Can cadmium be used as another proxy for volcanism?

The primary source of Cadmium (Cd) to the environment is volcanic emissions, emitted to the atmosphere in a wide range of particle size and chemical compounds that are easily dispersed by wind. Higher temperature emission sources also release elemental gaseous Cd that then cools and becomes bound to ash particles. Atmospheric transport of volcanic Cd emissions is a function of particle size as well as plume height, with transport of thousands of km possible. In the modern, Cd anomalies in Antarctic ice records are shown to be sourced from long-range transport of volcanic emissions, and Cd anomalies found in Greenland ice cores are shown to be generated by the Laki and Hekla eruptions.

When deposited into oceans Cd is highly soluble and has vertical concentration profiles strongly similar to those of nutrients (e.g. P), suggesting that despite its toxicity to higher life Cd acts as a nutrient for phytoplankton by replacing the Zn cofactor in the enzyme carbonic anhydrase under Zn limited conditions. Cd is exported to sediment as organic matter (OM) bound metal and is subsequently buried or fixed as insoluble Cd sulfide.

Studies of Cd in the geologic record are limited and tend to be focused on use of Cd as an indicator of nutrient levels or paleo-environmental conditions. Enhanced Cd concentrations have also been related to increased drawdown during ocean anoxic events (OAEs) related to enhanced OM preservation and the strong affinity of Cd for sulfide as well as an indicator of basin restriction. Examination of use as a proxy for volcanism has been limited.

Records of Cd levels in sediments deposited during Oceanic Anoxic Event 3 in Arctic Canada, show that at low concentrations (< 10 ppm) Cd has a negative correlation with Zn, consistent with Zn replacement. In contrast, at Cd concentrations above 10 ppm there is strong positive correlation with Zn, suggests a common source enriching both metals. Above 10 ppm, Cd concentrations also show a strong correlation with heulandite, a low temperature alteration product of volcanic glass, suggesting a volcanic origin. This is consistent with abundant bentonite beds with geochemical markers consistent with arc volcanism. This may explain the anomalous high Cd concentrations in late Cretaceous mudstones of northern Canada, and suggest that Cd could be another geochemical marker for enhanced volcanism. However, use of Cd may be more nuanced than other geochemical makers, such as Hg or Te, given its nutrient like properties.

These high Cd levels in OAE3 sediments contaminate surface and groundwaters in the modern, with linkage to human health issues, showing long term impact of volcanism on the environment.