From the UNIVERSITY OF BIRMINGHAM and the “thinnest of evidence” department comes this claim.
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.”
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The paper: Reconstructing vapor pressure deficit from leaf wax lipid molecular distributions
Estimates of atmospheric moisture are critical for understanding the links and feedbacks between atmospheric CO2 and global climate. At present, there are few quantitative moisture proxies that are applicable to deep time. We present a new proxy for atmospheric moisture derived from modern climate and leaf biomarker data from North and Central America. Plants have a direct genetic pathway to regulate the production of lipids in response to osmotic stress, which is manifested in a change in the distribution of simple aliphatic lipids such as n-alkanes. The Average Chain Length (ACL) of these lipids is therefore statistically related to mean annual vapor pressure deficit (VPDav), enabling quantitative reconstruction of VPD from sedimentary n-alkanes. We apply this transfer function to the Armantes section of the Calatayud-Daroca Basin in Central Spain, that spans the Middle Miocene Climatic Optimum (MMCO) and the Middle Miocene Climate Transition (MMCT). Reconstructed VPDav rises from 0.13 to 0.92 kPa between 16.5 and 12.4 Ma, indicating a substantial drying through the MMCT. These data are consistent with fossil assemblages and mammalian stable isotope data, highlighting the utility of this new organic molecular tool for quantifying hydrologic variability over geologic timescales.
Open access here: https://www.nature.com/articles/s41598-018-21959-w
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Some commenters here are unreasonably dismissive of the paper. There is no logical reason why we shouldn’t explore whether the composition of these compounds in sediments could be an effective proxy for humidity, and on the evidence they provide there is at least a prima facie case made that it can.
Whether it will have much bearing on the present climate debate is entirely another matter of course, but I don’t see that being claimed by the paper anyway.
One thing that surprises me is the inherent assumption that the mix of chain lengths found in the wax residues is independent of the plant species- and indeed different phenotypes within the species – in which they were produced. Pretty much everything rests on this. It might be right, and perhaps some work has been done to support it, but to me that’s counter-intuitive.