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The presence of phytoplankton parasites along the water column was explored at the Long Term Ecological Station MareChiara (LTER-MC) in the Gulf of Naples (Mediterranean Sea) in October 2019. Microscopy analyses showed diatoms dominating the phytoplankton community in the upper layers (0-20 m). Metabarcoding data from the water column showed the presence of Chytridiomycota predominantly in the upper layers coinciding with the vertical distribution of diatoms. Laboratory incubations of natural samples enriched with different diatom cultures confirmed parasitic interactions of some of those chytrids – including members of Kappamyces – with diatom taxa. The temporal dynamics of diatoms and chytrids was also explored in a three-year metabarcoding time-series (2011-2013) from surface waters of the study area and in sediment samples. Chytrids were recurrently present at low relative abundances, and some taxa found to infect diatoms in the incubation experiments were also identified in the ASV time-series. However, co-occurrence analyses did not show any clear or recurrent pairing patterns for chytrid and diatom taxa along the three years. The chytrid community in the sediments showed a clearly different species composition compared to the recorded in the water column samples, with higher diversity and relative abundance. The combination of observations, incubations and metabarcoding confirmed that parasites are a common component of marine protist communities at LTER-MC. Host-parasite interactions must be determined and quantified to understand their role and the impact they have on phytoplankton dynamics File1: VERDI_samples_parameters.xlsx - Physico-chemical variables obtained from CTD profile - Inorganic nutrients concentrations - Chlorophyll-a concentrations - Organic carbon and nitrogen concentrations - Phytoplankton abundances - Detections of chytrids File 2: VERDI_asv_table.tbl: ASV abundances from natural samples and incubations File 3: VERDI_tax_table.tbl: Taxonomic assignments of ASVs File 4: VERDI_asv_seqs.fa: Sequences of ASVs File 5: VERDI_incubations_images.zip - Compilation of images taken during incubations with diatoms - Physico-chemical variables obtained from CTD profile - Inorganic nutrients concentrations - Chlorophyll-a concentrations - Organic carbon and nitrogen concentrations - Phytoplankton abundances - Detections of chytrids - Metabarcoding ASV abundances from natural samples and incubations - Metabarcoding Taxonomic assignments of ASVs - Metabarcoding Sequences of ASVs - Compilation of images taken during incubations with diatoms - European Union’s Horizon 2020 research and innovation programme under grant agreement No 730984, ASSEMBLE Plus project. - Spanish MICINN Project SMART (PID2020-112978GB-I00) - The research program LTER-MC is funded by the Stazione Zoologica Anton Dohrn Peer reviewed

The database provides discrete measurements of carbon system parameters in water samples collected at 3 stations that form the marine time series GIFT during 33 oceanographic campaigns conducted over 2005–2021. Geographic coordinates of sampling stations are provided. Some physical data (i.e. pressure, temperature and salinity) are also included. Moreover, pH data obtained with a SAMI-pH sensor (Sunburst Sensors, LLC)) attached to a mooring line deployed in the Strait of Gibraltar for the years 2016 and 2017 are provided. During the cruises, a temperature and salinity profile was obtained with a Seabird 911Plus CTD probe. Seawater was subsequently collected for biogeochemical analysis using Niskin bottles immersed in an oceanographic rosette platform at variable depths (from 5 to 8 levels) depending on the instant position of the interface between the Atlantic and Mediterranean flows that was identified by CTD profiles. The biogeochemical variables shown in the database are pH in total scale at 25 °C (pHT25), total alkalinity (AT), and inorganic nutrients (phosphate, PO43and Silicate, SiO44−). pHT25 data were obtained by the spectrophotometric method with m-cresol purple as the indicator (Clayton & Byrne 1993). Samples were taken directly from the oceanographic bottles in 10 cm path-length optical glass cells and measurements were carried out with a Shimadzu UV-2401PC spectrophotometer containing a 25 °C-thermostated cells holder. Samples for AT analysis were collected in 500-ml borosilicate bottles, and poisoned with 100 μl of HgCl2-saturated aqueous solution and stored until measurement in the laboratory. AT was measured by potential titration according to Mintrop et al. (2000) with a Titroprocessor (model Metrohm 794 from 2005-2020 and model Metrothm 888 for 2021). Water samples (5 mL, two replicates) for inorganic nutrients determination were taken, filtered immediately (Whatman GF/F, 0.7 μm) and stored frozen for later analyses in the shore-based laboratory. Nutrients concentrations were measured with a continuous flow auto-analyzer using standard colorimetric techniques (Hansen & Koroleff 1999). 