515 3135 3595 modern 2339

Sea level higher than present 3500 years ago on the northern main Hawaiian Islands Eric E. Grossman and Charles H. Fletcher, III Department of Geology and Geophysics, School of Ocean and Earth Science and Technology University of Hawaii, 1680 East-West Road, Honolulu, Hawaii 96822 [email protected] the assumption that the Island of Oahu is stable and experienced a +6 m sealevel highstand (Ku et al. 1974) during the last interglacial. Oahu appears to have undergone uplift (0.03 - 0.07 mm/yr) during the Quaternary (Muhs and Szabo 1994; Grigg and Jones 1997), hence, last interglacial sea level on Oahu may have been closer to +2-3 m. Holocene sea level in Hawaii is argued to have never exceeded present (Easton and Olson 1976). This interpretation has been debated (Stearns 1977; Fletcher and Jones 1996). Here we reconstruct Holocene sea level above present that is in better agreement with sea-level histories on other Pacific islands (Pirazzoli 1991; Mitrovica and

ABSTRACT New data from an emerged coastal bench and associated fossil beach on Kapapa Island (Oahu), Hawaii, preserve a detailed history of middle to late Holocene sea level. These include 29 new calibrated radiocarbon ages and elevations indicating mean sea level reached a maximum position of 2.00 ± 0.35 m ca. 3500 yr B.P. These results correlate with additional evidence from Hawaii and other Pacific islands and provide constraints on Oahu’s long-term uplift rate (0.03-0.07 mm/yr), previously based solely on Pleistocene age shorelines. Our sea-level reconstruction is consistent with geophysical model predictions of Earth’s geoid response to the last deglaciation and with observations of increased Antarctic ice volume during the late Holocene. INTRODUCTION New information on the rate of global sea-level rise (2.1 ± 1.3 mm/yr; Nerem et al. 1997) continues the longstanding interest in the history and components of local relative sea levels.

Two important time periods serve as models of warmer climate and sea level position; the last interglacial ca. 125,000 yr B.P., and the middle Holocene ca. 5000 yr B.P. (Folland et al. 1990). Various studies in Hawaii and other areas (i.e., Edwards et al. 1993) rely on

157 40 W

Kapapa Isl.

Modern interitdal intertidal bench

21 25 N

Southeast Oahu Honolulu

leeward leeward

Mokulua Isls.

Hanauma Bay

3 1

3595 3618

E 3595 E 3135

4

Fossil beach 0

20 m

P

Emerged intertidal bench

E 515 E modern

P 2339

seaward seaward

Fossil beach stratigraphy (all ages are cal yr B.P.) 2

Trade winds

66 offshore offshore samples samples seaward seaward and and leeward leeward are are modern modern

Excavation Excavation pits pits P Samples E Emerged Emerged bench bnech samples samples Samples buried buried in in solution solution pipes pipes

Elevation (m, msl)

Figure 1. Geomorphology, stratigraphy, and geochronology of emerged fossil beach sediments on Kapapa Island, Oahu, Hawaii.

2

3

3.0

8

2.5

8

271

2.0

4

5

Key

153

1

277 324

6 339

359

4

6

339

Soil 8 2693 273 7 335 6

Sand 0

275

Cobble Eolianite

TABLE 1. NEW AGES OF HOLOCENE SHORELINE DEPOSITS ON OAHU, HAWAII 14C age†

Cal yr B.P.§ (2s )

Height (m msl)

Materials*

d 13C

Kapapa Island P. compressa, mid-unit (site 4) P. compressa, mid-unit (site 4) P. compressa, surface P. compressa, surface P. compressa, surface P. compressa, solution pipe P. reticulata, solution pipe P. compressa, solution pipe P. reticulata, solution pipe P. compressa, basal unit (site 3) P. compressa, basal unit (site 3) P. compressa, mid-unit (site 3) P. compressa, mid-unit (site 3) Pocillopora sp., upper unit (site 3) P. reticulata, upper unit (site 3) P. compressa, basal unit (site 1) Cypraea sp., basal unit (site 1) P. reticulata, basal unit (site1) P. compressa, mid-unit (site 1) P. compressa, basal unit (site 2) P. reticulata, basal unit (site 2) P. compressa, mid-unit (site 2) P. reticulata, mid-unit (site 2) P. reticulata, mid-unit (site 2) T. variablis, mid-unit (site 2) P. compressa, mid-unit (site 2) Pocillopora sp., upper unit (site 2) Pocillopora sp., upper unit (site 2)

