「人類世」(Anthropocene)：新地質紀元的候選者

原始網址：www.sciencedaily.com/releases/2016/08/160829094255.htm

「人類世」(Anthropocene)：新地質紀元的候選者

英國萊斯特大學地質學家參與在內的人類世工作小組(AWG)，將會於舉辦在8月27日至9月4日，地點為南非的第35屆國際地質大會上，提出人類世有資格作為新地質時期的證據總和以及他們的初步建議。

這個國際科學組織(包括萊斯特大學的地質學家Jan Zalasiewicz, Mark Williams、英國地質調查局的地質學家暨榮譽主席 Mark Williams、 人類學家Matt Edgeworth )自2009年起便開始著手分析人類世是否有資格能正式成為新的地質年代，此時代由人類對整個地球造成的影響所主導。AWG將會於開普敦舉行的國際地質大會中發表他們的初步發現以及建議。同時也會表明小組內部對圍繞人類世的各種重大問題進行投票後，不同意見的得票範圍。另外，他們也會規畫出使人類世正名的正式提案會依循何種途徑達成，並提出要達到這項目標還需完成的工作項目。

小組內部目前的主流意見如下所述：

• 如同 Paul Crutzen和Eugene Stoermer於2000年大力主張的，人類世這個概念在地質上已經成為事實。此現象的規模已經達到足以成為國際地層年代表(International Chronostratigraphic Chart)，或較為人所知的地質年代表(Geological Time Scale)中的一部分。
• AWG的主流意見提議將其位階定位在世/統。跟較低的位階(譬如期/階，也就是將其劃分成全新世的一部分)，以及像是紀或代這類更高的位階相比，世都比較受到偏好。在此層面上，跟其他所有地質時代表中的單位一樣，「人類世」(Anthropocene Epoch)代表一個純粹的時間單位，於此時形成的地層則對應至「人類統」(Anthropocene Series)這個地層單位。
•                若人類世處在這個位階則意味著全新世已然終結，但我們還是將人類世保留在第四紀與新生代之                               內。
• 人類造成的影響在地層紀錄中留下可供辨別的痕跡已經有數萬年的歷史，確切來說，甚至早在全新世開始之前。然而，要到20世紀中葉的「劇烈加速度」(Great Acceleration)時期，人類對地球系統造成重大且幾乎全球同步的改變才有明顯加劇的趨勢。刻印在近代沉積而成的地層當中最清楚且獨特的數種訊號也可以對應至20世紀中葉。
• 因此，20世紀中葉代表了最適切的人類世起始時間(同時也是人類統底部的形成年代)。
• 地球系統在人類世發生的顯著變化包含了侵蝕和沉積速率有加劇的跡象；碳、氮、磷及其他元素的化學循環發生了大規模擾動；全球氣候和海平面高度開始產生顯著變化；以及生物方面的影響，像是物種入侵至全球各處的速率到達前所未見的程度。這些變化當中很多就地質角度來看會持續很長一段時間，有些效應甚至是不可逆的。
• 上述以及相關過程已經在近代地層當中留下了一連串可以偵測到的訊號，包括塑膠、金屬鋁和混凝土的微粒、人造放射性核種、碳及氮同位素分佈模式的改變、飛灰微粒以及各種可能會成為化石的生物遺骸。這些訊號多數會成為能夠永久保存在地球地層當中的紀錄。
• 可以想見人類世的起始時間能用一個「全球標準地層年代」( Global Standard Stratigraphic Age, GSSA)來定義，即1945年這種以數字來表示的日期格式。或者是用較為普遍的方法來定義，即「全球界線層型剖面和點位」(Global boundary Stratotype Section and Point, GSSP)，在口語上更常被稱作「金色之釘」(golden spike)，這個實際存在的參考點位於一嚴格選定地點的地層剖面之上。 AWG的主流意見為尋找並選定一個符合條件的GSSP，因為在定義地質年代單位時此為最慣用且廣為接受的作法。
• AWG已經開始進行初步分析來辨識可以作為GSSP所在地點的沉積環境。這些沉積環境通常可找到各種地層訊號一起良好保存的沉積物，比方說未受擾動的的湖底或海底沉積物、一年形成一層的珊瑚骨骼、極地雪層或冰層、石筍等等。
• 接著我們會選擇數個地點進行採樣及進一步的分析，以完整描述此地層中跟人類世相關的訊號。藉由這個過程，我們期望可以找到一個或更多有望成為GSSP的適合地點，希望在接下來的2到3年之內我們便能完成整個程序。
• 這些結果會成為我們籌備正式提案時所用的基本骨架。這份正式提案會被呈交給我們的直屬上級組織，第四紀地層小組委員會(Subcommission on Quaternary Stratigraphy, SQS)來決議人類統是否能正式成為新的地層單位。若SQS以絕對多數決通過此提案，則還要繼續呈交給SQS的直屬上級組織，國際地層學委員會(International Commission on Stratigraphy, ICS)來進行後續表決，而所有贊成票仍須經過國際地質科學聯合會(International Union of Geological Sciences, IUGS)的執行委員會批准。
• 如果這些條件皆成立，則人類世便會正式成為地質年代表中的一份子。

