Old carbon contributes to aquatic emissions of carbon dioxide in the Amazon

Abstract

Abstract. Knowing the rate at which carbon is cycled is crucial to understanding the dynamics of carbon transfer pathways. Recent technical developments now support measurement of the 14 C age of evaded CO 2 from fluvial systems, which provides an important "fingerprint" of the source of C. Here we report the first direct measurements of the 14 C age of effluxed CO 2 from two small streams and two rivers within the western Amazonian Basin. The rate of degassing and hydrochemical controls on degassing are also considered. We observe that CO 2 efflux from all systems except for the seasonal small stream was 14 C-depleted relative to the contemporary atmosphere, indicating a contribution from "old" carbon fixed before ∼ 1955 AD. Further, "old" CO 2 was effluxed from the perennial stream in the rainforest; this was unexpected as here connectivity with the contemporary C cycle is likely greatest. The effluxed gas represents all sources of CO 2 in the aquatic system and thus we used end-member analysis to identify the relative inputs of fossil, modern and intermediately aged C. The most likely solutions indicated a contribution from fossil carbon sources of between 3 and 9 % which we interpret as being derived from carbonate weathering. This is significant as the currently observed intensification of weather has the potential to increase the future release of old carbon, which can be subsequently degassed to the atmosphere, and so renders older, slower C cycles faster. Thus 14 C fingerprinting of evaded CO 2 provides understanding which is essential to more accurately model the carbon cycle in the Amazon Basin.

Fluvial carbon export from a lowland Amazonian rainforest in relation to atmospheric fluxes

“…The fraction derived from fossil carbonate sources (estimated from Vihermaa et al . []) is likely to be less than 1% of the fluvial export but could be a maximum of 7% (Figure a). Rainfall contributions were equal to approximately 20% of the fluvial export.…”

“…In third‐ to fourth‐order streams in the Ji‐Paraná basin emissions ranged from 0.67 to 12.63 µmol C m −2 s −1 [ Rasera et al ., ]. CO 2 efflux from Stream 1 contains old carbon which most likely originates from carbonate weathering and is delivered in groundwater [ Vihermaa et al ., ]. Such carbon fractions constitute a nonecosystem‐derived component, and so a correction of 0.003 Mg ha −1 yr −1 is applied to the estimates of C efflux for comparison with the eddy covariance budgets.…”

“…Such carbon fractions constitute a nonecosystem‐derived component, and so a correction of 0.003 Mg ha −1 yr −1 is applied to the estimates of C efflux for comparison with the eddy covariance budgets. No relationship was found between [DOC], and the age of CO 2 evasion and Stream 2 with highest [DOC] had the youngest age, indicating that DOC was unlikely to contain old carbon [ Vihermaa et al ., ]. Further, the results suggested that in situ decomposition of POC was not the source of aged CO 2 efflux [ Vihermaa et al ., ].…”

“…A flux of this magnitude is larger than the losses from deforestation and land use change in South America of about 0.5 Pg C yr −1 [ Gloor et al ., ] and comparable to fossil fuel burning in the tropics 0.74 Pg C yr −1 [ Grace et al ., ] and therefore needs to be considered in any construction of the tropical carbon budget that aims to separate explicitly the different pathways of C movement. The aquatic efflux may also include a contribution of aged carbon, i.e., not recently fixed from the atmosphere by the ecosystem [ Clark et al ., ; Mayorga et al ., ; Vihermaa et al ., ]. If it is known that this occurs, a correction for the fraction of fossil carbon inputs must be made for, as otherwise the budget would overestimate the loss of recently fixed C.…”

Erosion of organic carbon from the Andes and its effects on ecosystem carbon dioxide balance

“…Alternatively, oxidation of POC biosphere during transit through river basins can fuel ecosystems downstream [Mayorga et al, 2005] and is a component of heterotrophic respiration, which is not included in ecosystem carbon budgets at the scale of forest plots [Malhi, 2012]. If the carbon is old, such as carbon stored over millennial time scales in soils [Galy and Eglinton, 2011;Hilton et al, 2015] and over longer time scales in sedimentary rocks as "petrogenic" POC (POC petro ) [Galy and Eglinton, 2011;Hilton et al, 2011a], its oxidation represents a source of CO 2 to the atmosphere not captured in most budgets [Bouchez et al, 2010;Hilton et al, 2014;Vihermaa et al, 2014Vihermaa et al, ]. et al, 2013Larsen et al, 2014], and because of the association between POC and clastic sediment, these locations also experience the highest measured rates of POC discharge [Milliman, 1995;Kao and Liu, 1997;Hicks et al, 2000;Lyons et al, 2002;Hilton et al, 2008a;Wheatcroft et al, 2010;Milliman and Farnsworth, 2011;Hilton et al, 2012;Stallard, 2012;Goñi et al, 2013;Hilton, 2017].…”