Act Your Age (or, What’s in a Number?)

This entry is written by Debbie Thomas, co-chief scientist of Expedition 378. It comes from her Expedition 378 Odyssey blog, which can be found here.

How do we determine the age of the sediment layers that we recover in deep-sea drill cores? And another: Why do we need to know the age of the layers of sediment in the cores we recover?

I’ll answer in reverse order – in the sequence of sediments that we are recovering at Site U1553, we know that the age of the sediments gets older as we drill deeper down through the layers because tectonic processes have not disrupted the vertical nature of the sediments. So as we drill, we know that we will encounter progressively older sediments, and thus we are confident that the events we recover deeper in a hole happened before the events that are recovered at shallower burial depths. But, how do we know if what we recover at Site U1553 is the same as an event known from another location in the ocean basins? How do we know how rapidly (or slowly) the sediments accumulated? In order to determine dates and rates, we must be able to assign numeric ages to the depths of our sediment cores. And this is far more challenging than you might imagine!

To determine the initial age of the sediment layers on board the JR, we staff the science party with experts in the disciplines of Micropaleontology and Paleomagnetism. In this first post on dating (geologic ages, not dinner and movies…), which may be the 5th post in the So Now What? series, I will focus on micropaleontology and the resulting biostratigraphy that it provides us to help constrain geologic ages. Stay tuned for a future installment on Paleomagic, er, I mean Paleomagnetism.

Micropaleontologists study the composition of the very small (you know, micro…) fossils that occur in the sediment samples. The evolutionary record of plankton (both zooplankton and phytoplankton) has left us a rich record of the geologic timing of first appearances (origination) and last occurrences (extinction) of species of many different taxonomic groups. Don’t worry, I will provide cool photos to illustrate!

Let’s start with the phytoplankton since they tend to command the spotlight – Calcareous nannofossils. Isabella and Rosie (with guest appearances from our Phys Propper, Heather) examine the calcaerous nannofossils, or nannos as we refer to them. As you might have guessed from their name, these single-celled photosynthetic organisms leave behind fossils that are made of calcium carbonate (CaCO3). Team Nanno examines the assemblage of the species in each sample by using a toothpick to smear a small amount of sediment on a microscope slide (the highly technical term for this is a toothpick sample):

A shot of a scientist's hands over a lab bench. She is leaning over a sediment-filled bowl with a slide in one hand. She is smearing a tiny amount of light grey sediment from a toothpick onto the slide.
Hand model, Rosie, prepares a smear slide from the core catcher sample in the micropaleo cereal bowl.

Rosie’s fingers always figure prominently in the posts, so here she is in her entirety collecting the cereal bowl of core catcher sample on the catwalk:

   A scientist looking at the camera and smiling brightly. She is wearing a hard hat and safety glasses, and she is holding a sediment sample in a ceramic bowl with both hands.
The smile says it all 🙂

A corridor in a lab. There are lab benches lining the walls, and each station has a microscope and a swiveling chair. There are two scientists in the back of the room - one is smiling at the camera and one is looking down. There is another scientist in the foreground looking into a microscope.
Isabella and Rosie in the back of the micropaleo lab discussing the nanno results or goofing off. Impossible to tell from this angle. Flavia in the foreground is desperately searching for planktonic forams. Keep reading.

They then use both plain and polarized light to identify the nanno species in the slides (the image below is polarized light to catch the cool optical properties of CaCO3):

Several nannofossils against a black background. They are mostly round and surrounded by smaller grains of sediment. The scale bar at the bottom shows each grain to be about 50 micrometers in diameter.

And this image is in plain light in which the CaCO3 appears clear (note that this image is not of the same sample as above):

A microscope image against a tan background. The sediment grains appear as translucent shapes on the slide. There are a few grains that show up more prominently and look like 6-sided stars.

Shipboard scanning electron microscope (SEM) images of these amazing little fossils reveal even cooler details about their structure:

A large assortment of nannofossils against a dark grey background. The fossils are surrounded by smaller grains of sediment.

