ordovician interval and preserve a record of slope-basinal deposition in the Cow Head Group near Cow Head. This unit consists of conglomerates and ribbon and laminated limestone interbedded with shale that is well exposed in sea cliffs at Cow Head Peninsula. These conglomerates, although prevalent throughout the section, vary in thickness and abundance stratigraphically and record both local disruption and large-scale episodic sedimentation events. Microfacies drilled for carbon isotope (δ13Ccarb) analysis of conglomerates reveal isotopic heterogeneity within individual samples, in some cases more than 1‰. While this might be an expected outcome of drilling multiple areas of a heterogeneous conglomerate hand sample, permil-level variability was observed both between individual clasts in a sample, between different parts of the same matrix, and between a clast and its surrounding matrix. No associated variation in δ18Ocarb or trace element distributions exists to suggest that this δ13Ccarb variability is the result of later-stage meteoric diagenesis. The δ13Ccarb variability suggests multiple sources of dissolved inorganic carbon (DIC) associated with carbonate precipitation for phases within these individual samples. These data indicate that processes such as local organic matter remineralization and early authigenic carbonate precipitation during lithification at the sediment–water interface (SWI) are either contributing to or controlling δ13Ccarb values in Cambrian carbonates, perhaps more so than at other intervals in Earth history.