Halifax: 19th - 21st October, 2015

VITALS
Ventilation, Interactions and Transports Across the Labrador Sea

Fixed Sampling Team

Objectives

  1. To determine the annual cycle of key dissolved gases (and relevant physical parameters) in the Labrador Sea using advanced sensor-based technologies and state-of-the-art discrete measurements.
  2. To relate the temporal and vertical changes of the concentration and inventory of these gases to key controls including air-sea gas exchange, mixing dynamics (lateral and vertical), re-stratification, and biological production/consumption.

Background

High-frequency measurements over an annual cycle of dissolved gases including oxygen and CO2 have been shown to be critical in infering rates of key biogeochemical processes associated with the breathing process, including air-sea gas exchange (e.g. Kihm et al., 2010), and biological production (e.g. Gruber et al. 2002; Kortzinger et al. 2008). Within the ocean, analysis of such time-series data using mass balances, while potentially powerful, is usually highly restricted by the absence of data with full depth resolution. Of particular importance is budgeting of gases within the ocean's surface mixed layer, as this is the layer that is affected directly by (and controls) air-sea gas fluxes and it is also the layer in which biological production tends to be highest. Such budgets require knowledge of the mixed-layer depth and, especially in cases where the mixed-layer is deepening, knowledge of underlying gas concentrations. In essence, while powerful, use of budgets for interpreting time-series of dissolved gases for estimating rates of biological and physic-chemical processes has been limited by a lack of measurement resolution, in particular resolution of vertical gradients.

For many processes, time-resolution of sampling is also critical. For example, both Wallace and Wirick (1982) and D'Asaro and McNeil (2007) measured very high inward fluxes of oxygen in association with the passage of storms. These high fluxes led to extreme supersaturations, in relatively shallow mixed layers, of moderately soluble gases such as oxygen and nitrogen via the process of air injection associated with breaking waves. However, the high supersaturations were rapidly dissipated after passage of a storm by both gas exchange and mixed-layer deepening, such that the signal of air injection was also rapidly dissipated from the mixed layer. Note that in the very deep mixed layers encountered in the Labrador Sea, the extreme fluxes associated with air injection may not drive such high supersaturations and rapid outgassing. Hence, high-frequency measurements of gases are required to measure concentration changes on the same timescales as the physical and biological processes that cause them. Modern sensor-based measurement technologies allow high temporal resolution measurements from fixed-point moorings or from drifters. However traditional mooring designs have trouble making long-term measurements close to the sea surface due to problems with bio-fouling and especially, risks to sensors located at or close to the surface associated with damage from storms, vandalism, etc. In addition, traditional fixed-point measurement platforms (oceanographic moorings) require multiple sensors to resolve vertical profiles, which increases overall cost and leads to concerns over calibration differences between sensors in determining gradients. Profiling floats provide several benefits, but they are usually not able to resolve very high-frequency variability.

The development of the SeaCycler technology at BIO, in conjunction with new, more reliable sensor technologies, allows the opportunity, for the first time, to measure depth-resolved profiles of dissolved gases such as CO2, oxygen, and even N2 with high frequency, in a region of deep convection, over long periods of time (e.g. a year). SeaCycler is, essentially, an intelligent, energy-efficient, underwater winch (Sea Cycler), which allows a package of sensors to be raised up to the surface from a safe sub-surface parking depth (~ 200m). When conditions at the surface are appropriate, a communication float can be released in order for 2-way data telemetry by Iridium satellite. Although relatively new, the SeaCycler has been tested in a number of demanding deployments in preparation for use in the US Ocean Observatories Initiative. The system has worked extremely well, and shows high reliability (see web page above).

We propose to deploy two moorings in the central Labrador Sea, one conventional and the other equipped with a profiling SeaCycler. Both will be deployed in close proximity (order 100km or less) to the long-term K1 mooring maintained by GEOMAR in Kiel (at 56.5°N, 52.6°W). This will establish a triangular mooring array, similar in concept to the Ocean Observatory Initiative's "Global Scale Nodes". The proposed experiment will provide unique data on processes and fluxes affecting CO2 uptake and storage as well as ventilation of the ocean interior with oxygen. At the same time, because a significant amount of the deployed equipment is developed and made in Canada (including the SeaCycler and a number of sensors), VITALS will show-case Canadian ocean measurement technologies (see letter of support from Rolls-Royce Canada). The similarity of the VITALS triangular array to the Ocean Observatory Initiative (OOI) Global Scale Nodes will permit comparison with other such nodes planned for the Irminger Sea and the North Pacific. The triangular array, together with the mobile samplers that are part of this proposal, will allow us to determine how spatial structure determines the gas exchange during convective events and during the re-stratification period. Finally, in addition to these full depth moorings, we will also examine deep-water exchange by adding one very short (100m) array within the deep trough in the central Labrador Sea, to supplement two existing moorings that are on the western (BIO) and eastern sides (Memorial/BIO) of the Labrador Sea.

