Scholarly article on topic 'Surface Water Geochemistry and Chemical Weathering Across Panama'

Surface Water Geochemistry and Chemical Weathering Across Panama Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Russell S. Harmon, W. Berry Lyons, Christopher B. Gardner, Steven T. Goldsmith, David T. Long, et al.

Abstract A geochemical study of rivers and streams was undertaken across Panama during 2005-09, from the Lago Bayano area in the east to the Costa Rican border in the west. Low overall dissolved solute contents (TDS = 145±160mg/L) suggest a short residence time for infiltrating precipitation in the weathering zone. Watershed lithology exerts the main control on riverine chemistry, with streams on marine sedimentary rocks having higher dissolved solids loads than those on igneous rocks, with the latter exhibiting the highest silica contents and increasing trends of total cations with increasing dissolved silica. This feature and the large degree of compositional overlap between large rivers and small tributary streams implies that chemical weathering of silicate materials in the soil zone is the predominant process determining the geochemistry of streams and rivers in this tropical environment. Silicate weathering rates (Casil+Mgsil+Na+K) range over more than an order in magnitude from 2.5 to 28.4 tons/km2/y, whilst H4SiO4 yields range from 7.1 to 65 tons/km2/y. Basin-wide CO2 consumption yields by silicate weathering, calculated from total cation content (corrected for sea salt contribution), basin area and discharge, are high on a global basis.

Academic research paper on topic "Surface Water Geochemistry and Chemical Weathering Across Panama"

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Procedia Earth and Planetary Science 7 (2013) 342 - 345

Water Rock Interaction [WRI 14]

Surface water geochemistry and chemical weathering across

Panama

Russell S. Harmon3*, W. Berry Lyonsb, Christopher B. Gardnerb, Steven T. Goldsmith® David T. Longd, Helena Mitasovaa, Susan Welchb,

Kathy Welchb

a Department of Marine, Earth, & Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695 b Department of Geological Sciences, Ohio State University, Columbus, OH 43210 c Department of Geography and the Environment, Villanova University, Villanova, PA 19805 d Department of Geological Sciences, Michigan State University, East Lansing, MI 48824

Abstract

A geochemical study of rivers and streams was undertaken across Panama during 2005-09, from the Lago Bayano area in the east to the Costa Rican border in the west. Low overall dissolved solute contents (TDS = 145±160 mg/L) suggest a short residence time for infiltrating precipitation in the weathering zone. Watershed lithology exerts the main control on riverine chemistry, with streams on marine sedimentary rocks having higher dissolved solids loads than those on igneous rocks, with the latter exhibiting the highest silica contents and increasing trends of total cations with increasing dissolved silica. This feature and the large degree of compositional overlap between large rivers and small tributary streams implies that chemical weathering of silicate materials in the soil zone is the predominant process determining the geochemistry of streams and rivers in this tropical environment. Silicate weathering rates (Casii+Mgsii+Na+K) range over more than an order in magnitude from 2.5 to 28.4 tons/km2/y, whilst H4SiO4 yields range from 7.1 to 65 tons/km2/y. Basin-wide CO2 consumption yields by silicate weathering, calculated from total cation content (corrected for sea salt contribution), basin area and discharge, are high on a global basis.

© 2013TheAuthors.PublishedbyElsevierB.V.

Selection and/orpeer-reviewunderresponsibilityoftheOrganizingandScientificCommitteeofWRI14- 2013 Keywords: Panama, tropical rivers, river chemistry, chemical denudation.

1. Introduction

Although the tropics cover some 22% of global land area, their hydrology and riverine geochemistry are not well understood. This is particularly true for the 80% of the tropics that receive high overall

* Corresponding author. Tel.: +44-1895-616192; fax: +44-1895-616015. E-mail address: Russell.S.Harmon@usace.army.mil.

1878-5220 © 2013 The Authors. Published by Elsevier B.V.

Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14 -doi:10.1016/j.proeps.2013.03.134

amounts of precipitation, but have an extended dry season that can result in inadequate water supply for human use (Finlayson et al., 2011). Wohl et al. (2012) have noted that hydrologic data collection in the tropics has declined in recent years despite the pressing need for more and higher quality measurements, and as climate-change projections predict more frequent and severe droughts and storms in the tropics for the future (IPCC, 2008). Also, the humid tropics are an important contributor to global chemical erosion, but few denudation studies have been conducted for tropical watersheds.

