Seasonal sea ice persisted through the Holocene Thermal Maximum at 80°N

The study addresses how Arctic sea ice responded to warmer-than-present conditions by reconstructing sea-ice cover during the Early Holocene, specifically the Holocene Thermal Maximum (HTM). Motivated by strong recent declines in sea ice and large uncertainties in model projections, the authors use marine geological archives to provide a long-term perspective on sea-ice variability, mechanisms, and resilience. Focusing on the northern Barents Sea (>80°N), a key Arctic–Atlantic gateway experiencing rapid change, the research tests whether seasonal sea ice persisted under elevated insolation and Atlantic Water influence, and evaluates associated oceanographic and ecosystem responses.

Prior work shows large spread among climate model projections for future Arctic sea-ice states, including timing of seasonally or perennially ice-free conditions, driven by differences in parameterizations, approaches, and internal variability. Paleo-records across the Barents Sea–Svalbard region document Early Holocene warmth linked to increased summer insolation, reduced summer sea ice, and variable Atlantic Water (AW) inflow, but with strong spatial heterogeneity and chronological challenges, especially in the northeastern Barents Sea. Marine archives south and east of Svalbard indicate periods of strong AW influence and high productivity, though proxy reconstructions (e.g., dinoflagellate cyst transfer functions vs foraminiferal assemblages and isotopes) sometimes yield differing reconstructions of sea-ice duration and ocean conditions. Few HBI-based sea-ice records exist near the study sites for the earliest Holocene; available records typically begin after ~9.5 ka BP, leaving a gap that this study helps to fill.

Two gravity cores from the northern Barents Sea were analyzed: 11GC (JR142-11GC; 81°49.08′N, 25°55.60′E; 539 m water depth; 213 cm length; collected 2006) and 06GC (HH15-06GC; 81°40.20′N, 25°57.43′E; 320 m water depth; 445 cm length; collected 2015). Cores were sampled at 1-cm resolution for highly branched isoprenoid (HBI) biomarkers and for benthic foraminiferal stable isotopes (δ18O, δ13C). Sea-ice biomarkers included IP25 and IPSO25; open-water phytoplankton biomarkers included HBI III and HBI IV (and reference to HBI II in ratio calculations). Biomarker extraction followed a modified HBI protocol: freeze-dried and homogenized sediments (~25 g) were saponified (KOH in methanol/water), extracted, purified via silica columns (including an additional hexane/silica clean-up), and quantified by GC–MS (Agilent 7890). An internal standard (e.g., cyclohexadecane) and an in-house reference sediment (Arctic HBI-rich) were used for quantification and quality control. Concentrations were normalized to sediment mass; procedural blanks were included. Semi-quantitative indices were derived: spring sea-ice concentration (SpSIC) combining IP25 with HBI II following established PIP25-style approaches to estimate spring sea-ice categories; and HBI T25 (ratio of HBI III to HBI IV) as a proxy for spring diatom blooms associated with the Marginal Ice Zone (MIZ), with thresholds referenced to modern Barents Sea conditions. Benthic foraminiferal stable isotopes (δ18O, δ13C) were measured to infer bottom-water properties, including AW influence. Chronologies were established using radiocarbon dating of benthic/mixed foraminifera (MARINE20 calibration, Bayesian age–depth modeling with Bchron); detailed depths, lab codes, and calibrated ages are reported, with 68% and 95% probability envelopes for age–depth models. Results are presented from the Younger Dryas through ~6 ka BP, with emphasis on 11.7–9.1 ka BP.