Another very important group of microfossils is the foraminera. Forams are single-celled zooplankton that produce CaCO3 shells, and these critters dwell at the sea-surface (planktonic forams) or on the seafloor (benthic forams). One of the most important analyses we perform back home in our laboratories involves dissolving the forams and analyzing the carbon and oxygen isotopes of the CaCO3. However, we try not to talk about dissolving the beautiful shells in front of the micropaleontologists…they need to study the intact fossil species in order to help determine the geologic age and depositional environment of our samples.

Preparing the foram samples requires a bit more time and effort than merely swiping a toothpick on a slide. The samples are washed and sieved in order to isolate the shells:

A scientist stands at a lab bench with her back to the camera. She is using a sprayer attachment on a sink to spray the sediment inside a sieve. Another person stands nearby, looking at Flavia and smiling.
Flavia sieving forams for benthic and planktonic analysis. Forams always make Lindy laugh.
A scientist sits in front of a microscope. There is a book open in front of her with detailed drawings of foraminifera. She is looking at the book and appears to be talking.
Bruna investigates the types of benthic forams in order to help us determine the water depth of Site U1553 back through geologic time. Note the black tray in the upper left portion of the microscope base. Each one of those sand-size grains is a single shell and Bruna can determine which species they are under the low-magnification scope.

Our next group of zooplankton is the Radiolaria. Rads are closely related to the planktonic foraminifera, with one key difference. The rads produce a skeleton made of silica (SiO2) and are glassy and clear instead of white. Below, Chris is isolating the rads from a core catcher sample and then we will see him at the microscope working on species identification:

A scientist stands at a chemical hood. He is holding a beaker with a small amount of liquid inside, and he is looking down at the beaker.
The same scientist at a computer. He is wearing a light green sweater. There is a picture of a radiolarian pulled up on the computer - the background of the microscope picture is the same color as his sweater!
Chris showcasing a stunning rad specimen, and his famous sweater (kept him warm on his most important geologic field campaigns!).


Ok, I’ve calmed down.

Once the micropaleontologists have assessed the assemblage of species in the samples, they work to assess their data using the established biostratigraphic framework in which the appearances and disappearances of the key species have been assigned numerical ages. This framework is the product of decades of studies at hundreds of locations, and continuously is being refined as we acquire new data and better radiometric ages of ash layers.

A large chart listing the time divisions of the Paleocene, with many scientific taxa listed for each one. Next to each scientific name, there is an up or down arrow.
Don’t panic, just zoom in toward the bottom to follow along with the text below.

Here is a quick example of how we apply this zonation scheme in real-time drilling decisions. Consider when we are drilling down through the Eocene (the second column of this chart) and approaching the Paleocene/Eocene boundary at ~56 million years ago. Typically it is Team Nanno on the lookout because they are able to make their slides so much more quickly than the other specialists. In this case, Team Nanno is on the lookout for the extinction of the Fasciculithus group (the downward facing triangle at 55.64 million years ago) and the appearance of Tribrachiatus bramletteii (the upward facing triangle at 55.86 Ma). While Team Nanno is desperately searching for these marker species, they have the added stress of one or both co-chief scientists breathing down their necks for confirmation in order to communicate the information to the drillers. Beautifully done, Rosie and Isabella!

Speaking of Isabella, our dear friend has produced a zonation scheme for the shipboard science party. Our personal appearances in the geologic record (notextinctions….) definitely span quite a range….

A chart labelled "Populi Stratigrafia: Official IODP Expedition 378 Time Scale." It is styled to look like a geologic time scale, but at closer inspection it has people's names on it. There are 3 eras: Immaturozoic, Maturozoic, and Curmudgeonozoic. The epochs in ascending age order are Puerilecene, Primocene, Oldishcene, and Wrinklescene. The stages in ascending order of age are Prepubescentian, Greenian, Ripean, Over-ripean, Rottenian, and Viagrian. There are many names listed in each stage. The names have microfossil names next to them and a year in parentheses.
Fortunately our ages are just numbers. Geologic ages are so much more. Grazie, cara Isabella!!

Until the next time,

Over-ripean DT (sigh)

A fuzzy green plushie, Little Cthulhu, being held over a microscope's eyepiece by a scientist, Ulla Röhl. Ulla appears to be talking. There is nothing under the microscope and it does not appear to be on.
Good grief, LC. You can’t see any forams because there isn’t even a sample under your scope.

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