When you want to study rocks, one of the main complications is to access to them! Most of the time, they are hidden by soil and plants on land and of course, by the water in oceans!
To deal with that problem, geologists have developed indirect ways to image the underlying rocks by using sound waves (called seismic waves) which are generated mechanically. The data collected during these seismic surveys provides accurate information to “see” buried features.
Many seismic surveys have been conducted in the Ross Sea, so scientists already have some idea of the geological history of that region over the last millions of years. The image above is from a US seismic survey that took place in 1990 (Courtesy of Prof. John B. Anderson, Rice University).
What is a seismic survey?
The idea is to use waves which are able to go through the water and the sediments and rock below. A sound wave is created by an “air gun” that is towed from a ship, just below the surface of the water. The acoustic waves travel down through the water and some of them travel into the layers of sediments beneath the sea floor.
When the waves meet a hard surface (the sea bed or a different type of sediment for example), some waves are reflected (which means that they come back toward the sea surface) and only some continue to travel down into the sediments and into the rocks, until they became too weak and the initial seismic energy is dissipated. On the drawing below, only the rays of reflected waves are shown for simplicity.
The reflected waves are recorded by an array of geophones attached to a cable towed by the ship. These arrays are called “streamers” and consist of long net-like bands with hydrophones (sort of like underwater microphones) spaced evenly along the streamer.
The information gathered by the geophones are then processed in order to obtain a seismic profile that can be analyzed by geologists.
Many seismic surveys have been conducted in the Ross Sea. This is an example of one of the seismic profiles obtained in 2005 under the Italian Antarctic Research Program (PNRA).
A seismic profile provides a lot of information to scientists at a regional scale. For example, they can map faults and major lithologic units, interpret depositional environments and infer the lithofacies and the ages of different units.
In the example below, we can assume that the orange layer is a sedimentary wedge that has formed below and in front of an ice sheet (represented here in blue) that was grounding on the sea floor, stripping off sediments and entrain them into the basal ice and finally drop them at the ice front.
Why are drilling expeditions so important anyway?
Because even if a seismic profile provides crucial knowledge of large-scale geological structures, it is different from a real geological cross-section in many ways:
– layers in a seismic section are imaging changes in surface reflectivity (eg density change) that are not always caused by a change of lithology, but they may be due to presence of fluid, gas, etc…
– more important, the vertical scale of the image is not the actual depth but it is time (the time corresponding to duration of the travel of the sound waves from the ship to the sediment layers and backward to the geophones)
The results of interpretation of seismic profiles need to be verified with drilling data in order to enhance the accuracy of the interpretation. What make drilling expeditions crucial is that they can provide the missing information needed to have a complete story of the area (real depth, age, rock type [lithology], etc.). But drilling is expensive and time consuming, therefore it is crucial to plan the best site locations by conducting seismic surveys before drilling. Only after having carefully studied the geometry of the strata buried below the sea floor can we confidently drill down to the target. This is also the main technique employed by industry to look for oil and gas reservoirs in hidden “sediment traps”.
“Même si les lampes s’éteignent, même si l’on me dit : il n’y a plus rien, je resterai pourtant. Il y a toujours à regarder.”
Benjamin, can you please introduce you and explain your research topics?
My name is Benjamin Keisling and I am part of the Sedimentology team onboard Expedition 374. I am a PhD Candidate in Geosciences at the University of Massachusetts Amherst in the USA.
My PhD research centers on ice sheet modelling, and is particularly focused on how we can use proxy data to constrain uncertain processes and parameters in ice sheet models.
In other words, I use geological data to test how well ice sheet model simulations represent the past. The models that best represent the available geologic data can then be used with more confidence to make future predictions of sea level rise.
It is commonly accepted that big changes in the past sea levels are linked to variations of the volume of the Antarctic ice sheet. Is that true, and in that context, how important is it to have a better knowledge of these past variations?
Big changes in past sea levels certainly reflect variations in the volume of the Antarctic ice sheet, and accurate prediction of future sea levels relies on better knowledge of these past variations.
For example, we often use the most recent warm “interglacial” period in Earth’s history (the Eemian, 125 thousand years ago) as an analogue for the present because the amount of carbon dioxide in the atmosphere was similar to preindustrial levels (around 280 parts per million). However, sea level records indicate that during the Eemian sea levels were 6–9 meters higher than they are today, which means that part of Antarctica must have melted.
But it’s very difficult from a modelling perspective to make enough of the ice sheet melt to explain those sea levels, so it means that our models are missing something, and that affects our ability to confidently model how the ice sheet will respond to warming today (when carbon dioxide levels are greater than 400 parts per million!) and into the future.
