filename: 2008gb003349_txts01.txt
created: October 14, 2008 by NG
last change: none

River fluxes
In steady-state, that part of the input of inorganic and organic
carbon into the ocean by rivers that escapes burial is released back
into the atmosphere as a flux of CO2 across the air-sea interface
[Sarmiento and Sundquist, 1992]. While the burial rate of carbon on
the seafloor of the deeper ocean is reasonably well established (about
0.1 PgC/yr as organic carbon and about 0.1 PgC/yr as CaCO3), the
burial in shallow sediments as well as the net input of carbon by
rivers beyond the river mouth is poorly established. One reason is
that most transport estimates of riverine carbon pertain to a location
far upstream of the river mouth, and therefore do not include the
myriad of transformation processes that occur in the estuaries and in
the very nearshore environments. The most commonly adopted estimate
for the river input of carbon is 0.4 PgC/yr in the form of organic
carbon, and 0.4 PgC/yr as inorganic carbon [Sarmiento and Sundquist,
1992]. Assuming a burial flux of 0.2 PgC/yr, requires a CO2 outgassing
of 0.6 PgC/yr in steady-state. More recent analyses suggest a much
larger input of organic carbon, perhaps as large as 1 PgC/yr [Richey,
2004], although it is unclear how much of this flux makes it past the
estuary. Nor is it known to which extent current river input estimates
reflect anthropogenically perturbed systems or pre-industrial
conditions. 

As the ultimate aim of the inversion is to estimate the contemporary
CO2 flux across the air-sea interface, we have to evaluate whether our
inversely based estimates include this steady-state outgassing of
river-derived carbon, or whether we need to apply corrections. The
answer to this question depends on two independent aspects: First, in
what respect does the DCgasex tracer reflect the gain of (inorganic)
carbon from rivers and its subsequent loss by air-sea exchange?
Second, what fraction of the global outgassing of river-derived carbon
is actually reflected in the data set employed in our study, whose
sampling locations are predominantly in the open ocean?

The answer to the first question depends critically on the carbon-to-
phosphorus ratio in both the inorganic and organic fractions of the
river supply. For clarity, we discuss separately how the inversion
attributes the organic and inorganic carbon that is added to the ocean
by rivers.  With regard to the input of dissolved inorganic matter,
rivers tend to have a very low ratio of phosphate to DIC [Meybeck,
1993]. As a result, DIC added to the ocean via rivers is reflected on
a nearly mol to mol basis as a gain in DCgasex. When that river
derived CO2 is eventually outgassed to the atmosphere, there is a
corresponding decrease in DCgasex. In a perfectly sampled ocean, a
perfect inversion, i.e. an inversion free from systematic biases, will
infer from this increase of DCgasex in coastal regions an uptake of
CO2 from the atmosphere, and will infer from the decrease in DCgasex
in the open ocean an outgassing of CO2, with the two fluxes balancing
each other.

With regard to organic carbon, there are similarities and differences.
As is the case for the river input of dissolved inorganic matter, the
phosphorus-to-carbon ratio in dissolved organic matter is much smaller
than the canonical stoichiometric ratio of marine organic matter
(Redfield ratio). Thus, the remineralization of this organic carbon
leads to an increase in DCgasex on a nearly mol to mol basis, which is
then interpreted by the inversion as an uptake of CO2 from the
atmosphere. This organic carbon derived CO2 will eventually outgas,
which will be reflected in DCgasex as a decrease, and hence correctly
attributed in the inversion to a sea-to-air flux.  Thus, analogous to
inorganic carbon, the air-sea fluxes estimated by a perfect inversion
of a perfectly sampled ocean will balance globally. There is an
important difference to the river input of inorganic carbon, however,
as organic carbon changes DCgasex only at the location where it is
remineralized. Thus, the riverine organic carbon signal is not
attributed to an air-to-sea flux in the region where the rivers enter
the oceans, but rather attributed to the ocean region where the
organic carbon is remineralized.  In summary, the oceanic inversion of
DCgasex tends to find a globally balanced flux even in the presence of
a steady-state outgassing of riverine carbon. This balance emerges
because the inversion incorrectly interprets the addition of carbon by
rivers as an air-to-sea flux, while it correctly determines the
sea-to-air flux associated with the outgassing of the riverine
carbon. Therefore, we need to subtract from the "raw" inversion
estimates the riverine carbon signal that was incorrectly attributed
by the ocean inversion to an air-to-sea flux.

To achieve this we add to our "raw" inverse fluxes a regionally
specific estimate of the net riverine carbon input (the total input of
river carbon minus the carbon that gets buried on the seafloor
[Jacobson et al., 2007]). Specifically, we adopt an estimate of 0.45
PgC/yr for the global total outgassing of riverine carbon based on the
analysis of Jacobson et al. [2007] and distribute this flux regionally
on the basis of the spatially resolved GEM-CO2 product, which is based
on the work of Ludwig et al. [1996] (see Jacobson et al. [2007] for
further details). The magnitude of the global adjustment as well as
its regionalization is uncertain, so that we assign an uncertainty of
±50% to these riverine carbon fluxes.

The answer to the second question, i.e. whether our network is
actually reflecting the input of riverine carbon and its subsequent
outgassing, turns out to be less important, despite the fact that we
are using primarily an open ocean network. This is because the
nversion misses not only the added inorganic river carbon signal in
the case of lacking data in coastal regions, but also the resulting
ocean outgassing signal. As a result, the lack of coastal data causes
no imbalances in the inverted signals. However, there is a reduction
of the river flux subtraction we need to apply to obtain net-air sea
fluxes (which is taken into account by Jacobson et al. [2007]).


References:

Jacobson, A. R., S. E. Mikaloff Fletcher, N. Gruber, J. L. Sarmiento, and
M. Gloor (2007), A joint atmosphere-ocean inversion for surface fluxes
of carbon dioxide: 1. Methods and global-scale fluxes, Global Biogeochem.
Cycles, 21, GB1019, doi:10.1029/2005GB002556.

Ludwig,W., J.-L. Probst, and S.Kempe (1996), Predicting the oceanic input
of organic carbon by continental erosion, Global Biogeochem. Cycles,
10(1), 23-41.

Meybeck, M. (1993), C, N, P and S in rivers: from sources to global inputs,
in Interactions of C, N, P and S, Biogeochemical Cycles and Global
Change, edited by R. Wollast, F. T. Mackenzie, and L. Chou, pp. 163-
193, Springer, Berlin.

Richey, J. E. (2004), Pathways of atmospheric CO2 through fluvial systems,
in The Global Carbon Cycle: Integrating Humans, Climate, and
the Natural World, edited by C. B. Field and M. R. Raupach, chap. 2, pp.
329-340, Island Press, Washington, D. C.

Sarmiento, J. L., and E. T. Sundquist (1992), Revised budget for the oceanic
uptake of anthropogenic carbon dioxide, Nature, 356, 589-593.