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

Abstract

Paired lithium-to-calcium (Li/Cashell) and oxygen isotope (δ18Oshell) 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/Cashell and δ18Oshell 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/Cashell and δ18Oshell 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/Cashell shows a weak positive correlation with growth increment width, indicating that physiological growth rate effects might in part control the ‘background’ Li/Cashell. Unambiguous seasonal δ18Oshell signals, used for a precise chronology for a live-collected specimen from the Gulf of Panama, imply that distinct parallel peaks in Li/Cashell and δ18Oshell mark the cold and highly productive coastal upwelling season during February to April. A comparison with environmental data reveals a negative correlation between Li/Cashell and water temperature, and a strong positive correlation between Li/Cashell and chlorophyll a in the Gulf of Panama. While a causal link between Li/Cashell and temperature remains uncertain, we argue that Li/Cashell 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/Cashell 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.