− Takafumi Matsui −
Earth's environment in the Jomon Period
Earth's climate in the last glacial period
Fig.1 The temperature variation based on the analysis
of the Antarctic boring ice cores drilled at Vostok Station
(Jouzel et al., 1996)It is widely known that the earth has experienced cold periods repeatedly. These cold ages are called "glacial periods", while intervening relatively warm periods are called "interglacial periods". At 21,000 years ago, the earth was in the middle of the last glacial period (LGM: Last Glacial Maximum). Analyses of boring cores taken from ice sheets in the Antarctic and Greenland and boring cores taken from the sea-floor and lake-floor suggest that the global average temperature was more than 5 Kelvin lower compared with the current value. An ice sheet means a glacier with the size comparable to a continent. In those days ice sheets covered the North America (the Laurentide Ice Sheet) and the northern part of Europe (the Fenoscandia Ice Sheet). The thickness of these ice sheets is estimated to have exceeded 3 kilometers at the central part.
The existence of large ice sheets lowers the global sea level because they fix a vast amount of water on the continents. The sea level during the LGM was considerably lower than the today's level, so that consequently most of the current continental shelf used to be dry land. At the time of LGM, Bering Strait, Indonesia and the Eurasian Continent, and Australia and New Guinea were connected by land bridges. The Inland Sea in Japan was not a "sea" since the sea floor was all above the sea level those days.Climate after the last glacial period
After the LGM, the global temperature began to rise and the water generated by melting of ice sheets streamed into the sea. This relatively warm period is called Bソlling/Allerソd Interval. However, the climate once regressed to that of the glacier period for about 1000 years around 12,000 B.P. This period is called Younger Dryas. The temperature changes before and after this period were very drastic. For example, the average temperature of Greenland changed 7 OC in only several decades.
Fig.2 The shoreline and the distribution of ice sheets of the northern hemisphere at the LGM. The shoreline of Japan at the LGM.
The climate turned to be warm again after Younger Dryas and it was about 6000 years ago that the most part of the northern hemisphere experienced the warmest climate (the hypsithermal interval). Ice sheets in the North America and North Europe have already faded away and have retreated to Greenland and Antarctic by this time. This means that the vast loads on the ground were removed and on the other hand new loads were applied to the sea floor. Consequently grounds began to rise while the sea floor began to sink. The ground of the central Fenoscandia is still uplifting at the rate of about 1 centimeters a year.
The environmental change during the Jomon Periods is characterized by this global warming and transgression. The ascent of sea level during the warm period at about 6,000 B.P. is seen in Japan ("Jomon transgression"). This is caused by the melting of ice sheets. The change in the sea level during the Jomon Periods suggests that the transgression stopped at some time and the sea level started to go down ("Jomon regression"). This regression has been considered to be due to the reaction of the mantle around Japan in response to the redistribution of loads to the ground and the sea.Other environmental variations
One of other processes that result in considerable global environmental change is an impact of a meteorite. The earth has been always exposed to a shower of meteorite impacts. Fortunately for us, most of them are too small to bring serious disasters.
However, sometimes there occurred a giant meteorite impact in the earth's history which resulted in significant environmental change. The most famous example is the one that caused the extinction of dinosaurs at the Cretaceous-Tertiary boundary.
Roughly speaking, the degree of hazard caused by an impact of a meteorite depends on size and impact velocity. The larger the meteorite is, and also higher the impact velocity is, the more violent the explosion is. However, statistically impact frequency of larger meteorites are rarer compared with that of small ones.
Barringer Meteor Crater (Arizona, USA), with a diameter of 1.2 kilometers, formed at about 50,000 years ago. The energy of this impact event is equivalent to the explosion of 20 Mega tons of TNT. This impact should have resulted in locally considerable disaster.
Impact energy equivalent to 104 Mega tons of TNT is necessary to result in a global scale disaster. The frequency of meteorite impacts with such energy is less than once in 100,000 years. No evidence of such a event have been found in the Jomon Periods so far. However, we should remind that such a large impact results in a serious influence on the earth environment once it had happened.
Large eruptions of volcanoes have also significant influence on the environment. For example, the eruption of Santorini in the Mediterranean Sea at 1,500 B.C. caused high tsunamis and gave a catastrophic disaster to Mediterranean civilizations.The sky in the Jomon Period
Fig.3 Barringer Meteor Crater
Fig.4 Canis Major (the Great Dog) at 5000 B.C. (left) and today (right).Stars that constitute constellations are called "fixed stars" because they seem to be fixed on the sky. Actually they moves in the sky in respective directions. However they look fixed because they are located so far from the earth. It is almost impossible to recognize their motions. This motions of stars across the sky are called the proper motions. The average proper motion of stars that are visible to human eyes is about 0.1 second degrees a year. This is no more than 0.005% of the diameter of the full moon.
A few stars show proper motions large enough to be recognized in a time scale of thousands of years. In 1718 Edmund Halley, who found Halley's Comet, notices that the positions of three bright stars were clearly different from the positions on old Greek catalogues. Since hundreds of other stars were in right places, Halley thought that this was not because the Greek astronomers made mistakes, but because they actually moved across the sky. Three stars with large proper motions that Halley found are Sirius, Procyon and Arcturs. Fig. 4 shows the constellation Canis Major at 5000 B.C. and today. You can see easily that the shape of the constellation changes because of the proper motion of Sirius.
