Geoscience Reference
In-Depth Information
Figure 9-9.
Selection of Danish amber pebbles
retrieved from the North Sea coast (see Color Plate 9-9).
From the collection of P. Laursen; photo by J.S. Aber.
Figure 9-10.
Buried wood in the lower till of the
Independence Formation, northeastern Kansas, United
States. Abundant wood in this till represents a spruce
forest that was overrun by an ice sheet about 700,000 to
600,000 years ago. From Aber, J.S. 1991. The glaciation
of northeastern Kansas.
Boreas
20, p. 297-314.
infrared absorption spectra to Baltic amber,
which is Eocene in age (Kosmowska-Ceranowicz
1999, 2009). Amber continues to erode from
Eocene deposits nowadays and is transported
onto beaches around the Baltic Sea and North
Sea, where it is collected by many people (Fig.
9-9). Baltic amber is also mined commercially
from a layer of marine glauconitic sand referred
to as “blue earth” (middle Eocene; Engel 2001)
in Poland, the Ukraine, and Germany, but the
largest operation by far is at Yantarny, Kalinin-
grad, Russia (Krielaars 2010). Fashioned and
polished amber objects and jewelry remain
popular today because of the gem's beauty and
rarity.
• Ability to acidify water may retard bacterial
decomposition and allow organic matter to
accumulate.
• Release of acid may weather underlying
mineral soil and lead to formation of a clay
pan that retains water.
Taken together, these capabilities favored
Sphag-
num
expansion and exclusion of most other
vegetation that could not tolerate low Eh and
pH and lack of nutrients.
Sphagnum
mires, co-
inhabited by spruce, tamarack, birch, heaths,
sedges, and pitcher plants, spread across vast
regions of northern North America and northern
Eurasia during interglacial and postglacial
periods. The last major glaciation (late Wiscon-
sin) took place between 25,000 and 10,000 years
ago, and this advance of ice sheets destroyed
most evidence for earlier wetlands in the glaci-
ated regions.
A few sites with older wetland strata were
preserved in protected spots, for instance in
northeastern Finland (Engels et al. 2010). In the
central United States, a spruce forest was overrun
by an advancing ice sheet in eastern Nebraska
and northeastern Kansas approximately 700,000
to 600,000 years ago (Aber 1991). Remains of
spruce wood as large as fence posts are pre-
served regionally in glacial deposits (Fig. 9-10).
The nearest comparable spruce forest today
is located 1200 km to the north in central
9.4 Pleistocene and Holocene wetlands
The late Cenozoic was a time of global cooling
that culminated in widespread, repeated, ice-
sheet glaciations during the Pleistocene Epoch
(
c
. 2.6 million to 10,000 years ago). The associ-
ated frequent changes in sea level and climatic
l uctuations placed strong adaptive pressures on
wetland communities, and
Sphagnum
moss came
to dominate mires in northern latitudes.
Sphag-
num
was preadapted to exploit oligotrophic
habitats (Greb, DiMichele and Gastaldo 2006):
• Extensive aerenchyma allows it to survive in
low-oxygen conditions.
• Compact form, overlapping leaves and rolled
branch leaves help it to retain water.
