Picking Foraminifera for Stable Isotope Analyses

Adriane here-

I am beginning to finish one of my dissertation chapters, which means I am starting on a new research project! But first, let me explain (for those who may not know) what a dissertation is: A dissertation is a compilation of three papers, or as we in academia call them, chapters. Each chapter is meant to eventually be published in a scientific journal, as each one is a separate research project or study. Some PhD programs may be different, but at my university, we usually have 3 chapters in our dissertations; in other words, in order to gain a PhD, we have to conduct 3 separate research projects.

Sea surface temperature map of the northwest Pacific Ocean (see inset map at top left). Here, the major ocean currents are labeled by white arrows. The Kuroshio Current flows along the eastern coast of Japan, where it meets the cold-water Oyashio Current. Around 35 degrees north, the Kuroshio Current flows into the Pacific Ocean, where it becomes the Kuroshio Current Extension. The three sites I’m working with are plotted on the map (Sites 1207, 1208, and 1209).

The new project that I am beginning is to reconstruct the ‘behavior’ of the Kuroshio Current Extension. This current, which I’ll call the KCE, is a western boundary current. Western boundary currents flow along the western edge of ocean basins. The KCE flows along the east coast of Japan, on the western side of the Pacific Ocean. Western boundary currents are quite important because they transport really warm waters from the equator northward to higher latitudes. This warm water that is transported towards the poles provides water vapor to the atmosphere. Thus, these currents, to some extent, control weather patterns (such as rain). But western boundary currents, and especially the KCE, are very important areas for wildlife as well. Where the KCE is forms what is called an ecotone. An ecotone is a region where two biological regions come into contact. Here, within the KCE, species that live in warm waters are able to mix with species that live in cooler waters. This makes for a very diverse (a lot of different species) area within the KCE. Even corals, who can only tolerate warmer waters, are found at their furthest poleward extent in the KCE region. And associated with corals and coral reefs are fish and their predators. So, the KCE region is an important region to local Japanese fisheries.

By now you might be wondering why all this background on the KCE matters. Because the area within the KCE is so important from a climate, biological, and economical perspective, it’s important to understand how the current will behave (shift to the north or south, increase or decrease transport capacity) under climate change. Right now, we have direct measurements of the KCE that indicate the current is beginning to slowly shift northward. But how much will the current shift? How will this affect the food chain in this region? To begin answering these questions, geoscientists often go back in time to investigate these systems during times of elevated global warmth. Thus, I will be reconstructing the sea surface temperature at three sites that cross the KCE during a more recent warm period in Earth’s history, called the mid-Pliocene Warm Period.

In 2001, there were three sites in the ocean that scientists collected sediment cores from. These three cores were collected to the north of, directly under, and to the south of the modern-day position of the KCE. I’m using sediment taken from the cores collected at these three sites to reconstruct the position of the current from 5 to 2.5 million years ago. But how will I do this?

When a specimen of Globigerinoides ruber or Globigerinoides obliquus is found from a sediment sample, it’s picked out with a paintbrush and some water and plopped into this labeled slide. The numbers at the top are the sample number.

To reconstruct sea surface temperatures, we need to measure stable isotopes of carbon and oxygen (read more about these two proxies on our ‘Isotopes’ page). Namely, oxygen is the most commonly used proxies to reconstruct temperature, and carbon is more commonly used to determine how productivity (or, more simply, how many nutrients were in the water column) through time. We measure isotopes of carbon and oxygen from the shells of planktic foraminifera.

This is one of the small weighing trays I use to weigh the specimens that are picked from the sediment samples. Each tray has a number associated with it (see sticker at bottom right).

The first step in this study is to ‘pick’ planktic foraminifera. This means that within each sediment sample within my 5-2.5 million year time interval, I sprinkle sediment into a tray and, with a paintbrush, literally pick out a certain species of foraminifera. The species that I’m using in this study is called Globigerinoides ruber. I just call them ‘rubers’ for short. This species of foraminifera is useful because it is still alive (extant) in today’s oceans, and because of that, scientists know exactly where this species likes to hang out in the water column. Rubers live in the upper part of the surface ocean, so they effectively record the conditions of the ocean’s surface, which is great!

Once I have picked out enough specimens from a sample (which ranges from 10 to 20), I weigh the specimens in an aluminum tray on a very sensitive scale. I need about 150 micro grams of rubers per sample for a good isotopic measurement.

After I have weighed the specimens, I then take my paintbrush and put them, one at a time, into a small plastic vial that is numbered. I also have a spreadsheet where I record all of the information, such as the number of specimens picked per sample, the empty weight of the aluminum tray, the weight of the tray and specimens (so that I can then calculate the weight of just the specimens), and the vial number that corresponds to each sample.

Plastic snap-top vials that each have a number. After the specimens are weighed in the aluminum trays, I then transfer them to these vials. I’ll mail these to another university, where the specimens will be analyzed.

I put the specimens in a vial with a very tight snap-cap because I will send all my samples to another university for isotopic measurements. We could do the measurements at my university, but the machine that we use to do this is not properly calibrated to make measurements off of foraminifera. But lucky for me, I have some awesome collaborators that do have machines that are finely tuned to take isotopic measurements from foraminifera!

Once vialed, the samples will be mailed off to the University of Missouri for isotopic measurements. It usually takes anywhere from 1-3 months for my collaborator there to run all my samples. When he is finished, I’ll receive a spreadsheet with the measurements. I’ll plot these data through time. Then, the fun part: I get to make interpretations about my data! I’ll use these data to track changes in the KCE through time, and also to correlate evolution and extinction events of planktic foraminifera to changes in sea surface temperature through time!

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