Marine diatoms record Late Holocene regime shifts in the Pikialasorsuaq ecosystem

The Pikialasorsuaq (North Water polynya) is an area of local and global cultural and ecological significance. However, over the last decades, the region has been subject to rapid warming and, in some recent years, the seasonal ice arch that has historically defined the polynya’s northern boundary has failed to form. Both factors are deemed to alter the polynya’s ecosystem functioning. To understand how climate-induced changes to the Pikialasorsuaq impact the basis of the marine food web, we explored diatom community-level responses to changing conditions, from a sediment core spanning the last 3800 years. Four metrics were used: total diatom concentrations, taxonomic composition, mean size, and diversity. Generalized additive model statistics highlight significant changes at ca. 2400, 2050, 1550, 1200, and 130 cal years BP, all coeval with known transitions between colder and warmer intervals of the Late Holocene, and regime shifts in the Pikialasorsuaq. Notably, a weaker/contracted pol..., A Calypso Square gravity core AMD15-Casq1 (543 cm) and corresponding box core (40 cm) were collected in 2015 from the central north NOW (77°15.035’ N, 74°25.500’ W, 692 m water depth) (Figure 1) during the ArcticNet Leg 4a, onboard the Canadian Coast Guard Ship Amundsen.
Core chronology: The core chronology is based on 11 accelerator mass spectrometry (AMS) dates on mollusc shells from the Calypso core, and 210Pb and 137Cs measurements on 20 samples from the box core (see Jackson et al. (2021) for more details). Here, all radiocarbon dates were calibrated using the latest marine calibration curve (Marine20; Heaton et al., 2020; Table S1). In Jackson et al. (2021), and using the Marine13 calibration curve, a local reservoir correction of 140 ± 60 years was applied based on measurements from a live marine mollusc specimen collected from the NOW before the mid-1950’s (McNeely & Brennan, 2005). Using the Marine20 calibration curve, this specimen now yields a reservoir offset of –4 ± 60 ..., , # Marine diatoms record Late Holocene regime shifts in the Pikialasorsuaq ecosystem

[https://doi.org/10.5061/dryad.cz8w9gj8p](https://doi.org/10.5061/dryad.cz8w9gj8p)

This dataset includes diatom counts (relative abundances, %) from core AMD15-Casq1. Diatoms were analyzed at a 1 to 10 cm sampling interval, which corresponds to an effective age resolution ranging from ca. 3 to 64 years (mean: 31 years). Absolute abundances are reported in valves per g of dry sediment. Fluxes were calculated by combining diatom concentrations (valves and spores g-1) with mass accumulation rates (g cm-2 yr-1).

## Description of the data and file structure

Diatom data are presented against depth and modelled age (years BP) in the sediment archive.

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