El Niño and the History of Eastern Pacific Reef Building

Abstract

Abstract The intense 1982-83 El Nino-Southern Oscillation (ENSO) event significantly raised sea surface temperature devastating eastern Pacific coral reefs. In the aftermath of this disturbance, slow recovery has led to extensive reef erosion. The death of corals and the subsequent reef erosion have stimulated a reevaluation of the history and the causes for the small, low diversity reefs of the eastern Pacific. Today's eastern Pacific reefs differ markedly in size and species composition and richness from this region's past reefs and other present-day Pacific reefs. The closure of the Panamanian isthmus in the Plio-Pleistocene marked a deterioration in the reef building environment in the eastern Pacific. With this seaway closed, the modern Pacific surface circulation developed, and the components necessary for the ENSO events came together. The termination of the Pacific-Atlantic exchange coincided with the beginning of glacial-interglacial cycles. Since the closure of the Panamanian seaway and the onset of the glacial cycles, the eastern Pacific has faced two different climatic states that restricted reef growth and development: one during glacial periods with cool waters and lowered sea-levels and the other during interglacial periods with higher sea-levels, warmer waters, and ENSO events. During this latest high sea-level stand, between 18 to 65 ENSO events of the 1982-1983 magnitude may have disturbed the eastern Pacific. An uplifted reef at Urvina Bay, Galapagos Islands provides an opportunity to determine how eastern Pacific reefs develop during sea-level high stands. Here, recurrent intense ENSO events start the coral communities on a cycle of death, erosion, and recolonization stunting long-term reef growth. ENSO events, acting in concert with other physical and biological forces, have prevented the buildup of a substantial reef-framework. During the transition to inter-glacial times, melting ice raised sea-level and improved the environmental conditions for coral-reef growth enabling coral recruits to colonize the shelf. In the western and central Pacific, where reefs survived subaerial exposure, corals colonized and built new reefs on these antecedent structures enabling carbonate accumulation to continue. Whereas in the eastern Pacific, where small reefs eroded during low sea-level stands, recruits had to settle on basaltic or other consolidated outcrops rather than on previous carbonate build-ups. New recruits face not only harsh reef building conditions (e.g., upwelling and intense grazing) but recurrent intense and lethal ENSO events. After coral mortality, bioerosion removed much of the coral build-up. This repeated process prevents the coral community from increasing in diversity or developing to a resistant structure that can withstand erosion after death. Thus, one generation's growth is not transferred to the next, and large, persistent reef frameworks are not constructed.