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.
Here, we present a high-resolution and well-dated sediment record from the northern Labrador Shelf that provides new evidence of the freshwater signature and the mechanisms involved in the Hudson Bay Ice Saddle collapse. A combination of Mg/Ca ratios and stable oxygen isotopes of benthic foraminifera indicate temperature changes and freshening of Labrador Shelf bottom waters.
About 60 specimens of I. helenae were handpicked from the 200–315 μm fraction in 5-cm intervals downcore, weighed, and crushed between two glass plates. The crushed samples were then split for Mg/Ca and stable isotope measurements (see below). About two thirds of each crushed sample was transferred into pre-leached Eppendorf vials and cleaned following the full protocol of Martin and Lea (2002), including a reductive and oxidative cleaning step and a final leaching step with 0.001 N HNO3. After dissolving and diluting the samples in 0.1 N HNO3, they were measured with an inductively coupled plasma-optical emission spectrometry (ICP-OES) instrument with radial plasma observation at the Institute of Geosciences, Kiel University. The analytical error of Mg/Ca analyses was 0.1% relative standard deviation and accuracy was monitored with reference material JCP-1. Additional trace elements (Fe, Al, and Mn) were monitored to exclude possible contaminated or coated samples from the dataset. Based on 15 duplicate down-core sample measurements, we obtained a standard deviation of 0.07 mmol/mol Mg/Ca, which translates into 0.9 °C with respect to temperature estimates. For bottom water temperature reconstructions, we applied the calibration of Skirbekk et al. (2016). As the calibration is based on a temperature range of 1–4 °C, it may become less accurate when temperatures exceed 4 °C, which is the case for 25% of the samples reported here.
About one third of the crushed samples of I. helenae were cleaned with ethanol absolute, decanted, and dried at 40 °C. Stable oxygen isotope analyses were carried out at the Leibniz Laboratory for Radiometric Dating and Stable Isotope Research in Kiel. A Finnigan MAT 253 mass spectrometer coupled with a Kiel IV carbonate preparation device was used and calibrated to the Vienna Pee Dee Belemnite (V-PDB) scale, which is applied to all δ18OCaCO3 analyses. Based on 15 duplicate down-core sample measurements, we obtained a standard deviation of 0.09‰. To estimate the δ18Ow (seawater), the value of calcitic δ18Oc (‰ V-PDB) was first translated into the V-SMOW scale by adding 0.27‰ and, together with the Mg/Ca-derived temperature, applied in the Shackleton equation T = 16.9 − 4.0 (δ18Oc − δ18Ow)58. To correct for ice volume (δ18Ow-ivc), a sea-level correction of 0.0083‰ per m (1‰ for 120m of sea level) was derived from the relative sea level curve by Austermann et al. (2013). Based on duplicates, δ18Ow-ivc values have a standard deviation of 0.31‰. The bottom water signals (Mg/Ca BWT°C, δ18Οc and δ18Οw-ivc) are based on infaunal benthic foraminifera I. helenae, which tend to live several centimeters deep in the sediment. This may explain the apparent lead of the bottom water signals compared to the sedimentological proxies Ca/Sr and a*(D65).
The freshwater signal itself is based on a combination of stable oxygen isotopes and Mg/Ca temperature estimates of the benthic foraminifera species Islandiella helenae, representing subsurface/bottom water conditions at about 200m water depth. The δ18Οc record is relatively steady, centered at 3.2 ± 0.3‰, with the exception of a large reduction to 1.6‰ at 1048–1043 cm depth and another decrease to 2.25‰ at 1013–993 cm depth. The Mg/Ca temperature estimates fluctuate between −1 and 6 °C and show warm peaks at 1100, 1050, 910, and 775 cm depth. The gradual warming from −1 to 3 °C between 1130 and 1100 cm depth and the warm peak to 6 °C at 1050 cm depth precede two cold spells to 0 °C, the first at about 1070 cm depth and the second cooling at 1040 cm depth. These two cold spells are supported by two freshenings evident in the temperature and ice volume corrected δ18Ow-ivc record. The δ18Ow-ivc record ranges from −2.5 to 0.5‰ and shows a minor minimum between 1073 and 1068 cm and a distinct minimum at 1048–1028 cm depth, corresponding to the subsurface water cold events, while the δ18Oc reduction at 993 cm depth is only reflected by a minor reduction in the seawater δ8Ow-ivc record.
The δ18Ow-ivc reduction of about −2.5‰ between 8.55 and 8.45 ka BP is the most pronounced benthic freshwater signal so far recorded in Holocene Labrador Sea sediments and implies an immense volume of freshwater to dilute the shelf waters down to 200m depth for about 100 years. Remarkably, the benthic foraminifera revealing the major freshwater peak at 1043 cm core depth have a radiocarbon age about 300–600 years older than those dated just below and above, respectively
