Geoscience Reference
In-Depth Information
subject is exceedingly complex, and considera-
ble scientii c uncertainty remains as to the role
of human activities for past events and trends
in Holocene climate.
found among carbonaceous sedimentary rock
associated with coal and lignite deposits that
date from Cretaceous and Tertiary times. Amber
provides the perfect time capsule with which to
examine extinct l ora and fauna, and reconstruct
past paleoclimates and wetland habitats. Amber
is valued as a beautiful gemstone today and as
far back as the Stone Age.
The late Cenozoic was a time of global
cooling that culminated in widespread,
repeated, ice-sheet glaciations. The associated
frequent changes in sea level and climatic l uc-
tuations placed strong adaptive pressures on
wetland communities, and
Sphagnum
moss
came to dominate mires in northern latitudes.
Much of what we know about paleoclimate
and environmental conditions of the latest
Pleistocene and Holocene was i rst discovered
in northern Europe based on wetland deposits
and fossils. Three main climatic phases are
well documented: 1) late-glacial/early Holocene
climatic amelioration, 2) mid-Holocene thermal
maximum, and 3) Neoglaciation of late
Holocene. These long-term trends were inter-
rupted by “abrupt events” during which sharply
colder climate returned
9.5 Summary
Wetlands have existed since the earliest records
of fossil life on Earth, and wetland life has
exerted a profound inl uence on the world's
environment and climatic system. Beginning
3.5 billion years ago, stromatolites (blue-green
algae) in coastal wetlands pumped oxygen
into the early ocean and atmosphere, which
eventually led to oxygen-rich environments for
most of the world. Given their situations, wet-
lands tend to accumulate organic-rich sediment
including diverse fossils and peat. Upon burial
these sediments may be transformed into lignite,
coal, and amber. The majority of coal preserved
in sedimentary strata of the continents was
created during two major pulses of accumulation
-
the Permo-Carboniferous (late Paleozoic) and
Cretaceous (late Mesozoic) through Tertiary
(Cenozoic).
Permo-Carboniferous coals accumulated on
the supercontinent Pangaea in two primary cli-
matic zones, equatorial and temperate, which
were separated by subtropical deserts. A massive
extinction event at the end of the Permian
caused a nearly total collapse of wetland eco-
systems worldwide. The Cretaceous Period rep-
resents the next great expansion of wetlands,
and it culminated with another massive extinc-
tion that affected all categories of life. Among
those plants and animals that survived, many
were obligate wetland inhabitants, which
suggest that wetlands may have served as refugia
during the extinction event. Following this
extinction, widespread and diverse mires devel-
oped during the Tertiary.
Amber is a chemical fossilization of tree
resin, and ancient resin was the trap that cap-
tured plant debris, small animals, and air
bubbles. The hardened resin was buried and
polymerized, changing into amber with exqui-
sitely preserved extinct organisms in three-
dimensional, lifelike positions. Most amber is
∼
8200 years ago and
during the Little Ice Age.
The exact timing of the three main Holocene
climatic phases differed somewhat in North
America and other parts of the world, but the
overall pattern is broadly similar globally. In
North America, vast proglacial lakes formed
during deglaciation, and sizable regions were
depressed below the marine limit. A major
change in vegetation took place rapidly in
eastern North America about 10,000 years ago,
which marked the shift from late glacial to early
Holocene climate. Continued climatic warming
led to the Altithermal of the mid-Holocene.
Across the continent, cooler and wetter climatic
conditions developed during the late Holocene.
Ice-cores from tropical alpine glaciers in
Africa and South America provide unique
records of climate change that coni rm other
types of paleoclimatic proxies. In combination
these records demonstrate signii cant changes
in tropical climate during the Holocene which
had major impacts on lakes and other wetland
habitats as well as early civilizations. Even in
