September 1: Detective work and carnivorous sponges at Axial
We started off today in what we thought would be a drained lava lake similar to those we had come across previously. At least this is how we read our bathymetric map that shows the topography of the seafloor in great detail (these maps show objects as small as one meter in size!). Typically drained lava lakes are cavities within sheet flows (a common lava flow type, literally occurring as thin sheets) and have rather distinctive drainback features on the interior wall. None of which we found here, to our great surprise. Instead the rim and the inside wall of that pond was comprised of pillow lava, with the pillows exposed in the wall nicely truncated and “chopped”, having broken off as talus. Pillow lava is another type of lava flow common in the deep sea, only it is not the type of flow morphology famous for the formation of lava lakes. So, what did we dive into? Another piece of the puzzle to solve… And this is what we are really doing out here—trying to put together the big puzzle of how volcanoes like Axial Seamount work. It is detective work in the end, and it requires answering a huge list of questions. The work usually begins with securing and investigating the crime scene—the geological information that can be observed on site such as the distribution and type of lava flows or thickness of the deposits produced from small explosive eruptions and proceeds. From previous expeditions and surveys we have already learned a fair amount about the geological relations on Axial Seamount. But, as science usually turns out to be some kind of Pandora’s box, the more questions you answer the more you get, and we must go out to the field again.
Today’s dive was really all about collecting samples to find out about the chemical composition of the lava and its age (in other words, the amount of years passed since the individual lava flows erupted). Of course, neither the composition nor the age can be directly determined on the ship; this will saved for labwork onshore. The composition of a lava flow is interesting as it provides many clues about the magmatic system. It tells us how long the magma was sitting around comfortable and cozy in the magma chamber before it got squeezed out through narrow dikes onto the cold seafloor. The individual ages of the lava flows help put the entire evolution of Axial Seamount into a time frame: how long has this seamount been active, do erupted lavas change with time, or in what time interval do eruptions occur? Some of the older eruptions on Axial Seamount we think happened about 30,000 years ago, just around the time when the period of the Neanderthals in central Europe came to its end. The latest eruption happened only three years ago. Now dating rocks is a tricky thing; commonly rocks “carry” internal clocks based on radioactive decay of specific elements, and these can be “read” in the lab. However, it turns out we cannot really apply this method to the rocks we are facing here (sometimes nature loves to make it just a little bit more complicated for us). So we have to use a little ingenuity. Shells of Foraminifera (tiny marine organisms) accumulate in the sediments on the seafloor and hence on every lava flow once it is in place, and their radiocarbon age is easy to determine, as long as you have enough material to work with. Rather than dating the rock, we date the base of the shell debris piling up on the lava flows. So our task for today is to collect rocks and the sediment covering the rocks.
The beautiful carnivorous sponge Lollipocladia tiburoni looks very much like a frilly version of its namesake – a lollipop. It was first discovered in 2006 by Chief Scientist Dave Clague using MBARI’s now retired ROV Tiburon at Vance Seamount in the Northeast Pacific. Jean Vacelet, a sponge taxonomist, later described the collected specimen as a new species in 2008.
Most sponges, including hexactinellid sponges (the most abundant sponges found in the deep sea) use an aquiferous system of water canals and specialized cells to generate a current throughout the body from which food, in the form of small single-celled animals and bacteria, is strained. Carnivorous sponges are unusual in that they lack this system, and instead use microscopic hooks to snare larger, flea-sized crustaceans which bump into them while drifting along in currents flowing past them, much like flying insects that get trapped in spider webs. The long filaments that emanate from the central disk are loaded with these food-catching hooks. Coincidentally these small hooks and other microscopic skeletal components are used to differentiate one species from all others.
Carnivorous sponges are primarily deep-sea animals and their unusual feeding style is understood to be an adaption to the food-poor deep sea, where maintaining an aquiferous system would be energetically inefficient. About 140 species of carnivorous sponges have been described worldwide.
Once back on shore, MBARI scientists will use powerful electron microscopes to verify that this is the species that we think it is. As with other sponges, the only way to truly know what species you have is to verify the size and suite of spicules that this animal possesses and compare those with previously published species descriptions.