Note: click on image for larger version!
Today's picture shows a sequence of samples that I've pulled up in
coring device that takes very small samples -- 1.7 cm in diameter.
They are from a soil that was about 1 meter deep, and ended in contact
with rock. It was in the center of a small depression in the
landscape, near the top of the largest mountain on the island. Each of
the 7 samples below is representative of a horizon or layer. Taken
together, they show that a soil formed here to a depth of about 50 cm,
and then was buried under another 50 cm of soil. Each horizon is
represented by one intact 2 cm section in the photo, and is given a
standard letter designation (I won't bore you with the explanation of
what each letter means).
A This is the "topsoil". It is dark in color and has a crumb
structure. Roots are abundant.
AB This layer still has mainly a crumb structure, but is a bit lighter
and has a few less roots.
BA This layer shows some dark areas of the above layers, but has fewer
roots and is mainly composed of an accumulation of yellow clay.
Bt This layer is dominated by reddish-yellow clay, which has
accumulated in "films". These films are thick and dominate the sample.
This clay has moved down from the surface soil, and has been deposited
here. This type of clay provides strong evidence of soil formation
processes.
2Ab Here we have a second topsoil, which has been buried. It has
similar structure to the topsoil above. While it is not as dark, it is
still brown rather than red or yellow in color.
2Bt This second layer of clay accumulation shows similar films and
color to the layer above -- the color is even more pronounced. This
layer suggests that the buried soil was stable for a long period.
2C At the very bottom of the profile, the corer brought up rock
fragments (reddish) in a matrix of clay. This represents soil forming
from the rapid weathering of the underlying rock.
I found soil profiles very similar to the above in 2 more locations,
and all 3 were close to a wetland area the rest of our team was
sampling. These profiles suggest a period of stable soil development
(perhaps under pre-Polynesian forest) and then a period of rapid
deposition that must have been fueled by erosion of the surrounding
landscape. It may be reasonable to guess that the upper 50 cm of soil
accumulated as a result of agriculture. Confirming this timing will
require further analyses back in the lab. But if the buried soil
represents pre-Polynesian forest and the upper soil contains evidence
of agriculture, then both will contain microfossil and chemical
information which will help us understand nearby wetland samples.
