Diel variability affects the inorganic carbon system in the sea-surface microlayer and influences air-sea CO2 flux estimates

Abstract

The ocean plays a crucial role in the global carbon cycle by absorbing and storing about one-third of anthropogenic carbon dioxide (CO2). It is estimated that the ocean has sequestered approximately 26 % of CO2 emissions over the last decade, resulting in significant changes in the marine carbon system and impacting the marine environment. The sea-surface microlayer (SML) plays a crucial role in these processes, facilitating the transfer of matter and energy between the ocean and the atmosphere. However, most studies on the carbon cycle in the SML have primarily addressed daily variability and overlooked nocturnal processes, which may lead to inaccurate global carbon estimates. We analysed temperature, salinity, pHT25, and pCO2 using data collected over three complete diel cycles during an oceanographic campaign along the Croatian coast near Šibenik in the Middle Adriatic. Our analysis revealed statistically significant differences (p<0.05) between daytime and nighttime measurements of temperature, salinity, and pHT25. Diel differences in pCO2, were also observed, with patterns largely driven by temperature effects and short-term mixing. These differences may be related to the occurrence of buoyancy fluxes, which are typically more pronounced during the day and could enhance CO2 fluxes, as observed with values of 1.98 ± 2.52 mmol cm-2 h-1 during the day, while at night, they dropped to 0.01 ± 0.02 mmol cm-2 h-1. These findings emphasise the importance of considering complete diurnal cycles to accurately capture the variability in thermohaline features and carbon exchange processes, thereby improving our understanding of the ocean's role in climate change.