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A series of drowned coral reefs form
terraces off Kawaihae on the island of Hawaii. |
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Coral reefs record glacial cycles
Terraces offshore of several of the Hawaiian Islands are sequences of ancient coral reefs. Reefs can grow fast enough to keep up with the continuous subsidence of the islands, but they "drown" and die when sea level rises suddenly, such as when glaciers melt. Software called "ReefGrow" has been developed as part of our project for modeling this sequence.
The timing of the interglacial periods in the Pleistocene is not well constrained by other paleoclimate records such as ice cores, but the ages of the tops of the reef terraces should coincide with major meltwater pulses as glaciers receded. The reefs off the Big Island date from 15,000 (the shallowest reef) to 400,000 years, and the reefs off Lanai, Oahu, and Niihau record the glacial cycles up to several million years ago. Intriguing, isn't it, that one goes to the tropics to study glacial cycles?
The paleoenvironments experienced by the fossil reefs provide other information about the history of the islands, such as evidence that giant tsunami hit Lanai in the past.
Our research on drowned coral reefs around Hawaii
The discussions below are paraphrased from abstracts of papers published by the Submarine Volcanism group.
Reef development during the last two glacial cycles
HAWAII - Drowned coral reefs on rapidly subsiding margins possess a unique archive of sea level and climate changes, generally unavailable from stable or uplifting margins. Using available field observations and sedimentary, radiometric age, and numerical modeling data, we propose a new model of submerged reef development around Hawaii during the last two glacial cycles (250 kyr). This model provides a quantitative predictive stratigraphy for the reefs that we argue, if drilled, will yield new information on sea level and climate changes, as well as coral reef response over the last 250 kyr. Comparing the observational and numerical modeling data, combined with sensitivity testing, we present our ‘‘best case’’ scenario for the evolution of the drowned lowstand reefs now at -400 (H2) and -150 m (H1). We find that growth rates of 2.5–2.85 m/kyr for the main shallow reef building facies, a subsidence rate of 2.5 m/kyr, and a variable basement substrate configuration best explain the observational data. Modeling of the internal stratigraphic succession of the reefs shows that the number and thickness of shallow reef units, as well as the frequency and duration of subaerial exposure and reef-drowning events, are sensitive to the frequency and amplitude of eustatic sea level variations but not the rate of subaerial erosion. H2 and H1 initiated growth during stable eustatic sea level conditions during highstands circa 222 ka (MIS7) and circa 126 ka (MIS5e), respectively. Both H2 and H1 have a long and complex growth history, growing episodically for ~90 kyr. Precessional (~20 kyr) and higher-frequency, suborbital eustatic sea level fluctuations dominate, with each reef experiencing repeated but brief (<5–10 kyr) drowning and subaerial exposure, producing a complex layer cake stratigraphy of shallow (0–30 m) coral reef units separated by either subaerial exposure horizons or thin, intermediate (30–60 m) coralgal units. Final drowning of H2 and H1 occurs during the penultimate (133–134 ka) and last deglaciation (12–14 ka). These findings are consistent with available age data and qualitative predictions of previous studies around Hawaii.