-1.2 -0.7 -1.5 -1.2 -0.8 -0.5 2.6 -1.6 -2.5 0.7 -2.3 -0.3 -0.2 -1.6 2.8 -1.4 2.0 0.4 -1.2 -2.4 2.9 -1.1 2.5 2.2 3.0 0.9 -1.4 -1.0

3050 ± 70 3160 ± 70 3420 ± 70 580 ± 105 3810 ± 50 3810 ± 55 3830 ± 55 2800 ± 80 3530 ± 60 3965 ± 65 3680 ± 60 3625 ± 55 3080 ± 60 2595 ± 50 3510 ± 55 3855 ± 100 3740 ± 55 3500 ± 55 3050 ± 60 3770 ± 55 3530 ± 55 3590 ± 55 3380 ± 65 2965 ± 80 3520 ± 55 2885 ± 55 2555 ± 55 1750 ± 50

2720 (2870-2450) 2785 (3025-2685) 3135 (3345-2890) 47 (287-0) 3595 (3786-3425) 3595 (3798-3421) 3618 (3815-3445) 2339 (2672-2143) 3278 (3448-3006) 3807 (4007-3573) 3440 (3626-3267) 3371 (3559-3207) 2733 (2893-2490) 2114 (2299-1951) 3253 (3417-3047) 3638 (3924-3380) 3492 (3682-3349) 3242 (3401-3027) 2718 (2855-2456) 3548 (3716-3366) 3278 (3441-3077) 3345 (3518-3156) 3075 (3292-2860) 2663 (2759-2340) 3264 (3431-3063) 2462 (2694-2325) 2064 (2276-1894) 1182 (1300-1023)

2.1 ± 0.15 2.1 ± 0.15 1.68 ± 0.15 1.68 ± 0.15 1.68 ± 0.15 1.53 ± 0.15 1.53 ± 0.15 1.43 ± 0.15 1.43 ± 0.15 1.81 ± 0.15 1.81 ± 0.15 2.12 ± 0.15 2.42 ± 0.15 2.73 ± 0.15 2.73 ± 0.15 1.92 ± 0.15 1.92 ± 0.15 1.92 ± 0.15 2.31 ± 0.15 1.78 ± 0.15 1.78 ± 0.15 2.16 ± 0.15 2.16 ± 0.15 2.54 ± 0.15 2.54 ± 0.15 2.54 ± 0.15 2.79 ± 0.15 2.79 ± 0.15

Beta-76292 Beta-76293 Beta-79905 AA-18218 AA-18219 AA-18221 AA-18223 AA-18225 AA-18226 AA-18227 AA-18228 AA-18229 AA-18231 AA-18232 AA-18233 AA-18234 AA-18235 AA-18236 AA-18237 AA-18238 AA-18239 AA-18240 AA-18241 AA-18242 AA-18243 AA-18244 AA-18245 AA-18246

South Mokulua Island P. compressa, base of beach

0.0

800 ± 50

305 (465-210)

0.50 ± 0.25

Beta-80884

(1s )

Lab No.

*Materials: Cypraea sp., Periglypta reticulata, Pocillopora sp., Porites compressa, Spondylus sparsispinosus, Tricolia variablis. †14C ages are conventional radiocarbon ages; ±1s error reflects counting statistics. §Cal yr B.P. (1950), are calibrated ages (calendar years before present) with 2s probability-age ranges.

