DOI: 10.5061/dryad.7h44j1023
title: Amplified seasonality in western Europe in a warmer world

description: Documenting the seasonal temperature cycle constitutes an essential step
 toward mitigating risks associated with extreme weather events in a future
 warmer world. The mid-Piacenzian Warm Period (mPWP), 3.3 to 3.0 million
 years ago, featured global temperatures approximately 3°C above
 preindustrial levels. It represents an ideal period for directed
 paleoclimate reconstructions equivalent to model projections for 2100
 under moderate Shared Socioeconomic Pathway SSP2-4.5. Here, seasonal
 clumped isotope analyses of fossil mollusk shells from the North Sea are
 presented to test Pliocene Model Intercomparison Project 2 outcomes. Joint
 data and model evidence reveals enhanced summer warming (+4.3° ± 1.0°C)
 compared to winter (+2.5° ± 1.5°C) during the mPWP, equivalent to SSP2-4.5
 outcomes for future climate. We show that Arctic amplification of global
 warming weakens mid-latitude summer circulation while intensifying
 seasonal contrast in temperature and precipitation, leading to an
 increased risk of summer heat waves and other extreme weather events in
 Europe’s future.
description: Specimen collection Seven
 fossil mollusk specimens were collected by one of the authors (S.G.) for
 analysis from temporary exposures of the Oorderen Member of the Lillo
 Formation in the Antwerp area (Belgium; see Fig. 1). These were specimens of <em>A. benedeni benedeni</em> (specimen ID: SG126 and SG127) from Deurganckdoksluis (51°16′49″N, 4°14′56″E; collected in 2013), <em>P. complanatus</em> (SG115) and <em>Glycymeris radiolyrata</em> (SG116) from Deurganckdok (51°17′24″N, 4°15′37″E; collected on 2 February 2001), and <em>Pygocardia rustica</em> (SG105), <em>Ostrea edulis</em> (SG113), and <em>Arctica islandica</em> (SG107) from Verrebroekdok (51°16′16″N, 4°12′53″E; collected in 1999–2000). At these localities, the Oorderen Member, from bottom to top, is divided into basal shell bed, <em>Atrina</em> level, <em>Cultellus</em> level (SG115 and SG116), and <em>benedeni</em> level (SG126 and SG127) (<em>37</em>). Specimens SG105, SG113, and SG107 were collected ex situ within the Oorderen Member. The estimated age range for the Oorderen Member is 2.72 to 3.3 Ma (see Fig. 1) (<em>35</em>, <em>67</em>). Paleoenvironmental context Field observations and a detailed assessment of the composition of the invertebrate fauna reveal that the fossil-bearing sediments of the Oorderen Member were deposited between 30 and 50 m water depth during warm, highstand intervals (<em>35</em>, <em>36</em>). Previous studies have argued that the SNS undergoes summer stratification during the mPWP, which would cause molluscan shell reconstructions to underestimate summer temperatures (<em>24</em>, <em>47</em>). However, the lack of dysoxic faunas in the Oorderen Member suggests the absence of persistent stratification (<em>35</em>). Furthermore, the presence of sedimentary structures indicative of tidal currents and strong vertical mixing during deposition of the Oorderen Member argues against a large surface to seafloor temperature gradient (<em>39</em>). Therefore, it is likely that the mollusks recorded temperatures close to the SSTs year round. This assessment is supported by the evidence of warm spring and cool autumn temperatures, suggesting a direct response of the water temperature experienced by the mollusks to the seasonal cycle in SATs (see Results; Fig. 3). However, we cannot fully exclude that spatially and temporally restricted temperature stratification did occur during the lifetime of the studied specimens (<em>24</em>, <em>47</em>). Note that, even if summer stratification occurred, the summer temperatures recorded by the mollusks would underestimate the true mPWP summer SST and the occurrence of enhanced summer warming during the mPWP would still be supported by the data. Specimen preparation Shells were partially embedded in epoxy resin before polished thick sections were prepared to expose a cross section through the axis of maximum shell growth [following (<em>68</em>); see fig. S3]. Preservation of the original shell calcite and aragonite was verified using a combination of scanning electron microscopy (SEM), cathodoluminescence microscopy, electron backscatter diffraction microscopy (EBSD), micro–x-ray fluorescence (μXRF), and x-ray diffraction [XRD; see (<em>67</em>) and figs. S4 to S12]. Only shells with excellent preservation as demonstrated by these methods were considered for clumped isotope analysis. Geochemical analysis Carbonate was sampled along transects in the direction of growth in transects on cross sections through the outer shell layers of all shells except <em>P. complanatus</em> (SG115) and <em>A. benedeni benedeni</em> (SG126 and SG127), whose thin outer shell layers necessitated sampling on the outside of the shell. A combination of handheld drilling (outside) and micromilling (in transects) was used to sample along narrow growth increments to minimize time averaging (Supplementary Materials). The number of replicates per specimen ranges from 61 to 157 (Fig. 2): SG105, <em>n</em> = 98; SG107, <em>n</em> = 63; SG113, <em>n</em> = 90; SG115, <em>n</em> = 100; SG116, <em>n</em> = 157; SG126, <em>n</em> = 79; SG127, <em>n</em> = 61. Some samples were replicated more than once, causing multiple replicates to originate from the same location in the shell, and some of these replicate measurements were