Cd isotopes in carbonates deposited during ‘OAE 2’: Assessment of a novel palaeo-productivity tracer

Cadmium (Cd) displays nutrient-type patterns in the modern ocean and has potential as a tracer of the efficiency of the ‘biological pump’ and its ability to transport CO₂ from the atmosphere to the deep ocean during intervals of extreme environmental change. This potential arises because phytoplankton preferentially incorporate lighter Cd isotopes under many oceanic conditions, leaving surface waters relatively enriched in heavier isotopes. As a consequence of this fractionation, Cd-isotope ratios have been shown to reflect nutrient availability and the intensity of primary productivity in the modern ocean. However, the ability of the Cd stable-isotope system to serve as a robust palaeo-productivity tracer is not yet well established.

Oceanic Anoxic Event 2 (OAE 2; ~94 Ma) represents a period of widespread environmental degradation and oceanic de-oxygenation, likely the result of increased volcanic activity, intensified marine and continental silicate weathering, augmented nutrient input to the ocean and elevated primary productivity. However, direct evidence for the availability of bio-limiting nutrients in the oceans and the role of primary productivity as a feedback mechanism to eventually re-stabilise climate is limited. Here we present the first Cd-isotope record for OAE 2, from the well-preserved and biostratigraphically well-constrained organic-lean pelagic carbonate section through the English Chalk at Eastbourne (UK). Contrary to expectations, Cd isotopes at Eastbourne do not seem to be controlled by surface ocean productivity, but likely reflect global sub-surface signatures. The isotopic record suggests an active biological pump during OAE 2 coupled with changes in ocean circulation on a global scale. Our new record proposes that the Cd-isotope proxy is powerful and potentially very important for unravelling environmental changes during deep time events.