by Staff Writers
Madison WI (SPX) Dec 20, 2016
Mother-of-pearl or nacre (pronounced nay-ker), the lustrous, tough-as-nails biomineral that lines some seashells, has been shown to be a faithful record of ancient ocean temperature.
Writing online Thursday, Dec. 15, in the journal Earth and Planetary Science Letters, a team led by University of Wisconsin-Madison physics Professor Pupa Gilbert describes studies of the physical attributes of nacre in modern and fossil shells showing that the biomineral provides an accurate record of temperature as the material is formed, layer upon layer, in a mollusk.
"We can very accurately correlate nacre tablet thickness with temperature," says Gilbert, explaining that mother-of-pearl is formed as mollusks lay down microscopic polygonal tablets of the mineral aragonite like brickwork to build layers of the shiny biomineral.
The work is important because it provides scientists with a new and potentially more accurate method of measuring ancient ocean temperatures, improving on methods now used with other biominerals to tease out the record of the environmental conditions at which the materials formed in the distant past.
"Everyone else measures temperatures in the ancient world using chemical proxies," says Gilbert, referencing methods that, for example, use ratios of isotopic oxygen locked into tiny fossil shells made by marine microorganisms known as Foraminifera to get a snapshot of ocean temperatures in the distant past.
The method devised by Gilbert and her collaborators is extraordinarily simple: using just a scanning electron microscope and a cross section of shell, it is possible to measure the thickness of the layered microscopic tablets that compose nacre in a shell. The thickness of the tablets, explains Gilbert, correlates with ocean temperature as measured in modern shells when ocean temperatures were known at the time the shells were formed.
The new work by the researchers from Wisconsin, Harvard, and the Lawrence Berkeley National Laboratory provides a novel physical approach to measuring past climate, says Gilbert, an expert in biomineral formation.
"If what you are measuring is a physical structure, you see it directly," says Gilbert. "You just measure nacre tablet thickness, the spacing of the lines, and it corresponds to temperature. When the temperature is warmer, the layers get thicker."
The new study looked at fossil samples of nacre as old as 200 million years from a mollusk in the family Pinnidae, large, fast-growing saltwater clams that live in shallow ocean environments. Today, as in the distant past, the bivalves are widespread in tropical and temperate coastal and shallow continental shelf environments.
The new method is potentially more accurate, Gilbert notes, because the chemistry of fossil shells can be altered by diagenesis. Diagenesis occurs over geologic time, during or after sediments rain down on ocean beds to form sedimentary rock. Fossil shells may partially dissolve and re-precipitate as calcite, which fills cracks in aragonite nacre, thus skewing the chemical analysis of a sample, if analyzed as a bulk sample.
"If the chemistry changes after the death of a fossil, the formation chemistry isn't necessarily preserved," says Gilbert. On the other hand, "if the physical structure is altered by diagenesis, you will notice immediately that nacre is no longer layered, and so you will know that it's not worth analyzing that area.
If just a few nacre tablets are preserved, their thickness can easily be measured" meaning the new technique can augment current geochemical methods used to assess past temperatures, and thus help reconstruct ancient climates, especially the shallow marine environments that preserve most of the world's invertebrate fossil record.
The family of mollusks in the new study has lived in the world's oceans for more than 400 million years, potentially leaving a clear record of ocean temperatures into the distant past. For purposes of assessing climate, the record is valuable because not only does it say something about past climate, but the data can also help modelers forecast future climate and environmental change.
"The only thing you can do to understand climate in the future is to look at climate in the past," Gilbert notes.
Rain out, research in
Rain out, research in
Drought tolerance in wheat experimental plants growing under rainout shelters. Image courtesy Surya Kant. For a larger version of this image please go here.
American Society of Agronomy
One way to grow crops in dry and drought-prone regions is to breed crop varieties that are better able to tolerate water stress. These crops can expand available arable land and increase food production.
Plant breeders and geneticists are continuously developing new crop varieties. But field-testing these new crop varieties to test whether they can actually tolerate water stress is challenging. Part of the challenge is that it is difficult to predict the timing and amount of rainfall, which can complicate experiments in the field.
So researchers turn to rainout shelters. These are structures designed to exclude rainfall from specific areas on agricultural fields. This allows experimentally-controlled water stress to be applied to the crops being grown in those areas.
In a new study, researchers from Agriculture Victoria in Australia describe a fully-automated, portable, and energy-independent rainout shelter. This new design will allow researchers to more effectively field test crop varieties for their tolerances to water stress.
"Developing tools to enable precise testing under natural field conditions is key for breeding water stress-tolerant crops," says Surya Kant, the lead author of the study. Field testing new crop varieties is vital. Experiments under more controlled conditions - such as in greenhouses - cannot always replicate the variable conditions found outdoors.
"There are always variations between field and greenhouse experiments," says Kant. "That is especially the case for drought tolerance research."
During field studies researchers often have to account for various soil types. In contrast, greenhouses often use premade potting mixes or a single kind of soil. There are also differences in plant density, competition with weeds, insects, pests and diseases. All of these differences add up and "results from greenhouse experiments can potentially be unreproducible in the field," says Kant.
The rainout shelters designed by Kant and his colleagues are built using steel arch frames and polyethylene covering. "This lightweight, robust design allows the structures to be portable," says Kant. "It also means that the shelters maintain durability in all weather conditions, especially high winds."
The rainout shelters are mounted on plastic road barriers. These barriers can be filled with water to act as foundations. They can be emptied for maximum portability when the shelters need to be moved.
"Most rainout shelters run on rails that are fixed to the ground, and are therefore non-portable," says Kant. "In contrast, our rainout shelters are portable. They can be relocated to another research station to allow for crop rotation and experimental site changes."
Availability of electric power supply is another issue for researchers using rainout shelters. This is especially the case when experimental sites are located in remote areas. Kant and his colleagues incorporated a portable solar power system in their design to ensure that all power is generated onsite. Independent power generation can minimize potential failures due to issues with existing power infrastructure.
The new rainout shelters also have other customized features, such as rain sensors and surveillance cameras. The rain sensors automatically deploy the shelters in the event of rain, such that no rainfall event is missed accidentally. The surveillance cameras allow researchers to monitor their experiments remotely.
The unique features on these rainout shelters can expand their use into more remote areas. This will potentially allow researchers to test crops for drought-tolerance and growth in previously unusable land.
Read more about these rainout shelters in Crop Science.
University of Wisconsin-Madison
Water News - Science, Technology and Politics
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