Don't Drink the Pore Water & Other Musings

Hi everyone,

We are still a couple days away from arriving at our next site off the northern coast of Papua New Guinea, which means more time for me to write. Woohoo! Things have been pretty nice for the past couple days. The paleontologists on the ship held an open house of sorts a couple evenings ago during which everyone else on the ship could come view some of the foraminfera and other tiny fossils that they have been looking at in the samples we've been bringing up to help determine the age of the sediments. It was great to have a chance to look at these fossils after having heard so much about them! Alas, my job on the expedition is not to look at fossils, but instead to squeeze water out of some sediment samples and analyze the chemistry of the water.


Wait a second, squeezing water out of sediments? Where does that water come from and why is it interesting? Imagine you are walking around on the seafloor and are able to pick up some sediment from the seafloor. It turns out that no matter what size of grains are in that sediment (e.g., sand or clay), the majority of what you would pick up would actually be just water - up to 70 or 80 percent water by volume! When sediment grains settle out on the seafloor, they do not do so in a very ordered manner, so there actually ends up being a lot of empty space between grains that gets occupied by water. This water is known as "pore water", as it occupies the "pore space" between grains in the sediment. The amount of pore space decreases as a given layer of sediment gets buried deeper and deeper by more sediments above, since the increased pressure added by the weight of the above sediments forces the grains to fit more closely together***. However, even sediments buried thousands of feet below the seafloor can be more than 20 or 30 percent water by volume, so there's always a bit of water that can be squeezed out.

To squeeze water out of sediment samples, we use a machine called a hydraulic press. When we place a sediment sample inside a squeezer assembly (basically, a metal cylinder with a hole at the bottom through which water can exit and a piston on top that presses down on the sediment), we use the force applied by this press to compact the sediments even more than they were beforehand and collect the pore water than exits as the amount of pore space decreases. The press can apply up to 30,000 ft lbs of force, or roughly 10,000 psi of pressure in our case. For comparison, the air pressure applied to keep a tire on your car inflated is typically ~30 psi. It's sort of like using a giant juicer where the sediment is your fruit and the water is your juice. Just don't drink the pore water, it's salty and not very nutritious for any living things larger than bacteria.

It probably seems a little silly to apply so much force and energy just to squeeze a bit of water out of some sediments. After all, there's a whole ocean of water above the sediments that we could sample without squeezing, right? However, the concentrations of various dissolved salts and nutrients within the sediments can tell us a lot about things that are going on within the sediments themselves. For example, there are a lot of microorganisms below the seafloor that live by eating the remains of dead organisms in the sediments and "breathing" with things like iron, nitrate, and sulfate instead of oxygen. Some of these chemical reactions in the sediments can affect the fossil material we're interested in using to tell us about climate change in Earth's past, so we gain a lot of important information on how much change the chemistry of the fossils has undergone by looking at the chemistry of the pore water.

Pore water can also tell us about the ice sheets that existed in North America and Europe around 20,000 years ago and why they may have disappeared. But more on that next time!

Staying afloat,
Dan

***You can actually demonstrate this the next time you go to the beach. If you step on wet sand, notice how a bit of water comes out of the sand as you put your weight on the sand to make a footprint. This water used to occupy empty space between the sand grains, and the compaction of the grains is what allows your footprint to remain intact once you step away!