Friday, February 12, 2010

Can Someone Clarify?

There's an interesting but somewhat confusing article in Der Spiegel today on new findings from speleothems in the caves of Mallorca. I had to go to Google maps to clarify the island's location; it's a little farther to the northeast than I remembered.

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Ah-ha! I had never noticed the "link" button in Google Maps before. Behold my first embedded map! The central finding is that sea levels through time can be measured by carefully examining the variations in composition of calcium carbonate caves deposits. So far so good. The geologists' work indicates that sea level was approximately one meter higher than today's level 81,000 years ago- at a time when the growth of glaciers should have been drawing sea levels down. Unexpected, perhaps even startling, but OK. Next, they claim that "The seas around Mallorco rose perhaps by as much as two meters in 100 years, according to Dorale's team." Now that is definitely startling, and raises lots of questions. But the thing that is really dissonant to me is this paragraph:
Mallorca is a good place to study these changes because the island barely moves, the scientists say. It's tectonically stable, and the buildup or melting of glaciers hasn't raised or lowered the island. The stalagtites and stalagmites, moreover, have have collected deposits of calcite from the ocean, and these deposits give up secrets like rings in a tree. Dorale's team dated the deposits by measuring the radioactive decay of uranium traces. "We've reconstructed sea levels with a high degree of precision," Dorale told SPIEGEL ONLINE.
Now I'm not really knowledgeable about the region, but I had been under the impression that most of the Mediterranean-European coast was considered seismically active. Is the eastern portion really considered stable enough to discount tectonic and isostatic changes in the land level compared to sea level? Over 81,000 years, even a small uplift averaging a hundredth of a millimeter a year would amount to 0.8 meters, most of the change identified. However, people don't get their work published in Science while overlooking such obvious concerns, so I do suspect I'm the one misinformed here.

Whatever the case, there are some interesting climatic implications to this article. One question raised at the outset is "What if glaciers melt faster than anyone has suspected?" That particular question has been getting a lot of attention in scientifically literate circles over the last couple of years. The arctic melt back of 2007 and recent work in Greenland and Antarctica tentatively suggest that glaciers can disappear faster than had been believed, so a better question might be, "How can or should we respond if glaciers melt faster than anyone has suspected?"

The accompanying photogallery only has four pictures, one of which is a generic glacier, but the three taken inside the caves, including the lead picture above, are lovely. The colors in the picture I linked blow me away.


Darius Whiteplume said...

I'd wonder (not being well studied here) if the Mediterranean had been sealed off by glacier action at the Strait of Gibraltar, allowing it to hold more water than when it has an outlet to the Atlantic.

As for seismic activity, I'd have to ask someone from Pompeii. ;-)

David Bressan said...

Dorale et al. 2010 wrote:

"Although we cannot be certain that peak MIS 5e sea level has been captured by our sampled encrustations at 2.6 m, the observation nonetheless suggests that tectonic motion is not a major confounding factor in our reconstruction of MIS 5a sea level, because it is implausible that MIS 5a deposits could be significantly elevated by tectonics while MIS 5e deposits were not. Our results at face value therefore indicate that between ~82 and 80 ka, sea level in Mallorca stood at ~ +1 m relative to present sea level (Table 1 and Fig. 2A)."

Maybe it´s reasonable to argue that if they have an up and down pattern like O16/O18 it shouldn´t be slow and constant tectonic, and neither earthquakes etc.

It will be interesting to correlate this new findings with italian speleotherms.

Lockwood said...

Darius- Glaciers didn't reach that far south, and while I don't know the depth off the top of my head, the strait is pretty deep. You would need a LOT of ice to block it.

Dave- Thanks! So you are implying that they ruled out gradual tectonism, but not a major single event? Vertical offset in the 1-2 m range seems plausible to me, but again, I don't know much about the tectonics of the region.