Wiggle Wrangling on a Core-Drilling Ship
A scientist explains how she lines up wiggles on a screen to recover the missing layers in cores drilled from the bottom of the ocean.
By Jenny Middleton
Research time on the R/V JOIDES Resolution is always in high demand due to the ship’s unique drilling capability. Because every hour on “the JR” is precious, all shipboard activities, including sediment core recovery and sample processing, operate 24 hours a day, 7 days a week, even on holidays. This means that everyone on board works 12 hours a day (and often more), every day, for the full 2 months of our Southern Ocean expedition. Although this may sound grueling to some, this is an exciting opportunity that many of us have been looking forward to for a very long time. (See Gisela Winckler’s recent post for more on why we’re here.)
To maximize ‘round-the-clock productivity, Carlos Zarikian (@cytheropteron), the IODP staff scientist for Expedition 383, has split the scientific party into two main shifts: noon-midnight (day shift) and midnight-noon (night shift). For each role on the day shift, from chief scientist to micropaleontologist, there is a counter part on the night shift that keeps the core flow moving while day shift gets a chance to rest, and vice versa. A lot can happen over the course of 12 hours on the JR, so everyone meets with their counterpart at the beginning and end of each shift to exchange notes and words of encouragement.
On this expedition, I am a stratigraphic correlator working midnight-noon. Since this is my first time on the JR, I’m learning a lot from our seasoned noon-midnight stratigraphic correlator, Christina Ravelo of UC Santa Cruz (@beringsea).
What do we stratigraphic correlators actually do?
Our drilling team recovers sediments from the seafloor in 10 meter (30 foot) increments, known as cores. Due to the complications of drilling in soft sediments, we typically lose half a meter (~2 feet) of material between each recovered core. This lost material causes gaps in our sedimentary record that, depending on the local sedimentation rate, can span anywhere from 1,000 to ~50,000 years of time. We have to drill multiple holes at each site and carefully offset our drilling depths between holes in order to recover the sedimentary intervals lost in these missing layers and generate complete climate records.
As stratigraphic correlators, we look for distinct changes in the density, magnetic susceptibility, and color records generated for each core by our physical properties and sedimentology teams. (See Julia Gottschalk’s recent post for more on the data we collect at sea.) These patterns show up as wiggles on a graph, and we use these markers to match sediment layers from one hole to the next. This matching process allows us to compare results between holes, to guide the drilling team in targeting missing layers, and to piece together a continuous sedimentary record, known as “the splice,” using the best-preserved intervals for each depth. Basically, we wrangle the wiggles.
In practice, this job is a lot of fun! Anticipation mounts as each new core is raised through miles of water from the seafloor to the surface. A small crowd gathers around the video feed from the rig floor to watch a countdown of the core’s position below the ship. We put on our required hard hats and safety glasses when we finally see the drill team raise the precious new core out of the drill pipe and start to lay it flat. As the drillers announce “core on deck,” we file out onto the catwalk to take our first look at this fresh slice of the past. The first thing we need to know is whether the core is full of 10 meters of mud, as expected, or if something has thwarted our plans. If the core is not full, we may need to change the drilling strategy.
If we’ve already completed the first hole at a given site, then the next thing we need to know is how the sediments inside each new core compare with the intervals we’ve recovered. Before I can answer this question, I must wait for the core to be cut into sections, laser etched and barcoded, and scanned on the Special Track Multi-Sensor Logger (see video below). Within about 15 minutes of receiving the core, the first section of data is available to view. Now people start to gather around my desk as I download these new wiggles and align them with our previous cores. What features do we see in this core? Are the data easily aligned or have the layers in this core been disturbed? Did we satisfactorily recover the missing layers of time? Do we need to change our target depth below the seafloor when we shoot the next core? These are the questions we must answer as we try to recover complete records of the past.
Jenny Middleton is a Lamont Postdoctoral Fellow at Columbia University’s Lamont-Doherty Earth Observatory. Follow her tweets about the expedition here.