discarded as outliers during data processing (see below). Measurements from <em>A. benedeni benedeni</em> (SG126 and SG127) include data from (<em>67</em>) as well as additional analyses on the same specimens carried out for this study. Small (70 to 160 μg) aliquots of each sample were digested in phosphoric acid for 600 s at 70°C in a Kiel IV carbonate preparation device, after which the resultant CO<sub>2</sub> gas was purified using two cold fingers and a Porapak trap (<em>69</em>) before the clumped isotope composition (Δ<sub>47</sub>) was analyzed using two Thermo MAT253 mass spectrometers. A long-integration dual-inlet (LIDI) workflow (<em>69</em>, <em>70</em>) was used for all except 53 aliquots from <em>P. rustica</em> (SG105); the latter aliquots were measured using sample-standard measurement cycles [“click-clack” mode; e.g., (<em>71</em>)]. Clumped isotopic ratios were corrected for intensity-based Δ<sub>47</sub> offsets based on intensity-matched ETH-3 standards (see section S3.1) before being standardized using the ETH standards (<em>72</em>), which were run in approximately 1:1 ratio with the samples (<em>73</em>) and corrected for <sup>17</sup>O concentration following (<em>74</em>). Long-term analytical precision was monitored using IAEA-C2 and Merck reference materials (typical standard deviation of 0.04‰; see table S2) after outlier removal based on metadata on instrument performance (see criteria in section S3.2). Seasonality reconstructions For each mollusk specimen, samples were internally dated relative to the seasonal cycle using ShellChron (<em>75</em>), after which measurements within a specimen were grouped in four 3-monthly bins (“seasons”). Summer was defined as the consecutive 3-month period with the lowest mean Δ<sub>47</sub> value, calculated separately for each specimen (see section S3.2 and figs. S20 to S32). The fact that these 3-monthly bins do not always line up with the minima and maxima in the stable oxygen isotope profiles through the shells shows that the common assumption that these extreme stable oxygen isotope values represent summer and winter seasons does not always hold true [see (<em>76</em>)]. Mean seasonal Δ<sub>47</sub> values were obtained through weighted averages of seasonal means and uncertainties considering uncertainty within and variability between specimens. Seasonal temperatures during the mPWP were reconstructed from Δ<sub>47</sub> for each seasonal group using the clumped isotope calibration by (<em>77</em>), propagating uncertainty on Δ<sub>47</sub> values following the procedure described in the supplement of (<em>78</em>). Climate model output Seasonal SST from the SNS was extracted from the corresponding local ocean grid cells (51°N to 55°N, 2°E to 4°E) in the PlioMIP2 model ensemble (see Fig. 2). SNS SST records were obtained from the same area within the ERSSTv5 product (<em>41</em>) and supplemented with local SST observations from the NIOZ monitoring station (<em>42</em>). Future projections matching the SSP2-4.5 scenario (<em>48</em>) were produced for CMIP6, and the regional SST outcomes were exported from the Intergovernmental Panel on Climate Change (IPCC) WG1 Interactive Atlas (<em>79</em>). Seasons were defined as weighted means of the 3 months containing the largest number of days in the season according to the astronomical definition: winter: January to March; spring: April to June; summer: July to September; autumn: October to December. Data processing was carried out using the open-source computational software R (<em>80</em>), and scripts are provided in (<em>81</em>).
description: # Amplified seasonality in western Europe in a warmer world. [Access this
 dataset on
 Dryad](http://datadryad.org/stash/share/aVcLka9SWsPNYZ1eBxtF24Gd4G2OAo9PJylgEhprWs8) This upload contains all data used for the manuscript "Amplified seasonality in western Europe in a warmer world" submitted for publication in Science Advances. ## Description of the data and file structure This upload contains files containing unprocessed data (in folders) as well as the outcomes of various steps of the data processing workflow (indicating by numbers in the names of the files). The dataset also contains a supplementary text file which explains the data processing workflow in detail. Missing data in the CSV and XLSX files are indicated with "NA". Dates are supplied in the format "dd/mm/yyyy hh:mm", in which d = day, m = month, y = year, h = hour and m = minute. Times are according to the European Central Time zone and in 24h format. ## Sharing/Access information This is a section for linking to other ways to access the data, and for linking to sources the data is derived from, if any. Links to other publicly accessible locations of the data: * [Eartharxiv](https://eartharxiv.org/repository/view/6399/) * [Zenodo](https://doi.org/10.5281/zenodo.8227319) Data was derived from the following sources: * [NOAA erSST](https://www.ncei.noaa.gov/products/extended-reconstructed-sst) * [PLIOMIP2](https://cp.copernicus.org/articles/12/663/2016/) * [CMIP6](https://pcmdi.llnl.gov/CMIP6/) * [NIOZ Monitoring Platform](https://www.nioz.nl/en/expertise/wadden-delta-research-centre/data-tools/long-term-ecological-time-series/sea-water-temperature) ## Code/Software All data processing code in this data produce are produced in the open-source computational software R. Scripts for all data processing steps are provided in this upload and include comments describing the processing steps.