Fluids and Pressure Solution in Rocks

Fluids are ubiquitous throughout the Earth’s crust. The majority resides within the pore space of sediment and sedimentary rocks. Naturally these fluids play a role in how these sedimentary rocks behave under greater and greater pressure. The most obvious example of this is the role the pore pressure can play in changing the observed stress on the rock. However, the role fluid chemistry plays in sedimentary compaction cannot be disregarded either. One prominent example is the phenomenon known as pressure solution observed in sedimentary rocks.

Pressure solution is a mechanism of compaction in sediment and sedimentary rocks. It involves the dissolution of grains in contact and under pressure. This is followed by the transport of the dissolved material away from the contacted surface, then precipitation in a lower stress region. This all leads to a slow filling of the pore space of the rock. Generally the chemical dissolution of the rock is not significant, as the fluids are in chemical equilibrium with the rock. Increased overburden stress with greater burial makes dissolution where the grains are in contact chemically favorable. As pressure increases, the grain contacts continue to dissolve until they become sutured together or even break down. This is even more significant if the minerals (ex. calcite) in contact are already fairly soluble in the presence of fluids.

Figure 1: Two proposed models for how pressure solution leads to increased compaction in sedimentary rocks.
(Zhang and Spiers, 2005)

Both microscopic and macroscopic evidence for pressure solution and its role in sediment compaction exist in nature. Under a microscope sutured and truncated grain contacts resulting from pressure solution are often observed in sedimentary rocks (Figure 2). On a larger scale, features such as styolites, created from continuous deformation via pressure solution, can be observed prominently in some carbonate formations.

Figure 2: Examples of sutured grains in carbonate grains.
From Imperial College Rock Library (https://wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewglossrecord.php?Term=pressure%20solution)

Figure 3: Styolites observed in a carbonate formation
From blogs.agu.org

But why is this important? Sedimentary rocks house a large amount of fluids that are highly relevant to humans, most notably groundwater and hydrocarbons. Human activities can change the fluid content of a given sedimentary formation, whether it be from disposing of wastewater or the extraction of petroleum. The injection of non-equilibrated water into chemically active formations can have disastrous results due to the disregard for chemical deformation. In extreme cases, you can see significant subsurface subsidence and fiscal costs. When thinking about how a sedimentary rock behaves, how it dissolves can be just as important as how it breaks.

How do we capture a ghost-like particle--geoneutrino

If I tell you that there are billions of particles passing through your body every second, what kind of particle do you think they would be? The answer is neutrinos. These nearly massless and uncharged particles travel at a speed closes to light. Because they can only interact through weak nuclear force, matters are virtually transparent to them. Geoneutrinos are electron antineutrinos, who are produced through beta-minus decay of radionuclides inside the Earth:

Figure 1. Beta-minus decay process (McDonough et al.) 

They can travel through the Earth unimpededly from their origins, and carry integrated information about the abundances of the radioactive sources, which will provide insights of the power drives mantle convection, plate tectonics and geo-dynamo. So, how do we catch these ghost-like messengers? For decades, geologists and physicists are working side by side to solve this problem. In 2005, the KamLAND team first reported the detection of geoneutrino (Araki et al. 2005).

Currently, there are five detectors of neutrinos, which are KamLAND (Japan), Borexino (Italy), SNO+ (Canada), JUNO and Jinping (both in China). ( McDonough et al. 2014) Geoneutrinos are detected by these giant underground scintillation detectors via inverse beta decay mechanism (IBD). IBD process involves an electron antineutrino enters detector, and interacts with a hydrogen atom (a free proton) in the hydrocarbon scintillator, producing a positron and a neutron:

Figure 2. Inverse beta decay process (McDonough et al.) 

This reaction requires the incoming geoneutrinos carry at least 1.806 MeV energies to initiate the process. There are only four isotopes that can emit geoneutrinos with sufficient energies to trigger the IBD detection inside our detectors, which are ²²⁸Ac and ²¹²Bi in the ²³²Th decay chain and ²³⁴Pa and ²¹⁴Bi in the ²³⁸U decay chain.

