Wednesday, October 3, 2012

Dating -- so how old are you really?

by Stacy Carolin

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.

In order to calculate the age, we use the measured values in the table above to solve a set of three first-order differential equations that model radioactive decay for a parent -> daughter -> grand-daughter system. To prove that this actually works, below is a picture of every age I've calculated on 4 different stalagmites that grew during the past 100,000 years (total of 92 dates, wow! drilled out, taken through chemistry, then run on the ICP-MS in 12 separate batches at Caltech over the past few years). And to my amazement, these ages have all come out in the right order (top is the youngest, bottom is the oldest)-- I am quite proud!

Hopefully I've convinced you that this process is very reliable and that I am capable of calculating the correct ages from the samples that I drilled. Ok, so now we are ready for our ages! Check them out (ages are in bold, absolute error is listed below):

Yay, now you're a scientist! What do you think? What are your thoughts? First, look at SC02! Based on these dates it grew 145mm (that's about 1/2 ft) in less than 1,000 years! That's incredible! The average growth rate of a stalagmite is usually between 5-20mm per 1,000 years (if we believe those dates-- I would like another few to prove it for sure.) Sadly though, it turned out to be too young for the Eemian period. And same for SC03. Darn it! Although because that 124,901 yr old age was not drilled at the very bottom, I can drill another date lower and hope we reach before 130,000 years ago for the oldest possible date. And unfortunately, SC12 turned out to be way too old. Oh well, he's out.

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 :)