A bunch of articles in today's issue of Nature use precise measurements of the composition of glacial air bubbles to extend the record of atmospheric gases (and airborne dust) back to 800,000 years before present. (Previously, the record "only" went back to 650,000 years before present.) Fully eight glacial cycles are seen in the new, expanded dataset. These new findings are all part of the European Project for Ice Coring in Antarctica (EPICA), and they offer some new insights, as well as additional confirmation of the close link between climate and past fluctuations in CO2 and CH4. Check it out.

Labels: climate change, CO2, global warming, ice

A new study in Geophysical Research Letters uses C-14 to date the shrinkage of the ice cap on Baffin Island, in Canadian Nunavut. Baffin Island is the fifth largest island in the world, located just west of Greenland.

As the (non-flowing) ice cap withers, it exposes vegetation which has been buried beneath the ice since ancient times. This organic matter can be dated using the relative proportion of isotopes of radioactive carbon-14 and its daughter product, stable nitrogen-14. The oldest date found so far is apparently 350 AD.

The researchers, mostly from the University of Colorado at Boulder, also used measurements of cosmogenic ("made by space") nuclides in the rocks on which the ice cap sat to figure out how long they had been uncovered by the ice. I'm not an expert on cosmogenic nuclide exposure dating, but it works something like a sun tan: how long have the rocks been exposed to the barrage of radiation from the sun? If they've been exposed for a long time, they build up a substantial amount of these "cosmogenic nuclides" that wouldn't be found in an unexposed sample of the same rock. In my local area of mid-Atlantic North America, a study by Paul Bierman, et al. (2004) used cosmogenic berylium-10 to date bedrock terrace levels along Mather Gorge, thereby revealing the incision history of the Potomac River.

However, this is the first time I've heard of carbon-14 used as a cosmogenic nuclide. The authors offer this justification: "In situ cosmogenic radionuclide inventories in rock surfaces provide an integrated record of periods of ice-cover and exposure at a specific site since the end of the last ice age. We utilize in situ cosmogenic 14C due to its short half-life. In situ 14C production is reduced by 85% under 6 m of ice and is completely attenuated under 35 m of ice. Any 14C that had accumulated in rocks prior to the last glaciation would have decayed below our background after 25 ka beneath the Laurentide Ice Sheet." Is this coming from nitrogen in the rocks, the same way carbon-14 is generated in the atmosphere? Or is some other element/isotope serving as the source material which then gets changed upon exposure to the sun? Enlighten me if you know! It builds up specifically in quartz, if that helps at all.

Anyhow, they've found that half the ice cap has melted in the past 50 years. Half. Yep.

References:

Anderson, R. K., G. H. Miller, J. P. Briner, N. A. Lifton, and S. B. DeVogel (2008), A millennial perspective on Arctic warming from 14C in quartz and plants emerging from beneath ice caps, Geophys. Res. Lett., 35, L01502, doi:10.1029/2007GL032057.

Bierman, P., E. Zen, M. Pavich, and L. Reusser (2004). The Incision History of a Passive Margin River, the Potomac near Great Falls. USGS Circular 1264: Geology of the National Capital Region—Field Trip Guidebook, Trip #6.

University of Colorado at Boulder press release on the study.

Labels: canada, global warming, ice

Casey went to Michigan last weekend to hang out with her sisters, and one thing they did was go snowshoeing/sledding at the YMCA camp where they all grew up. When they walked out to the lake, an interesting sight awaited them: a scum of ice balls floating in the water, and (presumably pushed by the wind), collected up along the shore. She brought back these three photos.
It's a neat phenomenon, and I'm not really too sure what to make of it. The question is: are these concentrically zoned features, like oolites, hailstones, or the hematite concretions seen in Friday's post? Or are they fragments of lake ice that got broken up and then rounded due to abrasion and jostling against one another in the waves? I'm guessing that the first hypothesis is correct, but unfortunately I don't have any way to test it from here in DC.

Some observations: the ice balls are pretty well sorted, pretty spherical, and well rounded. The ice balls are packed closer together closer to the shore. There also seems to be a size gradient from larger ice balls close to shore (right, in the image below) to smaller ice balls out into the open water of the lake (left, in the image below). If this isn't just a trick of the camera's perspective, could this correspond to increased growth due to closer packing (and thus more time lifted up out of the water into the cold Michigan air) close to the shore?
Finally, I note that in some areas, multiple ice balls have clumped together into a larger entity, as some oolites will do. This cohesion between ice balls is potentially the first step for making a solid layer of ice that will extend out from the shore over the whole lake.

Anyone else ever seen anything like this? What's going on?

Labels: ice, michigan