A unique warm-water oasis in the Siberian Arctic's Chaun Bay sustained by hydrothermal groundwater discharge

Chaun Bay (East Siberian Sea) has long been recognized for unusual benthic communities since the mid-20th century. A late-1980s expedition detailed boreal species previously unrecorded in the Siberian Arctic and described dominant cyclonic circulation, high in-bay primary production, and summer temperatures up to 12 °C near surface and seabed. An anomalous water layer with Pacific boreal–Arctic characteristics was noted, and it was hypothesized that boreal species may have arrived during a prior climatic optimum, but mechanisms enabling their persistence in this partially isolated, harsh environment were unclear. This study investigates the physical, geochemical and ecological processes underpinning this anomaly and tests whether submarine groundwater discharge (SGD) sustains a warm, nutrient-rich oasis.

Background work documented Chaun Bay’s atypical biota and hydrography, including boreal species presence and cyclonic circulation with locally high primary production despite poor riverine productivity. Globally, submarine groundwater discharge is traced by radium isotopes and often enriches coastal waters in heat, salinity, nutrients, and trace metals. Prior Siberian Arctic research identified SGD in the Laptev Sea using radium tracers. Pacific-influenced regions and Atlantic gateways are known to harbor boreal fauna in the Arctic, but comparable communities in the East Siberian Sea are rare, prompting examination of alternative local drivers such as hydrothermal SGD and mesoscale eddies.

Field campaigns: (1) October 2020 aboard R/V Akademik Oparin with 48 CTD stations measuring temperature, salinity, dissolved oxygen and collecting discrete samples at surface, bottom and intermediate depths; (2) April 2023 under land-fast ice near straits and Cape Naglyoynyn for vertical T–S profiles and ice thickness. A custom towed robotic system (Smart Fish) provided continuous thermohaline and oxygen measurements at 6–8 m depth along bay transects (data logged every 2 s). Radium isotopes: Large-volume (60–100 L) Niskin samples were filtered through MnO2 fibers; ex224Ra and ex223Ra were quantified by delayed coincidence scintillation counting (RaDeCC), with subsequent analyses to determine supported 228Th/228Ra and 227Ac corrections. Counting uncertainties followed published protocols. Radium ‘age’ was computed from 224Ra/223Ra activity ratios using known decay constants and an initial ratio from Laptev Sea submarine groundwater. Stable isotopes: δ18O and δD of water were measured using a Picarro L-2130-i with IAEA standards; reproducibility ±0.1‰ (δ18O) and ±0.5‰ (δD). Trace elements: Dissolved metals were measured by ICP-MS (Agilent 7700x) with daily optimization; standards prepared in 1% HNO3 + 0.5% HCl matrix. Ferromanganese coatings/nodules: Major and trace elements measured by EDXRF (Shimadzu EDX-800-HS) on pressed pellets; reproducibility ±5% (>1% concentration) and ±10% (<1%). Nutrients: Ammonium (indophenol), nitrate, nitrite, silicate, DIP by standard colorimetry; DIN = NH4+NO3+NO2; detection limits 0.01–0.02 µmol L−1; analytical error <10%. Carbonate system: TA by Bruevich open-cell titration (precision ±2 µmol kg−1), pH potentiometrically on total scale (precision ~0.004); pCO2 and ΩA calculated with CO2SYS using Mehrbach constants refit by Dickson & Millero and HSO4− dissociation constants. Geophysics: Marine towed proton magnetometer (MPMG-04) mapped the abnormal magnetic field Ta (observed minus normal minus variations), referenced with GPS and corrected using synchronous coastal magnetovariation records; Overhauser magnetometer recorded variations. Magnetotelluric results (from previous studies) informed resistivity structure. Biology: Zooplankton collected at 34 CTD stations using closing Juday net (180 µm) targeting pycnocline–surface and full-depth hauls; organisms identified, enumerated, staged, and biomass estimated via length–weight relationships. Macrozoobenthos sampled with Van Veen grab (0.12 m2, triplicates) plus trawls using Okkelman sledge; samples sieved (0.5 mm), preserved and identified; 41 quantitative and 51 qualitative stations, 27 trawls. Chlorophyll-a: 1.5 L filtered (0.8 µm), extracted in 90% acetone; spectrophotometric analysis correcting for pheophytin. Heat budget: Daily 3D fields (temperature, salinity, currents) from GLORYS12v1 and GOFS 3.1, and atmospheric fluxes from ERA5 were used to compute internal heat storage and boundary/surface/ice heat fluxes over Chaun Bay (volume V≈8.458×1010 m3) for 2019. Discrepancies between observed CTD temperatures and reanalyses were used to estimate geothermal SGD volume transport indirectly.

