Abstract:
With an accurate understanding of the lithologic dependence of pre-compaction porosity in pelagic sediments it should be possible to calculate mass accumulation rate histories of sedimentary phases, to generate proxy carbonate logs from core density scans and from seismic profiles, and to reconstruct the mechanical and chemical processes that occur during burial and lithification. We demonstrate that routinely made continuous Gamma Ray Attenuation Porosity Evaluator (GRAPE) wet bulk-density measurements of sediments show a positive relationship to carbonate content in a variety of pelagic environments. Mineralogically related differences in packing, rather than differences in grain density, appear to control wet bulk-density variations. A hyperbolic model calibrated at DSDP/ODP drill sites quantifies the relationship of density to carbonate content in different pelagic environments. The curvilinear relationships appear to converge to a similar wet bulk density (1.71 + or - 0.07 g/cm (super 3) , porosity = 58%) at 100% carbonate content, but they exhibit differing porosity at zero carbonate content. Wet bulk-densities for a given carbonate content depend on the non-carbonate mineralogy and grain size, and increase systematically from opal- to clay- to quartz/lithic fragment-dominated settings. Because contrasts in sediment wet bulk-density control the seismic response of unlithified sediments, it should be possible to generate carbonate profiles remotely in carbonate-rich settings after core calibration of the seismic system. Our model can generate long-proxy carbonate curves with a typical sample spacing of 0.6-1 cm from existing GRAPE data bases in equatorial Pacific, equatorial Atlantic, and northern Atlantic locations over the past 6 m.y. These time series are used to investigate the long term evolution of climate in the Milankovitch frequency band, as well as to reconstruct patterns of carbonate sedimentation over time. Sedimentation bears the imprint of Milankovitch cyclicity superimposed on long term trends. The switch in carbonate sedimentation from 41 kyr periodicity during the Matuyama chron to dominant 100 kyr periodicity in Brunhes chron previously described in the North Atlantic occurs in equatorial Atlantic and Pacific locations as well. The lithological dependency of porosity in uncompacted pelagic sediments also has important implications for understanding later compaction and diagenetic modification of pelagic sediments. In particular, the GRAPE observations suggest that differential burrow flattening observed in lithified limestone-marl alternations results from mechanical and pressure solution compaction of strata that had strong primary porosity differences, rather than from "diagenetic unmixing" of carbonate.