AWG的內部投票結果(35名成員)：

1. 人類世是否已經真實存在於地層紀錄當中？贊成：34。反對：0。棄權：1
2. 人類世是否應該正式成為新的地質年代？贊成：30。反對：3。棄權：2
3. 在地質年代表中的位階為？代：2。紀：1.5。世：20.5。亞世：1。期：2。亞期：0。空白：1。不確定：3。棄權：4
4. 人類世的起始年代為/人類統的底部形成於？約7000前：0。約3000年前：1.3。西元1610年的Orbis Spike^[1]：0。西元1800年左右：0。西元1950年左右：28.3。西元1964年左右：1.3。跨代(Diachronous)^[2]：4。不確定：0。棄權：0
5. 以GSSA還是GSSP來定義？GSSP：25.5。GSSA：1.5。不確定：8
6. 最主要的訊號為？金屬鋁：0。塑膠：3。煤灰微粒：2。二氧化碳濃度：3。甲烷濃度：0。碳同位素變化：2。氧同位素變化：0。核爆產生的放射性碳峰值：4。含鈽輻射塵：10。氮含量或氮同位素變化：0。生物地層證據(滅絕事件或化石組合改變)：0。其他(鉛、有機物永久汙染、人工製品化石(technofossil))：3。不確定：2。棄權：6

譯註：

[1]西元1610年的Orbis Spike：南極冰芯紀錄中的二氧化碳濃度有以西元1610年為中心的突然下降低峰值。16世紀時歐洲人前往新世界時帶來的疾病使美洲原住民大量死亡，造成原本用來農耕的大片土地重新長出森林或其他原始植被，它們吸收了大量二氧化碳而在冰芯中留下這道紀錄。研究人員以orbis，即拉丁文中的「世界」來稱呼這道峰值，因為其代表本來分隔在世上各地的人們開始產生了全球性的連結。

[2]跨代(Diachronous)：沉積學中，跨代意指一沉積岩層雖然其組成大致相同，但不同處形成的時間並不一致。海進海退或三角洲發育時形成的岩層常會有這樣的性質。對人類世來說，由於人類文明的進展在各地並不同步，像是用火、農耕以至於工業化，導致這些現象遺留在地層當中的痕跡於不同地點就象徵著不同年代。

'Anthropocene': Potential new geological time interval

The Working Group on the 'Anthropocene' (AWG), which includes University of Leicester geologists, will provide its summary of evidence and its provisional recommendations on a potential new geological time interval at the 35th International Geological Congress in South Africa between 27 August -- 4 September.

This international scientific body (that includes the University of Leicester geologists Jan Zalasiewicz, Mark Williams and honorary chair, the British Geological Survey geologist Colin Waters, and archaeologist Matt Edgeworth), has been active since 2009, analysing the case for formalization of the Anthropocene, a potential new epoch of geological time dominated by human impact on the Earth. The AWG is about to present its preliminary findings and recommendations at the International Geological Congress in Cape Town, at the same time indicating the range of voting opinion within the group on the major questions surrounding the Anthropocene. It will also map out a route towards a formal proposal on formalization, and indicate work that still needs be done to effect this.

Majority current opinion on the group indicates the following:

• The Anthropocene concept, as articulated by Paul Crutzen and Eugene Stoermer in 2000, is geologically real. The phenomenon is of sufficient scale to be considered as part of the International Chronostratigraphic Chart, more commonly known as the Geological Time Scale.