Reference: Webster, J.M., L.M. Wallace, D.A. Clague, J.C. Braga (2007) Numerical modeling of the growth and drowning of Hawaiian coral reefs during the last two glacial cycles (0–250 kyr), Geochem. Geophys. Geosyst., 8, Q03011, doi:10.1029/2006GC001415. [Abstract] [Article]
Diagenesis affects geochemistry of coral skeletons and requires careful analytical techniques
HAWAII - The geochemistry of coral skeletons may reflect seawater conditions at the time of deposition and the analysis of fossil skeletons offers a method to reconstruct past climate. However the precipitation of cements in the primary coral skeleton during diagenesis may significantly affect bulk skeletal geochemistry. We used secondary ion mass spectrometry (SIMS) to measure Sr, Mg, B, U and Ba concentrations in primary coral aragonite and aragonite and calcite cements in fossil Porites corals from submerged reefs around the Hawaiian Islands. Cement and primary coral geochemistry were significantly different in all corals. We estimate the effects of cement inclusion on climate estimates from drilled coral samples, which combine cements and primary coral aragonite. Secondary 1% calcite or ~2% aragonite cement contamination significantly affects Sr/Ca SST estimates by +1 °C and −0.4 to −0.9 °C, respectively. Cement inclusion also significantly affects Mg/Ca, B/Ca and U/Ca SST estimates in some corals. X-ray diffraction (XRD) will not detect secondary aragonite cements and significant calcite contamination may be below the limit of detection (~1%) of the technique. Thorough petrographic examination of fossils is therefore essential to confirm that they are pristine before bulk drilled samples are analysed. To confirm that the geochemistry of the original coral structures is not affected by the precipitation of cements in adjacent pore spaces we analysed the primary coral aragonite in cemented and uncemented areas of the skeleton. Sr/Ca, B/Ca and U/Ca of primary coral aragonite is not affected by the presence of cements in adjacent interskeletal pore spaces, i.e. the coral structures maintain their original composition and selective SIMS analysis of these structures offers a route to the reconstruction of accurate SSTs from altered coral skeletons. However, Mg/Ca and Ba/Ca of primary coral aragonite are significantly higher in parts of skeletons infilled with high Mg calcite cement. We hypothesise this reflects cement infilling of intraskeletal pore spaces in the primary coral structure.
Reference: Allison, N., A.A. Finch, J.M. Webster, D.A. Clague (2007) Palaeoenvironmental records from fossil corals: The effects of submarine diagenesis on temperature and climate estimates, Geochimica et Cosmochimica Acta, 71, 4693-4703, doi:10.1016/j.gca.2007.07.026. [Abstract]
Evidence of large tsunami on Lanai
LANAI - The origin of subaerial coral conglomerate deposits on the Hawaiian islands of Lanai and Molokai is controversial, primarily because these deposits are difficult to interpret and the vertical motion of these islands is poorly constrained. Based on bathymetry, dive observations, sedimentary and radiocarbon data from coralline algal dominated deposits from two submerged terraces at –150 and –230 m off Lanai, Lanai has experienced relatively little vertical movement over the last 30 ka. Using internally consistent age versus depth relationships, paleowater depths, and published sea level data, we estimate that Lanai has experienced maximum rates of uplift of 0.1 m/kyr or subsidence of 0.4 m/kyr over this period. Our analysis of possible uplift mechanisms, published geophysical, numerical modelling, and recent tide data suggests that this is also the maximum uplift rate for the last several hundred thousand years. Taken together these data support the interpretation that coral conglomerates at elevations higher than +35 m on Lanai are tsunami deposits with a minimum wave run up > 170 m, rather than shoreline deposits formed during the last two interglacials, then uplifted to their present elevations.
Reference: Webster, J.M., D.A. Clague, J.C. Braga (2006) Support for the Giant Wave Hypothesis: evidence from submerged terraces off Lanai, Hawaii, Int J Earth Sci, DOI 10.1007/s00531-006-0107-5. [Abstract] [Article]
Coralline algal deposits off Lanai indicate little vertical movement
LANAI - We present detailed bathymetry, remotely operated vehicle (ROV) and submersible observations, and sedimentary and radiocarbon age data from carbonate deposits recovered from two submerged terraces at -150m (T1) and -230m (T2) off Lanai, Hawaii. The tops of the terraces are veneered by relatively thin (<5 m) in situ accumulations of coralline algal nodule, coralgal nodule, Halimeda and a derived oolitic facies deposited in intermediate (30-60 m) to deep fore-reef slope settings (60-120 m). The data are used to develop a sedimentary facies model that is consistent with eustatic sea-level variations over the last 30 thousand years (ka). Both nodule facies on T1 and T2 initiated growth 30-29 ka following a fall in sea level of ~50 m and increase in bottom currents during the transition from Marine Isotope Stage 3 to 2. The nodules accreted slowly throughout the Last Glacial Maximum when sea-level was relatively stable. Drowning occurred during the early deglaciation (17-16 ka) and was marked by the complete drowning of coralline algal nodules facies on T2 and incipient drowning of coralgal facies on T1. Abrupt sea-level rise during the middle deglaciation, perhaps associated with global meltwater pulse 1A (14-15 ka), finally drowned the coralgal facies on T1, which in turn was overlain by a deep-water Halimeda facies or an oolitic facies derived from upslope. Our data indicates that Lanai has experienced relatively little vertical tectonic movement over the last 30 ka. Using paleobathymetric data derived from the sedimentary facies, age vs. depth relationships, and published sea-level curves, we estimate that Lanai could be either slowly uplifting or subsiding, but at rates <0.1 m/kyr (uplift) or <0.4 m/kyr (subsidence) over this 30 kyr period.
