Counting Deep Sea Sediments

Adriane here–

As paleontologists and paleoceanographers, sometimes the analyses we do involve complex equations, time-consuming geochemistry, or large amounts of computational time running models. But every now and then, we gather data using a method that is simple and fast. Today, I want to talk about one such method that I use quite often in my research. These data are called biogenic counts.

In previous posts, I’ve written about the deep-sea sediments I use in my research, such as sampling the cores we drilled from the Tasman Sea, and processing these samples once they are back in the lab. Each sample, which is stored in a small vial and represents 2 cm of the core (or 10 cubic cm of material), contain pieces of hard parts of plankton and animals, as well as minerals. These minerals and biogenic pieces, then, can tell us about our oceans and the life it held millions of years ago.

Biogenic count data is just that: I dump the sediment samples onto a tray and count the number of ‘things’ that are in that sample to determine the percentage of each ‘thing’ there. ‘Things’ in the sediment fall into a couple different categories: benthic foraminifera (foraminifera that live on the bottom of the seafloor), planktic foraminifera (foraminifera that float in the upper part of the water column in the open ocean), echinoderm spines (the hard parts of things like star fish and sea urchins), foraminifera fragments (pieces of foraminifera shell that are broken), sponge spicules (the hard parts of sponges that look like spiked glass), and I also make note of any minerals that are found in the sample. In one day, I do about 10 samples, which doesn’t seem like much but adds up everyday!

Below I’ll go  over the exact steps I take when performing biogenic counts:

A) An image of one of my jarred samples. B) The microsplitter used to split samples. Notice that the sample being poured in is split between the two cups on either side.

First, I take the jarred sediment and split the sample using a micro-splitter. A micro-splitter is a tiny contraption that equally ‘splits’ the sediment into two holders. Because each sample contains tens, maybe even hundreds of thousands of particles, there’s no way we could count all of that! So instead, splitting the sample down to a reasonable number of particles allows us to more accurately and quickly count the number of particles in each sample, which we can then use to get a percent of each ‘thing’ (e.g., benthic foraminifera, fragment, echinoderm piece) in each sample.

Generally, I try to split the sample until about 300 particles remain in one of the cups. This can take splitting the sample anywhere between 3-9 times, depending on how much sediment is in each sample to begin with. Once I have the ~300 particles, I then sprinkle them evenly onto a picking tray (a metal tray with a grid on it). I then count the number of each ‘thing’ on the picking tray. I keep count of each ‘thing’ using a counter, which makes the process very fast and easy!

An image of my picking tray with the sample sprinkled on it. Some of the major components, or ‘things’, in the sediment are labeled. Most of them are planktic foraminifera, which can be very small or larger. There are a few benthic foraminifera, several fragments, and only one piece of an echinoderm spine. Generally, planktic foraminifera are most common in these samples.

Once I have this information, I then put them into a spreadsheet to plot the data. One thing I haven’t mentioned yet is, why we do this and gather the biogenic count data. It’s actually very useful! We can use the percentages of each ‘thing’ in the sediment to calculate the ratio of planktic to benthic foraminifera. This tells us something about dissolution, or if the bottom waters were corrosive and dissolved the fossils, as benthic foraminifera are a bit more resistant to this corrosion than planktic foraminifera. I also calculate the planktic fragmentation index, which is another ratio which also indicates dissolution (the more dissolved a foraminifera is, the easier it is to fragment).

Thus, the biogenic count data is a quick but extremely useful method to determine the percent of each ‘thing’ in a sample, which can be used to infer something about the corrosive nature of bottom waters, which in turn can tell us something about ocean circulation from millions of years ago!




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