HR: 1340h
AN: PP13A-0588    [Abstracts]
TI: High Resolution Paleoclimate Records From Fossil Corals Recovered From Drowned Reefs Around Hawaii
AU: * Rothwell, A J
EM: arothwel@ucsc.edu
AF: University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AU: Webster, J M
EM: jwebster@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-9644 United States
AU: Ravelo, A C
EM: acr@ucsc.edu
AF: University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AU: Clague, D
EM: clague@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-9644 United States
AU: Potts, D
EM: potts@biology.ucsc.edu
AF: University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AB: Marine Isotope Stage (MIS) 11 (423-362 ka) was the longest sustained warm interval during the past 500 ky and has been implicated in the ­turn on­" of major reef provinces around the world (ie. Great Barrier Reef). Reconstructions of the oxygen isotopic composition of seawater (ŸO18Osw) and apparent ice volume indicate that peak sea-level conditions may have lasted until as late as 370 ka (Shackleton, 2000, Lea et al., 2000). Despite these advances little is known about short term climate variability during this unique interglacial episode. The submerged fossil coral reef terraces around Hawaii represent a unique and untapped archive of past mean background climate, in addition to seasonal and interannual climate variability in the North Pacific Subtropical Gyre (NPG) over the past 500 ky. These terraces initiated during the highstands and continued growing through periods of eustatic ice sheet growth, as island subsidence (2-3 m/ky) and falling sea levels kept similar pace, and stable shoreline conditions prevailed. Coral reef growth terminated during the major deglaciations as rapid sea-level rise and subsidence combined to submerge the reefs below the photic zone. In 2001, several massive fossil Porites corals were collected using the ROV Tiburon during the Monterey Bay Aquarium Research Institute­Ýs (MBARI) expedition to Hawaii. Sampling for the current project focused on two deep marine plunge pools on the East side of Kohala Volcano, Hawaii. Fossil samples were selected for isotopic analysis based on general appearance and the presence of annual growth bands in x-ray photographs. Each sample passed preliminary x-ray diffraction scans, showing that diagenetic alteration to calcite in the skeleton is $<$2%. Corals were micro-sampled along major growth axes, and analyzed to quantify their oxygen and carbon isotopic composition. One set of samples, from Plunge Pool 1,were dated at 377 ka using U/Th ratios, and another set from nearby Pool 2 is of unknown age, but likely also grew during MIS 11. Though ŸO18O in Porites corals is affected by sea surface temperature (SST) and ŸO18Osw, ŸO18O of precipitation in Hawaii is relatively high, and seasonal rainfall should not affect ŸO18Osw greatly. Modern samples (P. evermanni and P. lobata) recorded seasonal SST cycles of 2.45oC and 2.03oC, 96% and 80% as large as that reported for a 1o x 1o grid by Levitus (2.54oC). Fossil samples from Plunge Pool 1 recorded a similar seasonal SST range, while those of Pool 2 suggest reduced seasonality (1.3oC) in MIS 11 compared to today. Phasing of the ŸO18O and ŸO13C cycles in the fossil samples is similar to that of the modern samples, indicating similar environmental and biological factors during the two periods. To estimate the average SST during the time the fossil corals grew, residual ŸO18O for each specimen was calculated from the average ŸO18O values by 1) correcting for extension rate effects, 2) subtracting the Ÿ'ŸO18Osw between 377 ka and today, and 3) subtracting the average ŸO18O of the modern Porites lobata sample. Pool 1 sample ŸO18O residuals indicate that SST was 3oC cooler, while Pool 2 samples indicate 5oC cooler during MIS 11 compared to today. Further study will include use of Sr/Ca ratios as another SST proxy, and time series analyses of interannual variability in the records.
DE: 4870 Stable isotopes
DE: 4267 Paleoceanography
DE: 3309 Climatology (1620)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting