Where on (Google)Earth #5?
No clues. Go.
How does one play ‘Where on (Google)Earth?’
And to all you visitors who never play…just bite the bullet and get a dang Blogger account so you can leave comments. Do it here.
How do we know how old rocks are?
Thermochronic is gettin’ geochronic over at Apparent Dip with a wonderful post about geochronology that even I can understand.
If you are scientist and have family members or friends asking you to explain how we date rocks then read, bookmark, and re-read that post.
If you are not a scientist and just simply want to know for yourself, I think you’ll find it informative and understandable.
Plus, it has M&M’s involved in the explanation!
Just Science week — wrap-up
I’m tapped out.
Fortunately, this week fell during a time where I was only really busy instead of super-ultra-crazy busy. But, alas…I have no more ‘draft’ posts that have been sitting around for weeks/months waiting to be finished. I have no time to create one from scratch for a while.
Here’s a run-down of the week’s post…a hodge-podge of sedimentary research that i’m either actively engaged in or interested in:
JSW #1: Sediment thickness in the world’s oceans
JSW #2: Submarine Geomorphology
JSW #3: Experimental Sedimentary Systems
JSW #4: Learning about sediment dispersal from the age of single grains
JSW #5: Sediment transfer from the continent to the deep sea
The last post was cross-posted over at Deep Sea News…so I would like to thank them. They have a great blog, you should check it out.
These posts are not comprehensive reviews of the topics with extensive lists of references and such. For anyone that is interested in more, feel free to post a comment and I’d be happy to get some more information to you.
Finally, thanks to the folks at Just Science for making this happen.
Sediment Transfer from the Continent to the Deep Sea
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.
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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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This is post 4 of 5 for Just Science week (Feb 5th-9th).
Earlier posts this week:
Post #1: Sediment thickness in world’s oceans
Post #2: Submarine Geomorphology
Post #3: Experimental Sedimentary Systems
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If I could talk to the sand grains in an ancient sedimentary deposit (I guess I already do…so, more accurately, if they could hear me) I would ask: “Where did you come from? Where were you born and where did you grow up? Did you travel a lot before coming here?” If the sand grain could provide answers, we could then start to reconstruct the source areas of ancient sedimentary environments with great accuracy. Mountain belts get eroded…that material ends up as sediment…if you wanna learn something about ancient mountain belts, look at the sediments that they produced.
Zircons can, in some cases, answer some of these questions of provenance (albeit often with a lot of inference and interpretation…but, hey that’s what we do).
For example, we can extract these little suckers from a
sandstone (which is laborious and tedious process in and of itself…i’ll go into that another time) and then determine their age using radiometric dating. I am not even close to an expert on geochronometry … for this I refer you to another geo-blog, Apparent Dip, which has posts about this subject from time to time.
So, what do you do with the age of a zircon that is in a sedimentary deposit? Typically you date a bunch of grains (50-100) and end up with a distribution of ages. Some are old, some are young. Here’s an analogy: let’s say you have a few dollars of change in your pocket. Although they are all in your pocket now (the deposit) they probably have different dates on them and are of different morphologies (if they are all the same, that’s a pretty special situation).
We end up plotting the ages in a histogram and analyze the different groupings of ages. If we are lucky, we already know something about the potential source area and see if we can make some comparisions (if it hasn’t been completely eroded and/or consumed in some way).
Me and some colleagues presented some preliminary data last year at a geology meeting held in Mendoza, Argentina, called “Backbone of the Americas”. The figure below is an example of one of these histograms. These plots are preliminary and do not include the requisite error bars, number of grains considered statistically significant, etc. but are posted here as an example of the application (stay tuned for actual interpretations and results in the future).
On the x-axis is age (in millions of years) and relative probability on the y-axis. These four different plots are from different stratigraphic units within a preserved sedimentary basin (oldest deposits on bottom). A thorough comparison of significant peaks (i.e., lots of grains of similar age) from plot to plot can help us identify trends. One pattern in this particular example is the introduction of younger zircons (the peaks around 80 Ma) in the three upper plots as compared to the bottom one. In this case, the depositional age is very close to the youngest zircons here because the basin was fed by a volcanic arc. So, the younger depositional units were receiving new, baby (or toddler) zircons as sediment.
Anyway….this is just the tip of the iceberg. I’ll post again about this and provide links from my fellow geo-bloggers out there who do more of this kind of work are can probably explain it better.
If you want to know the ghory details of this preliminary dataset…you can read the abstract (PDF).
