Geoscience Reference
In-Depth Information
coast are of the present time and have, so to say, just emerged whereas
the soils located at 50-m altitude are more than 10,000 years old. It was
thus possible to study a chronosequence of podzolized soils in Finland
and to estimate that at least 5000 years are necessary for a typical Podzol
to develop (Mokma
et al.
2003). In Taiwan, the marine terraces represent
a sequence of the same duration (Tsai
et al.
2007).
2.3.5 Chronosequences on Offshore Bars
Here is a case where a marine bay is enclosed by a sandy offshore bar,
the sandbar, brought by ocean currents. This isolates a lagoon which,
fed with sediments by the streams that pour into it, dries up gradually
in spite of the fact that it sometimes is connected to the sea by a narrow
channel. Then the phenomenon recurs: a second sandbar is formed,
again isolating a lagoon (Fig. 2.8).
Old lagoon (clayey deposits)
Lagoon
Sand:
present-day
sandbar
Sand: old
sandbar
SEA
Channel
CONTINENT
Fig. 2.8
Evolution of a coast with sandbars.
The sandbars can be dated to the same time as the soils they carry.
This was done on the shores of Lake Michigan where 75 sandbars were
neatly lined up parallel to one another. The study showed that 3000
years were necessary for obtaining well-differentiated Podzols, even
though the beginnings of an E horizon are seen starting from 230 years
on (Barrett 2001).
2.3.6 Other Chronological Markers
There are historical and archaeological markers. But because of lack of
sufficient cooperation between pedologists and archaeologists, they have
rarely been exploited. The same example is always quoted: the ruins of
the Stara-Lagda fortress studied by Dokuchaev. A wall built of limestone
had a Rendzina on it. But the time-scale of phenomena related to human
activity is not useful, for it is generally too short.
