Lori Sentman December 5, 2013

Shoaling of the Isthmus of Panama 1 (Haug et al. 2004) Pacific -> Atlantic flow through the CAS Similar salinities between the Pacific and Atlantic oceans Tectonic plates began to converge Water exchange between Pacific and Atlantic became restricted and salinities began to diverge Gulf stream began to intensify Gulf Stream transports warm and salty water to the northern high latitudes Water cools and sinks as North Atlantic Deep Water, part of the Atlantic Meridional Overturning Circulation (AMOC) MiocenePlioceneHolocene

Timing of Closure Uncertain (ca. 16 to 2.7 Ma) Age (Ma) Sand MAR, Site 999 (g/cm 2 kyr) Sand MAR, Site 846 (g/cm 2 kyr) Site 999 (Caribbean) Site 846 (eq. east Pacific) (Haug and Tiedemann 1998, Fig. 2) (Haug et al. 2001, Fig. 2A) Carbonate sand-fraction MAR Age (Ma) Planktic foraminiferal Site 999 (Caribbean) Site 851 (eq. east Pacific) δ 18 O ( 0/00 ), G. Sacculifer salinity lower higher Widespread land mammal exchange between North and South America (“Great American Exchange”) dates final closure ca. 2.7 Ma (Webb 1997). Stable-isotope ratios and carbonate sand-fraction Mass Accumulation Rates (MAR) show divergence between Caribbean and equatorial eastern Pacific ca. 4.6 Ma (Haug and Tiedemann 1998, Haug et al. 2001).

Effect of Open CAS on AMOC Strength & Thermocline Depth (Zhang et al. 2012) 3 (Zhang et al. 2012, Fig. 3 ) 12 sensitivity experiments with 8 ocean/climate models Flow through CAS is from the Pacific->Atlantic Reduction in AMOC strength with opened CAS Deepening of equatorial Pacific thermocline with opened CAS scales linearly with reduction in AMOC in models without a dynamic atmosphere Wind-stress effects amplify the thermocline deepening induced by AMOC reduction AMOC Thermocline (Open CAS – Closed CAS)

Summary 4 Closure of the CAS was a gradual event occurring from around 16 to 2.7 Ma. Ocean sediment proxy data suggests a marked reorganization of the ocean circulation due to the shoaling of the Isthmus of Panama beginning around 4.6 Ma. In general, ocean climate models of varying complexity simulate reduced AMOC strength and a deepening of the equatorial eastern Pacific thermocline with an open seaway configuration. Wind-stress feedbacks in coupled atmosphere-ocean climate models amplify the thermocline deepening with an open seaway configuration. It is important to understand mechanisms of past climate change to make predictions of future climate change.