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518
Section: Astronomy
The Baikal Climate RECORD

The Baikal Climate RECORD

It has become a universal premise that the tens- to hundreds-kyr fluctuations of global climate are the response to orbital forcing associated with variations in the amount of solar radiation (insolation) in the Northern Hemisphere. Insolation changes with Earth’s orbit shape (eccentricity) and with the tilt of the Earth’s rotation axis relative to its orbital plane (obliquity).

Lake Baikal, with its continental interior position and long continuous deposition, stores a perfect undisturbed “archive” of Asian climates and is an exceptional paleoclimate site studied since 1990. During long and strong glaciations, the mountains around Baikal became thickly covered with ice. The growth of glaciers was attendant with degradation of rocks and influx of “glacial milk” into the lake. On the other hand, aridization possibly decreased the river input and the fluxes of solids to the lake. Since suspended load in rivers and glacial milk particles have different chemical compositions, the downcore patterns of sediment chemistry can be expected to reflect the redistribution of fluxes from different sources and record the climate change. Then, the geochemical record can be used to identify and investigate glacial/interglacial cycles in the Baikal watershed.

The Earth’s climate of the past 2.5 million years changed from warm to glacial periods, and was like the modern climate in-between the two extremes. During global cooling, the evaporated oceanic water condensed on continents producing huge ice sheets of more than two kilometers thick. Ice covered North America and a part of Europe to a latitude of 50—55 °N and northwestern Siberia at least to 62 °N. The sea level in cold times fell 100—130 m lower, lying bare the shelf areas and drying many small straits we know today.

It has become a universal premise that the tens- to hundreds-kyr fluctuations of global climate are the response to orbital forcing associated with variations in the amount of solar radiation (insolation) in the Northern Hemisphere. Insolation changes with Earth’s orbit shape (eccentricity) and with the tilt of the Earth’s rotation axis relative to its orbital plane (obliquity).

The climate change is recorded in marine sediments in oceans and in lake sediments on continents, which makes them valuable paleoclimate archives. Many climate records stored in marine sediments, Greenland and Antarctic (Vostok) ice cores, and loess sequences of China and Siberia bear periodicities of the same lengths as the orbital cycles of 100, 41, 23, and 19 kyr.

The very driving mechanism of the 10 to 100-kyr climate changes remains poorly understood but most models simulate it as a complex nonlinear ocean-air-land interaction in response to changes in boundary conditions (insolation is only one of them) interfered with internal feedbacks or threshold effects. However, the available climate models are becoming questioned in the light of recent direct measurements of ocean level indicating that insolation lags behind the marine signal: The ocean level rise anticipates insolation increase. Nevertheless, insolation — or rather its variations — is the key orbital climate agent.

Global cooling during glacial periods apparently decreased evaporation from the ocean surface and thus reduced the moisture input to the continents, to make the climate more arid. The wet/dry climate contrasts must have been especially prominent in continental Asia, which receives moisture with constant west Atlantic winds. Global winds were more intense and frequent during glacials, as evidenced by abrupt increase in dust contents in the respective intervals of Greenland and Antarctic ice cores.

Lake Baikal, with its continental interior position and long continuous deposition, stores a perfect undisturbed “archive” of Asian climates and is an exceptional paleoclimate site studied since 1990. During long and strong glaciations, the mountains around Baikal became thickly covered with ice. The growth of glaciers was attendant with degradation of rocks and influx of “glacial milk” into the lake. On the other hand, aridization possibly decreased the river input and the fluxes of solids to the lake. Since suspended load in rivers and glacial milk particles have different chemical compositions, the downcore patterns of sediment chemistry can be expected to reflect the redistribution of fluxes from different sources and record the climate change. Then, the geochemical record can be used to identify and investigate glacial/interglacial cycles in the Baikal watershed.

