May 13th, 2003; Leg 6, Port Stop

Today two groups of local high school and university students visited the R/V Western Flyer. Our morning group (see right), led by Gary Greene and Mike Foster,  included students from Jose Maria Morelos y Pavon High School and their instructor Alejandra Angeles-Perez; 4 graduate students from Carmona from Instituto Politecnico Nacional, Centro Interdisciplinario de Ciencias Marinas (CICIMAR) and their professor Dr. Gustavo Hernandes; and 2 undergraduate students from Universidad Autónoma de Baja California Sur (UABCS) and their instructor Dr. Rafael Riosmena-Rodriguez.  

Charlie Paull and Bill Ussler led the afternoon group (see left), who were from two departments at the Universidad Autonomas California Baja Sur (UACBS): 17 students from the Departamento de Ingeniería en Pesquerías and their instructors MC José

Luis Cervantes Díaz and MC Jesús Fiol; and one student from the Departamento de Geología Marina and Professor Mara Yadira Cortés Mtz. Both groups had the opportunity to see the ROV Tiburon, learn about how it is constructed, operated, and used by scientists, and tour the science labs, the Western Flyer bridge, and the control room for the Tiburon. Charlie Paull led the group to the rock slabs cut the previous day and held an impromtu tutorial on the dock on carbonate lithologies found in the deep sea. 

Charlie Paull explains textures found in the carbonate rock slabs collected during Leg 5. These rocks contain large quantities of authigenic carbonate produced by the activity of methane-oxidizing archaea in methane-rich sediments.

There is a new makeup to the science crew for Leg 6. New faces include Gary Greene, Mike Foster, Dave Caress, Debra Stakes, Meg Tivey, Antoine Page, Jorge Ledesma-Vasquez, and Alejandra Chee. Charlie Paull, Bill Ussler, and Rendy Keaten are the only veterans of the previous leg.  

– Bill Ussler, reporting 

Leg 6 objectives are primarily geological and exploratory in nature. Gary Greene has prepared the following discussion of our dive objectives and of our interest in rhodoliths. 

On a Roll: The Algae Tracers 

Several submarine canyons will be investigated along the southeastern margin of Baja California to determine their sedimentary activity and significance as conduits for transporting terrestrial and other materials to the deep basins of the Gulf of California. These canyons are of particular interest to both geologists and biologists as their heads lie in close proximity to extensive rhodolith beds, patches of living red algae that have a unique spherical or ball-like shape. This shape facilitates them to roll around the bottom, which is needed to keep them living, and thus allows them to easily roll downslopes or submarine canyons. Since rhodoliths grow in relatively shallow waters, in the photic zone of primarily tropical to sub-tropical environments, they are potentially excellent indicators of climatic conditions and marine depositional environments. Our intent is to use rhodoliths as natural tracers to document downslope sediment transport from known rhodolith banks or source areas.  

What is a rhodolith?
Rhodoliths, “red rocks,” are free-living, calcareous red algae that can form dense aggregations in shallow water (intertidal to ~ 200 meters) from pole to pole. Rhodolith beds are particularly abundant in subtropical waters like those of the Gulf of California. Accumulating observations indicate that these beds are, world-wide, among the “Big Four” marine communities dominated by macroalgae—the others being seagrass beds, kelp beds and forests, and coralline reefs. Living beds occur in areas that would otherwise be soft bottoms, providing complex, hard structure (albeit mobile) that serves as habitat for numerous invertebrates and seaweeds. Rhodoliths generally grow slowly (<1 mm/yr), and large ones are hundreds of years old. Where they are abundant, they are significant carbonate producers. Because they have a carbonate “skeleton,” they are one of the few groups of algae that preserve well as fossils. Paleoecologists have used fossil rhodolith deposits to indicate shallow water habitats of moderate water motion, and the shape of the rhodoliths may be used to estimate the amount of water motion. As mentioned above, we hope to use them as natural tracers of transport events such as hurricanes. If we can, then we will not only gain a better understanding of carbonate sedimentation, but also overall sedimentation rates based on the amount of terrestrial sediment deposited between rhodolith layers.  

Our intent is to collect cores of sediment on the slope and in the active channels of submarine canyons where rhodoliths may have rolled downslope or were carried down canyons in debris flows. It has been postulated that many rhodoliths are carried away from their growing grounds by severe storm events such as hurricanes. If that were the case, we would expect to see concentrations of rhodoliths in distinct layers within the cores we collect. We will look for such stratigraphy to document not only the past occurrences of hurricanes but also the periodicity of major sediment transport events along the western flank of the Gulf of California. 

Rhodoliths are thought to live about six months after they have been removed from their growing environment. Therefore, if a living rhodolith were found in one of our cores we would have a good timeline for determining the age of the sediment cored. The last major storm event in the southern Gulf was a hurricane in September of 2001. We are hopeful that we will see that event represented in one of our cores by the concentration of rhodoliths. This will also give us a timeline that we can use to determine recent sedimentation rate along the western marine margin of Baja California. 

In addition, we will collect rock samples along the steep fault scarps that form the eastern submarine margin of the Concepcion Peninsula and Espiritu Santo Island. These rocks should tell us much about the geologic history of the region, as they may represent rocks that were in existence prior to the rifting of Baja California from the Mexican mainland. As Baja ripped away from the North American continent, tear faults and other faults were produced that exposed the upper crust of the earth in this region. Our hope is that we will be able to collect rocks from these exposed fault faces that can then be used to determine the geological conditions that existed prior to and during rifting or seafloor spreading. 

A third objective of our cruise is to examine areas where we believe hot mineral-rich fluids are venting on the seafloor. For example, shallow nearshore faults in the Concepcion Bay area are venting hot (90° C) fluids and gas that have attracted a unique biological community, which are concentrated along the trend of the fault. We will investigate a major fault face offshore of the Concepcion Peninsula that may be associated with the same hot water source area. If vents or seeps are found, we expect to find chemosynthetic communities and other biology that are attracted to such seeps.