IDENTIFYING AUTHIGENIC CLAY FORMATION IN NEARSHORE TROPICAL SEDIMENTS: A NEW APPROACH USING LITHIUM ISOTOPES

Modern clay formation in the marine environment was proposed as a major control on oceanic chemistry over 30 years ago. Investigations of this hypothesized low temperature elemental sink were largely ignored after the discovery of large scale elemental cycling at mid-ocean ridges. However, recent experiments and observations have demonstrated that clays can precipitate rapidly in shallow tropical marine sediments. In this study, we examine the possibility that Mg-rich authigenic marine clays can be identified by their lithium isotopic compositions.

Li⁺ (0.78A) is a useful tracer of Mg⁺² (0.72A) in clays because it substitutes for Mg⁺² or occupies the space generated by Mg⁺²for Al⁺³ substitution in the octahedral layer. If seawater Mg⁺² is incorporated into authigenic clays, Li⁺ should also be fixed in the same mineral. During rock weathering ⁷Li is preferentially mobilized relative to ⁶Li so weathering products typically have low ⁷Li/⁶Li ratios in comparison to seawater, which has a uniform d⁷Li value of +32‰. For a given fractionation factor, seawater lithium fixed into sediment phases should have a diagnostic d⁷Li value.

We examined the pore waters, the clay-size particles, and the bulk sediments from the San Blas Archipelago, Panama, where recent Mg-rich marine clay formation is known to occur. We also analyzed soils, fluvial sediments, and estuarine sediments from the Caribbean Island of St. Lucia to determine the Li isotopic composition of clays prior to deposition in the marine environment. Lithium concentrations were analyzed by ICP-MS and the lithium isotopic compositions were analyzed by MC-ICP-MS (Nu Plasma).

Relative to the overlying seawater, the San Blas pore waters had lower Li concentrations and equal to or slightly higher d⁷Li values. The San Blas clays had lower d⁷Li values than those of the bulk sediments and had markedly different d⁷Li values than those of the terrestrial clays from St. Lucia. Our results show that it is possible to distinguish authigenic marine clays from terrestrially derived clays in some cases. This approach is likely to work best in tropical regions where intense chemical weathering has depleted the soils of Li and lowered its ⁷Li/⁶Li ratio.