Uncertainty in paleorecords built from foraminifera

We have a new paper out in Paleoceanography which describes a new computational toolkit that I built during my Ph.D. to characterize uncertainty in foraminiferal reconstructions of temperature and salinity. The toolkit, called Paleo Seawater Uncertainty Solver (or PSU Solver), constrains analytical, sampling, calibration, dating, and preservation-based uncertainty in reconstructions that use paired measurements of stable isotopes of oxygen (δ18O; apologies that I cannot make letters superscript in this interface) and magnesium-to-calcium (Mg/Ca) ratios in foraminifera (or forams: a type of plankton that deposit calcium carbonate shells). In the paper, we show how signficant this characterization of uncertainty can be while inferring climate processes in the past using forams.

δ18O has been measured in foram-calcite for over half-a-century to understand past climatic processes. In a closed system, changes in foram-δ18O ought to be governed by thermodynamics. However, in the open ocean, the foram-δ18O composition reflects not just the temperature of the seawater, but also the δ18O content of the seawater in which the foram grew its calcite shell (δ18Osw; where sw is in subscript and stands for 'seawater'). This parameter, δ18Osw, which may vary independent of temperature, can be used as a proxy for salinity changes, and on longer timescales, a proxy for ice-volume (more info here and here). The problem is, if you only measure the δ18O forams, how do you know which changes are due to temperature and which changes are due to δ18Osw of the ocean? In other words, the foram-δ18O signal is convoluted by temperature and salinity/ice-volume signals.

This is where the Mg/Ca paleothermometer comes into the picture: over the last two decades, researchers have shown that Mg/Ca ratios in forams can give quantitative insights into past seawater temperature changes i.e. foram-based Mg/Ca varies as a function of temperature! This appears to solve the deconvolution problem – if one measured both Mg/Ca and δ18O on co-deposited forams, then you could solve for δ18Osw and temperature of the seawater where the forams lived. Mathematically,

• δ18O = g(T, δ18O­sw)
• Mg/Ca = f(T)

So, with both of these measurements on the foram shells, one could tease out the δ18Osw and temperature of the seawater to infer past climate change. This technique has been used successfully to understand a variety of significant climate processes (we list many of them in our paper).

However, as more nuanced calibrations (i.e. as functions f and g above keep getting updated) and inferences are made from these paired measurements, and as climate model-data comparisons increasingly demand for more quantitative paleorecords, there is a strong need to quantify the uncertainty in the above deconvolution of temperature and δ18O­sw. Transferring analytical (/measurement/instrumental) errors, sampling uncertainty due to the number of foraminifera used in the measurement, calibration uncertainties, preservation effects, and age-model uncertainties into temperature and δ18O­sw space is not straightforward due to a variety of reasons, one of which is that the Mg/Ca-T relationship is non-linear in forams. Theoretical error propagation exercises predict uncertainties way too large to interpret even glacial-interglacial signals where such practices cannot handle complex issues such as the influence of salinity on Mg/Ca variability and the non-stationarity of the relationship between salinity and δ18O­sw.

In any case, all these things call for a computational approach to address quantitative uncertainty in foram paleoclimate. This is what PSU Solver does. The code is written in MATLAB and is available here, on Mathworks, or on GitHub. It can produce uncertainty profiles for temperature, δ18O­sw, and salinity paleorecords from paired foramininferal δ18O­ and Mg/Ca datasets. It can be as customized as the user wants it to be (or as simple as a user wants it to be) where the user dictates all options that they require. The user input can be as easy as time, δ18O,­ and Mg/Ca, if so desired.

Go ahead, download it, and try to play around with the code (here are the help files)! It can easily be coupled to more sophisticated algorithms or different sampling schemes as well. Furthermore, there’s an option to couple output from BACON to incorporate radiocarbon-based uncertainty in PSU Solver as well. I will have a post up shortly that explains how PSU Solver works in detail soon. Until then, here’s the type of plot that you could be producing right now (paleorecord from the Pigmy basin, Gulf of Mexico). Go try it out! We show that particular excursions in your reconstructions may (or may not) be sensitive to easily-changable options in the PSU Solver algorithm, and hence be an artifact of transposition issues. Do let me know if you have any questions/doubts/criticisms!

