55-8 Mercury Anomalies and Siliceous Volcanism in OAE-2 Sediments From the Cretaceous Western Interior Seaway (KWIS): An Example From the Lower Eagle Formation (LEF) in SW Texas

Session: Sequence Stratigraphic, Geochemical, and Geochronologic Correlation of the Cenomanian-Turonian Ocean Anoxic Event 2 (OAE2) in the Cretaceous Western Interior Seaway (KWIS) and the Gulf Coast

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Abstract:

Organic-rich OAE-2 sediments in the LEF are associated with anomalously high concentrations of Hg, attributed to LIP volcanism. The highest concentration of Hg (~750 ppb), occurs in LEF stratigraphy that also contains the highest density of altered siliceous volcanic tuffs (bentonites), coevally deposited with the organic-rich sediments.

Bentonite occurrences are ubiquitous throughout OAE-2 sediments in the KWIS, and a single LIP that may have produced this global event has not been identified. Despite thousands of kms of separation, other suggested sources of siliceous tuffs and Hg in the LEF include contemporaneous HALIP, CLIP, and the Greater Ontong Java and Kerguelen Plateau LIPs.  

Bentonite trace element (TE) data from the HALIP and Atascosa Trough (AT) in SW Texas have similarly evolved, but very different compositions. TE data from the AT suggests that tuffs in the LEF were not sourced from the HALIP, but from arc-related volcanism in SW Mexico.

Hg isotopes in ores from Terlingua (SW Texas) showed that the source of Hg was from proximal OAE-2 sediments in the LEF, having a similar Hg isotopic composition to OAE-2 sediments from Rehkogelgraben (Austria). Mass Independent Fractionation (MIF) of Hg isotopes (Δ¹⁹⁹Hg) from both locations preserve a “mantle-like” isotopic signature, similar to that of Continental Flood Basalts and/or Ocean Island Basalts. A similar Δ¹⁹⁹Hg isotopic signature could also be produced by arc-related volcanism involving marine sediments.

LIPs are very long-lived events characterized by the generation of enormous volumes of effusive basaltic volcanism and injection of volatiles (CO₂, H₂S/SO₂, and Hg⁰) into Earth’s upper atmosphere, where Hg⁰ is readily oxidized to Hg²⁺. With a residence time of ~1-year, Hg²⁺ may then be rapidly dispersed globally in Earth’s atmosphere.

Deposition of Hg also occurs in environments where there is a paucity of organic matter e.g., Rehkogelgraben. In the LEF for example, regional arc volcanism generated considerable air-borne material capable of removing Hg²⁺ from the atmosphere by electrostatic agglomeration of atmospheric Hg²⁺ with altered siliceous volcanic detritus. This represents a very simple and efficient mechanism for scavenging and depositing Hg. For the LEF, it is suggested that Hg was removed from the atmosphere by ash fall deposits produced by subduction-related volcanism in SW Mexico, and not from any specific contemporaneous LIP, which remain the primary source of atmospheric Hg.

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

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