As IBD activated, there will be two flashes of light occurs inside the liquid scintillation detector: the prompt flash is from the positron (product of IBD) and electron annihilation; and the second flash comes from the neutron (product of IBD) captured by a proton producing deuterium and generating 2.2 MeV of light. These two flashes are highly synchronized (only 200 microseconds away) and only accessible to geoneutrinos, which effectively eliminates most backgrounds. (Dye et al., 2012)

Figure 3. Structure inside detector (Chen et al., 2014)

Geoneutrino study is expecting a new era of detection ability since a new deterctor JUNO, which is about twenty times larger than KamLAND, will come online. With more data available, maybe in the coming decades we can better constrain the abundance of the heat producing elements like U and Th in the Earth. 

Fossil Fuels: How many T. Rex does it take to power a car?

It was once a common misconception that fossil fuels (also known as hydrocarbons or simply oil, gas, and coal) are the remains of dinosaurs, such as Tyrannosaurus Rex.  In fact, this belief was so prominent in the early oil and gas industry, that Sinclair Oil featured a green Brontosaurus as its mascot.  However, we now know that nearly all fossil fuels are remnants of bacteria and algae.  In order to produce the amounts of oil and gas required by our daily lives, these small organisms must have been present in significant quantities and were much more prominent than dinosaurs.   

The Brontosaurus used as Sinclair Oil's mascot during the 1960s. Image from American Oil and Gas Historical Society.

So where did these organisms live and how did they become extractable fossil fuels? Well, let's focus on the fossil fuels we use daily, oil and natural gas. Most of the organic material (organisms containing carbon) that became oil and gas were once bacteria and algae in the oceans, lakes, rives, and lagoons of primeval Earth.  These organisms were deposited on the bottoms of bodies of water when they died.  For the organic matter to become oil, they must first be preserved. Preservation occurs where there is a high productivity rate (i.e., organic matter being deposited quickly) and anoxic conditions (i.e., minimal oxygen).  If one of these conditions isn’t right, there either won’t be enough organic matter to produce oil or other living organisms will decompose the organic matter.  If both of these conditions are met and the organic matter is preserved, it can then be buried by the sediments that are deposited over them. As the organic matter is buried, the temperature of the surrounding rock increases, due to heating from Earth's interior, baking the organic matter at 80-150°C for millions of years.  The baking of these sediments is known as generation, during which the combination of temperature and time converts the organic matter into oil and natural gas.

Cross section of a petroleum system modified from Magoon and Dow (1994).  Red arrows indicate heating from below.  Blue arrows show migration of hydrocarbons through the reservoir. Dashed lined mark the extent of the oil trap. 

After the organic rich sediments have been converted into oil, they are now a potential oil deposit.  In order to become extractable, the oil must first migrate. Migration is the process where hydrocarbons, such as oil, move from the source rock, into a permeable and porous reservoir (e.g., sandstone).  After migrating the hydrocarbons may be entrapped, which is the process where hydrocarbons are trapped in a specific area due to that areas geometry.  Common hydrocarbon traps include folds, uplifts, and faults. In order for the hydrocarbons to stay in the trap an impermeable layer, or seal, is required to prevent fluid flow and loss of hydrocarbons.  At this point the organism has become oil and is in a prime position be extracted and ultimately power your car.  So how many T. Rex does it take to power your car? Well, a live one might be able to pull it along, but their remains won’t be able to keep it running.

A Solar Cell Solution?

Climate change has become an increasingly large concern among scientists and the general population. Many scientists agree that this crisis must be addressed and fixed within the next half century or it may become an irreversible process. Billions of dollars have been spent to address this issue and develop alternative energy sources. While no energy is entirely 'clean', solar energy provides the most environmentally friendly alternative energy.

