Seawater circulation and ocean composition

One of the reasons geochemists like myself are interested in studying North Pond is the tremendous amount of seawater that is being flushed through rocks in these places. Imagine that at any given time 1-2% of the ocean water is actually below the seafloor.  That water flows because there is a lot of heat in the deeper seafloor and it interacts with rocks it encounters along its flow path. Very slowly, the composition of the flowing water will change, and when it finally exits the seafloor, it has a composition that is somewhat different from the composition of the rest of the ocean. We call this process "seawater circulation" and believe that it plays an important role in explaining the composition of seawater. 

Everyone knows that seawater is salty. Besides the common rock salt (sodium chloride) there are other elements, like magnesium, calcium, and potassium dissolved in seawater. These elements are washed into the oceans by rivers and they are removed from seawater when minerals form within the oceans. There is a delicate balance between these sources and sinks. If we manage to understand that balance, we can explain the composition of the oceans today, understand changes in the ancient past and perhaps predict future changes. Seawater circulation within the seafloor is a critical part of that balance. It removes magnesium from and adds calcium to the oceans. The calcium can then combine with carbon dioxide and form limestone. This process has kept the levels of carbon dioxide in the atmosphere low throughout Earth’s evolution.  

But what does this have to do with microbes that we like to learn about in this expedition? We think that microbes make seawater and rock react faster, so they accelerate the process mentioned above. If this turns out to be true, we could show that microbial life in the deep sea is a major player in setting the composition of the oceans. To address this question, we will measure the changes in rock and water composition and relate it to how active microbes are in the subseafloor at North Pond.


water and heat flow

from Langseth et al. (1992)