Party Trick #565: Making an Ice Age Out of Pore Water

Greetings, friends!

We are presently in transit from our last couple sites to the west of Manus Island to our first of three planned sites along an area of the seafloor known as the Euaripik (rhymes with "terrific") Rise. This is a ridge along the seafloor that acts as a local high in topography; that is, it's sort of like an underwater mountain range. Still, its highest point is well over a mile beneath the sea surface!

Part of what I'll be doing at these next few sites is collecting pore water from the sediments to try to learn about how salty ocean was during the last ice age, or glacial period. How exactly does this work? First, think about all of the water that exists at Earth's surface: the water in the ocean's, underground, in rivers and lakes, and in the air around you. That's a lot of water, right? True, and it's also true that things like evaporation and precipitation can move this water around on Earth's surface. Most of water that falls out of the sky as precipitation actually evaporated from the ocean, but evaporation removes only the water from the ocean; the salts are left behind. This will become extremely important in a minute.

Twenty thousand years ago, parts of North America (including Canada, Wisconsin, and northern Illinois) and Europe were covered by a sheet of ice over a mile in thickness. Yeah, and you thought Midwest winters are brutal today! Anyways, the water in these ice sheets came from precipitation, most of which came from the ocean. But if a bunch of water that is in the ocean now was on land then, and the amount of water at Earth's surface was about the same, that means that there must have been less water in the ocean. And if there was less water in the ocean, that means the ocean must have been - you guessed it - saltier.

Now, we know from the above argument that the ocean as a whole must have been saltier at the end of the last ice age twenty thousand years ago; research by geologists suggests that sea level was actually over 300 feet lower then than it is today because so much water was taken out of the ocean and put on land as ice!* But exactly how much saltier is still unknown, and figuring out how much saltier different parts of the ocean were is really important for us to understand how the ocean and atmosphere may have transported heat differently during the Ice Age. This is in turn crucial for us to understand how and why ice ages start and end in the first place.

It turns out that looking at pore water from ocean sediments can help us tell how salty a given part of the ocean was during the last ice age. All water in ocean sediments ultimately starts as ocean water, meaning that if the ocean was saltier at a given time, the pore water in the shallowest sediments would also initially be saltier. This pore water and its salts get buried under more and more sediment over time. Although many of the salts in pore water participate in reactions in the sediment that change their concentration (or abundance), chloride (one of the ions that forms table salt, the most abundant salt in the ocean) does not. This means that as long as the pore water itself does not react with the sediment, pore water that starts off being saltier remains saltier as it is buried. And if we know how fast the sediment is being buried (that is, the sedimentation rate) and can measure the concentration of chloride (plus a few other things in order to make some minor corrections related to water temperature) - Presto! - we can tell how salty the ocean was during the last ice age!  

Whew! Hopefully you've made it to this point and are still awake. Now you can impress your friends and win over your enemies with your impressive knowledge of the salty glacial ocean** at cocktail parties. 

More cool party tricks next time!

- Dan

*This sea level drop was high enough to expose a strip of land that connected Alaska to Russia and would have allowed one to walk to Russia from Alaska and back....but shhh, let's not give Putin any ideas...

** "Salty Glacial Ocean" may or may not get confused for the name of a cocktail. Use with care.