Creating a High-Resolution Biostratigraphy

Adriane here-

A sea surface temperature map of the northwest Pacific Ocean, with warmer colors representing warm waters, and cooler colors representing cooler waters. The white lines with arrows are the major ocean currents , with the Kuroshio Current flowing along the coast of Japan. The three deep sea sites I’m using in my research are plotted on the map (black dots).

For the past two years, I’ve been conducting research into planktic foraminifera (‘foram’) evolutionary events in the northwest Pacific Ocean, specifically across the western boundary current known as the Kuroshio Current Extension (which I’ll call the KCE from now on). This is a dynamic area of the ocean, and is unique in that forams from warm waters are able to mix and mingle with cold water forams. This mixing of warm and cool species may lead to evolution of new species, but this process is poorly understood. So, part of my dissertation is to determine how important these western boundary currents, specifically the Kuroshio Current Extension, is in the creation of new plankton species. In doing this study, I am also creating a way to tell time using planktic foraminiferal biostratigraphy. OK, those were a lot of big words, so let me explain:

Biostratigraphy is composed of two primary words: bio, meaning life, and stratigraphy, which is a branch of geology concerned with the relative order of rocks and putting time into the rock record. So in short, biostratigraphy is using life to put time into the rock record, or using fossils to tell time. In my case, I use planktic foraminifera to tell time (read more about how I do that here). Commonly in biostratigraphy, we (paleontologists) create zones, which are blocks of time that are constrained by the evolution and extinction events of animals or, in my case, plankton. In the northwest Pacific, there are currently no detailed planktic foram biostratigraphies. Part of my research is to fix this problem!

To conduct a biostratigraphy and thus look at plankton evolution and extinction events, I’m working with sediment that was taken from three sediment cores. These cores were drilled from the north, directly under, and to the south of the modern-day position of the KCE in the northwest Pacific Ocean. The sites go back in time to ~15 million years ago, which is quite young compared to the rest of Earth’s history (4.6 billion years!). Each site contains minerals that aligned to the Earth’s magnetic pole when they were deposited on the seafloor. The direction in which these minerals align were measured by other scientists when the cores were drilled. It turns out that each core records almost all of the Earth’s changes in its magnetic pole. This is important because other scientists through the decades have worked hard to date each of these magnetic reversals. Thus, I can use these ages to construct an age model for each of my sites (an age model is where I assign an age to a certain depth in the core where a magnetic reversal happened; what I end up with is a plot where I can calculate the age at any depth in the core). This age model is important because I can then use it to determine precisely when a foram species evolved or went extinct at any of my three sites.

One of my cardboard slides. Each box contains a different species of foraminifera. On the side of the cardboard box, in purple ink, is the identifying code that tells me where exactly in the world and in the sediment core this sample is located. Currently, I have 414 of these!

The first step was to determine at what resolution I wanted to look at foram evolutionary events. I went with 30,000 years, on average. This means that every extinction or evolutionary event has an error of plus or minus 30,000 years. This seems like a lot, but in reality, it’s pretty good! After determining the resolution I wanted, I then used my age model to determine where within each core I wanted to request sediment samples from. All of the cores I use are stored in a facility in College Station, TX (read more about it here), and any scientist can order samples from the facility for free (it’s awesome!). The samples arrived within 2 months after I ordered them.

After I had sorted, sieved, and dried each sample to obtain foraminifera, my samples were ready to be used! I started at the warmest site, the one located to the south of the KCE, in the youngest sample. I sprinkled sediment from the sample onto a tray, looked at the sample under the microscope, and picked out with a small paintbrush every species I could identify. These specimens were placed on a specimen slide (a rectangular cardboard slide with 60 boxes) that had a thin layer of glue over it. In this way, the specimens from each sample stay on the slide, and can be looked at by researchers for years to come. I also have slide maps, or pieces of paper with 60 boxes printed on it where I label what species is in each box on the glued specimen slide. Picking one sample takes anywhere from 30 minutes to an hour, depending on how many species are present in the sample.

This is a figure with all of the evolution and extinction events that happen at my three sites. Site 1207 is on the left; 1208 in the middle; and 1209 on the right. In the middle is the Geologic Time Scale, representing the last 15 million years of earth’s history. The other two sites (1207 and 1209) are also plotted along time, with oldest at the bottom and youngest at the top. The species names that are in red are extinctions; the names in blue are evolution events.

It’s important to note that I did not look at all the samples that I ordered from College Station, TX. Instead, I did a ‘preliminary pass’ through every 10th or so sample. When I found a sample where a species evolved or went extinct, I then looked at the sample between that one and the next, and repeated that process over and over until I had constrained the event to +/- 30,000 years. I then repeated this process for the other two sites.

Once I had all the data, I plotted it up into several figures and spreadsheets to see where all the evolution and extinction events are taking place. Then, I looked at when several species that are commonly used to define zones among sites (these species are used because they are resistant to dissolution when ocean waters become acidic, they are large and easy to identify, and they occur in high numbers in each sample) evolved or went extinct. It turns out that although the three sites I’m using are close together (they span about 5 degrees of latitude), an evolution or extinction event in one species happened at different times across cores! This is a really cool result, as it means changes in the position of the Kuroshio Current Extension could have caused a species to migrate away or not able to live in the area anymore!

In addition to constraining plankton evolutionary events, I was also able to create zones for use in biostratigraphy bound by these evolutionary events. This is the first study that will have constrained plankton evolutionary events in the northwest Pacific Ocean at a high resolution, and the first time mid-latitude planktic foraminifera zones are calibrated (directly plugged into) the Earth’s magnetic record! I hope to publish these results later this summer in a scientific journal!

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