Evolution of shallow subsurface Atlantic nutrient and carbonate saturation state since the Last Glacial Maximum

Shallow subsurface Atlantic nutrient and carbonate chemistry respond to many oceanic processes, including deep ocean circulation changes. Changes in the Atlantic Meridional Overturning Circulation (AMOC) since the Last Glacial Maximum (LGM) affected the shallow Atlantic nutrients and carbonate chemistry, but high-resolution records from the shallow tropical Atlantic are from only a few sites and not replicated, and hence the timing and significance of millennial changes are poorly constrained. After reevaluating the optimal benthic foraminifera species for seawater nutrients and carbonate ion concentration ([CO₃^2-]) reconstructions with more core-top data and down-core inter-species comparisons, we present eight nutrient and five [CO₃^2-] reconstructions from the upper to intermediate-depth western Atlantic Ocean (~500 – 2000 m) to document changes in the shallow tropical-subtropical North Atlantic and trace changes in their southern- and northern-sourced regions.

The Demerara Rise and Florida Margin are among the first replicate nutrients and [CO₃^2-] records for the deglaciation, and are remarkably similar, confirming that these millennial changes represent regional signals of shallow tropical-subtropical North Atlantic. These high-resolution nutrients and [CO₃^2-] records provide new evidence for a weakened AMOC during Allerød and the Younger Dryas (YD), a diminished nutrient stream in upper North Atlantic during YD, and a millennial event at ~ 9 ka. We confirm that AMOC changes from ~18 to 8 ka are likely the main cause of nutrient and [CO₃^2-] changes in the shallow tropical North Atlantic. The long-term changes in [CO₃^2-] were additionally affected by rising atmospheric CO₂ since the LGM. Our results support the notion that changes in nutrients and carbonate chemistry can be affected by multiple factors, but a better understanding of their driving mechanisms and a combination of reconstructions may provide a more complete picture of AMOC changes since the LGM.