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961
Section: Biology
Weather Forecasting Back for 8 Million Years

Weather Forecasting Back for 8 Million Years

The global climate is highly unstable, and people witness climatic changes even within their life span. The Earth’s surface became about 0.5° С warmer for the past century. The temperature growth, quite notable on the global scale, decreased the snow area and caused an average sea level rise of 10—20 cm. Waning of Antarctica and Greenland ice sheets as a result of global warming may bring about another Deluge, which will flood large areas in Europe and Southeast Asia...

The causes of global climate disasters are, naturally, of interest to scientists and broad public. The clue to the present and the future of our planet is in its past. A coded record of the climate in the past is stored in sedimentary layers, which bear features of changes in the sea level, warm and cold oceanic currents, air circulation, and waxing/waning cycles of ice sheets. People only have to get and to decode the record. The first thing is to drill a hole in sediments and to recover a core. Say, a hole in the bottom of the world deepest lake…

It has been found out that the climate history of the past tens million years consists of intermittent glacial and interglacial rhythms, possibly, because insolation changes as the Earth orbits the Sun. The orbital forcing hypothesis proposed by Milankovitch was proved valid through studies of the ocean bottom sediments, especially, the isotope composition of oxygen in fossil foraminifers, primitive sea animals, relatives of amoebas.

The oxygen isotope climate proxy is due to isotope fractionation in the air and seawater. The normal oxygen 16O evaporates more readily than the heavier 18O, which resides in the air and seawater. A relative amount of 18O in the seawater increases during ice melting in warm periods and decreases during ice growth in cold spells. The oxygen isotope variations in foraminiferal shells that conserve the seawater composition were used to compile the standard δ18O curve that records changes in the global ice volume.

However, that was not the final solution of the paleoclimate problem, as the environment and ecosystems in different regions of the globe respond to a climate change quicker or slower and each in its own way. In addition to the orbitally-controlled insolation, climate responds to geological forcing, such as mountain growth and volcanism. Great amounts of ash ejected during the eruption of the Krakatau volcano in 1883 caused global cooling that lasted about two decades.

Long sedimentary climate archives are common to the continuous oceanic deposition, but variable continental processes most often leave geographically dispersed records, which are often difficult to bring together. Baikal, the world deepest and oldest extant lake, represents a unique repository of events recorded on a long timescale and at high resolution and is thus a perfect natural laboratory to study continental paleoclimates, and especially the climate of Eurasia.

Lake Baikal occupies the deepest rift basin in the center of the Baikal rift system that initiated after the India/Eurasia collision about 40 Myr ago. The lake consists of the three sub-basins: the shallowest (~900 m) North Baikal basin, the deepest (1634 m) Central Baikal basin, and the oldest South Baikal basin (~ 1400 m), with a submerged rise of Akademichesky Ridge between the North and the Central basins and the Selenga-Buguldeika saddle between the Central and the South basins. The Akademichesky Ridge is of tectonic origin, and the Selenga saddle is built mostly of sediments carried by the Selenga, the largest Baikal tributary.

« 5.1.1997.…The morning showed nothing threatening. The ship was moving and the ice field bordered a strip of clear water. The ice sheet seemed to be going to drift southward and let us through to the North. In afternoon, however, a transit ice block collided with the land fast ice and ice ridges began to grow. The block split into pieces of which some sank beneath the land ice and some rose to the surface. The ship heeled over. The ice chaos calmed down in half-an-hour and left the drilling complex blocked among ice ridges with their sails reaching over two meters high. The barge held its position, but the ship tilted to the left. We could not move and go on. The planned drilling work was in for failure and we thought of changing the site. To repair the heel, we had to chop off the three meters thick ice armor around the ship. The exhausting work finished in two days, and the ship took back its straight position.
28.1.1998.…After three weeks of drift together with a large ice floe we stopped for drilling in the center of the South Baikal basin… The core receivers were repeatedly jammed with sand from the uppermost sediments, and we had to pull the over 1400 m long rig up aboard the barge for cleaning out. That was an arduous job.
28.2.1998.…Only 40 meters of continuous coring has been possible. The coordination panel takes a decision to search for gas hydrates which hypothetically exist in the bottom sediments.*

The geographic position of Baikal in high latitudes, where insolation is the most variable due to the orbit inclination, makes the lake very sensitive to insolation changes. On the other hand, the enormous mass of water and great depth prevent the lake from freezing through, unlike most of smaller lakes in these latitudes, and maintain continuous climate-controlled deposition. Moreover, being located in the center of Asia in a highly continental climate, the lake is an ideal test site for seasonal climate variations.