Peltier 1991). Our history is based upon a detailed chronostratigraphic analysis of emerged Holocene shoreline features and the correction of a preexisting reef accretion curve for environmental factors and long-term uplift. The new sealevel curve clarifies geophysical models of geoidal history and deglaciation, as well as Oahu’s tectonic stability. HOLOCENE SEA LEVEL IN HAWAII Stearns (1935) cited an emerged bench ~2 m above mean sea level (msl) on Kapapa Island and Hanauma Bay (Oahu) as evidence of wave abrasion under a sea-level highstand 5000 yr ago, the Kapapa Stand of the Sea. He later based this age on a radiocarbon date of fossil coral (GX-2673, 3485 ± 160 14C yr B.P.) found at 1.95 ± 0.45 m in Hanauma Bay (Stearns 1977). Modern

wave overwash (Ku et al. 1974) and salt weathering (Bryan and Stevens 1993) have also been proposed to explain the Hanauma Bay bench. Using 63 14C dates from cores in the Hanauma Bay reef, Easton and Olson (1976) proposed that Holocene reef growth commenced ca. 7000 14C yr B.P., and that between 5800 and 3500 14C yr B.P. vertical accretion ensued at 3.3 mm/yr; lateral accretion at 22.2 mm/yr has dominated since 3000 14C yr B.P. They concluded Holocene sea level was never above present, despite evidence of reef truncation (reeftop age ~2500 yr B.P.) and fringing reef growth 2.4 - 3.0 m below msl (i.e., reef accretion postdated sea-level rise). According to Montaggioni (1988), sea level would have been 2 - 3 m higher at the reef crest and up to 15 m higher at the reef

front. Reef accretion records the lower portion of the tidal range (Hopley 1986). In addition, the Easton and Olson (1976) curve contains a depth bias of 0.5 - 1 m too low due to their placement of “recovered” samples lacking depth control (cored open cavities) at the base of cored intervals. Despite these conflicting interpretations, the Hanauma Bay reef accretion curve (Easton and Olson 1976) remains as the only Holocene sea-level curve for Hawaii and the central North Pacific. METHODS We use proxies of paleosea level from Kapapa and South Mokulua Islands, and Hanauma Bay to reconstruct middle to late Holocene sea level on Oahu. We radiocarbon dated coral clasts, mollusc shells, and sands obtained from four excavation sites in the Kapapa Island fossil beach and along two crossshore transects on the bench surface (Fig. 1, Table 1). Sediments of similar composition from offshore of Kapapa and South Mokulua Islands were also dated to establish the temporal variation between offshore and emerged deposits. Elevations, derived using differential Global Positioning System and Electronic Distance Measurement, are accurate to within 0.15 m and are reported relative to modern msl. All ages reported here are calibrated radiocarbon dates (Stuiver and Reimer 1993) and incorporate a 400 yr global marine reservoir correction (Bard et al. 1990) and regional 115 ± 50 yr correction for samples obtained in Hawaiian waters that deviate from the global mean (Stuiver and Braziunas 1993). FOSSIL BEACH GEOLOGY Kapapa Island is comprised of lithified eolianite. Its surface is wave-planed in the form of an emerged bench ranging from 1.5 to 1.9 m in elevation that is karstified and overlain by a laterite soil. A fossil beach above the bench and soil

Figure 2. Paleo-mean sea level between 3800 and 2200 yr B.P. is reconstructed from Kapapa and South Mokulua Islands. Minimum estimate (a) is drawn immediately above Kapapa Island samples collected from emerged platform and basal unit, representing deposition at mean lower low water. Alternatively, we estimate a paleo-msl position of 2.02 ± 0.35 m (b) by subtracting wave set-up (0.5 ± 0.2 m) and uplift due to flexure (0.17 m, 0.05 mm/yr for 3500 yr) from mid-point of fossil beachface (2.69 ± 0.15 m), a proxy for msl (Moberly and Chamberlain 1964). Wave set-up is based on deep water significant waves ranging from normal trade wind waves (height 1.46 m, period 8.63 s) dominant 75% of year, to annual maxima (height 3.9 m, period 12.0 s) (Fletcher and Jones 1996). In addition, we calculate a paleo-msl position 1.99 ± 0.15 m (c) by correcting height of emerged intertidal bench (1.76 ± 0.15 m), a proxy for mllw (Hopley 1986), to msl (0.4 m) and for flexural uplift (0.17 m). Our best estimate of paleomsl (d) peaks 2.00 ± 0.35 m ca. 3500 yr B.P. where (b) and (c) intersect dated samples and allow for a tidal range correction (0.4 m) above lower intertidal sediments, forming the basis of (a).

