Research Focus

I am trying to reconstruct the Holocene sea-level history in Hawaii and its impact on coastal environments and resources. As Earth's climate warmed following the last Ice Age ca. 18 ka (ka= "thousand years ago"), the great continental ice sheets retreated and decayed. The meltwater added to the ocean basins resulted in 120-130 m of 'eustatic' sea-level rise (global sea-level rise due entirely to the addition of meltwater). Along many coastlines, however, variations in local and regional tectonics have influenced the magnitude and timing of sea-level changes, confusing the eustatic signal. As a result, sea-level researchers focus on obtaining a detailed "relative" sea-level history for one individual location, which can then be compared to others within a region to understand the tectonic effects within that region. Only then can a regional sea-level record be obtained, and with several regional histories, a global trend can be established.

Implications

Sea-level histories derived from insular island settings far removed from tectonically active continental boundaries and the rebounding effects near ice sheet margins suggest that sea levels were higher than at present during the last 5000 years. This is important for two reasons:

1. The nearshore processes associated with recent sea levels shaped coastal
   environments and resources that are vital to our subsistence.
2. A recent sea-level highstand can serve as an analog for the predicted sea-level rise
   to accompany the uncertain climatic conditions of the 21st century.

Coral reef ecosystems, estuaries, wetlands, deltas, beaches, dunes, coastal aquifers, and important fish and wildlife habitats owe their existence to past and present nearshore processes. What will happen to them in the next century with 20-50 cm of global sea-level rise? Will they survive the natural changes and the demands human society places on them?

To understand the fate of coastal environments and resources we can closely monitor current changes taking places in the coastal zone and compare them to background conditions that existed prior to human impact. Along many coastlines, natural processes disguise past conditions as erosion removes geologic clues and accretion covers them up. The greatest challenge along most coastlines, however, is the human component which is systematically desturbing natural processes as a result of poorly managed and planned development that is leading to severe beach, wetlands and fisheries loss, coastal contamination and property inundation.

Kaneohe Bay, Oahu

Kaneohe Bay, Oahu, holds numerous geologic clues to past sea-level movements on Oahu. Shorelines are both incised into and deposited on, the flanks of the Hawaiian Islands above and below present mean sea level. These shorelines owe their origin to still stands of the sea during times past. By obtaining precise elevations and radiometric dates of intertidal biota that inhabited these shorelines, we can model sea-level movements through time. From these studies I hope to produce a Holocene sea-level curve for Hawaii and this region of the central North Pacific Ocean.

Kapapa Island, Oahu

To derive a Holocene sea-level curve for Hawaii and the central North Pacific region, I am loooking at various sea-level indicators from the islands of Kauai, Oahu, Molokai, Lanai and Maui. I am focusing on one location in particular to evaluate its significance as a sea-level indicator. I am trying to determine the nature of an emerged unconsolidated carbonate sand and cobble deposit on Kapapa Island in Kaneohe Bay, Oahu, Hawaii. It might owe its origin to a sea-level highstand on Oahu between 4 and 1.5 ka (ka = thousand years ago).

The unconsolidated carbonate deposit is stratified, sorted and asymmetric seaward. It sits 2-3 m above mean sea level, out of reach of present near shore processes on what appears to be a wave-abraded platform of lithified eolianite. Radiocarbon dates of sediments, coral clasts, and mollusc shells taken from the deposit show a systematic progression of ages with the oldest material (5.4 ka) near the center and the youngest (modern to 2.0 ka) near its seaward margin.

Recent storms have been observed to erode the deposit rather than furnish it with sediments. Offshore surveys seaward of the island and radiometric dates of subtidal sediment show that the region is sediment starved and that what little sediment exists for transport to the island is modern (0.1-0.5 ka). No modern samples have been recovered from the emerged deposit. The fact that the emerged deposit consists of stratified and sorted marine sands, coral clasts, and mollusc shells that that span several thousand years in age, suggests that deposition was episodic and time-transgressive rather than related to a single event.

Mechanisms

Investigations of the tectonic history of Oahu suggest that the island may have uplifted 0.2 m during the last 4,000 years due to lithospheric flexure resulting from the loading of the Pacific Plate at the hot spot at Kilauea. This explains only a small portion of the emergence of this deposit. Radiometric dates and stratigraphic analysis of the deposit sediments will help to determine its genesis and relation to past sea-level movements.

A mid- to late-Holocene sea-level highstand in Hawaii (and numerous locations throughout the Pacific Ocean) may be the result of climatic or geophysical responses to the decomposition of the last great ice sheets and subsequent redistribution of meltwater within the ocean basins. A response due to either equatorial ocean siphoning or hydroisostasy (or a combination of both) may have influenced Holocene shorelines in Hawaii and throughout the Pacific and Indian Oceans.

It is thought that the mid-Holocene climate optimum that occurred globally ca. 5.7 ka, may have led to thermal expansion of surface ocean waters. This climatic warm period is often considered an analog for future warming associated with an "enhanced greenhouse effect" owing to the burning of fossil fuels and changing land use practices. Sea-level rise of 20 cm is predicted for the double CO2 year ca. 2050 AD. Knowledge of the impacts of climate change and sea-level rise from the recent geological past may enable us to better predict the changes that will accompany the uncertain conditions in the future.

Publications (including abstracts)

1. Fletcher, C.H., Jones, A.T. (in press) Sea-level highstand recorded in Holocene shoreline deposits on Oahu, Hawaii. Journal of Sedimentary Research.

2. Fletcher, C.H., Grossman, E.E., Sherman, C.E., and Calhoun, R.S., 1995 Episodicity in sea-level change on Oahu, Hawaiian Islands. Abstracts with Programs, 1995 Annual Meeting, San Francisco, American Geophysical Union.

3. Fletcher, C.H., Grossman, E.E., Mullane, R.A., Sherman, C.E., Calhoun, R.S., and Rooney, J., 1995. Sea-level change in Hawaii: post-glacial episodicity, late Holocene emergence, and modern beach loss. extended abstract, National Science Foundation, Global Sea Level Change Workshop, Key Biscayne, FL.

4. Fletcher, C.H., Grossman, E.E., Mullane, R.A., Sherman, C.E., and Calhoun, R.S., 1995. Late Quaternary sea-level and coastal sedimentation patterns in Hawaii. Abstracts with Programs, 1995 Annual Meeting, New Orleans, LA, Geological Society of America.

5. Grossman, E.E., and Fletcher, C.H., 1994. Evidence for an emerged (?) shoreline of mid- to late Holocene age on Oahu, Kauai, and Molokai, Hawaii. Abstracts with Programs, p. A-306, 1994 Annual Meeting, Seattle Washington, Geological Society of America.

Address

Go to:

• My Personal Page
  
• The SOEST Coastal Geology Group Page

[ SOEST home | G&G home | faculty | students | staff ]

This page is maintained by ericg@soest.hawaii.edu