Geological Oceanography Cruise (OS499) is one of four cruise electives available to Marine Biology and Marine Science majors.  Minimal prerequistes encourage students at all stages of their degree programs to take the half-semester courses.  The variety of student interests and abilities in these courses in many ways mirrors those found on typical oceanographic research cruises.  

Students in this class, team taught by Professors Boucher and Sahl, recently studied Penobscot Bay geology using several instruments and techniques.  Their tasks included side scan surveying of the seafloor, piston coring, multicoring, sediment trapping, sediment analysis, and transmissometry.  

What did they discover? Read on to find out!

	During the first class, students learn a "turbidity maximum" may exist in the Penobscot River estuary.  Exceptionally high turbidity occurs in an area of this estuary, possibly due to large amounts of suspended sediments.  Turbidity maxima in other estuaries (regions where salt and fresh waters mix) are created by circulation patterns and topography.  They influence estuarine properties, including water chemistry, sediment accumulation, and fisheries recruitment. In search of a turbidity maximum, the class will map turbidity and measure sedimentation rates on two cruises.  Students here have just calculated currents and tidal excursion (the horizontal movement of water over a tidal cycle) in preparation.
	Another group proposes a cruise track.  A cruise plan, based on the discussion that follows, will be done for homework.  For the cruise to succeed and its science accomplished, currents, boat speed, distances and time must be factored into the cruise plan. These students estimate that during the five hour cruise they can deploy sediment traps at two stations, while measuring light transmission, salinity, and temperature at four stations.  Planning is a valuable skill; offshore cruises cost scientists many thousands of dollars per day.
	After agreeing to a cruise plan, students turn their attention to the sediment traps.  The traps are half-meter long PVC pipe with fittings that attach to moorings; they collect falling sediments in the water column.  Two traps will be moored per station for two weeks; one near the surface and one in deep water.  Here, Dr. Sahl supervises the preparation of mooring lines long enough for the selected sites.
	On the first cruise, students prepare to deploy a mooring with two traps, just upstream of the high turbidity region encountered previously.  Another mooring will be sited in the suspected turbidity maximum.  Heading back downstream, students will deploy a CTD and transmissometer along a path that hopefully cuts through the turbidity maximum.  The instruments will provide nearly instantaneous data on light transmission, salinity, temperature and density in the estuary.
	In the field, and later in class, the cruise data are examined.  This section plot shows light transmission along the cruise transect.  Though low (blues and purples are 20-30% light transmission), the values are not as low as on previous cruises (0-10% transmission).  The turbidity maximum, if present, may be a seasonal feature or may change in response to river discharge.
	A week later, while the traps are still collecting sediments, students learn about side scan sonar, working further south in Penobscot Bay.  The "fish" is the red torpedo-shaped object suspended from the A-frame.  It sends sound pulses to the seafloor, then detects the reflected sound.  The sound data are converted to pictures of the seafloor by a deck unit in the vessel's cabin. Jeremy Shambaugh, a student in the course, will use the side scan in his senior project to map the seafloor near Pond Island, Maine.  He will compare sonar information about the seafloor to actual samples.
	Jessica Clifford tends the side scan fish during a deployment. Jessica holds its  Kevlar communication cable, through which the sound data are transmitted to the deck unit.  Cable is let out or brought in as needed to maintain the proper slack.
	The return trip up Penobscot Bay to collect the sediment traps.  On this third cruise, the class will once again make light, temperature, and salinity measurements using the CTD and transmissometer.  Shown at left, they are housed inside a protective frame known as a rosette.
	A sediment trap mooring has just been located.  Large tidal currents have submerged is surface float, making sighting of the trap difficult.  The float will be caught with a boat hook, and its line attached to a winch.  Each trap will be brought on board, kept upright, and transported back to the lab for analysis.
	Yvette Kowalczyk and a sediment trap on the way to the trap holder. Students in the background signal the winch operator to bring the second trap on board.  All four traps have surprising amounts of sediment; draining each trap partially (standard procedure) cannot be done.  In typical Penobscot Bay conditions, the traps would have collected about an inch of sediment; these are as much as two thirds full!
	Several weeks later, the samples have been dried, weighed, and partially analyzed (and displayed here in plastic bags).  The results are astounding -- one trap (from the suspected turbidity maximum) collected over a kilogram of sediment in two weeks time.  That's roughly a pound of sediment falling on an area equivalent to the footprint of a typical laptop computer per week!  Does a turbidity maximum exist?  Probably, but the results are not conclusive.  Construction of the new Waldo-Hancock bridge, near the study site, coincided with trap deployment and could have contributed large amounts of sediment runoff.  Future classes will return to the site once the bridge is finished.
	The remainder of the course is spent learning new techniques.  Here, Yvette holds a sediment core obtained an hour earlier in Smith Cove with a multicorer. Students plan to section it into layers using a threaded extruder, measure pH and Eh in the sections, and then extract the sections' porewaters.
	The core will be sectioned in a nitrogen glove bag, but only after the technique for uncapping it and placing it only the extruder is thought through!     Wes Gapp (front right) places a temporary seal on the plastic glove bag.  Air is purged from the bag by filling and emptying it several times with nitrogen gas.  The open core top will be introduced into the bag when purging is nearly complete.  Nitrogen is inert, and will protect the core's sediment from oxidation and other chemical changes.  Students will place their hands into the bag's gloves when they take turns sectioning the core.
	During their last class, students operate a piston corer. Here, they discuss how it penetrates sediments once its trigger hits the seafloor.  This is a small corer designed for coastal and lake work.  Piston corers used in the deep sea are much larger, with a thousand pounds of weight in the core barrel, but operate similarly.
	Once on board, the core cutter (the part that pushes into the seafloor first) will be removed.  The core is inside a metal barrel attached to the corer's body, which the students are positioning on deck.
	The core is examined in lab.  About a meter in length, the core records roughly a hundred years of Penobscot Bay history.  In one core, several fish vertebrae and small gravel were found.