Historic carbon burial spike in an Amazon floodplain lake linked to riparian deforestation near Santarem, Brazil

The forests along the Amazon Basin produce significant quantities of organic material, a portion of which is deposited in floodplain lakes. However, potentially important effects of ongoing deforestation in the watershed on these carbon fluxes is still poorly understood. Here, a sediment core was extracted from an Amazon floodplain lake to examine the relationship between carbon burial and land cover/use. Historical records from 1942 and satellite data from 1975 were used to calculate deforestation rates between 1942 and 1975, and 1975 to 2008 in four zones with different distances from the margins of the lake and its tributaries (100, 500, 1000 and 6000-m buffers). Sediment accumulation rates were determined from the 240+239Pu signatures and the excess 210Pb method, reaching near 3.8 and 4.2 mm year−1 in the last 60 and 120 years respectively. The average carbon burial rates ranged between 100 and 350 g C m−2 year−1, with pulses of high carbon burial derived from the forest vegetation, as indicated by δ13C and δ15N signatures, which corresponded to heavy deforestation in the 1940 and 50s. Finally, our results revealed a potentially important spatial dependence of the OC burial in Amazon lacustrine sediments in relation to deforestation rates in the catchment. These deforestation rates were more intense in the riparian vegetation (100-m buffer) during the period 1942–1975 and the larger open water areas (500, 1000 and 6000-m buffer) during 1975–2008. The continued removal of vegetation from the interior of the forest was not related to the peak of OC burial in the lake, but only the riparian deforestation around 1950. Our novel findings suggest the importance of abrupt and temporary events in which some of the biomass released by the deforestation, especially restricted to areas along open water edges, might reach the depositional environments in the floodplain of the Amazon Basin.