However, the primary concern is that up to this point, solar cell technology has failed to be implemented on the large scale required to make any sort of appreciable environmental impact. According to the Institute for Energy Research, solar energy makes up less than 1% of the total energy consumption in the U.S. In order to ameliorate the burden we are placing on the Earth's atmosphere, it is imperative that we find a way to increase the amount of solar energy produced. To do this, there needs to be a viable option, in terms of cost and efficiency. The good news is we might have developed a viable option: Copper-Indium-Gallium-Selenide thin-filmed solar cells (CIGS).

Figure 1. Flexible CIGS solar cells are lightweight and easy to install.

Traditional silicon solar cells have dominated the photovoltaic market in the past and currently controls ~90% of the current market, and has a relatively high efficiency of around 25%, according to the National Renewable Energy Laboratory (NREL). So what's the drawback? In the words of Mark Pinto, Applied Material's executive vice president for energy and environmental solutions, "with solar, it's all about cost."

Cost is the primary reason why silicon solar cells has failed. Their high cost has rendered them an ineffectual option for a major alternative energy source. Cost is also an issue with the most efficient solar cells, such as the multi-junction solar cells, which offer the 46% efficiency (via NREL). Because the cost of these solar cells are so high, there is little to no incentive for producers to produce these cells in large quantities, thus solar energy use in the United States is almost negligible. The failure of the silicon solar cell comes at the cost of the Earth's environment.

CIGS is very different from the silicon cells. They belong to a group of solar cells, which utilize new thin-film technology. One advantage of these thin-film cells is that they are flexible and are thus easier to install. Oppositely, silicon solar cells are rigid and require a much stronger substrate (i.e. roof) to attach it to. Additionally, CIGS is significantly cheaper to produce mostly because it can be produced in a one step process.

Figure 2. Thin-film solar cells are diverse due to their flexibility. Here they are shown laid over roof shingles via Paul Martin.

Previously, the primary concern with CIGS was its relatively low efficiency. However, recent advances in solar technology has increased the efficiency of CIGS up to 16-23% (PVinsights and NREL), which is remarkably close to the silicon solar cells. As a result of the relatively low cost of CIGS and its steadily increasing efficiency, Brad Mattson, CEO of Siva Power explains that CIGS offers the "holy grail in solar: silicon-like efficiencies at low thin-film costs". Therefore, CIGS offers what no other solar cell currently offers.

It is imperative that we continue to develop CIGS technology in order to implement it on a large scale. By doing so, we will be able reduce environmentally harmful emissions and, consequently, start to improve our current climate crisis. With steady increases in CIGS efficiency and cost over the past decade, this solar cell solution may become a reality sooner than we think.

“Listening” to the Landscape: The Essence of Environmental Seismology

An Overview

Environmental seismology is a fairly new branch of geology, but its already improving our understanding of landscape-shaping processes. Let's take a look at the advantages (and disadvantages) of this promising field. I'll focus on recent applications in glaciers, rivers, and landslides.

What is Environmental Seismology, Anyways?

Environmental seismology focuses on historically unwanted surface-generated vibrations. For the past century, seismologists have primarily focused on earthquakes produced by faults deep within earth, ignoring smaller vibrations produced at earth’s surface as unwanted background noise. However, recent advances in seismometer portability, sensitivity, and computation power are changing that.

A Sample of Recent Applications

Glaciers: By deploying seismometers onto glacial ice sheets, scientists are able observe glacial processes in remote areas or deep under the ice. For instance, glacial “earthquakes” are felt when icebergs break off and go into the ocean with their heavy side up. As they flip over to put the heavy side down, they can crash into the ice sheet, generating an amount of energy equivalent to up to two Hiroshima atomic bombs.  The energy released produces magnitude 4-5 “earthquakes” that are detectable from thousands of miles away, but sound different than classic earthquakes caused by faulting.  By counting these events and estimating their size, scientists are working to estimate the amount of ice going into the ocean from ice sheets on land (Fig. 1). This information on the rate of ice loss is needed for predictive climate models and estimating sea level rise.

