Microfabric and Physical Properties of Deep-Sea High Latitude Carbonate Oozes
The major change in pelagic sedimentary facies observed in the Southern Ocean has been the sequential replacement of a calcareous ooze facies by a siliceous ooze facies. These biogenous oozes are accumulations of the skeletons and skeletal fragments of calcareous and biosiliceous phytoplankton and zooplankton (nannofossils and foraminifers versus diatoms and radiolaria, respectively), which once lived in the upper water column. This “rain” of microfossil skeletons to the seafloor was synchronous with the establishment of a zonally distributed oceanographic circulation pattern around Antarctica. The facies changes discussed in this study are associated with (1) the opening of the Drake Passage, (2) the development of the Antarctic Circumpolar Current system, (3) increases in the volume and geographic extent of the Antarctic ice sheets and, (4) the expansion of cold, nutrient-rich water masses around Antarctica.
The time-averaged mass accumulation of biogenous oozes at southern high latitudes is dependent on the dynamic interactions and feedbacks among the oceans, ice sheets, land masses, atmosphere, and biological systems. These interactions and feedbacks are likewise influenced by external and internal forcing mechanisms, including variations in Earth’s orbital parameters, solar-driven processes, and plate tectonics. Therefore, these pelagic oozes contain a wealth of potential paleoclimatic and paleoceanographic information expressed (1) as variations in the measured physical properties and (2) as changes in sedimentary microfabric.
Variations in shipboard physical properties (wet bulk, dry bulk and grain density, shear strength, porosity, and water content) and sediment composition from Ocean Drilling Program (ODP) Sites 689, 690, 747, 748, 749, and 751 are evaluated using a scanning electron microscope (SEM) to observe changes in the sedimentary microfabric of individual samples. Step-loaded consolidation tests were also performed using relatively thin, laterally confined sediment samples in Anteus back-pressure oedometers. The results from these tests are presented with information on the composition, index properties, and age of the individual samples.
The consolidation of open-ocean biogenous oozes is a process that is primarily governed by the increasing vertical overburden stress caused by the successive accumulation of sedimentary material. The initial porosity of these high-latitude oozes is largely determined by the amount and type of diatoms present. Downhole compositional and textural changes have a significant influence on the behavior of individual subunits and on the observed porosity reduction. In general, the consolidation tests described in this chapter indicate that the biosiliceous oozes are normally consolidated to overconsolidated, whereas the calcareous oozes are normally consolidated to underconsolidated.
Trends in the mass physical properties of high-latitude, biogenous sediments reflect variations in the lithologic composition and texture of microfossil assemblages. Studies of these trends at widely spaced geographic locations can help researchers to understand the temporal and spatial variability of the Southern Ocean, an important oceanographic system during the Neogene. A systematic analysis of the sedimentary microfabric observed in these sequences can provide additional constraints on the paleoceanographic interpretations suggested for southern high-latitude environments.
KeywordsVoid Ratio Ocean Drill Program Drake Passage Centric Diatom Consolidation Test
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