On board, sedimentologists use the lithologies of the cores to estimate advances and retreats of the ice sheet during periods warmer than today, especially during Miocene and middle Pliocene. Can you explain how these data will be helpful in your work?
We know from many records that the size of the Antarctic ice sheet has fluctuated in the past, and models do a good job of recreating some of that variability. However, we are still lacking fundamental knowledge about what caused the ice sheet to retreat during past warm periods, and the sediments we are collecting can give us insight into that.
In Antarctica today, the ice mostly loses mass at its floating ice shelves (like the Ross Ice Shelf), where icebergs calve off and warm ocean waters melt the underside of the ice. Because Antarctica is so far south, it rarely melts from the surface – it stays below freezing even during the summertime. But there is a big debate now about whether the large variations in the size of the ice sheet are triggered by melting from the ocean or melting from the atmosphere.
Through studying the sediments we are collecting on Expedition 374, we will get a better idea of what the oceanic and atmospheric conditions were like before the ice sheet retreated, and this gives us information we can use to evaluate our models. In addition, it is notoriously difficult to constrain how certain parameters in our models have changed throughout geologic time, for example, the shape of the ice-sheet bed or how slippery it is. The sediments that we are describing can give us direct insight into these important parameters, so that our models have a better shot at getting the right answers for the right reasons.
We want to know how the sea levels will evolve in the future. And Expedition 374 is looking 25 My behind us! How can having a better knowledge or our past help us to predict the future when the atmospheric CO2 concentration will increase?
Geological data provide an invaluable source of information for models, because they give us targets to test models against. We don’t know exactly what will happen in the future, but geological data can tell us what happened in the past, and right now, the future is looking more and more like the Miocene and middle Pliocene. The amount of carbon dioxide in Earth’s atmosphere hasn’t been as high as it is today since at least the middle Pliocene, and during that time the ice sheet was probably smaller and very dynamic, causing oscillations in sea level on the order of 10 meters. We need to understand what drove these oscillations and how they affected the rest of the global climate system in order to predict when and how they will happen in the future.
If you want to learn more about what is a numerical modelling of ice sheet, you can visit this webpage:
From last time, it seemed like studying Earth’s ancient magnetic field was not so complicated, right?
Oh, but maybe I omitted a few important details about how the paleomagnetists on board have to deal with some complications… Their ultimate target is to determine the age of the sediment. They do that by determining the polarity of Earth’s magnetic field: sometimes, it is normal (like today), sometimes it is reversed. The paleomagnetists measure the inclination (the dip of the magnetic field) specifically to determine the polarity of the field. They want to know whether the field goes into or comes out of Earth. In the southern hemisphere, the field comes out of Earth when the polarity is normal and the inclination is negative, and it goes into the Earth when the polarity of the field is reversed, during which the inclination is positive. Saiko and Tim are therefore always trying to figure out what the inclination of the sediment is.
However, this is not straightforward. When the cores arrive on deck and then into the hands of our paleomagnetists, there are more magnetic directions present in the core than just the one of the ancient magnetic field…
To make it simple, you can consider that there are 2 types of magnetic particles: the “good” ones and the “bad” ones. Yes, just like in a cowboy movie… The good ones are magnetically hard and are not disturbed by the small magnetic fields present during the drilling operations. The “bad” ones are magnetically soft. They align in the direction of every small magnetic field that is present during their trip from under the seafloor to the paleomagnetists’ lab.
All “soft particles” thus carry an paleomagnetic signal that is linked to the coring. This “overprint” is not useful when Tim and Saiko try to determine the direction of the ancient magnetic field. This overprint is sometimes so strong that it masks the original signal of the strong particles!
Our paleomagnetists have to remove that overprint to get to the real signal.
The problem is that most natural samples contain a combination of soft and hard magnetic particles… Our paleomagnetists want to get rid of the weak particles! How? They can’t dig out the bad particles, because then they would destroy the core and change the orientation of the good ones as well! They have work out a non-destructive method!
How does it work?
The big idea is to cancel out the overprint magnetization of the soft particles in order to only measure the magnetization of the hard ones. Remember that the soft ones are very susceptible to small magnetic fields. We can therefore change the orientation of the weak particles by applying a small magnetic field. This is similar to what happens in the drill string. Now we need to find a way to randomize the orientation of the soft particles so their combined effect is zero.
We do this by demagnetizing our samples, using an alternating magnetic field. This means that we generate a field that very quickly switches in direction whilst slowly dissipating to zero. This field is so small that it only affects the soft particles and not the hard ones. Depending on the properties of the soft particles, they align with the field at different times throughout the demagnetization.
