Cryogenic brines as a diagenetic fluid: using clumped isotopes to reconstruct the cementation history of sediments in the ANDRILL 2A core

Abstract:

The ANDRILL 2A core contains a succession of Neogene glaciomarine deposits, cemented by up to 20 wt% carbonate. Isotopic analysis of the cements yielded extremely negative d¹⁸O values, indicating either formation from isotopically negative fluids or at extremely high temperature. In outcrop, such values could be interpreted as being the result of meteoric diagenesis, but the lack of any exposure horizons in the core precludes such an interpretation. A previous study of the pore fluids described a brine below 200m, with an extremely negative δ¹⁸O value (c. -10‰), probably formed by batch-freezing seawater on the continental margin. The present study integrates ∆_47­ values of the cements with traditional approaches to further assess the nature of diagenetic fluids and processes. Isotopic data suggest three sources of carbonate: marine, methane reducing, and the aforementioned brines. Marine carbonate indicates δ¹³C and water δ¹⁸O within the range typical of seawater (c. -1‰), whereas the cryogenic brines show more negative values. A few samples exhibited extremely low δ¹³C values, the lowest below -25‰; the only feasible source for these cements would be the oxidation of methane. The shallow cements’ signatures diminish with depth as cryogenic brines begin to dominate the isotopic signal. Biogenic materials show an increased influence of this brine at depth, the deepest buried shells are isotopically indistinguishable from adjacent cements. These analyses show that these cryogenic brines play a major role in the diagenetic history of this site. Clumped isotopic results support previous interpretations using traditional methods, which have identified cryogenic brine as a major cementing agent in the subsurface of Southern McMurdo Sound. Because cryogenic brines have likely formed throughout Earth history, results have the potential to change the way diagenesis is evaluated in sedimentary successions that formed in polar environments.