Conservative behavior of δ¹³C values of dissolved inorganic carbon in the Cananéia-Iguape estuarine-lagoon complex (southeastern Brazil)

The carbon isotope composition of Dissolved Inorganic Carbon (δ¹³C_DIC) is a useful tool to study the carbon budget in estuarine settings. Here we present the first set of δ¹³C_DIC data from the Cananéia-Iguape Estuarine Complex (São Paulo, Brazil), a changing environment undergoing freshening at a rate of the order of 770 m³ s^-1. We measured δ¹³C_DIC values of water at 15 stations distributed across the area, both during neap and spring tide, in May 2015 and April 2017. The spatio-temporal variability of δ¹³C_DIC suggests a conservative mixing of two water endmembers, freshwater with δ¹³C_DIC = −10 ‰ (V-PDB) and seawater with δ¹³C_DIC = +2.0 ‰ (V-PDB). Linear regression of δ¹³C_DIC vs. salinity predicts the δ¹³C_DIC values of open ocean surface water from the nearby Santos Basin and suggests that the freshwater and seawater endmembers mix at 1:1 proportion in the estuarine complex. Mixing was the dominant process governing the behavior of δ¹³C_DIC values, whereas photosynthesis, degradation of organic matter, and CO₂ uptake or outgassing did not play a significant role. In particular, phytoplankton production had no impact on δ¹³C_DIC, suggesting that the ecosystem was exempt from eutrophication. Long-term freshening in the NE sector of the estuary did not significantly impact the δ¹³C values of DIC. These results underline the need for long-term measurements of δ¹³C_DIC in the study area to contribute to the understanding of the evolution of the estuarine complex, whose freshening trend could be enhanced by extreme precipitation events linked to climate change.

Keywords:
Carbon isotopes; Water geochemistry; Tidal mixing; Freshwater input