Carbonate sediment supply on oceanic islands: A model and its applications Jodi N. Harney Charles H. Fletcher University 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 varies with 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 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 CaCO 3 production rates of sediment- producing organisms Calculate annual sediment production

Substrate Mapping Line 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 Physiographic zones

Measuring coral growth and bioerosion Rates consistent with those published for Hawaiian reefs (e.g. Grigg 1995) GPRe = 2.8 kgm -2 y -1 GPRfb = 10.7 kgm -2 y -1 GPRm = 8.4 kgm -2 y -1 Bioerosion (Bz) = 0.2–1 kgm -2 y -1

GPR Ho = 6.5 kgm -2 y -1 GPR F = 0.1–0.4 kgm -2 y -1 GPR apg = 10 kgm -2 y -1 Halimeda Benthic forams (and micromolluscs) Articulated coralline algae Clear plants from a measured area of seafloor; remove organic matter; measure CaCO 3 content in kgm -2 Collect samples of rubble; remove living organisms; measure CaCO 3 content in kgm -2 Collect individual living clumps; remove organic matter; measure CaCO 3 content in kgm -2 Measuring standing crop of direct producers Rates consistent with those in literature GPR M = 0.1–0.4 kgm -2 y -1

Rates of CaCO 3 production and erosion Gross Production Rates (kgm -2 y -1 ): –1.0 = GPR e (encrusting coral) = GPR m (massive coral) = GPR sb (stout-branching coral) = GPR fb (finger-branching coral) = GPR ace (encrust. coralline algae) = B z (bioerosion rate by zone) Sources include: Grigg 1982, 1995, 1998; Agegian 1985 Direct Production Rates (kgm -2 y -1 ): 0.3– – – – –17.8 = GPR Hd Halimeda discoidea = GPR Ho Halimeda opuntia = GPR M micromolluscs = GPR F benthic forams = GPR apg 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 (m 2 ) Rugosity (expresses reef topography, R = 1–4) Percent living coral cover: C e encrusting (Porites lobata, Montipora patula, M. verrucosa) C m massive (Porites lobata) C sb stout-branching (Pocillopora meandrina) C fb finger-branching (Porites compressa) Percent coralline algae cover: C ace encrusting (Porolithon onkodes and others) C apg articulated (Porolithon gardineri) Percent Halimeda cover: C Hd H. discoidea C Ho H. opuntia Organism abundance by zone

Gross production by coral (each growth form: e, m, sb, fb) G e = C e A h GPR e Habitat area (m 2 ) A h = A s R Gross production by all coral forms G c = G e + G m + G sb + G fb Gross production by encrusting coralline algae G ace = C ace A h GPR ace Equations for gross framework production For each zone: Total unconsolidated sediment produced by bioerosion of reef framework (kgy -1 ) S F = (G c + G ace ) B z

Direct production by Halimeda S H = C H A h GPR H Habitat area (m 2 ) A h = A s R Direct production by forams S F = C F A h GPR F Direct production by micromolluscs S M = C M A h GPR M Equations for direct sediment production For each zone: Direct production by articulated coralline algae S apg = C apg A h GPR apg TOTAL sediment production (kgy -1 ) S T = S F + S D Sum of all direct sediment production sources S D = S H + S F + S M + S apg

Sediment production by zone Coral garden (SCG) S F = 34 x 10 3 kgy kgm -2 y -1 S D =1.5 x 10 3 kgy kgm -2 y -1 Seaward reef platform (S1) S F =329 x 10 3 kgy kgm -2 y -1 S D = 142 x 10 3 kgy kgm -2 y -1 Nearshore hardgrounds (NH) S F =121 x 10 3 kgy kgm -2 y -1 S D =110 x 10 4 kgy kgm -2 y -1 Rate of sediment production by Kailua reef complex = Range 0.3 – 2.0 kgm -2 y -1 Avg.0.86 kgm -2 y -1 (~700 cm 3 )

S F = 2982 ± 179 x 10 3 kgy -1 S D = 4498 ± 565 x 10 3 kgy -1 S T = 7480 ± 744 x 10 3 kgy -1 (average = 0.86 kgm -2 y -1 ) Total Sediment Production convert to volume AS V = 7039 ± 1172 m 3 y -1 Annual Sediment Volume

Holocene sediment budget, Kailua Bay Total Sediment Storage ± 2174 x 10 3 m 3 41 ( ± 7) % Total Holocene Sediment Production ± 5862 x 10 3 m 3 Sediment Lost (or unaccounted for) ± 8036 x 10 3 m 3 59 ( ± 7) % Applications

Coastal and carbonate dynamics Total calcareous sediment production Per reef surface area 41% stays in system, 4% goes to beach 7039 ± 1172 m 3 y -1 = m 3 m -2 y -1 Annual beach replenishment rate Net seasonal shoreline change, Kailua Beach (Gibbs et al. 2000) = 115 m 3 y m 3 m -1 beach length = 172,000 m 3 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 m 3 each year for 5000 years = 18.8 x10 6 m year carbonate sediment supply = 21.5 x 10 6 m 3 Holocene Shoreline Progradation

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 m 3 Kailua SS = 7039 m 3 y -1 System = 41% of budget Beach = 4% of budget Replacement rate ~ 115 m 3 y -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 m 3 y -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