Sedimentology!

Today was a thrilling day for the sedimentologists. It was the first day of gravity coring. We cored in a deep basin (>1000 m water depth) where based on our CHIRP data we hypothesized sediment deposited due to the recent earthquake activity. It is possible to link sediment layers to recent or historical events based on the type of sediment (i.e., grain size), how it is deposited, and using geochemistry to determine its age. We began coring at 11am Monday morning and were finished around 9pm, with nearly full cores (182 and 133 cm). We are really hopeful because these two core sites were 2.5 km from each other and both cores reveal very similar event layers. During a high energy event, such as an earthquake, the heaviest sediments are deposited first. Therefore, we would expect sands, shell fragments, gravels to be at the bottom of the event layer. The finest sediments would remain suspended in the water column until they eventually settle to the bottom of the seafloor. Visually, this lithology is noted in both cores, at similar depths. Contacts in sediment cores mean there is a notable change in sediment type with depth. Both cores revealed several contacts at comparable depths, meaning these areas were influenced by the same physical processes. Perhaps something as large as the recent earthquake. We will bring these cores back to the laboratories at Columbia University and Queens College to study them for geophysical properties, better understand their sedimentology using laboratory techniques, and determine if the sedimentation was recent and how fast they accumulated (using radionuclides). Radionuclides adsorb to sediment particles and provide us with a chronology because they decay at a particular known rate.  We can determine what the activity is of radionuclides associated with a particular sediment depth and then back calculate to determine the age of these sediments. Radionuclides such as Beryllium-7 with a half life of 53 days or thorium-234 with a half life of 30 days are useful for understanding if sedimentation was recent. In other words, if we find that our surface sediments have any beryllium or thorium activity, it means they were deposited by this earthquake that occurred on January 12.
Pictures provide you with a visual of how we collected gravity cores. It takes several hours to collect one core because water depths were about 1700 m and the wire is deployed from 20-40 m/min. However, when that core returns and there is sediment inside of it, we all rejoice!
Another picture is included of what some believe are eroded hillsides from the recent earthquake activity.

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