Chaun Bay, situated in the East Siberian Sea (ESS), has long been recognized for its unusual marine ecosystem, characterized by high biological productivity and the presence of boreal species not typically found in the Siberian Arctic. Since the mid-20th century, expeditions have documented the bay's unique benthic communities, revealing an abundance of species more commonly associated with warmer, more southerly waters. These observations sparked hypotheses about the mechanisms supporting this seemingly anomalous ecosystem in such a climatically harsh environment. One suggestion was that these boreal species migrated to the region during a past climatic optimum, but how they persist in this partially isolated and cold environment has remained a mystery. This study aims to investigate the underlying physical and biogeochemical processes responsible for maintaining the exceptional biodiversity and warmth within Chaun Bay, specifically focusing on the potential role of hydrothermal submarine groundwater discharge.

Previous research on Chaun Bay highlighted the presence of boreal species, a cyclonic current structure, and high primary productivity despite the surrounding low productivity of the Kolyma River and other smaller rivers. Water temperatures reaching 12°C in summer near both the surface and bottom were observed, along with an unusual water layer resembling the boreal-Arctic Pacific water mass. However, the lack of a clear explanation for the persistence of these non-Arctic species fueled the need for a more comprehensive investigation into the underlying mechanisms supporting this unique ecosystem. Studies focusing on submarine groundwater discharge (SGD) as a source of nutrients and heat in other regions provided a framework for exploring this potential mechanism in Chaun Bay. Prior knowledge on radium isotopes as indicators of SGD further guided the investigation.

This research involved two expeditions to Chaun Bay. The first, in October 2020 aboard the R/V *Akademik Oparin*, encompassed a series of shipboard measurements including physical (temperature, salinity, dissolved oxygen), biogeochemical (radium isotopes, dissolved metals, nutrients, pH, pCO2), and geological (sediment samples) data. These measurements were taken at various stations across the bay using a CTD package with Niskin bottles, a towed robotic system ("Smart Fish") for continuous profiling at 6-8 meters depth, and geological grabs. The second expedition in April 2023 involved hydrological observations from land-fast ice in the straits between Chaun Bay and the ESS, and off Cape Naglyoynyn within Chaun Bay. The methods employed included: * **Hydrological surveys:** Conductivity-Temperature-Depth (CTD) profiles, Smart Fish towed robotic system for continuous measurements of temperature, salinity, and dissolved oxygen. * **Radium isotope measurements:** Delayed coincidence scintillation counting method used to quantify 224Ra, 223Ra, and 228Ra isotopes. * **Trace element analysis:** Inductively coupled plasma mass spectrometry (ICP-MS) used to determine trace metal concentrations. * **Nutrient analysis:** Standard colorimetric methods for ammonium, nitrates, nitrites, dissolved silicates (DSi), and dissolved inorganic phosphorus (DIP). * **Total alkalinity (TA) and pH measurements:** Titration indication method for TA and potentiometric method for pH. * **pCO2 and ΩA calculations:** CO2SYS program used to calculate partial pressure of CO2 and saturation states of aragonite. * **Geomagnetic surveys:** Measurements using a marine towed proton magnetometer to identify subsurface geological structures. * **Zooplankton sampling:** Juday net used to collect samples, with organisms identified and measured. * **Macrozoobenthos sampling:** Van Veen grab and Okkelman sledge used for quantitative and qualitative sampling, with species identified and enumerated. * **Chlorophyll-a analysis:** Spectrophotometric method using membrane filters. * **Heat budget calculations:** Utilizing oceanographic reanalyses (GLORYS12v1 and GOFS 3.1) and atmospheric reanalysis (ECMWF ERA5) to assess heat balance.

The study revealed several significant findings: 1. **Hydrothermal SGD identified:** Temperature and salinity anomalies, enriched in radium isotopes and dissolved metals, were discovered near Cape Naglyoynyn and the mouth of the Chaun River, strongly indicating hydrothermal SGD. The radium isotope data further supported this conclusion via the activity ratio of 224Ra/228Ra and 'radium age' calculations. 2. **Cyclonic eddy identified:** A cyclonic eddy mixes the warm, nutrient-rich SGD waters with oxygen-rich surface waters, creating a unique water mass within the bay. 3. **Elevated biological productivity:** The bay showed significantly higher chlorophyll-a concentrations, zooplankton biomass, and benthic abundance and species diversity compared to the surrounding ESS. Benthic communities were rich in boreal species, including those of Pacific origin. 4. **Presence of Thysanoessa krill:** Large populations of *Thysanoessa* krill, typically absent in the Arctic except for areas influenced by Atlantic or Pacific inflow, were observed, suggesting a permanent population in Chaun Bay, possibly even serving as a source for the wider ESS shelf. 5. **Volcanogenic heat source proposed:** Geomagnetic and magnetotelluric surveys revealed a positive linear magnetic anomaly associated with a potentially volcanogenic heat source at depth, which provides the heat driving the hydrothermal SGD. 6. **Heat budget inconsistency:** Heat budget calculations using oceanographic and atmospheric reanalyses revealed an inconsistency, suggesting an additional, unaccounted-for heat source consistent with the hydrothermal SGD. An approximate volume transport of geothermal source was estimated (200 m³/s using GOFS3.1 data and 60 m³/s using GLORYS12v1 data). 7. **Relationship between SGD, eddy, and biological communities:** The study demonstrates a clear relationship between the hydrothermal SGD, the cyclonic eddy, and the rich biological communities. The eddy's mixing action distributes the heat and nutrients from the SGD, fostering high productivity in the benthic and pelagic zones.

The findings conclusively demonstrate that the unique warm-water oasis in Chaun Bay is maintained by hydrothermal SGD. The interaction of this SGD with a cyclonic eddy is key in creating a highly productive ecosystem in the otherwise less-productive Siberian Arctic. The high salinity and nutrient input from SGD, along with the eddy's mixing effect, explain the presence of boreal and Pacific species that are not typically found in this region. The results highlight the significant impact of SGD on Arctic ecosystems and suggest that such seemingly localized phenomena may play a much larger role in shaping the broader Arctic environment than previously recognized. The findings also underscore the importance of considering submarine groundwater discharge when modeling Arctic heat budgets. Future research should investigate the long-term variability of the hydrothermal SGD and the cyclonic eddy, and their response to climate change.

This study provides compelling evidence for the crucial role of hydrothermal SGD in sustaining a unique and biologically rich 'Arctic oasis' in Chaun Bay. The interaction of SGD with a cyclonic eddy, enhanced by underlying volcanogenic structures, creates a favorable environment for boreal species and supports unusually high biological productivity. The results challenge our understanding of Arctic ecosystems and highlight the significance of undersea processes in shaping the Arctic environment. Future studies should focus on the long-term stability and climate sensitivity of this system and explore the potential for similar phenomena in other regions of the Arctic.

The study's observations were limited to two specific periods (October 2020 and April 2023), which may not fully capture the annual variability of the SGD flow and eddy dynamics. Further, the heat budget calculations relied on existing oceanographic and atmospheric reanalysis data, which might not fully represent the complexity of the Chaun Bay system. Additionally, while the study identifies a likely volcanogenic heat source, direct confirmation requires further geophysical investigation.