LCZO -- Nutrient Fluxes, Stable Isotopes -- Mg concentrations and isotopic signatures -- Bisley -- (2009-2011)

OVERVIEW

Description/Abstract

In order to assess the effects of critical zone processes on Mg concentrations and isotopic signatures of tropical streams, we studied a well constrained, highly weathered andesitic volcaniclastic catchment in the Luquillo Critical Zone Observatory, Puerto Rico. Our results indicate that dissolved Mg concentrations and isotope ratios in the regolith pore water are mainly controlled by rain input, with weathering inputs being more important at sites with thinner regolith (2.7–0.9 m deep) and at depth (>8 m) on a thick ridgetop regolith (∼10 m). In addition to mixing of precipitation and weathering-sourced Mg, an isotopic fractionation process is taking place between dissolved Mg and the regolith, likely during dissolution or recrystallisation of Fe(III)-(hydro)oxides under alternating redox conditions. Bulk regolith is isotopically heavier than both the bedrock and the exchangeable fraction (δ26Mgregolith-bedrock = +0.03 to +0.47‰), consistent with the preferential incorporation of heavy 26Mg into secondary minerals with some exchange of sorbed Mg with isotopically lighter pore water. Magnesium concentrations in the stream show a typical dilution behaviour during a storm event, but the [Mg] – δ26Mg pattern cannot be explained by mixing of rain and pore water; the data are best explained by a steady-state fractionation model with α = 1.00115. During baseflow the stream has δ26Mg = +0.01‰, higher than any of the water samples or the bedrock. In-situ analysis of the Mg isotopic composition of bedrock minerals points at the dissolution of Mg-rich chlorite (δ26Mg = +0.19‰) as the most likely source of this isotopically heavy Mg, with mass balance calculations indicating chlorite dissolution is also the main source of Mg to the stream. Overall, our study highlights the importance of atmospheric input of nutrients to the vegetation in tropical areas covered by thick, highly leached regolith, whereas the Mg flux and Mg isotopic signature of watershed exports are dominated by bedrock dissolution delivered to the stream through deeper, usually un-sampled critical zone pathways.

Creator/Author

Buss, Heather L.| Pogge von Strandmann, Philip A.E.|Chapela Lara, Maria |Schuessler, Jan A.| Moore, Oliver W.

CZOs

Luquillo

Contact

Miguel Leon, Miguel.Leon@unh.edu

Subtitle

The influence of critical zone processes on the Mg isotope budget in a tropical, highly weathered andesitic catchment

SUBJECTS

Disciplines

Biogeochemistry

Topics

Nutrient Fluxes|Stable Isotopes

Subtopic

Mg concentrations and isotopic signatures

Keywords

Weathering|Mg isotopes|Mg cycle|Critical zone|LCZO|Puerto Rico

Variables

magnesium|magnesium-25 stable isotope ratio delta|aluminium oxide|calcium oxide| potassium oxide|magnesium oxide|sodium oxide|silicon dioxide|Niobium| titanium dioxide|iron oxide|chromium oxide|manganese oxide|Quartz|Hematite|Goethite|Kaolinite|Illitea|Chlorite|silicon|aluminium|calcium

Variables ODM2

Magnesium|Aluminum|calcium oxide|potassium oxide|magnesium oxide|sodium oxide|silicon dioxide|Niobium, total|Iron, ferric|chromium oxide|manganese oxide|Quartz|Goethite|Clay|Silicon

TEMPORAL

Date Start

2009-05-08

Date End

2011-05-21

SPATIAL

Field Areas

Bisley

Location

Bisley

North latitude

18.322

South latitude

18.3108

West longitude

-65.7485

East longitude

-65.7349

REFERENCE

Citation

Chapela Lara, M., Buss, H. L., Pogge von Strandmann, P. A. E., Schuessler, J. A., & Moore, O. W. (2017): The influence of critical zone processes on the Mg isotope budget in a tropical, highly weathered andesitic catchment. Geochimica et Cosmochimica Acta. DOI: 10.1016/j.gca.2016.12.032

Publications of this data

Chapela Lara, M., Buss, H. L., Pogge von Strandmann, P. A. E., Schuessler, J. A., & Moore, O. W. (2017). The influence of critical zone processes on the Mg isotope budget in a tropical, highly weathered andesitic catchment. Geochimica et Cosmochimica Acta http://dx.doi.org/10.1016/j.gca.2016.12.032

CZO ID

7171