Publication Date:
2024-01-09
Description:
We analyzed geotechnical and acoustic properties of sediments from Lau Basin backarc Sites 834, 835,838, and 839 to examine the large-scale heat and fluid circulation in the Lau Basin. We hypothesize that a feedback system exists in which sediment properties, particularly permeability, affect large-scale heat and fluid circulation and are, in turn, affected by the circulation of fluids.
Sediments in the Lau backarc basin display none of the expected trends in physical and acoustic properties as a function of depth and are probably underconsolidated. The sediments can consolidate normally, however, as shown by measuring both compressional- and shear-wave velocity in a controlled laboratory triaxial setting. In-situ underconsolidation might occur because high sedimentation rates could prevent the sediments from dewatering and consolidating normally, or water lost during the consolidation process could be rapidly replaced by fluids circulating through the system convectively.
Although initial sedimentation rates were extremely high in the Lau Basin, laboratory measurements indicate that the overlying clayey nannofossil oozes are sufficiently permeable to allow the passage of fluids through the sediments on the scale of several years; however, a comparison of vertical and horizontal components of flow indicates that horizontal fluid flow is 3 orders of magnitude greater than vertical flow. The highly permeable vitric silts and sands probably dominate the horizontal component of fluid flow.
Measured vertical heat flow at these sites is much lower than would be predicted by a simple cooling model, suggesting that heat is flowing laterally. A large difference in measured heat flow exists between the lowermost stratigraphic units of Sites 834 and 835, indicating that lateral fluid flow may be occurring. The chemistry of pore fluids measured at Sites 834, 835, and 838 shows an increase and subsequent decrease in dissolved calcium with depth within the sediment column, again suggesting lateral fluid flow. The dissolved-calcium profile at Site 839 shows a seawater signature and no consistent trend downhole, indicating possible downwelling of bottom water throughout the entire sediment column.
The volume of fluid required to (1) dissipate the excess heat expected but not measured and (2) prevent the sediments from consolidating normally in these Lau backarc sites is very small, about 8 m**3 at Site 834. Very small gradients are required to replace this amount of fluid lost during the 5.5 m.y. of sediment deposition. In general, sites of downwelling are much more diffuse and cooler than sites of upwelling. Site 839 probably is a downwelling site. The remaining three sites possibly are located in a zone of lateral fluid flow within a large-scale, convective circulation cell.
Keywords:
135-834A; 135-835A; 135-838A; 135-839A; Density, wet bulk; Depth, reconstructed; DEPTH, sediment/rock; DRILL; Drilling/drill rig; DSDP/ODP/IODP sample designation; Elevation of event; Event label; Joides Resolution; Latitude of event; Leg135; Lithology/composition/facies; Longitude of event; Ocean Drilling Program; ODP; Permeability; Porosity; Sample code/label; South Pacific Ocean; Specific gravity; Velocity, compressional wave; Velocity, shear wave; Void ratio; Water content, wet mass
Type:
Dataset
Format:
text/tab-separated-values, 221 data points
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