The presented data originates from the 1306cm long gravity core MSM45-19-2 (58°45.68 N, 61°56.25 W, 202m water depth) taken during R/V Maria S. Merian cruise MSM45 in August 2015 at 202 m water depth on the northern Labrador Shelf, northeast Canada, northwest Atlantic. In order to fully understand the dynamics that contributed to the establishment of modern Labrador Sea circulation, detailed reconstructions of the major paleoceanographic changes through the Holocene are critical. Therefore, this study aims to provide new information on bottom (i.e. subsurface) water mass composition as well as on changes in primary productivity and sea-ice conditions in the LC during the Holocene. Reconstructions are based on alkenone biomarker concentrations to provide a better understanding of the dynamics of different water masses that contributed to the establishment of modern Labrador Sea. circulation.
Long-chain alkenones are biosynthesized by haptophytes (e.g. the coccolithophorid Emiliania huxleyi) living in surface waters. They consist of 37 carbon atoms with 2, 3, or 4 double bonds (C37:2, C37:3, and C37:4, respectively). Alkenone analyses were carried out at the Biomarker Laboratory at Kiel University. In total, 290 samples were analyzed in 5-cm intervals. Long-chained alkenones (C37) were extracted from 2 to 3 g of homogenized bulk sediment, using an Accelerated Solvent Extractor (Dionex ASE-200) with a mixture of 9:1 (v/v) of dichloromethane: methanol (DCM: MeOH) at 100°C and 100 bar N2 (g) pressure for 20 min. Extracts were cooled at −20°C and brought to near dryness by Syncore polyvap at 40°C and 490 mbar. For the identification and quantification of C37:2, C37:3, and C37:4 compounds, we used a multi-dimensional, double gas column chromatography (MD-GC) set up with two Agilent 6890 gas chromatographs (Etourneau et al., 2010). The addition of an internal standard prior to extraction (cholestane (C27H48) and hexatriacontane (C36H74)) allowed quantification of the three different compounds in nanograms per gram dry bulk sediment. From peak areas of the different compounds, the proportion of each unsaturated ketone was calculated. The analytical precision is 1% for the relative abundances of C37:4 and 1.5 (ng/g) for the alkenone concentrations, respectively.
Alkenone concentrations (sum of C37:2, C37:3, and C37:4) of 10–30 ng/g in core MSM45-19-2 are at the lower detection limit and represent only a fraction of the concentrations that are commonly found in other subpolar to tropical areas (500–1500 ng/g; e.g. Blanz et al., 2005), where coccolithophore blooms occasionally occur under sea ice–free conditions. These low concentrations in combination with unusually high proportions of tetra-unsaturated C37 ketones (C37:4) prevent the estimation of reliable sea surface temperature (SST) values in the lower section of the core. Very low alkenone concentrations (<20 ng/g) were found in the deeper part of the core between 1300 and 600 cm, with the exception of a peak (up to 28 ng/g) at about 800 cm. Generally, higher alkenone abundances with larger fluctuations characterize the upper section of the core (<600 cm).
We differentiated four distinct climatic periods in the western Labrador Sea. From 8.9 to 8.6 ka BP, the Labrador Shelf was dominated by polar water outflow from Baffin Bay and covered by perennial sea ice. Between 8.6 and 7.4 ka BP, a strong subsurface inflow of warmer Atlantic water masses is ascribed to an intensification and redirection of the West Greenland Current. At 7.4 ka BP, the decreased influence of Atlantic water masses on the Labrador Shelf marks the establishment of winter convection leading to the formation of Labrador Sea Water in the central basin. Concurrently, an intensified polar water outflow through the Canadian Gateways strengthened the inner Labrador Current, and higher primary productivity suggests longer spring blooms because of a shorter sea-ice season during the Holocene Thermal Maximum. In the late Holocene after 3 ka BP, periodic fluctuations of primary productivity may tentatively be correlated with stronger and weaker northwesterly winds.
