One of the many projects I work on involves the study of climate change in the fossil record. I’ve put a bit of it on-line here. What I’ve published thus far deals mostly with interpreting general climatic and environmental factors using bulk geochemistry (all isotopes) from rocks and the fossil contained therein. That is to say, I take a big rock or fossil and grind it (or part of it) down into a single sample. I analyze that and call that a ‘average’ for that entire rock layer.
It turns out that clams (and mollusks in general) do a good job of recording environmental signals not just in bulk, but on a fine scale, such that we can see yearly, monthly, even daily records of weather.
Remember now, weather is that’s going on with the atmosphere at a given place at a given time. It’s raining or snowing or sunny. Climate is on average, what can be expected in an area on a given day. Another way to think of it is this: Weather determines what you wear today. Climate determines what you have in your closet.
I’m interesting in looking at overall climate patterns. Things like the seasonality of precipitation. Does it rain mostly in the spring, or in the winter? Or may temperature patterns. It’s not just knowing the average annual temperature, but how much hotter the hottest part of the year is over the coldest part of the year. Things like this offer great insight into the overall climate of an area. Was it monsoonal climate? Was it tropical? If we can look at seasonal changes, we can figure this out.
Mollusks contain such records!
Mollusks, like clams or mussels, grow incrementally. They have growth lines, not unlike rings in a tree. As the clam grows, new layers are added to the edge of the shell and the old layers are preserved. The minerals that formed as the shell grew and the growth lines formed record the environment the the mollusk was living in at that time. This mineral shell is not remodeled as the animal gets bigger. New layers are just added on to the older ones.
So what’s getting recorded?
Mollusk shells are made up of calcium carbonate minerals, either calcite or aragonite. Both have the chemical formula CaCO3. The carbon and oxygen in the carbonate (the CO3 part) come from the environment, from the water that the shell is growing in. Isotopes of carbon (Carbon-12 versus Carbon-13) record information about plant life and ‘productivity,’ which is a general term for how much photosynthesis is going on in the water. Oxygen isotopes (Oxygen-16 versus Oxygen-18) vary with temperature and the amount of precipitation (among other things), and provide a general sense of ‘weather.’
A single mussel can live many years, and keep a continuous environmental record over that time. For us, the scientists, it’s just a matter of getting that geochemical (isotopic) information out of the shell. This is where a very fine dental burr, an extremely patient undergraduate student, and a mass spectrometer come in.
Growth lines are evident on the exterior of the shells. To extract the seasonal information, one simply has to collect powder from the shell at regular intervals tracing the growth lines. Two great students, Julia Voronov in 2007 and the Jen Morey in 2011, collected such samples from several shells, and both of them wrote senior theses on their results. (I’m happy to say both of them have also gone on to graduate school and are doing really well!)
So what does it look like?
Below is a shell that has been sampled. You can’t really see the sample grooves (my students are so careful!) so I’ve outlined them with dashed lines. This is a typical mussel from the Hanna Formation.
The isotopic data can then be summarized into a graph like this one (data from the same shell).
Several things can be read off a graph like this. For one, you could look at the absolute values for carbon and oxygen. You can calculate averages. These numbers can be fairly straightforward with carbon, since the carbon probably comes right out of the water. It is possible that the isotopic values for carbon can be altered by the metabolic processes of the mussel itself, but this can be accommodated fairly easily.
Oxygen is significantly trickier. Oxygen isotopes vary with temperature, humidity, amount of precipitation, and (importantly for lake-dwelling organisms) evaporation. In general, I ignore the actual values of oxygen and think more about the overall pattern, which itself is plenty complex.
Another thing, which winds up being important in this analysis, is any correlation between oxygen and carbon. Do carbon values go up when oxygen values do? Or does one go up when the other goes down? Also, if there is a correlation either way, are the changes always simultaneous, or is there a lag. Does oxygen start to drop and then carbon sometime later?
Such patterns can be telling and may indicate increases in aridity, or temperature, or changes in the annual timing of precipitation. If one is interested in climate change, this is what you’re interested in.
Right now, I’m in the beginning phase of preparing a manuscript studying climate change using mussel shells across the Paleocene-Eocene Thermal Maximum, to address what abrupt climate change did to seasonal patterns at to also assess whether or not climatic ‘recovery’ (cooling back down) meant returning to the original climate patterns or if things were forever changed.