Data data data!!! Much apologies for keeping everyone waiting!
Let's just remind ourselves of what we we're looking for so we know whether to be excited or sad when we finally get these dates :) I want to find stalagmites that formed during the Eemian period. This is why: Technically, the earth is currently in an "Ice Age" and has been for the past 2.5 million years (Antarctica is covered with ice as we all know). But "Ice Age" climates vary between "glacial" states (90% of the time) and shorter "interglacial" states (warmer and less ice, like today). These fluctuations are due to astronomical cycles (solar energy and orbital variations), the composition of the atmosphere (greenhouse gases), and changes in ocean currents. The interglacial state we are currently in is called the Holocene, which has been surprisingly stable for the past 11,000 years. The earth's last interglacial state was the Eemian, which began about 130,000 years ago and lasted for about 10,000 years. For many reasons (see previous post) it would be great to know how these two individual warm interglacial periods compare. So we are looking somewhat blindly for stalagmites (our time capsules of tropical climate knowledge) that formed throughout the Eemian period (130,000 to 120,000 years ago).
Below are the best-guess stalagmites that I chose to try to date for this project. These are scanned images of the "slabs" that I talked about slicing in previous writing. The tops will be the youngest, and the bottoms the oldest, since stalagmites grow from bottom up over time. The arrows show where I drilled out my sample.
Data collection from the ICP-MS is incredibly complicated and complex, and not very relevant here, so I am not going to discuss. I will just share the final counts of each element that we were looking for (U-238, U-234, Th-230) in each of the samples, which we will assume I calculated correctly from the ICP-MS output data. Notice how many more U-238 there are in all stalagmites (which is the most abundant uranium type found naturally because it has a half-life of a whopping 4.5 billion years! Recap: U-238 decays into U-234, which then decays into Th-230, which then decays into another element... that's why they are all technically radiogenic.) Th-230 are the smallest in number (Th-230 has a half-life of only 75,000 years), thus any Th-230 contamination from dirt/mud will likely cause the most error in our ages.
So what do we do from here? Sigh, all that work, and I still desperately need more samples, there's no way I can learn about the Eemian period if I only have half of it represented with one lonely stalagmite. It is best to have at least 3 records so that I can compare the results and make sure they are all reproducible. So, happily, it's time to go back to the caves (in Malaysian Borneo!) and collect even more fallen stalagmites, and hope that a few that I bring back just happened to have grown during the past 115,000-135,000 years.
Hey what a coincidence! We are headed to Borneo in less than two weeks! And then back to Caltech. YES! Stay tuned :)