Carbonate sediment supply on oceanic islands: A model and its applications

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 Geophysics

2. OUTLINE • Introduction, objectives, and approach in Kailua Bay, Oahu • Methods • Applications • Conclusions • Substrate mapping • Physiographic zonation • Sediment production

3. Sediment Budgets Quantitative estimates of sources, sinks, fluxes, losses of sediment within a defined system • among primary controls of coastal morphology and evolution • affects development of beaches, dunes, reefs • can be instrumental in predicting and interpreting coastal behavior

4. Beaches in Hawaii (Moberly et al. 1965) Dark detrital grains derived from volcanic rocks Calcareous skeletal remains of reef-dwelling organisms Relative proportion varies with local conditions

5. On oceanic islands in low latitudes, calcareous sediment supply is controlled by shallow-marine carbonate productivity (reefs and associated settings)

6. 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

7. Multispectral imagery (Isoun et al. 1999)

8. Sediment composition and age Harney et al. 2000. Coral Reefs 19:141–154.

10. 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

11. Substrate Mapping Line transect method Each transect map provides >50 variables that describe: • distribution and abundance of substrate types (rubble, sand, dead coral, living coral, coralline algae, Halimeda) • reef topography (rugosity) • community structure • species composition • growth form 52 sites mapped in Kailua Bay

12. Physiographic zones • 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

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

14. Measuring standing crop of direct producers Halimeda Clear plants from a measured area of seafloor; remove organic matter; measure CaCO3 content in kgm-2 GPRHo = 6.5 kgm-2y-1 Benthic forams (and micromolluscs) Collect samples of rubble; remove living organisms; measure CaCO3 content in kgm-2 GPRF = 0.1–0.4 kgm-2y-1 GPRM = 0.1–0.4 kgm-2y-1 Collect individual living clumps; remove organic matter; measure CaCO3 content in kgm-2 Articulated coralline algae GPRapg = 10 kgm-2y-1 Rates consistent with those in literature

15. Gross Production Rates (kgm-2y-1): 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 = GPRHdHalimeda discoidea = GPRHoHalimeda opuntia = GPRMmicromolluscs = GPRFbenthic forams = GPRapgarticulated coralline algae 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 Comparable to data from sources including: Drew & Abel 1985, Payri 1988, Hillis 1997 (Halimeda) Hallock 1981, 1984 (forams) Agegian 1985 (artic. coralline algae) Rates of CaCO3 production and erosion

16. Organism abundance by zone 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. discoidea CHo H. opuntia

17. Habitat area (m2) Ah = As R Gross production by coral (each growth form: e, m, sb, fb) Ge = Ce Ah GPRe Gross production by all coral forms Gc = Ge + Gm + Gsb + Gfb Gross production by encrusting coralline algae Total unconsolidated sediment produced by bioerosion of reef framework (kgy-1) Gace = Cace Ah GPRace SF = (Gc+ Gace)  Bz Equations for gross framework production For each zone:

18. Habitat area (m2) Ah = As R Direct production by Halimeda SH = CH Ah GPRH Direct production by forams SF = CF Ah GPRF Direct production by micromolluscs Direct production by articulated coralline algae Sum of all direct sediment production sources SM = CM Ah GPRM Sapg = Capg Ah GPRapg SD = SH + SF + SM + Sapg Equations for direct sediment production For each zone: ST = SF+ SD TOTAL sediment production (kgy-1)

19. Sediment production by zone 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 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 reef platform (S1) SF = 329 x 103 kgy-1 0.35 kgm-2y-1 SD = 142 x 103 kgy-1 0.13 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)

20. Total Sediment Production convert to volume ASV = 7039 ± 1172 m3 y-1 Annual Sediment Volume 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 )

21. Applications Total Holocene Sediment Production 35196 ± 5862 x 103 m3 Total Sediment Storage 14375 ± 2174 x 103 m3 41 (± 7) % Sediment Lost (or unaccounted for) 20821 ± 8036 x 103 m3 59 (± 7) % Holocene sediment budget, Kailua Bay

22. Coastal and carbonate dynamics Total calcareous sediment production Per reef surface area 41% stays in system, 4% goes to beach 7039 ± 1172 m3 y-1 = 0.0007 m3m-2y-1 Annual beach replenishment rate Net seasonal shoreline change, Kailua Beach (Gibbs et al. 2000) = 115 m3 y-1 43 m3 m-1beach length = 172,000 m3 annual flux Difference in rates of beach supply and shoreline change is 3 orders of magnitude

23. HANALEI, KAUAI • 5000 year carbonate sediment supply = 21.5 x 106 m3 • Holocene progradation history required additional calcareous sediment supplied by transport from Anini reef: 3760 m3each year for 5000 years = 18.8 x106 m3 Holocene Shoreline Progradation

24. KIHEI, MAUI Shoreline Change • 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

25. LANIKAI, OAHU + 12,000 m3 Beach Renourishment Kailua SS = 7039 m3y-1 System = 41% of budget Beach = 4% of budget Replacement rate ~ 115 m3y-1 Replacement time ~ 100 y

26. 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

27. Mahalo