Unusual and Uncharted
By: Whitney Rogers
The earth was once vastly different than what we have now. During the last Ice Age, for example, what eventually became the southern coast of California bore a different conformation that it does today. During E/V Nautilus’ Southern California leg the team will look at the inner and outer borderlands. The borderlands extend 100-200km off the California coastline, displaying unusual geography, with lots to explore, including ridged-boundaries and basins lying between these faults.
There are diffuse boundaries between the North American and Pacific plate that lie along the San Andreas fault. About 20% of that famous boundary’s activity takes place off shore. The San Andreas fault runs through Los Angeles southward, possibly through San Diego. Understanding the offshore portion of the fault is important with regard to public preparedness: evaluating hazards, such as tsunamis and earthquakes, is vitally important to the millions of people who call Southern California home. Incomplete multibeam bathymetric coverage has revealed large marine slides, which calls for deep sea exploration of this area.
Even though the borderlands are within America’s Exclusive Economic Zone (EEZ), they remain largely unexplored. The academic, federal and private communities have investigated the area for over 50 years; however, bathymetric coverage is less than 50% of the region. Most of the points of interest can only be mapped or discovered through multibeam sonars, and therefore much of our understanding of this area is based on remote sensing and circumstantial evidence. The Corps of Exploration will launch the vehicles to explore geological and biological targets, while filling bathymetric gaps with multibeam mapping.
Geologically, this area is unusual. Some of the basins we will be diving around are anoxic, with little macroinvertebrate life growing within them. This means their sediment integrity is intact, which can provide a chronological look at past climate change. Intact sediment cores allow scientists to reference local historic events to the oceanic conditions happening globally on the same timescale. It also provides supreme opportunity for geological samples, which we are collecting throughout our cruise leg.
One of the particularly interesting dive sites of this leg is Sixty Mile Bank. This flat top bank rises to 130 meters in depth. Scientists believe this flat bank was likely sitting at sea level during the Last Glacial Maximum (the most recent glacial period). This was a period in Earth’s climate history during the last glacial period where ice sheets were at their greatest extension (thickness) and the sea level was at its lowest. This maximum thickness was reached about 30,000 calendar years ago. We can measure these previous extensions through evidence in sediments laid down by sea level changes all over the world.
The E/V Nautilus team targeted this site because many questions remain about the Sixty Mile Bank. The bank rests between the inner California borderland and the outer California borderland, two different geological terrains. The steep slope off its western edge may be controlled by faults and contain exposed basement rocks. Basement rocks are the rocks that lie below sedimentary platforms. Geologists are interested in these rocks because they are the foundation and oldest metamorphic and/or igneous rock that forms our oceanic crusts. We dove to collect samples every 100m, and towed across the base of the bank to begin up the slope’s ascent. We repeated this process when diving on Velero Bank and Northeast Bank.