Stratigraphic unmixing reveals repeated hypoxia events over the past 500 yr in the northern Adriatic Sea

In the northern Adriatic Sea and in most semi-enclosed coastal regions worldwide, hypoxiainduced by eutrophication in the late 20 th century caused major die-offs of coastal marineorganisms. However, ecosystem responses to hypoxia over longer centennial scales are unclearbecause the duration of direct observations is limited to a few decades and/or the temporalresolution of sedimentary archives is compromised by slow sedimentation and bioturbation.To assess whether perturbations of ecosystems by hypoxia recurred over centuries in thenorthern Adriatic Sea, we evaluate the timing and forcing of past hypoxia events based onthe production history of the opportunistic, hypoxia-tolerant bivalve Corbula gibba , using 210 Pb data, radiocarbon dating, amino acid racemization, and distribution of foraminifers insediment cores that capture the past 500 yr in the Gulf of Trieste. Unmixing the stratigraphicrecord on the basis of 311 shells of C. gibba , we show that the reconstructed fluctuations inabundance do not correlate with abundances in the raw stratigraphic record. We find thatproduction of C. gibba has undergone major decadal-scale fluctuations since the 18 th century,with outbreaks corresponding to density of more than 1000 individuals per square meter.These outbreaks represent long-term phenomena in the northern Adriatic ecosystem ratherthan novel states characteristic of the 20 th century eutrophication. They positively correlatewith centennial-scale fluctuations in sea-surface temperature, indicating that the hypoxiaevents were coupled with water-column stratification rather than with nutrient enrichment.