Sustained ocean observations provide the foundation for much of the chemical and physical oceanographic research that’s taking place around the world. Such observations, collected over the years and sometimes decades, give scientists insight into global cycles, regional variability and seasonal trends, and long-term changes in ocean chemistry.
In a recent study published in the Oceanography Special Issue on Changing Ocean Chemistry, a team of researchers led by Professor Nicholas Bates (BIOS Senior Research Scientists and Director of Research) collated data from seven independent carbon dioxide (CO2) time series to elucidate changes in the global ocean carbon cycle. With data from the Caribbean Sea, North and South Pacific Ocean, and the Western, Eastern, and North Atlantic Ocean, the study was able to assess multiple ocean carbon cycle measurements from periods spanning 15 to 30 years. This is the first time that scientific information from seven time-series has been assembled into one study to examine coordinated changes in seawater chemistry across the global ocean. It also marks a unique collaboration between scientists in Bermuda, US, Venezuela, Spain, Iceland and New Zealand.
By comparing differences between the recorded observations and the average climatological seasonal values at each time-series site, the researchers were able to account for the irregularly spaced sampling times. In doing so, they uncovered valuable information about the seasonality of the seawater CO2 carbonate system and changes in ocean chemistry due to the uptake of anthropogenic (human produced) CO2. Study findings include:
• Most of the ocean time-series sites were annual net sinks for atmospheric CO2, especially during boreal summer in the subpolar gyre and boreal winter in the subtropical gyre.
• Some sites seasonally transitioned to being net sources of CO2 to the atmosphere for a few months each year (e.g., the Western and Eastern Atlantic locations).
• The cohort of time-series exhibit similar changes in ocean chemistry in response to the release of human produced CO2 into the atmosphere and its subsequent uptake by the oceans.
• The ability of subtropical to subpolar surface waters to absorb CO2 (the buffering capacity) has gradually reduced over time.
• The surface waters of the ocean are slowly becoming less alkaline in a process called ocean acidification, with long-term decreases in pH and saturation states for CaCO3 minerals such as aragonite, which are important to calcifying marine organisms and ecosystems.
Dr. Bates notes that, “Although the time-series considered in this study are based in different geographical locations and oceanic biomes, these sites provide some of the best information we have about temporal variability of ocean CO2, giving us unprecedented understanding of the response of the ocean carbon cycle to natural variability and anthropogenic perturbation.”