River geochemistry, chemical weathering, and atmospheric CO₂ consumption rates in the Virunga Volcanic Province (East Africa)

Abstract

We report a water chemistry data set from 13 rivers of the Virunga Volcanic Province (VVP) (Democratic Republic of Congo), sampled between December 2010 and February 2013. Most parameters showed no pronounced seasonal variation, whereas their spatial variation suggests a strong control by lithology, soil type, slope, and vegetation. High total suspended matter (289-1467 mg L 21 ) was recorded in rivers in the Lake Kivu catchment, indicating high soil erodibility, partly as a consequence of deforestation and farming activities. Dissolved and particulate organic carbon (DOC and POC) were lower in rivers from lava fields, and higher in nonvolcanic subcatchments. Stable carbon isotope signatures (d 13 C) of POC and DOC mean d 13 C of 222.5& and 223.5&, respectively, are the first data to be reported for the highland of the Congo River basin and showed a much higher C4 contribution than in lowland areas. Rivers of the VVP were net sources of CH 4 to the atmosphere (4-5052 nmol L 21 ). Most rivers show N 2 O concentrations close to equilibrium, but some rivers showed high N 2 O concentrations related to denitrification in groundwaters. d 13 C signatures of dissolved inorganic carbon suggested magmatic CO 2 inputs to aquifers/soil, which could have contributed to increase basalt weathering rates. This magmatic CO 2 -mediated basalt weathering strongly contributed to the high major cation concentrations and total alkalinity. Thus, chemical weathering (39.0-2779.9 t km 22 yr 21 ) and atmospheric CO 2 consumption (0.4-37.0 3 10 6 mol km 22 yr 21 ) rates were higher than previously reported in the literature for basaltic terrains.

Variations in dissolved greenhouse gases (CO<sub>2</sub>, CH<sub>4</sub>, N<sub>2</sub>O) in the Congo River network overwhelmingly driven by fluvial-wetland connectivity

“…Note that TA values from the Congo are generally very low compared to other large rivers globally (Meybeck, 1987), due to the large proportion of relatively insoluble rocks on the catchment (70 % of metamorphic rocks) and a small proportion of low soluble rocks such as siliciclastic sedimentary rocks (10 % mainly as sand stone), unconsolidated sediments (17 % as sand and clays) and a very small proportion of highly soluble volcanic rocks (1 %). The high TA values in the Lualaba are due to a larger proportion of volcanic rocks in high-altitude areas, such as the Virunga region, which is rich in volcanic rocks (including basalts) and has been shown to be a hotspot of chemical weathering (Balagizi et al, 2015).…”

“…However, there can be large errors associated with the computation of the dissolved CO 2 concentration from pH and total alkalinity (TA) for low alkalinity waters in particular so that high-quality direct measurements of dissolved CO 2 concentration are preferred (Abril et al, 2015), although scarce. In the tropics, research on GHGs in rivers has mainly focussed on South American rivers and on the central Amazon in particular (Richey et al, 1988(Richey et al, , 2002Melack et al, 2004;Abril et al, 2014;Barbosa et al, 2016;Scofield et al, 2016), while until recently African rivers were nearly uncharted with a few exceptions (Koné et al, 2009(Koné et al, , 2010.…”

Aging of basalt volcanic systems and decreasing CO<sub>2</sub> consumption by weathering

“…For the Lesser Antilles, a magmatic contribution of 23 % to 40 % to the CO 2 consumed by weathering was identified . High 13 C-dissolved inorganic carbon (DIC) values suggest that magmatic CO 2 contributes significantly to the alkalinity fluxes from the Virunga system (Balagizi et al, 2015). The magmatic CO 2 contribution derived from volcanic calcite dissolution in Iceland was estimated to be about 10 % of the alkalinity fluxes for the studied area (Jacobson et al, 2015).…”

“…Basalt areas, despite their limited areal coverage, contribute significantly to CO 2 sequestration by silicate rock weathering (Gaillardet et al, 1999;Dessert et al, 2003;Hartmann et al, 2009). The sensitivity of basalt weathering to climate change (Dessert et al, 2001Coogan and Dosso, 2015;Li et al, 2016) supports a negative weathering feedback in the carbon cycle that maintains the habitability of the Earth's surface over geological timescales (Walker et al, 1981;Berner et al, 1983;Li and Elderfield, 2013). Changes in volcanic weathering fluxes due to emplacement of large volcanic provinces or shifts in the geographic distribution of volcanic fields associated with continental drift may have contributed to climate change in the past (Goddéris et al, 2003;Schaller et al, 2012;Kent and Muttoni, 2013).…”

Supplementary material to "Short Communication: Aging of basalt volcanic systems and decreasing CO<sub>2</sub> consumption by weathering"

“…The Virunga Volcanic province is located in eastern central Africa and is related to the East African Rift system. The geologic map of this area was compiled out of four different maps from Smets et al (2010), Balagizi et al (2015), Petricec et al (1971) and De Mulder et al (1986).…”