carbonate sediment supply on oceanic islands: a model and its applications jodi n. harney charles h....
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![Page 1: Carbonate sediment supply on oceanic islands: A model and its applications Jodi N. Harney Charles H. Fletcher University of Hawaii Dept. of Geology and](https://reader036.vdocuments.mx/reader036/viewer/2022062620/5519d3d35503468b0c8b48e8/html5/thumbnails/1.jpg)
Carbonate sediment
supply on oceanic
islands:
A model and its
applicationsJodi N. Harney
Charles H. FletcherUniversity of Hawaii
Dept. of Geology and Geophysics
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OUTLINE
Introduction, objectives, and approach in Kailua Bay, Oahu
Methods
Applications
Conclusions
Substrate mapping Physiographic zonation Sediment production
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Quantitative estimates of sources, sinks, fluxes, losses of sediment within a defined system
Sediment Budgets
• among primary controls of coastal morphology and evolution
• affects development of beaches, dunes, reefs• can be instrumental in predicting and interpreting
coastal behavior
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Beaches in Hawaii
Calcareous skeletal remains of reef-dwelling organisms
Dark detrital grains derived from volcanic
rocks
(Moberly et al. 1965)
Relative proportion varieswith local conditions
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On oceanic islands in low latitudes, calcareous sediment supply is controlled by shallow-marine carbonate productivity (reefs and associated settings)
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Kailua Bay, Oahu carbonate reef
complex 0–25 m water depth
200-m wide paleostream channel bisects platform
seaward mouth opens onto 30–70 m deep sand field
high-resolution central portion is MS imagery
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Multispectral imagery (Isoun et al. 1999)
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Sediment composition and age
Harney et al. 2000. Coral Reefs 19:141–154.
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Approach
Map distribution and abundance of carbonate producers across the reef complex
Define physiographic zones in terms of benthic communities
Measure CaCO3 production rates of
sediment-producing organisms
Calculate annual sediment production
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Substrate MappingLine transect method
• distribution and abundance of substrate types (rubble, sand, dead coral, living coral, coralline algae, Halimeda)
• reef topography (rugosity)
• community structure
• species composition
• growth form
Each transect map provides >50 variables that describe:
52 sites mapped in Kailua Bay
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each with a suite of biogeological characteristics based on mapping data collected within zone
zone area measured using image analysis software and corrected for reef rugosity
Physiographiczones
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Measuring coral growth and bioerosion
Rates consistent with those published for Hawaiian reefs (e.g. Grigg 1995)
GPRe =2.8 kgm-2y-1
GPRfb =10.7 kgm-2y-1
GPRm =8.4 kgm-2y-1
Bioerosion (Bz) =0.2–1 kgm-2y-1
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GPRHo =6.5 kgm-2y-1
GPRF =0.1–0.4 kgm-2y-1
GPRapg =10 kgm-2y-1
Halimeda
Benthic forams(and micromolluscs)
Articulatedcoralline algae
Clear plants from a measured area of seafloor; remove organic matter; measure CaCO3 content in kgm-2
Collect samples of rubble; remove living organisms; measure CaCO3 content in kgm-2
Collect individual living clumps; remove organic matter; measure CaCO3 content in kgm-2
Measuring standing crop of direct producers
Rates consistent with those in literature
GPRM =0.1–0.4 kgm-2y-1
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Rates of CaCO3 production and erosion
Gross Production Rates (kgm-2y-
1):
2.8
8.4
6.7
10.7
2.6
0.2–1.0
= GPRe (encrusting coral)
= GPRm (massive coral)
= GPRsb (stout-branching coral)
= GPRfb (finger-branching coral)
= GPRace(encrust. coralline algae)
= Bz (bioerosion rate by zone)
Sources include:Grigg 1982, 1995, 1998; Agegian 1985
Direct Production Rates (kgm-2y-
1):
0.3–3.0
6.4–6.7
0.05–1.8
0.05–0.1
10.0–17.8
= GPRHd Halimeda discoidea
= GPRHo Halimeda opuntia
= GPRM micromolluscs
= GPRF benthic forams
= GPRapg articulated
coralline algae
Comparable to data from sources including:Drew & Abel 1985, Payri 1988, Hillis 1997
(Halimeda)Hallock 1981, 1984 (forams)
Agegian 1985 (artic. coralline algae)
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For each zone, mapping data is pooled and averaged:
Habitat area (m2)
Rugosity (expresses reef topography, R = 1–4)
Percent living coral cover:Ce encrusting (Porites lobata, Montipora patula, M.
