Looking for life in the deep Earth
Nearly a mile underground in the Northeast part of England, there are crystals and life, so that’s where I went.
I was part of a NASA Jet Propulsion Lab team to go deep underground in search of bacteria, and if found, determine if they are alive or dead. If they are viable, then what is the mineral-microbe interaction? What role do minerals have in life preservation? Also, from where did these halobacteria come? Were they part of the original salt rocks, did they come from somewhere else, did humans bring them from the surface? Lots and lots of questions that need answers; but they are all important because they will help with search for life outside of Earth. To do this work, I went to the Boulby Mine with the NASA-JPL Origins and Habitability Lab.
The Boulby Mine is the deepest mine in all of the United Kingdom, and at approximately 1.4 km (0.9 miles) deep, polyhalite ore is being extracted. Several decades ago, this massive salt mine was a significant producer of potash fertilizer for agriculture. Unfortunately, the potash ore was a mix of sylvite (potassium chloride, the potash) and halite (sodium chloride, bad for crops), and this ore had to be processed to remove the halite. Sylvite was shipped to the farms, and the halite was applied as road salt. The discovery of polyhalite (polyhalite means many salts) meant the mine could be much more economic. Polyhalite is a calcium, magnesium, potassium sulfate mineral that has all the beneficial ingredients for plants, and therefore much more functional and economical for farmers to use. It doesn’t require any chemical treatment or post-processing, and so can be taken directly to the farm after crushing. Polyhalite is found in the deepest parts of the Boulby mine. Above this is a thick halite deposit, and above that is the potash deposit.
These salt deposits are sediments that were laid down near the end of the Permian period, about 250 million years ago, at the bottom of what was called the Zechstein Sea. The sea was vast. How big was it? The sediments from the sea stretch from modern-day Eastern England to Belarus, and from Southern Norway to the Czech Republic. As the Earth’s tectonic plates shifted, the sea eventually became closed off from the rest of the world’s ocean water, and it slowly evaporated. As the water evaporated, the water became saltier and saltier, and eventually became completely saturated with salt. Crystals began to form and sink to the bottom of the sea. This process continued over many millions of years, producing an enormously thick layer of salts. Depending on the chemistry of the seawater, you get different types of salts, and the rocks under England and the North Sea have recorded these different mineralization sequences. These salts are primarily polyhalite, halite, sylvite (but there are others), and all the salt layers are capped by shale and sandstone.
As the salt crystals are forming, tiny bits of fluids get trapped inside (or included). These fluid inclusions, and whatever is in the water at the time of entombment, are preserved until the crystals are dissolved again. Studying the trapped liquid is extremely important, because they serve as a record of the water chemistry at the time of formation, and therefore, what the environment was like while the water was evaporating 250 million years ago. Also, if there was anything in the water at the time of encapsulation, it would be trapped in the crystal as well. You probably won’t find fish or bugs in the fluid inclusions because these tiny pockets of trapped brine are tens to hundreds of micrometers. Also, when the water is at this level of salinity, not much can live in it. On Earth today, we know that halophiles are about the only things that can thrive in such salty environments (like Searles Lake in California), and these halobacteria are what we are looking for in the deep-time rocks under England.
It’s important to look at life in minerals, and life around the minerals in these very briny and salty environments, because this is what we think might exist on other planets and moons. So by examining what’s happening here in this mine, we can then extrapolate that information to other parts of the solar system.
Since we have only come back from the field (trip dates were March 1st through 9th, 2020), we are still analyzing the minerals and brine fluids that we collected. Several brine pools were sampled, some natural and some man-made using salt from the mine. Using a digital holographic microscope named Shamu, we were astonished to find that all these waters contained halophiles. Even more surprising, the halophiles became much more active when they were exposed to light. For a deep earth halobacteria to be actively doing photosynthesis (at least seemingly) is quite remarkable. The next step is to sequence the DNA and compare them to halophiles that we know today.
Lots of unusual minerals are located in the mine and within fluid inclusions. The mine itself is well known to have a huge quantities of sylvite, halite, and polyhalite. However, there are nodules of boracite around (being a hard mineral wreaks havoc on the mining tools) the potash areas of the mine. And even stranger minerals such as higardite and volkovskite occurances are scattered through the mine.
Aaron Celestian is Curator of Mineral Sciences at the Natural History Museum of Los Angeles, Affiliate Research Scientist at NASA Jet Propulsion Lab, and member of the Origins and Habitability Lab. He researches how minerals interact with their environments and with living things, and how those minerals can be used to solve problems like climate change, pollution, and disease.
Photograph Credits: Title photo by the Luleå University rover team (used with permission) who were down in the mine with me, and all other photographs were taken by me unless noted in the caption.
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