OC103 Lesson #11: Paleoceanography and the Record of Climate Change As sediments accumulate on the seafloor they create a record of what was going on in that area of the ocean over time. Think of it as similar to your dirty laundry pile: the items you used most recently are on the top of the pile, and digging down into the pile is like going back in time until you reach the items at the bottom of the pile that you wore the longest ago. If you were to carefully observe the order of the clothes in your pile, you could recreate the exact order of what you wore (since you last did laundry), which could tell you how rainy or cold it was, how many parties, weddings, and funerals you went to, etc. Marine sediments can be treated in the same way because they record past conditions of ocean and climate, as well as what is happening on any nearby land. For example: • Turbidites record underwater mud avalanches, and possibly the frequency and size of earthquakes that serve to shake loose and start a mud avalanche • Volcanic ash layers in deep-sea sediments record volcanic activity in an area • The rise of a mountain chain on land is reflected by changes in sediments dumped into the ocean by rivers that drain the new mountain chain This lesson describes how marine sediments can be used to decipher past ocean and climate conditions and answer important questions about how those conditions might change in the future. Oceanographers collect sediments from the seafloor for two types of studies: 1. to see what is being deposited under the most recent conditions, which we can correlate with our historical observations of conditions, and 2. to use these correlations to look at older sediments to determine how the conditions varied over time. Because the most recent sediments are deposited on top of older sediments, digging or drilling down into the sediments is like going back in time. The most common way to sample into the sediments is to use a coring device, which is basically a sturdy pipe with a heavy weight on one end (see figure at right). The pipe is lowered to the seafloor with the weight at the top so that the weight shoves the pipe into the soft sediments. A set of one-way fingers at the lower end of the pipe allows the sediments to be shoved up into the inside of the hollow pipe, but when the pipe is pulled back out of the seafloor the fingers keep the sediment sample from falling back out of the pipe. The pipe is then raised back onto the ship and the sediments are extracted from the pipe in one long cylindrical piece. This cylinder of sediments can be several meters or more long, and depending on how quickly sediment accumulated in the area you just sampled, may represent thousands or millions of years of sediment accumulation. By looking carefully at the sediments and looking for features such as those we covered in the last lesson, it is possible to reconstruct thousands or millions of years of ocean conditions in that area.Turbidites Once the sediment core is extracted from the coring device, it is split in half lengthwise so that there is a convenient flat surface to look at. Layering is often visible in the sediment core, and the oceanographer will carefully examine each of the layers to determine what kind of sediments it contains. Sometimes this is obvious to the naked eye based upon color or sizes of individual grains, or sometimes it is necessary to examine a sample of a sediment layer under a microscope. Next are three examples of different types of sediment in some cores from the Pacific Ocean. All three of these cores are part of the extensive collection of sediment cores from all over the world that are stored at the Northwest Core Repository on the OSU campus. This facility contains thousands of cores collected by scientists from OSU and elsewhere, and are made available to any scientist wishing to study the marine sediment record. • The history of large earthquakes in the Pacific Northwest from turbidites. The image at right is of part of a core, collected not far off of the coast of Oregon, containing turbidites (terrigenous sediments deposited by turbidity flows). [The core is about 3 inches wide, and the section shown in this image is about 5 inches tall. Ignore the funny circles and holes imprinted into the sediments, they are from a color scanner that was pressed up against the sediments to register their color characteristics.] The 2-inch thick layer where the penny rests is sandy, while the smoother looking layers above and below the sandy layer are silty muds (remember the particle-size distinction between sand and silt from last lesson). The sandy layer settled out of the turbidity flow quickly because the particles are larger, and then the finer particles settled out more slowly, adding the silty layer on top of the sand. The silty layer below the sand is the top of an earlier turbidite, and it also has a sandy layer beneath it (but not visible in this image). In fact, if you could see this entire core (which is several meters long) you would see that this silt-on-top-of-sand layering repeats over and over, which means that there were a series of turbidites in this area. Dr. Chris Goldfinger at OSU is studying cores similar to this one to determine how often these turbidity flows occurred, and if they occurred simultaneously along the entire coasts of Oregon and Washington. The shaking associated with large earthquakes is known to trigger turbidity flows, so figuring out how often and how widespread these turbidites are may reveal the frequency and size of large earthquakes that occurred along the coast of Oregon and Washington over the past several thousand years (so far we have only been able to use a coring device long enough to penetrate about 10,000 years worth of sediment, but longer coring devices may be used in the near future to obtain longer records of turbidites).  ~3” Tracking the size and frequency of volcanic eruptions. The image below shows a portion of a core from the deep ocean, too far off of the coast of Oregon for turbidity currents to reach (the core has a blotchy, wrinkly look because it is covered with plastic wrap to keep it from drying out). Most of the core consists of reddish-brown clay (which is what you see in most of this image) that is made up of wind-blown dust and sediment particles so tiny that they could be transported far from land by wind and ocean currents