Tidal influence on methane (CH4) and carbon dioxide (CO2) fluxes in the lower Amazon River

Abstract

Organic materials with upland or water column origins are continuously moved, processed, and deposited along the Amazon River. The organic matter degradation results on CO2 and CH4 production, which may oversaturate the water column or shallow groundwater and consequently diffused to atmosphere. The lower Amazon River is made up of a complicated network of channels that are bordered by flooded forests influenced by tides and seasonal water stage variation. Extensive forest areas are prone to periodical flooding both due to South American Monsoon System and daily tidal floating. Therefore, flooded forests from the lower Amazon River play an important role as particulate and dissolved carbon source exported through secondary channels until it reaches the main channel of Amazon River and the Atlantic Ocean. Aquatic environments in Amazonia as well as those that experience seasonal floods have recognized increased effectiveness of organic matter degradation, leading to CH4 formation on the soils or anoxic sediments of flooded areas as well as CO2 in the water column. The hydrologic and biogeochemical processes in the lower Amazon River are interlaced and are factors turning this region critical to the particulate organic matter transport to Ocean and an important source of CO2 and CH4 to the atmosphere, which is critical to the Amazon carbon budget. There are, nevertheless, a number of knowledge gaps, particularly on the temporal and spatial dimensions of CO2 and CH4 emission from flooded forests of the lower Amazon River. The purpose of this investigation is to identify the sources of organic matter in substrates of flooded forests of the lower Amazon River and to assess the spatial and temporal fluctuation (daily and seasonal) of CO2 and CH4 emission. Goals of the project include determining the %C, %N, C:N proportion and ´13C organic sediments, ´13C-CO2 and ´13C-CH4, CO2 and CH4 concentrations in channel waters draining flooded forests and CO2 and CH4 emissions to the atmosphere. This will support the lower Amazon River carbon balance construction, theme of the project "Biogeochemical Blind Spots Along the Lower Amazon Continuum: From Earth to Atmosphere and Ocean" (FAPESP process 2018/18491-4). (AU)