Reference: Webster, J.M., D.A. Clague, J.C. Braga, H. Spalding, W. Renema, C. Kelley, B. Applegate, J.R. Smith, C.K. Paull, J.G. Moore, D. Potts (2006) Drowned coralline algal dominated deposits off Lanai, Hawaii: carbonate accretion and vertical tectonics over the last 30 ka. Marine Geology 225: 223-246. [Abstract] [Article]
Drowning of coral reef associated with melting of glaciers
HAWAII - Sea-level rise was not smooth and continuous when the glaciers melted, and we propose that it was during the meltwater pulses, periods of dramatic, unusually fast rise of sea-level, that the reef-building corals could not grow vertically fast enough and died. We present evidence that the drowning of the -150 m coral reef around Hawaii was caused by rapid sea-level rise associated with meltwater pulse 1A (MWP-1A) during the last deglaciation, when sea-level rose about 35 meters in less than 500 years.
New U/Th and 14C accelerator mass spectrometry dates, combined with reinterpretation of existing radiometric dates, constrain the age of the coral reef to 15.2-14.7 ka (U/Th age), indicating that reef growth persisted for 4.3 thousand years following the end of the Last Glacial Maximum at 19 ka. The drowning age of the reef is roughly synchronous with the onset of MWP-1A between 14.7 and 14.2 thousand years ago. Dates from coralline algal material range from 14 to 10 cal ka (calibrated radiocarbon age), 1-4 thousand years younger than the coral ages. A paleoenvironmental reconstruction incorporating all available radiometric dates, high-resolution bathymetry, dive observations, and coralgal paleobathymetry data indicates a dramatic rise in sea level around Hawaii ca. 14.7 thousand years ago. Paleowater depths over the reef crest increased rapidly above a critical depth (30-40 m), drowning the shallow reef-building Porites corals and causing a shift to deep-water coralline algal growth, preserved as a crust on the drowned reef crest.
Reference: Webster, J.M., D.A. Clague, K. Riker-Coleman, C. Gallup, J.C. Braga, D. Potts, J.G. Moore, E.L. Winterer, and C.K. Paull (2004) Drowning of the -150m reef off Hawaii: a casualty of global meltwater pulse 1A? Geology, 32(3): 249-252. [Abstract] [Article]
Coral ages and island subsidence
HILO - A 25.8m-thick section containing coral fragments occurs directly below a ~1340 year old surface lava flow (the Panaewa flow) in the Hilo drill hole. Coral samples have been dated by 230Th/U methods. The unit is 9790 years old at the bottom and 1690 years old at the top, and was deposited in a shallow lagoon behind an actively growing reef. This unit is underlain by a lava flow, which is itself underlain by more coral dating (by 14C) 10,340 years old. The age-depth relations of the dated samples compare with proposed sea level curves, after allowance for island subsidence is considered (island subsidence averages 2.2 mm/yr for the last 47 years based on measurements from a tide gage near the drill hole, or 2.5-2.6 mm/yr for the last 500,000 years based on the ages and depths of a series of drowned reefs offshore west of Hawaii). The age-depth measurements of these coral fragments are more consistent with eustatic sea levels as determined by coral dating at Barbados and Albrolhos Islands than those based on oxygen isotopic data from deep sea cores.
The Panaewa lava flow entered a lagoon covered with coral debris, extended 2 km into Hilo Bay and ended in a steep scarp. It was 30.9m thick at the drill site, 11m of which was above sea level, and has now subsided to 4.2m above sea level there. It demonstrates how a modern lava flow entering Hilo Bay would not only change the coast-line but could extensively modify the offshore shelf.
Reference: Moore, J.G., B.L. Ingram, K.R. Ludwig, and D.A. Clague (1996) Coral ages and island subsidence, Hilo drill hole, Journal of Geophysical Research, 101(B5): 11,599-11,605. [Article]
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