2. Methods for processing the data: 3. Instrument- or software-specific information needed to interpret/reproduce the data, please indicate their location: 4. Standards and calibration information, if appropriate: 5. Environmental/experimental conditions: 6. Describe any quality-assurance procedures performed on the data: 7. People involved with sample collection, processing, analysis and/or submission, please specify using CREDIT roles https://casrai.org/credit/: Chief Scientists -I.Emma Huertas/Susana Flecha; Hydro: Who -Susana Flecha/David Roque/Silvia Amaya-Vías/Angélica Enrique; Nuts: Who -Manuel Arjonilla/ Status - final; Silicate and Phosphate Autoanalizer Hansen and Koroleff (1999) This research was supported by the COMFORT project that has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 820989 (project COMFORT, "Our common future ocean in the Earth system – quantifying coupled cycles of carbon, oxygen, and nutrients for determining and achieving safe operating spaces with respect to tipping points).” Funding was also provided by the European projects CARBOOCEAN (FP6-511176), CARBOCHANGE (FP7-264879), PERSEUS (FP7-287600) and the Junta de Andalucía TECADE project (PY20_00293). The dataset is subject to a Creative Commons License Attribution-ShareAlike 4.0 International. F.F.P. was supported by the BOCATS2 (PID2019-104279GB-C21) project funded by MCIN/AEI/10.13039/501100011033. SAV was supported by a pre-doctoral grant FPU19/04338 from the Spanish Ministry of Science, Innovation and Universities. Peer reviewed

[Methods] Experiment locations and climate Trans-Mediterranean translocation of Posidonia oceanica fragments took place between Catalunya (Spain), Mallorca (Spain) and Cyprus in July 2018 and were monitored until July 2019 (Fig. 1). Sea surface temperature data for each transplant site were based on daily SST maps with a spatial resolution of 1/4°, obtained from the National Center for Environmental Information (NCEI, https://www.ncdc.noaa.gov/oisst ) (Reynolds et al. 2007). These maps have been generated through the optimal interpolation of Advanced Very High Resolution Radiometer (AVHRR) data for the period 1981-2019. Underwater temperature loggers (ONSET Hobo pro v2 Data logger) were deployed at the transplant sites in Catalunya, Mallorca and Cyprus and recorded hourly temperatures throughout the duration of the experiment (one year). In order to obtain an extended time series of temperature at transplant sites, a calibration procedure was performed comparing logger data with sea surface temperature from the nearest point on SST maps. In particular, SST data were linearly fitted to logger data for the common period. Then, the calibration coefficients were applied to the whole SST time series to obtain corrected-SST data and reconstruct daily habitat temperatures from 1981-2019. Local climate data was also compared to the global thermal distribution of P. oceanica to assess how representative experimental sites were of the thermal distribution of the species (Supplementary materials). Collectively, seawater temperatures from the three locations span the 16th - 99th percentile of temperatures observed across the global thermal distribution of P. oceanica. As such Catalunya, Mallorca and Cyprus are herein considered to represent the cool-edge, centre and warm-edge of P. oceanica distribution, respectively. Transplantation took place toward warmer climates and procedural controls were conducted within each source location, resulting in six source-to-recipient combinations (i.e. treatments, Fig. 1). Initial collection of P. oceanica, handling and transplantation was carried out simultaneously by coordinated teams in July 2018 (Table S1). Each recipient location was subsequently resampled four times over the course of the experiment, in August/September 2018 (T1), October 2018 (T2), April 2019 (T3) and May/June 2019 (T4, Table S1). Between 60-100 fragments were collected for each treatment. A fragment was defined as a section of P. oceanica containing one apical shoot connected with approximately five vertical shoots by approximately 10-15 cm of rhizome with intact roots. Collection occurred at two sites within each location, separated by approximately 1 km. Within sites, collections were conducted between 4 – 5 m depth and were spaced across the meadow to minimise the dominance of a single clone and damage to the meadow. Upon collection, fragments were transported for up to one hour back to the nearest laboratory in shaded seawater. Handling methods In the laboratory, fragments were placed into holding tanks with aerated seawater, at ambient temperature and a 14:10 light-dark cycle. All shoots were clipped to 25 cm length (from meristem to the