Seismic interpretation of pelagic sedimentation regimes in the 18–53 Ma eastern equatorial Pacific : basin-scale sedimentation and infilling of abyssal valleys

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 12 (2011): Q03004, doi:10.1029/2010GC003347.Understanding how pelagic sediment has been eroded, transported, and deposited is critical to evaluating pelagic sediment records for paleoceanography. We use digital seismic reflection data from an Integrated Ocean Drilling Program site survey (AMAT03) to investigate pelagic sedimentation across the eastern-central equatorial Pacific, which represents the first comprehensive record published covering the 18–53 Ma eastern equatorial Pacific. Our goals are to quantify (1) basin-hill-scale primary deposition regimes and (2) the extent to which seafloor topography has been subdued by abyssal valley-filling sediments. The eastern Pacific seafloor consists of a series of abyssal hills and basins, with minor late stage faulting in the basement. Ocean crust rarely outcrops at the seafloor away from the rise crest; both hills and basins are sediment covered. The carbonate compensation depth is identified at 4440 m by the appearance of acoustically transparent clay intervals in the seismic data. Overall, we recognized three different sedimentation regimes: depositional (high sedimentation rate), transitional, and minimal sedimentation (low sedimentation rate) regimes. In all areas, the sedimented seafloor mimics the underlying basement topography, although the degree to which topography becomes subdued varies. Depositional regimes result in symmetric sedimentation within basins and subdued topography, whereas minimal sedimentation regimes have more asymmetric distribution of sediments within topographic lows and higher seafloor relief. Regardless of sedimentation regime, enhanced sediment deposition occurs within basins. However, we observe that basin infill is rarely more than twice as thick as sediment cover over abyssal hills. If this variation is due to sediment focusing, the focusing factor in the basins, as measured by 230Th, is no more than a factor of ∼1.3 of the total vertical particulate rain.This research is supported by NSF grants OCE‐07253011 and OCE‐0851056 (M. Lyle and M. Tominaga) and NERC grant NE/C508985/2 (N. C. Mitchell)

What Controls Opal Preservation in Tropical Deep-Sea Sediments?

Measurements of opal preservation in deep sea sediment cores have been presented in three ways: the opal concentration as a fraction of total dry weight (%opaltot), the opal concentration normalized to calcite‐free dry weight (%opalcalcite‐free), and me opal accumulation rate (opal MAR). It is tempting to interpret changes in these indices as indicators of rates of biological production in past oceans. Based on theoretical constraints, we argue that in typical tropical and subtropical sediments, both %Opalcalcite‐free and opal MAR reflect a significant artifact of dilution by other phases. Thus the band of high %Opalcalcite‐free in the equatorial Pacific appears to be caused in large part by the high %Calcite in that region, rather than by high opal productivity. The best candidate for a reliable paleoproductivity proxy appears to be %Opaltot. Unfortunately, present‐day %Opaltot data from tropical and subtropical regions show little or no systematic trend with the rain rate of opal. Pore water silica concentration data reveal that the apparent pore water opal solubility is not constant but correlates regionally with the rain rate of opal to the seafloor. A model that treats opal as a single homogeneous phase with a single well‐defined solubility product predicts a strong dependence of opal concentration on rain rate (in stark contrast to the data), and a constant asymptotic pore water Si. Two models representing opal as multiple heterogeneous phases with different solubilities are able to reproduce the observed asymptotic pore water Si/rain rate relationship, but not the lack of rain rate trend in the opal concentration data. Only by assuming a systematic trend in the quality of opal (i.e., the solubility) as a function of opal production, can we reproduce the observed pattern of opal preservation. The implication of this study is that changes in opal preservation in the geologic record cannot simply be interpreted in terms of changes in surface ocean productivity until our understanding of opal diagenesis can be improved