- Hydrothermal SGD identification: Distinct near-bottom warm and saline anomalies were found in October 2020 near Cape Naglyoynyn (+2 to +3.5 °C; salinity >30 psu at 10–15 m) and south of the Pevek Peninsula (15–20 m), persisting under ice in April 2023. These features contrasted with surrounding 0 °C surface waters and were absent elsewhere in Chaun Bay and adjacent ESS to 37 m depth. - Radium tracers: Near Cape Naglyoynyn, 224Ra = 20.6–31.7 dpm 100 L−1, 223Ra = 1.1–2.5 dpm 100 L−1, 228Ra = 24–67 dpm 100 L−1 versus bay averages of ~10.2, 0.6 and 47 dpm 100 L−1, respectively, indicating SGD. 224Ra/228Ra ≈ 1 near sources declined to ~0.1 away consistent with decay; radium ‘age’ increased from ~0.3–0.5 d near sources to ~7.2–10.2 d NW, tracing cyclonic dispersion. A second SGD area was detected near the Chaun River mouth (e.g., ex224Ra 31.7; ex223Ra 1.3; 228Ra 62 dpm 100 L−1) with lower salinity due to freshwater influence. - Eddy dynamics: Sequential radium ages delineated an almost closed cyclonic eddy and a secondary anticyclonic flow. Estimated current speeds from radium-age pathways: ~0.16, 0.28, 0.26, and 0.43 kn (the latter likely affected by sampling timing). Satellite imagery (July 2020) showed co-located eddies, suggesting quasi-stationary mesoscale structures. - Geochemistry: SGD areas were enriched in dissolved metals (e.g., Fe, Mn, Ti) relative to the bay average. Extensive ferromanganese coatings formed on seabed surfaces; Fe/Mn ≈ 1.54 near Cape Naglyoynyn indicates mixed hydrogenetic–hydrothermal origin. Marine sediments exhibited elevated 226Ra, Th, and 40K relative to the wider ESS. - Nutrients and carbonate system: SGD near Cape Naglyoynyn supplied ammonium and phosphorus; DIN/DIP vs salinity showed anomalously low ratios at SGD (inorganic P influx) and high ratios near Pevek Town (anthropogenic N). SGD zones showed reduced pH; surface waters generally acted as a CO2 sink due to cooling, but SGD elevated near-bottom pCO2 locally. - Stable isotopes: δ18O–δD relationships defined a steep Groundwater Line with 18O-depleted, marine-like δD signatures at the seabed in a ‘C’-shaped pattern around the eddy core, indicating marine-derived thermal groundwater likely heated at depth and re-emerging via convective circulation. - Geophysics: A positive linear magnetic anomaly with local maxima (10–23 nT) and widening near Cape Naglyoynyn/Chaun River indicates a magnetized volcanic intrusion (~10 km wide, expanding to ~35 km). Magnetotelluric data revealed very low resistivity (<50 Ω·m) at 5–50 km depth, consistent with a magma body as a heat source. - Heat budget and SGD flux: 2019 reanalysis-based heat budgets (GLORYS12v1, GOFS 3.1) showed inconsistencies between boundary fluxes and observed internal heat storage, implying an unaccounted heat source. Comparing CTD to reanalyses yielded average ΔT ≈ 0.3 °C (GOFS) and 0.1 °C (GLORYS), implying geothermal SGD volume transports of ~200 m3 s−1 and ~60 m3 s−1, respectively (assumed source ΔT ~50 °C). - Biology: Near SGD/eddy-influenced zones (Cape Naglyoynyn, south of Pevek Peninsula, Chaun River mouth), benthic abundance reached 3,433–12,292 ind m−2, biomass 161–363 g m−2, and 29–52 species per grab (up to 55 per trawl), versus central-bay values of 200–908 ind m−2, 5–40 g m−2, and 11–14 species per grab. Overall, 102 (grab) and 143 (trawl) species were recorded, dominated by molluscs (Astarte spp.). Dominant molluscs were up to twice larger near anomalies. Chlorophyll-a peaked at 9.4 mg m−3 (surface) and 7.4 mg m−3 (bottom), comparable to spring-bloom levels in Chukchi/Barents seas and far exceeding typical ESS interior values (<1 mg m−3). Salinity was elevated (29–31 psu), suppressing typical Arctic brackish zooplankton and favoring Pacific-origin species. Boreal taxa included Mytilus trossulus, Einhornia crustulenta, Tegella anguloavicularis, Leucon kobjakovae, and Pagurus capillatus. Thysanoessa spp. krill were abundant in 2020 and observed under 1.5 m ice in 2023, suggesting a local, persistent population.

