Charles H. Fletcher

The Holocene sea-level highstand in the equatorial Pacific: analysis of the insular paleosea-level database

Abstract A review of the literature provides 92 estimates of the middle to late Holocene sea-level highstand on Pacific Islands. These data generally support geophysical model calculations that predict a +1 to 3 m relative sea-level highstand on oceanic islands due to the Earth’s rheological response to the melting of the last continental ice sheets and subsequent redistribution of meltwater. Both predictions and observations indicate sea level was higher than present in the equatorial Pacific between 5000 and 1500 y B.P. A non-linear relationship exists between the age and elevation of the highstand peak, suggesting that different rates of isostatic adjustment may occur in the Pacific, with the highest rates of sea-level fall following the highstand near the equator. It is important to resolve detailed sea-level histories from insular sites to test and refine models of climatic, oceanographic, and geophysical processes including hydroisostasy, equatorial ocean siphoning, and lithospheric flexure that are invoked as mechanisms affecting relative sea-level position. We use a select subset of the available database meeting specific criteria to examine model relationships of paleosea-surface topography. This new evaluated database of paleosea-level positions is also validated for testing and constraining geophysical model predictions of past and present sea-level variations.

The Predictive Accuracy of Shoreline Change Rate Methods and Alongshore Beach Variation on Maui, Hawaii

Abstract Beach erosion has direct consequences for Hawaiis tourist-based economy, which depends on the attraction of beautiful sandy beaches. Within the last century, however, beaches on Oahu and Maui have been narrowed or completely lost, threatening tourism and construction development. In order for the counties and state of Hawaii to implement coastal regulations to prevent infrastructure damage, it is necessary to find a statistically valid methodology that accurately delineates annual erosion hazard rates specific to Hawaii. We compare the following erosion rate methods: end point rate (EPR), average of rates (AOR), minimum description length (MDL), jackknifing (JK), ordinary least squares (OLS), reweighted least squares (RLS), weighted least squares (WLS), reweighted weighted least squares (RWLS), least absolute deviation (LAD), and weighted least absolute deviation (WLAD). To evaluate these statistical methods, this study determines the predictive accuracy of various calculated erosion rates, including the effects of a priori knowledge of storms, using (1) temporally truncated data to forecast and hindcast known shorelines and (2) synthetic beach time series that contain noise. This study also introduces binning of adjacent transects to identify segments of a beach that have erosion rates that are indistinguishable. If major uncertainties of the shoreline methodology and storm shorelines are known, WLS, RWLS, and WLAD better reflect the data; if storm shorelines are not known, RWLS and WLAD are preferred. If both uncertainties and storm shorelines are not known, RLS and LAD are preferred; if storm shorelines are known, OLS, RLS, JK, and LAD are recommended. MDL and AOR produce the most variable results. Hindcasting results show that early twentieth century topographic surveys are valuable in change rate analyses. Binning adjacent transects improves the signal-to-noise ratio by increasing the number of data points.

