Mountain building can promote the sequestration of atmospheric carbon dioxide (CO2)
by the erosion of organic matter from the terrestrial biosphere (OCbiosphere), and
its transport in sediment-laden rivers to sedimentary deposits. However, erosion
and exhumation can expose rock-derived organic carbon to chemical weathering.
Oxidation of this petrogenic organic carbon (OCpetro) is a source of CO2which is very
poorly constrained. Here, the OC budget of a mountain belt is quantified to better
understand whether erosion in mountain belts results in a net source, or sink of
CO2.
The erosion, transport and offshore fate of OCbiosphere has been tracked through
mountain river systems in Taiwan, using a suite of elemental (C, %), stable isotope (δ13C)
and radiocarbon (Δ14C) measurements. During floods at high suspended sediment
concentrations, OCbiosphereand OCpetrois transferred rapidly to the deep ocean. Marine
sediments show evidence for efficient, long-term preservation of OCbiosphere. Large amounts
of terrestrial OC and sediment are also delivered to the surface ocean, dispersing OC
over a larger area. Marine sediments sourced by this delivery mechanism have
C, δ13C and Δ14C values which suggest >70% of the terrestrial OC is preserved
offshore.
To quantify OCpetro oxidation rates in mountain catchments, the trace element Rhenium
(Re) is used as a proxy. Existing measurements of physical erosion in Taiwan rate also allow
the controls on OCpetro oxidation to be assessed. Re is associated with OCpetro in rocks and
following oxidation during chemical weathering forms a soluble anion which contributes to
the dissolved load of rivers. Soils in Taiwan confirm that Re loss is coupled to OCpetro loss
during weathering, and so the dissolved Re flux is used to provide a first order
estimate of the corresponding release of CO2 in river catchments. OCpetro oxidation
increases with erosion rate. It is likely that the OCpetro oxidation rates estimated from
dissolved Re flux are an upper bound. Nevertheless, the estimated CO2 release from
Taiwan by OCpetro oxidation does not negate the estimates of CO2 sequestration by
burial of OCbiosphere offshore. The findings from Taiwan are compared to OC
transfers estimated for the Himalaya, where OCpetro oxidation in the mountain
belt remains unconstrained. Together, these cases suggest that mountain building
in the tropics can result in a net sink of OC which sequesters atmospheric CO2. |