Millennial-scale surface water mass radiocarbon reservoir age changes recorded on the California Margin

Santa Barbara Basin is well known for producing high quality paleoclimate reconstructions, however the validity of correlations between climate change recorded in the basin and other regions rests upon the accuracy of age models. Two independent well established chronologies based on varve counting and radiocarbon dates do not agree over the last 2 millennia. Here we test the validity of two assumptions associated with the two chronologies: 1) The regional reservoir age of marine surface waters 641±119 years) does not vary through time; and 2) laminae couplets preserved in basin sediments are annual and counted with high precision. In this high-resolution ¹⁴C study of SBB sediments we compare 49 mixed planktonic foraminifer and 21 terrestrial organic carbon ¹⁴C dates to the varve chronology, to extend the high-resolution paleoclimate chronology of the basin back ~2,000 years. We show evidence that both assumptions may be incorrect. First, regional reservoir ages appear to fluctuate through time, varying between 525-750 years (ΔR = 125-350 year). Second, an apparent consistent (r² = 0.95) undercounting of varves occurs between ~1700 and 400 AD based on a new varying ΔR ¹⁴C chronology, therefore suggesting that some laminae couplets may not be annual. However, the consistency of the offset between the two chronologies suggests that a correction can be applied to the lamination counts to account for missing years. As the variable reservoir ages determined by comparison of planktonic foraminiferal radiocarbon with terrestrial ¹⁴C are not observed in benthic-planktonic paired ¹⁴C differences, we suggest they reflect changes in the reservoir age of the upper (500 m) water column on the North American margin. Diffusion of young atmospheric carbon into the upper water column may be enhanced ~1400 years BP due to stronger winds or reduced sea ice cover in the North Pacific. Alternately diffusion of old oceanic carbon from deep water masses might be enhanced ~500 years BP by changes in meridional overturning circulation.