Tidal wetland record of holocene sea-level movements and climate history

Abstract Paleontological, geochemical, and lithological indicators of former marine and nonmarine influence are correlated between 13 cores from a tidal wetland (Wolfe Glade) on the southeast coast of Delaware Bay. Twenty new radiocarbon dates are used to establish the chronostratigraphy of marsh facies. Sediment iron content, weight loss on ignition, and remains of plants, diatoms and foraminifera (known to tolerate specific tidal inundation cycles) are used to describe cored facies originating from supratidal, intertidal and subtidal salt marsh subenvironments. Pollen assemblages are used to infer local Holocene climate fluctuations and to reconstruct paleoenvironments during marsh evolution. The record of paleoclimatology suggests the Atlantic Chron was warm with variable humidity in the region, whereas the Subboreal was dominated by cool and humid conditions. The Subatlantic was warmer, and initially dryer, with a cool humid phase prior to about 1 ka, returning to warmer, humid conditions in the last several centuries. Local relative sea-level movements are characterized by five rapid, short-term episodes when the rate of sea-level rise accelerated relative to the rate of marsh aggradation. These episodes are recorded by transgressive facies contacts at 5.3 ± 0.2 ka, 4.4 ± 0.2 ka, 3.25 ± 0.2 ka, and 1.8 ± 0.2 ka, sidereal years. An earlier transgressive facies contact dated 6.9 ± 0.2 ka is probably not the product of a true sea-level movement. Sea-level movements are recorded throughout the marsh as palustrine or high marsh peats or peaty overlain by lower intertidal or subtidal marine deposits. Several features suggest that these episodes are local relative transgressions produced by short-term accelerations in the rate of sea-level rise relative to marsh aggradation: the contemporaneity and apparent suddenness of marine inundations: the sequence of facies indicating marine drowning; the presence of similar events in marshes elsewhere in Delaware Bay; and the marsh-wide extent of indicative facies transitions. We propose that the rapid and frequent sea-level movements observed in Wolfe Glade are the result of surges and relaxations in the Gulf Stream (and associated spin-up and partial collapse of the North Atlantic gyre) in response to winds generated by changes in the North Atlantic atmospheric thermal gradient associated with Holocene climate fluctuations.

Sea-level Highstand Recorded in Holocene Shoreline Deposits on Oahu, Hawaii

ABSTRACT Unconsolidated carbonate sands and cobbles on Kapapa Island, windward Oahu, are 1.4-2.8 (± 0.25) m above present mean sea level (msl). Agreeing with Stearns (1935), we interpret the deposit to be a fossil beach or shoreline representing a highstand of relative sea level during middle to late Holocene time. Calibrated radiocarbon dates of coral and mollusc samples, and a consideration of the effect of wave energy setup, indicate that paleo-msl was at least 1.6 (± 0.45) m above present msl prior to 3889-3665 cal. yr B.P, possibly as early as 5532-5294 cal. yr B.P., and lasted until at least 2239-1940 cal. yr B.P. Hence, the main phase of deposition on Kapapa Island lasted a minimum of c. 1400 yr and possibly as long as c. 3400 yr. No modern samples have been recovered from the fossil beach on Kapapa Island, and samples from potential source sites offshore of the island show modern ages, indicating that sediments on the island are not deposited by modern-era storm and tsunami overwash. Because antecedent sediments are uncommon offshore but common on the island, deposition must have been time-transgressive rather than related to a single event. Radiocarbon ages of coral and mollusc clasts from a breccia lining an emerged (1.4 ± 0.25 m msl) intertidal notch, cut into emerged coralline-algal carbonate of presumed last interglacial age, on south Mokulua Island (15 km to the southeast of Kapapa Island) correlate to the history recorded on Kapapa Island. Calibrated ages range from 2755-2671 to 3757-3580 cal. yr B.P. (averaging c. 3100 cal. yr B.P.) suggesting that a higher than present sea level formed the notch prior to 3757-3580 cal. yr B.P. A storm or tsunami origin for the features on Kapapa and south Mokulua islands is highly unlikely. Their age and elevation indicate, instead, a history of higher relative sea level (and subsequent fall) on windward Oahu during the middle to late Holocene. This history is consistent with geophysical models of postglacial geoid subsidence over the equatorial ocean first predicted by Walcott (1972) and later refined by Clark et al. (1978) and Mitrovica and Peltier (1991).

