Clams’ feeding activity, calcification rates, and overall shell density have declined in concert with decreasing sea surface temperatures and the long-term falling sea level trend since the Holocene peak about 6000 years ago.
A team of scientists (Cheli et al., 2021) have suggested Earth’s surface temperatures were “up to 4°C” warmer during the Holocene Climate Optimum (HCO) 9000 to 5000 years ago. Back when Holocene CO2 levels were only in the 265 ppm range (~6500 years ago), the northern Adriatic Sea’s surface temperatures are ~1.5°C warmer than they are now.
With the warmer sea surface temperatures, more water was locked up on land as ice instead of filling ocean basins. Consequently, the ocean coasts stretched much further inland compared to where they do today.
For example, the ancient biblical city of Ur used to sit on the coast of the Persian Gulf 6000 years ago, when sea levels were 2-3 meters higher than they are now – a highstand “almost wholly the consequence of the water-load term” (Lambeck, 1996). Today the remains of Ur’s coastal city past can be found about 200 km further inland from the modern shoreline.
Image Source: Lambeck, 1996
Cheli and colleagues have determined the Po (Italy) coastline in the northern Adriatic Sea has expanded seaward about 20-30 km since the Mid-Holocene due to the cooling SSTs and more extensive land glaciation that exists today.
As a consequence of the cooler sea surface temperatures and lowered sea levels, the clams residing in Po’s waters now have to contend with 2-6°C cooler riverine water temperatures, “increasing the metabolic cost for calcification” (Cheli et al., 2021) and weakening their shell density.