Linking Ocean Variabilty to Phytoplankton Changes Over the Last 2000 Years off N. Iceland

International audienceUnderstanding ocean variability at decadal to century time-scale is a key issue of climate research, yet it remains poorly described partly because high resolution marine records are lacking. Recent observations also suggested that climate-driven oceanic changes may also affect marine productivity reflecting adjustments of the pelagic ecosystem. To explore these issues we investigated sediment cores (a Calypso and a box core) off North Iceland at unprecedented high temporal resolution (2 to 5 years). The area lies in a sensitive boundary region with strong climatic gradients between the Arctic and the North Atlantic realms. Surface hydrology is affected by warm, high salinity Atlantic water (Irminger current-IC) and cold, low salinity water of the East Icelandic Current (EIC), as well as by sea ice and drifting ice exported from the Arctic Ocean and East Greenland. Using biomarker proxies we reconstructed, over the past two millennia, high frequency surface ocean properties, such as Sea Surface Temperature (SST) and patterns of major phytoplankton groups. Records from a box core covering the last 80 years and comparison with instrumental data allowed to improve constrain of the proxies used. The SST curve reveals 4 major oscillations of about 20-25 years, depicting also the Great Salinity Anomaly (GSA). It reflects ocean circulation changes with varying impact of Polar and Atlantic waters (EIC, IC). Similarities with the NAO (North Atlantic Oscillation) index suggest links between STT and westerlies intensity. Phytoplankton biomarker profiles (specific phytosterols and alkenones) are consistent with biological data acquired during annual surveys. They show significant increase in periods when Atlantic waters and subsequent mixing processes prevail during positive NAO phase, compared to low levels when Polar waters and stratified conditions occur. Over the last two millennia the SST curve depicts a broad cooling trend towards present, steepening over the last 500 years. Prolonged warm or cool centennial intervals are recognizable among which the Medieval Warm Period (MWP), the Little Ice Age (LIA), the Roman Warm Period (RWP) and cooler Dark Ages. The SST signal also shows intermittent 20-25 year oscillations that are likely reflecting the ocean response to wind forcing, presumably the NAO. Phytoplankton biomarkers show important high frequency oscillations. Diatom sterols infer a broad decrease of siliceous productivity since 2000 yr BP with minimum values during the LIA, and a sharp rise after 200 yr BP. Such a trend is not observed for coccolithophorid biomarkers (alkenones), which show low centennial scale variations since 1200 yrs BP, along with high abundance between 1600 and 1200 yrs BP. Similar trends are observed for dinoflagellate biomarkers (dinosterol). Overall, planktonic biomarkers show multidecadal to centennial scale fluctuations inferring modifications of phytoplankton community structure. These could be linked at least partly to hydrological changes indicated in the SST curve, namely modifications of the relative importance of IC and EIC, which likely reflect the ocean response to NAO forcing