Controls on the Mg Cycle in the Tropics: Insights from a Case Study at the Luquillo Critical Zone Observatory

To better constrain the mechanisms controlling short-term Mg dynamics in the tropics, we sampled critical zone compartments of a catchment covered by thick, highly weathered regolith. Our Mg and δ26Mg data indicate that rain is a main source of Mg throughout the regolith, and we do not observe Mg isotope offsets in vegetation/surficial pore water. In addition to rain and weathering inputs, a heavy isotope excursion at ∼1 m depth indicates a fractionation process, likely sorption-desorption or clay dissolution. Stream water δ26Mg reflects inputs from rain and a heavy source, likely differential weathering along deep bedrock fractures.