147-18 Controls on the Barium Isotopic Composition of Authigenic Carbonates at Methane Seeps

Session: Climate, Ocean and Environmental Changes Through Earth History: From Marine and Terrestrial Proxies to Model Assessments (Posters)

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The barium (Ba) isotopic composition of carbonate minerals (δ¹³⁸Ba_carb) is a promising new geochemical tracer for past Ba cycling. However, further study of the potential influence of early diagenetic processes on δ¹³⁸Ba_carb records remains an important avenue for this new tracer’s development. Marine methane seep environments are sites of intense Ba cycling during early diagenesis. Seeps may be responsible for much of the flux of Ba from the sedimentary subsurface into seawater, while the authigenic carbonate minerals precipitating at seeps may represent an archive of Ba cycling. This study presents δ¹³⁸Ba_carb data of authigenic carbonate collected at eight methane seeps from the South China Sea and the Gulf of Mexico. The obtained δ¹³⁸Ba_carb values range widely between −0.15‰ to 0.37‰. Under conditions of low upward flux of Ba, identified by low Ba/Ca ratios of carbonate minerals (<100 mmol/mol), δ¹³⁸Ba_carb values exhibit a negative covariation with Ba content, suggesting variable transitions between the influence of seawater (low Ba/Ca and high δ¹³⁸Ba) and the influence of sediments (high Ba/Ca and low δ¹³⁸Ba). Aragonite-dominated carbonates tend to show higher δ¹³⁸Ba_carb values than high-Mg calcite-dominated carbonates. This pattern suggests a greater influence of seawater during aragonite precipitation caused by its formation at shallower depths compared to high-Mg calcite. Accordingly, low δ¹³⁸Ba_carb values and relatively high Ba/Ca molar ratios are interpreted to reflect the release of Ba from sediments. However, under conditions of increased upward flux of Ba, seep carbonates display highly variable δ¹³⁸Ba_carb values independent of Ba/Ca molar ratios, suggesting that additional processes (i.e. re-precipitation of ¹³⁴Ba-enriched barite) control porewater barium isotope compositions. Taken together, these new observations showcase the complexity of Ba isotope systematics at marine seeps, governed by the mixing of Ba from different sources and mineral dissolution and authigenesis (carbonate minerals and barite).

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7979

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