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Month: December, 2014

8 December 2014: “Shaping” the river depth

IMG_3787

First meeting before disembarking

Because it takes a long time to process samples from the field, I just got my first major data points from the river sediments we collected this past March and July. The data measure (1) what percentage of the sediment is and (2) the composition of carbon isotopes in each sample which I will represent by the symbol δ13C. These metrics are useful for making broad statements about where the river organic carbon comes from (e.g., algae vs. various plant types), how these vary with season, and what further analyses of greater specificity need to be done.

Once you start plotting the data in different graphs, you can see different shapes and forms of it emerge across the times and in locations of the river’s cross-section that we sampled. The most obvious shape so far is how the percent organic carbon and δ13C values of sediments between the river surface and depths closer to the river bed. The same shift by depth occurs in March and July.

We are not the first to see this, but it somewhat comforting to verify previous scientist’s observations that the Amazon River is more complicated than it looks. It means that you can’t just take one measurement at one time – one sample – and say that this data point scales with all the carbon moving through the river system towards the ocean. It means that the solid material which the river carries in the surface can be significantly different in composition from the material it carries at depth. What we see in the surface can come from different sources or parts of the drainage basin, and can also interact with the river environment differently on its way towards the Atlantic Ocean.

This is a typical complexity of all large, deep rivers. The volume of water in flux is so great that the denser (which tends to be heavier and larger) sedimentary material always ends up deeper in the river. Scientists have tried to fit all this mathematically into one equation, called the Rouse profile equation, named after the scientist who formulated it (Rouse 1950). The equation allows you to calculate the amount of sedimentary material (or a specific chemical measurement in it) at any depth of a river so long as you have one known measurement at one depth. The unique Rouse number, a constant in the equation, encompasses all change with depth expected to occur in that specific river.

Like all scientific models of the real world, the Rouse profile is a simplification. It does not always work. But it is more accurate and practical than assuming that large rivers are all the same with depth. And when it does, it means that one need not take measurements at all depths of a large river because you can calculate, or “model” them, instead (Bouchez et al. 2010).

Carl Johnson, our lab manager, emailed us the data just this past Friday. It’s perfect timing to have fresh data to discuss as we disembark on the boat today. Here’s our course for the coming week. I will reveal the sequence of these sites later on.

Screen Shot 2014-12-08 at 11.02.49 AM

from: http://www.globalrivers.org/expeditions/2014-amazon-river-expedition/

References:

1. Bouchez, J., Metivier, F., Lupker, M., Maurice, L., Perez, M., Gaillardet, J., & France‐Lanord, C. (2011). Prediction of depth‐integrated fluxes of suspended sediment in the Amazon River: Particle aggregation as a complicating factor.Hydrological Processes25(5), 778-794.

2. Rouse H. 1950. Engineering Hydraulics. Wiley: New York.

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7 December 2014: Faces of the Amazon Dream

IMG_3682Harbor by Universidade Federal do Oeste do Pará – Santerem, July 2014

It is probably no coincidence that boats on the Tapajos River in Santarem are named Amazon Dream. Now on my fourth visit to Brazil in the past 13 months, I have come to realize that the concept of the Amazon dream is more than a sporadic phenomenon in my world. It would be an over-statement to compare it to formal anthropological patterns like the American Dream. Nonetheless, it pervades more facets of my life, and captivates more people, colleagues and friends in my community than I was aware of before it became part of my own graduate thesis.

Starting today, I will join thirty explorers in Manaus, the largest city of the Brazilian Amazon, to discuss what comprises the Amazon Dream and navigate through the confluence of two major Amazon tributaries, the Negro and Solimões Rivers. Our team largely consists of faculty from Woods Hole Oceanographic Institution, Woods Hole Research Center and collaborating institutions in Brazil and Siberia: from light-hearted and passionate students to researchers like Dr. Bernhard Peucker-Ehrenbrink, who was one of the early pioneers of the Global Rivers project at Woods Hole Oceanographic Institution, and Dr. José Mauro Sousa de Moura, who leads monthly river sampling efforts from Santarem. The group also includes people who support earth science from several other angles: affiliates of the Woods Hole organizations who have been drawn in different ways to the research there, and Chris Linder, who has mastered the skill of conveying the excitement of earth science through the still (and sometimes moving image).

Dream is such a non-scientific word because it sounds vague and theatrical, an over-dramatization of emotions over logic. With that said, knowing myself and my advisors in graduate school, who have made it their career to study the global significance of river systems on Earth, scientific dreams may be dramatic, but they are also very specific and testable. I have only just scraped the surface of understanding why the diverse group of scientists I am joining in Manaus could share a similar attraction to the Amazon River Basin.

Given the size of the drainage basin, there is a lot of room to dream here. Depending on how you count, over ten tributaries meet the eastward flowing Amazon River on its way to the Atlantic Ocean. They come from a wide expanse of South America: the Andes in the far West, the elevated Guyana and Brazilian shields to the North and South, and the tropical lowland forests that the hug the main stem right in the middle. By the time the Amazon River meets the Atlantic Ocean farther to the East, it has carried the unique imprints of each tributary. If you manage to look at the discharge with just the right lens (e.g., stable isotope chemistry, traces of lignin or sediment load), you can find the diverse natural and human histories that river water from distinct tributaries has absorbed during its journey towards the main stem.

Gaillardet et al. 1996

The geographical extent of the Amazon River tributaries (Gaillardet et al. 1996)

At the same time, there is nothing like a biodiversity hotspot and a massive organic carbon pool to prove that one need not travel far, or function at the large scale, to actually dream big in a place like this. The Amazon rainforest and river systems have a way of concentrating a lot in a small space. If you ever find yourself in primary rainforest, try to tally each distinct flora you can see, or pick apart every unique sound you hear coming from the tree canopy. Or, re-visit all the questions one can ask by visiting one site on the river, like Óbidos. These challenges are comparable to counting all the stars in the night sky; they can occupy people for their entire careers, or for their entire lifetime.

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Rain forest in the Xingu River basin, one of the Amazon River’s most Eastern tributaries

In the coming days, as I traverse the largest river in the world among new company, it will be our shared learning experience and challenge to characterize what it means to dream about the Amazon River and, more broadly, other river systems in the world that compare and contrast with the Amazon in significant ways. Notably, this boat is not named Amazon Dream; perhaps that would be thematic overkill.

Reference:

Gaillardet, J., Dupre, B., Allegre, C. J., & Négrel, P. (1997). Chemical and physical denudation in the Amazon River Basin. Chemical geology142(3), 141-173.