Evidence of El Niño/la Niña–southern Oscillation Variability in the Neogene- Pleistocene of Panama Revealed by a New Bryozoan Assemblage-based Proxy

Here we explore how fossil cheilostome bryozoans can demonstrate El Niño/La Niña–Southern Oscillation (ENSO) variability in ancient tropical environments of the tropical eastern Pacific and southwestern Caribbean when used collectively to produce frequency distributions of estimates of mean annual ranges in temperature (MARTs) via zooid-size MART analysis (zs-MART). The approach is based on linking variation in the sizes of constituent zooids in bryozoan colonies with the temperature regimes in which the zooids developed. The shapes of frequency distributions of zs-MART estimates from modern environments are consistent with known upwelling and non-upwelling environments and ENSO variability. Data from fossil colonies provide evidence that ENSO variability characterized Caribbean environments in what is now present-day Panama during the Miocene and Pliocene (thus supporting neither a permanent El Niño nor a permanent La Niña in the Pliocene), but not the Pleistocene after the Isthmus of Panama closed. Western Atlantic data provide further evidence for ENSO variability in the Pliocene of Florida but not Virginia. Our study shows the potential of bryozoan assemblages to infer variation in seasonal regimes thus encouraging the further development of this proxy for inferring ENSO variability. The ability to infer the quasi-periodic climate patterns expected from the El Niño/ La Niña–Southern Oscillation (ENSO) is of great importance if we are to better understand the wider impacts of present-day and future global climate regimes. Extreme weather and changes in ocean circulation linked with the El Niño cycle are responsible for collapses in South American fisheries (e.g., Arntz and Tarazona 1990), droughts and fires, monsoons and tropical cyclone activity in Australasia (Evans and Allan 2006, Nicholls and Lucas 2007), cycles of malaria in India and South America (Kovats et al. 2003), and may increase the risk of civil wars in tropical countries (Hsiang et al. 2011). There is some evidence that ENSO variability is influenced by global warming and that more frequent and stronger El Niño events may pertain in the future (Zhang et al. 2008, Yeh et al. 2009) with potentially far-reaching consequences. The existence of the El Niño cycle in ancient environments is therefore of considerable applied interest because these environments may serve as analogues for future conditions. In this respect it is widely appreciated that the climate during the Pliocene Warm Period is of particular relevance because it was a time of sustained global warmth when the land-sea configuration, continental positions, patterns of ocean circulation, and biota were similar to the present day (Dowsett and Robinson 2009). Evidence that the Pliocene Warm Period was characterized by a permanent El Niño (Wara et al. 2005, Fedorov et al. 2006, Ravelo et al. 2006) or a permanent La FastTrack➲ publication