過去の痕跡を調べる ２０２０年４月５日（日）
大阪の子ども達は、長い春休みになってしまいそうです。今のところ、２か月間のお休みとなっています。午前中は家にいるようにしている子ども達も、午後になると公園に出てきて体を動かしています。近くの児童公園の管理者として、草花に水やりをするときに、子ども達が遊ぶ小さな滑り台の手が触れる所にも、水をかけるようにしようかなと思います。
今日は、大和川の堤防までフィールドワークに出かけました。鳥は、ムクドリの写真を撮ることができました。ウが、川面を低空飛行していましたが、それは遠いので写真に撮ることはできません。昆虫は、テントウムシの幼虫、モンシロチョウ、モンキチョウ、ベニシジミの写真を撮りました。今日は、２００ｍｍ望遠レンズを持って行ったので遠くから写真が撮れます。植物は、ノヂシャ、満開のソメイヨシノ、ムラサキツユクサ、キツネノボタン、ボケなどの写真が新たに増えました。
大和川の川原は、ナヨクサフジの紫色の花と、セイヨウカラシナの黄色の花が広がっていてとても綺麗です。また、堤防の上に植えられたソメイヨシノは満開でした。不要不急のお出かけは控えると大阪府は発表しているので、大和川の堤防の上を歩いている人はあまりいません。申し訳ないなと思いながらも、人通りが少ない道を選びながら、お散歩をかねたフィールドワークをしました。
今日は、大和川の川原で土器を見つけました。昔、大和川の近くの小学校に勤務している時、子ども達と一緒に川原で土器をたくさん見つけたことがありました。川原を１時間ほど歩きまわって、バケツにいっぱいの土器片を拾いました。堺市の歴史博物館に持って行き、本物の土器かどうか調べてもらいました。縄文土器、弥生土器、須恵器などが混じっていて、川の上流から流されてきたものでした。大和川の付け替えの時、川底にあった遺跡から流されてきているものだそうです。移動してきているものなので、とても貴重だということでもないようですが、大事にしてくださいと言われました。今回は、その時のことを思い出して、１０分間ほど大和川の川原で土器を探すと、すぐに２、３個、見つけることができました。今もあるようです。昔、木津川の川原を歩いた時も、土器片を見つけたことがありました。いつの時代のものか分かりませんが、たしかに土器のようでした。奈良県の盆地の中を流れる川の川原でも、土器が見つけられると思います。機会があれば、一度チャレンジしてみたいと思います。さらに、土器の発見と同じように、和泉山脈から流れ出てくる川の石から、中生代白亜紀の化石も見つけられるかもしれません。露頭からの発見ではないので、価値は半減ですが、恐竜の歯などは、こんな所から発見されることがよくあります。和泉山脈近くの川原で、化石探しもしてみたいです。
＜新聞の記事から＞
●南極に森林や湿地 南極大陸は、約９０００万年前（白亜紀半ば）は、温暖で森林や湿地があったことが推定されると、ドイツのアルフレッド・ウェゲナー極地海洋研究所などの国際研究チームが４月３日までに英科学誌ネイチャー電子版に発表した。当時は南極大陸を覆う氷床がなく、大気中の二酸化炭素濃度が現在よりはるかに高かったと考えられている。沿岸の海底下を観測船で掘削し、９３００万年前から８３００万年前の土を採集して、その中に温暖な地域に生息する植物の根や花粉などが多く見つかった。当時の気候を復元すると、年間平均気温は１２～１３度。年間降水量は１１２０ｍｍと推定された。夏には月間平均気温が１８，５度、河川や沼の最高水温が２０度に達したと考えられる。
＜アルフレッド・ウェゲナー極地海洋研究所のホームページでの発表＞
A sensational discovery: Traces of rainforests in West Antarctica
●90 million-year-old forest soil provides unexpected evidence for exceptionally warm climate near the South Pole in the Cretaceous[01. April 2020]
An international team of researchers led by geoscientists from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, have now provided a new and unprecedented perspective on the climate history of Antarctica. In a sediment core collected in the Amundsen Sea, West Antarctica, in February 2017, the team discovered pristinely preserved forest soil from the Cretaceous, including a wealth of plant pollen and spores and a dense network of roots. These plant remains confirm that, roughly 90 million years ago, the coast of West Antarctica was home to temperate, swampy rainforests where the annual mean temperature was ca. 12 degrees Celsius – an exceptionally warm climate for a location near the South Pole. The researchers surmise that this warmth was only possible because there was no Antarctic ice sheet and because the atmospheric carbon dioxide concentration was significantly higher than indicated by climate models to date. The study, which provides the southernmost directly assessable climate and environmental data from the Cretaceous and poses new challenges for climate modellers around the globe, was released today in the journal NATURE.
