/Image1182.gif) Short（HP/2011/10）による『The Earth as a Planet』から |
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地球（Earth）は誕生して約４６億年が経過しているが、その概略的な変遷についての情報を集めている。 歴史年表（Chronological Table）的な内容は地質年代（Geologic Time Scale）によって表現されているが、こちらは『年代測定法とは』のページを参照。 地球の進化史（History of Evolution）上で特筆されることがいくつかある。１つめは、地球誕生後の早い時期に起こったとされるジャイアント・インパクト（Giant Impact）と月の誕生（Formation of Moon）である。２つめは、約４０億年前頃と推定されている生命の誕生（Origin of Life）である。３つめは、同じ頃に始まったと考えられているプレート・テクトニクス（Plate Tectonics）である。４つめは、２７億年前頃に起こったと考えられている核のダイナモ対流の開始（Start of Dynamo Convection in Outer Core）と地球磁場の誕生（Formation of Earth's Magnetic Field）である。これ以外にも、海の誕生（Formation of Ocean）、生命の爆発的進化（Explosive Evolution of Organism）、生物の陸上進出（Spreading on Land）、オゾン層の形成（Formation of Ozone Layer）、氷河時代〔スノーボール・アース（Snowball Earth）も含めて〕の到来（Coming of Ice Age）、などさまざまの重要な出来事が知られている。 |
| リンク |
| 地球 |
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Short（HP/2011/10）による『The Earth as a Planet』から |
 図1 放射性起源同位体比に基づいた地球内部の化学的構造モデル（Kaneoka, 2008）． Fig. 1 A model of the chemical structure of the Earth's interior based on radiogenic isotopes（Kaneoka, 2008）. 兼岡（2008）による 『“時間”を武器に地球を探る― 年代測定の現状と，同位体比による地球深部の化学的環境の解明―』から Kaneoka, I.（2008）: On the degassing state and the chemical structure of the Earth's interior inferred from noble gas isotopes-Past and recent views. Geochemical Journal, 42, 3-20. |
 図4: 地球初期進化の概念図. 岩波講座地球惑星科学13 巻. 横畠（2001）による『地球史セミナー第1 回地球史概論』から |
 図2: 地球史年表. (a) 従来の時代区分, (b) 地球史7 大事件に基づく時代区分. 川上(2000), p 33. 横畠（2001）による『地球史セミナー第1 回地球史概論』から |
| 大気 |
/Image1360.gif) Fig. 7. Some estimates of Cenozoic and Cretaceous atmospheric CO2 levels, based on different proxies and models. Proxy values often represent special extreme variations (e.g. sudden CO2 rise after release of methane from decomposition of clathrates; sharp CO2 decline after burial of organic carbon). Note that estimates based on paleosol carbonates are generally higher than estimates based on other proxies. Many proxies agree on Neogene values, but the spread of estimates increases markedly in the Paleogene and Cretaceous. Most researchers assume that during most of the Cretaceous atmospheric CO2 levels were between 2 and 8 times present. R values on right axis are ‘times pre-industrial.’ Numbers in parentheses are estimates for the time in the future when these levels will be reached, assuming the entire 5000 Gt reserves of petroleum, natural gas, and coal will be used at the increasing rate determined from past history (1958.2008). Data from Cerling (1991), Ekart et al. (1999), Freeman and Hayes (1992), Haworth et al. (2005), Pagani et al. (1999), Pearson and Palmer (2000), Retallack (2001), Royer et al. (2001b), Berner and Kothavala (2001), Wallmann (2001), and Yapp and Poths (1996). Hay(2011)による『Can humans force a return to a ‘Cretaceous’ climate?』から |
/Image1171.gif) Fig. 2 Evolution of the Sun's luminosity with time in the last 4.6 Ga, as predicted by the computer model of Gough (1981). The increase is around 7% every Ga. The young Earth therefore experienced a fainter Sun 〜30% less powerful 4.5 Ga ago than at present |
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/Image1172.gif) Fig. 3 Long-term evolution of the relative concentrations of CO2 and CH4, as well as of O2. To be noticed are (1) the rapid decrease in methane coinciding with an O2 rise, leading to the Huronian glaciation (2.2-2.4 Ga) and (2) the drop in CO2 at the beginning of the Neoproterozoic leading to major glaciation (750 Ma). (From Kasting 2004) |
