Bio: On the Rocks is the blog from the Department of Geology (School of Natural Sciences) of Trinity College Dublin. You can read original –and many other– posts here: http://ontherocks.ie/
Posts by OnTheRocks:
Being on a ship for two months has its ups and downs. But when you’re drilling and recovering sediments that haven’t seen daylight for millions of years, you can bet both the ups and the downs are thrilling.
My research involves the study of modern diagenetic (“rock-forming”) processes in order to better understand how to interpret evidence of these processes in the geologic record. In particular, I’m interested in placing better constraints on the cycling of sulfur in deep ocean sediments. Such cycling involves processes like the reduction of sulfate (SO42-) to sulfide (H2S), the oxidation of sulfide back to sulfate, and the formation of solid phase sulfides like pyrite (FeS2).
Knowledge of how the redox cycling of sulfur proceeds in modern sediments is used widely in making models of the oxygenation state of Earth’s oceans. However, most of this knowledge has come from the shallow continental shelf – i.e., the small fraction of the seafloor easily accessible via wading on foot or with small boats. So, I’m working to learn how deep ocean sediments factor into the modern sulfur cycle and how their role may have changed through geologic time. Much of this work involves measuring the abundance of different sulfur isotopes in various dissolved and solid phases within sediments, as processes within the sulfur cycle tend to “fractionate” these isotopes such that the products formed from these processes differ in their isotopic composition from the reactants used to make them.
As part of my research, I’m currently sailing on one of the world’s only drill ships devoted exclusively to marine scientific research, the JOIDES Resolution, as part of International Ocean Discovery Program (IODP) Expedition 363. We’re drilling sediments from a variety of deep ocean sites amongst the islands of Southeast Asia to understand the regional responses of the Western Pacific Warm Pool (WPWP), the world’s largest reservoir of warm surface water, to local and global changes in climate over the past 15 million years. Fortunately for me, the sites are also well suited to study the sulfur cycling in deep ocean sites with a wide range of sedimentation rates, organic matter contents, and input of terrestrial material.
Accomplishing the research goals of the 30+ scientists on the ship requires an incredible feat of modern engineering: the ability to drill and retrieve cores of sediment from up to two thousand meters below the seafloor in water that is hundreds to thousands of meters deep. To do this, sections of pipe are connected together and extended beneath the ship until the seafloor is reached. A drill with a hole in the middle for collecting a sediment core can be dropped through the pipe to commence the retrieval of the sediments. As you might imagine, floating away from a site while being attached to it with an extremely long and rigid pipe would be disastrous, so our ship is equipped with a special “dynamic positioning” system that can maintain our position within a range of a few tens of meters over long periods of time.
My job as an inorganic geochemist on the ship is to help extract pore waters from slices of the sediment cores brought up to the surface and to measure the concentrations of various dissolved constituents within those waters. We have a surprisingly well-equipped chemistry lab on board the ship that allows us to measure the concentrations of major cations and anions like sulfate, calcium, magnesium, and potassium, as well as minor constituents like ammonium, iron, and silica. These concentrations can tell us a lot about the rate of organic matter degradation and any mineral precipitation or dissolution reactions that are occurring in the sediments. We are also collecting and preserving water for measurements that cannot occur on the ship due to a lack of instrumentation (e.g., measurements that require mass spectrometers).
Life at sea is quite different from life at home, but not unpleasant by any means. The 125 people on board are generally divided into two 12-hour shifts to allow the ship to operate 24 hours a day, seven days a week. Each person shares a room with someone who is working on the opposite shift so that everyone can sleep as undisturbed as possible. Four meals are served a day, and we even have creature comforts like a gym, a movie room, and a TV lounge with satellite phone and Internet. But the biggest challenge so far has been staying away from the dessert cooler and the soft-serve ice cream machine. They’re dangerous!
We’re currently about halfway through our nine week expedition and are slowly moving north from Papua New Guinea towards a deep ocean high in bathymetry called the Eurapik Rise. We’ll drill a few sites on our way there before ending in Guam in early December. What sorts of exciting surprises will we find in the sediments we recover? Only time will tell.
Dan Johnson is a Ph.D. student in Geochemistry at the California Institute of Technology in Pasadena, CA. You can find more details about his shipboard experience at his blog, danontheboat.blogspot.com, and about IODP Expedition 363 at iodp.tamu.edu.
See original post here: http://ontherocks.ie/2016/11/20/science-at-sea/
On the Rocks blog: http://ontherocks.ie/
Have you ever thought about what your house is actually made of? Before starting my Ph.D. I had never taken this question into consideration. However, for thousands of home owners across Ireland, the structure of their homes is under threat from the very stuff it is made of. Concrete. But why exactly is this commonplace material causing problems for these unlucky few? To answer this question, I will have to start by raising another: what is concrete?
Concrete is a synthetic, man-made rock composed of water, cement and aggregate. For the concrete novices out there, aggregate is the crushed rock which forms the large portion of concrete, while cement is powdered rock made of limestone and clay which is used to form the glue that binds the aggregate together. So, how does it all form? As you can see below, concrete starts off life as a pile of aggregate floating in a soup of cement and water. As time passes, the mix begins to dry and crystals of calcium-silica-hydrate begin to form and eventually surround the aggregate, binding it together and giving concrete its strength.
Now that we’re up to speed on concrete, we can finally start pointing fingers as to what is causing the problem. As you’ve probably guessed from the title of this post, the problem lies in the aggregate. Aggregate is crushed rock that forms a large portion of the concrete structure, up to 75% in most cases. Aggregate is usually inert, meaning it is unreactive and ideally should be very stable. However, for some unfortunate home owners across Ireland, the aggregate in their house can look something like the picture below. This is not the aggregate you want for making concrete. It clearly contains an abundance of a gold coloured mineral called pyrite.
Pyrite is an iron disulphide, meaning its crystal structure is made up of groups of iron atoms attached to two sulphur atoms. When this mineral comes into contact with moisture or water, oxidation tends to occur. This oxidation creates a sulphuric acid which is then free to react with the calcium that is abundant in most of the aggregate used in Ireland. The combination of calcium, sulphur and oxygen brings together all the components necessary for gypsum to form. Gypsum is a salt and like most salts it has a habit of expanding. When this expansion is confined inside of building material it can lead to a process called pyritic heave. In other words, it can lead to widespread cracking and expansion of concrete throughout the home.
So, where does my research come into all this? We want to investigate whether the different crystal structures of pyrite have a bearing on the degree of pyritic heave. For instance, is the presence of cubic pyrite worse than framboidal pyrite or does the greater surface area of the spherical framboids make them the main culprit. Similarly, we also want to investigate the elemental signature of the household pyrite and attempt to match this signature with that of the pyrite found in the quarries. Achieving this would allow us to identify the quarries that are supplying a poor standard of aggregate and make sure that this material does not end up in people’s homes.
By Tadhg Dornan, Ph.D. student at Trinity College Dublin conducting iCRAG funded research within the Raw Materials spoke.
See original post here: http://ontherocks.ie/2016/11/12/concrete/
On the Rocks blog: http://ontherocks.ie/