Scholarly article on topic 'Isotopic Geochemistry of Panama Rivers'

Isotopic Geochemistry of Panama Rivers Academic research paper on "Earth and related environmental sciences"

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{Panama / "tropic weathering" / "riverine geochemistry" / "stable isotopes" / Sr-isotopes.}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Russell S. Harmon, Gerhard Wörner, Michael J. Pribil, Zoltán Kern, István Fórizs, et al.

Abstract River water samples collected from 78 watersheds rivers along a 500-km transect across a Late Cretaceous-Tertiary andesitic volcanic arc terrane in west-central Panama provide a synoptic overview of riverine geochemistry, chemical denudation, and CO2 consumption in the tropics. D/H and 18O/16O relationships indicate that bedrock dissolution of andesitic arc crust in Panama is driven by water-rock interaction with meteoric precipitation as it passes through the critical zone, with no evidence of a geothermal or hydrothermal input. Sr-isotope relationships suggest a geochemical evolution for Panama riverine waters that involves mixing of bedrock pore water with water having 87Sr/86Sr ratios between 0.7037-0.7043 and relatively high Sr-contents with waters of low Sr content that enriched in radiogenic Sr that are diluted by infiltrating rainfall to variable extents.

Academic research paper on topic "Isotopic Geochemistry of Panama Rivers"

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Procedia Earth and Planetary Science 13 (2015) 108 - 111

11th Applied Isotope Geochemistry Conference, AIG-11 BRGM

Isotopic Geochemistry of Panama Rivers

Russell S. Harmona*, Gerhard Wörnerb, Michael J. Pribilc, Zoltän Kernd, Istvän Forizsd, W. Berry Lyonsc, Christopher B. Gardnerc, and Steven T. Goldsmithd

aUSACE-ERDC International Research Office, Ruislip, UK HA4 7HB and North Carolina State University, Raleigh, North Carolina 27695, USA

b University of Göttingen, 37077 Göttingen, Germany cUS Geological Survey, PO Box 25046, MS973, Denver, CO 80225 dHungarian Academy of Sciences, Budapest, H-1112, Hungary eThe Ohio State University, Columbus, Ohio 43210 USA fVillanova University, Villanova, PA 19085 USA

Abstract

River water samples collected from 78 watersheds rivers along a 500-km transect across a Late Cretaceous-Tertiary andesitic volcanic arc terrane in west-central Panama provide a synoptic overview of riverine geochemistry, chemical denudation, and CO2 consumption in the tropics. D/H and 18O/16O relationships indicate that bedrock dissolution of andesitic arc crust in Panama is driven by water-rock interaction with meteoric precipitation as it passes through the critical zone, with no evidence of a geothermal or hydrothermal input. Sr-isotope relationships suggest a geochemical evolution for Panama riverine waters that involves mixing of bedrock pore water with water having 87Sr/86Sr ratios between 0.7037-0.7043 and relatively high Sr-contents with waters of low Sr content that enriched in radiogenic Sr that are diluted by infiltrating rainfall to variable extents.

© 2015TheAuthors.Published by ElsevierB.V.This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the scientific committee of AIG-11

Keywords: Panama: tropic weathering: riverine geochemistry: stable isotopes, Sr-isotopes.

1. Introduction and Background

The weathering mafic to intermediate silicates at the Earth's surface, plays a key role in the global geochemical cycle as a global carbon sink. Chemical denudation across the humid tropics is characterized by two distinguishing features that combine to control bedrock weathering - the continuous interaction of bedrock and saprolite with large amounts of precipitation and intense and continuous biological activity within the critical zone that together produce a highly-leached and acidic regolith. Situated between 7-10°N and 77-83°W, the Isthmus of Panama comprises a 77,082 km2 landscape of abundant small mountain rivers that receives a large spatial variation in rainfall because of its unique geographic location as the land bridge between North and South America. Therefore, Panama offers an

1878-5220 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the scientific committee of AIG-11

doi : 10. 1016/j .proeps .2015.07.026

ideal setting in which to investigate the origin of riverine solutes and evaluate the controls on silicate weathering and CO2 consumption in a tectonically active region of the humid tropics.

