Secondary Minerals Drive Extreme Lithium Isotope Fractionation During Tropical Weathering

Abstract

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.