The study of a Posidonia australis sedimentary archive has provided a record of changes in element concentrations (Al, Fe, Mn, Pb, Zn, Cr, Cd, Co, As, Cu, Ni and S) over the last 3000 years in the Australian marine environment. Human-derived contamination in Oyster Harbor (SW Australia) started ~100 years ago (AD ~1900) and exponentially increased until present. This appears to be related to European colonization of Australia and the subsequent impact of human activities, namely mining, coal and metal production, and extensive agriculture. Two contamination periods of different magnitude have been identified: Expansion period (EXP, AD ~1900-1970) and Establishment period (EST, AD ~1970 to present). Enrichments of chemical elements with respect to baseline concentrations (in samples older than ~115 cal years BP) were found for all elements studied in both periods, except for Ni, As and S. The highest enrichment factors were obtained for the EST period (ranging from 1.3-fold increase in Cu to 7.2-fold in Zn concentrations) compared to the EXP period (1.1-fold increase for Cu and Cr to 2.4-fold increase for Pb). Zinc, Pb, Mn and Co concentrations during both periods were 2- to 7-fold higher than baseline levels. This study demonstrates the value of Posidonia mats as long-term archives of element concentrations and trends in coastal ecosystems. We also provide preliminary evidence on the potential for Posidonia meadows to act as significant long-term biogeochemical sinks of chemical elements.