Isotopic analysis of conodonts and their host limestones sampled between two regionally extensive, altered volcanic ash layers in eastern Laurentia shows that a 454 Ma epeiric sea maintained large lateral differences in Nd and C isotope compositions. This is consistent with inferred temperature-salinity-defined epicontinental water masses and restricted circulation between epicontinental and oceanic environments. Because the majority of old marine fossils and sedimentary rocks are known from epeiric seas, some isotope excursions in ancient marine strata may originate from expansion and contraction of geochemically distinct epicontinental water masses, rather than global-scale changes in the state of the earth-ocean system. Data Repository item 9861 contains additional material related to this article.

[1] Abstract: Bioalteration of Quaternary to Early Cretaceous basaltic glass from pillow lavas of the upper oceanic crust can be documented in Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ ODP) samples from shallow to deep drill holes from the north to central Atlantic Ocean, Lau Basin, and Costa Rica Rift, a wide range of marine settings. Biogenerated textures are rooted in fractures and occur as two main types, a granular type and a tubular type. The granular type, common at all depths within the volcanic pile, appears as solid bands, semicircles or irregular patches of individual and/or coalesced spherical bodies, mostly 0.2-0.6 mm in diameter, with irregular protrusions into the fresh glass. The tubular type is more common at deeper levels in the crust and consists of thin tubes, sometime branching bodies, mostly 20 -30 mm long and are more common at deeper levels. The upper crust displays a large variability in the relative importance of biotic to abiotic alteration, and the degree of bioalteration appears to decrease with depth. Thus the fraction of bioalteration of the total alteration of the glass ranges from 20 -90% in the upper 300 m down to a maximum of 10% at about 500 m depth. This might be due to a natural variability in the abundance of bioaltered glass or to biased sampling from low drilling recovery of relatively young crust. The proportion of bioaltered to abiotically altered glass does not show any systematic variations with age of the crust. Thus bioalteration lasts as long as abiotic alteration, i.e., for as long as water is available to the hydration of the oceanic crust. Evidence from heat flow measurements suggests that hydrothermal circulation lasts until at least 70 Ma, and thus the deep biosphere is likely to expand at least into crust of this age.

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