Pubsplained #2: How many forams for a good climate signal?


Thirumalai, K., J. W. Partin, C. S. Jackson, and T. M. Quinn (2013), Statistical constraints on El Niño Southern Oscillation reconstructions using individual foraminifera: A sensitivity analysis, Paleoceanography, 28(3), 401–412, doi:10.1002/palo.20037. (Free Access!)


#Pubsplained #2: We provide a method to quantify uncertainty in estimates of past climate variability using foraminifera. This technique uses numerous, individual shells within a sediment sample and analyzes their geochemistry to reconstruct seasonal and year-to-year variations in environmental conditions.

Here is a link to our code.


This plot shows how uncertainty in IFA statistics decreases (but not all the way!) as you increase the number of foraminiferal shells analyzed.

This plot shows how uncertainty in IFA statistics decreases (but not all the way!) as you increase the number of foraminiferal shells analyzed.

Planktic foraminifera are tiny, unicellular zooplankton that are widely found in the open ocean and can tolerate a large range of environmental conditions. During their short (2-4 weeks) lifespan, they build shells (or tests) made of calcium carbonate. The tests fall to the seafloor and continually become covered by sediments over time. We can access these foraminiferal tests using sediment-cores and analyze their geochemistry to unravel all sorts of things about past ocean conditions.

Typically, ~10-100 shells of a particular species are taken from a sediment sample, and collectively, analyzed for their isotopic or trace metal composition. This procedure is repeated with each subsequent sample as you move down in the core. Each of these measurements provides an estimate of the "mean climatic state" during the time represented by the sediment sample. In contrast, individual foraminiferal analyses (IFA), i.e. the geochemistry of each shell within a sample, can provide information about month-to-month fluctuations in ocean conditions during that time interval. The statistics of IFA have been used to compare and contrast climate variability between various paleoclimate time periods.

There are many questions regarding the uncertainty and appropriate interpretation of IFA statistics. We addressed some of these issues in this publication. We provided a code that forward-models modern observations of ocean conditions and approximates, with uncertainty, the minimum number of foraminiferal tests required for a skilled reconstruction. In other words: "how many shells are needed for a good climate signal?"

Armed with this algorithm, we tested various cases in the Pacific Ocean to obtain better estimates of past changes in the El Niño/Southern Oscillation, a powerful mode of present-day climate variability. We found that the interpretation of IFA statistics is tightly linked to the study location's climate signal. Namely, we found that the ratio of seasonality1 to interannual variability2 at a site controlled the IFA signal for a given species occurring throughout the year. We then demonstrated that this technique is far more sensitive to changes in El Niño amplitude rather than its frequency.

In the central equatorial Pacific, where the seasonal cycle is minimal and year-to-year changes are strong, we showed that IFA is skillful at reconstructing El Niño. In contrast, the eastern equatorial Pacific surface-ocean is a region where El Niño anomalies are superimposed on a large annual cycle. Here, IFA is better suited to estimate past seasonality and attempting to reconstruct El Niño is problematic. Such a pursuit becomes more complicated due to changes in the past synchrony of El Niño and seasonality.

Our results also suggest that different species of foraminifera, found at different depths in the water column, or with a particular seasonal preference for calcification, might have more skill at recording past changes in El Niño. However, care should be taken in these interpretations too because these preferences (which are biological in nature) might have changed in the past as well (with or without changes in El Niño).

You can use our MATLABTM code, called INFAUNAL, to generate your own probability distributions of the sensitivity of IFA towards seasonality or interannual variability for a given sedimentation rate, number of foraminifera, and climate signal at a core location in the Pacific. Do let me know if you have any difficulties running the code!

1 - The difference in environmental conditions between summer and winter, average over multiple years

2 Changes from year-to-year (could be winter-to-winter or summer-to-summer etc.) within the time period represented by the sediment sample

Sediment Traps and Plankton Tows

A very pretty Cocodrie sunset with the R/V Pelican.

Better late than never I suppose, but I wanted to document our short research cruise aboard the R/V Point Sur that happened early last month. It was another routine sediment trap operation, but this one really stood out because we happened to see some really cool critters at sea!

As usual, the northern Gulf of Mexico sediment trap crew (Eric Tappa from the University of South Carolina, Julie Richey and Caitlin Reynolds from the USGS, myself, and another helping hand from Michael Lis, an undergraduate student from UT Austin working with me) made our way down to LUMCON at Cocodrie, Louisiana and boarded the R/V Pelican to start our transit to the sediment trap mooring site. However, things did not go as planned (as is common in the field) and our overnight transit was halted short, as we had already turned back towards port after leaving it only some hours before! It turned out that there were some serious engine problems on the ship which entailed something we could not afford - delays. Luckily, as we were sulking around LUMCON taking an in-depth look at their impressive library, the R/V Point Sur came to our rescue and their crew graciously agreed to charter our operation. 

Relief as the sediment trap comes up to the surface. Picture credit: Michael Lis.

Relief as the sediment trap comes up to the surface. Picture credit: Michael Lis.

This time, apart from sediment trap redeployment operations, we had an additional exciting task: using the plankton tow. A plankton tow is a vital tool used to sample plankton that floats freely in the surface ocean. It is, in essence, a giant net with a fine mesh that we can trawl around and look at what comes up. We intended to sample some plankton at the sediment trap site. In any case, here’s a picture collage of some of the amazing creatures we were lucky enough to catch in our plankton tow:

I'm really excited about the upcoming science from this research trip. Hopefully, we can find some interesting results! Until my next voyage, I will certainly miss being out at sea!

Our Recent Paper in Four Tweets

As promised, this is the tl;dr version of my previous post, where I have tried to reduce our open access paper into four tweet-length snippets per sub-heading. Here goes:

The History: A particular plankton shell used to reconstruct climate is purported to have 2 morphs that live in different depths of the ocean.

The Importance: If true, previous studies that attempt to quantify past oceanic climates from non-selective morphs of that plankton species are biased.

The Study: We analyzed pairs of extreme & intermediate morphs, & with a model, found that all morphs live in the top (<30 m) part of the ocean. 

The Implications: We conclude that morph-based uncertainty in this species when used for studying ancient (Holocene) climates is little-to-none.