Panama is an 'S-shaped', E-W oriented isthmus, located between 7.2-9.6°N latitude and 77.2- 83.0°W longitude, that forms a land bridge between South and Central America. Elevations in Panama range from sea level up to almost 3500 m. Excluding the low area in which the Panama Canal is located, the Panamanian landscape consists predominantly of interior mountains and dissected upland plains that separate the rolling hills and plains of the Caribbean and Pacific coastal zones. The landscape of Panama is dominated by tropical forest that is interrupted at lower elevations by urban development and extensive commercial woodlands, pastures and agricultural lands, except for the semi-arid and largely deforested Azuero peninsula. Overall, Panama's climate is typical of the low-latitude humid tropics in that it is characterized by near-uniform average temperature, abundant rainfall and a distinct seasonality of precipitation. Because of the strong influence of the Inter-Tropical Convergence Zone, about 90% of annual precipitation in Panama falls during May to December, followed by a marked dry season from January to April. Rainy season storms generated in the Caribbean tend to track from north to south, so that there is a pronounced N-S precipitation gradient across the Panamanian isthmus. Although year-to-year variations in total precipitation can be significant, an important consequence of the presence of the Cordillera Central as a longitudinal spine running from E-W across Panama is that the Atlantic coastal region receives an annual average of >3000 mm of precipitation compared with <2000 mm for the Pacific coastal region on the leeward side of the continental divide.

2. Results and Discussion

Over 800 samples were collected during 2005-09 from 127 named rivers, plus tributary streams and soil seeps, in 29 of Panama's 52 hydrographic basins in a series of sample transects that extended from the Bayano region in the east to the Costa Rica border in the west that included the Panama Canal Zone, the Azuero and Burica Peninsulas, the Cordillera Central in Chiriqui Province, and the Bocas del Toro region. The bedrock geology of the areas sampled consists of magmatic rocks of Late Cretaceous-Miocene age, Tertiary marine sediments and Quaternary volcanics (Worner et al., 2005; Wegner et al, 2010). Rivers were sampled during both wet and dry seasons, with most rivers sampled multiple times. Temperature, conductivity and pH were measured in the field at the time of sample collection and dissolved constituents (F, Br, Cl, NO3, SO4, PO4, Si, Ca, Mg, Na, K, Li, Sr, & Ba) subsequently analysed in the laboratory at The Ohio State University by ion chromatography and ICP-OES. Bicarbonate was estimated by charge balance. Five water types (i) soils seeps and springs (SS), (ii) low-order tributary streams (LOTS), (iii) high-order tributary streams (HOTS), (iv) major tributary rivers (MTR), and (v) main-stem rivers (MSR) were assigned into one of five different geological domains according to the predominant bedrock terrain of the watercourse: (i) Quaternary volcanics (QV) with an adakitic chemical affinity, (ii) Miocene (MIR) and (iii) Paleocene-Eocene (PEIR) igneous rocks with an island arc character, (iv) Cretaceous igneous rocks (CIR) with a volcanic arc affinity constructed on proto-arc crust and uplifted portions of oceanic crust of the Caribbean plate, and (v) Tertiary marine sedimentary rocks (TS).

Overall, Panama stream and river waters are predominantly of the Ca-HCO3- type. Respective ranges, mean values, and coefficients of variation for T (oC), TDS (mg/L), and pH are: 12.7-34.6 (25.1±2.6, 10.4%); 6-1901 (145±160, 110%); and 5.05-8.72 (7.46±0.59, 7.9%). Average dissolved solids contents (in mgL-1) are: Si = 15.8±6.3, Ca2+ = 11.2±8.6, Mg2+ = 4.7±3.9, Na+ = 9.7±15.9, K+ = 1.2±2.9, Sr = 0.12±0.22, Cl- = 8.2±23.4, NO3- = 0.38±1.35, and SO42- = 5.9±12.3. Other chemical constituents are

present in Panama soil seeps, streams, and rivers at levels of <0.1 mg/L. Compositional variability reflects both the nature of the lithology from which drainage is occurring and time since the last strong precipitation event. The highest dissolved solids loads and variations are observed in small streams draining the Tertiary sedimentary terranes. Only minor compositional differences are observed for the streams and rivers in igneous terranes, which exhibit roughly equivalent silica values. CIR streams and rivers are enriched in Ca, Mg, and Na relative to those in PEIR, MOV, and QV terranes, reflecting the presence of marine sediments in the accretional arc rocks of this terrane. A general trend of increasing cation contents is observed from SS to LOTS to HOTS that is reversed for the MTR and MSR. The TES streams exhibit the most difference among water types for a specific geological domain. Piper diagrams reveal differences among the water domains, that can be classified as follows: TES and CIR waters are Ca-HCO3 dominant with the latter enriched in Ca, Mg, and Na; the PEIR and MIR waters are Ca/Mg-HCO3 dominant with the PEIR waters more Mg enriched, and the QV waters are HCO3 dominant. The cause of these differences can mainly be attributed to the degree the domain waters are influenced by different weathering reactions, mixing with end-member solutions such as sea aerosols, and/or the possible presence of water of different hydrochemical facies within a water type. There is a strong relationship between H4SiO4 and total cation concentrations throughout the data set. This strongly suggests that the chemical weathering of silicate materials is the major process controlling the geochemistry of Panama streams and rivers. Bicarbonate concentrations above that which can be explained by carbonate mineral dissolution, also supports this notion. Together, these data indicate that the chemical weathering patterns and transport through Panama watersheds are complex and vary with the seasonality of rainfall and hydrologic regime. A general downstream increase in total dissolved solids was observed in major rivers sampled at multiple points regardless of watershed lithology. Total cation concentrations were higher during base flow conditions. Gibbs and Schoeller plots indicate that lithology exerts the major control on riverine chemistry in Panama. Streams and rivers developed on magmatic rocks exhibit the highest silica and increasing trends of total cations with increasing dissolved silica. This feature and the large degree of compositional overlap between SS, LOTS, and HOTS with MTR and MSR indicates that chemical weathering of silicate materials in the soil zone and at the soil-rock interface is likely the predominant process determining the geochemistry of streams and rivers in this tropical environment.

