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Arctic Expedition Fall 2022 – Log 3

Arctic Expedition Fall 2022 – Log 3

A slice of the seafloor:

Maureen Walton and Virginia Brake

Coring is perhaps the most in-depth activity we will do on the Araon, as it provides a rare chance to extract a vertical tube of “time” that could represent just a few years to hundreds, thousands, or even hundreds of thousands of years. Core locations are determined based on a careful examination of our existing data coverage, scientific objectives, and research interests. We have three types of coring devices available for our research: gravity core, multi-core, and push core.

The gravity cores are the longest, and therefore deepest, and most likely oldest, samples we will collect. The gravity coring device consists of a headstand weight (approximately one ton) connected to a six-meter (about 20 feet) metal core barrel deployed from the A-frame at the stern of the vessel. Contained within the core barrel are clear plastic tubes called liners that are 10 centimeters (four inches) in diameter and designed to hold the cored sediment. The gravity corer is deployed from the A-frame using a wire, and when it is close to the seafloor, it “freefalls” for a short distance and pierces the seafloor, filling the liners inside the core barrel with sediment. Because the sediment is so heavy, a “core catcher” at the bottom of the barrel holds the sediment inside the barrel on the ascent. Once pulled back onto the deck, the core liners are removed from the metal barrel and cut into 1.5-meter (about five feet) long sections for processing.

Gravity coring device being deployed. Photo by V. Brake
Animation of a gravity corer in action. Animation by A. Normandeau, GSC
Sectioned cores during porewater extraction. Photo by V. Brake
Looking down on remnants of cores after subsampling for microbiology. Photo by J. Paduan

The Araon has a well-appointed core laboratory (the “wet lab”) where cores are subsampled. On this cruise, the geochemists (like Roberto Gwiazda from MBARI) have been sampling porewater by drilling through the plastic core liner into the sediment and inserting syringes that pull fluid from the sediment. Among other things, geochemists can analyze the fluids for various isotopes to tell how fresh the porewater is and where it might have come from. After porewater sampling, sometimes a day or two later, sediment cores are split lengthwise (“hotdog style”), photographed, and described by geologists onboard (like Jeff Obelcz and Maureen Walton from NRL) in their notes. After splitting, the sediments can also be subsampled and further analyzed to better understand things like mineral content, microbiology, and age. Most analyses of subsamples will occur onshore after the cruise. Half of the core remains untouched and is archived as a reference section.

The past three Araon expeditions in the Arctic have used box cores to get “shallow depth” samples of the subsurface. New to this Arctic mission on the Araon is the multi-core system, which consists of eight core liners placed around a central console that are collected simultaneously. These liners are about the same diameter as the gravity cores (10 centimeters or four inches) but much shorter, less than one meter (three feet) long. The advantage of the multi-core over the box core is that we can collect more material over a larger area. The multi-core is very good at preserving the sediment-water interface; this is important for scientists who study the interaction of seawater and the seafloor as well as very recent sedimentary processes. Like the gravity cores, multi-cores are split lengthwise, described, photographed, and subsampled after collection.

Unlike the gravity and multi-cores, the locations where the MiniROV collects push cores are not predetermined. The MiniROV can collect up to seven push cores per dive using the robotic arm. Each core is seven centimeters (2.75 inches) in diameter and up to 20 centimeters (eight inches) long. When we settle on a good site to core, we typically plan for two push cores, but MBARI’s Charlie Paull often says to MiniROV pilot Frank Flores, “Let’s grab another one” to ensure we have plenty of good samples from each site for different analyses. Push cores from the MiniROV are extruded from the plastic core liner by slicing the sediments horizontally into one centimeter (nearly half an inch) subsamples that are then placed into labeled plastic Ziploc bags. These subsamples will be later analyzed onshore to determine things like sedimentation rate.

As of September 6, 2022, the mission has collected 10 gravity cores, 14 multi-cores, and seven sets of push-cores from MiniROV dives, with more of each to come.