• Majority AWG opinion is for assignation as an Epoch/Series. This option is preferred over either a lower rank (e.g. Age/Stage, i.e. as a subdivision of the Holocene) or a higher rank such as a Period or Era. In such a step, and in common with all other geological time units, the Anthropocene would comprise both a 'pure time' unit (an Anthropocene Epoch) and an equivalent unit of strata (an Anthropocene Series).
•     If the Anthropocene is adopted as an Epoch, this would mean that the Holocene has terminated, but that we             remain within the Quaternary Period and Cenozoic Era.
• Human impact has left discernible traces on the stratigraphic record for thousands of years -- indeed, since before the beginning of the Holocene. However, substantial and approximately globally synchronous changes to the Earth System most clearly intensified in the 'Great Acceleration of the mid-20th century. The mid-20th century also coincides with the clearest and most distinctive array of signals imprinted upon recently deposited strata.
• Hence, the mid-20th century represents the optimal beginning of a potential Anthropocene Epoch (and, simultaneously, the base of the Anthropocene Series).
• Changes to the Earth System that characterize the potential Anthropocene Epoch include marked acceleration to rates of erosion and sedimentation, large-scale chemical perturbations to the cycles of carbon, nitrogen, phosphorus and other elements, the inception of significant change to global climate and sea level, and biotic changes such as unprecedented levels of species invasions across the Earth. Many of these changes are geologically long-lasting, and some are effectively irreversible.

• These and related processes have left an array of signals in recent strata, including plastic, aluminium and concrete particles, artificial radionuclides, changes to carbon and nitrogen isotope patterns, fly ash particles, and a variety of fossilizable biological remains. Many of these signals will leave a permanent record in the Earth's strata.
• The Anthropocene beginning might conceivably be defined by a Global Standard Stratigraphic Age (GSSA), i.e. a numerical age that can be expressed as a calendar date such as 1945. Or more, conventionally it could be defined by a Global boundary Stratotype Section and Point (GSSP), which is more colloquially a 'golden spike', and is a physical reference point in strata at one carefully selected place. Majority opinion on the AWG is to seek and choose a candidate GSSP, as this is the most familiar and widely accepted method of defining geological time units.

• The AWG has already begun the process of identification of potential GSSPs, by initial analysis of the general environments in which the best combinations of stratigraphic signals may be found (e.g. undisturbed lake or marine sediments, annually banded coral skeletons, polar snow/ice layers, speleothems and so on).

• This will lead to selection of sites for sampling and further analysis, to provide full descriptions of relevant signals in the strata, a process that we hope will lead to the identification of one or more suitable candidate sites for a GSSP. We would hope to complete this process over the next 2-3 years.

• This would then form the basis for the preparation of a formal proposal, to our immediate parent body, the Subcommission on Quaternary Stratigraphy (SQS), on defining a formal Anthropocene unit. If the SQS recommends this by supermajority vote, the proposal will go on to its parent body, the International Commission on Stratigraphy (ICS) to be voted on, with any vote in favour still needing to be ratified by the Executive Committee of the International Union of Geological Sciences (IUGS).
• If all of these conditions can be fulfilled, then the Anthropocene would become a formal part of the Geological Time Scale.

Results of AWG Vote (35 members):

1. Is the Anthropocene stratigraphically real? For: 34, Against: 0, Abstain: 1
2. Should the Anthropocene be formalised? For:30, Against: 3, Abstain: 2
3. Hierarchical level of the Anthropocene? Era: 2, Period: 1.5, Epoch: 20.5, Sub-epoch: 1, Age: 2, Sub-age: 0, None: 1, Uncertain: 3, Abstain: 4
4. Base/beginning of the Anthropocene? ~7ka: 0, ~3ka: 1.3, 1610 Orbis: 0, ~1800: 0, ~1950: 28.3, ~1964: 1.3, Diachronous: 4, Uncertain: 0, Abstain: 0
5. GSSA .v. GSSP? GSSP: 25.5, GSSA: 1.5, Uncertain: 8
6. What is the Primary Signal? aluminium: 0, plastic: 3, fuel ash particles: 2, carbon dioxide concentration: 3, methane concentration: 0, carbon isotope change: 2, oxygen isotope change: 0, radiocarbon bomb spike: 4, Plutonium fallout: 10, Nitrate concentration / nitrogen isotope change: 0, Biostratigraphic: extinction/ assemblage change: 0, Other (lead, persistent organic pollutants, technofossils): 3, Uncertain: 2, Abstain: 6.

引用自：University of Leicester. "'Anthropocene': Potential new geological time interval." ScienceDaily. ScienceDaily, 29 August 2016.