Radiocarbon content of pre-bomb marine mollusks and variations in the C-14 reservoir age for coastal areas of the Barents and Kara seas, Russia

Fourteen mollusks, collected alive between 1900 and 1945 from the Russian Parents and Kara seas, were analyzed by AMS C-14 dating to evaluate variations in the C-14 marine reservoir for arctic coastal sites, which is important for correcting ages in paleoenvironmental time-series and advancing understanding of the exchange of carbon. The C-14 ages on the mollusks reveal a range of marine reservoir values (R(t)) from 159 C-14 yr to 764 C-14 yr. The oldest R(t) values of 764 to 620 C-14 yr are for the bivalve Portlandia arctica, which often inhabit cold and low salinity waters and muddy substrates. The depleted C-14 content for this bivalve reflects possibly the incorporation of old carbon from freshwater inputs and/or the consumption of old organic matter from the underlying sediments and pore waters. Other mollusks with sessile habitats and pelagic food sources gave significantly lower R(t) values between 159 and 344 C-14 yr. The youngest R(t) values indicate enrichment in C-14 and may partially reflect enhanced transfer of C-14-enriched CO2 from the atmosphere to the ocean surface with wind-generated wave agitation. This study underscores that a variety of processes can lead to variable C-14 depletion and enrichment of surface waters yielding a ca. 600 year age span for contemporaneous arctic mollusks. There may be added uncertainty in the C-14 reservoir correction for deposit-feeder species such as Portlandia sp. and perhaps for certain benthic foraminifera (e.g. Nonion labradoricum) because these taxa often incorporate old organic matter from the substrate. A reservoir correction of greater than or equal to 700 years may be more appropriate for infaunal, deposit-eater species, particularly in glacier-dominated environments. Mollusks and foraminifera with sessile habits and pelagic food sources should be selected preferentially for C-14 dating, because their shells may more closely reflect the C-14 content of the global-ocean mixed layer.