tip of the longest leaves), to standardise initial conditions and reduce biomass for transportation. For transport by plane or ferry between locations, fragments were packed in layers within cool-boxes. Each layer was separated by frozen cool-packs wrapped in wet tea towels (rinsed in sea water). All fragments spent 12 hrs inside a cool-box irrespective of their recipient destination, including procedural controls (i.e. cool-cool, centre-centre and warm-warm) to simulate the transit times of the plants travelling furthest from their source location (Fig. 1a). On arrival at the destination, fragments were placed in holding tanks with aerated seawater at ambient temperature as described above in their recipient location for 48 hrs, prior to field transplantation. Measurement methods One day prior to transplantation, fragments were tagged with a unique number and attached to U-shaped peg with cable-ties. Morphological traits for each fragment were measured and included: 1) length of the longest apical leaf, width and number of leaves 2) total number of bite marks on leaves of three vertical shoots per fragment, 3) number of vertical shoots, 4) leaf count of three vertical shoots per fragment and 5) overall horizontal rhizome length. A subset (n=10) of fragments per treatment were marked prior transplantation to measure shoot growth. To do this, all shoots within a single fragment were pierced using a hypodermic needle. Two holes were pierced side-by-side at the base of the leaf/top of the meristem. Transplant methods All transplant sites were located in 4 – 5 m depth in area of open dead-matte, surrounded by P. oceanica meadow. In Mallorca and Cyprus, fragments were distributed between two sites, separated by approximately 1 km. In Catalunya, a lack of suitable dead matte habitat, meant that all fragments were placed in one site. Fragments were planted along parallel transects at 50 cm intervals and with a 50 cm gap between parallel transects (Fig. S1). Different treatments were mixed and deployed haphazardly along each transect. Resampling methods and herbivory On day 10 of the experiment, a severe herbivory event was recorded at both warm-edge translocation sites. Scaled photos of all fragments were taken at this time to record the effects of herbivory on transplants. At the end of each main sampling period (T0 – T1, T1-T2 and T3 – T4), all pierced fragments were collected and taken back to the laboratory to measure shoot growth. At T1, T2 and T3, additional sets of fragments (n = 10 per treatment) were marked using the piercing method to record growth in the subsequent time period. In addition, at T1 and T3, n = 20 shoots within the natural meadow at each site were marked to compare growth rates between the native meadow and transplants. Underwater shoot counts and a scaled photo was taken to record fragment survivorship, shoot mortality, bite marks, and shoot length among all remaining fragments within each site and sampling time. In the laboratory, morphological measurements (described above) were repeated on the collected fragments and growth of transplant and natural meadow shoots was measured. Growth (shoot elongation, cm d-1) of the marked shoots was obtained by measuring the length from the base of meristem to marked holes of each leaf (new growth) of the shoot and dividing the leaf elongation per shoot by the marking period (in days). For each shoot, total leaf length (cm shoot-1) and the number of new leaves was also recorded. The rate of new leaf production (new leaves shoot-1 d-1) was estimated dividing the number of new leaves produced per shoot and the marking period. New growth was dried at 60 ºC for 48 hrs to determine carbon and nitrogen content of the leaves, and carbon to nitrogen (C:N) ratios. Carbon and nitrogen concentrations in the new growth leaf tissue was measured at the beginning of the experiment and each subsequent time point for each treatment. Nutrient analyses were conducted at Unidade de Técnicas Instrumentais de Análise (University of Coruña, Spain) with an elemental analyser FlashEA112 (ThermoFinnigan). Underwater photos of shoots were analysed using ImageJ software (https://imagej.net). Maximum leaf length on each shoot in warm-edge transplant sites (cool-warm, centre-warm and warm-warm) were recorded for the initial (day 10) herbivore impact, T1, T2 and T3 time-points and related to transplant nutrient concentrations. Herbivore impact was estimated as the proportional change in length of the longest leaf relative to initial length at T0. Thermal stress Long term maximum temperatures were recorded as the average of annual maximum daily temperatures in each transplant site, averaged between years from 1981-2019. Maximum thermal anomalies were calculated as the difference between daily temperatures in a recipient site over the course of the experiment and the long-term maximum temperature in the source site for each corresponding population. ‘Heat stress’ and ‘recovery’ growth periods of the experiment were defined as T0 -T2 (July-October) and T2-T4 (November-June), respectively, corresponding to periods of positive and negative maximum thermal anomalies. Thermal anomalies experienced by the different transplant treatments were plotted using the ‘geom_flame’, function in the ‘HeatwavesR’ package (Schlegel & Smit 2018) of R (version 3.6.1, 2019) . 