Paired Li/Ca and δ¹⁸O peaks in bivalve shells from the Gulf of Panama mark seasonal coastal upwelling

Paired lithium-to-calcium (Li/Ca_shell) and oxygen isotope (δ¹⁸O_shell) ratios were measured in four aragonitic Megapitaria aurantiaca bivalve shells from the tropical Eastern Pacific and evaluated with respect to their linkage to seasonal coastal upwelling. The Li/Ca_shell and δ¹⁸O_shell of two specimens from the upwelling-dominated Gulf of Panama are positively correlated and consistently reveal large seasonal variations of up to ~300 μmol/mol and ~3‰, respectively. The Li/Ca_shell and δ¹⁸O_shell of two shells from the adjacent non-upwelling Gulf of Chiriquí are not correlated and show a reduced seasonal variability rarely exceeding 20 μmol/mol and ~1.0‰, respectively. In some M. aurantiaca specimens the Li/Ca_shell shows a weak positive correlation with growth increment width, indicating that physiological growth rate effects might in part control the ‘background’ Li/Ca_shell. Unambiguous seasonal δ¹⁸O_shell signals, used for a precise chronology for a live-collected specimen from the Gulf of Panama, imply that distinct parallel peaks in Li/Ca_shell and δ¹⁸O_shell mark the cold and highly productive coastal upwelling season during February to April. A comparison with environmental data reveals a negative correlation between Li/Ca_shell and water temperature, and a strong positive correlation between Li/Ca_shell and chlorophyll a in the Gulf of Panama. While a causal link between Li/Ca_shell and temperature remains uncertain, we argue that Li/Ca_shell peaks in M. aurantiaca are consistent with the concept of an incorporation of additional Li provided by diatom-dominated phytoplankton blooms during coastal upwelling. Our findings suggest that Li/Ca_shell in M. aurantiaca may serve as a new proxy for seasonal coastal upwelling, the potential of which should be tested for other mollusk species and upwelling regions.