Methods

  1. We will deploy a SeaCycler and associated sensor/telemetry package at a mooring location in the central Labrador Sea for the period of 2 annual cycles stating either in Fall 2014 or, more likely, early summer 2015. The sensor package will include measurements of dissolved oxygen, partial pressure of CO2, gas tension, fluorescence and transmission, temperature and salinity (RBR and/or Seacats sensors) (see Gases section below). The SeaCycler sensor package will profile the upper 200 m of the water column twice per day, with the potential for periodic sampling at higher frequencies. Data will be telemetered over Iridium (c. 2 MB per day) when conditions permit release of the Seacycler's communications float. A McLean Moored Profiler will be deployed below the SeaCycler and equipped with a similar sensor package to allow for profiling of the depth range 2000 m to ca. 200 m. Data from the McLane profiler will be communicated to the SeaCycler via an inductive mooring controller designed by Scripps Institute of Oceanography. The 2-way Iridium communication allows for alteration of the data collection frequency, etc.. The SeaCycler mooring will be deployed for 2 years (2 deployments and recoveries).

    Initial deployment will make use of a loan of the one existing prototype SeaCycler which is owned by Prof. Uwe Send of the Scripps Institute of Oceanography and which has been extensively tested. This unit will be deployed in the NE Pacific by the USA’s Ocean Observatory Initiative (OOI) until Spring / Summer 2014 after which it will be transported to Rolls-Royce Canada’s workshop in Dartmouth, NS where it will be refurbished and equipped with a new instrument package for the Labrador Sea deployment. The exact timing of this deployment is likely to be during the BIO’s 2015 cruise to the Labrador Sea, but an earlier deployment will be attempted if a suitable ship is available. Recovery is likely to be during a cruise of the German vessel Merian, run by Kiel, to the Labrador Sea in summer 2016. The mooring deployment will be designed and conducted through a cooperation among scientists from BIO, the CERC group at Dalhousie, the Send group and Johannes Karstensen and Arne Körtzinger from GEOMAR. In particular, the Scripps mooring design, capable of withstanding the demanding conditions of the Labrador Sea, will be adapted and transferred to Canada. The multi-institution partnership will allow for information exchange, sharing of expert personnel and experience and offers additional opportunities for deployment / recovery cruises.

    Our aim is to use separate funding opportunities to purchase a 2nd SeaCycler for the second deployment. This newer system, with improved capability, will be developed in consultation and with the cooperation of scientists from the CERC group, BIO, Uwe Send's group at Scripps, and Rolls-Royce Canada and will allow next generation technology to be developed and extensively tested. If funding does not materialise, then we will refurbish and redeploy the Scripps SeaCycler (as per attached budget).

  2. The SeaCycler mooring will be in addition to a long-term mooring maintained in the deep Labrador Sea by GEOMAR in Kiel (at K1; 56.5°N, 52.6°W).). We will contribute 3 optodes for oxygen measurements and a fluorometer for deployment on Kiel’s K1 mooring which otherwise is presently equipped only with physical sensors and which extends to a nominal depth of c. 90m.

  3. An additional conventional mooring will be deployed in 2014 and outfitted with a number of gas and physical sensors in order to measure annual time-series of T, S and dissolved gases such as O2 at high frequency over a period of c. 3 years.

    Exact spacing and positioning of these VITALS moorings, and allocation of sensors to the different moorings will be discussed and decided at an international mooring workshop that will be hosted by VITALS in year 1. This will be attended by groups from Scripps and Kiel, and open to other international groups that might be interested in contributing to the experiment. A similar mooring array, also with a SeaCycler is likely to be deployed by the OOI in the Central Irminger Sea in coming years. The combined mooring work will allow for a high level of international cooperation and provide an unprecedented view of the seasonal variability of NW Atlantic region and its influence on biogeochemistry and gases.

    Combining data from multiple physical, biological, and gas sensors will give us an unprecedented opportunity to determine what controls carbon and oxygen in forming deep-waters. The first step will be to analyze the data using idealized, one-dimensional, mixed layer / convection models (deGrandpre et al. 2006; Hamme and Severinghaus 2007; Wallace and Lazier, 1988). Meteorological data from large-scale weather models such as FNMOC or NCEP supplemented by satellite observations will be used to drive the model, with CTD profiles further constraining mixed layer depth, deepening rates, and restratification. High-resolution chlorophyll and light availability / transmission data will be used together to estimate the effects of biological production and respiration on dissolved carbon and oxygen (Alkire et al. 2012). Gas tension data can be combined with oxygen to yield dissolved N2 pressures (Emerson and Stump 2010), which we will analyze with wind speed and mixed layer depth to determine site-specific bubble injection rates. These constraints on physical mixed-layer, biological, and gas exchange processes will be brought together to analyze the oxygen and pCO2 data. The second step will be to assess the impact of lateral inputs to the gas budgets. Gas sensors and current estimates from the other fixed moorings, gliders, and floats will be examined to determine adjustments to the one-dimensional budget and the uncertainties induced by lateral variations. Shipboard sections of discrete carbon, alkalinity, oxygen, isotopes, N2/Ar, and other physical/biological quantities from AR7W will supplement the lateral analysis. Finally, the carbon and oxygen budgets will also be investigated through more realistic, three-dimensional modelling of the system (see Modelling team).

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