Tuesday, November 22, 2011

Migrating dinosaurs and oxygen isotopes

ResearchBlogging.orgA recent paper made a claim that dinosaurs must migrate, based on the oxygen isotopes in dinosaur teeth. This paper is both awesome and flawed.

The awesome part:

It is hard to measure oxygen isotopes in teeth. Teeth are a mixture of organic matter and two minerals: calcium carbonate, and hydroxyapatite. The organic matter contains oxygen, the carbonate portion contains oxygen, and the hydroxyapatite contains oxygen in two different parts of the mineral; bound water (the “hydroxyl” part, and phosphate ions.

None of these phases are stable in groundwater, and they generally get replaced by other minerals such as silica during the fossilization process. That is why most of the dinosaur teeth you see in museums are black. Even if the teeth aren’t fossilized, the different components will exchange oxygen with groundwater at different rates. Depending on the groundwater chemistry. So simply finding appropriate samples from the Jurassic- 150 million years ago, is not easy.

Secondly, teeth are hard to analyse for oxygen isotopes in lab, because you need to make sure that the oxygen from the different materials isn’t mixed, especially if some of the oxygen has been compromised. This can also be tricky.

Most oxygen has an atomic mass of 16, from having 8 protons and 8 neutrons. However, about 2 in a thousand oxygen molecules have 2 extra neutrons, giving a mass of 18 amu. This oxygen-18 ( abbreviated 18O) evaporates slightly more difficultly and condenses more easily than normal oxygen-16 (16O), so rainfall is generally depleted in 18O. This gives what scientists refer to as a negative δ18O value, which basically means that the rain water has a lower 18O/16O ratio than seawater. As air cools, more and more 18O rains out, so that snow has a strongly negative δ18O deviation (figure 1).


Figure 1. Tropical lowland rainfall is generally slightly negative in δ18O (left), while mountain snow in generally highly negative (right).

This leads to the flawed part of the paper.

Fricke et al. (2011) state that because their dinosaur teeth have a variation in δ18O, the dinosaurs must have migrated from lowlands to uplands (figure 2).


Figure 2. oxygen isotopic variation in dinosaur teeth in interpreted as arising from migration.

But as the low δ18O snow melts, it forms low δ18O rivers (figure 3). In an environment with seasonal rainfall, local, tropical rain could give a modest δ18O depletion, white water draining from high mountains would have a strong δ18O depletion. This is exactly what Lambs et al. (2005) see in modern day India: The Ganges river, has a δ18O value of -5, while the Bramhaputra, which flows into India from Tibet, has a δ18O value of -11. Despite their different sources, both rivers empty out into the same river delta. So in this case, the water is migrating, by flowing down hill.


Figure 3: Water can move as well.

An animal which drank from locally fed streams and ponds during a wet season, but retreated to a river with a distal source in the dry season, would also have a δ18O anomaly like that of a migrating dinosaur. This would also explain why the dinosaur had more negative δ18O values when it died; the rock which contained the fossils was a river sand.

This is seen by Dettman and Lohmann (2000) in rocky mountain oysters (fossilized bivalves, you pervs). Shellfish fossils have a δ18O value that ranges from -5 to -23, all in the same sedimentary sequence. Nobody interprets this as evidence for oyster migration. Rather, it is thought to be caused by rivers with very different source characteristics feeding the same depositional setting. Just like the modern Ganges delta.

So my opinion is that the analytical work and sample selection were very good, but the interpretation is a bit simplistic.

Fricke, H., Hencecroth, J., & Hoerner, M. (2011). Lowland–upland migration of sauropod dinosaurs during the Late Jurassic epoch Nature DOI: 10.1038/nature10570

1. David L. Dettman and, & 2. Kyger C Lohmann (2000). Oxygen isotope evidence for high-altitude snow in the Laramide Rocky Mountains of North America during the Late Cretaceous and Paleogene Geology, 28 (3), 243-246

Lambs, L., Balakrishna, K., Brunet, F., & Probst, J. (2005). Oxygen and hydrogen isotopic composition of major Indian rivers: a first global assessment Hydrological Processes, 19 (17), 3345-3355 DOI: 10.1002/hyp.5974

6 comments:

Schenck said...

Cool finding, I'm doing a paper for a class on istopes used in the study of dinosaurs and I have a few previous ones from Fricke, this should be helpful, as is your discussion! Thanks!

Isotopic said...

I don't think there is much calcium carbonate in teeth, or at least enough that it's a concern. A more significant problem is the presence of carbonate in apatite, as it is more easily exchanged.

Isotopic said...

Did you actually read the paper? I don't mean this in a perjorative way, I know that sometimes it's difficult for you to get a hold of full-text. The issue that you bring up is dealt with in the paper, and at any rate, they make it clear in the text that although they believe their interpretation is the most parsimonious, it's not unique.

Regarding my prev. comment, they DID primarily measure the d18O values in the apatite carbonate (don't confuse this with a carbonate mineral like calcite), which is normal b/c it's tough to measure the phosphorous-bound oxygen.

Chuck said...

I read it, and at no point do they constrain the river d18O. They looked at evaporation-related pedogenic sediments, but those would presumably reflect locally-derived ephemeral water sources, since they have to dry out to form. At least that's how I saw it.

I didn't see any analyses of bivalves or crocodiles or other obviously non-migratory creatures.

I had a brief skim of the supp mat, but didn't dig into the details.

I'll give you the Apatite thing, as I'm not really up on how the CO2 is incorporated.

Barbara said...
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Sarju said...

this paper made me more confused....i thought 13C-18O-rich carbonate in a tooth mineral is related to temperature at which the tooth is formed. Animals with high body temperatures incorporate less 13C-18O-rich carbonate in their enamel than do animals with low body temperature ...and dinosaurs like sauropods have less like in mammals...nyz can any one of you guyz make me/ give me any paper to study on for recent finding? thanks