Dating (and Re-Dating) Coral Rubble

by Pamela Grothe

I’m back at it, dating more coral rubble, but this time using the U-series method. Last time I left off I had just finished dating many of the coral rubble using the fast-screen radiocarbon dating method. In all, I ended up dating 36 pieces of rubble from Christmas Island, 28 corals from the Palmyra collection, and 20 different coral from the Line Islands that were previously dated by U-series.

To recall, back in March I was dating corals with known U-series ages using the rapid screening radiocarbon method. Of the 20 different control corals, we found three corals with large differences between the two different types of ages, where two different corals disagreed by 1,000 years and one coral by about 3,000 years! However, the other 17 corals seemed to behave fairly well, at least within error of the radiocarbon fast screening age (which can end up being a couple hundred of years in either direction because of the large error in the rapid screening method itself and in addition the uncertainty in the calibrated age correction). This is a little unnerving considering all paleo-applications rely on the accuracy of the dates. Which date is telling us the truth? Why are they so different? Were samples mislabeled or did something happen geochemically to cause one or both of these dating methods to misbehave?

Before I move on to addressing these questions, first I want to present a first glance of the age distribution of the coral rubble on Christmas Island (based on the fast-screen radiocarbon dates). To our initial surprise, they are YOUNG! Many came back post-bomb, meaning they are younger than 1950. And others are only a couple hundred years old. Only a few were older than a thousand years old, and most of those were from larger fossil coral that the team drilled from people's lawns (with permission of course), not the rubble. At first I was a bit disappointed because I am really interested in nailing down mean tropical Pacific climate through the Holocene (really anything older than the last thousand years). However, this collection has taught us a few things and could be useful in filling critical gaps in the central tropical Pacific climate during that last Millennium, such as the Little Ice Age (~1500-1800AD) and the Medieval Climate Anomaly (~900-1200AD).

Map of age distributions for Christmas Island fossil coral rubble.

The oldest rubble samples come from further back in the rubble piles. It makes intuitive sense that younger coral rubble would be closer to the shore since they are most likely washed up storm deposits. This will guide our next sampling trip (in August perhaps?? Stay tuned!), focusing on grabbing samples from the back ridges and grabbing samples further down in the rubble pile to try and find several thousand year old samples.

That said, the existing coral rubble dates are all radiocarbon dates, which may contain some biases when applied to this type of material. This is why I am now at the University of Minnesota performing U-Th dating on the radiocarbon-dated coral rubble. U-Th dates have much smaller age errors than radiocarbon dates and there are fewer problems with this method so we generally trust them more. First, I am interested in if the corals that dated post-1950 are really that young (they do look pristine under the microscope!). One problem with Christmas Island and radiocarbon dating is that in the mid-1900’s thermonuclear bomb testing occurred on the island (YIKES!), which could theoretically make the corals “hot”, or in other words add a ton of radiocarbon to them, making them date "young" using the radiocarbon method. We don’t really notice this in any of our control dates but we can’t say for sure if it hasn’t randomly affected certain corals. Another problem we have to consider is that the samples for dating were taken within 2 cm of the exposed coral surface. The next time we visit our large coral archive at Christmas Island, we will sample in the interior of the rubble sample, just in case.

So here I am, at the University of Minnesota for my second two-week dating excursion and I’m sure with many more to come since dating is a heavy part of my research. I will be dating 48 corals with the U-series dating method, 3 of which are the coral with large discrepancies between radiocarbon and U-series dates, 23 of the Christmas coral rubble samples that were dated with the radiocarbon method, 2 samples from Palmyra whose radiocarbon dates fell near the Medieval Climate Anomaly, and 16 additional corals of unknown age from the Christmas Island rubble collection. All of these dates are being performed in Dr. Larry Edward’s lab with lots of help from Dr. Hai Cheng.

My goal is to really nail down these issues so that I can leverage both dating methods in order to get as many dates as I can, quickly and accurately, for my paleoclimate reconstruction work.

Stay tuned next week when I’ll go into more details about the U-series dating method for corals.  In the meantime you can brush up on your U-series dating from Stacy’s blog post last year (A Lesson on Dating...)!

Until then, CHEERS!