El Niño Influence on Holocene Reef Accretion in Hawai‘i

New observations of reef accretion from several locations show that in Hawai‘i accretion during early to middle Holocene time occurred in areas where today it is precluded by the wave regime, suggesting an increase in wave energy. Accretion of coral and coralline algae reefs in the Hawaiian Islands today is largely controlled by wave energy. Many coastal areas in the main Hawaiian Islands are periodically exposed to large waves, in particular from North Pacific swell and hurricanes. These are of sufficient intensity to prevent modern net accretion as evidenced by the antecedent nature of the seafloor. Only in areas sheltered from intense wave energy is active accretion observed. Analysis of reef cores reveals patterns of rapid early Holocene accretion in several locations that terminated by middle Holocene time, ca. 5000 yr ago. Previous analyses have suggested that changes in Holocene accretion were a result of reef growth ‘‘catching up’’ to sea level. New data and interpretations indicate that the end of reef accretion in the middle Holocene may be influenced by factors in addition to sea level. Reef accretion histories from the islands of Kaua‘i, O‘ahu, and Moloka‘i may be interpreted to suggest that a change in wave energy contributed to the reduction or termination of Holocene accretion by 5000 yr ago in some areas. In these cases, the decrease in reef accretion occurred before the best estimates of the decrease in relative sea-level rise during the mid-Holocene high stand of sea level in the main Hawaiian Islands. However, reef accretion should decrease following the termination of relative sea-level rise (ca. 3000 yr ago) if reef growth were ‘‘catching up’’ to sea level. Evidence indicates that rapid accretion occurred at these sites in early Holocene time and that no permanent accretion is occurring at these sites today. This pattern persists despite the availability of hard substrate suitable for colonization at a wide range of depths between 30 m and the intertidal zone. We infer that forcing other than relative sea-level rise has altered the natural ability to support reef accretion on Hawaiian insular shelves. The limiting factor in these areas today is wave energy. Numbers of both large North Pacific swell events and hurricanes in Hawai‘i are greater during El Niño years. We infer that if these major reeflimiting forces were suppressed, net accretion would occur in some areas in Hawai‘i that are now wave-limited. Studies have shown that El Niño/Southern Oscillation (ENSO) was significantly weakened during early-mid Holocene time, only attaining an intensity similar to the current one ca. 5000 yr ago. We speculate that this shift in ENSO may assist in explaining patterns of Holocene Hawaiian reef accretion that are different from those of the present and apparently not related to relative sea-level rise. Although conditions of water temperature and clarity, irradiance, and substrate are sufficient in many coastal areas around the main Hawaiian Islands to support viable and accreting coral reef communities, their distribution is much more limited. Living coral and coralline algae communities are often restricted to a patchy, scattered veneer, resting unconformably on a Pleistocene fossil reef substrate (e.g., Sherman et al. 1999). A num305 1 This project was funded in part by NOAA via the Hawai‘i Coral Reef Initiative (Award no. NA16OA1449) and the U.S. Geological Survey (Award no. 98WRAG1030). Manuscript accepted 27 May 2003. 2 Department of Geology and Geophysics, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822. 3 U.S. Geological Survey, Pacific Science Center, Santa Cruz, California 95064. Pacific Science (2004), vol. 58, no. 2:305–324 : 2004 by University of Hawai‘i Press ber of researchers have reported that Holocene reef accretion in Hawai‘i has been predominantly a function of sea level and exposure to wave energy (e.g., Grigg 1998, Grossman 2001). New observations of the timing of Holocene reef accretion from several locations show that accretion leading up to middle Holocene time occurred in areas where today it is precluded by the wave regime. The dramatic decrease in accretion at these locations, occurring ca. 5000 yr ago, is insufficiently explained solely as the product of relative sea-level rise and suggests an increase in wave energy in Hawaiian waters at that time. Environmental Controls sea level. Relative sea level has played a dominant role in controlling reef growth in the Hawaiian Islands on scales of thousands of years. Eustatic sea level at the Last Glacial Maximum, ca. 21,000 yr ago, is estimated to have been between 113 m and 135 m below the current level (Clark et al. 2001). In Hawai‘i, as seen in Figure 1, relative sea level is believed to have risen rapidly from the Last Glacial Maximum to its current level ca. 5000 yr ago and continued rising to about 2 m above that. The Holocene highstand culminated ca. 3000 yr ago (Fletcher and Jones 1996) before dropping back down below the current level (Grossman and Fletcher 1998) before the advent of tide gauge recording. Coral reefs can grow in Hawai‘i only between the critical depth of 30 m (Grigg and Epp 1989) and the intertidal zone, with optimal growth occurring at about 12 m (Grigg 1998). Thus, dramatic changes in sea level during Holocene and late Pleistocene time have exerted obvious constraints on reef growth. Smaller fluctuations in sea level associated with the middle Holocene Kapapa Stand of the sea (þ2 m) are proposed to have enabled and terminated reef growth by changing both the area covered by seawater and amount of wave energy able to reach specific reef areas (Grigg 1998). At current rates, relative sea-level rise ranges from 13 cm per century on the island of Kaua‘i to 32 cm per century on the island of Hawai‘i, with other locations falling between these extremes (Permanent Service for Mean Sea Level, 2003). Over periods of up to a few centuries the relatively small changes in sea level are, given the 12 m depth for optimal growth, unlikely to have substantial effects on reef growth, except in limited areas near the modern tidal range. wave energy. The primary factor responsible for curtailing Holocene reef growth on scales of years to as much as a few centuries is exposure to wave energy. The moderate energy provided by northeasterly trade winds and south swell usually benefits reefs in Hawai‘i by enhancing circulation and nutrient uptake (Grigg 1998). Although occasional Figure 1. Dates and ages of potential sea-level indicators and modeled sea-level curve for O‘ahu from Grossman and Fletcher (1998). Letters show age and depth of samples listed in Table 1: K, Kailua; L, Lono; H, Hikauhi; P, Punalu‘u; M, Mānā. Note that Punalu‘u samples near the sea-level curve are from non–in situ samples. PACIFIC SCIENCE . April 2004 306