Age of riverine carbon suggests rapid export of terrestrial primary production in tropics

Abstract

The balance between the storage of vascular plant carbon in soils, oxidation to carbon dioxide, and export via rivers affects calculations of the strength of terrestrial ecosystems as carbon sinks. The magnitude and timescale of the riverine export pathway are not well constrained. Here we use radiocarbon dating of lignin phenols to show that plant‐derived carbon carried by suspended sediment of the Mekong River is very young, having been produced within the last 18 years. Further, this plant‐derived carbon remains young during times of the year when bulk carbon varies from modern to over 3000 radiocarbon years old. Our results demonstrate that primary‐production derivatives are exported rapidly and suggest that the age of riverine lignin is similar to estimates of the residence time of terrestrial organic carbon in tropical catchments. These results are relevant for modeling predictions of the influence of the terrestrial biosphere on atmospheric carbon dioxide levels.

Carbon dynamics and CO<sub>2</sub> and CH<sub>4</sub> outgassing in the Mekong delta

“…Within the freshwater zone (salinity < 1), DOC values (2.4 ± 0.2 mg L −1 , n = 19) were within the range (0.9-5.1 mg L −1 ) reported by Huang et al (2017), and δ 13 C-DOC values (−27.8 ± 0.6 ‰, n = 19) were again consistent with a dominance of terrestrial C3 vegetation inputs and close to values reported by Martin et al (2013) slightly upstream in the Lower Mekong. The δ 13 C values were significantly lower in DOC than POC for the same samples in October 2004 (Fig.…”

“…Reported seasonal cycles of remotely sensed chlorophyll a concentration also indicate higher phytoplankton biomass and primary production in October compared to April and December (Gao et al, 2013;Loisel et al, 2017). The δ 13 C-POC values in the freshwater part of the delta (salinity < 1) from the three sampling campaigns averaged −26.7 ± 0.7 ‰ (n = 34), distinctly higher than the data from Ellis et al (2012), which averaged −29.8 ± 0.9 ‰, but similar to data collected by Martin et al (2013;average −26.4 ‰) at the same site as the Ellis et al (2012) study. These δ 13 C-POC values are consistent with the expected dominance of terrestrial C3 vegetation in the riverine organic carbon load.…”

Where Carbon Goes When Water Flows: Carbon Cycling across the Aquatic Continuum

“…The age of OM and CO 2 in rivers varies drastically across space and time due to the complex mobilization dynamics previously discussed and heterogeneity in physiochemical characteristics across not only latitudes, but within a basin (Raymond et al, 2004). For example, the radiocarbon age of POM in the Mekong River varies from ∼3000 years old during the low water period to modern during the high water period (Martin et al, 2013). DOM and particulate lignin phenols, on the other hand, were ubiquitously young throughout the hydrograph, indicating a rapid cycling of the majority of the DOM pool and, likewise, fresh vascular plant derived OM in the particulate phase (Martin et al, 2013).…”

“…For example, the radiocarbon age of POM in the Mekong River varies from ∼3000 years old during the low water period to modern during the high water period (Martin et al, 2013). DOM and particulate lignin phenols, on the other hand, were ubiquitously young throughout the hydrograph, indicating a rapid cycling of the majority of the DOM pool and, likewise, fresh vascular plant derived OM in the particulate phase (Martin et al, 2013). Similar observations have been made in the Amazon basin, where DOM ages were also ubiquitously young, whereas POM ranged from old in the headwaters to modern in the lowland rivers likely due to a dilution of old petrogenic carbon with fresh litter and rapid cycling of DOM (Mayorga et al, 2005).…”

Reviews and syntheses: Anthropogenic perturbations to carbon fluxes in Asian river systems: Concepts, emerging trends, and research challenges

“…It is very difficult to evaluate overall pollution impacts on the mainstem Mekong River as very few systematic measurements of the nutrient and C cycles of the Mekong River have been made, except for the short reaches of the lower Mekong in Laos 405 and Cambodia (Alin et al, 2011;Ellis et al, 2012;Martin et al, 2013) Richey et al, 1990;Mayorga and Aufdenkampe, 2002) and high-alkalinity 410 rivers such as the Mississippi and Yellow River (1.5 -3 mM; Raymond and Cole, 2003;Ran et al, 2015a). The annual DIC flux of the Mekong was 3.95 Tg, with DIC and alkalinity both negatively correlated with discharge (J. Richey, unpublished data).…”