DOI: 10.5061/dryad.g1jwstqv4
title: Data for: Unprecedented shift in Canadian High Arctic polar bear food web unsettles four millennia of stability

description: Stable carbon (δ13C) and nitrogen (δ15N) isotope analysis was conducted on
 modern and archaeological polar bear bone collagen from the Canadian
 Arctic Archipelago to investigate potential changes in polar bear foraging
 ecology over four millennia. Polar bear δ13C values showed a significant
 decline in the modern samples relative to all archaeological time-bins,
 indicating a disruption in the sources of production that support the food
 web, occurring after the Industrial Revolution. The trophic structure,
 indicated through δ15N, remained unaltered throughout all time periods.
 The lower δ13C observed in the modern samples indicates a change in the
 relative importance of pelagic (supported by open-water phytoplankton)
 over sympagic (supported by sea ice-associated algae) primary production.
 The consistency in polar bear δ13C through the late Holocene includes
 climatic shifts such as the Medieval Warm Period (MWP, A.D. 950–1250) and
 the early stages of the Little Ice Age (LIA, A.D. 1300–1850). These
 findings suggest that polar bears inhabit a food web that is more pelagic
 and less sympagic today than it was through the Late Holocene. We suggest
 that modern, anthropogenic warming has already affected food web structure
 in the Canadian Arctic Archipelago when modern data are contextualized
 with a deep time perspective.
description: Ancient bone samples were
 collected from 35 polar bears from 10 archaeological sites in the region
 inhabited by the Lancaster Sound polar bear subpopulation.
 The archaeological samples came from
 pre-Dorset (N= 10; 4000–2800 BP), Dorset (N= 15; 1500–700 BP), and Thule
 (N= 10; 700–500 BP) sites, and consisted of several different anatomical
 elements but each element represented a distinct individual. The
 archaeological samples could not be sexed or aged. Modern bone samples (N=
 11; 1998–2007 CE) were obtained from 11 individuals harvested within a 150
 km radius of the communities of Ikpiarjuk (Arctic Bay), Aujuittuq (Grise
 Fiord), and Qausuittuq (Resolute) between 1998 and 2007. The modern
 samples consisted of polar bear bacula, collected as a mandatory sample by
 subsistence hunters. The modern samples were, therefore, male and ranged
 in age from three to 8-years-old.  For the archaeological specimens, chunks of bone
 weighing ~200 mg were sampled using an NSK dental drill equipped with a
 diamond-tipped cutting wheel. Samples were demineralized in 16×100 mm
 glass culture tubes with 0.5 M HCl at room temperature. After the samples
 were demineralized they were rinsed three times with Type I water
 (resistivity &gt;18.2 MΩ·cm). The samples were treated with 0.1 M NaOH
 to remove humic contaminants from bones that exhibited a dark coloration.
 After 30 min, the NaOH was removed and samples were rinsed twice with Type
 I water. The samples were then placed in a solution of 0.01 M HCl and
 placed in a dry bath at 75˚C for 36 h, to solubilize the collagen. The
 collagen was extracted from the modern specimens using the same protocol
 with the following exceptions. First, powdered bone was removed from the
 samples using a Dremel equipped with a rotary burr. Because modern bone
 contains significant quantities of lipids whereas ancient bone does not,
 the samples were first treated with 2:1 chloroform:methanol under
 sonication for 1 h. The powdered samples were demineralized in 0.5 M HCl
 for 4 h. Demineralization in HCl does not alter the stable carbon or
 nitrogen isotopic composition of the bone collagen.
 The solution containing the
 solubilized collagen was then transferred into pre-weighed 4 ml glass
 vials and freeze-dried. Collagen samples (0.45−0.55 mg) were weighed into
 tin capsules for analysis with a EuroEA 3000 (Euro Vector SpA) Elemental
 Analyzer coupled to a Nu Horizon (Nu Instruments) continuous flow isotope
 ratio mass spectrometer at the Water Quality Centre at Trent University.
 Ten percent of samples were analyzed in duplicate to assess homogeneity.
 Analytical sessions were calibrated using international standards USGS40,
 USGS41a and USGS66. Accuracy and precision were assessed with in-house
 check standards: SRM-1 (caribou bone collagen), SRM-2 (walrus bone
 collagen), and SRM-14 (polar bear bone collagen). The standard uncertainty
 across analytical sessions was calculated to be ±0.13 ‰ for
 δ<sup>13</sup>C and ±0.24 ‰ for
 δ<sup>15</sup>N.
 <strong>Data
 Treatment</strong> Modern bone samples may be particularly prone to
 contamination with lipids under certain circumstances. Despite lipid
 extraction with a commonly used protocol, we observed a correlation
 between the atomic C:N ratios and the δ<sup>13</sup>C
 values of the collagen even though our highest C:N ratio was 3.30. This
 observation is consistent with increasing quantities of residual lipids
 causing lower δ<sup>13</sup>C
 values as C:N ratios rise. No additional collagen from the modern
 specimens was available for further chemical pretreatment after the
 initial analyses. We therefore applied a mathematical correction to the
 modern bone collagen δ<sup>13</sup>C
 values using the following equation:
 <strong>Equation
 1</strong><strong>:</strong> 
 δ<sup>13</sup>C<sub>lipid corrected</sub> =
 δ<sup>13</sup>C<sub>measured</sub> +
 (10.122)*(C:N<sub>measured</sub> -
 C:N<sub>expected</sub>)
 δ<sup>13</sup>C<sub>measured</sub> and C:N<sub>measured</sub> are the isotopic and elemental compositions determined for the sample prior to any adjustments. We used an expected C:N ratio for mammalian bone collagen of 3.23 (Guiry and Szpak, 2020), and 10.122 represents the slope of the regression line when plotting our modern C:N<sub>measured</sub> and δ<sup>13</sup>C<sub>measured</sub>. When comparing the δ<sup>13</sup>C values of modern and ancient samples, it is important to correct for the global decrease in δ<sup>13</sup>C of atmospheric and oceanic CO<sub>2</sub> since the beginning of the industrial revolution. A correction was calculated using Equation 2: <strong>Equation 2: </strong> ?<sup>13</sup>?<sub>?????</sub> = (?<sub>????? ????</sub>) × ? (? − 1850)<sup>?</sup> where α is the annual rate of decrease in δ<sup>13</sup>C specific to the water body (we used a value of 0.014 ‰ for the Northwest Atlantic), <em>y</em> is the year of sample harvest and <em>b</em> is the global oceanic decrease in δ<sup>13</sup>C (0.027 ‰). The carbon isotopic compositions of the modern samples have undergone correction through Equations 1 and 2 and the revised carbon isotope values are denoted δ<sup>13</sup>C<sub>corr</sub>. Both of these corrections have been applied to account for confounding causes of low δ<sup>13</sup>C values in modern samples to avoid an exaggeration of any differences between modern and ancient samples since both of these adjustments increase the δ<sup>13</sup>C values of the modern samples.