Physical properties, compressional-wave velocity and consolidation characteristics of ODP Holes 133-815B and 133-817B

Sites 815 and 817 were drilled near the Townsville Trough during Leg 133 of the Ocean Drilling Program. The physical properties, compressional-wave velocity, and consolidation characteristics indicate that the periplatform carbonate sediments maintain more water content and lower compressional velocity near the Queensland Plateau than the clayey hemipelagic sediments, which have a clay content of up to 60%. Bulk density, void ratio or porosity, water content, and compressional-wave velocity are shown to have a linear relationship with burial depth. Between 3.5 and 5 Ma (about 100-500 mbsf), these physical properties maintained a constant rate vs. the depth in core because of the fast sedimentation-rate effect at Site 815. However, compressionalwave velocity still increases downward in this section. The clay content in this section causes an increase of bulk modulus and compaction effect. At Site 817, scarce terrigenous mud content and abundant carbonate content (88%-97%) cause a straight line relationship between physical properties and burial depth. During the consolidation test, we show that dominant micritic particles may cause faster acoustic velocity than sediments composed mainly of coccoliths. The bulk modulus ratio increasing rate in the clay-rich carbonate sediments is almost 4.5 times higher than in the clay-free periplatform carbonate sediments.

Calcium carbonate stratigraphy of ODP Site 177-1090

A total of 776 sediment samples were measured for percent CaCO3 using a coulometer. These data are compared with percent blue reflectance (450-550 nm) measured with the Oregon State University split-core analysis track. In previous studies percent blue reflectance has been an excellent proxy for percent CaCO3 and in this study shows many of the main depositional trends (i.e., a 100-k.y. cycle, with a 55% reflectance range is evident in the upper 900 k.y., underlain by sediments exhibiting a 40-k.y. cycle with only a 30% reflectance range). Between ~21 and 5 Ma the average percent reflectance decreases from ~35% to ~8%. A similar decrease is also recorded between ~24 and 22 Ma. Percent CaCO3 trends closely match those of the percent blue spectral reflectance. This is especially well shown in the 100-k.y. cyclicity and in the interval between 24.5 and 21.5 Ma. In both intervals CaCO3 analyses are abundant. An exception occurs in the interval between 2 and 5 meters composite depth (~193 and 240 k.y.). There, percent CaCO3 and percent reflectance are out of phase. The lack of agreement is not likely to be due to a very wet core, in which water would dominate the spectral reflectance instead of sediment, or to problems with the composite depth slice. The discrepancy remains unexplained and provides clear evidence that when noninvasive measurements are used as proxies for chemical measurements they must be substantiated by the actual chemical or physical measurements.

(Table 2) Calcium carbonate content in deep-sea surface sediments

The reconstruction of paleocarbonate ion concentrations provides an important constraint on the contribution of the CaCO3 cycle to the decrease in atmospheric CO2 content during glacial time. Such reconstructions have been challenging because each of the existing paleo-[CO3]2- indices has serious limitations. In this study, we reexamine the Broecker-Clark CaCO3 size index by analyzing the <20 µm, 20 to 38 µm, and 38 to 63 µm fractions in sediments from the Ontong-Java Plateau and the Ceara Rise. Scanning electron microscope analyses demonstrate that the less than 20 µm CaCO3 is dominated by coccoliths and the greater than 20 µm CaCO3 is dominated by foraminifera. Our results clearly indicate that the coccoliths are far more resistant to dissolution than the foraminifera. Referenced to a core top sample from 2.31 km depth in a core top sample from 4.04 km depth on the Ontong-Java Plateau, ~70% of the foraminifera CaCO3 was dissolved as opposed to only ~7% of the coccolith CaCO3. We found that the dissolution of foraminifera shells did not produce a significant amount of fragments smaller than 63 µm in size, and thus the Broecker-Clark size index is not a measure of the extent of fragmentation. Rather, it is a measure of the extent of differential dissolution of foraminifera relative to coccoliths. On the basis of these results, we propose a new dissolution index which involves the ratio of dissolution-susceptible foraminifera CaCO3 to total CaCO3.