Multicentennial Record of North Atlantic Primary Productivity and Sea-Ice Variability Archived in Coralline Algal Ba/Ca

Accelerated warming and melting of Arctic sea-ice has been associated with significant increases in phytoplankton productivity. However, satellite-based chlorophyll estimates and in-situ instrumental observations of marine productivity have only been available in select regions for the last few decades. Barium has previously been shown to be severely depleted in surface waters due to biological scavenging during periods of intense phytoplankton blooms. Here we present an annually-resolved multicentennial record of coralline algal barium-to-calcium ratios (Ba/Ca), as a proxy for North Atlantic phytoplankton productivity associated with sea-ice variability in Labrador, Canada extending well into the Little Ice Age (LIA). Coralline algal Ba/Ca demonstrates statistically significant correlations to observational and proxy records of sea-ice cover, extent, and transport variability, and shows a persistent pattern of covariability that is broadly consistent with the timing and phasing of the Atlantic Multidecadal Oscillation (AMO). Higher (lower) algal Ba/Ca values are interpreted as decreased (increased) primary productivity coinciding with cooling (warming) sea surface temperatures and the expansion (melting) of sea-ice. Comparison to a tree-ring proxy AMO index demonstrates more frequent positive Ba/Ca excursions (indicating reduced productivity) associated with AMO cool phases during the LIA, followed by a decline in Ba/Ca beginning in 1910 to present. Our multicentennial record of coralline algal Ba/Ca ratios indicates that the sharp rise in primary productivity since 1950 in the Subarctic North Atlantic is unprecedented in the last 365 years, and is in agreement with recent observations of increasing productivity in the Arctic Ocean. The ongoing increase in phytoplankton productivity is expected to fundamentally alter marine biodiversity and trophic dynamics as warming and freshening of the surface layer is projected to intensify over the coming century.