Just Science #5: Sediment Transfer To The Deep

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[For the 5th installment of Just Science, I asked Brian Romans a PhD candidate in the Department of Geological & Environmental Sciences at Stanford University to post on his research. You can catch his blog at http://bromans.blogspot.com/]

As we all know, the deep sea contains fantastic records of ancient oceanic conditions. The deep sea also holds clues about the continents. In this case, we can use deep sea sediments to better understand how Earth surface systems respond to climatic fluctuations. The inherent relief between continental and ocean plates drives the transfer of sediment from the shoreline to the deep ocean. A grain of sand lodged from a decomposing rock in the mountains may spend a long time making its way down a river system, or being swashed around at the coast, but ultimately the deep sea is the final resting place. In other words, this is as low as it can go. Combine this with a high volume of sediment over time and the result is an accumulation (sometimes several kilometers thick) for geologists to examine.

Studies of sediment transfer within this context has been coined "source-to-sink" and involve the integration of several Earth science disciplines including sedimentology, geomorphology, hydrology, mineralogy/petrology, geochemistry, marine geophysics, and others. A big chunk of my current research is a collaborative source-to-sink project between Stanford University and the U.S. Geological Survey. We are focused on the sediment records housed in the deep marine basins of the California Continental Borderland region offshore southern California (Fig. 1)(1). The wrenching effects of the San Andreas transform fault system have created a highly segmented seascape with valleys, ridges, mountains (some of which stick out as islands), and deep basins. Using multibeam bathymetry, seismic-reflection profiles, and core samples, we can map the distribution and flux of continentally-derived sediment in these basins.

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Figure 2 is a seismic-reflection profile from the Santa Monica Basin showing the nature of the basin fill (2). High-resolution mapping of the sea floor reveals a complex geomorphology complete with canyons, leveed channels, and fans.

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Figure 3 is a perspective image(3) of Hueneme submarine canyon, the main sediment feeder to this basin.

So, what are we finding out in these studies? A radiocarbon-dated Ocean Drilling Project core in Santa Monica Basin is tied to the seismic-reflection survey providing time constraints to the maps of sediment distribution. We then calculated the volumes of sediment that had accumulated over the last 7,000 years. The average flux over this time is approximately 3 million tons of sediment per year, which is a lot. But more interesting than the absolute numbers, is the variability of this rate at shorter time scales (hundreds of years). A couple thousand years ago, the sediment flux rate increases by a factor of five and is then much more variable from then on. What is causing this variability in flux? This is the primary question we are working on now. Some paleoclimate records for the California coast (4) indicate a shift from weaker and fewer El Niño's to stronger and more frequent El Niño's around this same time. Since the main source of sediment to this basin is a river we can begin to connect these climatic fluctuations directly to the record of sediment flux.
These preliminary results are from a recent presentation at the AGU conference(5) in December 2006. This study will be submitted for publication soon.

Ultimately, the record of sediment transfer that is stored in the deep sea (modern or ancient) will tell us a great deal about what was happening on the continent regarding the interactions of tectonism, climate, and Earth surface processes.

References:

1 Perspective image created in GeoMapApp, a fantastic freeware program for exploring the world's bathymetric database. Download here: http://www.marine-geo.org/geomapapp/

2 Normark, W.R., D.J.W. Piper, and R. Sliter, 2006, Sea-level and tectonic control of middle to late Pleistocene turbidite systems in Santa Monica Basin, offshore California: Sedimentology, v. 53, p. 867-897. Explore this dataset online at: http://pubs.usgs.gov/of/2006/1180/index.html

3 Bathymetry of the northeastern Channel Islands: http://walrus.wr.usgs.gov/pacmaps/ci-persp.html

4 Barron, J.A., L. Huesser, T. Herbert, and M. Lyle, 2003, High-resolution climatic evolution of coastal northern California during the past 16,000 years: Paleoceanography, v. 18, no. 1.

5 Romans, B.W. and Normark, W.R., 2006, Distribution and rates of terrigenous sediment accumulation on the Hueneme submarine fan in the late Holocene (4.3 ka - present), Santa Monica Basin, California: AGU December 2006 Meeting.

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