Speaker –  Russell Harmon, MEAS-Earth Science (host – E. Hyland)

Seminar Title – Linking Silicate Weathering to Riverine Geochemistry – Case study from a mountainous tropical setting in west-central Panama

Abstract – Comprising a diversity of common igneous and sedimentary lithologies, the landscape of Panama reflects its origin and evolution as an inter-oceanic island arc. This landscape developed and matured over the past 100 My as a consequence of tectonic interactions of the Caribbean, South American, Cocos, and Nazca plates that resulted in the creation of the isthmus of Panama and formation of the Central American Land Bridges 2-3 million years ago. The geochemistry of 71 watersheds across a 500-km transect in the mountainous area of west-central Panama provides insight into controls on weathering and rates of chemical denudation and CO2 consumption across an igneous arc terrain in the tropics. Stream and river compositions across central Panama are generally dilute, with a mean TDS value of 118±91 with bicarbonate and silica the predominant dissolved species. Solute contents and stable isotope compositions are consistent with dissolution of igneous rocks present in Panama by meteoric precipitation, with geochemical signatures largely acquired in river headwater regions. Rock alteration profiles document that weathering proceeds primarily by dissolution of feldspar and pyroxene. Sr-isotope ratios are dominated by Sr derived from the igneous basement, with only minor contributions from other sources. Cation weathering yields (Casil+Mgsil+Na+K as tons/km2/y) span about an order in magnitude, from 3 to 32 tons/km2/y. CO2 consumption ranges from 166 to 1157 x103 mol/km2/y, falling towards the upper end observed globally, and are consistent with higher CO2 consumption rates on mafic to intermediate igneous terrains. Strong positive correlations of chemical denudation and CO2 consumption are observed with precipitation, mean watershed elevation, extent of land surface forest cover, and physical erosion rate.

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