State of the Planet

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Drilling Ancient Mud from Seafloor No Easy Task

A sediment core is secured along the ship’s rail for sampling.
A sediment core is secured along the ship’s rail for sampling.
Watching winch tension as a core is pulled out of the seafloor.
Scientists monitor how hard the cable is pulling as a sediment core is pulled out of the seafloor. Too much pull will stretch the cable and could cause it to break, leaving the sediment corer on the bottom of the ocean.

Yesterday we left our first study region with new samples from the seafloor and a healthy respect for the ocean currents that can erode sediment deep in the ocean.  The samples will be useful for our research but we had to work for them.  The seafloor we surveyed was heavily eroded and we had to look carefully before finding sites that were promising enough to sample.  Even then we ran into difficulties getting the sediments back to the ship.

We spent several days surveying the seafloor using instruments on the ship to identify possible sites for sampling.  We looked for flat areas where we could see layers of sediment below the seafloor.  These layers show up in the echoes from sound pulses in a type of measurement called seismic reflection (see previous blog post).  Unfortunately much of the region we surveyed has deep gullies with no sediment layers.  Ocean currents have scoured these regions leaving no sediment for us to core.  We finally located several small areas that had a hint of sediments and one big pile of sediment we thought would be our best chance for samples.

We use a sediment corer to take samples of the seafloor.  The corer is a long tube with heavy weights on top that push the tube down into the seafloor.  When the tube is pulled out it removes a long cylinder of sediment that we bring back to the surface.  The corer is lowered on a steel cable at about 1.5 miles per hour and takes more than an hour to reach the seafloor.  At 150 feet above the seafloor, a mechanical trigger releases the corer from the cable and 5,000 pounds of steel rocket towards the bottom.  The weight and speed push the corer up to 30 feet into the sediments.  Then we have to pull the corer back out.  Sometimes this is easy but if the sediments stick to the corer it can take almost 20,000 pounds of pull to free the tube and slide it out.

Section of sediment core
A section of sediment core showing changes from clay sediments at the bottom to sandy sediment on top.
Sorting foraminifera shells
Foraminifera shells a few millimeters across can be sorted with a fine-tipped paintbrush. The different species of foraminifera can be used to determine the age of the sediments.

The other important step in coring is to keep the sediments inside the tube on their two-mile trip back to the surface.  This seems obvious but we ran into troubles with the very first core we took.  Usually a ring of metal fingers in the bottom of the core (called a core catcher) keeps the sediment inside the tube.  However, the sediment we were coring contained a lot of sand-sized shells that was washing out of the tube leaving us with no sediment by the time the corer reached the surface.  To prevent this, we added a sock of fabric around the core catcher to keep the sand from washing out.  Bingo!  The fabric kept the sand in the corer and we started recovering sediments to study.

When the sediment corer arrives at the ocean surface it is laid horizontally along the ship’s rail where we take a sample of the sediment in the core catcher to determine the age of the bottom of the core.  This age is determined by looking for a striking, pink colored shell made by a type of plankton called foraminifera. This pink foraminifera was abundant in the Pacific Ocean until 120,000 years ago, so if we find pink shells we know the sediments are at least 120,000 years old.  We will do more detailed analyses later but this age gives us our first peek at how much time it took for the sediments to accumulate.

Next, we cut the core into smaller sections that are easier to handle and the core is split open so we can see how the sediment looks.  We study its color, texture and composition before storing it in a refrigerated container aboard the ship.  At the end of the cruise we will send the container to the Deep-Sea Core Repository at Lamont-Doherty Earth Observatory where the sediments will be preserved for researchers around the world to study.

We are now steaming south to the equator to start a new survey to find the right locations to drill more sediment cores.

Sock inside the core catcher
Katherine Wejnert from The Georgia Institute of Technology samples the sock inside the core catcher.
Cutting a sediment core into sections.
Steve Hovan (Indiana University of Pennsylvania) and Allison Jacobel (Columbia University) cut a sediment core into sections.
Preparing to take notes on the sediment composition.
Christine King (University of Rhode Island) prepares to take notes about a new sediment core.
Microscopic examination of sediments
Jennifer Hertzberg (Texas A & M University) determines how old the sediments are by looking for a pink-shelled species of foraminifera that lived in the Pacific Ocean 120,000 years ago.
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11 years ago

an one of important record for seaman like me, you are right,drilling and become a seaman is not easy task , must have more skill, learn sailing and swimming is a must