SIMS analysis of fine-scale biogeochemical proxies in the Panoche-Tumey Hills paleoseep (PTHP)

We have used secondary ionization mass spectrometery (SIMS) analysis of carbonate cements to investigate microchemical environments in fossiliferous regions of the Paleocene-aged PTHP. The cements are part of a paragenetic sequence that is commonly recognized in paleoseep environments. The early Paleocene PTHP studied here is one of largest recognized paleoseeps in the world, and lies above a well-exposed sandstone injectite feeder system at the edge of a former forearc basin. Analyses were focused on early authigenic cements surrounding or replacing well-preserved fossils in order to characterize fine-scale variations in stable carbon and oxygen isotopes. In addition, we collected data from transects across abiotic pipe structures to assess whether these preserve long-lived histories of seep fluids. Errors for δ18O were less than 0.3‰ (1σ) and for δ13C were 2.1‰ to 3.4‰. Our results include the lightest carbon isotope values from the PTHP to date (δ13C = -58‰), and ranges of up to 63‰ in δ13C and 10.5‰ in δ18O in single samples. Large isotopic differences were observed over distances of ≤200 micrometers. The lightest carbon isotopes occur in cements adjacent to fossil bivalves, and Teredolites trace fossils in permineralized wood. Locally, bivalve shells have been replaced by carbonate with similarly light values. Transects from early to late cements in abiotic pipe structures consistently show increasing δ13C and δ18O values from the rims to the cores of the pipes. One sample seems to preserve a long fluid history (as preserved in the isotope proxies), and a crossplot of δ13C vs. δ18O shows a mixing trend that is consistent with early carbon derived from the process of carbonate reduction, followed by a gradual increase of diagenetic fluids from dissolution of marine carbonate. This trend also tracks a gradual decrease in Mg content from rim to core observed in several pipe structures. These observations suggest that in a given seep locality, inorganic pipe structures may preserve a long history of fluid flow from early bacterially-dominated processes to later diagenesis of more deeply buried marine carbonate material. The early cements also have light oxygen isotope signatures. With the exception of minor amounts of Mg and Fe, the cements analyzed in this study are relatively pure and show little textural or chemical evidence for late meteoric diagenesis. Given the lack of evidence for post-marine alteration of these early cements, the light oxygen isotope values may reflect early seep fluid signatures. Our results suggest that in the PTHP: a) carbon sources shifted significantly during seep evolution, b) very light carbon and light oxygen were coupled during the early stages of seepage and c) abiotic conduits contain the most complete records of seep fluid composition and evolution. These trends may be typical of other carbonate-rich seep systems. It is clear from these high-resolution analyses that there can be large geochemical variations over tens to hundreds of micrometers in seep cements. Thus, SIMS analysis is a powerful tool in detailing fine-scale isotopic trends and variations that bulk analysis or even micromilling may miss.