3. Silicate Weathering, H4SiO4 Yields, & CO2 Consumption

Silicate weathering rates and H4SiO4 yields were determined using a dataset consisting of samples from two regions: 38 spot samples from 28 rivers trans-Panama region during 2006-2007 and 35 spot samples from five rivers for the trans-isthmus region of the Panama Canal watershed during 2005-2009. Prior to its use in calculations, the precipitation-corrected dataset was further corrected for non-silicate contribution of Ca and Mg using previously established continental silicate end member ratios of 0.35 for Ca/Na and 0.24 for Mg/Na (Gaillardet et al., 1999). A multi-step process was employed whereby individual cations in the corrected dataset were first multiplied by the average daily discharge value provided by either Empresa de Transmisión Eléctrica (ETESA) or the Autoridad del Canal de Panama in order to produce an instantaneous denudation flux. Plots comparing the instantaneous denudation fluxes with respective discharge values were then compiled separately for the two regions in order to produce specific elemental flux determination equations. Justification of this approach was supported by high correlation values observed in the instantaneous denudation fluxes/discharge comparisons for both the trans-Panama (Ca=0.88; Mg=0.78; Na=0.77; K=0.70; and Si=0.84) and Panama Canal region (Ca= 0.89; Mg=0.89; Na=0.89; K =0.77, and Si=0.95). Finally, long-term ETESA monthly discharge values (15+ years) were substituted into the equations and calculated values were subsequently divided by watershed area to produce silicate weathering rates and H4SiO4 yields for 37 rivers in the trans-Panama region and 5 rivers in the Panama Canal region, respectively.

Silicate weathering rates fluxes (Casli+ Mgsli + Na + K) range over more than an order in magnitude from 2.5 to 28.4 tons/km2/y, with the highest average values identified with the Chagres and Bayano portions of the Panama Canal region (21.2 and 20.1 tons/km2/y, respectively) and the Chiriqui and Bocas del Toro regions of western Panama (12.5 and 13.9 tons/km2/y, respectively). This is not surprising, as the elevated topography of these regions has been shown to play an important role in increasing average annual precipitation (Harmon et al., 2009). The idea of lithologic control on the magnitude of chemical weathering observed may be further supported by the andesitic Veraguas and Cocle regions, which exhibit the next highest average rates (9.8 and 6.1 tons/km2/y, respectively) despite their relative lack of precipitation. The range of variation for H4SiO4 yields was slightly larger, from 7.1 to 65 tons/km2/y and exhibited similar regional trends. In both cases, the Rio Pequini of the Chagres region exhibited the highest overall values. Panama H4SiO4 yields were similar to those observed for andesitic active margin terrains of 12 to 33.6 tons/km2/y for the Taranaki region of New Zealand and 3.1 to 55.4 tons/km2/y for Dominica (Goldsmith et al. 2008, 2010).

CO2 consumption was calculated by taking the precipitation and silicate corrected Na+K+Mg(2x)+Ca(2x) annual flux values and dividing by watershed area. CO2 consumption rates range from 130 to 1370 x103 mol/km2/y. These values fall toward the upper end of the ranges observed for actively eroding convergent settings, i.e. 100-320 x103 mol/km2/y for the Himalayas and 220-1000 x103 mol/km2/y for the Andes by Edmond and Huh (1997) and within the low to middle ranges for andesitic volcanic settings of 1100-1400 x103 mol/km2/y for Martinique and Guadalupe by Rad et al. (2006) and 190-1575 x103 mol/km2/y for Dominica by Goldsmith et al. (2010). The fact that these CO2 consumption rates fall at the lower end of those determined for basaltic terrains of 1300-4400 x103 mol/km2/y for Reunion by Louvat and Allègre (1997) and 580-5240 x103 mol/km2/y for the Deccan traps by Dessert et al. (2001) may be representative of comparatively lower runoff as well as lithology-limited reaction kinetics.

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