P.S. If you are not a super expert in coding (which I am not either), PSU Solver has been written keeping us in mind: it is not meant to intimidate! Try it out...! Thanks to Julie Richey for being patient with the algorithm and also to Chris Maupin for testing PSU Solver.

Another Cruise to the Northern Gulf of Mexico

As another autumn rolled by, it was time for another research cruise into the northern Gulf of Mexico for sediment trap research. As I've blogged before, we collaborate with the USGS St. Petersburg Marine Science Center and try to help out with their long-running sediment trap program whenever we can. The sediment trap is a device placed underwater between the sea surface and seafloor that captures pelagic sedimentation (any biological/sedimentological material produced above it, that drifts into the device and gets 'trapped'). The trap has several cups where one cup is open at any time and collects material for two-to-four weeks, after which it closes and another cup is opened. Every six to ten months (depending on how it is programmed), we have to go out to where the trap is moored, retrieve it by releasing its anchor, collect all the previous samples, perform routine maintenance, and redeploy the trap with new cups.

The last sediment trap cruise I was on (March '15) proved to be quite an ill-fated mission: the trap failed to come up to the surface after the signal was sent to release its anchor (after several tries)! This not only meant that the device was lost, either tangled up at the bottom of the ocean somewhere or drifted off into some foreign land (most likely the latter), but that our last 10 months of samples were lost! Such is the mercurial nature of observational field research.

Luckily, USGS scientists Julie Richey and Caitlin Reynolds managed to procure a new sediment trap! The purpose of this mid-November trip was to deploy the new sediment trap in the same location as the last one. Our motley crew, apart from the usual suspects of myself, Julie, Caitlin, and the illustrious Eric Tappa from USC, included Eric's student Chris, and graduate students Natasha and Allison from our lab. We were to sail, as usual, on the RV Pelican, out of LUMCON at Cocodrie, Louisiana. However, weather had other plans for us.

Port of Gulfport

Gulfport Sunset.

As is the case with field work, rigid plans solidified months ago, can vaporize instantly: due to weather, the RV Pelican would be severely delayed in coming back to Cocodrie such that our schedule (flights et al) couldn't be accommodated. Instead we had to go to Gulfport, MS to board the RV Point Sur - an equally capable vessel (if not more - it had just been to Antarctica some time ago!) On the way back though, to catch our flights, we would be dropped back in Cocodrie.

This being the case, we set sail around 5PM on the 18th of November from Gulfport. Coincidentally, on the flight from Austin to Dallas, I was seated (by random chance) next to chemical oceanographer Jessica Fitzsimmons, who had also sailed on the Point Sur! From Dallas, Natasha, Allison, and I took a flight to NOLA, from where we drove out to Gulfport. Though the weather in Gulfport was dreary the entire day, not really helping the city's rough look, the clouds paved way for a brilliant sunset as we made our ~22 hr journey to the trap location.

Eric Tappa with the new sediment trap. Credit: Julie Richey

Caitlin, Chris, and I inspecting the sediment trap prior to deployment. Credit: Eric Tappa

Deploying the trap is not the most tedious job out at sea. However, rough weather can complicate things, and every step you take has to be an extremely careful one. This being the case, Eric, who's had plenty of experience with these conditions and instruments, guided us (and the crew) through a safe and successful deployment. All of us watched for the two buoys to plop down beneath the sea-surface, but the 10-12 ft of swell and the intermittent rain prevented us from identifying the exact moment.

Sailing on top of a warm-core eddy, as seen by the sea-surface height map (left) and the sea-surface temperature map (right). Hot stuff!Source: CCAR 

Generally, when we were at site, we also make measurements about the salinity, temperature, and oxygen content of the seawater with a CTD, that also collects waters at different depths. In this process, we realized that we were actually atop a warm-core eddy! This was quite exciting, as it was my first time (I study these systems) being in one, and accordingly, the water temperature was a sizzling 27ºC in November. 

The scientific party.

Amidst a modest yet uncomfortable swell, we made it back to Cocodrie early Friday morning, after spending our second night on the Point Sur. The next cruise to collect all these biweekly-to-monthly cup samples that will be collected by the new sediment trap will be in May. Hopefully, we will get to do some good science with these samples!