Six samples collected offshore of the seaward and lee sides of Kapapa Island (this study; Fletcher and Jones 1996) dated modern. The occurrence of only fossil age sediments on the island and only modern ages offshore, suggests that the onshore and offshore sediment reservoirs are temporally distinct, and that the fossil beach represents an episode of deposition by littoral processes that no longer operate on the surface of the island. Thus, it is unlikely that modern or historical tsunamis or storm surge are responsible for the formation of the fossil beach. Furthermore, the age distribution and grading of beach sediments suggest that depostion was gradual and sustained over 2500 yr. SEA-LEVEL RECONSTRUCTION We interpret the stratigraphy of the beach as a transgressive-regressive sequence recording the landward migration of the foreshore toe during a sea-level rise followed by the seaward translation of this setting during the subsequent fall. A region of coarse sedi-

ment, located at the toe where the greatest amount of wave energy is expended on the beach face, can be observed on most modern beaches including those on Oahu and the lee of Kapapa Island. This is a depositional analog for the Holocene stratigraphy on Kapapa Island. On South Mokulua Island, eight ages of coral clasts and molluscs (2700 3600 yr B.P.) were obtained from a carbonate breccia preserved in a paleosealevel notch carved into and postdating an emerged reef of presumed last interglacial age (Fletcher and Jones 1996). Our GPS surveys correct the elevation of the breccia, which is removed from present-day nearshore processes, from 1.7 ± 0.25 m to 1.57 ± 0.15 m. As on Kapapa Island, samples in the active littoral zone are modern. Tide-gauge data in Hawaii show that the islands undergo differential landsea movements, perhaps due to lithospheric flexure, resulting from volcanic loading over the hotspot which depresses the sea floor, creating a moat and distal arch around the load. Model predic-

3.5 Paleo-mean sea level on Oahu: 2.00 ± 0.35 m ca. 3500 yr B.P. 3

2.5

Elevation (m, msl)

rises to 3.6 m at the berm crest. In cross-section it is asymmetric and has a steep offshore-sloping beach face that is the product of recent erosion. Interannual extreme storm and wave events erode the beach face and do not deposit sediments onto the island. The seafloor surrounding Kapapa Island is an abraded bedrock surface of Pleistocene-age limestone, largely devoid of loose sediment. The fossil beach is composed of lowsloping, fining-upward stratified layers of cobbles and sands that are younger in age upward and seaward. The sequence consists of a distinct clastsupported basal layer of cobbles and shells overlain by medium to coarse sands; a middle unit of cobbles and shells; an upper unit of sands with occasional cobbles; and top soil. The oldest ages (~3800 yr B.P.) are in the basal layer, while the youngest (~1200 yr B.P.) appear just below the berm crest. Sample dates are normally distributed with a modal age of 3595 yr B.P. (~3500 yr).

2

s

midpoint beachface (2.69 m)

u t

b

u emerged platform (1.76 m)

1.5

seaward edge of platform

1

0.5

0 0

c a - minimum b - beachface proxy c - emerged intertidal platform proxy d - best

d a

Corrections

Observations modern beach

1000

Kapapa Isl. S. Mokulua Isl. upper-unit modern beach mid-unit basal unit emerged platform samples from pipe

2000

3000

Age (cal yr B.P.)

s Set-up (wind/wave)

(Fletcher and Jones, 1996)

(lithospheric flexure) u Uplift (Muhs and Szabo, 1994) t Tide (msl-mllw)