Formed by tectonic amalgamation, uplift, and faulting at an active plate margin from Late Cretaceous through Late Tertiary time, Panama consists of a diverse mosaic of typical constructional andesitic arc lithologies. Tholeiitic to low-K calc-alkaline basalts and andesites dominate extrusive suites, whereas gabbros to granodiorites are the most common intrusive rocks. Dacite, rhyolite, and granite are locally important lithologies. Tertiary ignimbrites are present in central Panama and explosive andesitic-dacitic volcanism occurred at isolated sites during the Quaternary. Panamanian igneous rocks belong to three petrogenetic suites: oceanic island, island arc, and adakitic. Geomorphologically, western Panama lies with a portion Central American volcanic arc that has undergone only limited erosion and, therefore its landscape is characterized by Late Tertiary-Holocene volcanic structures, elevations that exceed 3500m, and steep topography. Exposed rocks in western Panama comprise volcanic arc lavas, pyroclastics, and volcaniclastic sediments. Erosion has been more extensive across east central Panama, where elevations are typically below 1000m and incision has reached into the deeper Late Cretaceous-Mid-Tertiary sections of the arc to expose submarine volcanic and plutonic rocks. Although having very different trace element signatures, the mineralogical and, therefore, the major element character of Panamanian volcanic and intrusive igneous rocks is comparable, falling along the typical magmatic differentiation trend from mafic to silicic compositions. Thus, the solute signature of Panamanian rivers should be broadly similar in terms of major element composition, except where watersheds are developed on terrains dominated by high-Si igneous rocks or on marine carbonate lithologies.

Almost 500 rivers are present in Panama, most of which are small mountain rivers that originate in the Cordillera Central. Stream and river compositions across Panama are generally dilute and mildly alkaline, having a median electrical conductivity value of only 165 mS/cm. In general, drainages formed on igneous lithologies have lower dissolved content loads than those developed on marine sedimentary and mixed geology terrains. Silica is the predominant aqueous species in the stream and rivers sampled. Overall, the cation inventory across the watersheds samples is Ca~Na>Mg>K. No systematic differences are observed for either cation or anion compositions, except for waters draining sedimentary rocks. The general consistency of elemental composition and ranges of compositional variation observed for Panama are similar to that for small mountain rivers across the Caribbean basin. Cation weathering yields range over more than an order in magnitude from 2.6 to 22.1 tons/km 2/y, whilst silicate weathering yields range from 6.9 to 56 tons/km2/y. CO2 consumption rates for Panama range from 166 to 1157x103 mol/km2/y, falling toward the upper end of those observed globally and similar to rates observed across the Caribbean basin in Puerto Rico and the Lesser Antilles1,2.

2. Stable Isotopes

H- and O-isotope ratio ranges for 111 stream and river samples are -67.2 to -3.2%o for 8D and -9.8 to -1.6%o for S18O, with respective averages of -38±11%o and -6±1.5%o. Isotopic variations observed at specific sites reflects the sequential distillation condensation history of individual air masses, i.e. a 'rainout' effect', as they move from their Caribbean sources in the ITCZ across the Isthmus of Panama3. Precipitation stable isotope compositions has been measured for two sites in the in the trans-isthmus region of central Panama during the IAEA Global Network for Isotopes in Precipitation program. Monthly mean precipitation at these two sites has highly correlated isotopic compositions, described by the relationship 8D = 7.5S18O + 5.4, which conforms closely to the 'Global Meteoric Line' (GMWL4) of 8D = 8S18O +10. Together, all rainfall and surface water data from this study are highly correlated (R2 = 0.97) and well described by the relationship SD = 7.3S18O + 6, which is essentially equivalent to that for Costa Rica surface waters5 of SD = 7.6S18O + 10.5. This suggests that surface waters in Panama can be viewed as unmodified rainfall, with spatial and temporal trends in the isotopic composition of precipitation directly translated to shallow subsurface waters that discharge to streams and rivers. This would not be the case if the Panama streams and rivers contained a significant component of old ground water or hydrothermal water that had undergone significant water-rock interaction, both processes that would cause a mixing of meteoric and non-meteoric waters and the consequent displacement of waters away from the GMWL6. It is, therefore, concluded that

surface waters across central Panama are meteoric precipitation, produced through a simple evaporation-condensation process7 that have had only have short subsurface residence times that have not affected their H- and O-isotopic signatures.

