The Stable Isotope Story
I am now pretty convinced that climatic warming caused the whitings (calcite precipitation events) we see in Lake Erie! What made up my mind? And why should anybody care?
Stable isotope data was the key. It indicated that there were high levels of evaporation, high calcite precipitation rates, and high primary productivity.
Hold on -- what's a stable isotope? Good question, I'm glad you asked. Isotopes are "varieties" of elements: they all have the same number of protons, but a different number of neutrons. This means they behave almost the same, but not quite. Since they have different numbers of neutrons, isotopes have slightly different masses. This means that lighter isotopes will tend to evaporate more easily, and be preferred by organisms for use in photosynthesis, etc. For example, if lots of light oxygen isotopes are evaporating, then more heavy isotopes get left behind, making the water "heavier." We can measure the stable isotope composition of different materials (in this case, mud and shells) and compare it to an international standard to get a ratio. This ratio can help us figure out what was happening in the environment at the time!
It sounds like a bit of a stretch, I know. And it is true that different factors can cause the same change in an isotopic composition -- oxygen isotopic composition of lake water could be made really heavy by lots of evaporation, OR also by an influx of heavy water from another lake, or from precipitation, or groundwater ... so how do we know? Well, we can never really KNOW 100% (one of the frustrating things about trying to understand the past), but we can make a very good guess. It all comes back to that multi-proxy approach I was talking about in earlier entries. We use LOTS of variables, combined, to make a complete picture. Put it all together, and see what makes the most sense.
Before I tell the Lake Erie isotopes story, one more piece of background:
We measured stable isotopes on two materials: mud and shells. The shells were made by fingernail clams, which are benthic organisms (ie, they live on the lake floor). The mud is not so easy to source, but since it was very fine-grained and calcitic, we had a hunch that it was precipitated out of the lake's upper waters (the fancy geologic word for this water layer is the "epilimnion." use it at a party, your friends will be impressed). Thus, we expect to see the shell isotopes telling us something about what was happening in the bottom waters, and the mud isotopes to tell us about the surface.
Here's what we found in the sedimentary record about 2,900 yrs ago:
1. Shells -- higher ratios of heavy Carbon (Carbon-13) to light Carbon (Carbon-12) (a "positive" trend), and higher ratios of heavy Oxygen (Oxygen-18) to light Oxygen (Oxygen-16).
2. Mud -- slight increase in Carbon 13/12 ratios, and lower ratios of Oxygen 18/16 (a "negative" trend).
What does it mean?
If you're not a geologist you're probably tired of reading this stuff by now, so I'll make is short and sweet as possible. Heavier Carbon isotopic compositions usually mean that there's a lot of photosynthesis going on -- lots of little phytoplankton sucking up the light isotope, leaving the heavy isotope behind in the water. So we have conclusion 1: lots of primary productivity.
Interpreting the oxygen is a bit trickier (hold on to your hats) ... when calcite precipitates out of water, it likes to use the light oxygen isotope, and tends to leave the heavy stuff behind. Evaporation (as we already talked about) also preferentially removes light oxygen, which also makes remaining water isotopically heavier. Nice story, does it fit? Yes! We see lighter oxygen ratios in the precipitated calcite (the mud) and heavier oxygen ratios in the shells (which reflect the composition of the water).
So climate was warmer, lake levels were lower, there was lots of calcite precipitation and lots of primary productivity.
Not a bad story, for a pile of mud.