The result is that all soft particles are now aligned in random directions so that their combined effect is zero. Fortunately, the magnetically hard particles don’t care about this small field and their magnetization does not change, which means these particles still have a record of Earth’s ancient magnetic field. So, the only signal we have left is the good one!
It is now time to analyze some measurements made by our team during Expedition 374. Please read again our first post of “Paleomagnetism for Rookies” for all details about inclination and the polarity of Earth’s magnetic field.
This is an example of how a drilling overprint influences the real signal in sediments. The blue line is the inclination before demagnetization. You can see that all inclination values are positive. Remember that we are still drilling on the South Hemisphere close to Antarctica. Does that pattern mean that this sample was deposited during a period of reversed polarity?
No. Tim and Saiko demagnetized the sample and removed the drilling overprint. You can see that the inclination is now negative. This means that the sediments were deposited during a period of normal polarity!
Conclusion: Don’t judge a book by its cover; read its contents first. Similarly, don’t interpret the magnetic polarity without first removing overprints. Only then will the secrets of the past be revealed!
Some physical properties of the cores and what they can tell us about sediments
Earth has experienced many cycles of climate change throughout its geologic history. Records of these past climates can be found in the sediments that we drill during Expedition 374. In order to learn more about past climatic conditions, we have to decipher the information recorded in the cores.
Scientists on board the JOIDES Resolution try to steal some of the secrets of the cores by measuring the physical properties of the sediments.
What are the physical properties measured on board the JOIDES Resolution? What kind of information can they provide?
Magnetic susceptibility is the degree to which a material can be magnetized in a small external magnetic field (not exceeding 0.5 mT).
Magnetic susceptibility is an indicator for changes in the composition of the sediments. Sediments can come from the erosion of rocks on the continent (terrigenous sediment such as sand and clay) and/or they can have a biogenic origin (which means that they form from the remains of once living organisms that die and fall to the seafloor, the test (shells) of plankton for example).
The range of magnetic susceptibility values gives us an idea of the proportion of terrigenous versus biogenic particles, which is a clue for their source. If the sediment is mainly composed of terrigenous particles, the magnetic susceptibility will be higher than if the sediment is mainly biogenic, when magnetic susceptibility will be close to zero.
“Gamma ray attenuation bulk density”
Bulk density is a measure of mass per unit volume (g/cm3).
This measurement uses a gamma ray radiation, which interacts with the sediment or the rock. The attenuation of gamma radiation is linked to the density of the material: the stronger the attenuation, the denser the sediment.
Changes in density are also linked to the porosity of the sediments which represents the volume of the open spaces between sediment/rock grains compared to the total volume of the rock.
A change in the bulk density is often an indicator of changes in the type of sediments (mineral composition, grain size, etc.…).
“Natural gamma radiation”
Some radioactive isotopes (uranium, thorium, and potassium) are present naturally in some sediments. Thorium and potassium are usually found in clays, whereas uranium is present in clays and organic-rich materials. Gamma rays are emitted spontaneously from atomic nuclei during radioactive decay and this natural radioactivity is recorded by the “Natural Gamma Radiation Logger” on the JOIDES Resolution.
In general, the natural radioactivity is a useful tool to measure the amount of clays in the sediments.
You are now ready to be part of the physical properties’ group on the JOIDES Resolution! These are some results that we record during Expedition 374. It is your turn to play!
Question A: Using the magnetic susceptibility, which sediments have a higher content of terrigenous particles:
sediments from period 1
sediments from period 2
both have the same content in clays
Question B: Using the record of gamma ray attenuation bulk density, we can say that the sediments from period 1 have:
higher porosity than those of period 2
the same porosity as those of period 2
lower porosity than those of period 2
Question 3: Using the natural gamma radiation, we can say
sediments of period 1 have higher clay content than those of period 2
sediments of period 2 have higher clay content than those of period 1.
sediments of period 1 have the same clay content as sediments of period 2
Answers: A.2 ; B.3 ; C.2
All your answers were correct? Well done!
The sediments of period 1 are diatom-rich mud formed mainly by accumulation of diatoms and with high porosity. They are deposited in open-water conditions which are good for the proliferation of diatoms.
The sediments of period 2 are diamictite. They form by accumulation of terrigenous sediments brought by the ice sheets and have a high content in clay, sand, and gravel. The climate was much colder than during period 1 as the same location was covered by an ice shelf.
We can conclude that the passage from sediments of period 2 (older) to sediments of period 1 (younger) is linked to warming temperatures.
Now you are ready to apply for an expedition on the JOIDES Resolution !