1. The prevalence of local adaptation and phenotypic plasticity among populations is critical to accurately predicting when and where climate change impacts will occur. Currently, comparisons of thermal performance between populations are untested for most marine species or overlooked by models predicting the thermal sensitivity of species to extirpation. 2. Here we compared the ecological response and recovery of seagrass populations (Posidonia oceanica) to thermal stress throughout a year-long translocation experiment across a 2800 km gradient in ocean climate. Transplants in central and warm-edge locations experienced temperatures >29 ºC, representing thermal anomalies >5ºC above long-term maxima for cool-edge populations, 1.5ºC for central and <1ºC for warm-edge populations. 3. Cool, central and warm-edge populations differed in thermal performance when grown under common conditions, but patterns contrasted with expectations based on thermal geography. Cool-edge populations did not differ from warm-edge populations under common conditions and performed significantly better than central populations in growth and survival. 4. Our findings reveal that thermal performance does not necessarily reflect the thermal geography of a species. We demonstrate that warm-edge populations can be less sensitive to thermal stress than cooler, central populations suggesting that Mediterranean seagrasses have greater resilience to warming than current paradigms suggest. Australian Research Council, Award: DE200100900. Horizon 2020 Framework Programme, Award: 659246. Fundación BBVA. Peer reviewed

The values of natural abundance of stable isotopes were measured in 13 micronekton fish species sampled during the MAFIA cruise (North Atlantic, April 2015). This dataset contains the values obtained for carbon and nitrogen in bulk tissues, and nitrogen values in amino acids. Length data and the number of individuals analysed for each species are also provided. Mesopelagic fishes were collected using a ''Mesopelagos” net (5x7 m mouth opening, 58 m total lenght) equipped with graded-mesh netting (starting with 30 mm and ending with 4 mm) and a multi-sampler for collecting samples from 5 different depth layers (Olivar et al., 2017). For C:N and stable isotope analyses, individual fish were eviscerated, freeze-dried and weighted. Aliquots of muscular tissue (or whole individuals for species of small size) were analyzed in an elemental analyzer (bulk tissues, Olivar et al., 2019) or a gas chromatograph (derivatized amino acids, Mompeán et al., 2016) coupled to isotope-ratio mass spectrometers. This research was funded by projects MAFIA (CTM2012-39587-C04), BATHYPELAGIC (CTM2016-78853-R), and QLOCKS (PID2020-115620RB-100) from the Plan Estatal de I+D+I (Spain), projects SUMMER (Grant Agreement 817806) and TRIATLAS (Grant Agreement 817578), from the European Union (Horizon 2020 Research and Innovation Programme), and the support through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S).

The values of natural abundance of stable isotopes were measured in 13 micronekton fish species sampled during the BATHYPELAGIC cruise (North Atlantic, June 2018). This dataset contains the values obtained for carbon and nitrogen in bulk tissues, and nitrogen values in amino acids. Length and biomass data for each individual analyzed are also provided. Fishes were collected using a ''Mesopelagos” net (5x7 m mouth opening, 58 m total lenght) equipped with graded-mesh netting (starting with 30 mm and ending with 4 mm) and a multi-sampler for collecting samples from 5 different depth layers (Olivar et al., 2017). Individual fish were eviscerated, freeze-dried and weighted. Aliquots of muscular tissue (or whole individuals for species of small size) were analyzed in an elemental analyzer (bulk tissues, Olivar et al., 2019) or a gas chromatograph (derivatized amino acids, Mompeán et al., 2016) coupled to isotope-ratio mass spectrometers. Carbon analyses were made before and after removal of lipids with a mixture of trichloromethane:methanol:water. This research was funded by projects BATHYPELAGIC (CTM2016-78853-R) from the Plan Estatal de I+D+I (Spain), SUMMER (Grant Agreement 817806) and TRIATLAS (Grant Agreement 817578), from the European Union (Horizon 2020 Research and Innovation Programme), and Grant Number IN607A2018/2 from the Axencia Galega de Innovación (GAIN, Xunta de Galicia, Spain).