4000

5000

6000

DISCUSSION The emerged intertidal bench at Kapapa Island (1.76 ± 0.15 m) has important implications for Oahu’s tectonic stability and past interglacial sea levels. An intertidal bench forming today at Kapapa Island is a proxy for mean lower low water (mllw) (Hopley 1986) and a modern analog for the emerged bench above. If the emerged bench is older than Holocene, its surface must have been planed off ca. 125 000 yr ago, when sea level was last near its present position. Conservative uplift of 0.03 mm/yr would place the bench ~2 m higher than its present elevation. As our data show, a better explanation is that the emerged bench surface is Holocene, abraded by the Kapapa highstand beginning ca. 5000 yr B.P. and peaking ~2 m above present ca. 3500 yr B.P. (Fig. 3). Our reconstruction provides late-Quaternary support for Oahu’s mean uplift rate. A regressive marine contact within the Hanalei (Kauai) coastal plain (Calhoun and Fletcher 1996) suggests the Kapapa highstand has regional validity.

Holocene sea level on Oahu (Hawaii) 4

ICE-4G

2

(2)

(3) ?

0

present msl beachrock

(1)

?

-2

Elevation (depth) (m, msl)

tions (Watts and ten Brink 1989) suggest that Oahu may be in the vicinity of the upward-flexing arch, which is consistent with the long-term uplift of the island. We reconstruct middle to late Holocene sea level on Oahu using three different methods (see caption, Fig. 2). The first relies on the relationship of the surveyed fossil beach samples to sea level. The other two are based on the relationship of geomorphic surfaces, the midpoint of the fossil beach face (a proxy for msl) and the surface of the emerged intertidal bench (a proxy for mllw), to sea level and nearshore pro-cesses. The convergence of these estimates between 1.99 and 2.02 ± 0.35 m ca. 3500 yr B.P. is strong support for the calculated tradewind wave set-up correction (Fletcher and Jones 1996) and the average long-term uplift rate of 0.05 mm/yr.

-4

? -6

-8 (3)

Proposed sea level (this study) Kapapa Isl. (this study)

-10

Windward Oahu (Fletcher and Jones, 1996) Kauai (Calhoun and Fletcher, 1996) Hanauma reef cores (Easton and Olson, 1976) Hanauma reef surface (Easton and Olson, 1976)

-12

(1)

Hanauma reef growth curve (best fit) y = -4.394 + 0.005* x -1.611e-06 *x 2+ 1.144e-10 * x 3 R = 0.99; y = depth (m), x = age (cal yr B.P.)

(2)

Coral habitat correction (Montaggioni, 1988)

fore reef

Flexural Uplift (Muhs and Szabo, 1994)

-14

0

ICE-4G (Peltier, 1994, 1996)

1000

2000

3000

4000

5000

6000

7000

8000

Age (cal yr B.P.) Fig. 3 by the minimum and best paleo-msl Figure 3. Holocene sea-level curve for Oahu is constrained reconstruction (envelope) of figure 2, and the reinterpretation of Hanauma Bay reef accretion history (triangles, Easton and Olson 1976). Reef sample depths are corrected from mean lower low water to mean sea level (0.4 m) and for uplift. Sample ages are calibrated to calendar years (Stuiver and Reimer 1993) using Easton and Olson’s “inner skeleton” ages for dated replicates. A third-order polynomial best fit (R = 0.99) to reef surface samples (open triangles) is the best model of maximum vertical accretion and minimum limit (1) for sea level. To enable coral survival, however, minimum sea level (1) must have been above these data points. Applying Montaggioni’s 2.5 m habitat correction, this minimum limit is elevated (2), intersecting our reconstructed sea-level highstand ca. 3000 yr B.P. In addition, the timing of transgression inferred from reef accretion assumes that the coral record tracked sea level, and factors including time lag for coral establishment, rate and magnitude of biological and mechanical erosion, and true depth of “recovered” coral remain uncertain. These considerations have the effect of shifting the minimum limit earlier in time (e.g., toward right). Our proposed sea-level curve (3), provides a testable hypothesis of sea-level movements on Oahu. An envelope representing the range of flexural uplift (hatching) shows the amount of uplift a shoreline feature of given age has undergone; maximum at 8000 B.P. is 0.24-0.56 m.

