Sediment cores are tubes of marine sediments that are collected vertically from the sea floor. These tubes of mud provide valuable records of climatic and environmental change. But, before we can begin to interpret these changes the sediment cores go through an organized stream of measurements and description, this all takes place in the sediment core lab on the forward deck of I/B Oden. For the last month we have been busy collecting sediment cores from Petermann Fjord and northern Nares Strait. Very little is known about this remote area, but during the last month our knowledge has vastly increased! Nares Strait is an important gateway between the Arctic and Atlantic oceans. The Nares Strait gateway was blocked in the last glacial period by a large stream of glacier ice, part of which entered the area from the Petermann Glacier. Today, the Petermann Fjord houses one of the last glacier outlets of the Greenland Ice Sheet, this terminates in a floating ice shelf. Changes that we observe and measure in our sediment cores will allow us to determine when the ice stream from the Petermann Glacier retreated from Nares Strait back into the fjord. These records will also allow us to reconstruct the history of the floating ice shelf.
The core lab team. L-R Maziet Cheseby, Maureen Walczak-Davies, Joe Stoner, Anne Jennings, and Laurence Dyke.
Once a sediment core enters our lab it is scanned using a multi-sensor track (MST); the different instruments reveal the properties of the unopened opaque tubes of sediment and give us the first clues as to what lies inside. Next, the core is taken to the splitting tent, this is where our work begins. We split the thick PVC core barrel using a circular saw that runs along a custom-built track, this ensures the cores are cleaved into two perfect halves. We then run a ‘cheesewire’ through the sediment and gently tease open the core. This is our ‘moment of truth’ and the first time anyone has set eyes on the sediment inside; our concentration rapidly cedes to curiosity as we glance up from splitting to see what we have got.
The electric saw we use to split sediment cores. It is set at the perfect height to cut the core liner without disturbing the sediment inside.
A freshly split core with several large cobbles that miraculously made it inside the piston core barrel.
We then focus on describing and recording changes in the properties of the sediment within each core, we use our eyes and hands to examine the cores. We record our observations of sediment color, texture, and sedimentary structures as we work our way systematically along the sediment cores. This is called a core description and it provides a firm and unchanging basis for further work. We also photograph each core using a line scanning camera, these provide a reference image for each core that supplement our descriptions.
Line scans of sediment cores from our study area in Petermann Fjord and Nares Strait.
As part of our core description we take small samples of the sediment from each sediment section to get a glimpse of what lived in the water column and on the seafloor as the sediments were being deposited. We sieve these small samples to look at the sand fraction through a dissecting microscope. Through the microscope we have seen foraminifera (protists that leave behind a calcareous shell), ostracods (small crustaceans), sponge spicules, echinoid spines, large diatoms (algae that have a siliceous shell), and small mollusk shells. The calcareous lifeforms, especially the foraminifera, will be useful, both for studying changes in the environment, but also provide the basis for establishing a chronology (a relation between depth in the core and time) that allows us to assess how the environment has changed over time. One interesting discovery of this work has been the presence of Cibicidoides wuellerstorfi; this species of foraminifera is normally found in the deep ocean – it is a surprise to find it here in the high Arctic.
Cibicidoides wuellerstorfi, a deep ocean dweller that has made its way into the high Arctic.
So far we have opened and described 24 sediment cores (over 77 m!). We have 3 more cores in the queue and several more that are planned. All of the cores are stored in D-tubes for the long return trip to Oregon State University Core Repository. From here they will be worked on from scientists around the world – we will be working for the next months and years to unravel the history that is contained within them.
Cores racked in D-tubes, these cores will be transported back to the OSU Core Repository in Corvallis, Oregon.
Written by: Anne Jennings and Laurence Dyke