CALIBRATION OF ARAGONITE SHELL LOSS RATES AND TIME-AVERAGING IN ARCTIC SEABEDS: BENTHIC ACTIVITY MAKES A TOUGH SITUATION TOUGHER

The Alaskan continental shelf is characterized by high macrobenthic biomass, producing fully bioturbated seabeds with abundant dead shell assemblages, but the scale of time-averaging in such high-latitude, cold-water settings is unknown. From under-saturated overlying waters (OW), especially when combined with bioturbation and sediment irrigation, we expect higher postmortem loss rates and high-sequestration rates, and thus younger median shell ages than in tropical and temperate shelves, where median shell ages are ~50 years and maximum ages are 10-20 ka. AMS-calibrated, amino acid racemization was used to date 213 shells from two aragonitic bivalve genera (Macoma and Nuculana) collected from the top 10 cm of 50-100m deep seabeds at three sites, each characterized by a different combination of benthic activity and OW aragonite saturation. We find that all specimens of Nuculana were <1600 years old with median 50 years, and all Macoma shells were <850 years with median age <50 years. All assemblages experience fast initial loss rates, with decadal half-lives similar to those in warmer seabeds, but much slower net sequestration rates: a long tail of old shells does not develop, thus shortening the time-averaging window for paleobiologic inference. Within the Arctic study area, the highest loss rates are in the northern Bering Sea and southeastern Chukchi Sea, both well-documented centers of high organic flux to the seabed and high benthic oxygen demand, implying high aerobic decomposition and porewater acidification. Such conditions exacerbate shell loss, even though the strongest seasonal aragonite undersaturation of OW is found elsewhere. These results quantify, for the first time on geological (14C) time-scales, (1) high aragonite loss rates in Arctic seabeds, (2) that loss arises from low sequestration, and (3) within this generally aggressive setting, loss rates are highest where benthic activity is highest, rather than where OW is most undersaturated, contrary to usual suppositions.