Wigley and Rapier (1987) proposed that an increase of 1 °C in global mean surface temperature (the best estimate of temperature during the middle Holocene climatic optimum, Folland et al. 1990) could raise sea level 4-8 cm due to thermal expansion of the water column. This leaves another ~1.9 m of the Kapapa Stand to be explained by other processes. The equatorial ocean siphoning model (ICE-4G, Peltier 1994, 1996) is consistent with our interpretation of middle to late Holocene sea level in Hawaii. However, it still overpredicts the height of msl in excess of 1 m, and misidentifies its culmination by ~1500 yr. This suggests that our data may not capture the oldest portion of the highstand peak (e.g. fossil beach has been eroded), and/or that the parameters controlling the timing and magnitude of the ICE-4G highstand need refinement. In fact, increased rates of ice accumulation during the late Holocene that may have led to Antarctic ice-volume expansion equivalent to a ~1.0 ± 0.2 m sea-level lowering (Goodwin 1998), would require a revision of the glacioeustatic histories prescribed by the geophysical models including ICE-4G. CONCLUSION

We have constructed a new Holocene sea-level curve for Oahu showing mean sea level higher than today between ~5000 and ~2000 yr ago with a maximum ~2 m above present ca. 3500 yr ago. This history should be used to test and constrain models of postglacial sea-level movements requiring the amplitude and timing of the highstand apex. Our reconstruction of Holocene sea level using three independent lines of evidence, including the elevations and ages of fossil beach sediments, the mid-point of the paleobeach face (a proxy for msl), and the surface of the emerged intertidal platform (a

proxy for mllw), confirms that Oahu is undergoing long-term uplift of ~0.05 mm/yr, consistent with lithospheric flexure associated with volcanic loading at the hotspot. Adjustments to the Hanauma Bay reef history (Easton and Olson 1976) for coral habitat range (Montaggioni 1988) and uplift nearly reconcile previous discrepancies in observed and predicted Holocene sea-level movements on Oahu. ACKNOWLEDGMENTS

Support for this research was provided by National Science Foundation (NSF) grant EAR9317328, U.S. Geological Survey grant 1434-94-A-1029, National Geographic Society grant 5273-94, and the University of Hawaii (Department of Geology and Geophysics Harold T. Stearns Fellowship and William T. Coulbourn Award). REFERENCES CITED Bard, E., Hamelin, B., Fairbanks, R. G., and Zindler, A., 1990, Calibration of the 14C timescale over the past 30,000 years using mass spectrometric U-Th ages from Barbados corals: Nature, v. 345, p. 405-410. Bryan, W. B., and Stevens, R. S., 1993, Coastal bench formation at Hanauma Bay, Oahu, Hawaii: Geological Society of America Bulletin, v. 105, p. 377-386. Calhoun, R. S., and Fletcher, C. H., 1996, Late Holocene coastal-plain stratigraphy and sealevel history at Hanalei, Kauai, Hawaiian Islands: Quaternary Research, v. 45, p. 47-58. Easton, W. H., and Olson, E. A., 1976, Radiocarbon profile of Hanauma Reef, Oahu, Hawaii: Geological Society of America Bulletin, v. 87, p. 711-719. Edwards, R. L., Beck, J. W., Burr, G. S., Donahue, D. J., Chappell, J. M. A., Bloom, A. L., Druffel, E. R. M., and Taylor, F. W., 1993, A large drop in atmospheric 14C/12C and reduced melting in the Younger Dryas, documented with 230Th ages of corals: Science, v. 260, p. 962-968. Fletcher, C. H., and Jones, A. T., 1996, Sea-level highstand recorded in Holocene shoreline deposits on Oahu, Hawaii: Journal of Sedimentary Research, v. 66., p. 632-641. Folland, C. K., Karl, T., and Vinnikow, K.Y. A., 1990, Observed climate variations and change, in Houghton, J. T., Jenkins, G. J., and

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