The zinc isotopic composition of siliceous marine sponges: Investigating nature's sediment traps

•We measure zinc isotopes in hexactinellid and demosponge sponge opal.•We relate the Zn isotopic composition of the spicules to environmental parameters.•Hexactinellid Zn isotopes reflect deep-ocean seawater values.•Demosponge Zn isotopes reflect internal fractionation processes.•Sponge spicule Zn isotopes show potential for use as paleoenvironment archives.

The zinc (Zn) content and isotopic composition of marine biogenic opal have the potential to yield information about the nutrient availability, utilization and export of particulate organic matter from surface to deep waters. Here, we report the first measurements of the Zn isotopic composition of deep-sea marine sponge skeletal elements - spicules - collected in the Southern Ocean. Our results highlight different Zn uptake and isotopic fractionation behavior between the two major siliceous sponge clades (hexactinellids and demosponges), which is most likely linked to sponge feeding strategy. Hexactinellid spicule Zn isotopic compositions are not fractionated with respect to seawater, most likely due to Zn transport via the open internal structure of the sponges. In contrast, demosponge spicules exhibit a wide range of Zn isotopic compositions that are related to the opal Zn concentration, most likely reflecting variable Zn isotope compositions in the organic matter particles on which they feed, and internal fractionation processes.