One great advantage of field work in geochemistry is the ability to gain meaningful information from the environment by the simple act of being there. No analytical chemistry necessary. I remember how my advisor Phoebe showed me patches of silvery blue water along the darker back drop of the Southern Ocean surface water during our 2012 cruise on the R/V Revelle. These discolorations indicated dense populations of coccolithophores on ocean surface. Using the naked eye, we were able to hypothesize the chemical features of the water before us.
Several visual observations were similarly immediate and insightful on the Amazon main stem, as well. Upon arriving at the Óbidos site on our first full day, before even taking my first sample – long before actually analyzing it in lab back at home— it was clear that this water was packed with sediment, and by proportion, organic carbon. It was quickly apparent that the amount of carbon I collected per 100 liters at each Fazenda Tanguro stream I visited last November would not measure up to the quantity we could retrieve from filtering the same volume of sediment from the Amazon River waters.
Amazon River sediment clogged on a filter. Though it looks like milk-chocolate, I can assure you it certainly does not taste of it.
As a consequence, the orange murkiness of Amazon River discharge also gave away the unique logistical hurdles of sampling water here in April. On this first day at Obidos, we collected 100 liters of surface discharge near a center point of the river. Using pressure by manual bike pumps, we forced murky orange water through filters designed to collect all particles above 0.22 microns (1 thousandth of a millimeter!) in diameter. The filters clogged after just 1.5 liters. Because a large proportion of the work involved in sample processing during field work is manually exchanging clogged filters for new ones, the greatest challenge ahead was filtering the equivalent of 50 2-liter soda bottles before our trip’s end.
Fortunately, we have an efficient group of scientists on this river boat. João Felipe II is equipped with a diligent team of researchers from the Wood Hole Research Center and Universidade Federal do Oeste do Pará, quick learners of the act of water filtration. With just two bike pumps and three to five scientists, we are able to master a network of 4 filtration cylinders, passing a total of ~9 liters of water through filters at a time.
The filtration team. Even Kaka from Brazil’s 2010 National World Cup team lent a helping hand!
The great sediment concentrations in the river mean that we will go home with more than enough organic material to investigate its origin and transformation in the Amazon River system. We will be able to find specific biological molecules that have been produced by plants, riverine algae, and aquatic bacteria.
More importantly, we will have enough material from these samples to measure the relative proportions of two carbon isotopes in these specific molecules: carbon-12 and carbon-13. Isotopes are atoms of the same element that differ by the number of neutrons in their nucleus. Carbon-12 is the most common carbon isotope in nature; it has six protons and six neutrons inside its atomic nucleus (hence, 12 indicates the sum of protons and neutrons). By contrast, far less common is carbon-13, which is slightly heavier, as it has seven neutrons in its nucleus.
Living things incorporate different proportions of molecules with carbon-12 and carbon-13 when constructing or transforming organic matter. For example, during photosynthesis, plants use less energy in absorbing carbon dioxide with carbon-12 than carbon dioxide with carbon-13. Organic molecules produced by different plant types will demonstrate unique ratios of carbon-12 to carbon-13 based on these preferences.
Resolving the isotope ratios of specific biological molecules in our samples will help us track the history of organic carbon in the river, from its point of production to its journey into the river system and thereafter.