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