The extracellular concentration of H2O2 in surface aquatic environments is controlled by a balance between photochemical production and the microbial synthesis of catalase and peroxidase enzymes to remove H2O2 from solution. In any kind of incubation experiment, the formation rates and equilibrium concentrations of reactive oxygen species (ROSs) such as H2O2 may be sensitive to both the experiment design, particularly to the regulation of incident light, and the abundance of different microbial groups, as both cellular H2O2 production and catalase–peroxidase enzyme production rates differ between species. Whilst there are extensive measurements of photochemical H2O2 formation rates and the distribution of H2O2 in the marine environment, it is poorly constrained how different microbial groups affect extracellular H2O2 concentrations, how comparable extracellular H2O2 concentrations within large-scale incubation experiments are to those observed in the surface-mixed layer, and to what extent a mismatch with environmentally relevant concentrations of ROS in incubations could influence biological processes differently to what would be observed in nature. Here we show that both experiment design and bacterial abundance consistently exert control on extracellular H2O2 concentrations across a range of incubation experiments in diverse marine environments. During four large-scale (>1000 L) mesocosm experiments (in Gran Canaria, the Mediterranean, Patagonia and Svalbard) most experimental factors appeared to exert only minor, or no, direct effect on H2O2 concentrations. For example, in three of four experiments where pH was manipulated to 0.4–0.5 below ambient pH, no significant change was evident in extracellular H2O2 concentrations relative to controls. An influence was sometimes inferred from zooplankton density, but not consistently between different incubation experiments, and no change in H2O2 was evident in controlled experiments using different densities of the copepod Calanus finmarchicus grazing on the diatom Skeletonema costatum (<1 % change in [H2O2] comparing copepod densities from 1 to 10 L−1). Instead, the changes in H2O2 concentration contrasting high- and low-zooplankton incubations appeared to arise from the resulting changes in bacterial activity. The correlation between bacterial abundance and extracellular H2O2 was stronger in some incubations than others (R2 range 0.09 to 0.55), yet high bacterial densities were consistently associated with low H2O2. Nonetheless, the main control on H2O2 concentrations during incubation experiments relative to those in ambient, unenclosed waters was the regulation of incident light. In an open (lidless) mesocosm experiment in Gran Canaria, H2O2 was persistently elevated (2–6-fold) above ambient concentrations; whereas using closed high-density polyethylene mesocosms in Crete, Svalbard and Patagonia H2O2 within incubations was always reduced (median 10 %–90 %) relative to ambient waters.

The speciation of dissolved iron (DFe) in the ocean is widely assumed to consist almost exclusively of Fe(III)-ligand complexes. Yet in most aqueous environments a poorly defined fraction of DFe also exists as Fe(II), the speciation of which is uncertain. Here we deploy flow injection analysis to measure in situ Fe(II) concentrations during a series of mesocosm/microcosm/multistressor experiments in coastal environments in addition to the decay rate of this Fe(II) when moved into the dark. During five mesocosm/microcosm/multistressor experiments in Svalbard and Patagonia, where dissolved (0.2 µm) Fe and Fe(II) were quantified simultaneously, Fe(II) constituted 24 %–65 % of DFe, suggesting that Fe(II) was a large fraction of the DFe pool. When this Fe(II) was allowed to decay in the dark, the vast majority of measured oxidation rate constants were less than calculated constants derived from ambient temperature, salinity, pH, and dissolved O2. The oxidation rates of Fe(II) spikes added to Atlantic seawater more closely matched calculated rate constants. The difference between observed and theoretical decay rates in Svalbard and Patagonia was most pronounced at Fe(II) concentrations <2 nM, suggesting that the effect may have arisen from organic Fe(II) ligands. This apparent enhancement of Fe(II) stability under post-bloom conditions and the existence of such a high fraction of DFe as Fe(II) challenge the assumption that DFe speciation in coastal seawater is dominated by ligand bound-Fe(III) species.