verrucosa)Cm massive (Porites lobata)
Csb stout-branching (Pocillopora meandrina)
Cfb finger-branching(Porites compressa)
Percent coralline algae cover:Cace encrusting (Porolithon onkodes and others) Capg articulated (Porolithon gardineri)
Percent Halimeda cover:CHd H. discoideaCHo H. opuntia
Organism abundance by zone
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Gross production by coral(each growth form: e, m, sb, fb)
Ge = Ce Ah GPRe
Habitat area (m2) Ah = As R
Gross production by all coral forms
Gc = Ge + Gm + Gsb + Gfb
Gross production by encrusting coralline algae
Gace = Cace Ah GPRace
Equations for gross framework production
For each zone:
Total unconsolidated sediment produced by bioerosion of reef framework (kgy-1)
SF = (Gc + Gace) Bz
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Direct production by Halimeda SH = CH Ah GPRH
Habitat area (m2) Ah = As R
Direct production by forams SF = CF Ah GPRF
Direct production by micromolluscs
SM = CM Ah GPRM
Equations for direct sediment production
For each zone:
Direct production by articulated coralline algae
Sapg = Capg Ah GPRapg
TOTAL sediment production(kgy-1)
ST = SF + SD
Sum of all direct sediment production sources
SD = SH + SF + SM + Sapg
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Sediment production by zone
Coral garden (SCG)
SF = 34 x 103 kgy-1
0.39 kgm-2y-1
SD = 1.5 x 103 kgy -1
0.01 kgm-2y-1
Seaward reefplatform (S1)
SF =329 x 103 kgy-
1
0.35 kgm-2y-1
SD = 142 x 103 kgy-1
0.13 kgm-2y-1
Nearshore hardgrounds (NH)
SF =121 x 103 kgy-
1
0.19 kgm-2y-1
SD = 110 x 104 kgy-1
1.81 kgm-2y-1
Rate of sediment production by Kailua reef complex =Range 0.3 – 2.0 kgm-2y-1 Avg. 0.86 kgm-2y-1 (~700 cm3)
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SF = 2982 ± 179 x 103 kgy-1 SD = 4498 ± 565 x 103 kgy-1
ST = 7480 ± 744 x 103 kgy-1
(average = 0.86 kgm-2y-1 )
Total Sediment Production
convert to volume
ASV = 7039 ± 1172 m3 y-1
Annual Sediment Volume
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Holocene sediment budget, Kailua Bay
Total Sediment Storage
14375 ± 2174 x 103 m3
41 (± 7) %
Total Holocene Sediment Production
35196 ± 5862 x 103 m3
Sediment Lost(or unaccounted for)
20821 ± 8036 x 103 m3
59 (± 7) %
Applications
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Coastal and carbonate dynamics
Total calcareous sediment productionPer reef surface area
41% stays in system, 4% goes to beach
7039 ± 1172 m3 y-1
= 0.0007 m3m-2y-1
Annual beach replenishment rateNet seasonal shoreline change,Kailua Beach(Gibbs et al. 2000)
= 115 m3 y-1
43 m3 m-1 beach length
= 172,000 m3 annual flux
Difference in rates of beach supply and shoreline change is 3 orders of
magnitude
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HANALEI,KAUAI
•Holocene progradation history required additional calcareous sediment supplied by transport from Anini reef: 3760 m3 each year for 5000 years = 18.8 x106 m3
•5000 year carbonate sediment supply = 21.5 x 106 m3
HoloceneShorelineProgradation
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KIHEI,MAUI
•Erosion along the south Kihei coast is linked to the northward transport of coastal sediments
•In the last century, a volume equivalent to 1600 years of carbonate sediment production has migrated from south Kihei northward
Shoreline Change
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LANIKAI,OAHU
+ 12,000 m3
Kailua SS = 7039 m3y-1
System = 41% of budgetBeach = 4% of budgetReplacement rate ~ 115 m3y-1
Replacement time ~ 100 y
Beach Renourishment
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CONCLUSIONS Carbonate sediment supply is an important factor
in the behavior and evolution of coastal margins; depends on reef productivity; can be estimated using a field-based model
Annual rates of sediment supply are instrumental in developing sediment budgets and understanding coastal behavior over space and time
In Kailua, carbonate sediments are produced at a rate of 7039 ± 1172 m3y-1; 41% of those produced in the last 5000 years remain stored in bay and coastal plain
The Kailua model is the most comprehensive, field-based effort on the largest system to date; first for Hawaii; can be applied to other reef systems
Rates at which reefs produce sediment are slow compared to rates of shoreline change
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Mahalo