• Sea-ice persistence: Continuous presence of sea-ice biomarkers (IP25, IPSO25) indicates seasonal sea ice persisted through 11.7–9.1 ka BP during the HTM in the northern Barents Sea (>80°N). SpSIC reconstructions show predominantly intermediate spring sea-ice concentrations across this interval.
• Semi-quantitative sea-ice levels: Site 06GC generally exhibits higher SpSIC (~20–55%) compared to the more northerly 11GC (~10–40%) between 11.7 and 9.1 ka BP. After ~9.1 ka BP, SpSIC increases at both sites, reaching predominantly extensive sea-ice conditions (>50%) by ~8.3 ka BP.
• Atlantic Water influence: Elevated benthic δ18O values during the Early Holocene (ca. 5‰ at 06GC; slightly lower, ~4.8‰ but increasing at 11GC) together with the presence of Cassidulina neoterica at 06GC indicate AW-derived bottom waters reached both sites during the HTM.
• High productivity: Pelagic HBI biomarkers (HBI III, HBI IV) peak around ~9.1 ka BP, indicating high phytoplankton productivity and a nearby MIZ. Mean Early Holocene HBI III concentrations are comparable to modern Barents Sea MIZ surface sediments (06GC: ~9.3 mg g−1; 11GC: ~11.1 mg g−1 vs modern ~12.9 mg g−1).
• Bloom proxy: HBI T25 values are mostly below the spring-bloom threshold at 11GC (HBI T25 < 1), with sporadic blooms (HBI T25 > 1) at 06GC during the Younger Dryas and near ~9.3 ka BP, suggesting episodic spring blooms near the MIZ.
• Regional variability: Differences in SpSIC and isotopic values between the sites likely reflect varying local influences of AW, proximity to the Svalbard coast (fast ice/meltwater effects), and stratification, consistent with broader spatial heterogeneity documented across regional archives.

Findings show that, despite elevated summer insolation and AW inflow during the HTM, the northern Barents Sea maintained seasonal sea ice with intermediate spring concentrations through 11.7–9.1 ka BP. This persistence indicates that warmer-than-present conditions did not eliminate seasonal sea ice in this high-latitude sector, likely owing to stratification that buffered cold, fresh surface layers from warmer, saline subsurface AW. The co-occurrence of sea-ice and open-water biomarkers suggests a dynamic MIZ in the vicinity, fostering high primary productivity even as sea ice persisted. Elevated benthic δ18O values and presence of AW-indicator benthic taxa corroborate AW influence below the pycnocline, while ratio-based indices (SpSIC, HBI T25) track shifts toward more extensive sea ice after ~9.1 ka BP, consistent with regional cooling trends post-HTM. The results underscore substantial regional heterogeneity in Early Holocene sea-ice states across the Barents Sea–Svalbard system and provide critical constraints that can inform and validate models projecting future Arctic sea-ice trajectories under continued warming and Atlanticification.

This study provides the first high-resolution, HBI biomarker-based reconstructions of Early Holocene sea ice from the high Arctic northern Barents Sea (>80°N). It demonstrates that seasonal sea ice with intermediate spring concentrations persisted through 11.7–9.1 ka BP during the HTM, despite warmer-than-present conditions and AW inflow, and that biological productivity was high with a nearby MIZ. After ~9.1 ka BP, sea-ice conditions strengthened, becoming predominantly extensive by ~8.3 ka BP. These reconstructions extend the observational baseline for Arctic sea ice, offer process insights (e.g., stratification buffering AW warmth), and provide data to constrain numerical models of future sea-ice change. Future research should refine spatial coverage (especially south/east of the sites), improve chronological resolution in sediment-poor sectors, and further calibrate biomarker ratios against modern observations to reduce uncertainties.

• Spatial coverage: Few Early Holocene HBI records exist near the study sites; many regional archives begin after ~9.5 ka BP, limiting broader spatial extrapolation for the earliest part of the interval.
• Regional sediment and dating constraints: Some areas (e.g., NE Barents Sea) have scarce/low-resolution Holocene sediments and chronological issues, complicating regional synthesis.
• Proxy-specific uncertainties: Individual HBIs can be affected by production, export, and preservation; ratio-based indices mitigate but do not eliminate uncertainties. Calibration of SpSIC and HBI T25 to modern analogues, while improved, carries error (e.g., RMSE) and threshold uncertainties.
• Site-specific heterogeneity: Differences in local AW influence, stratification, and coastal effects (fast ice/meltwater) may not represent broader regional conditions.
• Model comparison limits: While results inform models, inherent internal climate variability and differing model parameterizations complicate direct comparisons between paleo reconstructions and simulations.