Influence of Transient Bottom Water Temperature Variations on Geothermal Heat Flow Measurements From the Bellingshausen Sea, West Antarctica, and the Baltic Sea

Geothermal heat flow (GHF) data provide important constraints for ice-sheet flow dynamics as GHF affects sliding conditions at the ice-bed contact and englacial temperatures. However, marine measurements of the geothermal gradients can get distorted down to ∼3–10 m below seafloor by annual bottom water temperature variations. First-ever geothermal data from the Bellingshausen Sea shelf, West Antarctica, are presumably affected by temperature variations in the modified Circumpolar Deep Water (mCDW) over annual and multidecadal periods. As magnitude and resolution of temperature data are low in the Bellingshausen Sea, we test a 1D water-sediment model on Baltic Sea data, which includes long-term water temperature and semiannual GHF measurements. The model approximates the measured sub-bottom temperatures satisfactorily, although sparse data in Antarctica lead to uncertainties in reconstructed bottom water temperatures. Distortions in the geothermal gradients of the Bellingshausen Sea can be modeled using annual water temperature variations of ±0.03–0.15°C. However, the spatial heterogeneity of mCDW temperatures, recorded by heat flow lance sensors, shows no connection to geothermal gradient distortions. Therefore, the mCDW temperature variations are likely not strongly seasonal but are local changes of similar magnitude. Higher-resolution water temperature records are needed to quantify uncertainties through annual water temperature variations in the current measurement-derived GHF of 47–84 mWm−2 in Ronne Entrance and 21–57 mWm−2 in Eltanin Bay. Multidecadal ocean warming reduces the geothermal gradient by 16%–55% in the Bellingshausen Sea and leads to a reversed geothermal gradient in the Baltic Sea. This highlights the need to correct marine GHF for environmental factors.