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/Image1173.gif) Fig. 6 This curve depicts the most recent results from the GEOCARB model of Berner for the evolution of CO2 concentration in units of PAL (1 PAL = 280 ppm). The grey envelope shows the importance of uncertainties. The window inside the vertical lines corresponds to the Cretaceous-Palaeocene period. (From Berner 1994) |
/Image1174.gif) Fig. 7 Reconstruction of atmospheric CO2 concentrations for the Mesozoic and the Cenozoic since 250 Ma, derived from two different models GEOCARB (circles Berner 1994) and GEOCLIM (squares Donnadieu et al. 2006). The major difference between these two models is the evaluation of the sink. Large uncertainties remain independently of the models used. (Reconstruction from compilation by Royer D.A. 2006) |
/Image1175.gif) Fig. 9 Evolution of the biodiversity and of atmospheric CO2 (crosses) during the Triassic and the Jurassic periods. The red and black curves correspond to calcareous plankton evolution during the same period. The dashed curve corresponds to the GEOCARB III model. (Godde´ris et al. 2008) |
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| Ramstein（2011）による『Climates of the Earth and Cryosphere Evolution』から | |
/Image1169.gif) Figure 1: Changing concentrations of atmospheric carbon dioxide with time on Earth in response to the steady increase in solar luminosity (after Kasting 1993). The amount of past CO2 that has been calculated in this figure (using the model of J.F. Kasting, 1993) is the concentration required to keep the surface warm enough for liquid water to exist even with past lower solar output. PAL = present atmospheric level of CO2. The Moon formed between 4.5 and 4.45 billion years ago (dark blue field). The Earth could have started with an atmosphere extremely enriched in carbon dioxide (gray field). The rise in land plants after 500 million years ago ("C3" on the figure) defines a inimum limit for CO2 in the air after that time. /Image1170.gif) Figure 2: This plot (after Kasting, 1993) shows estimates of changing concentrations of free oxygen in the atmosphere over geologic time. The area (in light blue) represents the range in possible concentrations of oxygen based on model calculations, and the study of ancient soils (paleosols), fossil organisms, and marine sediments that only form in the absence of oxygen. These sediments are preserved as Banded Iron Formations (BIF) and only appear in the geologic record up to about 1.8 billion years ago. Although it is not known when photosynthesis began, it is clear that photosynthesis only became an important producer of oxygen in the atmosphere late in Earth’s history. PAL = present atmospheric level of O2. Mojzsis（HP/2011/10）による『Life and the Evolution of Earth’s Atmosphere』から |
/Image1168.gif) 大気中の酸素とオゾン濃度の進化 JAXA（HP/2011/10）による『超伝導サブミリ波リム放射サウンダ（SMILES）』から |
/Image1160.gif) 井上（HP/2011/10）による『酸素発生型光合成と藻類』から |
/Image1183.gif) Short（HP/2011/10）による『The Earth as a Planet』から |
/Image1165.gif) O2 build-up in the earth's atmosphere. Red and green lines represent the range of the
estimates while time is measured in billions of years ago (Ga).