3. Strontium Isotopes

Sr-isotopic compositions can be used to identify and constrain the 87Sr/86Sr ratio of potential sources of dissolved Sr. Sr contents of streams and rivers in Panama vary from 0.02 to 21.1 ^M. L-1, with 31 samples having

87 86 87

Sr/ Sr ratios between 0.70350 to 0.70527. Waters draining granitic terrains are enriched in Sr compared to those draining mafic compositions. Panama waters define a strongly linear array on a 87Sr/86Sr vs. 1/Sr plot that is best explained by 2-component mixing, with respective end-members of (i) a silicate rock reservoir characterized by high-Sr content and low 87Sr/ 86Sr ratios and (ii) a Sr-deficient (high Ca/Sr) source or strongly diluted waters derived therefrom that is comparatively enriched in radiogenic Sr. The mean value and range of 87Sr/ 86Sr =

0.70365.0.00018 for arc rocks of Cretaceous to Quaternary age in Panama8 is unexpectedly small given the large compositional variation from gabbro to granite and basaltic andesite to dacite present across the across the Panama volcanic arc. This implies a single, well-constrained sub-arc mantle source for the rock-derived end-member. Processes involving radiogenic reservoirs that, in principle, might influence the Sr-isotopic composition of river waters and thus explain the binary mixing trend are: (1) atmospheric deposition of Sr transported by sea spray and precipitation9, (2) dissolution of marine sedimentary carbonates, (3) preferential leaching from Rb-rich micas in granites, (4) leaching of secondary calcite present in the Cretaceous and Miocene submarine, hydrothermally-altered volcanic rocks, and (5) atmospheric dust transported from the African Sahel across the Atlantic Ocean and Caribbean Sea to Panama10. Although the atmospheric delivery of Sr to coastal regions of the tropics has been documented9, 11, a seawater-derived source for the mixing component (source 1) in Panama is unlikely, as there is no correlation between the Sr-isotopic composition of waters and their Cl contents. Marine carbonates that are enriched in radiogenic 87Sr (source 2) occur only in a few restricted places in Panama and, therefore, cannot be the geographically widespread radiogenic component that is required as a ubiquitous mixing component in waters draining volcanic arc landscape across west-central Panama. Preferential leaching of radiogenic Sr from Rb-rich minerals in granites (source 3) or secondary interstitial hydrothermal calcite source (source 4) are both reasonable possibilities for producing variably radiogenic Sr-isotope signatures in stream waters. A large portion of the basement of the Cordillera Central of Panama has been constructed from submarine volcanic and intrusive rocks that, in places, were affected by sea-water hydrothermal alteration. Any carbonate deposited from marine hydrothermal fluids in altered submarine igneous rocks would be dominated by the seawater Sr-isotope composition at the time of its formation. Simple mixing calculations have been performed to test these hypothesis. The dispersion

87 86 87 86

of data is observed on a plot of Sr/ Sr vs. Ca/Sr that is distinct from the well-defined array in the Sr/ Sr versus 1/Sr plot, thus ruling out a simple two-component mixing model. This modeling leads to the conclusion that the

87 86 87

higher Sr/ Sr ratios observed for streams draining granites and their Ca/Sr ratios are best explained by Sr-enriched waters from dissolving hydrothermal seawater carbonate, with a possible minor contribution from Sahara-derived dust. Waters from Sr-poor and Rb-rich minerals in the granites, marine carbonates and modern seawater are

87 86 87 86 87

less likely sources. In order to satisfy the combined °'Sr/ouSr-1/Sr and °'Sr/ouSr-Ca/Sr relations, the °'Sr-rich end-member waters need to be both subsequently heavily diluted with modern rainwater and their Ca/Sr ratio need to be substantially increased by preferential retention of Sr.

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