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

AbstractTo 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

Lithological influences on contemporary and long-term regolith weathering at the Luquillo Critical Zone Observatory

Lithologic differences give rise to the differential weatherability of the Earth’s surface and globally variable silicate weathering fluxes, which provide an important negative feedback on climate over geologic timescales. To isolate the influence of lithology on weathering rates and mechanisms, we compare two nearby catchments in the Luquillo Critical Zone Observatory in Puerto Rico, which have similar climate history, relief and vegetation, but differ in bedrock lithology. Regolith and pore water samples with depth were collected from two ridgetops and at three sites along a slope transect in the volcaniclastic Bisley catchment and compared to existing data from the granitic Río Icacos catchment. The depth variations of solid-state and pore water chemistry and quantitative mineralogy were used to calculate mass transfer (tau) and weathering solute profiles, which in turn were used to determine weathering mechanisms and to estimate weathering rates. Regolith formed on both lithologies is highly leached of most labile elements, although Mg and K are less depleted in the granitic than in the volcaniclastic profiles, reflecting residual biotite in the granitic regolith not present in the volcaniclastics. Profiles of both lithologies that terminate at bedrock corestones are less weathered at depth, near the rock-regolith interfaces. Mg fluxes in the volcaniclastics derive primarily from dissolution of chlorite near the rock-regolith interface and from dissolution of illite and secondary phases in the upper regolith, whereas in the granitic profile, Mg and K fluxes derive from biotite dissolution. Long-term mineral dissolution rates and weathering fluxes were determined by integrating mass losses over the thickness of solid-state weathering fronts, and are therefore averages over the timescale of regolith development. Resulting long-term dissolution rates for minerals in the volcaniclastic regolith include chlorite: 8.9 × 10‾¹⁴ mol m‾² s‾¹, illite: 2.1 × 10‾¹⁴ mol m‾² s‾¹ and kaolinite: 4.0 × 10‾¹⁴ mol m‾² s‾¹. Long-term weathering fluxes are several orders of magnitude lower in the granitic regolith than in the volcaniclastic, despite higher abundances of several elements in the granitic regolith. Contemporary weathering fluxes were determined from net (rain-corrected) solute profiles and thus represent rates over the residence time of water in the regolith. Contemporary weathering fluxes within the granitic regolith are similar to the long-term fluxes. In contrast, the long-term fluxes are faster than the contemporary fluxes in the volcaniclastic regolith. Contemporary fluxes in the granitic regolith are generally also slightly faster than in the volcaniclastic. The differences in weathering fluxes over space and time between these two watersheds indicate significant lithologic control of chemical weathering mechanisms and rates

Secondary Minerals Drive Extreme Lithium Isotope Fractionation During Tropical Weathering

Lithium isotopes are used to trace weathering intensity, but little is known about the processes that fractionate them in highly weathered settings, where secondary minerals play a dominant role in weathering reactions. To help fill this gap in our knowledge of Li isotope systematics, we investigated Li isotope fractionation at an andesitic catchment in Puerto Rico, where the highest rates of silicate weathering on Earth have been documented. We found the lowest δ7Li values published to date for porewater (−27‰) and bulk regolith (−38‰), representing apparent fractionations relative to parent rock of −31‰ and −42‰, respectively. We also found δ7Li values that are lower in the exchangeable fraction than in the bulk regolith or porewater, the opposite than expected from secondary mineral precipitation. We interpret these large isotopic offsets and the unusual relationships between Li pools as resulting from two distinct weathering processes at different depths in the regolith. At the bedrock‐regolith transition (9.3–8.5 m depth), secondary mineral precipitation preferentially retains the lighter 6Li isotope. These minerals then dissolve further up the profile, leaching 6Li from the bulk solid, with a total variation of about +50‰ within the profile, attributable primarily to clay dissolution. Importantly, streamwater δ7Li (about +35‰) is divorced entirely from these regolith weathering processes, instead reflecting deeper weathering reactions (>9.3 m). Our work thus shows that the δ7Li of waters draining highly weathered catchments may reflect bedrock mineralogy and hydrology, rather than weathering intensity in the regolith covering the catchment.Plain Language Summary: Weathering is the process by which rocks are altered at the Earth's surface, transforming fresh minerals into clays with some loss of chemical elements to rivers and eventually oceans. Understanding how intense weathering is now, and has been in the past, is important because it supplies nutrients for ecosystems and is part of the Earth's long‐term carbon cycle (and thus, climate regulation). To do this, geochemists have developed tracers of weathering intensity, of which Li isotopes (expressed as δ7Li) are considered to be the best. However, we know little about the behavior of Li isotopes in the tropics, where weathering is the most intense. To help make δ7Li a more robust tracer, we sampled a 10 m deep soil profile at a tropical catchment in Puerto Rico where rocks are dissolving very fast. We found that weathering here is so intense that clays are continuously dissolving, producing the lowest δ7Li values ever recorded on Earth, but that the stream water draining the catchment does not reflect these values. Our work thus expands the range of known values of this tracer and warns geochemists that δ7Li in rivers might not be directly related to weathering intensity in tropical catchments.Key Points: Lowest δ7Li values reported to date in nature (porewater = −27‰; bulk regolith = −38‰; exchangeable Lithium (Li) = −50‰). Large isotopic differences driven by clay precipitation, dissolution, and re‐precipitation processes. Li isotopes may not be appropriate tracers of weathering intensity for very highly weathered catchments.National Science Foundation (NSF) http://dx.doi.org/10.13039/100000001Alexander von Humboldt‐Stiftung (Humboldt‐Stiftung) http://dx.doi.org/10.13039/100005156https://doi.org/10.4211/hs.28acde53dc5549f4a6e5d820364dd21

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

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