C. (IMAGE) Aarhus University Caption Data on δ13C, δ15N and mercury in narwhal tusks measured in layers corresponding to the period 1968-2010. The cover of summer sea ice for the same period is also shown. Credit Aarhus University, Denmark Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.