The element composition of Baikal sediments was first studied by scientists from the Limnological Institute SB RAS (Irkutsk) in 1997 when a core spanning an interval of about 1 million years was analyzed at the Siberian Synchrotron Radiation Center (SSRC) of the Budker Institute of Nuclear Physics (Novosibirsk). Indeed, the element concentrations turned out to undergo periodic changes producing rhythmic downcore patterns with 96, 72, 54, 41, 23 and 19 kyr cycles.

Besides the orbital rhythms, global climate is known to oscillate at relatively short periods. Which are the ways and causes of these abrupt changes? Interest in millennial-scale climate instability has quickened recently in view of evident rapid global warming. The man-induced forcing of this warming, however, remains uncertain, as well as the predicted future climate scenarios.

Global warming occurred also in the past. The Greenland ice cores bear signature of multiple events within the glacial period between 11 and 74 kyr BP in which temperature rose for 10—15 °C in ten to fifty years. That warming was almost hundred times more rapid than now! The warm spells lasted from 400 to 1 000 years, and then the temperature rapidly returned to the glacial level. The 60-kyr long glaciation was intervened by at least nineteen warm events. Traces of those climate anomalies have been discovered recently in many places of the Northern Hemisphere and appear to be asynchronous with similar events in the Southern Hemisphere.

The Siberian Branch of the Russian Academy of Sciences has worked much to conserve Lake Baikal, a widest known wonder of Siberian nature, for the living and coming generations. The ban of the Siberia-Pacific oil pipeline has been the most recent accomplishment. Siberian scientists argued against the unreasoned pipeline construction along the Baikal shore and along the Trans-Siberian and Baikal-Amur railways already since the early project stage and continued to fight when the decision had been made.

Many opponents of the project explained that the area was seismically active and the pipeline risked to be destroyed by a large earthquake to let oil spill into the clear Baikal water, which would be fatal for the unique flora and fauna of the lake. However, that reason was not convincing enough. The line partisans pretended that the pipe could be made perfectly strong and as safe as to withstand any great earthquake.

Therefore, we chose other key words and invoked landslide and mudflow hazard. The area designated for the line construction is not only highly seismic but is especially very wet and subject to frequent and heavy rain- and snow-falls. Hence, the local earthquakes are attendant with landslide events which may throw down the hillside — together with all pipe-safety constructions and the pipe itself — into the lake. Events of this kind occurred at some sites of the Baikal-Amur railway and called forth building new bridges. No less serious is the threat from mudflows, dreadful streams of rocks and mud produced by rapidly melting snow or rain showers. Beyond the Baikal shore, mudflow hazard persists all along the seismic zone of the Baikal-Amur railway that was presumed to be traversed by the pipeline.

Finally, our arguments were successful. It was decided to move the line 400 km away to the north. That was also an economic gain as the line approached the oil and gas fields. This success is one of few but meaningful examples when the government had to consider scientists’ opinion though it was against the interests of big business.

The causes of these abrupt and frequent excursions are not quite clear but they are independent of insolation anyway. Some evidence has been found lately of about ten global water- and air-circulation events when sudden changes in N — S marine transport (Golfstream is its one branch) and in transport of heat from the equator to the North Atlantic high latitudes were attendant with abrupt warming or cooling in Greenland.

If scientists find a convincing proof that the past abrupt climate changes were of global scale, it may cast doubt on the uniqueness of the current global warming. Solving the problem requires high-resolution paleoclimate data from all regions worldwide, and Siberia remains among the least explored areas. To bridge the gap, we measured the concentrations of elements in the Baikal sedimentary record at every 1 mm to a decadal resolution (~20 years). The high-resolution elemental analysis of the Baikal cores became feasible due to a scanning device mounted on a SR-XRF (Synchrotron Radiation X-Ray Fluorescence) station. The scanner was designed at SSRC and provided geochemical records of Siberian climates at thousands of core horizons. This work would be impossible without cooperation of specialists in different sciences, which is the basic guiding principle in the RAS Siberian Branch.

Еvgeny L. Goldberg, Candidate in Physics and Mathematics (Limnological Institute, Siberian Branch of the Russian Academy of Sciences, Irkutsk Science Center)

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