Water Column Methods

 

          We had the opportunity to collect water column data during six 4-day cruises aboard the R.V. Pelican that bracketed low and high river discharge conditions (Fig. 1).  The corresponding cruise dates were: October 19-22, November 20-23, December 12-15, 2003 and March 6-9, April 20-23, and May 20-23, 2004.  While aboard the Pelican, we collected water column samples using the Pelican’s CTD rosette at 15 stations with 12 stations located on the continental shelf, 2 stations in Barataria Bay, and one station at the Southwest Pass (Fig. 2).  We also collected box cores (<1m) and Kasten cores (3m) at three designated sites to analyze sediment-water column interactions.

 

 

                   

          Fig. 1:  Mississippi River discharge from 8-03 to 6-04.                   Fig 2:  Site map illustrating designated water column sampling stations.

 

          Sample collection and on-board analysis of samples was a 24 hour a day process that required a coordinated effort from everyone in the research group.  The goals of our sampling lay-out and cruise schedule was to obtain a water column inventory of 222Rn, to collect cores to analyze porewaters for 222Rn and 223Ra/224Ra, and to collect 4He/3He/3H samples for tracer and groundwater age data.  The CTD rosette consists of 12-20 L cylinders that are independently fired at specific depths in the water column.  222Rn samples were collected by transferring water from the CTD rosette into a 6 L bottle that was immediately sealed to prevent gas loss.  The samples were then analyzed in the on-board radon lab using a technique similar to Mathieu et al. (1988).  This technique consists of sparging the sample with helium and collecting 222Rn in a trap at liquid nitrogen temperature.  The 222Rn is then transferred to a Lucas cell and counted with a photomultiplier tube.  Porewater 222Rn samples were collected by weighing a specific section of the core and transferring it into a 6 L bottle that was pre-filled with 6 L of radon-free water and analyzed as described above.  Porewater radium isotopes were collected from box cores and Kasten cores that were centrifuged on-board and the pore fluids were collected and analyzed at Tulane University.  4He/3He/3H samples were collected from the CTD rosette using the same methodology described in the groundwater methods section and analyzed at the UM Noble Gas laboratory.  The pictures below illustrate some of the methodology used to collect water column samples.

 

 

 

                

Mike standing beside the CTD rosette.                                                              Dail holding a core that was subsampled from the box core.

 

               

Clay collecting a sediment sample for porewater 222Rn.                                 The on-board radon lab.  Extraction lines to the left and

                                                                                                                                   counters in the background.

 

             

Dan, Erin, Clay, and Reide sampling a core using an extruder.                     The on-board centrifuge was used to extract pore fluids for

                                                                                                                                radium isotope analysis.

 

               

Brion, Kat, and Troy sampling from a Kasten core.                                          The crew preparing to sample the Kasten core.