Radiocarbon dating of Arctic marine sediment is often challenging due to the low availability of calcareous fossils. Consequently, bulk organic matter dating has at times been used to establish sediment core chronologies. Yet, radiocarbon dates based on bulk organic matter often appear to deviate vastly from dates based on fossils, mainly caused by input of carbon from other sources, including terrigenous organic matter. In this study, we aim to examine the link between the composition of the bulk organic matter and the age offsets between the bulk radiocarbon dates and those obtained from calcareous foraminiferal tests. All samples are taken from the marine sediment core AMD14-204C from offshore Upernavik (eastern Baffin Bay). The radiocarbon dates for bulk organic matter are on average ~3000 years older than the radiocarbon dates based on foraminifera, but with changing age offsets throughout the record. To investigate the cause of this age offset and its variations over time, we applied X-ray Fluorescence analysis, stable isotopes, organic pyrolysis and microscopic organic petrology to examine the distribution and characterization of the organic matter. The results show that the older organic matter includes clastic input of reworked sedimentary rocks potentially originating from West Greenland and/or the Canadian Arctic Archipelago. Changes in the input of contemporary marine algal produced organic matter versus both terrigenous input and reworked ancient organic matter appear to control the age offsets between the bulk and foraminifera dates. Low Hydrogen Index and low δ13Corg together with high Oxygen Index, indicative of high influence of terrigenous organic matter, seem to correspond to samples with the largest age offsets; 1000-2000 years greater than in other samples. To examine the cause of the variations in the age offsets, a new quantification of the autochthonous organic matter as a fraction of the TOC was calculated. This shows that samples with the largest age offsets contained the lowest fraction (as low as ~12 %) of autochthonous organic matter in the TOC.

Data includes under ice temperature (C) and salinity (psu) measurements from each sampling day, measured by using with RBR Concerto® and CastAway-CTD® sensors.

Relative abundances (%) of dinoflagellate cysts from the top 0,5 cm surface sediment from the Belcher Islands Archipelago, south-eastern Hudson Bay, Canada on April 15th, 2019, using a Kajak-Brinkhurst gravity corer. Dinoflagellate cyst quantifications were done using a light microscope (Zeiss Axio Imager.A2 upright research microscope, University of Helsinki) with bright-field optics at a x600 magnification.

Highly branched isoprenoid (HBI) concentrations (ng/g) from the top 0,5 cm surface sediment from the Belcher Islands Archipelago, south-eastern Hudson Bay, Canada on April 15th, 2019, using a Kajak-Brinkhurst gravity corer.

Pigment concentrations (nmol/l) from the bottom 5 cm of modern sea ice samples from the Belcher Islands Archipelago, south-eastern Hudson Bay, Canada between April and May 2019 using a Kovacs Mark II core barrel with an internal diameter of 9 cm.

Relative abundances (%) of diatoms from the top 0,5 cm surface sediment from the Belcher Islands Archipelago, south-eastern Hudson Bay, Canada on April 15th, 2019, using a Kajak-Brinkhurst gravity corer. The surface sediment data includes relative diatom abundances (%). Diatom quantifications were done using a light microscope (Zeiss Axio Imager.A2 upright research microscope, University of Helsinki) with phase contrast optics at a x1000 magnification.

Relative abundances (%) of dinoflagellate cysts from sediment trap samples from the Belcher Islands Archipelago, south-eastern Hudson Bay, Canada between April and July 2019 using an automated Gurney Instruments Baker-style, cylindrical sediment trap (aperture 0.032 m2). The sediment trap was deployed at 40-m depth, 30-m above the seafloor and the rotation for the trap was set to 8-day intervals. Dinoflagellate cyst quantifications were done using a light microscope (Zeiss Axio Imager.A2 upright research microscope, University of Helsinki) with bright-field optics at a x600 magnification.

Diatom concentrations (valves/g) from sediment trap samples from the Belcher Islands Archipelago, south-eastern Hudson Bay, Canada between April and July 2019 using an automated Gurney Instruments Baker-style, cylindrical sediment trap (aperture 0.032 m2). The sediment trap was deployed at 40-m depth, 30-m above the seafloor and the rotation for the trap was set to 8-day intervals. Diatom quantifications were done using a light microscope (Zeiss Axio Imager.A2 upright research microscope, University of Helsinki) with phase contrast optics at x1000 magnification.

TOC and N fluxes (mg/m2/day) from sediment trap samples from the Belcher Islands Archipelago, south-eastern Hudson Bay, Canada between April and July 2019 using an automated Gurney Instruments Baker-style, cylindrical sediment trap (aperture 0.032 m2). The sediment trap was deployed at 40-m depth, 30-m above the seafloor and the rotation for the trap was set to 8-day intervals.

Relative abundances (%) of diatoms from modern water column samples collected from the Belcher Islands Archipelago, south-eastern Hudson Bay, Canada between April and May 2019 using a plankton net (Ø 18 cm, 10 µm, 30-m net haul). Diatom quantification was done using a light microscope (Zeiss Axio Imager.A2 upright research microscope, University of Helsinki) with phase contrast optics at a x1000 magnification.