Effect of tectonic closure of the American Seaway on oxygen minimum zone in the tropical Pacific from model simulations

The world’s largest oxygen minimum zone (OMZ) resides in the eastern tropical Pacific where poorly ventilated “shadow zone”, created by stagnant tropical cyclonic gyre is complemented by intensive biological consumption of dissolved oxygen, thereby promoting oxygen deficiency in this region. The present-day continental configuration with the presence of the Isthmus of Panama prevents water mass exchange between the tropical Pacific and the Caribbean Sea, shaping the climate and marine biogeochemistry features in the eastern tropical Pacific. The tectonic closure of the American Seaway during the mid-Miocene to mid-Pliocene epoch (~16-3 Ma BP) is thought to be a key factor for the development of stagnating conditions in the eastern equatorial Pacific leading to emergence of tropical Pacific OMZs . 

This study aims at investigating the impact of the CAS opening on the large-scale ocean circulation and oxygen minimum zone in the tropical Pacific. To this end, we performed a series of sensitivity experiments with the global climate model KCM where a sill of the open CAS was set at different depths, ranging from shallow to deep levels. Our results confirm previous studies that Panamanian gateway closure during the Pliocene may have led to an intensification of the Atlantic Meridional Overturning Circulation (AMOC) due to a termination of fresh-water supply from the tropical Pacific to the North Atlantic. It was found that the CAS opening drives eastward subsurface flow in the northern tropical Pacific. This, in turn, facilitates stronger west-to-east oxygen supply and subsequent overall oxygen enrichment in the subsurface Pacific waters with strongest anomalies observed in the eastern tropical Pacific. In addition, the marine net primary production is slightly weakened in this region due to an export of nutrients to the Caribbean Sea through the open Panamanian gateway. This, in turn, leads to a weaker export of particulate organic carbon towards the ocean interior, and, therefore, to lower biological consumption of oxygen in this region.