DOI: 10.5061/dryad.cz8w9gj8p
title: Marine diatoms record Late Holocene regime shifts in the Pikialasorsuaq ecosystem

description: 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 polynya
 during the Roman Warm Period and Medieval Climate Anomaly caused the
 diatom community to reorganize via shifts in species composition, with the
 presence of larger taxa but lower diversity, and significantly reduced
 export production. This study underlines the high sensitivity of primary
 producers to changes in the polynya dynamics and illustrates that the
 strong pulse of early-spring cryopelagic diatoms that makes the
 Pikialasorsuaq exceptionally productive may be jeopardized by rapid
 warming and associated Nares Strait ice arch destabilization. Future
 alterations to the phenology of primary producers may disproportionately
 impact higher trophic levels and keystone species in this region, with
 implications for Indigenous Peoples and global diversity. 
description: 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 <em>Amundsen</em>.
 <strong>Core
 chronology</strong>: The core chronology is based on 11 accelerator
 mass spectrometry (AMS) dates on mollusc shells from the Calypso core, and
 <sup>210</sup>Pb and <sup>137</sup>Cs 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
 years. In line with this reduced reservoir offset for the Marine 20 (vs.
 Marine13) calibration curve, and owing to the lack of a regional ΔR term
 for the polynya (Pieńkowski et al., 2023), no additional reservoir age
 correction (i.e., ΔR=0) was applied. A mixed age-depth model was
 constructed using the bacon-package in R (Blaauw & Christen,
 2011). Accordingly, the composite core covers the last ca. 3800 cal years
 BP. We note that the new calibration only resulted in negligible changes
 compared to the age model presented in Jackson et al. (2021). 
 <strong>Diatom
 analyses</strong>: Sediment samples for diatom analysis were
 prepared following the protocol described in Crosta et al. (2020).
 Approximately 0.3 g of dry sediment was treated with an oxidative solution
 composed of hydrogen peroxide (H2O2), distilled water and tetrasodium
 pyrophosphate (decahydrate,
 Na<sub>4</sub>O<sub>7</sub>P<sub>2</sub>-10H<sub>2</sub>O) in a warm bath (~65°C) for several hours until the reaction ceased. The residue was then rinsed repeatedly with distilled water by centrifugation (7 min at 1200 rpm). Hydrochloric acid (HCl, 30%) was used to remove the carbonate content. The residue was again rinsed several times until neutral pH, and microscopy slides were mounted in Naphrax©. In each sample, ca. 300 diatom valves were identified to the lowest taxonomic level possible. Resting spores of <em>Chaetoceros</em> were counted, but not included in the relative abundance calculations. Census counts were done using a light microscope (Olympus BX53, UNB) with dark field, phase contrast optics and oil immersion, at 1000X magnification. We followed the counting rules presented in Crosta and Koç (2007): specimens were counted when at least half of the valve was observed, with the exception of <em>Rhizosolenia</em> and <em>Thalassiothrix</em> taxa that were only counted when the spine-like proboscis or appendix was visible, respectively.
description: # 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<sup> </sup>of dry sediment. Fluxes were calculated by combining diatom concentrations (valves and spores g<sup>-1</sup>) with mass accumulation rates (g cm<sup>-2</sup> yr<sup>-1</sup>). ## Description of the data and file structure Diatom data are presented against depth and modelled age (years BP) in the sediment archive. ## Sharing/Access information n/a ## Code/Software n/a