Linking silicate weathering to riverine geochemistry—A case study from a mountainous tropical setting in west-central Panama

Abstract

Chemical analyses from 71 watersheds across an ~450 km transect in west-centralPanama provide insight into controls on weathering and rates of chemical denudation and CO 2 consumption across an igneous arc terrain in the tropics. Stream and river compositions across this region of Panama are generally dilute, having a total dissolved solute value = 118 ± 91 mg/L, with bicarbonate and silica being the predominant dissolved species. Solute, stable isotope, and radio genic isotope compositions are consistent with dissolution of igneous rocks present in Panama by meteoric precipitation, with geochemical signatures of rivers largely acquired in their upstream regions. Comparison of a head water basin with its entire watershed observed considerably more runoff production from the high-elevation upstream portion of the catchment than in its much more spatially extensive downstream region. Rock alteration profiles document that weathering proceeds primarily by dissolution of feldspar and pyrox ene, with base cations effectively leached in the following sequence: Na > Ca > Mg > K. Control on water chemistry by bedrock lithology is indicated through a linking of elevated ([Na + K]/[Ca + Mg]) ratios in watersto a high proportion of catchment area silicic bedrock and low ratios to mafic bedrock. Sr-isotope ratios are dominated by basementderived Sr, with only very minor, if any, contribution from other sources. Cation weather-ing of Ca sil + Mg sil + Na + K spans about an order in magnitude, from 3 to 32 tons/km 2 /yr. Strong positive correlations of chemical denudation and CO 2 consumption are observed with precipitation, mean watershed elevation, extent of land surface forest cover, and physical erosion rate.

Geochemistry of the Lake Chad Tributaries Under Strongly Varying Hydro-climatic Conditions

“…consumption, because its presence in solution is almost entirely due to silicate weathering (Négrel et al 1993;Gaillardet et al 1997;Amiotte Suchet et al 2003;Harmon et al 2016). It is predominant in the Chari and Logone rivers, with a mean annual value of 159 to 153 μmol/L respectively, which is similar to the average world river concentration of silica, but lower than for the Niger river.…”

Physical controls and ENSO event influence on weathering in the Panama Canal Watershed

“…between weathering fluxes and LULC practices in tropical SMR watersheds have been limited in scale to date 10,11,13 , due to low temporal resolution geospatial data. However, annual LULC data [forest and mosaic (i.e., forest plus croplands)] measured for the PCW on an annual scale by the ESA Climate Change Initiative (CCI) allowed for their direct comparison with cation values over the 17-year period (The presence of large-scale cropland in only one of the watersheds prevented a comparison with LULC practice).…”

“…In a search for controls, studies have recognized the strong positive feedback between physical and chemical weathering [1][2][3] and noted the importance of underlying volcanic lithologies on maintaining elevated chemical yields [4][5][6][7][8][9][10][11] . Others have identified strong correlations/associations between weathering yields and precipitation/ runoff 2,3,9,10,12 and, more recently, land use/landcover practices 10,11,13 . Yet, despite their important contribution to silicate weathering, few datasets for SMRs exist at high temporal resolution 10,14 and/or duration 15 for such high-yield terrains.…”

“…Our observed lack of statistically significant differences in discharge points to the importance of lithology, as well as other variables, in maintaining high chemical erosion rates. For example, watersheds on the north side of the canal are largely underlain by mafic to intermediate volcanic rocks compared to the largely sedimentary cover for those on the south side 11 . The importance of volcanic lithology in maintaining disproportionately high cation yields was previously documented as part of an isthmus-wide study 10,11 .…”

“…For example, watersheds on the north side of the canal are largely underlain by mafic to intermediate volcanic rocks compared to the largely sedimentary cover for those on the south side 11 . The importance of volcanic lithology in maintaining disproportionately high cation yields was previously documented as part of an isthmus-wide study 10,11 . While we observed a negative, and somewhat variable statistically significant relationship between cation yields and basin-wide mean annual temperature (r avg = − 0.39), this is likely due to increased cloud cover associated with precipitation events.…”