Why Lake Erie is special

Lake Erie is by far the shallowest Great Lake: only 15 meters deep in the western basin, 20 meters in the central basin, and 64 in the eastern basin, as compared to the others, which are hundreds of meters deep. This means that Lake Erie is much more sensitive to changes -- like the littlest sibling who over-reacts to everything.

I think Lake Erie's shallow basins and resulting sensitivity are the reason that its sedimentary record (ie, mud) is different than those from other lakes in the region. We see whiting deposits (lots of fine-grained calcite) in Lake Erie and not Ontario, Huron, or the Finger Lakes because climate changes that happened in the region had a much stronger impact on Lake Erie than the others. Certain conditions are necessary for calcite precipitation (also called "whiting events" or "whitings"), and Erie was simply pushed over the edge, while the other lakes were more stable.

So where is this edge? How do we define it? And what pushed Lake Erie over this threshhold, inducing calcite precipitation? There are two possible interpretations.

CLIMATE
Warmer temperatures (jet stream shift?)increased water temperature, thus increasing CaCO3 saturation (ie, the point where water just can't hold any more dissolved ions).

LAKE LEVELS
The outlet sill eroded, lowering lake level, which increased Ca++ weathering from the shoreline, which increased CaCO3 saturation.

I like the climate explanation better, because I think temperature is the primary control on whitings, and lake level drops can't account for the degree of warming that is indicated by the other sediment properties. I also like the warm climate explanation because it is consistent with the increase in primary productivity (ie, more little green floaty organisms living, photosynthesizing, and dying) indicated by our stable isotopes. And finally (the icing on the cake), there have been several studies linking cyanobacteria abundance and whitings -- more cyanobacteria means more whitings (as described in previous entry). Altogether this seems to be a stronger interpretation, with lots of proxy data fitting together like puzzle pieces.

Of course I could be wrong. Before I officially root for this interpretation in my final paper (and in the article that we will hopefully publish!) I'm going to try to see if it's possible to quantitatively analyze how much a temperature change would have impacted cyanobacteria, and calcite solubility. In science, a self-consistent interpretation is good, but a self-consistent interpretation with numbers backing it up is better ...

Regardless of what caused these whiting events, the fact remains: Lake Erie contains an extensive carbonate record that is absent in all the other lakes in the region. Since carbonate materials are extremely valuable for reconstructing past climates, I think these sediments will be key to understanding environmental changes in the Great Lakes/New York area.

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Comments

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Posted by: Stephen
Posted on: April 27, 2006 10:02 PM

Do you think the lowering of the water (exposing the shore) would cause carbonate precipitation to that extent in Erie? Do you think it would need more surface area to bring Ca down to the lake (that is, instead of just the shore, but the areas surrounding the lake as well)?

Or maybe it's a combination of warmer temperatures that brought the increase in biological activity PLUS the lowering of the lake through a change in weather patterns to cause carbonates to precipitate.

Although, that wouldn't make sense for weathering to increase (presumably by rain) and still have a lake lose water.

I don't know. I don't have a Ph.D in geology (only an interest), but your blog is fun to read.


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Posted by: Katherine
Posted on: April 28, 2006 11:43 AM

I agree -- I think the second interpretation is stronger: higher water temperatures would increase primary productivity (abundance of cyanobacteria, which increases calcite precipitation) AND also increase evaporation, lowering lake levels and making the lake waters even more susceptible to warming (potentially, a positive feedback).

I've also just gotten some new stable isotope data that further supports the "warm climate" explanation ... will write a new entry on it soon!

Thanks for your interest in my work :)

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Posted by: Tupac
Posted on: October 2, 2006 01:02 PM

Very good article!

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