6 weeks 4 days from now
Recovering Sediments from the F-hole!
Submitted by Alison Mote on Tue, 07/02/2013 - 12:19
Saturday marked the mid-point of Exp. 341 and we celebrated "Hump Day" at our second deep sea site in the Gulf of Alaska, U1418! Today is our 32nd day at sea, and I think I can safely say that we're all very familiar with the color gray. I think I'm even dreaming in gray, and "Shades of Gray" has taken on a new meaning.
The first four cores at U1418 (A-D) were all pretty similar, so much so that our crossover meetings were beginning to feel a bit like the movie "Groundhog Day." Although, I have to admit that it never gets old hearing Ellen Cowan, one of our fantastic sedimentologists, recount the “gray to greenish-gray mud with interbedded silts, diatom oozes, and occasional volcanic ash layers” that our sedimentology team has been describing. Or hearing Joe Stoner, one of our paleomagnetists, report that our sediments are still "normal," which, in terms of magnetic polarity, means that they are younger than 780,000 years.
It’s no big surprise that our first four cores are very similar since the holes are only offset by about 20 meters. We do this to ensure that we’ve recovered a complete record of sediments below the seafloor at each site, and to create an accurate depth scale below the seafloor. Our stratigraphic correlators, Alan Mix and Guillaume St-Onge, are masters of correlating sedimentary layers between holes and report that everything has been matching up very nicely.
We started drilling our final and deepest hole at site U1418 on Sunday night. Yesterday the drilling team used the RCB drill bit to drill down to ~260 meters below the sea floor, without recovery, to reach the approximate depth of the D-hole (~300 m). I’m happy to report that we started recovering cores again in the middle of the night last night, although it was a bit of a challenge for the drilling team. The first two cores that arrived on the catwalk were empty because something was blocking the drill bit. After some troubleshooting, the drilling team de-plugged the obstruction in the core barrel and the subsequent core was filled with beautiful glacial-gray mud. Our drillers do some pretty amazing work to get our sediments onto the deck!
Here we go, F-hole!
The target depth for the F-hole is ~900 meters below the sea floor, and we’re all pretty excited to see some new sediments as we go deeper into the hole and back in geologic time (especially the sedimentologists who spend 12 hours a day describing the cores!). Can you believe that the sediment cores pictured above were buried hundreds of meters below the ocean floor until just a few hours ago? Hundreds of thousands of years in the making and we're seeing them for the first time! Pretty neat.
There are some beautiful metamorphic drop stones in this core, which were probably carried to the deep sea from the mountains of Alaska by ice. We also see diamictite layers, which are poorly sorted sedimentary layers that may have been deposited by submarine landslides in the geologic past. Another observation our scientists have made at this site is that the sediments at depth are much younger than our previous site, evidenced by the presence of relatively young fossil assemblages. This means that the rates of sedimentation over the past several hundred thousand years were very high in this part of the ocean.
Speaking of this part of the ocean, did you know that we are drilling into the 4th largest submarine fan in the world?! It’s called the Surveyor Fan (I know, I've been holding out on you...sorry!). We'll discuss the geologic wonder that is the Surveyor Fan more next time!