picardBy Evan

As a modeller, nothing is more frustrating than a lack of data that allows you to constrain your model. One could say that my entire PHD project is an exercise in futility, due to a sheer lack of data (that doesn’t stop me from trying). 😉

My experience today (Monday), represented one of those days where I could not find the data that I was after. I wanted to find a proper eustatic (i.e. global) sea level curve for the penultimate glaciation (a period between about 190 and 120 thousand years ago). This turned out to be far more difficult than I imagined it would be.

The first lead I got was this paper by Shackleton in 2000. This paper mainly focuses on oxygen isotope record for the past 400,000 years, which indirectly is a proxy for sea level change. Essentially, because oxygen-16 has a smaller mass than oxygen-18, it preferentially gets evaporated from oceans. When ice sheets form on land, the proportion of oxygen-18 in the ocean increases. By calibrating the oxygen-18/oxygen-16 ratio curve with observed sea-level, it is possible to infer what it was further in the past. Not a direct measure, but it is a start.

Oxygen-18 change during the past 400,000 years, and part of the curve calibrated with actual observed sea level - from Shackleton (2000)
Oxygen-18 change during the past 400,000 years, and part of the curve calibrated with actual observed sea level – from Shackleton (2000)

The actual sea level observations were what I was after. The references to those dates indicate the correspond to Marine Isotope Stage 5 – which is after the penultimate glaciation finished. Still no direct observations.

The next article I came across was by Lea et al (2002). They collected oxygen isotope and Mg/Ca data from a core off near the Galapagos Islands. The curve is actually pretty similar to that of Shackleton’s; unsurprising due to them using similar methods. However, there are still no direct observations of sea level during the period of time I am interested in.

Sea level for the past 350,000 years, according to Lea et al (2002).
Sea level for the past 350,000 years, according to Lea et al (2002).

Another paper is by Siddall et al (2003). They also used oxygen isotopes as a proxy for sea level, but in the Red Sea. The Red Sea is quite shallow, so it is very sensitive to changes in sea level, causing large changes in chemistry. The results are similar to the other two papers.

Eustatic sea level infered from two cores recovered from the Red Sea - from Siddall et al 2003)
Eustatic sea level inferred from two cores recovered from the Red Sea – from Siddall et al (2003)

Well, reconstructing sea level based on indirect evidence of oxygen isotopes is all fine and dandy (I see no reason to dismiss these reconstructions outright), but I was after some direct data. Where are the coral based reconstructions, like for the past 20,000 years? The first paper I found with some actual data from this period was by Scholz et al (2007). They showed data from Barbados for marine isotope stage 6.5, corresponding to about 175,000 years ago. This was a period when sea level was about 50 m lower than present.

Sea level data for marine isotope stage 6.5 - from Scholz et al (2007)
Sea level data for marine isotope stage 6.5 – from Scholz et al (2007)

If you look at the data and compare to the sea level reconstructions made by oxygen isotopes, you can see why I was hesitant to accept them at face value.  Only a few data actually fit the oxygen isotope reconstructed curves within their error limits. This could just as easily be a problem with the uranium/thorium dating of corals (much of Scholz’s study focused on eliminating samples that gave anomalously young dates due to transport of uranium within the fossil corals from chemical processes).

In the end, the above figure was about all I could find for observations of sea level change in the interval I wanted. I did find data corresponding to the interglacial periods preceding and following the penultimate glaciation, but they aren’t what I was after (I assume there was not much ice in North America during those time periods). So yes, frustration abounds due to a lack of data, and there is not much I can do about it, except to move on and do something different with my time.