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!

Take It to Print

by Stacy Carolin
We have a paper out (!) And all that text will likely cause a little headache, so I’ve condensed it down to the important talking points –in common tongue-- to bring to the dinner table tonight: 

Link to Science paper

The author examining a fallen stalagmite. Credit:  Syria Lejau

Background:
The new published record was created from stalagmites collected in Gunung Mulu National Park on the island of Borneo, which sits on the equator in the western Pacific. Variability in the isotopic chemistry of the rainwater over our site is directly related to variability in the rainfall amount in the region (see lab-colleague Jessica Moerman's recent study in EPSL). Stalagmites form from rainwater that drips through soil and limestone into cave chambers, and the isotopic chemistry in the rainwater is captured and maintained in the calcite formations over tens of thousands of years. The four stalagmites used in this study grew during most of earth's last ice age (100,000 years ago to 15,000 years ago), and reveal how rainfall amount over Borneo changed (or did not change) in response to various atmospheric and oceanic changes, both global and local, during this period.

Points (1)-(5) on published

Figure 2 --click for larger

(1) Ice-age-cycle global temperature and CO2 variations did not greatly influence rainfall amount over Borneo -- Like the Chinese stalagmite records, the 100,000-year-long Borneo record does not have a "sawtooth" shape famously characteristic in the global temperature, atmospheric CO2, and sea level records spanning several ice age cycles. We conclude then that on these multi-thousand-year timescales, those variables are not directly influencing rainfall amounts over Borneo.

(2) Abrupt sea level drop / sunda shelf exposure did not cause a synchronous shift toward dry (or wet) conditions in Borneo -- Sunda Shelf emergence (shallow continental shelf connecting Borneo to Southeast Asia) has been implicated in shaping western tropical Pacific rainfall amount in previous studies, but we find little correlation between the Borneo records and an index of Sunda Shelf exposure over the last 100,000 years. For example, significant variations in the Borneo records during the earlier glacial interval (90,000-70,000 years ago) bear little resemblance to reconstructed sea level changes, most notably ~76,000-71,000 years ago, when a 60 meter drop in sea level almost doubled the size of the exposed shelf without any corresponding shift in rainfall amount.

(3) Rainfall variability over Borneo closely followed solar radiation cycles over the equator (seasonal solar energy flux increase = rainfall amount increase, and vice versa) -- The Borneo records suggest that solar precessional cycles (meaning change in solar energy flux over the equator due to cyclical change in the orientation of earth's tilted axis) may be the dominant source of Borneo rainfall amount variability on thousand-year timescales. For example, today the tilted axis points the northern hemisphere toward the sun (northern hemisphere summer) when the earth is furthest from the sun in its elliptical orbit, while 12,000 years ago the tilted axis pointed the southern hemisphere toward the sun (northern hemisphere winter) when the earth was furthest from the sun in its elliptical orbit. These cyclical changes on thousand-year timescales vary the seasonal amount of solar energy over Borneo, which the records suggest in turn do drive changes in the amount of rainfall over Borneo.

(4) The Borneo record’s lack of many abrupt climate shifts that are present in almost all northern hemisphere records implies a selective response of Borneo rainfall to high-latitude abrupt climate change forcing -- The dominant paradigm to explain the famous global millennial-scale climate shifts first discovered in the Greenland ice cores is that they are driven from the North Atlantic region, either from weakening of the Atlantic ocean circulation or from a dramatic albedo change due to sea ice cover, which in turn shifts the northern hemisphere to cooler/drier conditions and makes the southern hemisphere warmer/wetter. The absence of several of these abrupt climate change signals in the Borneo record represents a clear challenge to our understanding of how the tropical Pacific is involved in the mechanism(s) that forced abrupt climate change during earth’s last glacial state.

(5) The largest millennial-scale anomaly in the 100,000-year Borneo record is coincident with the Toba super eruption -- The Toba eruption was the largest volcanic eruption in the past 2.5 million years and climate responses to the eruption are uncertain. Many anthropologists believe that the volcanic winter following the super-eruption decimated most “modern human” populations at that time, and caused a bottleneck in human evolution. The abrupt shift toward dry conditions over Borneo (within age error of the volcanic eruption) that then lasted for a millennium is interestingly mirrored in the Chinese stalagmite record. Further studies are needed to determine if these events are at all related.

Thanks to Kim, Jess, Jessica, Syria, Jenny, and all the rest for helping put this beast together, from field to print! What an experience!