Boron paleosalinity proxy for deeply buried Paleozoic and Ediacaran fossils
Abstract Boron content is an indicator of paleosalinity for Mesozoic and Cenozoic clayey sediments, but is compromised by clay diagenesis in deeply-buried sediments of Paleozoic and Ediacaran age. This study of North American, Russian, and Australian, Paleozoic and Ediacaran fossils showed tight covariance of B (ppm) and K (wt%), confirming common wisdom that boron is carried largely by illlite-smectite clays. Ratios of B/K >40 μg/g distinguish modern marine and freshwater sediments, but this discrimination threshold declines to 21 μg/g for sequences with bituminous coal just below the anchimetamorphic zone, and to 5 μg/g for sequences within the anchimetamorphic zone toward greenschist facies. Marine and non-marine threshold B/K values can be predicted from Weaver and Kubler indices of illite crystallinity, as a measure of B depletion during deep burial and metamorphism. Deviations from that expectation (ΔB/K-Weaver and ΔB/K-Kubler) are positive in marine rocks, and negative in non-marine rocks. Trilobites, stromatolites and other marine fossils were correctly identified as marine by this proxy, and fossil land plants identified as non-marine, though in some cases by small margins because of declining B content with increased clay crystallinity. Cryogenian to Devonian problematic quilted fossils (Dickinsonia, Arumberia, Rangea, Pteridinium, Ernietta, Aspidella, Rutgersella, Protonympha) are indistinguishable from Devonian to Triassic plants using this proxy, and significantly different from trilobites, brachiopods, ammonites and other securely marine fossils.