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Book Review

MAC-EXP: A new sediment corer designed to maintain in situ pressure conditions

Jackson, K., Witte, U., Chalmers, S., Anders, E., & Parkes, J. (2017). A System for Retrieval and Incubation of Benthic Sediment Cores at In Situ Ambient Pressure and under Controlled or Manipulated Environmental Conditions. Journal of Atmospheric and Oceanic Technology, 34(5), 983-1000.

Introduction:

Microbial life in deep-sea sediments has an important role in biogeochemical cycling of nutrients stored in organic matter. Unfortunately, it is difficult and expensive to reach and sample remote sea-floor sites, miles below the sea surface. Furthermore, organisms in cores that are recovered from the deep-sea can be damaged or undergo physiological changes because of the difference in pressure between the seafloor and the ocean surface. This is problematic because the traditional method for investigating deep-sea microbial diversity and roles is through cultures grown in the laboratory, and several organisms that live below 1000 meters (where pressure levels are 10MPa and greater) are only successfully cultured if they have not experienced depressurization.

A group of ocean engineers confronted the problem of depressurization of deep-sea sediment cores by designing a core collection system called the Multi-Autoclave Corer Experiment (MAC-EXP). It is a pressure-coring experimentation and cultivation system that collects cores from up to 3500 m water depth and seals them in individual pressure chambers. At the surface the cores can connect to a laboratory incubation system that enables control of oxygen and temperature conditions. The MAC-EXP approach is designed to save money and to have a deployment method similar to a multicorer.

MAC-EXP Design:

The MAC-EXP is designed to be reliable, easy to deploy, usable to practical working depths, inexpensive, use materials that have minimal impact on the biology in the sediment (acrylic), and cause minimal disturbance in the core top. The system’s function is to maintain constant pressure in the chamber and to incubate the core with a constant supply of oxygen (15.5 mmol/m^2/day) through recovery.  It is equipped with a temperature and oxygen sensor, and a core sleeve that can be packed with ice to help ensure constant temperature. For studies in the laboratory, the system is equipped with ports to connect to external water circulation systems and to conduct tracer experiments. The corers are attached vertically to a frame that sits horizontally on the seabed.  A caveat to the design is that the system may not work correctly if the surface is not horizontal.  To assess the success of a coring operation a clear, unpressurized sleeve is included on the frame.

The system was assembled with stainless steel, aluminum bronze (protected from the inside of the sealed chamber as to prohibit copper from compromising the organisms), acrylic, and a strong and flexible nylon that was soaked in cyanoacrylate glue thinned with acetone (to eliminate porosity). The frame plus corers weighs approximately 1 ton. For technical details about the mechanical design and construction please refer the thorough descriptions in sections 3-7 of Jackon et al. (2017).

Field Tests:

The MAC-EXP underwent the first field tests during the Spring of 2016 in the Goban Spur and Rockall Trough areas of the North Atlantic Ocean.  Before any tests were performed 25 repairs were required to get the corer to a functional state. The engineers encountered some significant obstacles once testing began. For one, there was an unanticipated force required to extract the cores from the seafloor. The added weight was equivalent to lifting 550 pounds per core; with four cores that adds 1 ton of extra weight to the winch cable. Unfortunately, the original cleat set up was not strong enough to hold the load so only 2 cores were extracted at a time and only with the use of extra lines and cleats.  There were also issues with the valves improperly sealing, allowing water to leak in, so O-rings were added the design to prevent seepage.  A major design flaw that the engineers encountered was using ropes to trigger many of the system elements. In the field, there were many issues with lines slacking, getting caught on stationary parts, and becoming entangled with other ropes. Material issues were identified with the stainless steel, which consumed a fair amount of oxygen from the incubation system, and with the sleeves, which were ineffective at maintaining constant temperature.

By the end of the expedition the MAC-EXP successfully recovered three pressurized cores during two successful deployments. Although there are modifications needed, the MAC-EXP has the potential to advance the study of microbial processes and biogeochemical cycling in deep-sea sediments by enabling the collection of cores that are maintained in in situ conditions.

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