克勞福德湖被選為辨識人類世起始時間的主要標記

 原文網址：https://www.southampton.ac.uk/news/2023/07/crawford-lake-anthropocene.page

一組國際研究團隊選擇該地最能代表「人類世」(Anthropocene)——一個地質新時代的開始。

克勞福德湖。圖片來源：Sarah Roberts

人類世工作小組推舉加拿大的克勞福德湖為人類世的全球界線層型剖面和點位(Global Boundary Stratotype Section and Point, GSSP)。GSSP是科學家在經年累月堆積起來的岩層當中，找到一個國際公認的參考點，做為一個新的地質年代——紀或世的開始。

有些地質學家提議我們現在所處的時代為「人類世」，這個新的地質世代表人類活動已經成為影響地球氣候與環境的主要因子之一。

此概念對於我們如何看待自身對地球的衝擊有重大意涵。但是學界在人類世的議題上有所分歧，包括人類世的起始時間、有什麼證據，以及人類的影響是否深遠到足以構成一個新的地質年代——其時間跨度通常為數百萬年。

為了解答這些問題，國際地層委員會(ICS)成立了人類世工作小組。

「克勞福德湖底下的沉積物對於過去數千年提供了絕佳的紀錄，顯示出發生在近代的環境變遷，」人類世工作小組的秘書，倫敦大學學院的Simon Turner博士表示。「湖水化學與生態的季節性變化形成了年層，經過採樣之後可以從中得出人類的歷史活動所造成的多種標記。克勞福德湖這類地點會如此重要，是因為在此形成的地質文獻可以精準地把資訊儲存下來，而對應到歷史上的全球環境變遷事件。由於世界各地其他地方看到類似的環境變遷事件時都要和GSSP互相比對，因此這些標準地點的紀錄必須穩固而且能重複得到就顯得相當重要。」

從珊瑚礁到冰原，團隊前往世界各地在不同類型的環境蒐集一段又一段的岩芯樣品。其中一部份再送往英國南安普敦國家海洋中心之內，屬於南安普敦大學的GAU放射性分析實驗室進行分析。研究人員在此處理樣品之後，偵測其中是否有人類對環境造成影響的關鍵印記：鈽。

南安普敦大學環境放射化學的主席Andrew Cundy教授是人類世工作小組的成員，他解釋：「鈽的出現是一道鮮明的指標，向我們指出人類什麼時候成為一種主要的營力，而在地球各個角落留下獨一無二的『指紋』。」

「鈽在自然界的量相當稀少。但在1950年代早期第一顆氫彈試爆之後，我們便能在世界各地的岩芯樣品中看到鈽含量以前所未有的速度增加並達到高峰。接著在1960年代中期禁止核試驗條約生效之後，我們又可以看到鈽含量逐漸降低。」

其他人類活動的地質標記包括了燃煤電廠產生的大量飛灰、高濃度的重金屬(如：鉛)、出現塑膠纖維與碎片……等。這些標記都對應了「大加速」(The Great Acceleration)——意指20世紀中期開始，從交通運輸到能源使用等人類活動的許多面向都有了突飛猛進的成長，直到今日仍方興未艾。

研究人員在分析數百個樣品之後提名克勞福德湖的岩芯為GSSP，以及把人類衝擊以高解析度呈現出來的類似紀錄為輔助地點。從這些地點得出的證據最近會呈交給國際地層委員會，他們明年將決定是否要把人類世正式命名為一個新的地質世。

 

Crawford Lake chosen as the primary marker to identify the start of the Anthropocene epoch

An international team of researchers has chosen the location which best represents the beginnings of what could be a new geological epoch, the Anthropocene.

The Anthropocene Working Group have put forward Crawford Lake, in Canada, as a Global Boundary Stratotype Section and Point (GSSP) for the Anthropocene. A GSSP is an internationally agreed-upon reference point to show the start of a new geological period or epoch in layers of rock that have built up through the ages.

It’s been proposed by some geologists that we are now living in the Anthropocene – a new geological epoch in which human activity has become the dominant influence on the world’s climate and environment.

The concept has significant implications for how we consider our impact on the planet. But there is disagreement in the scientific community about when the Anthropocene began, how it is evidenced and whether human influence has been substantial enough to constitute a new geological age, which usually span millions of years.

To help answer these questions, the International Commission on Stratigraphy (ICS) set up the Anthropocene Working Group.

“The sediments found at the bottom of Crawford Lake provide an exquisite record of recent environmental change over the last millennia,” says Dr Simon Turner, Secretary of the Anthropocene Working Group from UCL. “Seasonal changes in water chemistry and ecology have created annual layers that can be sampled for multiple markers of historical human activity. It is this ability to precisely record and store this information as a geological archive that can be matched to historical global environmental changes which make sites such as Crawford Lake so important. A GSSP is used to correlate similar environmental changes seen in other sites worldwide, so it is critical to have a robust and reproducible record at this type locality.”

The team has gathered core sample sections from a variety of environments around the world, from coral reefs to ice sheets. Samples from a range of these sites were then sent for analysis to the University of Southampton’s GAU-Radioanalytical labs at the National Oceanography Centre in Southampton. Researchers there processed the samples to detect a key marker of human influence on the environment – the presence of plutonium.

Professor Andrew Cundy, Chair in Environmental Radiochemistry at the University of Southampton and member of the Anthropocene Working Group, explains: “The presence of plutonium gives us a stark indicator of when humanity became such a dominant force that it could leave a unique global ‘fingerprint’ on our planet.

“In nature, plutonium is only present in trace amounts. But in the early-1950s, when the first hydrogen bomb tests took place, we see an unprecedented increase and then spike in the levels of plutonium in core samples from around the world. We then see a decline in plutonium from the mid-1960s onwards when the Nuclear Test-Ban Treaty came into effect.”

Other geological indicators of human activity include high levels of ash from coal-fired power stations, high concentrations of heavy metals, such as lead, and the presence of plastic fibres and fragments. These coincide with ‘The Great Acceleration’ - a dramatic surge across a range of human activity, from transportation to energy use, starting in the mid-20th century and continuing today.

From the hundreds of samples analysed, the core from Crawford Lake has been proposed as the GSSP, along with secondary supporting sites that show similar high-resolution records of human impact. Evidence from the sites will now be presented to the ICS, which will decide next year whether to ratify the Anthropocene as a new geological epoch.