Carbonate sediment supply on oceanic islands: A model and its applications
description
Transcript of Carbonate sediment supply on oceanic islands: A model and its applications
Carbonate sediment
supply on oceanic
islands:
A model and its
applicationsJodi N. Harney
Charles H. FletcherUniversity of Hawaii
Dept. of Geology and Geophysics
OUTLINE
Introduction, objectives, and approach in Kailua Bay, Oahu
Methods
Applications
Conclusions
Substrate mapping Physiographic zonation Sediment production
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
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
On oceanic islands in low latitudes, calcareous sediment supply is controlled by shallow-marine carbonate productivity (reefs and associated settings)
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
Multispectral imagery (Isoun et al. 1999)
Sediment composition and age
Harney et al. 2000. Coral Reefs 19:141–154.
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
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
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
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
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
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)
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
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
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
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)
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
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
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
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
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
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
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
Mahalo