Tuesday, March 27, 2012

Zunli Lu, Ikaite, Global Warming, Europe, Antarctica

Scientists use rare mineral to correlate past climate events in Europe, Antarctica

New study published in April issue of Earth and Planetary Science Letters

...To understand the present, scientists look for ways to unlock information about past climate hidden in the fossil record. A team of scientists led by Syracuse University geochemist Zunli Lu has found a new key in the form of ikaite, a rare mineral that forms in cold waters. Composed of calcium carbonate and water, ikaite crystals can be found off the coasts of Antarctica and Greenland.
"Ikaite is an icy version of limestone," say Lu, assistant professor of earth sciences in SU's College of Arts and Sciences. "The crystals are only stable under cold conditions and actually melt at room temperature."
It turns out the water that holds the crystal structure together (called the hydration water) traps information about temperatures present when the crystals formed. This finding by Lu's research team establishes, for the first time, ikaite as a reliable proxy for studying past climate conditions. The research was recently published online in the journal Earth and Planetary Science Letters and will appear in print on April 1. Lu conducted most of the experimental work for the study while a post-doctoral researcher at Oxford University. Data interpretation was done after he arrived at SU.
The scientists studied ikaite crystals from sediment cores drilled off the coast of Antarctica. The sediment layers were deposited over 2,000 years. The scientists were particularly interested in crystals found in layers deposited during the "Little Ice Age," approximately 300 to 500 years ago, and during the "Medieval Warm Period," approximately 500 to 1,000 years ago. Both climate events have been documented in Northern Europe, but studies have been inconclusive as to whether the conditions in Northern Europe extended to Antarctica.
Ikaite crystals incorporate ocean bottom water into their structure as they form. During cooling periods, when ice sheets are expanding, ocean bottom water accumulates heavy oxygen isotopes (oxygen 18). When glaciers melt, fresh water, enriched in light oxygen isotopes (oxygen 16), mixes with the bottom water. The scientists analyzed the ratio of the oxygen isotopes in the hydration water and in the calcium carbonate. They compared the results with climate conditions established in Northern Europe across a 2,000-year time frame. They found a direct correlation between the rise and fall of oxygen 18 in the crystals and the documented warming and cooling periods.
"We showed that the Northern European climate events influenced climate conditions in Antarctica," Lu says. "More importantly, we are extremely happy to figure out how to get a climate signal out of this peculiar mineral. A new proxy is always welcome when studying past climate changes."

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A team of scientists led by geochemist Zunli Lu from Syracuse University in New York state, has found that contrary to the ‘consensus’, the ‘Medieval Warm Period’ approximately 500 to 1,000 years ago wasn’t just confined to Europe.
In fact, it extended all the way down to Antarctica – which means that the Earth has already experience global warming without the aid of human CO2 emissions.
At present the Intergovernmental Panel on Climate Change (IPCC) argues that the Medieval Warm Period was confined to Europe – therefore that the warming we’re experiencing now is a man-made phenomenon.
However, Professor Lu has shown that this isn’t true – and the evidence lies with a rare mineral called ikaite, which forms in cold waters...
The research was recently published online in the journal Earth And Planetary Science Letters and will appear in print on April 1.

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Trace elements in carbonate and paleo-redox
Global warming will affect marine ecosystems in complex ways. The lower oxygen solubility at higher temperatures could lead to a decrease in dissolved oxygen (deoxygenation). This would severely impact ocean life... Forecasting ocean chemistry changes during perturbation of the climate system relies on the development of computer models and their calibration using proxy data on the state of the oceans. Development of geochemical proxies thus is key to enable us to use geological records to predict potential consequences of global warming. I'm interested in developing new and sensitive proxies for oxygenation, to investigate changes in oceanic redox conditions during abrupt warm events and oceanic anoxic events in the geological past. Currently, I'm focusing on iodine proxy in carbonate rocks and micro-fossils.

Early diagenesis and authigenic carbonate (ikaite)
Complex biogeochemical reactions take place within the buried marine sediments, altering compositions of both solid phase sediments and their ambient porewaters... I'm interested in ikaite crystals (CaCO3·6H2O), a metastable hydrated carbonate mineral, that can precipitate rapidly in shallow sediment columns at relatively low temperature. Ikaite releases the hydration water at higher temperature, but may remain its macro-structure (pseudomorph, called glendonite) in outcropped old sedimentary rocks. Ikaite and glendonite have been used as indicators of cold bottom waters. The hydration water of ikaite also records oxygen isotopic composition of bottom water (or porewater), and may provide interesting information about past climate changes.

Porewater modeling
As a low temperature geochemist, I find that models are very useful tools for interrogating analytical data. I investigated glacial meltwater events in Antarctic fjords during the Anthropocene, by adapting the pioneering approach of modeling trends in δ18O in the porewaters of deep-sea cores, previously used to constrain the size of ice sheets during the Last Glacial Maximum. I hope to apply this approach in fjords of different sedimentary settings to reconstruct the glacier history and allow insight into the sensitivity of polar glaciers to abrupt warming events...

Gas hydrate, carbon and fluids in subduction zones
Large amount of methane are produced in marine sediments during organic matter decomposition and stored in gas hydrates. Gas hydrates may be an alternative energy source but global warming may also cause the dissociation of gas hydrates and massive release of gas...