Buried Soils!

Yesterday and today, we collected some dryer sediments that appear to have accumulated over hundreds or thousands of years. In these areas, soils were forming as the sediments were accumulating. Recognizing soil formation processes plays a critical role in allowing us to identify samples that are suitable for our study.

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.

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Troy Baisden

Was Collapse Inevitable on Easter Island (Rapa Nui)? Reconstructing a Civilisation's Failure is a Marsden Programme Troy Baisden is involved in.

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