How well do stable isotopes really work?

Defining uncertainty and error in planktic foraminiferal oxygen isotope measurements

Andrew Fraass and Christopher Lowery

Summary by Andy Fraass, website collaborator

Figure 1. Two different locations in the ocean. Temperature and salinity combine to give us the δ18O values. On the left are a few planktic foraminifera in their rough depth habitats (where each species likes to hang out in the water column) (Shiebel and Hemleben, 2005). Drawings are modified from Kennett and Srinivasan (1983) and Schiebel and Hemleben (2005).

Data: Chris and I developed a model to understand how good planktic foraminiferal isotopes actually are at recording temperature, and how important it is that a scientist uses a bunch of tests to measure the isotopes, rather than just a few. There’s actually no data in a traditional sense in this paper; we went back to theory, statistics, and math.

Methods: Chris and I wrote a theoretical model. Planktic foraminifera make their shells in a certain depth (or depths) in the water, and that depth has a certain chemistry. The model allows the scientist to say that the forams are mostly growing in a certain season and at a range of depths. Then the scientist has to decide to include how well the organism records the water chemistry (technically called ‘vital effects’), if the shell has its chemistry altered in the sediment (diagenesis), and a few other things including if there’s a chance that a different species (with a different ecology) got mixed in.

Results: Good results from stable isotope studies come from about 15 or so shells in an analysis, but it’s very dependent on the species and what the ocean structure is like.

Why is this study important? Given all the things that could go wrong, of which the parameters in the model are a part, it’s honestly a little surprising that planktic stable isotope records give the same results as the model. They do, which other folks have shown repeatedly. What Chris and I show here is that as long as you put in enough shells when you’re doing your analysis, then the record actually records what we think it does!

The big picture: Studying the ocean is tough, especially when we’re talking about the ocean from tens of millions of years ago. This paper helps show that despite statistical concerns some of us had with it, we’re doing a good job at recording the past.

Citation: Fraass, A. J., and C. M. Lowery (2017), Defining uncertainty and error in planktic foraminiferal oxygen isotope measurements, Paleoceanography, 32, 104–122, doi: 10.1002/2016PA003035.

References:

  • Kennett, J.P. and Srinivasan, M.S., 1983. Neogene planktonic foraminifera: a phylogenetic atlas. Hutchinson Ross.
  • Schiebel, R. and Hemleben, C., 2005. Modern planktic foraminifera. Paläontologische Zeitschrift, vol. 79(1), p.135-148.

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