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Holocene Reef Accretion, Southwest Molokai, Hawaii Mary Engels
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Acknowledgements: - Chip Fletcher, Craig Glenn, Jane Schoonmaker - Chris Conger, John Rooney, Joe Reich - Family - Coastal Geology Group - SOEST - USGS, Mike Field, Curt Storlazzi and Eric Grossman - Khaled Bin Sultan Living Oceans Foundation - Hawaii Coral Reef Initiative
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Overview: I. Introduction: Objectives and Strategy II. Modern Ecosystem Investigations i. Data collection: Methodology ii. Data Analysis: Substrate and coral types iii. Conclusions: Model of modern ecosystem zonation for SW Molokai III. Drill Core Investigations i. Data collection: Methodology ii. Data Analysis: Facies identification and distribution iii. Conclusions: Facies interpretation IV. Synthesis V. Conclusions
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Objectives: What is the spatial variability of the Molokai modern reef ecosystem? What is the influence of Holocene sea level on the Molokai reef system? Exploratory investigation of the geologic history of the Molokai fringing reef system.
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OVERVIEW OF THE SOUTHWEST SHORE OF MOLOKAI: LONO HARBOR HALE O LONO SITE: -High wave exposure -Sparse coral cover -Shore parallel ridge and runnel morphology HIKAUHI SITE -Medium wave exposure -Abundant coral cover -Shore normal spur and groove morphology HIKAUHI SITE HALE O LONO SITE LAAU POINT 0 km 0 mi 1 km 1 mi
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Strategy: Principle of Uniformitarianism : “the present is the key to the past” Application: Modern reef ecosystem zonation provides a model for understanding Holocene reef accretion.
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Strategy: Principle of Uniformitarianism : “the present is the key to the past” Application: Modern reef ecosystem zonation provides a model for understanding Holocene reef accretion. Survey and model modern ecosystem zonation (PRESENT)
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Strategy: Principle of Uniformitarianism : “the present is the key to the past” Application: Modern reef ecosystem zonation provides a model for understanding Holocene reef accretion. Survey and model modern ecosystem zonation (PRESENT) Identification of lithofacies from drill cores (PAST)
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Strategy: Principle of Uniformitarianism : “the present is the key to the past” Application: Modern reef ecosystem zonation provides a model for understanding Holocene reef accretion. Survey and model modern ecosystem zonation (PRESENT) Identification of lithofacies from drill cores (PAST) Comparison of lithofacies and modern ecosystem model to determine depositional environments (PRESENT + PAST)
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Strategy: Principle of Uniformitarianism : “the present is the key to the past” Application: Modern reef ecosystem zonation provides a model for understanding Holocene reef accretion. Survey and model modern ecosystem zonation (PRESENT) Identification of lithofacies from drill cores (PAST) Comparison of lithofacies and modern ecosystem model to determine depositional environments (PRESENT + PAST) Reconstruct paleoecosystem zonation from lithofacies distribution (PRESENT+PAST = RECORD)
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Strategy:
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Overview: I. Introduction: Objectives and Strategy II. Modern Ecosystem Investigations i. Data collection: Methodology ii. Data Analysis: Substrate and coral types iii. Conclusions: Model of modern ecosystem zonation for SW Molokai III. Drill Core Investigations i. Data collection: Methodology ii. Data Analysis: Facies identification and distribution iii. Conclusions: Facies interpretation IV. Synthesis V. Conclusion
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Modern Ecosystem Investigations: Methodology 10 m benthic surveys-Line Intercept Technique. Recorded every change in substrate type. Coincident (or nearly so) with drill cores. 19 surveys from Hikauhi, 27 surveys from Hale O Lono HIKAUHI HALE O LONO 0 km 0 mi 1 km 1 mi
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Modern Ecosystem Investigations: Data Analysis
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Modern Ecosystem Investigations: Results
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Modified from Storlazzi et al. (in press) Study Sites
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Modern Ecosystem Investigations: Results Modified from Storlazzi et al. (in press) Study Sites
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Modern Ecosystem Investigations: Results Modified from Storlazzi et al. (in press)
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Modern Ecosystem Investigations: Conclusions
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Overview: I. Introduction: Objectives and Strategy II. Modern Ecosystem Investigations i. Data collection: Methodology ii. Data Analysis: Substrate and coral types iii. Conclusions: Model of modern ecosystem zonation for SW Molokai III. Drill Core Investigations i. Data collection: Methodology ii. Data Analysis: Facies identification and distribution iii. Conclusions: Facies interpretation IV. Synthesis V. Conclusions
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Shore normal transects Water depth 4 m to 21 m Where possible cores started on live coral. 14 cores from Hikauhi 10 cores from Hale O Lono. Cores sub-sampled for radiocarbon dating, X-ray diffraction analysis and thin section analysis Drill Core Investigations: Data Collection: Methodology
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157d 16’ 0” 157d 15’ 0” 21d 4’ 30” 157d 16’ 0”157d 15’ 0” 21d 5’ 30” 21d 4’ 30” 5.5 m 8.5 m 14.0 m 17.7 m 21.0 m Drill Core Investigations: Data Collection Hale O LonoHikauhi 157d 10’ 45” 157d 9’ 45” 21d 5’ 30” 21d 4’ 45” 4.0 m 5.5 m 9.1 m 14.3 m 19.8 m 9.8 m 6.1 m 10.7 m 18.3 m 12.8 m 0 km 0 mi 0.5 km 0.5 mi 0 km 0 mi 0.5 km 0.5 mi
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Drill Core Investigations: Data Analysis Lithofacies (facies) A: Encrusting coral-algal bindstone B: Mixed skeletal rubble C: Massive coral framestone D: Unconsolidated floatstone E: Branching coral framestone
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Drill Core Investigations: Data Analysis
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A: Encrusting coral-algal bindstone High-Energy B: Mixed skeletal rubble High-Energy
