數千年前急劇發生的氣候變遷成因為何？新研究推翻了之前的假說

原文網址：www.sciencedaily.com/releases/2016/12/161219115304.htm

數千年前急劇發生的氣候變遷成因為何？新研究推翻了之前的假說

約莫12,900年前發生了一起急劇的氣候變遷事件，也促成了新仙女木期開始。以往認為這起事件的肇因為一或多顆彗星/火流星撞擊北美大陸，但發表於《第四紀科學期刊》的兩篇新研究卻否決了這門假說。

在新仙女木期(Younger Dryas)之前，氣候正從冰河期狀態逐漸暖化成跟今日相仿的溫度，之前覆蓋北美的廣大冰層也已經完全消融。然而，在大約12,900年前，氣溫卻突然驟降回冰河期的狀態，並持續了將近1200年。於此同時，北美洲的猛瑪象和乳齒象也邁入滅絕。

這兩篇論文挑戰了撞擊理論中，其他科學家曾發表並用來支持該理論的兩條線索。第一條線索是新仙女木期最初堆積的沉積物中，奈米尺寸的鑽石含量有增加現象。另一條線索則是從一處重要考古場址的古火焰(paleofire)證據中，得出新仙女木期開始時曾發生相當嚴重的野火事件。支持者主張第一條線索中的鑽石為撞擊事件的產物，第二條線索則代表此事件引發了遍及整個北美大陸的野火。

兩篇新論文皆指出用來支持這兩條線索的證據和解釋並站不住腳。「撞擊說的擁護者發表說他們找到了六方白碳石(lonsdaleite)，這種罕見的鑽石型態通常跟衝擊作用有關。然而，我們卻證實他們把石墨烯和石墨烷的多晶聚集體(polycrystalline aggregate)誤認成了六方白碳石。」其中一篇論文的第一作者Tyrone Daulton博士說。「此外，我們也證明了撞擊說擁護者發表的奈米鑽石含量測量結果具有重大缺陷。在新仙女木期開始時，並沒有證據可以指出奈米鑽石的含量有出現高峰，因此無法推論當時曾經發生過撞擊事件。」

第二篇論文的第一作者Andrew Scott教授表示：「新仙女木期是因撞擊事件而導致的想法，確實相當有趣也非常引人注目。但是過去數年以來，越來越多的方法論研究成果，卻無法支持這則故事。我們的研究顯示過往被認為代表了這起事件的記號，實際上有很多是辨識錯誤或判讀錯誤之下的結果。」

Studies refute hypothesis on what caused abrupt climate change thousands of years ago

Two new studies in the Journal of Quaternary Science refute the hypothesis that one or more comets/bolides struck North America approximately 12,900 years ago triggering rapid climate change and the start of the Younger Dryas period.

Prior to the Younger Dryas, the climate had gradually warmed from glacial conditions to near modern temperatures, and the massive ice sheets in North America were in full retreat; however, approximately 12,900 years ago, temperatures rapidly plummeted and returned to glacial conditions for about a 1200 year long period. Also about this time, the mammoths and mastodons became extinct in North America.

The two papers challenge two lines of evidence reported and used by others to support the impact theory. One is the report of elevated concentrations of nanometer-sized diamonds in sediments deposited at the onset of the Younger Dryas. It is claimed that these diamonds were formed during an impact. The other is the interpretation that paleofire evidence at a key archaeological site demonstrates massive wildfires at the beginning of the Younger Dryas. It is claimed that the impact caused wildfires that spanned the continent.

Each paper shows that the evidence and interpretations supporting these two lines of arguments do not stack up. "Impact proponents report the rare form of diamond, lonsdaleite, that is usually associated with shock processing; however, we show that they misidentified polycrystalline aggregates of graphene and graphane as lonsdaleite," said Dr. Tyrone Daulton, lead author of one of the papers. "Further, we show that the nanodiamond concentration measurements reported by impact proponents are critically flawed. There is no evidence for a spike in the nanodiamond concentration at the onset of the Younger Dryas to suggest that an impact event occurred."

Prof. Andrew Scott, lead author of the second paper said, "The idea of a Younger Dryas impact was an interesting one that has drawn much attention; however, increasingly methodological research over the past few years has failed to corroborate that story. Our research has shown that many of the markers for such an event have been misinterpreted or misidentified."

原始論文：1. Tyrone L. Daulton, Sachiko Amari, Andrew C. Scott, Mark Hardiman, Nicholas Pinter, R. Scott Anderson. Comprehensive analysis of nanodiamond evidence relating to the Younger Dryas Impact Hypothesis. Journal of Quaternary Science, 2016; DOI: 10.1002/jqs.2892

2. Andrew C. Scott, Mark Hardiman, Nicholas Pinter, R. Scott Anderson, Tyrone L. Daulton, Ana Ejarque, Paul Finch, Alice Carter-champion. Interpreting palaeofire evidence from fluvial sediments: a case study from Santa Rosa Island, California, with implications for the Younger Dryas Impact Hypothesis. Journal of Quaternary Science, 2016; DOI: 10.1002/jqs.2914

引用自：Wiley. "Studies refute hypothesis on what caused abrupt climate change thousands of years ago." ScienceDaily. ScienceDaily, 19 December 2016.