Enriched Seawater Delivery System to Support In Situ Ocean Acidification Experiments using Carbon Dioxide for pH Adjustment of Seawater
Abstract
A series of Free Ocean CO2 Enrichment (FOCE) experiments are underway or are in planning to perform in situ ocean acidification research at a number of locations around the world. One of the most challenging locations is in Monterey Bay at the site of the Monterey Accelerated Research System, the United States test facility for cabled observatories. This site is located at 890 m deep and 4 0C within the local oxygen minimum zone and approximately 50 kilometers from shore. At this depth and temperature the behavior of liquid CO2 presents various challenges that had to be addressed in order to provide the low pH seawater needed for the FOCE apparatus to perform as desired. To solve this challenge a team of engineers and scientists at the Monterey Bay Aquarium Research Institute (MBARI) have developed a standalone device referred to as the Enriched Seawater Delivery System. Simple injections of seawater saturated at one atmosphere with CO2 demonstrated that the FOCE unit itself performs as designed. However, providing a consistent source of CO2 enriched pH altered seawater within the design criteria proved to be an imposing problem which when solved could have a broader impact in the oceanographic community. The decision was made to build a stand-alone device separate from the FOCE flume to perform in situ CO2 experiments in conditions where CO2 hydrate can form. Challenges to be over-come by this work included: (1) liquid CO2 is buoyant at the prescribed depth; (2) minimizing the formation of hydrates while manufacturing the CO2 enriched seawater. Because CO2 hydrate is denser than seawater, management of the phases and stability of liquid CO2 was necessary to prevent clogging within the delivery system. Our earliest field experiments demonstrated that containing and controlling the CO2 and the CO2-enriched seawater is difficult and makes the metering of the enriched fluid with on demand milliliter per second precision an extremely challenging problem. The Enriched Seawater Delivery system is currently deployed and operating successfully in the Monterey Bay. The unit is expected to deliver CO2 enriched seawater for approximately 4 weeks with each 200 liter load of liquid CO2 to the FOCE experiments and providing a -0.3 pH offset for over a year with regular (monthly) refills. We outline the science requirements, the technical challenges, and engineering implementation required to accomplish enriched seawater delivery from the system. We present data demonstrating the desired pH offsets as a function of the FOCE internal water velocity and the deep-sea performance to date.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMOS33B1660K
- Keywords:
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- 4271 OCEANOGRAPHY: GENERAL / Physical and chemical properties of seawater;
- 4294 OCEANOGRAPHY: GENERAL / Instruments and techniques;
- 4813 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Ecological prediction;
- 4894 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Instruments;
- sensors;
- and techniques