Presenter: Brice Loose (University of Rhode Island)

Description:
Shelf water in the Ross Sea is a precursor to Antarctic Bottom water, which occupies the largest volume of the deep ocean, sequestering vast amounts of heat and carbon.  The physical and biogeochemical properties of deep water reflect the biogenic gas cycling between the atmosphere and the deep sea.  Here we use inert noble gas measurements (He, Ne, Ar, Kr, Xe) from late austral fall, together with ∆O₂/Ar to estimate biological oxygen from the Ross Sea and reveal how sea ice production and glacial ice influence deep water formation, while blocking ventilation.  Dense shelf water with neutral density greater than 28.27 kg m^-3 of Terra Nova Bay polynya contained minimal glacier melt in contrast with sea ice fractions of nearly 1.2%, indicating nearly 15 m of ice production per square m of water column, which suggests annual average ice production of 5.9 cm d^-1. Integrated sea ice formation in the Ross polynya was two thirds less or 0.5 % by volume. While these physical properties are important to the thermohaline circulation, they play a relatively small role in restoring the oxygen deficits in dense shelf water.  Instead, summer biological activity restores 67% or 60 umol/kg of the oxygen deficit in Ross Sea dense shelf waters. In comparison, physical processes of air-sea exchange, air bubbles and ice contribute less than 5% or 5 umol/kg to the restoration of oxygen.  These results highlight the importance of summertime productivity to deep water properties.  Yet, despite the intense productivity, dense shelf water in the Ross Sea returns to the abyssal ocean with a nearly 20% undersaturation in oxygen, revealing that neither primary production nor gas exchange can completely restore preformed oxygen to saturation equilibrium.  These ventilation processes may further weaken the oxygen end member in shelf water, as changes in ice cover and glacier melt enhance surface stratification and influence the ventilation of old circumpolar water.