Warm-water mollusc assemblages from northern Chile (Mejillones Peninsula): new evidence for permanent El Niño-like conditions during Pliocene warmth?

Abstract

Although results have been controversial, understanding the tropical Pacific climatic state during the Pliocene warm interval (c. 4.5-3.0 Ma) is crucial if insight is to be gained into the dynamic processes of present and future global warming. In the multi-proxy effort to reconstruct ancient climates, a critical role can be played by palaeoclimatic evidence provided by the spatial and temporal distribution of temperaturesensitive marine molluscs. Shallow-water strata of the Mejillones Peninsula, northern Chile (238S), contain dense faunal assemblages in which molluscs exclusive to, or characteristic of, Pliocene deposits (Chlamys simpsoni, Chlamys vidali, Chorus blainvillei, Concholepas nodosa, Fusinus remondi, Herminespina mirabilis) coexist with surprisingly abundant and varied populations of extant warm-water species (Bulla punctulata, Cerithium stercusmuscarum, Olivella sp., Turbo cf. fluctuosus, Anomia peruviana, Argopecten ventricosus, Donax peruvianus, Dosinia ponderosa, Mexicardia procera, Undulostrea megodon), most of which have their current southern zoogeographical limit at 68S. These tropical elements are reliable indicators of nearshore marine conditions and their abundant occurrence implies that sea surface temperatures (SST) along the northern Chile coast were at least 2 8C warmer in the mid-Pliocene than at present, and that these very different conditions lasted long enough to allow stable colonization of the area. Such a significantly warmer SST pattern strongly resembles general climatic conditions accompanying modern El Niño events, when warm tropical waters propagate southward along the western margin of South America; this supports the existence in this area of persistently El Niño-like conditions during the mid-Pliocene.

Evidence of El Niño/La Niña–Southern Oscillation Variability in the Neogene-Pleistocene of Panama Revealed by a New Bryozoan Assemblage-Based Proxy

“…However, since the TEP of Panama is characterised by El Niño variability in the present day, the presence of molluscs typical of this region in northern Chile in the Pliocene in fact is in keeping with our findings. Thus, together our study and that of Ragaini et al (2008) suggest that the Peruvian Province was warmer in the Pliocene than at present as reflected by the extension of present day Panamanian molluscs into the Peruvian Province and that ENSO variability broadly characterised both Central and South American regions. Unfortunately, at present we lack bryozoan material from the Pliocene of South America to examine this further.…”

“…At face value it is curious that our data appear to conflict with inferences of a permanent El Niño in the Pliocene of northern Chile based on molluscan assemblage composition in the Peruvian Province (Ragaini et al 2008). These Pliocene molluscan assemblages are suggested to provide evidence for permanent warm conditions based on their similarity with molluscan assemblages observed today in the Panamic Province.…”

“…Our study thus complements recent evidence that rejects a permanent El Niño in the Pliocene based on analyses of monthly-resolved fossil coral skeletons using δ 18 O and salinity proxies >35 yrs of growth in two specimens (Watanabe et al 2011) and coupled climate model simulations that demonstrate ENSO variability in the Pliocene (e.g., Haywood et al 2007, Scroxton et al 2011, von der Heydt et al 2011. These studies contradict those inferring a permanent El Niño in the Pliocene using geochemical and faunal-based estimates of sea surface temperatures derived from foraminifera, alkenones (e.g., Wara et al 2005, Fedorov et al 2006, and the presence of warm-water molluscs in northern Chile (Ragaini et al 2008). Our data, in the same way, reject the lesser supported hypothesis for a permanent La Niña in the Pliocene (Rickaby and Halloran 2005).…”

The PRISM (Pliocene palaeoclimate) reconstruction: time for a paradigm shift

“…The potential exists to add bryozoan MART data and isotopic series from selected molluscs from coeval marine deposits along coastal Peru (Pisco Basin) and Chile (Mejillones Peninisula and the Coquimbo region [115]). These signal carriers/proxies would provide temporally instantaneous point estimates of seasonality, structure and range of the annual cycle of surface temperature along with traditional palaeontological analysis of assemblages.…”

Geological map of the Miocene-Pleistocene successions of the Mejillones Peninsula, Northern Chile

“…Based on planktic foraminiferal biostratigraphy (Ibaraki, 2002;Tsuchi et al, 1988), sediments of this unit have been generally assigned to the Pliocene age. Throughout the peninsula, they contain dense faunal assemblages in which molluscs exclusive to, or characteristic of, Pliocene deposits coexist with surprisingly abundant and varied populations of warm-water species (Ragaini, Di Celma, & Cantalamessa, 2008), most of which have their current southern zoogeographical limit at 68S (thermally anomalous molluscan assemblages of Ortlieb, Díaz, & Guzmán, 1996).…”