With all this dredging, I'm becoming familiar with the deep rumbling of the bow thruster and the sudden jerks of the ship when there are big pulls on the wire. After all, we are currently transiting to our 13th dredge target. There are just four full days of dredging left, but at this rate we should be able to get a lot more done.
Conditions have varied somewhat since we entered the Gulf, but last night the sea looked as calm as a lake, with the moonlight reflecting off the water. Only slightly marring the weather were visits by numerous seabirds, who must see the New Horizon as an ideal place to rest. This proclivity is unfortunate for the crew, whom I overheard musing about the benefits of working on deck in ponchos.
Today we entered the Farallon Basin (check ship track for exact location) in the southwestern Gulf. The Farallon Basin has a spreading axis at its center, so the seafloor here is oceanic crust, transitioning into thin, rifted continental crust at the basin's edges. Spreading centers in the Gulf, like this one in Farallon, are different from more well known spreading axes like the Mid-Atlantic Ridge (MAR) and East Pacific Rise (EPR), since they form as deep rift valleys rather than bathymetric highs. Some of these rift valleys are over 3,000 meters deep! One difference is that the Gulf spreading centers are so close to land. Unlike the MAR or EPR, these spreading centers are inundated by massive amounts of sediment that pour into the Gulf. One familiar source of terrigenous input to the Gulf is the Colorado River, which empties into the northern Gulf (though no longer consistently, as a result of overuse for irrigation). Further south, especially in the eastern Gulf, the Mexican Sierra Madre Occidental mountains are a more important source of sediment.
The second major source of sediment is pelagic rain from highly productive surface waters. This "rain" is actually organic debris from biota. High surface water productivity is a result of strong wind-driven upwelling, which causes phytoplankton blooms. Diatoms, or microscopic algae, are one of the major components of these blooms. They are also one of the most important photosynthesizers in the ocean. When conditions are right, diatoms can reproduce extremely quickly. Upwelling provides these conditions by replenishing nutrient-depleted, warm surface waters with deeper waters that are filled with important nutrients like nitrate and phosphate from the decomposition of organic matter. Diatoms form a test (or outer shell) made of silica, so sediments in the Gulf are silica-rich.
All this sediment is important because it influences the way new ocean crust forms. Out on the MAR or EPR, underwater eruptions bring basalts into contact with cold water, causing the formation of pillow lavas. These distinctive shapes form as the outer surface of the lava cools and hardens into a roundish pillow shape, at which point hotter lava breaks through and builds another pillow on top. My advisor, Dick Norris, has a fantastic visual aid for demonstrating pillow lavas, which I highly recommend should you have an extra bag of marshmallows handy while camping. After squishing up the marshmallows into one mass (handily achieved by sitting on the bag), simply throw the gooey ball onto a campfire. Next, watch as the top of the marshmallow ball burns; then, flows of heated marshmallow break the surface and begin to form pillows. Anyway, those layers of sediment on the Gulf seafloor prevent the formation of pillow lavas. Magma can't extrude onto the seafloor surface; rather, it injects vertically into the sediments as dikes, then spreads out horizontally as sills. I'll get into sills more later, since this is a favorite topic of Jared's.
Our most recent dredge was over a seamount in the Farallon basin that looked curiously like a sombrero. Our next target will be another seamount, 1,900 meters deep and surrounded by a moat-like ring. Our rock collection is growing rapidly!