Simulating the impact of the Panamanian seaway closure on ocean circulation, marine productivity and nutrient cycling

The closure of the Panama Isthmus, ∼14–3 Ma, caused large reorganizations of ocean circulation. Here we use a global climate–ocean ecosystem model to investigate the effects of the closing gateway on ocean circulation, marine productivity and nutrient distributions. Several sensitivity experiments with different sill depths and vertical diffusivities are performed. Consistent with previous model studies, we find constricted throughflow due to shallowing of the sill leads to intensification of the North Atlantic thermohaline overturning. We demonstrate a strong coupling between the flows through the tropical gateways of Panama and Indonesia in a way that reduced outflow of upper ocean Pacific waters via the Panama gateway into the Atlantic is compensated by increased flow through the Indonesian Archipelago. The simulated rates of North Atlantic Deep Water formation strongly depend on the vertical diffusion in the model, particularly for a deep sill. For the first time, we document by model results shifts in nutrient distributions associated with reorganizations of ocean circulation with repercussions on marine productivity patterns. Reduced flow of nutrient-rich sub-surface waters from the Pacific into the Atlantic reduces biological productivity in the North Atlantic. In the eastern tropical Pacific restriction of the nutrient-rich outflow leads to nutrient accumulation which in turn maintains a strong increase in productivity. These results seem to be largely consistent with the paleoproductivity proxy record. A massive drop of opal accumulation rates, however, as found in North Pacific sediments for the time of the final closure (∼2.75 Ma) cannot be simulated. Generally, global marine net primary productivity (NPP) is found to increase with proceeding gateway closure.