植物葉片上變成化石的蠟質可以成為瞭解古代氣候的新工具

原文網址：www.sciencedaily.com/releases/2018/03/180302090955.htm

植物葉片上變成化石的蠟質可以成為瞭解古代氣候的新工具

一篇發表在《科學報告》(Scientific Reports)的新研究描述了一種估算古代大氣水含量的新方法：利用植物葉片上變成化石的蠟質。

當地表和大氣變得更加溫暖，大氣中的濕氣――也就是水蒸氣的含量也會增加。由於水蒸氣是一種相當大量的溫室氣體，因此瞭解其增幅對於預測未來的氣候來說是很重要的。此外，大氣中的水氣也會影響降雨事件的模式和強度。

藉由研究地球歷史上氣候比現在還溫暖許多的時期，可以探討溫度和水氣含量之間的關係，為此需要一種方法來估算古代大氣的水氣含量。

伯明罕大學的Yvette Eley博士解釋：「如果我們想要瞭解在氣候比現今炎熱許多的情況下地球的運作方式，就得研究數百萬年之前的時期。困難之處在於這些氣候相當溫暖的時期，遠比我們從南極冰芯得到的最古老氣候紀錄(不到100萬年)還要久遠許多。」

想知道如此久遠以前跟大氣有關的氣候條件，譬如雨量和大氣含水量是相當具有挑戰性的。現在使用的方法包括利用在土壤中形成的碳酸鈣結核，或者是哺乳類牙齒化石的化學成分。但這兩種方法的共通難處是它們所用的材料在古代沉積物中都相對稀有。

Eley博士補充：「我們運用新方法來定量古代大氣的水氣含量。這個方法是依據植物葉片的基本性質，以及它們用來保護葉片的蠟質層如何因應環境用水壓力而改變。葉片的蠟質非常堅韌且不易分解，因此常常能在古代河流、湖泊甚至是海洋沉積物中以生物標記化合物(biomarker compound)的形式發現它們。」

運用這類植物蠟質化合物來估算古代水氣含量的方法可以克服其他方法中的限制條件，因為經常發現植物蠟質的沉積物和土壤的年代可以一路追溯至數千萬年前甚至是數億年前，且它們的形成環境也相當多樣。

康乃迪克大學綜合地質科學中心的副教授Michael Hren的研究團隊，在數篇研究中證實這種新工具可以適用於現今美國各地和中美洲的土壤。研究顯示葉片蠟質化合物的化學性質和大氣中的水含量有明確的對應關係。

Hren博士表示：「氣團中的實際水量和氣團最多能容納的水量之間的差距稱為蒸氣壓差(vapour pressure deficit)。我們發現土壤中有機物的分布情形和此參數似乎有強烈的關聯。」

Eley和Hren運用這項新的指標來重建西班牙中部1500萬至1700萬年前的空氣含水量。

雖然這段期間的氣候條件比工業革命前的還要溫暖，但它其實是通往現代氣候發展過程中的其中一個冷化階段。新的數據證實了氣候模型的預測：大氣冷化和大氣裡的水氣減少會同步發生。跟其他用來探討此區溫度和降雨的獨立指標所得到的結果相比，大氣水氣含量的重建結果也與它們一致。

Eley博士表示：「研究結果給予我們信心這項指標確實可以運作，而我們有充分的理由相信在之後探討更久遠的過去時，同樣可以運用這項指標。我們希望未來的研究成果可以給我們直接了當的數據，來驗證我們對於全球暖化、大氣水含量和降雨系統之間的關係是否理解正確。」

Fossilised plant leaf wax provides new tool for understanding ancient climates

New research, published in Scientific Reports, has outlined a new methodology for estimating ancient atmospheric water content based on fossil plant leaf waxes.

As the Earth's surface and atmosphere warm, the amount of moisture -- water vapour -- in the atmosphere will increase. Understanding the size of this increase is important for predicting future climates as water vapour is a significant greenhouse gas. Atmospheric moisture content also influences the patterns and intensity of rainfall events.

The relationship between temperature and moisture content can be explored by the study of intervals in Earth's history when climates where significantly warmer than those seen in modern times, which necessitates a method for estimating ancient atmospheric moisture content.

Dr Yvette Eley, from the University of Birmingham, explained, "If we want to understand how the Earth would work with a climate substantially warmer than today, we have to study intervals millions of years in the past -- made difficult because these warm climates are much older than our oldest climate records from Antarctic ice cores (less than one million years old)."

To try and understand climate properties related to the atmosphere -- like rainfall and atmospheric moisture content -- in such ancient times is very challenging. Existing methods, using calcium carbonate concretions that form in soils, or the chemistry of fossilised mammal teeth, are both hampered by their relative rarity in ancient sediments.

Dr Eley added, "Our new approach to quantifying ancient atmospheric moisture content relies on the fundamental properties of plant leaves, and how they alter their protective waxy coverings in response to water stress. These leaf waxes are tough and resistant, and are regularly found as what we call biomarker compounds in ancient river, lake and even marine sediments."

A method of estimating ancient moisture content based on these plant wax compounds overcomes the limitations of other methods because plant waxes are commonly found in soils and sediments stretching back tens or even hundreds of millions of years and across many environments.

The validity of this new tool was proven in studies of modern soils across the US and Central America, carried out by the research team of Associate Professor Michael Hren in the Center for Integrative Geosciences at the University of Connecticut. These studies showed a clear relationship between the chemistry of these waxy compounds and the amount of moisture in the atmosphere.

"What we see is that the distribution of organic compounds preserved in soils seems to be strongly related to the difference between how much water is in an air mass, and how much the air mass can hold, or what is known as the vapour pressure deficit," says Dr Hren.

Eley and Hren then applied their new proxy to reconstruct atmospheric moisture content in Central Spain during an interval 15 to 17 million years ago.

Although consistently much warmer than pre-industrial conditions, this interval marks one of the cooling steps that led to the development of the modern world. The new data confirms the expectations of climate models, that atmospheric cooling is coupled to less atmospheric moisture. The reconstructed changes in atmospheric moisture also align with results from other independent proxies used to investigate changes in temperature and rainfall in the region.

Dr Eley said, "This gives us the confidence that our proxy works, and we have every reason to believe that it will do so for future exploration into the even deeper past. We hope the results of this exploration will provide direct data to test our understanding of the relationship between global warming, atmospheric moisture content and rainfall systems."

原始論文：Yvette L. Eley, Michael T. Hren. Reconstructing vapor pressure deficit from leaf wax lipid molecular distributions. Scientific Reports, 2018; 8 (1) DOI: 10.1038/s41598-018-21959-w

引用自：University of Birmingham. "Fossilised plant leaf wax provides new tool for understanding ancient climates." ScienceDaily. ScienceDaily, 2 March 2018.