Numerical models of diagenesis, sediment properties, and pore fluid chemistry on a paleoceanographic transect: Blake Nose, Ocean Drilling Program Leg 171B

A critical issue in many paleoceanographic studies is the potential of diagenetic processes to compromise the fidelity of geochemical proxies (e.g., delta O-18) in deep-sea carbonate sediments and microfossils. A recently established strategy for dealing with this issue is to drill depth transects in anomalously shallowly buried and therefore presumably well-preserved target sediments. Here we report the results of an experiment designed to test the validity of this approach. We apply a simple one-dimensional numerical model to complex pore water chemical profiles (for Ca, Mg, Sr, Sr-87/Sr-86, and delta O-18) observed in a shallowly buried paleoceanographic transect of paleogene and Cretaceous age drilled on Blake Nose during Ocean Drilling Program Leg 171B. We show that it is possible to reproduce these complex pore water profiles simply by invoking (1) a deep-seated zone of chemical reaction well below the recovered sections and (2) minimal chemical alteration within the overlying recovered sediments but significant restriction of advective flow by embedded low-porosity layers. These results suggest that the sediments recovered during Ocean Drilling Program Leg 171B can be considered to be virtually chemically inert at their present burial depth (their carbonate recrystallization histories having been largely limited to events during early, similar to 10 Ma timescale, burial. These findings support the strategy of selecting unusually shallowly buried sections for paleoceanographic transect drilling. Ironically, however, the very attraction of targeting such sections (the lack of overlying sediments) dictates that it is not possible to use pore water chemistry to constrain the early diagenetic histories of such sections because of the diffusion of the pore water signal through the sediment-water interface. Diffusional loss may also account for the absence of pore water chemical anomalies associated with the ooze-chalk transition at Blake Nose. However, our work suggests that traditional interpretations of this boundary as the inferred locus of maximum rates of carbonate recrystallization require reevaluation.