First stages of degradation of Limacina helicina shells observed above the aragonite chemical lysocline in Terra Nova Bay (Antarctica)

The observation of badly preserved shells of the pteropod Limacina helicina in deep (880 m) sediment traps moored in the polynya of Terra Nova Bay was the first inspiration that led us to investigate the chemical and physical processes affecting the aragonitic shells of these organisms upon their death. For this purpose we determined the chemical saturation state of CaCO3 in seawater from Total Dissolved Inorganic Carbon (TCO2) and Total Alkalinity (TA) data. The results showed that the saturation level for aragonite was located at about 1000 m depth, hence below the trap level. A few lines of evidence concur to suggest aragonite dissolution above the chemical lysocline in the investigated area: (i) fluxes of Limacina helicina shells near the bottom were very low, despite the large abundance of these pteropods in the upper 200 m of the water column, (ii) the shells collected near the bottom showed a state of advanced chemical degradation, in contrast to the shells collected by the shallow trap, which appeared intact and well preserved and (iii) carbonate fluxes observed in the bottom trap corresponded to only 1% of fluxes measured in the shallow one. The presence in the studied area of HSSW seems to be the main factor inducing the degradation of the shells. These results suggest an underestimation of the CaCO3 budget in the deepest waters and significant implications for the mechanisms influencing the inorganic carbon cycle.