定義人類世

 原文網址：https://www.u-tokyo.ac.jp/focus/en/press/z0508_00233.html

海洋物質的微量放射性標示了現代的起點

透過海洋沉積物和珊瑚裡的微量放射性物質，研究人員首次能以高度量化的方法來定義「人類世」(Anthropocene)。就人類對地球環境和生態系的衝擊來說，「人類世」從實際層面上標記了其中的重大轉折，使得許多領域的研究人員對此都十分看重。海洋沉積物與珊瑚骨骼記錄了核試爆產生的放射性落塵，研究人員將兩者結合起來，顯示出世界各地進行核試爆的期間與前後，海洋環境出現了明顯的變化，他們將這段時期定義為人類世的起點。

珊瑚的年輪 這張珊瑚剖面的掃瞄影像顯示出它的生長輪。每道生長輪對應到不同年份，因此研究人員可以找出特定的大氣變化事件標記來探討其發生時間。 圖片來源：橫山祐典等人，2022

日本西南方的別府市有許多值得造訪的理由，它以歷史悠久的溫泉、風景秀麗的沙灘以及活力十足的傳統祭典而聞名。不過東京大學大氣與海洋研究所的教授横山祐典、別府灣計畫的主持人——愛媛大學的副教授加三千宣與同僚，最近一次前往別府市並不是為了沉浸當地的氛圍，而是探討當地海床某些令人感興趣的地方。跟所有人都愛好的水上活動聽起來不太一樣，他們前來尋找50多年前太平洋的核彈試爆所產生的證據——鏷，以及人類活動留下來的其他化學印記。

「別府灣的沉積物在安靜的海床上堆積成層，使其成為太平洋周圍把人為衝擊的紀錄良好保存的地點之一。比方說二十世紀中葉核試爆產生的含鏷落塵就在這裡保存了下來，」橫山教授表示。「我們的任務是尋找落塵的明確跡象，時間為1950年代到大部分試爆終止的1963年。我們從灣區採集了岩芯樣品，裡面有清楚的訊號顯示來自落塵的鏷。不只如此，我們從石垣島採集的珊瑚骨骼也含有落塵。將沉積物與珊瑚互相比對可以讓我們更精確的定年沉積物中看到的訊號。」

即使沉積物樣品可以記錄過往環境變遷留下的證據，但它們卻可能堆積得相當雜亂，而且很容易受到擾動。因此研究人員才需要和珊瑚樣品交互參照——就像樹木一樣，珊瑚的成長方式會在內部留下獨特的環狀構造，每一圈對應到一個生長年。這兩種沉積物都有各自的用途：沉積物傾向捕捉大範圍的環境資訊，但比較不精確；珊瑚靠著生長輪可以提供非常精確的年代，但是對於過往的環境變遷資訊相對較少。據此，研究人員在珊瑚骨骼找到1954年的落塵留下的確切證據，而同一顆珊瑚裡的其他標記也能在沉積物中發現，意謂在沉積物中看到的大範圍化學訊號可以推測是在同一年發生。

別府灣 沉積物樣品的採集地點——別府灣的空拍影像。圖片來源：橫山祐典等人，2022

橫山教授和同僚如此想找到核試爆落塵留下的地質與化學紀錄，因為這是重新定義何謂「現代」的提議中的一部份。我們所處的歷史時期廣義來說被稱為「全新世」，大約開始於12000年前。然而，自從工業革命以來人類對地球造成的空前影響已經證明足以留存於世，使得歷史、生物、大氣科學以及許多相關領域的研究人員，試著創造一個更加精確的定義來描述當今的後核武時代，也就是他們口中的「人類世」。

「要在我們的樣品中分析鏷是很困難的。在我們探討的這段時期，總共有三噸的鏷釋放到大氣與海洋，但它們的散播距離非常遠而且範圍很廣，因此我們尋找的訊號實際上微弱得難以置信，」橫山教授表示。「儘管如此，這項成果的重要之處不只是為人類世提供了更加穩固的定義；我們的研究方法可以成功運用，代表也能用來改良海洋和氣候模型，甚至可以探討過去的海嘯以及其他地質災害。」

