Sediment Cores

In a depositional environment such as Kawai Nui, material collecting on the bottom accumulates in chronological order: the earliest deposited material (the oldest) is deepest, while the most recently deposited material (present day) is on top. Careful examination of cores taken through bottom sediment layers will reveal the history of the aquatic environment in that location. Various methods can be used to determine what kind of environment was present when a layer was deposited and, in some cases, actually establish an age for a particular layer. Basic descriptions of sediment cores from Kawai Nui include the following.

Cores taken in the marsh (Stearns and Vaksvik, 1938) revealed silty clayey marls interbedded with coral detritus and alluvium to depths of 30 m. This alluvium (Takasaki et al., 1969) is reworked older alluvium from upper valleys in the area. The older upper valley alluvium is the erosional product of a former high stand of sea level.¹

Borings and probes of the marsh were reported by Dames and Moore (1961). These showed dark grey marine clay and marine sand with shells down to about 38 m without finding firm rock. The marine sediments were overlain by an organic slurry, which was overlain by peat. The peat, in turn, graded into the live roots of the present vegetation. The Dames and Moore report contained core and probe data from three previous surveys as well as their own work.¹

Basin Stratigraphy

The bottom layer of a complete core begins with marine debris: calcareous sand and fragments of tropical reef organisms such as corals and mollusk shells. Identification of these skeletal remains can be a useful means of characterizing the ancient marine environment that existed here [MORE: CLICK HERE]. This reef layer extends quite deep (see description above) and represents the period of time when this area was a marine embayment. Reef development gradually filled in the ancient valley created by stream erosion when sea level stood much lower than it does today. At Na Pohaku o Hauwahine, the top of the reef layer lies about 1.5 m (4 ft) below the ground surface. It is deeper away from the shore, reflecting the shape of the ancient basin, which is presumably comprised of a thin layer of colluvium and alluvium above basalt bedrock (Moye, 2002).

The lagoon layer: marine sands are overlain locally by fine-grained lagoonal clays and extensively by marsh sediments composed of terrestrial clay, silt, and decomposed to peaty organics. The weight of the overlying sediments and surface water has compacted the lagoonal clays and lower marsh sediments, forming a high porosity but low permeability aquitard throughout much of the Kawai Nui basin. These compacted clays are remarkably dense and "dry" in core samples.²

The marsh layer is comprised of peat and extends from near the present-day surface down to the clayey layer. This peat layer represents accumulated plant matter as Kawai Nui filled in from a more-or-less open lake environment to a vegetation-choked marsh. Dead plant material is preserved because conditions (low oxygen and low pH) within the peat hinder microbiological decomposition. This preservation allows us to examine plant remains at different depths to infer changing patterns of vegetation in the area of the core. Seeds are especially useful, because they can be identified to species, and their abundance in a particular layer presumably indicates something about abundance in the ancient marsh [MORE: CLICK HERE]. Pollen grains are also used in this manner, although because pollen grains may travel on the wind some considerable distance, the grains present may tell us more about the surrounding forest and grass lands than about the marsh itself.

¹ Linda L. Smith. 1978. Development of emergent vegetation in a tropical marsh. MS Thesis, U.H., Botany. p. 16.

Dames and Moore, Inc. 1961. Proposed land development, Kawainui Marsh, Kailua, Hawaii. Prep. for Trousdale Construction Co.,Honolulu.
Stearns, H.T. and K.N. Vaksvik. 1938. Records of the drilled wells on Oahu, Hawaii. Bull. No. 4, Hawaii Div. Hydrography, Honolulu.
Takasaki, K.J., G.T. Hirashima, and E.R. Lubke. 1969. Water resources of Windward Oahu, Hawaii. Water Supply Pap. No. 1894. U.S. Geol. Surv., Honolulu.

² Robert J. Moye. 2002. Preliminary analysis of the hydrology of the Kawai`nui [sic] Basin, Kailua Ahupua`a, Ko`olaupoko District, O`ahu. unpub. rept. prep. for `Ahahui Malama i ka Lokahi. 17 p.