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Sediment Transfer from the Continent to the Deep Sea

February 9, 2007

This is post #5 for Just Science week.
Today’s installment is also cross-posted over at Deep Sea News today.

 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 (image at top of post [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. The image above 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. The image below 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.

4 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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5 Comments leave one →
  1. Lab Lemming permalink
    February 9, 2007 4:49 pm

    I’m gonna have to disagree with you here:

    “As we all know, the deep sea contains fantastic records of ancient oceanic conditions.”

    The deep sea record tells us nothing at all about ancient oceans, because the very oldest deep sea basins are only 4% of the age of the Earth. They are great for the recent past, but useless for ancient stuff.

    “ultimately the deep sea is the final resting place.”

    As mentioned previously, the deep sea floor is a transient feature, so any sediment that ends up there will soon be transported to a subduction zone, scraped off, and stacked back up on a continent again. And I reckon that continental redidence times are, in general, considerably longer than oceanic ones. In otherwords, sand grains spend most of their lifetime as continental/ cratonic sediments locked up in sandstone, and only briefly go for dips down to the sea floor before getting tossed back up as a thrust sheet.

  2. Brian permalink
    February 9, 2007 5:40 pm

    Fair enough.

    I should’ve quantified the time frames I’m talking about with this kind of research. It’s all relative.

    I’m not talking about deep geologic time here…instead of the word
    “ancient oceans” I should’ve said “past oceans” or something else.

    A lot of source-to-sink studies are quantifying physical/chemical transfer of material over the last century. Some are going into glacial-interglacial time frames and some into the millions of years time frame. So when I said ‘ancient oceans’ I didn’t mean Archean or anything.

    You say: “They [deep sea records] are great for the recent past, but useless for ancient stuff.” I’m talking about paleoceanography…yes, of course we can’t study the truly ‘ancient’ because there are no records. But, is there not a distinction between paleoceanography and oceanography? Furthermore, if you read the Deep Sea News blog, most of it is biological topics of the modern ocean. I was asked to write a post that delves into marine geology by the organizers of that blog

    So I guess same goes with the ‘ultimate resting place’ comment. Of course it doesn’t stay there forever…this is all within the context of investigating sediment transfer and how it relates to other Earth surface processes. So in the last paragraph of the post, I mention studying these patterns back to 7,000 years ago. If you study the modern ocean and/or modern Earth surface processes, say the last century at most, then that time frame is only ~1.5% of the last 7,000 years.

    So, the lesson is we all oughtta put a ballpark number with anything we are talking about…at least within an order of magnitude. The Earth is so dynamic across such a broad temporal scale that nearly everything can be described as a transient feature and we won’t be able to communicate with each other with the terms ‘ancient’ and ‘recent’.

  3. CJR permalink
    February 10, 2007 12:06 pm

    Well, it does depend on your definition of ‘ancient’ – Lemming is just showing his Precambrian bias here ;-)

    Deep sea sediment records provide you with continuous and (relatively) unaltered records going back into the Cenozoic, preserving the changes accompanying the transition from a much warmer world to our current one with permanent polar ice caps – that’s a pretty useful thing to have. You get much older rocks on the continents, of course, but the sequences are cut up by faulting and are often more altered, so our knowledge of those older times is a lot less detailed and harder to interpret. Some of us like the challenge though!

  4. Lab Lemming permalink
    February 10, 2007 4:36 pm

    Well, the previous post was about sedimentary zircons that ranged from a few tens to 200 million years in age, so I assumed that the implied timescale would be the same.

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