24.1.1996. …We found ourselves among ice ridges. All people on board were standing near the ship sides, at the head or on the captain’s bridge gazing at the nature’s work. What an unforgettable picture! A small ship in the ocean of alive moving ice blocks which break and ride over one another building enormous fields of ridged ice. Everybody feels somewhat unprotected, being in the ice wilderness and helpless against before nature, which is not to play with.

These auspicious circumstances taken together allowed the lake to accumulate a kilometers-thick bottom fill that stores a unique archive of geological and climate events for over 30 million years. At last, the remarkable archive of Cenozoic environments in Central Asia, continuous over the past five to eight million years, has waited until people came to study it…

The history of the Baikal Drilling Project (BDP) began over ten years ago with a multinational multidisciplinary programme of geological and geophysical exploration. Prior to the drilling experiments, considerable joint efforts were made to study the nature of sedimentation using examination and sampling with Pisces manned submersibles, piston coring of shallow sediments, and high-resolution multichannel seismic profiling of the whole sedimentary section.

Seismic imaging showed the greatest sediment thickness in the southern and central sub-basins (up to 8 km) and the thinnest fill (4 km) in North Baikal. Sedimentation rate in different parts of the lakes was estimated as 0.03 to 0.2 mm/yr: the slowest on the Akademichesky Ridge and the fastest on the Selenga-Buguldeika saddle and in deep lake basins. Distinctly stratified shallow sediments promised to be an informative account of paleoclimate events.

In 1989, Prof. D. Williams from the University of South Carolina (USA), one of the founders of the Baikal International Center for Ecological Research (BIСER), suggested the idea of deep drilling on Lake Baikal. The project was supported by Prof. Sh. Horie from Japan, who coordinated the first drilling on Lake Biwa. Later the team extended with German and Japanese scientists. The Japanese part made a large group headed by Dr. T. Kawai and created a separate association for Baikal studies.

Drilling was performed by the Russian Nedra drilling team that leaded the continental drilling programme in Russia and drilled the world deepest borehole on the Kola peninsula. The Russian part included several groups, mostly from the academic institutions of the Irkutsk Science Center (the Institute of Geochemistry, the Limnological Institute, and the Earth Crust Institute).

The Nedra engineers in a special care about the lake ecology designed a nature-friendly drilling complex Baikal. Of various models we chose an ice-based drilling platform, mounted on a barge driven to the selected site by a research vessel which then serves as a lodge. In its final version, the drilling complex mounted in 1997 on a 1000 ton barge was capable of drilling up to 1000 m long holes in 900 to 1000 m of water. Experts of the oceanic and continental drilling from the US and Germany find the methods and quality of Baikal drilling superior to many analogs worldwide.

A special thing is navigation and positioning the complex in the drill site at the very beginning of the ice season. One has to know the weather forecast and monitor the freezing process on the lake. The weather information for BDP came from satellite images obtained and deciphered at the Institute of Solar and Terrestrial Physics (Irkutsk).

Five BDP campaigns since 1993 recovered an over 1.5 km thick layer-cake of bottom fill which spans over eight million years. Cores from seven boreholes in different parts of the lake revealed the deposition history in different environments. The first core from the site near the mouth of the Buguldeika showed that the river used to flow out of Baikal into the Lena about fifty thousand years ago. The core is made up of dense sediments rhythmically alternated between detrital clay material and fine silt rich in diatoms. Coarser sediments in the core bottom may record the environment of a riverbed.

Deposition on the Akademichesky Ridge surrounded by deepwater basins was different. According to the drilling evidence, the basins have existed for at least 5 million years and isolated the rise from the land influence. Therefore, the sedimentary column contains only sediments precipitated from water. The sediments are about 1000 m thick and likewise display rhythmic layering.

The cores from the central South Baikal basin differ from the two previous ones. They are intervened with gravel-sand flashes of land provenance (sandy turbidites) carried by ephemeral streams far seaward. Such sedimentary flows are known on the ocean margins, where they result from the so-called “avalanche sedimentation”. They carry abundant plant detritus into the lake which becomes buried at great subbottom depths to make a source of hydrocarbon compounds. These compounds are pressurized and conserved as gas hydrates under the great load of deep water.

The cores from the three subbasins formed in different deposition environments have different implications for problems of paleoclimate, continental rifting, evolution of sedimentary basins, including generation of hydrocarbons, etc.

