The Gulf Stream carries warm water across the Atlantic Ocean: north from the tropics, across the equator, and into the upper latitudes, where it cools, sinks, and flows back toward the tropics. This pattern is known as the Atlantic Meridional Overturning Circulation (AMOC), one element of the global ocean circulation commonly referred to as the ocean conveyor belt. The AMOC is closely tied to the climate in the North Atlantic and may be susceptible to global warming.
Here a new study by Gottschalk et al. explores the link between AMOC dynamics and calcium carbonate saturation in the Atlantic Ocean, a measure of how corrosive water masses are for calcareous shells of marine organisms at the seafloor. Their carbonate remains may accumulate in marine sediments when they sink to the ocean floor. The burial and preservation of carbonate are an essential component in the global carbon cycle and are closely tied to ocean circulation reorganizations and atmospheric carbon dioxide levels.
The researchers used sediment cores from the Cape Basin, located west of South Africa, to reconstruct changes in carbonate preservation over the past 400,000 years, which covers the last four glacial-interglacial cycles. The researchers used millimeter-scale X-ray fluorescence to analyze calcium/titanium ratios (Ca/Ti) in the sediment cores. The Ca/Ti ratio corresponds closely to carbonate weight percentages in the sediment samples and serves as a proxy for the carbonate saturation of deep water masses in the South Atlantic.
The study found that fluctuations in carbonate preservation reflect abrupt changes in the AMOC, which are linked to past global climate changes. AMOC changes redistribute carbonate ions, and the patterns showed that, for example, carbonate preservation peaks were associated with Northern Hemisphere warm intervals (both long and strong as well as short and weak) during the last four glacial periods. The authors suggest that abrupt changes in carbonate preservation in the South Atlantic hint at an oceanic mechanism linking North Atlantic climate anomalies and variations of the global carbon cycle.
The results from the marine sediments offer new insights into the dynamics of the AMOC during past glacial periods. The study can help clarify how sensitive the AMOC was to past climate changes and how it might have affected atmospheric carbon dioxide concentrations in the past. (Paleoceanography and Paleoclimatology, https://doi.org/10.1029/2018PA003353, 2018)
—Aaron Sidder, Freelance Writer