Glider vehicles are now perhaps some of the most prolific providers of real-time and near-real-time operational oceanographic data. However, the data from these vehicles can and should be considered to have a long-term legacy value capable of playing a critical role in understanding and separating inter-annual, inter-decadal, and longterm global change. To achieve this, we have to go further than simply assuming the manufacturer’s calibrations, and field correct glider data in a more traditional way, for example, by careful comparison to water bottle calibrated lowered CTD datasets and/or “gold” standard recent climatologies. In this manuscript, we bring into the 21st century a historical technique that has been used manually by oceanographers for many years/decades for field correction/inter-calibration, thermal lag correction, and adjustment for biological fouling. The technique has now been made semi-automatic for machine processing of oceanographic glider data, although its future and indeed its origins have far wider scope. The subject of this manuscript is drawn from the original Description of Work (DoW) for a key task in the recently completed JERICO-NEXT (Joint European Research Infrastructure network for Coastal Observatories) EU-funded program, but goes on to consider future application and the suitability for integration with machine learning. Refereed 14.A Sea surface salinity Subsurface salinity TRL 8 Actual system completed and "mission qualified" through test and demonstration in an operational environment (ground or space) Manual (incl. handbook, guide, cookbook etc) Standard Operating Procedure 2019-12-03

Predators can induce changes in the diel activity patterns of marine copepods. Besides vertical migration, diel feeding rhythms have been suggested as an antipredator phenotypic response. We conducted experiments to assess the non-lethal direct effects of the predator Meganyctiphanes norvegica (northern krill) on the diel feeding patterns of the calanoid copepod Centropages typicus. We also analysed the influence of seasonal photoperiod and prey availability on the intensity of copepod feeding rhythms. We did not detect any large effect of krill presence on the diel feeding behaviour of copepods, either in day-night differences or total daily ingestions. Seasonal photoperiod and prey availability, however, significantly affected the magnitude of copepod feeding cycles, with larger diel differences in shorter days and at lower prey concentrations. Therefore, the role of non-lethal direct effects of predators on the diel feeding activity of marine copepods remain debatable and might not be as relevant as in freshwater zooplankton The research leading to these results received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 730984, ASSEMBLE Plus project. The open access publication fee was covered by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI). This study was framed within the project FERMI (CGL2014–59227-R, MINECO/AEI/FEDER, UE), and M.O. was supported financially by an FPU grant (FPU15/01747) from the Spanish Ministry of Education, Culture and Sport Number of experiment, date of experiment, temperature, bottle volume, prey species, prey species AphiaID, copepod species, copepod species AphiaID, predator species, predator species AphiaID, day/night, predator/no predator, prey concentration, prey size, number of copepods per bottle, copepod size, number of predators per bottle, predator size, incubation time, number of copepod faecal pellets, faecal pellet volume Peer reviewed

Although it has been demonstrated that the speed and magnitude of the recent Arctic sea ice decline is unprecedented for the past 1450 years, few records are available to provide a paleoclimate context for Arctic sea ice extent. Bromine enrichment in ice cores has been suggested to indicate the extent of newly formed sea ice areas. Despite the similarities among sea ice indicators and ice core bromine enrichment records, uncertainties still exist regarding the quantitative linkages between bromine reactive chemistry and the first-year sea ice surfaces. Here we present a 120 000-year record of bromine enrichment from the RECAP (REnland ice CAP) ice core, coastal east Greenland, and interpret it as a record of first-year sea ice. We compare it to existing sea ice records from marine cores and tentatively reconstruct past sea ice conditions in the North Atlantic as far north as the Fram Strait (50–85∘ N). Our interpretation implies that during the last deglaciation, the transition from multi-year to first-year sea ice started at ∼17.5 ka, synchronously with sea ice reductions observed in the eastern Nordic Seas and with the increase in North Atlantic ocean temperature. First-year sea ice reached its maximum at 12.4–11.8 ka during the Younger Dryas, after which open-water conditions started to dominate, consistent with sea ice records from the eastern Nordic Seas and the North Icelandic shelf. Our results show that over the last 120 000 years, multi-year sea ice extent was greatest during Marine Isotope Stage (MIS) 2 and possibly during MIS 4, with more extended first-year sea ice during MIS 3 and MIS 5. Sea ice extent during the Holocene (MIS 1) has been less than at any time in the last 120 000 years.