Holocene Reef Accretion, Southwest Molokai, Hawaii Mary Engels

Holocene Reef Accretion, Southwest Molokai, Hawaii

Mary Engels

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

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


ii. Data Analysis: Facies identification and distribution

iii. Conclusions: Facies interpretation

IV. Synthesis

V. Conclusions

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.

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 SITEHALE O LONO SITE

LAAU POINT

0 km

0 mi

1 km

1 mi

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.

Strategy:


Application:



Survey and model modern ecosystem zonation (PRESENT)

Strategy:


Application:




Identification of lithofacies from drill cores (PAST)

Strategy:


Application:





Comparison of lithofacies and modern ecosystem model to determine

depositional environments (PRESENT + PAST)

Strategy:


Application:





Comparison of lithofacies and modern ecosystem model to determine

depositional environments (PRESENT + PAST)

Reconstruct paleoecosystem zonation from lithofacies distribution

(PRESENT+PAST = RECORD)

Strategy:

Overview:










IV. Synthesis

V. Conclusion

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

HIKAUHIHALE O LONO

0 km

0 mi

1 km

1 mi

Modern Ecosystem Investigations: Data Analysis

0% 20% 40% 60% 80% 100%

% of 100

> 10 m

5 to 10 m

< 5 m

> 10 m

5 to 10 m

< 5 m

Hal

e O

Lon

o

H

ikau

hi

Substrate types

Fossil reef (FR) - bare

Fossil Reef w/ sand

FR w/ encrusting coralline algae (CA)

Coral - living

Dead coral - bare

Dead coral w/ encrusting CA

Rubble

Sand

Zoanthid

Modern Ecosystem Investigations: Data Analysis

0% 20% 40% 60% 80% 100%

% normalized to 100

>10m

5m to 10m

< 5m

>10m

5m to 10m

< 5m

Hal

e O

Lon

o

Hik

auhi

Coral types

Encrusting M. flabellata

Encrusting M. patula

Encrusting M. verrucosa

Encrusting P. lobata

Finger branching P. compressa

Massive P. lobata

Pavona varians

Platy P. lobata

Stout Branching P. meandrina

Modern Ecosystem Investigations: Results


Modified from Storlazzi et al. (in press)

Study Sites


Modified from Storlazzi et al. (in press)

Modern Ecosystem Investigations: Conclusions

Overview:










IV. Synthesis

V. Conclusions

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

157d 16’ 0” 157d 15’ 0”

21d 4’ 30”

157d 16’ 0” 157d 15’ 0”

21d 5’ 30”21d

5’ 3

0 ”21

d 4’

30”

• 5.5 m

• 8.5 m• 14.0 m

• 17.7 m• 21.0 m

Drill Core Investigations: Data Collection

Hale O Lono Hikauhi

157d 10’ 45”

157d 10’ 45”

157d 9’ 45”

157d 9’ 45”

21d 5’ 30”

21d

5’ 3

0 ”21

d 4’

45” 21d 4’ 45”

4.0 m5.5 m

9.1 m

14.3 m

19.8 m

9.8 m

6.1 m

10.7 m

18.3 m12.8 m

0 km

0 mi

0.5 km

0.5 mi

0 km

0 mi

0.5 km

0.5 mi

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


A: Encrusting coral-algal bindstone High-Energy

B: Mixed skeletal rubble High-Energy




C: Massive coral framestone Mid-Energy





D: Unconsolidated floatstone Mid-Energy


E: Branching coral framestone Low-Energy




D: Unconsolidated floatstone Mid-Energy


910 cal yr BP


4,812 cal yr BP

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


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


Rapid sea-level rise


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



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


-Erosion

-Changes in wave shadowing


Modern Shoreline

-10 m shoreline

-20 m shoreline

Overview:










IV. Synthesis

V. Conclusions

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

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

Overview:










IV. Synthesis

V. Conclusions

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 environmentalconditions, 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.

Mahalo!

Holocene Reef Accretion, Southwest Molokai, Hawaii Mary Engels

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