How important are continental margin sediments in the carbon cycle? How much of the primary production of organic matter is mineralized in bottom sediments, and how much of it is buried? Those are some of the questions that the members of the Canadian JGOFS's benthic group who embarked on the CSS Parizeau in Rimouski on December 5, 1993, for a cruise in the Gulf of St. Lawrence and the Scotian Shelf were trying to answer. The group consisted of Anna Maria Hatcher, Sandor Muslow, Bryan Schofield and Don Webb (Dalhousie University), Takeshi Araki (McGill University), Myriam Bourgeois (UQAM), Marc Olivier (UQAR), Shaojun Zhong and Bjorn Sundby (INRS), Norman Silverberg (IML), and Angela Hillemyr and Anders Tengberg (University of Gothenburg). Our task was to collect and process samples of sediment and sediment pore water and measure fluxes across the sediment-water interface of chemicals relevant to the carbon cycle.

It was not without apprehension that we embarked on this cruise, for the Gulf of St. Lawrence in December is not known for its good weather, and Bert Klein and the JGOFS pelagic studies group who had just disembarked from the Parizeau in Rimouski had suffered weather so bad that they had been forced to spend several days in a harbor on the Magdalene Islands riding out a storm. However, when we arrived at our first station, in the Laurentian Trough south of Anticosti Island, the sea was calm, the sky was blue and with a temperature just barely below the freezing point, we had ideal conditions for checking out our procedures, making sure that everything worked, and establish a working routine for the rest of the cruise.

To complete the over-the-side portion of our operations required 10-12 hours at each station. We would start with a series of four bottle casts with 30 litre Niskin bottles to bring up bottom water for experiments and analyses, and a CTD cast to collect information about the salinity and temperature structure of the water column and the current velocities in the bottom water. This would be followed by coring until four 20x30 cm box cores and two sets of eight 10-cm diameter Multicores had been collected successfully. It was the first time we deployed the Multicorer, a new type of sediment sampler that takes up to eight individual cores simultaneously. Our Multicorer is the first of its kind in Canada, and we had acquired it for the JGOFS program because of its reputation for collecting sediment samples with the sediment-water interface intact. The reputation was not exaggerated and once we had ironed out a few bugs, we were collecting cores of outstanding quality.

Each sample that arrived on deck was processed immediately. After determining the oxygen concentration in the bottom water samples, a portion of the water was set aside for use with the core incubations and the rest was passed through manganese columns for later analysis of radium. Water was filtered for chlorophyll analysis and for collecting suspended particulate matter. Some of the box cores were subsampled with smaller cores. Some of these were preserved and returned to the laboratory for a flume study of sedimentation, others were incubated at the in-situ temperature in order to measure the fluxes of oxygen, carbon dioxide, nutrients etc. across the sediment-water interface. We also incubated cores taken with the Multicorer in an incubator designed specifically for these cores. Since this was a new technique, we judged it wise to carry out the older procedure at the same time in order to make sure that we can relate results from different cruises. In addition to measuring the rate of oxygen uptake by incubation of cores, we also measured the distribution of oxygen in the sediment pore water, using polarographic oxygen microelectrodes. The multicores were excellent for this purpose. Multicores were also used to determine the abundance of benthic macro-organisms as a function of depth. Together with the depth distribution of radionuclides, which are used as tracers of sedimentation and biological mixing obtained on the same cores, the data on infauna type and abundance will provide information on bioturbation in these sediments.

One of the box-cores was placed in a large nitrogen filled glove box and subsampled layer by layer from the surface down. A portion of each subsample was loaded into squeezers and squeezed with gas pressure to separate the pore water from the sediment. This is a long and laborious procedure which takes over 24 hours to complete for each core. The pore water samples are used for measurements of the chemicals that participate in the carbon cycle, such as carbonate species, nutrients, dissolved organic carbon, amino acids, sulfate and sulfide. Phosphate, ammonia, and total inorganic carbon were measured on board, using flow injection analysis; other analyses are under way. Smaller portions of each subsample were preserved for analysis of porosity, grain size distribution, organic and inorganic carbon, iron, manganese, phosphorus, pigment analysis, and bacterial activity.

The weather remained good until we had completed the work on our third station, in the Cabot Strait. Steaming west along the coast of Nova Scotia it became apparent that we might not be able to complete both of the two stations that remained since a gale was approaching rapidly. We decided to head for the Emerald Basin, and, if necessary, abandon the planned station further out on the Continental Slope. That turned out to be a good decision, for when we brought the last core from the Emerald Basin on board, the wind was picking up and the waves were building rapidly. Before heading for Halifax, we checked the state of a sediment-trap that had been moored in the Emerald Basin in May 1993 and which had not responded to previous attempts to release it from its anchor. The mooring was still there; we could see it clearly on the echo sounder and we could communicate with its acoustic release, but it did not respond to our attempts to release it. We are hoping that the Navy will see our predicament as an opportunity for an exercise and recover the trap for us.

Completing four out of five stations during a cruise that requires so much over-the- side work is a success by anyone's standards, so it was a happy ship that entered Halifax Harbor. Although good weather had a great deal to do with our success, we nevertheless owe much to the ship's personnel. The Parizeau is blessed with an excellent crew who goes out of its way to be helpful, and the ship's Master, Captain James Dockrill, took a strong interest in the scientific program and made sure we got all the help and support we needed.

Most of the samples we collected during this cruise were preserved and taken back to our respective laboratories where they are now being put through the painstaking chemical and biochemical analyses that are required to produce the information we seek. However, because some analyses were actually completed on board, it is possible to give at least a preliminary answer to the questions we asked when we embarked on the cruise. Benthic respiration at the four stations we occupied, measured as the oxygen uptake rate, ranged from 1.5 to 3.0 mmol O2/m2/d, which correspond to about 10% of the primary production of organic matter. We estimate the burial of organic carbon to be roughly 10% of the benthic respiration. This means that about 10% of the primary production is respired in the bottom sediment and 1% is buried. These figures are preliminary and will be refined as our analyses get completed. It is nevertheless clear that the bottom sediments do play an important role in the carbon cycle.