Controls on the Cd-isotope composition of Upper Cretaceous (Cenomanian–Turonian) organic-rich mudrocks from south Texas (Eagle Ford Group)

Abstract

The isotopic composition of Cd buried in marine sediments may preserve valuable palaeoenvironmental information on past ocean redox conditions or biological cycling. It is unclear, however, how the Cd-isotope composition of the sedimentary record reflects these processes. In this study, new Cd-isotope data are presented, along with δ13C, and Cd, Mo and TOC concentrations, from organic-rich mudrocks from the southern Western Interior Seaway (WIS), spanning the Cenomanian–Turonian stages within the Eagle Ford Group of the Maverick Basin, Texas, USA. Relationships between [Cd/TOC], δ114Cd, and MoEF indicate that sedimentary Cd was derived from organic matter with additional contributions from CdS formed in euxinic water masses. Local redox conditions exerted a primary control on the δ114Cd composition of these deposits, with high δ114Cd values attributed to near-quantitative removal from seawater in euxinic environments. Lower δ114Cd values in non-euxinic environments may reflect isotopically light Cd associated with organic material due to partial remineralization. These observations imply that δ114Cd values of samples deposited in demonstrably euxinic conditions may be used to constrain the δ114Cdseawater coming into the Maverick Basin at this time and give a composition of 0.28 ± 0.11‰ (2 SD) for the Early Cenomanian. Samples from an interval of peak-organic carbon burial globally, namely Oceanic Anoxic Event 2 (OAE 2), show anomalous δ114Cd and [Cd] patterns compared to the rest of the data, implying a perturbation to the dissolved Cd pool. The data presented in this study demonstrate that sedimentary Cd isotopes preserve valuable information on the extent of Cd burial into sulfide-bearing sediments at both local and global scales.