The fifth and final day of the American Geophysical Union meeting was my busiest by far.
In the morning I attended several talks in a geomorphology session about Transient Landscapes. The idea is that there are landscapes that can approach a steady state in which the forces of uplift and erosion balance. While this perfect balance is likely never achieved, it is an important concept in geomorphology because it allows researchers to investigate the departure from this equilibrium state. One thing I've always wondered about this -- and a few talks alluded to it -- is the timescale of investigation. All landscapes are transient at some timescale.
I also went to a talk in the Monitoring of Fluvial Systems session about the use of motion-sensing tagged particles. I've often daydreamed about a smart particle (a 'smarticle' if you like) that is essentially a hydrodynamically correct RFID particle that communicates its position in real time. We aren't quite there yet but the talk I saw summarized their use of a system in which cobbles (particles ~65-250 mm in diameter) in a river were tagged. A capsule that contains a motion-sensing chip is placed into a hole that is drilled into the cobble. When the cobble is in motion -- it needs to completely flip over not just vibrate -- the pulse rate changes and thus records the motion. In this case, the researchers combined this particle tracer work to help constrain a numerical model for a river restoration project. Once you start pondering the future of this technology it's difficult to not get very excited.
In the afternoon I presented my own poster and then ended up chairing the late afternoon session with a friend and colleague (I filled in for another friend who had to leave the meeting unexpectedly). My poster was in a session called Source to Sink Insights into Integrated Sedimentary System Evolution. The goal of the session was to highlight research linking sedimentary systems from where sediment is generated (in eroding mountains) to where it ends up being deposited (in depositional basins). You might think of the buzz-phrase 'source to sink' as similar to 'cradle to grave'.
My poster presented some preliminary data of a sediment budget for the past 15,000 years in a southern California sedimentary system. We compared sediment accumulation rates from the offshore basins to erosion rates in the onshore mountains at 1,000-year timescales. How did we determine these rates? For the sediment sinks we mapped volumes of sediment (using seismic-reflection data) that were tied to cores with radiocarbon ages. For the erosion rates, we used the abundance of cosmogenic radionuclides (10Be to be specific) in river sands. The basic idea is that cosmic rays bombard the Earth's surface and produce these nuclides within the rock (in this case, in quartz). If the landscape is eroding quickly, then the abundance is low -- if the landscape is eroding slowly, then the abundance of these nuclides is high. There is, of course, a lot more to the theory and application of this method (e.g., check out this brief introduction if you want to learn more).
Although there are some significant uncertainties with these data I need to address before thinking about submitting a paper, the preliminary findings are intriguing. If the data hold up under more scrutiny, we found that there is more accumulation in the basins than can be accounted for by the erosion in the source area. Essentially, we have a surplus of sediment in this 15,000 year sediment budget. This begs the question -- where is the 'extra' sediment coming from? Quantifying the volumes, rates, and contributions of sediment from the various components of they system allows us to evaluate the effect on the preserved deposits. As a stratigrapher, one of my main goals with this kind of work is to investigate to what degree varying rates influence the preserved stratigraphy. I think this is an open question at this point.
I had a lot of visitors to my poster. Many people -- both those who work with the sedimentary rock record (stratigraphers) and those who work with eroding landscapes (geomorphologists) -- came by to discuss the work. I got a lot of great feedback from other experts pointing out some issues with the data I didn't even think of. And that is exactly the point of AGU. Don't be afraid to share preliminary data and your ideas.
The rest of the afternoon was the oral part of the same session, which had several great talks. The integrative nature of the session resulted in talks that spanned everything from the drainage history of North America for the past 50 million years to small-scale observations in a modern submarine canyon and everything in between. So, perhaps a bit of a hodge-podge, but most framed their particular study within this larger theme of linking different segments of the sediment transfer system together.
This year's AGU meeting was incredible. I absolutely love it. It's long, it's intense, it can be chaotic and very busy. The first day had nearly 20,000 people attend. This is amazing -- 20,000 Earth and space scientists all in the same location sharing ideas and coming up with new ideas. I can't wait until next year.
Speaking of taking a break, I'm won't be blogging for the next week and half while I enjoy the holidays. But, I've got some good stuff coming in the early part of the new year so stay tuned.
