A compound-specific isotopic approach to quantifying the source of terrestrial organic matter transported by a large river

University Of Southern California, Los Angeles CA

Investigators

Abstract

The erosion of organic matter from the land surface and its transport by rivers reflects a central link between Earth's biological and geological systems. Pinpointing the sources of organic material within large river systems has been difficult but is crucial for understanding and quantifying the carbon cycle, which is an important aspect of the climate system. The goal of this project is to contribute valuable new information about the sources of organic matter in rivers by identifying the isotopic composition of specific organic molecules, namely the waxy molecules from plant leaves, and by tracing these molecules through their transport downstream. These same molecules are also used for past climate reconstruction, and this work will gather information about their sourcing, integration and preservation that will be important for understanding climate signals. The research will be based in the Kosñipata-Madre de Dios river system, in the Andean headwaters of the Amazon River in Peru. The Amazon is one of the planet's largest rivers and most important biogeochemical systems, and yet the source and fate of organic material eroded from the Andes remains unclear. Isotopic analysis of carbon in particulate organic matter (POM) transported by rivers has already found wide application and contributed to our understanding of the carbon cycle. Previous efforts have particularly focused on analyses of bulk carbon (e.g., stable carbon isotopic and radiocarbon composition of bulk POM), representing the average composition of a complex mixture. This new project will look at the isotopic composition of specific organic molecules within river POM, particularly the plant leaf waxes, as well as the plants from which they are derived, and soils in which they are stored. Leaf wax compounds are useful as they are derived only from terrestrial biomass, with no complicating input from aquatic organisms. The main focus will be on the stable hydrogen isotope composition (delta D) of leaf wax, which reflects the isotopic composition of the source water used by plants. This will be complemented by radiocarbon analyses through collaboration with Woods Hole Oceanographic Institution. Sampling will occur across a gradient in elevations within the Kosñipata-Madre de Dios. Because there is an elevation gradient in the delta D of precipitation in the Kosnipata-Madre de Dios, as in many other mountainous environments, investigators hypothesize that the leaf wax composition can act as a source fingerprint, to identify where POM is derived, and to assess how this varies spatially, from high to low elevation, and across the foreland floodplain, as well as temporally, during storm events. This information will be vital to constraining the use of biomarker isotopic composition for paleo-climate reconstruction, and for quantifying and tracing Andean organic carbon in the Amazon system. The research program will facilitate a wide-ranging international and inter-disciplinary collaboration, with research and education activities in three countries (US, Peru, and UK). The Peruvian fieldwork is supported by collaborations with colleagues at the field stations and university in Peru, and in turn the PIs will work with colleagues in Peru to provide research-based educational materials at the field stations for a range of undergraduate and public visitors as well as lecture material for undergraduate and graduate degree courses in the US and in Peru. Student teaching and learning will be a vital component of the project, with research opportunities for 1 PhD and 4 undergraduate students at the University of Southern California to be involved in field and laboratory research on important questions about the carbon cycle and the climate system.