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-δ¹⁵N) are elevated in the ETNP relative to the Caribbean Sea throughout the Early Pliocene. Whereas ETNP FB-δ¹⁵N shows no long-term trend across the Early Pliocene, FB-δ¹⁵N 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-δ¹⁵N to indicate progressive isolation of their subsurface nutrient pools due to CAS shoaling. The oxygen isotopic composition of seawater (δ¹⁸O_sw) derived from planktonic foraminifer δ¹⁸O 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 δ¹⁸O_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.