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Drill Core Investigations: Data Analysis A: Encrusting coral-algal bindstone High-Energy B: Mixed skeletal rubble High-Energy C: Massive coral framestone Mid-Energy
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Drill Core Investigations: Data Analysis A: Encrusting coral-algal bindstone High-Energy B: Mixed skeletal rubble High-Energy C: Massive coral framestone Mid-Energy D: Unconsolidated floatstone Mid-Energy
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Drill Core Investigations: Data Analysis E: Branching coral framestone Low-Energy A: Encrusting coral-algal bindstone High-Energy B: Mixed skeletal rubble High-Energy C: Massive coral framestone Mid-Energy D: Unconsolidated floatstone Mid-Energy
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Drill Core Investigations: Data Analysis
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910 cal yr BP
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Drill Core Investigations: Data Analysis
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4,812 cal yr BP
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Drill Core Investigations: Conclusions Decreasing age of reef toward shore Distinct facies change between ~8,100 cal yr BP and ~7,900 cal yr BP Youngest recorded age ~4,800 cal yr BP Sequence is exposed, not buried under continued accretion
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Drill Core Investigations: Conclusions Decreasing age of reef toward shore Distinct facies change between ~8,100 cal yr BP and ~7,900 cal yr BP Youngest recorded age ~4,800 cal yr BP Sequence is exposed, not buried under continued accretion
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Drill Core Investigations: Conclusions Possible causes: Decreasing age of reef toward shore (long term, 8,000 years), -Local relative sea-level rise (Hawaiian Islands): island subsidence, localized oceanic thermal expansion -Eustatic sea-level rise (Global): Glacial melting, thermal warming, basin volume changes Distinct facies change (short term, 200 years change ~8,100 cal yr BP): -Rapid local relative sea-level rise (Hawaiian Islands): island subsidence, localized oceanic thermal expansion -Rapid eustatic sea-level rise (Global): Catastrophic Rise Event III, 8.2 ka event
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Drill Core Investigations: Conclusions
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Rapid sea-level rise
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Drill Core Investigations: Conclusions Rapid sea-level rise
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Drill Core Investigations: Conclusions Rapid sea-level rise
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Drill Core Investigations: Conclusions Rapid sea-level rise
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Drill Core Investigations: Conclusions Decreasing age of reef toward shore Distinct facies change between ~8,100 cal yr BP and ~7,900 cal yr BP Youngest recorded age ~4,800 cal yr BP Sequence is exposed, not buried under continued accretion
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Drill Core Investigations: Conclusions Possible causes: Youngest age recorded, ~4,800 cal yr BP -Change in environmental conditions: change in El Nino, Southern Oscillation (ENSO) patterns, changes in wave shadowing (increasingly vertical topography), Penguin Bank Sequence is exposed, not buried under continued accretion -Erosion -Changes in wave shadowing
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Drill Core Investigations: Conclusions Modern Shoreline -10 m shoreline -20 m shoreline
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Overview: I. Introduction: Objectives and Strategy II. Modern Ecosystem Investigations i. Data collection: Methodology ii. Data Analysis: Substrate and coral types iii. Conclusions: Model of modern ecosystem zonation for SW Molokai III. Drill Core Investigations i. Data collection: Methodology ii. Data Analysis: Facies identification and distribution iii. Conclusions: Facies interpretation IV. Synthesis V. Conclusions
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Synthesis Spatial variability of the Molokai modern reef ecosystem? Increasing wave energy, by depth changes and exposure to North Pacific swell, = decreased coral cover and a shift from delicate coral morphologies to robust coral morphologies
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Synthesis Influence of Holocene sea level on the SW Molokai reef system? -Early Holocene sea-level rise caused a landward migration of reef accretion centers. -Rapid sea-level rise between ~8,100 cal yr BP and ~7,900 cal yr BP changed depositional environments at Hale O Lono. -Stabilization of late Holocene sea level forced lateral progradation of reefs at Hikauhi Spatial variability of the Molokai modern reef ecosystem? Increasing wave energy, by depth changes and exposure to North Pacific swell, = decreased coral cover and a shift from delicate coral morphologies to robust coral morphologies
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Overview: I. Introduction: Objectives and Strategy II. Modern Ecosystem Investigations i. Data collection: Methodology ii. Data Analysis: Substrate and coral types iii. Conclusions: Model of modern ecosystem zonation for SW Molokai III. Drill Core Investigations i. Data collection: Methodology ii. Data Analysis: Facies identification and distribution iii. Conclusions: Facies interpretation IV. Synthesis V. Conclusions
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Drill Core Investigations: Late Holocene accretion at Hikauhi has consisted of lateral progradation of the reef structure. In the early Holocene at Hale O Lono, the reef back-stepped under rising sea levels which likely resulted from a combination of both local and global influences. The distinct facies change at Hale O Lono between ~8,100 cal yr BP and ~7,900 cal yr BP is likely the result of rapid global sea-level rise, perhaps associated with CREIII or the 8.2 ka event. Reef accretion at Hale O Lono terminated at ~4,800 cal yr BP due to changes in environmental conditions, possibly related to a reduction in wave shadowing by Laau Point or ENSO changes. The transgressive sequence at Hale O Lono remains exposed, possibly due to a decrease in wave shadowing by Laau Point Conclusions: Modern Ecosystem Investigations: On a small scale (by site), substrate and coral types change with depth. On a large scale (western end of Molokai), substrate and coral types change with proximity to Laau Point, the refraction point for North Pacific swell.
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