Figure 1: Glacial earthquakes in Greenland increased during the 2000 - 2005 observation period. This could represent an increase in ice loss from Greenland into the ocean. Figure from Ekström, Nettles, and Tsai (2006).

Rivers: River processes can be observed without ever getting wet by using near-river seismometers. Information from river-produced vibrations can also expand our understanding of how a river wears down the landscape. One of the most effective river erosion processes is thought to be the damage to bedrock done by large bedload cobbles tumbling along the bottom of a river. The rate of erosion is related to the amount of bedload. However, measuring the amount of bedload is difficult because it happens infrequently (a few times a year) and during large floods (when getting in a river is downright dangerous), so an easier and safer way to measure bedload is needed. With near-stream seismic monitoring, several researchers have been able identify bedload movement events by seeing the rumbling vibrations produced by the cobbles hitting the bottom of the river. These vibrations are different than what is produced by the water by itself, and researchers are working to try to relate the vibrations to the total amount of bedload. This research has taught us that most bedload is transported before the peak of a flood, helping to pinpoint when the most river erosion occurs (Fig. 2).

Figure 2: A) Seismic river observation includes both turbulent flow and bedload noise. B) Bedload transport noise is observed mostly before the peak of a flood event, suggesting river erosion is more effective during this period. Figure from Larose et al. (2015).

           Landslides: Landslides and related gravity-driven phenomena (mudslides, rockfalls, debris flows, avalanches, etc.) are hazardous and significant landscape-shaping processes that are best observed from a distance. By measuring the “sound profile” of a landslide using seismometers, scientists are working to estimate the total amount of material moved by the landslide event (Fig. 3). Emergency response agencies are interested in using this sound profile information for developing better real-time response alerts in remote areas. A widely-spread seismometer network can also be used to determine important characteristics of landslides in remote areas, including the speed, frequency of landsliding, and even the landslide trigger mechanism.

Figure 3: Example “sound profiles” for different sources of seismic vibrations. The left figures show the seismometer response to each event. The right figures show the frequency distribution of the events. Red on these figures is louder sounds. Note that the figures have different time domains (x axis is changing). Studying these "sound profiles" will improve natural disaster alerts in remote areas. Figure from Burtin, Hovius, and Turowski (2016).

Evaluation of Environmental Seismology Methods

Advantages:

• Continuous Measurement: No waiting around for infrequent but significant events like iceberg calving, floods, or landslides.

• Non-Contact: Keeps scientists out of harm’s way while studying hazards

• Difficult Processes Revealed: Listening to vibrations gives new insights into the physics of landscape processes.

Disadvantages:

• Potential Interference: Road noise, precipitation, and unwanted vibrations sometimes make it difficult to focus on the process of interest, especially if it is a quiet process.

• Seismic Properties Required: Knowing about the seismic properties of the  soil, rock, or ice through which the vibrations is required for this type of study, but this information is difficult to obtain.

• Non-Localized Measurement: Vibrations travel a long way, so each seismometer can be observing close by and far away processes at the same time, leading to confusion in the interpretation of signals

Looking Ahead

Environmental Seismology has already produced insights into landscape-shaping processes, and is likely to become a larger field as seismometers become cheaper, more sensitive, and easier to use. Moving forward, the field needs additional research into how each process physically produces vibrations. It also requires calibration with traditional measurement techniques. With this additional research the field will continue to develop our understanding of landscape-shaping processes.

References

Burtin, A., Hovius, N., Turowski, J. ,2016, Seismic monitoring of torrential and fluvial processes: Earth Surface Dynamics, v. 4, p. 285-307. http://doi.org/10.5194/esurf-4-285-2016

Ekström, G., Nettles, M., and Tsai, V. C., 2006, Seasonality and increasing frequency of Greenland glacial earthquakes: Science v. 311, p. 1756–1758. doi:10.1126/science.1122112

Larose, E., Carrière, S., Voisin, C., Bottelin, P., Baillet, L., Guéguen, P., Walter, F., Jongmans, D., Guillier, B., Garambois, S., Gimbert, F., Massey, C., 2015, Environmental seismology: What can we learn on earth surface processes with ambient noise?: Journal of Applied Geophysics, v. 116, p. 62-74, http://dx.doi.org/10.1016/j.jappgeo.2015.02.001.

