ABSTRACT: The strontium (Sr) isotopic compositions in the HCl-soluble and insoluble fractions of sediments impacted by penguin and seal excreta in the maritime Antarctic were analyzed. The HCl-soluble phase of the sediments impacted by penguin guano has a high ⁸⁷Sr/⁸⁶Sr ratio (average 0.7087), close to the ratio in seawater (average 0.7092), suggesting that the ⁸⁷Sr/⁸⁶Sr ratios in the sediments are significantly impacted by penguin guano. The ⁸⁷Sr/⁸⁶Sr ratios in the HCl-soluble phase of the sediments impacted by seal excreta vary between 0.705066 and 0.706117, with an average of 0.705507, higher than those of the local bedrock, but lower than the ratio in modern seawater; they are interpreted to be a mixture of Sr from seal excreta (30 to 50%) and the weathering products of the local bedrock (50 to 70%). The Sr contributions from penguin guano and seal excreta were calculated using the 2-member isotope mixing equation, and they have a significant correlation with the historical penguin population size as estimated from other geochemical analyses and the seal hair density in the sediments, respectively. These results suggest that the ⁸⁷Sr/⁸⁶Sr ratios in the HCl-soluble fraction of sediments impacted by penguin or seal excreta can potentially be used as an indirect and new proxy for the historical population size of these sea animals in the maritime Antarctic.