 

Defining the Anthropocene

Radioactive traces in ocean materials mark the start of the modern age

For the first time, researchers can offer a strong quantitative definition for the start of what is known as the Anthropocene, thanks to traces of radioactive material in marine sediments and corals. The Anthropocene period is considered important by researchers in many fields as it effectively marks a milestone for humanity’s impact on Earth’s environment and ecosystem. Researchers combined records of nuclear fallout from atomic tests present in ocean sediments and coral skeletons. These records show a clear change in the ocean environment before, during and after a period of worldwide atomic testing, which the researchers have defined as marking the beginning of the Anthropocene.

There are many reasons to visit the resort city of Beppu in southwest Japan, famous for its historical hot springs, beautiful beaches and vibrant traditional festivals. However, during a recent trip there, Professor Yusuke Yokoyama from the Atmosphere and Ocean Research Institute at the University of Tokyo, Michinobu Kuwae, associate professor at Ehime University and leader of the Beppu Bay project, and their colleagues were visiting not to soak up the local ambience, but to investigate some interesting properties of the local seafloor. It might not sound like everyone’s preferred beach activity, but they were looking for evidence of plutonium from nuclear bomb tests in the Pacific Ocean that took place over 50 years ago, and other chemical imprints left by humanity.

“Beppu Bay is one of several areas around the Pacific that contain well-preserved records of humanity’s impact on the environment because of sediments that form layers on the quiet seafloor. For example, plutonium fallout from nuclear testing during the mid-20th century is preserved,” said Yokoyama. “Our task was to find clear indications of fallout from the 1950s up to 1963 when testing largely stopped. We took core samples from the bay area, and there are clear signals of the plutonium from fallouts. However, we also collected coral skeletons from the island of Ishigaki, southwest of Okinawa, which contained fallout. Comparing sediments to corals allows us to more accurately date the signatures we see in the sediments.”

Even though sediment samples can record evidence of past environmental changes, they are deposited so haphazardly and can be disrupted easily. This is why the team needed to cross-reference the core samples with coral, as coral, like trees, grow in such a way that their internal structure leaves distinct rings corresponding to each year of growth. The reason both kinds of records are useful is because sediments tend to capture a broad range of environmental information but lack precision, and coral, thanks to their growth rings, can offer great historical precision, but don’t capture as much information about past environmental change. Thanks to this, the team found clear evidence in coral skeletons of fallout from 1954; other markers in that coral also found in the sediments meant that the wider range of chemical signatures in the sediments could be linked to that same year.

The reason Yokoyama and colleagues are keen to find geological and chemical records of nuclear fallout is because they are part of an initiative to redefine how we refer to the modern age. Our period of history is broadly known as the Holocene, which extends back around 12,000 years. However, humanity’s unprecedented impact on the Earth since the industrial era began has proved so monumental that researchers in many fields relating to history, biology, atmospheric science and more, seek to create a new precise definition for this modern post-nuclear era, which they are calling the Anthropocene.

“It was challenging to analyze plutonium within our samples, as during the period in question, 3 tons of plutonium were released into the sea and atmosphere, but those 3 tons dispersed far and wide. So we’re actually looking for incredibly small signatures,” said Yokoyama. “Nevertheless, this work is important not just to solidify the definition of the Anthropocene, but also because the successful use of our method means it could also be used to improve ocean and climate models, or even help explore past tsunamis and other geological hazards.”

原始論文：Yokoyama, Y., Tims, S., Froehlich, M., Hirabayashi, S., Aze, T., Fifield, L.K., Koll, D., Miyairi, Y., Pavetich, S., & Kuwae, M. Plutonium isotopes in the North Western Pacific sediments coupled with radiocarbon in corals recording precise timing of the Anthropocene. Scientifc Reports, 2022 DOI: 10.1038/s41598-022-14179-w

引用自：The University Tokyo. “Defining the Anthropocene.”