The Life-Cycle of a Sediment Core aboard the I/B Oden

A few days ago my husband Paul and I celebrated our second wedding anniversary by splitting cores collected from Nares Strait and Petermann Fjord.  In spite of the high-velocity mud spatter, this ‘celebration’ was fairly sentimental for us: we got to know each other splitting cores on the exact same circular saw frame, nearly nine years ago.  The scenery was a little different on the cruise where we met, working in the Indian Ocean off the coasts of Thailand and Sumatra in the wake of the 2006 earthquake and tsunamis.  Icebergs and hanging glaciers are a distinct rarity in the tropics.  However, the reason we go to sea remains the same.  Paul is a technician for the Oregon State University Coring Group, a National Science Foundation-funded facility that provides global support for oceanographers who need samples of the seafloor.  I am an oceanographer and marine geologist with a passion for pre-historic environmental reconstructions, and I’m sailing on the Petermann 2015 expedition to help with shipboard processing of the sediment cores.

 

Mo&Paul

(Maureen and Paul Wlaczak)

 

Before there were satellites, icebreakers, sailing ships, Inuit migrations, or polar bears, there was mud.  As long as there have been continents rising out of Earth’s oceans, wind and water have been working to break them down and wash the rocks back into the sea.  Over time, these sedimentary deposits accumulate like pages of a book, with the beginnings of the story at the bottom.  We can use the composition, lithology, chemistry, and incorporated microbiology of these sediments to help us reconstruct the local environment at the time they were deposited.

 

SedimentLayers

(Layers of packed sediment can be seen in Petermann’s fjord walls)

 

The coring team has set out to recover potentially continuous records of thousands of years of ice advance and retreat, as well as the oceanic conditions that accompanied changes in ice behavior.  This will allow us to place observations of the modern climate and oceanography in the context of the regional history of the glaciers and ocean moving out of the last ice age and through the Holocene.  The Holocene is the geologic period spanning the past ~10,000 years, during which time most human civilizations developed.

 

To recover long, continuous sequences of marine sediment we are deploying a variety of coring devices from the deck of the Icebreaker Oden.  The smallest and most whimsical of these devices is called a ‘multi-core’. This instrument is comprised of 8 short plastic tubes on a rosette suspended in a lander with big broad feet that let it perch on the soft sediment surface like a swamp hen.  The multi-core is lowered over the side of the ship and into the water, and when it reaches the seabed the lander stops moving and the line attaching the instrument to the ship goes slack.  The cores then slowly slide part-way into the mud, capturing the interface between sediment and water.  Samples of the delicate surface of the seafloor are very important, as they allow us to calibrate many of the proxies that we will use in the context of the modern environment.

 

The next size of corer we are deploying is a gravity corer, which we lower into the sediment to collect a single longer sample of material.   This is a simple, robust device, called a gravity corer because it settles into the seafloor under its own weight (augmented by around 500 kg of lead).   This will bring up a sample of seafloor typically ranging in length from 1-5 meters.

 

OLYMPUS DIGITAL CAMERA

(Gravity corer. Photo courtesy of Martin Jakobsson)

 

Depending on the results of deploying the gravity corer, we may decide to deploy the piston corer.   If you’ve ever played with your soda as a kid, sealing your thumb against the top of your straw to pull a thin straw-full of your drink out of your cup, you have a very intuitive understanding of what the piston in the piston core is doing. The piston core is weighted with around 1,000 kg of lead and rigged with an extra length of coiled slack wire, attached to a trigger mechanism that is released when the corer is about 3 meters from the bottom.  This allows the piston core to free-fall the last few meters through the water column, achieving a speed that frequently allows for greater penetration of the seafloor than is possible with the standard gravity corer.

 

(Rudimentary animation of how a piston corer is triggered and dropped into the sediment)

 

The longest core we have recovered so far on this expedition is roughly 9 meters, but depending on deployment conditions and sediment type, variations of this kind of corer can recover continuous records exceeding 40 meters.

 

OLYMPUS DIGITAL CAMERA

(Piston corer on its way to be deployed)

 

PistonCore_CU

(The piston corer is deployed off the fantail of the Oden. Photo courtesy of Martin Jakobsson)

 

When the cores are hauled up to the deck of the ship, they begin their first day as scientific storytellers. Our fantastically organized core curator, Maziet Cheseby, will record details of the core recovery and length as the core is removed from the barrel of the coring device and divided into approximately 1.5 meter long sections.  These smaller sections are then logged for a variety of ‘physical properties’ like sediment density and magnetic susceptibility, which give us basic information on the sediment composition and allow us to begin the process of correlating the cores, allowing us to compare separate core samples from a single site.

 

Once this logging is complete, pore waters may be sampled, and then they’re off to be split lengthwise, creating a working half for scientific sampling and an archive half that will be preserved as long as possible. The section halves will be cleaned and described, photographed, and then carefully wrapped and prepared for shipment in a refrigerated container back to the OSU Marine Geology Repository, where the real fun begins.

 

Core_AnneMicrobio

(Anne Jennings begins core descriptions and will take small samples to look at microbiology under a microscope)

 

Core_Imaging

(Recording hi-def images of the cores)

 

Once back in Oregon, the cores will be subjected to further non-destructive analysis, and then sub-samples of the sediment will be distributed to research scientists at institutions around the world for study.  We anticipate the results of this expedition will markedly improve our understanding of past, present, and future arctic climate.

 

CoresRacked

(Core halves packed, labeled, and ready for shipment)

 

Written by: Maureen Walczak

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