Charles Darwin was the first to hypothesize that volcanic islands, atolls, and their varying types of reefs all had a common origin:

• islands were formed by the buildup of repeated lava flows from volcanoes; these islands have barrier reefs
• volcanoes became quiescent and began sinking; these islands have fringing reefs with a lagoon between the ocean and the island
• when the volcano sank completely, all that remained near sea level was the fringing reef, which is now an atoll.

The upper meter is kaolinitic clay. At the very top there are calcite fossils of shallow marine organisms, marine organic matter, and pyrite with a marine sulfur isotope signature. After the soil sank below sea level, the marine waters of the lagoon became anoxic, and reduced the upper part of the paleosol.

The black color diminshes in intensity with depth as organic matter and pyrite levels decline, taking a sharp drop at about 94 cm in this core.

The next NINE meters of paleosol comprise strong redoxomorphic features, or mottling. Mottles are composed of khaki goethite (FeOOH), lurid red hematite, and pale green to white kaolinite. These features indicate alternate wetting and drying. During wet phases, bacteria in the soil consume all of the oxygen, causing the Eh to drop below 0; reducing conditions.

Under reducing conditions, iron is reduced to its ferrous (Fe²⁺) mobile form. The iron moves (segregates) to concentrate in some areas, leaving iron-depleted zones behind.

When the water level drops again after days to weeks of no rain and ongoing plant transpiration, oxygenated air from the soil surface can once again move into the pore space of the soil, causing oxidation of the iron.

The more wet/dry cycles occur, the more iron segregation can occur, and the more intensely mottled the soil will become.

Below the redoxomorphic zone is a 3-meter thick pallid zone. Here, the colors are paler, and overall the section is lower in iron than the overlying redoxomorphic zone.

Such a thick (9 m) redoxomorphic zone overlying a pallid zone, suggests that rather than water levels fluctuating over 13 m several times a year, probably the water table lowered as soil formation proceeded. If the groundwater level fluctuated only a meter or so, but declined over time over the 9+ m, a thick redoxomorphic zone could form.

When streams eroded downward through the landscape and intersected the level of what would become the pallid zone, water flowed laterally out of the bottom of the soil profile and into the stream, taking much of the iron with it and leaving behind the pallid zone. Later submergence of the soil profile allowed alteration of some of the remaining iron to siderite (FeCO3), outlined when you move the cursor over the image at right.

An earlier stage of redoxomorphic conditions occurs at Site 873 on Wodejebato Guyot. LEss iron has been leached from between the iron-enriched zones.

Less of the upper part of the soil was preserved here, as the site was closer to the outer edge of the guyot, and probably experienced more wave erosion than the centrally-located Limalok Guyot site (Site 871).

The most striking example of how submergence of the island and deposition of lagoonal sediment caused reduction of these iron-rich paleosols occurs at Site 874B on Wodejebato Guyot.

This material is classic C horizon palesol: it looks just like the basalt it altered from, but is composed entirely of kaolinite and smectite, with the paleo-vesicles infilled with vermiform kaolinite (white specks in both gray and red zones).

The sequence of development of the strongly redoximorphic paleosols on the Pacific guyots, Limalok and Wodejebato, may have proceeded like this:

1) A volcanic island formed over a hotspot as repeated lava flows built up the volcanic edifice from the seafloor, until the edifice breached the sea surface.

Big Island of Hawaii is a good example of this.

2) Almost as soon as the lava flows breacehed the sea surface, and proceeding until the island drifted off of the hot spot and became quiescent (no more volcanic activity), plants colonized the island, rain fell, and soil formation began.

The younger flows of Big Island of Hawaii are a good example of this.

3) As weathering proceeds, the soil deepens. Erosion lowers the level of the island, causing groundwater levels to drop, and the redoxomorphic zone deepens.

Although not mottled, the soils in central Oahu, Hawaii (right image), are very thick.

5) Water levels rise again, upward from the base of the soil profile, as the island submerges. Siderite forms in the pallid zone.

At right, a thin section from the pallid zone showing siderite coating an iron glaebule.

6) Lagoonal deposits form on top of what is left of the soil (almost certainly the very tops were eroded during submergence), forming the reduced zone that migrated downward into the soil profile from above.

At right, Christmas Island of the Pacific is a modern atoll. Roll mouse over the image to see the lagaoon.