A decline in molluscan carbonate production driven by the loss of vegetated habitats encoded in the Holocene sedimentary record of the Gulf of Trieste

Carbonate sediments in non-vegetated habitats on the NE Adriatic shelf are dominated by shells of molluscs. However, the rate of carbonate molluscan production prior to the 20th century eutrophication and overfishing on this and other shelves remains unknown because (1) monitoring of ecosystems prior to the 20th century was scarce and (2) ecosystem history inferred from cores is masked by condensation and mixing. Here, based on geochronological dating of four bivalve species, carbonate production during the Holocene is assessed in the Gulf of Trieste, where algal and seagrass habitats underwent a major decline during the 20th century. Assemblages of sand-dwelling Gouldia minima and opportunistic Corbula gibba are time-averaged to > 1,000 years and C. gibba shells are older by >2,000 years than shells of co-occurring G. minima. This age difference is driven by temporally disjunct production of two species coupled with decimeter-scale mixing. Stratigraphic unmixing shows that Corbul...