EL NIÑO–SOUTHERN OSCILLATION INFLUENCE ON BIOFACIES DYNAMICS OF THE SHALLOW-WATER CARBONATE SYSTEMS OF THE GALÁPAGOS ARCHIPELAGO

ENSO is a periodic climatic and oceanic event caused by anomalies in sea surface temperature and nutrients over the eastern tropical Pacific (ETP). Recurring ENSO events have a significant impact on climate and the ecosystems of the Circum Pacific region. In the marine realm, ENSO is known for altering temperature and nutrient patterns, affecting the pelagic food chain, and causing widespread bleaching of corals due to temperature stress. This is particularly evident in the Galápagos Archipelago, Ecuador (GAL), which sits in proximity to the oscillation event, and contains carbonate assemblages, which range from coral-algal dominated (photozoan) to molluscan-dominated (heterozoan). Following the strong ENSO events of 1982-1983 and 1997-1998, the GAL experienced widespread heat-related coral degradation, with minimal recovery and increases in coralline algal abundance in the southern mesotrophic (mixed photozoan-heterozoan) archipelago. To date, however, these observations lack statistical scrutiny, and the environmental drivers of these shallow benthic ecosystems as a whole, remain poorly understood. Here we calibrate point-counted shallow water carbonate sediments, collected from seven islands – covering the geographical extent of the GAL – to oceanographic parameter data (2002-2014) obtained for each site. Linear regression tree analysis – combining biogenic hierarchical cluster analysis results with environmental data – was performed to determine the prominent drivers of the biogenic distribution within the GAL sediments. Additional statistical models were constructed to determine the predominant biogenic categories driving the sedimentological patterns observed in the cluster analyses. Results show that the minimum chlorophyll-a and the maximum sea surface temperature, which are strongly influenced by El Niño events, are dominant drivers shaping recent shallow water carbonate sediment biofacies in the GAL. These findings indicate a relatively long-term influence of ENSO on the evolution of these modern, time-averaged, carbonate sediments, and suggest that the post 1982/83 El Niño changes from coral to coralline algal abundance are related to recent strong ENSO cycles, which may be forcing the mesotrophic environments of the southern GAL to a sedimentological tipping point.