The mid-Cretaceous time interval, from ca. 115 million to 80 million years ago, is not only considered the age of the dinosaurs, but was also the warmest period in the past 140 million years. Sea surface temperatures in the tropics at this time were likely as high as ca. 35 degrees Celsius, and sea level was 170 metres higher than today. Yet we still know very little about environmental conditions in the Cretaceous south of the polar circle, since there are virtually no reliable climate archives that extend that far back in time. The new sediment core offers the team of experts the first chance to reconstruct the West Antarctic climate during the warmest interval of the Cretaceous, thanks to the unique evidence it contains.
In the sediment core, which the team collected with the University of Bremen’s seafloor drill rig MARUM-MeBo70 near the Pine Island Glacier on an RV Polarstern expedition, they found pristinely preserved forest soil from the Cretaceous. “During the initial shipboard assessments, the unusual colouration of the sediment layer quickly caught our attention; it clearly differed from the layers above it. Moreover, the first analyses indicated that, at a depth of 27 to 30 metres below the ocean floor, we had found a layer originally formed on land, not in the ocean,” reports first author Dr Johann Klages, a geologist at the AWI.
●Evidence of a swamp landscape rich in vegetation
Yet it didn’t become clear just how unique the climate archive truly was until the sediment core was subjected to X-ray computed tomography (CT) scans. The CT images revealed a dense network of roots that spread through the entire soil layer of fine-grained clay and silt, and which was so well-preserved that the researchers could make out individual cell structures. In addition, the soil sample contains countless traces of pollen and spores from various vascular plants, including the first remnants of flowering plants ever found at these high Antarctic latitudes.
“The numerous plant remains indicate that 93 to 83 million years ago the coast of West Antarctica was a swampy landscape in which temperate rainforests grew – similar to the forests that can still be found, say, on New Zealand’s South Island,” explains co-author Prof Ulrich Salzmann, a palaeoecologist at Northumbria University in Newcastle upon Tyne.
The results of the vegetation analysis puzzled the researchers: under what climatic conditions could temperate rainforests have formed back then at a geographic latitude of roughly 82 degrees South? Even during the Cretaceous, the Antarctic continent was at the South Pole, which means the region where the forest soil originated was subject to a four-month polar night; for a third of every year, there was no life-giving sunlight at all.
“To get a better idea of what the climate was like in this warmest phase of the Cretaceous, we first assessed the climatic conditions under which the plants’ modern descendants live,” says Johann Klages. The researchers subsequently searched for biological and geochemical temperature and precipitation indicators in the soil sample, on the basis of which they could reconstruct the air and water temperature in the West Antarctic rainforests, as well as the amount of precipitation they received.
●Numerous analyses, one result: In the Cretaceous, Antarctica was ice-free and extremely warm
The outcomes of the various analyses fit together like the pieces of a puzzle: Roughly 90 million years ago, there was a temperate climate just 900 km from the South Pole. The annual mean air temperature was ca. 12 degrees Celsius; in other words, back in the Cretaceous, the average temperature near the South Pole was roughly two degrees warmer than the mean temperature in Germany today. Summer temperatures were ca. 19 degrees Celsius on average; water temperatures in the rivers and swamps reached up to 20 degrees; and the amount and intensity of rainfall in West Antarctica were similar to those in today’s Wales.
The researchers then used this new vegetation, temperature, and precipitation data from West Antarctica as target values for simulations of the mid-Cretaceous climate. Their calculations with a palaeoclimate model revealed that the reconstructed conditions could only be achieved when (1) the Antarctic continent was covered with dense vegetation, (2) there were no land-ice masses on the scale of an ice sheet in the South Pole region, and (3) the carbon dioxide concentration in the atmosphere was far higher than previously assumed for the Cretaceous. “Before our study, the general assumption was that the global carbon dioxide concentration in the Cretaceous was roughly 1000 ppm. But in our model-based experiments, it took concentration levels of 1120 to 1680 ppm to reach the average temperatures back then in the Antarctic,” says co-author and AWI climate modeller Prof Gerrit Lohmann.
Accordingly, the study shows both the enormous potency of the greenhouse gas carbon dioxide, and how essential the cooling effects of today’s ice sheets are. “We now know that there could easily be four straight months without sunlight in the Cretaceous. But because the carbon dioxide concentration was so high, the climate around the South Pole was nevertheless temperate, without ice masses,” explains co-author Dr Torsten Bickert, a geoscientist at the University of Bremen’s MARUM research centre.
The big question now is: if it became so warm in the Antarctic back then, what caused the climate to subsequently cool so dramatically to form ice sheets again? “Our climate simulations haven’t yet provided a satisfactory answer,” says Gerrit Lohmann. Finding the causes of these tipping points is now a key challenge for the international climate research community.