29 March 2014: Starting the rainy season
With the last trip to Brazil five months behind us, we arrive again for the next phase of sampling from the Amazon River Basin. For one week, I will visit one to two major sites on the main stem of the Amazon River with a team of scientists from Massachusetts-based Woods Hole Research Center and Brazil-based Universidade Federal do Oeste do Pará (UFOPA). These sites, Óbidos and Alter do Chão (a.k.a. the Caribbean of Brazil), sit a few degrees south of the equator, and wind through the northern states of Amazonas and Par, the quintessential Amazon rain forest, inside the “lungs” of our planet. Our visit coincides strategically with the start of the rainy season (and the end of Carnaval season), as daily rainstorms and river discharge are on the rise.
Typical river boats docked along the Rio Tapajós in Santarem, Pará, near its meeting with Rio Amazonas.
At its core, the current mission is the same as my first trip in November. We return in search of sediments of organic carbon suspended in river discharge. Alongside the other scientists traveling from Woods Hole, we bring about two hundred pounds of equipment in our luggage. Once again, a disproportionately large fraction of this weight is comprised of bicycle-pump-powered devices for catching and filtering river water.
But, this time, we are sampling the main stem of the Amazon River, and by this fact alone, I expect this trip to differ dramatically. The suspended sediments I collected from Fazenda Tanguro in November flowed through small streams with limited catchment areas, through which underground water and rainfall drained into the river. This meant that the particles of organic carbon in my sediments have a unique terrestrial origin. If I collected sediments from a stream that flowed solely through soy cropland, for example, I could quickly surmise that most of the organic material in my samples came from soy agriculture.
Sediments suspended in the main stem of the Amazon River tell a broader story. There is a reason that station Óbidos has been visited by scientists of Amazonia for several decades. The flow rate of the river through this site varies by a factor of three, from ~100,000 m3/sec during the dry season (November – January) to ~300,000 m3/sec during the wet season (April – July) (Moreira-Turcq et al. 2013), the volume of 50 to 150 million soda bottles flowing through the river each second. Because water at station Óbidos has traveled extensively across the Amazon landscape, from the high Andes in Peru to the wide floodplains below, scientists have used measurements from Óbidos alone to learn about large-scale interactions between entire river network and the global carbon cycle.
Alter do Chão, which sits near the confluence of the Tapajos River tributary and the Amazon River, similarly draws water from an extensive catchment area. As a consequence, the organic carbon in suspended sediments from the Tapajós and Amazon River waters pools together a variety of sources from different landscapes. As we are interested in the role of different landscape types on the movement of carbon through the river system, isolating the origin of this organic matter is not trivial.
For this reason, the measurements that stem from this trip are ever the more valuable alongside the samples we collect from more isolated sites like Fazenda Tanguro. What comparisons can we draw between the large scale and the small scale within the Amazon River network? What is the pooled of effect of land use change spanning across all of Amazônia, one of the greatest terrestrial carbon sinks on Earth, on the movement of organic carbon from land to river to sea?