Stable isotopes of Heliconoides inflatus pteropod shells record near-surface conditions in the Cariaco Basin

Pteropods are a group of aragonitic, pelagic gastropods found throughout the world’s oceans. The pteropod fossil record extends back to the Cretaceous, however, their potential as palaeoceanographic proxies remains underdeveloped.

Here we present stable isotopic measurements from the shells of 67 Heliconoides inflatus specimens from 17 sediment trap samples (1996 – 1998) in the Cariaco Basin, Venezuela. These measurements were compared to hydrographic measurements from the CARIACO timeseries, and the calculated δ¹⁸O of seawater. Pteropod shell condition, a measure of the amount of dissolution that a pteropod shell has undergone, was assessed to determine whether dissolution affects shell stable isotopic composition.

The isotopic composition of pteropod shells does not correlate with pteropod shell condition, suggesting that pteropods are robust recorders of ocean chemistry. We find that H. inflatus calcifies at an average depth of 49 m in the Cariaco Basin, shallower than the most recent calcification depth estimate for this species (75 m). The isotopic composition of an additional 48 H. inflatus shells from a Cariaco Basin core top sample (192 m water depth; ~1000 yr BP) were also measured. Core top specimens are enriched in both δ¹³C (0.4 ‰) and δ¹⁸O (0.2 ‰) relative to sediment trap specimens capturing an increase in seawater temperature (δ¹⁸O) and an increase in isotopically light carbon associated with the burning of fossil fuels (δ¹³C) over the ~1000 years between the two samples. Radiocarbon dates of planktic foraminifera (G. sacculifer) and pteropods (H. inflatus) from the same stratigraphic level in the core varied by 880 years. This discordance is currently unexplained, though similar non-overlapping ages have been reported in other studies.

This research reaffirms that the stable isotopic composition of pteropod shells are reliable recorders of water column chemistry, and can preserve a signal of environmental variability in the shallow sedimentary record.