Biology Reference
In-Depth Information
centration). When land is reduced, less silicate rock is available for weather-
ing and that affects the following chemical transformations:
+
+
CO
2
weathering of CaSiO
3
(e.g., wollastonite)
CaCO
3
SiO
2
and
+
+
CO
2
weathering of MgSiO
3
(e.g., enstatite)
MgCO
3
SiO
2
That is, during periods of lower sea levels, as during glacial intervals, in-
creased erosion takes place and the trend is for less CO
2
and lower tem-
peratures. With decreased weathering of silicate rocks, less CO
2
is removed,
more accumulates in the atmosphere, and higher temperatures result (as in
the Cretaceous).
The reconstruction of past sea levels is based on acoustic signals sent
through subsurface strata and refl ected back in patterns determined by the
thickness, density, and porosity of the rock. If the sedimentary layer is solid,
indicating an interval of deposition (
high sea levels), the refl ected ampli-
tude, frequency, waveform, and velocity is different than if the rock is un-
consolidated, indicating a time of exposure to erosion. By examining seis-
mic profi les from localities worldwide, geologists have constructed curves
showing periods of major inundation by the sea followed by regressions
(fi g. 3.1). There were widespread inundations of at least 100 m throughout
the Cretaceous, Paleocene, and early Eocene (Miller et al. 2005), a regres-
sion around 30 Ma (Oligocene) corresponding to early formation of glaciers
in the Arctic and especially on Antarctica circa 33.5-34 Ma (Pearson et al.
2009), and rapid fl uctuations after about 5 Ma, when late Pliocene and
Quaternary glaciations were underway. Fossil fl oras and faunas can be plot-
ted on the curve to give an indication of their relationship to this particular
environmental factor (fi g. 3.1). There have been absolute or eustatic fl uc-
tuations in the level of the sea through geologic time as a result of climate
change (glaciation) and nonclimatic factors (ocean water displacement
from subsurface volcanism). There have also been relative changes in sea
level due to upward or downward movement of the land. Either way, the
curve constitutes a useful context for viewing biotic history.
On a fi ner time scale, sea levels are presently rising 30 to 40 cm per
century, and this portends increasing trouble for coastal areas. As noted
by Pilkey and Cooper (2004), the recent rush to the shore, with “devel-
opments” on beachfronts and wetlands, and the massive population in-
creases on deltas and coastal lowlands, requiring associated subsurface
=
