When Lago Cardiel Was Larger
Lago Cardiel is much larger than I expected; we can barely see the opposite shore. This side is ringed by low hills and opens up like an apron, broad and gently sloping. As soon as we come through the hills, we notice well-defined former shorelines, which look like soap residue rings around a bathtub.
Shorelines make great natural roads, just as glacial outwash fields are apparently well-suited for airport runways because they are flat and drain well. JFK is built on a glacial outwash plain, Mike tells me.
We drive along until we spot an outcropping of basalt rock and carbonate tufa. Littered over this surface are the tiny white shells of dead snails which once lived near the shore. Their presence indicates that the lake shoreline must have been this high at some point. The shells must have been left when the lake receded.
An old snail shell that was embedded in the old shoreline, in a layer of dark beach cobbles. Above the shell is the newer shoreline. Credit: Ale Borunda.
The lake has grown and shrunk many times over the past 20,000 years. When it grows, it leaves behind a “bathtub ring” of residue around its edge: pebbles smoothed by wave action, carbonate tufas, the remnants of the things that lived at the shore. There are at least three distinct bathtub rings (each of which represents a time when the lake surface was higher than it is today) that we can see in a Google Maps printout we’ve been carrying around. But Scott Stine, a researcher who did his Ph.D thesis on Lago Cardiel, has identified other shorelines beyond the three we can see now.
To date these shorelines, Stine used the carbon-14 dating method on organic material linked to various shorelines to determine their age. His data suggested that the highest shorelines occurred about 10,000 years ago, at a time when the rest of South America was hot and dry. That would mean that this lake got really, really huge, mainly because of rainfall. Jay and Mike are unsure this happened, and think that instead the high shorelines may be older than everyone thinks. Maybe the lake was big about 20,000 years ago—the peak of the last ice age? Dating the shells and tufas again may resolve this question. So we’re looking for the highest shorelines, which is trickier than it might seem. We also have to find good material to date. That’s why Jay nearly jumped up and down with glee when we started finding shells. They’re great for doing carbon-14 analysis since we know the fossils once lived in the former lake. Jay and Mike call out questions as they search. Maybe this is the 55 meter shoreline? Are there other calcifying organisms that lived in the lake?