Multiple independent lines of evidence converge to show that Chaun Bay’s anomalously warm, saline, nutrient- and metal-rich waters originate from hydrothermal submarine groundwater discharge. Elevated short- and long-lived radium isotopes, characteristic 224Ra/228Ra ratios, and short radium ages pinpoint SGD sources near Cape Naglyoynyn and the Chaun River mouth. Dissolved metal enrichments and ferromanganese precipitates, together with geomagnetic anomalies and low resistivity at crustal depths, indicate a volcanogenic heat source driving convective circulation and heating of intruding marine waters at depth. Stable isotope signatures (δ18O-depleted with marine-like δD) support a marine origin for the groundwater modified by water–rock interaction and potential CO2 exchange. A quasi-stationary cyclonic eddy mixes thermal groundwater with oxygen-rich surface waters and confines heat near the seabed via convergent flow around the eddy core, creating and maintaining a warm, oxygenated, nutrient-rich habitat. This hydrographic structure yields high primary production (elevated chlorophyll-a), enhanced zooplankton biomass, and exceptionally rich benthic communities with boreal taxa typically associated with Pacific/Atlantic inflows. The persistence of Thysanoessa spp., including under winter ice, implies a self-sustaining population potentially seeding the broader East Siberian shelf. Heat budget imbalances in reanalyses underscore the climatic relevance of SGD-derived heat, suggesting Chaun Bay may ventilate heat to the adjacent ESS. Together, these findings resolve the long-standing question of how boreal species persist in Chaun Bay and highlight hydrothermal SGD–eddy coupling as the sustaining mechanism of this ‘Arctic oasis.’

The study identifies and characterizes hydrothermal submarine groundwater discharge as the primary driver of a unique warm-water ‘Arctic oasis’ in Chaun Bay. Evidence from radium tracers, dissolved metals, nutrient and carbonate chemistry, stable isotopes, geomagnetics, and heat-budget analyses reveal marine-derived groundwater heated by a volcanogenic source and recirculated to the seabed. A quasi-stationary cyclonic eddy entrains and mixes this groundwater with oxygenated surface waters, enhancing productivity and supporting diverse, boreal-influenced benthic and zooplankton communities, including a likely persistent Thysanoessa population. The inferred geothermal SGD heat flux is substantial and may influence regional shelf temperatures. Future work should include sustained year-round observations to quantify temporal variability of SGD and eddy dynamics, direct measurements of geothermal heat and fluid flux, coupled physical–biogeochemical modeling to assess ecosystem impacts, and genetic/ecological studies of boreal taxa to resolve colonization, adaptation, and connectivity across the Arctic shelves.

- Temporal coverage is limited to one late-autumn cruise (October 2020) and a brief under-ice survey (April 2023); longer-term observations are needed to confirm eddy quasi-stationarity and SGD seasonality. - Indirect estimation of geothermal SGD flux relies on reanalysis–observation temperature differences and assumed source temperature; direct flux measurements were not made. - Spatial sampling constraints (e.g., unsafe ice near Cape Naglyoynyn, shifting eddy core) and differing durations of CTD vs Smart Fish surveys introduce uncertainties (e.g., radium-age pathway distortions). - Krill presence in winter was observed visually without quantitative sampling due to logistical constraints. - Potential confounding sources (e.g., anthropogenic N near Pevek) complicate nutrient ratio interpretations locally.