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

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 delta C-13, 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], delta Cd-114, 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 delta Cd-114 composition of these deposits, with high delta Cd-114 values attributed to near-quantitative removal from seawater in euxinic environments. Lower delta Cd-114 values in noneuxinic environments may reflect isotopically light Cd associated with organic material due to partial remineralization. These observations imply that delta Cd-114 values of samples deposited in demonstrably euxinic conditions may be used to constrain the delta Cd-114 seawater coming into the Maverick Basin at this time and give a composition of 0.28 +/- 0.11 parts per thousand (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 delta Cd-114 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. (C) 2020 Elsevier Ltd. All rights reserved.