Wikipedia（HP/2011/10）による『Great Oxygenation Event』から |
 理科ネットワークの『地球と生命の誕生』の中の『暗い太陽のパラドックス』から 約46億年前に誕生した太陽は核融合反応により輝いているが、初期は暗かった（現在の7割程度の明るさ）と予想されている。地球の表面温度（全地球平均温度）は、主に太陽光度の大きさによって決まると考えると、初期の地表面温度はマイナス数10℃になってしまう。現在知られている証拠は、そのような低温環境を示さない。これが『暗い太陽のパラドックス』である。これは、初期の地球大気には大量の二酸化炭素が存在したため、その温室効果によって暖められ、地表面温度は適度な値であったと説明されている。 |
/Image1166.gif) 過去の地球大気組成 倉本（2004）による『海と惑星はなぜ存在するのか 惑星大気起源論への招待』から |
| 海 |
/Image1191.gif) Fig. 1. Compilation of new and previously published oxygen isotope data for screened chert samples over geologic time. All samples except the 3800 Ma examples are for unmetamorphosed cherts or cherts metamorphosed no higher than Greenschist Facies. The history of long-lived continental cratons is taken from Rogers (1996) and the horizontal lines represent periods of stability of specific cratonic masses identified by him. The 10‰ variation in δ18O for cherts at any given time is caused by the presence of low 18O meteoric waters present during diagenetic chert formation and/or formation of cherts during deeper burial at elevated temperatures. The overall increase in δ18O with time is interpreted as global cooling over the past 3500 Ma. New data are given in Tables 1 and 2. Published data are from Knauth and Lowe (1978, 2003), Force et al. (1986), Gao and Land (1991), Suchecki and Hubert (1984), Kenny and Knauth (1992), Beeunas and Knauth (1985), Winter and Knauth (1992) and references cited therein. /Image1192.gif) Fig. 2. Halite accumulations in the Phanerozoic. Data were originally listed by geologic time period (Zharkov, 1981; Land et al., 1988) and are centered here on the absolute age of these designated periods. Precambrian halite has never been inventoried. The Neoproterozoic value estimated here is based on the assumption that the enormous accumulations in Australia, Oman, Iran and Pakistan are spread over basins larger in extent than that hosting the Luann salt, USA (180 Ma). Most of the known salt was clearly deposited in two large pulses, one in the interval 180-250 Ma and one in the interval 500-700 (?) Ma. Sequestration of these “saline giants” requires a rare combination of geographic, geologic, climatic, oceanographic and depositional conditions. /Image1193.gif) Fig. 3. Dissolved oxygen in seawater as a function of salinity and temperature for the present atmosphere (1 PAL O2) and an ancient one with 10% of the present atmospheric level of O2 (0.1 PAL) using experimentally determined algorithms by Weiss (1970). An example threshold level of 0.5 ml/l for dissolved O2 required for Neoproterozoic metazoans is shown together with example trajectories for the evolution of dissolved O2 in seawater with time. Curve B invokes a minimum salinity in the Archean and a relatively steady decrease throughout the Precambrian. Curve A is for high initial salinity with little decline until deposition of the great Neoproterozoic saline giants. Under an atmosphere of 0.1 PAL O2 and temperatures of 15-25 ℃, reduction of ocean salinity by deposition of these salt deposits could shift dissolved O2 to values above that required for metazoan respiration, as shown by the arrow labeled “Neoproterozoic”. Knauth（2005）による『Temperature and salinity history of the Precambrian ocean: implications for the course of microbial evolution』から |
| 生命 |
  〔東北大学理学部自然史標本館の『ツアーコース』の中の『地球生命の進化』から〕 |
| ジャイアント・インパクト |
/Image1180.gif) Figure 2: The effect on Earth of the giant impact that formed the Moon. a, A giant planetary embryo collides with the nearly complete Earth. b, A magma disk is in orbit about Earth, while blobs of iron from the planetary embryo settle down through the mantle to join the existing core. c, The outermost part of the magma disk coalesces to form the Moon as the result of radioactive cooling, while the rest falls back to Earth. Inside Earth, the mantle nearest the core has partly solidified, and the mantle might acquire a layered structure. Stevenson(2008)による『A planetary perspective on the deep Earth』から |
 The Earth's core was formed in two stages, which could explain why scientists have been coming up with two different dates for its age (Image: Universe Today/NASA) 〔Anna Salleh氏によるGiant impact added to Earth's coreから。ABCの『News in Science』の中のページ（Thursday, 27 October 2005）。〕 |
 図５ 月のマグマオーシャン起源仮説．（a）表層が融解していた地球に、（b）天体が衝突し、（c）地球自転が加速されて一部が分離し、（d）月を形成する。高橋（1993）より。 奥地・高橋（1994）による『地球化学的月起源論』から |