AGE AND COMPOSITION OF CARBONATE SHOREFACE SEDIMENTS, KAILUA BAY, OAHU, HAWAII

Abstract The origin, age, and dynamics of carbonate sediments in Kailua Bay on Oahu, Hawaii, are described. The shoreface (from shoreline to 4 km offshore) consists of a broad (5 km2) fringing coral reef ecosystem bisected by a sinuous, shore-normal, sand-filled paleostream channel 200–300 m wide. The median grain diameter of surface sands is finest on the beach face (<0.3 mm) and increases offshore along the channel axis. Kailua sands are >90% biogenic carbonate, dominated by skeletal fragments of coralline algae (e.g. Porolithon, up to 50%) followed by the calcareous green alga Halimeda (up to 32%), coral fragments (1–24%), mollusc fragments (6–21%), and benthic foraminifera (1–10%). Sand composition and age across the shoreface are correlated to carbonate production. Corals and coralline algae, principal builders of the reef framework, are younger and more abundant in sands along the channel axis and in offshore reefal areas, while Halimeda, molluscs, and foraminifera are younger and more dominant in nearshore waters shoreward of the main region of framework building. Shoreface sediments are relatively old. Of 20 calibrated radiocarbon dates on skeletal constituents of sand, only three are younger than 500 years b.p.; six are 500–1000 years b.p.; six are 1000–2000 years b.p.; and five are 2000–5000 years b.p. Dated fine sands are older than medium to coarse sands and hence may constitute a reservoir of fossil carbonate that is distributed over the entire shoreface. Dominance of fossiliferous sand indicates long storage times for carbonate grains, which tend to decrease in size with age, such that the entire period of relative sea-level inundation (∼5000 years) is represented in the sediment. Despite an apparently healthy modern coral ecosystem, the surficial sand pool of Kailua Bay is dominated by sand reflecting an antecedent system, possibly one that existed under a +1–2 m sea-level high stand during the mid- to late Holocene.

Sea level higher than present 3500 years ago on the northern main Hawaiian Islands

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 Oahus 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 Earths geoid response to the last deglaciation and with observations of increased Antarctic ice volume during the late Holocene.

Annual and interannual changes on a reef-fringed pocket beach: Kailua Bay, Hawaii

Abstract Historical aerial photographs and topographic survey sheets are used to establish a 70-year shoreline history (1926–1996) for Kailua Beach, Oahu, Hawaiian Islands. The shoreline has migrated seaward over this period at an average rate of 0.5 m/yr, with a maximum net accretion along the beach of 58.7 m and a maximum net erosion of −13.2 m. Net accretion has taken place even while sea levels have risen on the order of 0.1 m. Semi-annual and monthly beach profile surveys (1995–1999) at seven transects reveal short-term variations of shoreline position, sand volume, and beach shape. A relationship between beach width and corresponding sand volume fluctuations, established from the beach profile data, is applied to the historical shoreline change data to establish a history of sand volume fluctuations. Results show that Kailua has experienced a net accretion of 673 000 m3 of sand over the period 1926–1996, with average annual rates of volume change varying between 6.8 m3/m/yr and −0.1 m3/m/yr. The most recent period (1989–1996) shows a net volume increase of 41 000 m3. Given the lack of sand inputs at the ends of the beach, exchange with offshore deposits is a likely mechanism for long-term accretional trends. Seasonal fluctuations in Kailua Beach morphology dominate the variability with a response to seasonal wave state that varies along the length of the beach in magnitude and sign. At least four alongshore zones are observed, with the first and third zones exhibiting high/low sand volumes during the summer/winter, and the second and fourth zones exhibiting opposite behavior with high/low volumes during the winter/summer. Although seasonal sand accumulation varies along the beach, the overall beach profile is largely maintained. Moreover, changes in sand volume occur in phase over the subaqueous and subaerial sections of the beach. This behavior suggests that longshore rather than cross-shore sand transport is important at annual time scales. A simple seasonal transport pattern is proposed to account for these observed fluctuations, which depends in part on the topography of the offshore reef.