Ambient Seismic Noise: How to Image Earth Interior without Earthquakes?

Seismology is a powerful tool to image Earth interior structure because seismic waves can travel through Earth at different depth and carry valuable information about Earth’s  structure. However, earthquakes usually happen at active tectonic regions like plate boundaries which is a big limit to our study regions. If we want to study a seismically quiet region, we have to use teleseismic waves which will lose much high frequency information because they are easily attenuated. Is there a way to study Earth interior structure without earthquakes?  Yes, it is ambient seismic noise!

Figure 1. Ubiquitous ambient noise recorded by two stations (R. Weaver, 2005) .

Ambient seismic noise is ubiquitous ground motions excited by human activities, oceanic micro-seismicity or wind. Ambient seismic noise is usually thought to be useless in traditional seismology studies. However, recent studies prove that surface waves and body waves can both be extracted from ambient noise by calculating their cross-correlations, which means we can also use noise to study Earth’s  structure. When the ambient noise travels through two different stations, the seismograms of these two stations will have certain amount of coherence. The coherence can be retrieved by calculating the cross-correlations of these two seismograms. The waveform of the cross-correlations are similar to that of earthquakes because it also contains different body wave phases and surface waves. Ambient noise is independent from earthquakes so we can use seismograms recorded at any station, at any time. It can largely increase our data source.

 Figure 2. Surface wave and body wave extracted from ambient noise (Prieto et al., 2012).  

Ambient seismic noise is a new tool to study Earth’s structure at different depth. Surface wave is the most obvious waves extracted from ambient noise and it can be used for high resolution tomography for crust. Moreover, some core phases like PKIKP² can also be extracted so we can use them to study the anisotropy of Earth’s core. This study used to be limited by the location of earthquakes and stations because it requires to sample Earth from different azimuth. It only requires to have stations at different latitude with ambient noise method. Ambient noise is also popular at oil industry because the companies can save huge amount of money without making any explosions for oil exploration. Ambient noise is a huge supplement to traditional seismology study because it breaks the time and space limits of earthquakes and offers a more flexible way to image Earth interior structure.

Figure 3. Surface wave tomography from ambient noise (Shapiro et al., 2005). 

Recipe for making a Solar System

Consider the process of baking cupcakes…you mix together sugar, butter, eggs, and flour. You stir thoroughly to mix the batter well. The batter then goes into separate cupcake tins and put in the oven. In many ways, the process of forming our Solar System is similar to baking cupcakes. However, we do not know the recipe; we can only infer it by studying the cupcakes – well, actually the crumbs.

Imagine if planets were cupcakes...what was the batter like that formed them?

http://hubpages.com/food/Fun-Cupcake-Recipe-Make-Solar-System-Cupcakes

 For planetary geologists, the crumbs are meteorites – fragments of planetary objects (cupcakes) that were made from planetary “batter.” However, not all meteorites are the same. For example, some have more flour, while others have more egg. This raises many questions. Was the batter well mixed? Was the batter divided and further modified? Were some cupcakes made and then additional ingredients, like chocolate, were added to the remaining batter? These differences in meteorites are referred to as nucleosynthetic anomalies.

             Planetary geologists study nucleosynthetic anomalies by measuring the isotopes of different elements in a wide range of meteorites. Isotopes can provide information about the type of ingredients used, or if multiple brands of the same ingredient were used. They can also narrow down the order in which the ingredients were added. Through additional isotopic studies of meteorites, the recipe for our Solar System will hopefully eventually be constrained.  

The steps to form our Solar System. Step A represents the “batter” and steps B and C represent mixing. In step D, the “cupcakes” have formed. What is in the “batter” in step A? And how well was it mixed?

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