The cores from the Akademichesky Ridge, where deposition is controlled uniquely by climate change, are especially valuable for paleoclimate studies. The Buguldeika cores document the history of rifting and provide a higher resolution climate record due to faster sedimentation than on the Akademichesky Ridge. The sediments of the Posolsk Bank in South Baikal contained gas hydrates which have never been encountered before in freshwater basins. This discovery was the main accomplishment of the 1997 campaign.

Once a core that stores a message from the distant past has been recovered, scientists are to decipher the signals using sensitive climate proxies. Unlike the oceanic cores, where the climate history is recorded in biogenic carbonates, the climate-driven fundamental ecosystem changes in low-mineralized Baikal can be inferred from biogenic silica. The greatest part of biogenic silica occurs in frustules of diatom algae, the oldest and most numerous Baikal dwellers. Diatom-rich mud in the layered cake of the Baikal cores alternates with diatom-barren clay. It was hypothesized that diatomaceous mud formed in warm climates of interglacials and clay deposited in cold glacial times.

21.1.1999.…Winter drilling on Lake Baikal is a difficult thing. Every time nature tests people for strength. Today we fixed the barge to the right point and cut a small hole in the ice to balance its motion onto the barge under strong wind. Yet the ice began to ridge up rapidly at 2 p. m. An ice block struck against the barge like a large hammer. Right away the skipper told water had penetrated into one bilge. A 15 cm thick jet of water spurted out from a hole in the ship plating. While we were trying to patch the hole up with canvas the bilge became full of water. The ice block deformed the bilge wall and cut a long fracture from the deck to the bottom. The patching failed. We tried to pump water out but our two drill pumps could not keep up. Then we decided to weld the top of the hole but had to leave the repairing work on the main hole for healing numerous small fractures which let water into the neighboring bilges.
22.1.1999.…A ship came to our rescue. The captain and his mates cut stakes and planks, took all rags we had and tried to close up the hole from outside. They had finished by 1 p. m., the drilling people set up the third pump, and the water began to fall. That was the first victory.
24.1.1999.…The repairing work came to its culmination. We decided to make a box in the bilge near the hole, to weld it and to armor it with cement. The box was ready by the evening and was so good that we passed on to other smaller defects. The barge had been put to rights.
30.1.1999.…Drilling started.
31.1.1999.…The first high-quality core recovered in the morning. Sedimentation rate in the site is 30 cm/Kyr. Thus the climate history of the Posol’sk Bank can be resolved to fine details, the subtlest temperature variations are well pronounced. In spite of the frost of — 30° C and fear for the shortage of fuel, the work was a success, and drilling reached a subbottom depth of 350.5 m.»*
* Kuz’min M. I. In the ice field of Lake Baikal, Vinogradov Institute of Geochemistry (Irkutsk), GEO, Novosibirsk, 2001, 140 pp.

In addition to the biogenic silica and diatom abundance, there are other proxies of paleoclimates, such as magnetic susceptibility, spore-pollen stratigraphy, and various organic and inorganic geochemical indicators. The magnetic susceptibility curve is based on different magnetic properties of clay and silt.

Deciphering paleoclimate signals requires precise age ties. An age model as a clue to the history of sedimentation and paleoclimates is developed using paleomagnetic measurements as the most reliable dating tool. Age-depths relationships are checked against the geomagnetic timescale of normal and reverse polarity. The latest switch to reverse polarity, when the Southern and Northern poles change places, occurred about 800 Kyr BP.

Different proxies in the Baikal archive of continental climates were compared with one another and correlated to the marine oxygen isotope record of the orbitally-forced climate. The biogenic silica (and, respectively, diatom count) and whole-core magnetic susceptibility curves match the climate trends reported from the oceanic sites. The climate events recorded in both oceanic and continental archives show the same periodicity of 100, 44, 23, and 19 Kyr cycles. Therefore, the climate in different regions of the globe for the past 5 million years changed under the effect of a single astronomic cause.

The general paleoclimate trends, supported by the specious composition of diatoms and spore-and pollen assemblages, reveal cold spells in the Late Cenozoic at 2.8—2.5 and 1.8—1.4 Myr BP which are also recorded in the oceanic archive and in cores from Alaska, Iceland, Europe, West Siberia, and elsewhere.

The climate record stored in the bottom fill of the Earth’s deepest lake shed light on the age and conditions of global climate hazards in the Cenozoic associated with volcanic events in Central Asia as well as in the whole Northern Hemisphere.

The unique continuous record of continental climates over the past 8 m. y. obtained for the first time can be used for reference in correlation of dispersed continental sections.

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