OXYGEN AND CARBON ISOTOPES IN ADAMUSSIUM COLBECKI: δ13C PROXY FOR SEA-ICE PERSISTENCE?

The Antarctic scallop Adamussium colbecki is a promising proxy for sea-ice persistence and can potentially resolve subannual seawater conditions characteristic of annual and multiannual sea ice. Alternating groups of widely- and narrowly-spaced striae (small ridges on valve surfaces) are thought to indicate seasonal growth differences: wide groups in summer, narrow groups in winter. Shell oxygen (δ¹⁸O_s) and carbon (δ¹³C_s) in striae groups may therefore reflect seasonal seawater conditions. We expect lower δ¹⁸O_s in wide summer striae groups under both annual and multiannual sea ice if glacial meltwater mixes through the water column. We also expect higher δ¹³C_s in wide striae groups under annual sea ice but not under multiannual sea ice, as phytoplankton blooms post sea-ice breakout enrich seawater δ¹³C_DIC.

Scallops were collected from two sites in western McMurdo Sound (Ross Sea) located ~30 km apart: Explorers Cove (EC) has multiannual sea ice and Bay of Sails (BOS) has annual sea ice. Adults were collected live by divers at 9–18 m depth in 2008 from EC and BOS. Additional juveniles (< 2 yrs) were collected from EC in 2016. Two adults each from EC and BOS and two 2016 juveniles were serially sampled for stable isotopes. δ¹³C_s decreases over ontogeny due to metabolic effects; the linear trend was removed to enable seasonal comparison. Detrended residuals are referred to as δ¹³C_{s det}.

Mean δ¹⁸O_s (~3.7‰) is not different in narrow and wide striae groups under either annual or multiannual sea ice, suggesting negligible glacial meltwater mixing at depth and minimal seasonal temperature change at both sites. δ¹⁸O_s values are within expected equilibrium range and decrease over ontogeny, suggesting increased growth during warmer temperatures in older scallops. In contrast, mean δ¹³C_{s det} is ~1‰ higher in wide summer striae groups than narrow winter striae groups under annual sea ice at BOS, but not different between striae groups under multiannual sea ice in EC adults. δ¹³C_{s det} is also higher in wide summer striae groups from 2016 EC juveniles, however sea ice broke out at EC in 2015, so juveniles experienced annual-like sea-ice conditions. Seasonal differences in δ¹³C_s suggest that carbon isotopes coupled with striae width in A. colbecki may be a good proxy for sea-ice persistence in Antarctica both in modern and fossil assemblages.