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JSW #3: Experimental Sedimentary Systems
This is the third post for the Just Science week (Feb 5th-9th). See Monday’s and Tuesday’s posts.
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On the first post for this week, I said I would stay within a theme of deep-marine sedimentation….well, i’m gonna break from that for this post just because the topic today is just that cool.
For a few years i’ve been following the work at the St. Anthony Falls Labratory, University of Minnesota, in Minneapolis. And a good friend of mine who works there was kind enough to give us a tour in December. This place does all sorts of cool stuff…like super-cavitation research (!)…but, here i’m gonna show you a few images from their experimental set-ups for studying sedimentation.
So, basically what we got here are some very sharp scientists who have taken ‘playing in a sandbox’ to the ultimate level. They have two main experimental tanks that interest me the most: a delta basin and the experimental Earthscape basin, or “Jurassic Tank”.
The above image is a deposit growing in the delta basin. Remember this is just a couple meters across. These experiments run for weeks and they photograph at equal time increments so they can make a time-lapse movie at the end to see the evolution of the system. But the real beauty of this is the quantitative information. At other increments during the run time, they scan the deposit with an extremely precise laser and collect data about the topography of the feature. As the deposit is buried by further deposition, they can relate this information to patterns of river avulsion and lobe-switching identified in natural systems.
The image to the right is a plan-view image from “Jurassic Tank”, which is an experimental basin a few meters wide by a few meters long. What makes it unique is its ability to create differential subsidence in almost any imaginable pattern. The “basement” of the basin is actually an interlocking framework of numerous plates that each move independently from each other. With this they can run experiments with similar subsidence patterns that are seen in Earth’s sedimentary basins.
And, like the delta basin, the progress of the filling is scrutinized and measured. At the end, the entire basin fill is carefully sliced centimeter by centimeter and high-res photos are taken of the cross-section. The goal is to then relate the explicit knowledge of the sedimentation to the resultant stratigraphy. In the “real world” we typically only have the stratigraphy remaining…and it’s up to geologists to interpret the processes that generated it.
These experiments are helping us think in different ways regarding what actually controls the patterns we observe and map in stratigraphy.
Check out their web resources:
– St. Anthony Falls Lab (SAFL), is part of
– National Center for Earth Surface Dynamics (NCED)
– NCED Research Projects
– a link to some movies
– data repository for the stratigraphy experiments
This short article from Science in 2000 does a much better job at explaining the goals of particular research projects utilizing these facilities (note: you’ll probably need a university or personal license to download).
Space junk and ring of debris
I had to interrupt the JSW (Just Science week) posts because I wanted to show this quite striking graphic of space junk around our planet from the New York Times.
The red ring of debris is from the recent satellite destruction shenanigans by the Chinese….who’s brilliant idea was that?
Click on the above image to see full size and then go here to view the interactive feature at New York Times website.
JSW #2: Submarine Geomorphology
This is the second post for the Just Science week (Feb 5th-9th).
See the first post here.
from USGS; see more here
The resolution and coverage of both sea-floor mapping and subsurface seismic-reflection technology has grown by leaps and bounds over the last couple of decades. The above image is a perspective image of the Los Angeles, California area onshore topography and offshore bathymetry. Note the relatively flat continental shelf (green), very defined shelf edge, and variably steep continental slope leading to the deep sea (dark purple). The shelf edge and shelf is incised by submarine canyons that deliver terrigenous (i.e., derived from continent) sediment to the deep ocean. Many of these canyons and channels are sitting out at the shelf edge and are inactive now. During Last Glacial Maximum (~18,000 years ago) sea levels were much lower and the shoreline was essentially at the shelf edge. Some of these canyons are still active at present, however. On Friday of this week I will have a post about research I’ve been doing recently studying the flux of terrigenous sediment to one of these basins.
get original image here
This next image (above) is a map-view of the bathymetry offshore of Monterey, California (please check out MBARI‘s website for more great images of the sea floor). Monterey Bay is in the upper right of the image with the head of Monterey submarine canyon smack-dab in the middle between Santa Cruz to the north and the town of Monterey to the south. Monterey Canyon is a huge feature…follow it out seaward and the canyon transitions into a submarine channel with a beautiful meander that’s barely on this image. The distance from the canyon head to that meander, called Shepherd Meander, is approximately 125 km (75 mi) to give you a sense of scale. The water depth at that meander is approximately 3400 m (11,150 ft). Monterey Canyon is one of the most studied and well monitored submarine canyons in the world. Turbulent gravity-driven flows carrying a lot of sand are responsible for carving out this fantastic feature over long time scales (hundreds of thousands to millions of years). This paper from 2004 by colleagues of mine, Andrea Fildani and Bill Normark, is a great resource for understanding the geologic history and sedimentary processes related to the formation of Monterey Canyon.