Holocene evolution of an estuarine coast and tidal wetlands

Modern facies-distribution patterns, extensive core data, and chronostratigraphic cross sections provide a detailed history of Holocene inundation within the Delaware Bay estuary and sedimentation in adjacent coastal environments. Flooding of the estuary occurred with rising sea level as the shoreline retreated northwest along a path determined by the pre-transgression topography. Simultaneous migration of an estuarine turbidity maximum depocenter provided the bulk of fine sediments which form the coastal Holocene section of the estuary. Prior to 10 Ka, the ancestral bay was predominantly a tidal river, and the turbidity maximum depocenter was located southeast of the modern bay mouth. By 10 Ka, lowlands adjacent to the ancestral channel of the Delaware River were flooded, forming localized tidal wetlands, and the depocenter had initiated high rates of fine-grained sedimentation near the present bay mouth. At that time, coastal Holocene strata began to onlap the interfluve highlands. By 8 Ka, the fine-grained depocenter had migrated northwest along the main channel of the Delaware River, although the widened mouths of tributary valleys continued to be active sites of sediment accumulation. Following the passage of the fine-grained depocenter, coarse-grained sediments accumulated along the coast in response to increased wind-wave activity. During the middle Holocene, portions of the estuarine coast began to resemble modern geomorphology, and washover barrier sands and headland beach sandy gravels accumulated along the southwest shore. The late Holocene was characterized by erosional truncation and submergence of aggraded coastal lithofacies and by planation of remnant highland areas. Knowledge of the eroded Holocene section is fragmentary. At present, continued sea-level rise is accompanied by deposition of tidally transported muds in coastal environments and deposition of sandy sediments in some offshore regions. An unconformity marks the base of the developing open estuarine sequence of coarse clastic lithofacies and denotes the end of coastal accumulations. Modeling of coastal-lithofacies transitions identifies specific lithofacies complexes in the Holocene stratigraphic section which were influential in the evolution of the coast. Development of the Holocene section of the estuary coast involved both constructive, or aggradational, and destructive, or erosional, phases.

Decorrelating remote sensing color bands from bathymetry in optically shallow waters

We have developed a simple technique to decorrelate remote sensing color band data from depth in optically shallow water. The method linearizes color band data with respect to depth by subtracting an optically deepwater value from the entire waveband under consideration and taking the natural logarithm of the result. Next, this linearized waveband is rotated about the model 2 regression line computed against a bathymetry band. The rotated color band is decorrelated from water depth. We demonstrate the technique for a small area of Kailua Bay, Oahu, HI, using Quickbird multispectral and Scanning Hydrographic Operational Airborne Lidar Survey LIDAR data. Results indicate that color band data are effectively decorrelated from depth, while bottom reflector variability is maintained, thus providing the basis for further analysis of the depth-invariant wavebands. The primary benefit of our technique is that wavebands are rotated independently, preserving relative spectral information.

Holocene Reef Development Where Wave Energy Reduces Accommodaton Space, Kailua Bay, Windward Oahu, Hawaii, U.S.A.

ABSTRACT Analyses of 32 drill cores obtained from the windward reef of Kailua Bay, Oahu, Hawaii, indicate that high wave energy significantly reduced accommodation space for reef development in the Holocene and produced variable architecture because of the combined influence of sea-level history and wave exposure over a complex antecedent topography. A paleostream valley within the late Pleistocene insular limestone shelf provided accommodation space for more than 11 m of vertical accretion since sea level flooded the bay 8000 yr BP. Virtually no net accretion ( 6 m. Coral framestone accreted at rates of 2.5-6.0 mm/yr in water depths > 11 m during the early Holocene; it abruptly terminated at 4500 yr BP because of wave scour as sea level stabilized. More than 4 m of rudstone derived from the upper fore reef accreted at depths of 6 to 13 m below sea level between 4000 and 1500 yr BP coincident with late Holocene relative sea-level fall. Variations in the thickness, composition, and age of these reef facies across spatial scales of 10-1000 m within Kailua Bay illustrate the importance of antecedent topography and wave-related stress in reducing accommodation space for reef development set by sea level. Although accommodation space of 6 to 17 m has existed through most of the Holocene, the Kailua reef has been unable to catch up to sea level because of persistent high wave stress.