A hypersaline haven
Shark Bay, a World Heritage site on the westernmost point of Australia, contains a unique marine environment. A combination of high evaporation, low rainfall, and restricted water flow from the open ocean creates a steep salinity gradient. In the bay's outermost regions, salinity is similar to normal seawater at 35-40 parts per thousand (ppt). But in the semi-enclosed inner basins of Hamelin Pool and L'Haridon Bight, the water becomes hypersaline, reaching 56-70 ppt, nearly double the salt concentration of the open ocean.
This extreme environment excludes most marine species that would typically graze on or compete with the organisms that thrive here. A common inhabitant is a tiny bivalve mollusc, the Hamelin cockle (Fragum erugatum). This cockle flourishes in such numbers that its shells form entire beaches, accumulating over thousands of years into vast deposits called coquina. These shells, along with the fossilized remains of microscopic single-celled organisms called foraminifera, are a detailed chemical record of the region's past climate.
A library written in shell
The science of reading this archive is called sclerochronology, which analyzes the physical and chemical variations in the hard tissues of organisms. As a Fragum erugatum cockle grows, it builds its calcium carbonate (CaCO₃) shell layer by layer, forming growth increments similar to tree rings. The chemical composition of each layer reflects the water conditions at the time of its formation.
Specifically, scientists analyze the ratio of heavy to light oxygen isotopes (¹⁸O to ¹⁶O) within the shell's carbonate structure. In this hot, arid region, low rainfall and high evaporation increase the water's salinity and the concentration of the heavier ¹⁸O isotope. When rainfall increases, the bay's water becomes less saline and the ¹⁸O concentration drops. This isotopic ratio, locked into the shell layers, records historical rainfall and evaporation. By sampling both living cockles and ancient shells from sediment cores, researchers can reconstruct a high-resolution climate timeline for the past 8,000 years, identifying everything from seasonal changes to long-term drought patterns.