Three-dimensional seismic-reflection technology allows us to create maps of buried
geomorphic features. The images shown at right and below clearly show beautiful meandering channels that were once on the sea floor (in very deep water) and are now in the subsurface. By analyzing ‘slices’ within the seismic volume we can map the evolution of the geomorphology through time. These images from a recent paper from Posamentier & Kolla (2003).
Utilized together, bathymetric images from the modern sea floor and seismic-reflection images from the buried sea floor are spawning new branches of geomorphology that are focused on deep-marine processes and evolution. I envision more and more collaborative efforts in the future among subaerial (i.e., on land) and submarine researchers of Earth surface processes.
Posamentier and Kolla, 2003, Seismic geomorphology and stratigraphy of depositional elements in deep-water settings, Journal of Sedimentary Research, Vol. 73, No. 3
JSW #1: Sediment thickness in the world’s oceans
For my first post of JSW (Just Science week, Feb 5-9), i’m going to introduce my field of study, sedimentary geology, with an illustration. Much of the rest of this week, I will be discussing more than you’ve ever wanted to know about the sedimentation along continental margins. Actually, I have no idea what i’ll be posting about…that’s my guess…you’ll have to stay tuned.
The above figure, which can be found here, shows the thickness of sediment in the world’s oceans and marginal seas. I love this map…it is really great to get a global view like this. Like many others investigating patterns of nature, it is necessary to look at all scales. This post starts out big….the whole globe. Subsquent posts will zoom in spatially as well as pull back temporally and get a sense of past conditions.
One of the first things to focus on is the continents of North and South America. At this scale the configuration (and history) of the tectonic plates clearly governs the patterns (see map of plate boundaries below). Note the asymmetry of sediment thickness….the eastern margins of the land masses have thick accumuations (the hotter colors) of sediment whereas the western margins do not. The active western margins consist of a subduction margin in South America, transform margin in southern North America, and subduction margin in northern North America at a very large scale. This activity, although it creates a lot of high relief nearshore producing sediment, is not the bes
t place to preserve sediment over long time scales.
The accumulations along the eastern, or “passive” margins, are so thick because they’ve been piling up for 10s of millions of years. These two strikingly different tectonic settings should be kept in mind.
Another notable pattern on the map of sediment thickness is the Bengal submarine fan. This fan, which sits in the Bay of Bengal between India and southeast Asia is the single largest accumulation of sediment on the planet (extending for nearly 30 degrees of latitude from the margin!).
Big deal. So, you may be asking so what…why should we study these accumulations of sediment? I hope to show you with a series of posts why I think it’s interesting from a scientific standpoint to figure these things out. From the small-scale processes of transporting/depositing sediment to the spectacular geomorphology present on the sea floor to the implications to studying Earth history.
And, by now, i’m sure most of you may have also noticed the spatial coincidence of some of the thicker accumulations with petroleum provinces. This is no coincidence for sure….where you pile up a bunch of sediment over a long period of time you get the perfect host for our world’s (for better or for worse) lifeblood. I’m not going to go too much into this particular application of sedimentary geology…at least not this week. My aim is higher….I want to learn as much as I can from these sediments/rocks. I’m a data junkie…I’ll admit it. I want to observe and characterize information about the Earth….i’ll take it where I can get it. I’ve been working on a post about ‘peak oil’ from a sedimentary geologist’s perspective….but not this week. This week, i’ll be using this information to tell you something (hopefully) interesting about our planet.
map of tectonic plates found here
"Just Science" week on this blog (Feb 5th-9th)
I’ve joined a bunch of science-related bloggers in a week designed to be about Just Science. That is, posts will not be diatribes about the misuse of science by government (not that that’s not fun), not rants about intelligent design in public schools (not that that doesn’t deserve ranting), not a reference and/or commentary on a mainstream article, etc. … But a post about some actual science. The goal is at least a post a day.
Good idea!
I’m not sure i’ll make a post a day, but i’m sure gonna try.
Here’s the feed: http://www.justscience.net/?feed=rss2
Clastic Detritus 
