Upwelling and Freshwater Runoff Events Recorded in the Brazilian Bivalve Semele Casali

This project documents the long-term variability of freshwater runoff and upwelling events in coastal waters and the South Brazil Current of the Brazilian Bight over the last 1,000 yrs. Our methodology integrates amino acid racemization dating with trace element and stable isotope analyses to obtain high resolution time series of environmental proxies from numerous individually dated bivalves. Specimens of the aragonitic infaunal tellinid bivalve Semele casali were collected from two locations (10m and 30m depth) in Ubatuba Bay. Shell material grown in these water masses (background conditions) should display minimal variation in Mn/Ca and Ba/Ca and have δ¹⁸O values from -1.3‰ to +2.2‰, and δ¹³C values from +1‰ to +2‰. Shell material grown during upwelling events will be exposed to the colder nutrient-rich South Atlantic Central Water and should exhibit an increase in Ba/Ca, a larger range of δ¹⁸O values, and a notable decline in δ¹³C values. Shell deposited during freshwater runoff events should display concurrent increases in Mn/Ca and Ba/Ca and major concurrent decreases in δ¹⁸O and δ¹³C values.

Trace element profiles from 19 valves (0-944 years BP) suggest most bivalves (16) grew during background conditions, two grew during upwelling conditions, and one grew during freshwater runoff conditions. Stable isotope values for the background valves overlap with, though range slightly lower than, expected equilibrium values (δ¹⁸O: -2.1‰ to +0.6‰ and δ¹³C: -0.2‰ to +1.4‰). Stable isotope values for a valve that grew during runoff conditions were higher than expected (δ¹⁸O: -1.0 to +1.1 and δ¹³C: +0.7 to +2.1). The stable isotope results suggest that the bivalves grew primarily in standard marine background conditions but were occasionally exposed to other water masses.

These initial results suggest that past freshwater runoff and upwelling-induced productivity events can be detected and reconstructed in Holocene shell deposits.