Mongolian Lake Systems Recorn Past Climate Change
John Peck, Assistant Professor
University of Akron
John Peck was an assistant marine research scientist at GSO for three years until he took a tenure-track position at the University of Akron in August 2000. He worked as a technician for Jim Kennett, then earned MS and PhD degrees with Bob McMaster and John King, respectively.
Mongolia, a landlocked country in central Asia,
is located in a region of the highest degree of continentality (seasonal contrast)
on Earth (see Fig. 1). The extreme continental climate of Mongolia is reflected
in the annual temperature range of about 45°C, with limited precipitation
(generally less than 250mm per year) occurring largely in July and August. The
climate in the Mongolian region represents an important transition from the
subarctic in the north to the Gobi Desert in the south. The lakes we are studying
are located in the semiarid, north-central portion of Mongolia.
One quarter of Mongolia's approximately two million
people live in the capital city of Ulaanbaatar and three quarters live in the
countryside in small towns or in gers, felt and canvas tents (see Fig. 2). The
steppe grasses are intensely grazed by approximately 20 million domesticated
horses, sheep, goats, cows, camels, and yaks. Droughts have had major impacts
on livestock and people in Mongolia. It is critical to have a better understanding
of the frequency and magnitude of past climate change in Mongolia in order to
effectively manage its natural resources. Previous studies of meteorological
instrument records have been used to identify the spatial pattern and timing
of drought in Mongolia. Since 1940, at least half of Mongolia's provinces have
experienced a severe summer drought once every three to five years. An important
goal of our study is to use lake sediments to extend the shorter Mongolian instrument
and tree ring climate records back in time to assess the range of natural climatic
variability in Mongolia. Understanding the range of past natural climate variability
provides an indication of the types of future climate change that are possible
in Mongolia.
Lake/watershed systems have the potential to
yield important insights on past climatic conditions for a variety of reasons.
A lake, particularly if it has a deep central basin, can accumulate sediment,
which yields a continuous geologic record. Continuous geologic records are often
difficult to obtain on continents because much of the land surface is subjected
to erosion or episodic deposition. Some lakes rapidly accumulate sediment rich
in organic matter that is suitable for radiocarbon dating. These lakes can yield
a well-dated, high-resolution record. The sediment particles that accumulate
within a lake have origins that are both autochthonous (e.g., diatoms produced
within the lake itself) and allochthonous (e.g., pollen from terrestrial plants,
soil erosion, and wind blown dust from outside the lake). Therefore, lake sediments
can provide a record of both the lake and the watershed ecosystem responses
to climate change. Lakes that are located near major ecosystem boundaries are
particularly sensitive recorders of climate. As climate changes, ecosystem boundaries
shift, and new vegetation, soil conditions, and lake levels are established
at the site. These changes, in turn, influence the type of sediment that accumulates
in the lake. Several of the lakes we are studying are closed lakes that have
no river flowing from them and represent a sensitive balance between climate
parameters such as evaporation, precipitation, and temperature. Small changes
in these parameters can result in large sedimentary and geochemical limnological
changes.
Changes to the Mongolian Constitution in 1992
and a democratically elected government in 1996 brought opportunities for scientists
from the United States to collaborate with Mongolian scientists. A group of
scientists from the Mongolian Academy of Sciences led by Dr. Khosbayar has worked
with U.S. scientists including John King (GSO), Sarah Fowell (University of
Alaska), Doug Williams (University of South Carolina), and me to study the climate
signal recorded in Mongolian lake sediments. This group is conducting an interdisciplinary
study of sediment cores recovered from five lakes during two field trips (see
Fig. 3). Lake Telmen, one of the five lakes sampled, illustrates well how lake/watershed
systems provide insight to past climate change.
In the Lake Telmen region today, the mean temperature
in January is -32°C (-26°F) and the mean temperature in July is 12°C
(54°F). The mean annual precipitation is 250mm. The lake lies near the boundary
between the forest-steppe and steppe ecosystems. Lake Telmen, a closed lake
basin, is slightly salty (about 4g per L salinity). In July, a well-developed
thermocline at 10m depth corresponds with a large increase in turbidity generated
by dense patches of plankton found at this depth. As this organic matter decomposes
and descends, it lowers dissolved oxygen to hypoxic levels (2.36 mg per L) in
the bottom water. Bottom-dwelling organisms, such as worms, typically inhabit
lake floors and mix the sediment as they feed. However, in the low-oxygen conditions
of the deep water in Lake Telmen, benthic organisms cannot survive to mix the
sediment. Sediment cores from the central basin of the lake show fine laminations
less than 1mm thick. Two laminae are deposited per year and the resulting couplet
is referred to as a varve. During the warm productive summer months, photosynthesis
by plants in the lake depletes the lake waters of CO2,
induces supersaturation with respect to calcium, and results in a sediment lamina
comprising abundant calcium aggregates and very small (less than 8 microns)
calcite crystals. When the lake surface is frozen, from the fall to the spring,
the second lamina of the varve couplet accumulates. This lamina is composed
of amorphous organic matter and clay particles that settle from the water column.
The absence of sediment mixing by benthic organisms preserves the varves, and
yields a high-resolution geologic record from Lake Telmen sediments.
From 6,210 to 3,960 years ago, as determined
by radiocarbon dating, Lake Telmen was between 15 and 20m shallower than it
is at present. Coarser-grained sediment containing pollen from vegetation indicative
of arid conditions and poor unstable soils accumulated in a small lake that
covered about 40 percent of the present lake area. Without extensive vegetation
cover, numerous sand dunes in the Telmen vicinity were created by the wind.
With an increase in moisture balance (precipitation minus evaporation), the
level of Lake Telmen rose. Sediment cores collected in 12.8m of water contain
an ancient soil at their base that was inundated by the rising level of the
lake about 3,570 radiocarbon years ago. The lake level had risen to the extent
that mixing generated by the wind did not reach the deeper water; thus hypoxic
water conditions became established and varved sediment accumulated. By 2,250
radiocarbon years ago, the lake level had risen to 9.95 m above the present
lake surface, and wave action cut a high-stand terrace into the hillsides around
the lake. Varved sediment accumulated over a much larger area that was now in
deeper water. Consistent with the sedimentological evidence, pollen from specific
land plants suggests that the climate was more humid between 3,960 and 1,500
radiocarbon years ago. Lake levels declined beginning 1,500 years ago. Three
periods of stable lake level are recorded by wave-cut terraces on headlands
and relict beaches in embayments at 5.27m (dated at 1,400 years ago), 3.25m,
and 0.70m above the present lake surface. Through an interdisciplinary study
of lake systems, we are able to reconstruct the history of moisture balance
in central Mongolia. Ongoing studies of the fossils (diatom, pollen, and others),
the sedimentology, the mineralogy, and the sediment geochemistry will give us
more insight on the nature of lake and watershed ecosystem response to past
climate change. By studying five lake systems, we will be able to document regionally
coherent climate signals preserved in the sediment record. In addition to defining
the long-term trends in climate change as we have done through lake level histories,
it is important to look at how rapidly these changes can occur. For example,
due to unusually heavy snow fall and spring rains in 1993, Lake Telmen rose
approximately 1.5m with the addition of 55km3 of water from the surrounding
mountains. Understanding the rate at which changes impact ecosystems has great
importance because it determines how quickly Mongolian society will need to
adjust to new environmental conditions.