Hydrological processes in the humid tropics differ from other regions in having greater energy inputs and faster rates of change. In this Review it is argued that understanding of the key hydrological interactions there remains limited, and a vision of future research designed to address these shortcomings is outlined.

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The hydrology of the humid tropics

Terrestrial carbon export via inland aquatic systems is a key process in the global carbon cycle It includes loss of carbon to the atmosphere via outgassing from rivers, lakes or reservoirs and carbon fixation in the water column as well as in sediments This review focuses on headwater streams that are important because their stream biogeochemistry directly reflects carbon input from soils and groundwaters that becomes superimposed by additional inputs further downstream Major drivers of carbon dioxide partial pressures (pCO2) in streams and mechanisms of terrestrial dissolved inorganic, organic and particulate organic carbon (DIC, DOC, and POC) influxes are summarized in this work Our analysis indicates that the global river average pCO2 of 3,100 ppmV is more often exceeded by contributions from small streams when compared to rivers with larger catchments (>500 km2) Because of their large proportion in global river networks (>96 % of the total number of streams), headwaters contribute large – but still poorly quantified – amounts of CO2 to the atmosphere Conservative estimates imply that globally 36 % (ie 093 Pg C yr-1) of total CO2 outgassing from rivers and streams originate from headwaters We also discuss challenges in determination of CO2 sources, concentrations and fluxes To overcome uncertainties of CO2 sources and its outgassing from headwater streams on the global scale, new investigations are needed that should include groundwater data Such studies would also benefit from applications of integral CO2 outgassing isotope approaches and multi-scale geophysical imaging techniques

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1002/2016RG000547

A review of CO2 and associated carbon dynamics in headwater streams: a global perspective

Mountain building results in high erosion rates and the interaction of rocks with the atmosphere, water and life. Carbon transfers that result from increased erosion could control the evolution of Earth’s long-term climate. For decades, attention has focused on the hypothesized role of mountain building in drawing down atmospheric carbon dioxide (CO2) via silicate weathering. However, it is now recognized that mountain building and erosion affect the carbon cycle in other important ways. For example, erosion mobilizes organic carbon (OC) from terrestrial vegetation, transferring it to rivers and sediments, and thereby acting to draw down atmospheric CO2 in tandem with silicate weathering. Meanwhile, exhumation of sedimentary rocks can release CO2 through the oxidation of rock OC and sulfide minerals. In this Review, we examine the mechanisms of carbon exchange between rocks and the atmosphere, and discuss the balance of CO2 sources and sinks. It is demonstrated that OC burial and oxidative weathering, not widely considered in most models, control the net CO2 budget associated with erosion. Lithology strongly influences the impact of mountain building on the global carbon cycle, with an orogeny dominated by sedimentary rocks, and thus abundant rock OC and sulfides, tending towards being a CO2 source. By increasing erosion, mountain building can steer the evolution of atmospheric carbon dioxide (CO2) and global climate. This Review expands from the canonical focus on silicate weathering to consider the net carbon budget of erosion, including both CO2 sinks (silicate weathering, organic-carbon burial) and CO2 sources (oxidative weathering).

/pdf/mountains-erosion-and-the-carbon-cycle-26vm6hrhf1.pdf

Mountains, erosion and the carbon cycle

Temperature dependence of basalt weathering

. Geological sequestration of atmospheric carbon dioxide (CO2) can be achieved by the erosion of organic carbon (OC) from the terrestrial biosphere and its burial in long-lived marine sediments. Rivers on mountain islands of Oceania in the western Pacific have very high rates of OC export to the ocean, yet its preservation offshore remains poorly constrained. Here we use the OC content (Corg, %), radiocarbon (Δ 14Corg) and stable isotope (δ13Corg) composition of sediments offshore Taiwan to assess the fate of terrestrial OC, using surface, sub-surface and Holocene sediments. We account for rock-derived OC to assess the preservation of OC eroded from the terrestrial biosphere and the associated CO2 sink during flood discharges (hyperpycnal river plumes) and when river inputs are dispersed more widely (hypopycnal). The Corg, Δ14Corg and δ 13Corg of marine sediment traps and cores indicate that during flood discharges, terrestrial OC can be transferred efficiently down submarine canyons to the deep ocean and accumulates offshore with little evidence for terrestrial OC loss. In marine sediments fed by dispersive river inputs, the Corg, Δ14Corg and δ 13Corg are consistent with mixing of terrestrial OC with marine OC and suggest that efficient preservation of terrestrial OC (>70%) is also associated with hypopycnal delivery. Sub-surface and Holocene sediments indicate that this preservation is long-lived on millennial timescales. Re-burial of rock-derived OC is pervasive. Our findings from Taiwan suggest that erosion and offshore burial of OC from the terrestrial biosphere may sequester >8 TgC yr−1 across Oceania, a significant geological CO2 sink which requires better constraint. We postulate that mountain islands of Oceania provide a strong link between tectonic uplift and the carbon cycle, one moderated by the climatic variability which controls terrestrial OC delivery to the ocean.

/pdf/preservation-of-terrestrial-organic-carbon-in-marine-z6pi053lw3.pdf

Preservation of terrestrial organic carbon in marine sediments offshore Taiwan: mountain building and atmospheric carbon dioxide sequestration

https://hal-insu.archives-ouvertes.fr/insu-00536476/document

Comparison of dissolved inorganic and organic carbon yields and fluxes in the watersheds of tropical volcanic islands, examples from Guadeloupe (French West Indies)

E. Lloret

Papers

Comparison of dissolved inorganic and organic carbon yields and fluxes in the watersheds of tropical volcanic islands, examples from Guadeloupe (French West Indies)

Dynamic of particulate and dissolved organic carbon in small volcanic mountainous tropical watersheds