Early Pliocene Shoaling of the Central American Seaway Reconstructed From Foraminifera‐Bound Nitrogen and Oxygen Isotopes

The formation of the Isthmus of Panama closed the Central American Seaway, severing the only Late Cenozoic low‐latitude connection between the Pacific and Atlantic Oceans. Here we clarify the Early Pliocene (5.3–3.6 million years ago [Ma]) sequence of events associated with the shoaling of the Central American Seaway based on differences in upper ocean biogeochemical properties between the eastern tropical North Pacific (ETNP) and the Caribbean Sea. Foraminifera‐bound nitrogen isotopes (FB‐ δ 15 N) are elevated in the ETNP relative to the Caribbean Sea throughout the Early Pliocene. Whereas ETNP FB‐ δ 15 N shows no long‐term trend across the Early Pliocene, FB‐ δ 15 N in the Caribbean Sea declines by ∼0.5‰ between 4.6 and 4.5 Ma, and by an additional ∼1‰ between 4.35 and 4.25 Ma. We interpret the divergence between ETNP and Caribbean Sea FB‐ δ 15 N to indicate progressive isolation of their subsurface nutrient pools due to CAS shoaling. The oxygen isotopic composition of seawater ( δ 18 O sw ) derived from planktonic foraminifer δ 18 O and Mg/Ca shows a small but variable gradient between the ETNP and Caribbean Sea over the Early Pliocene, with a trend toward a larger δ 18 O sw gradient after 4.25 Ma. We suggest that the development of persistent chemical differences in both thermocline nutrients and surface waters between the ETNP and Caribbean Sea after 4.1 Ma reflects the cessation of basin‐scale oceanic exchanges across the Central American Seaway.