ABNORMALLY HIGH NEAR-SURFACE POROSITIES AT THE BASIN SITES

The porosity of sediments gradually decreases with depth as a result of compaction. This phenomenon can be described by Athy's (1930) relation

= ₀e^–Z/L,

where ₀ is the surface sediment porosity, Z is the depth, and L is the characteristic decay constant. We used the MAD-derived porosities at Sites 1244, 1245, 1246, 1251, and 1252 for this analysis only. As seen in Figure F13, the porosity generally decreases at all sites with depth following Athy's relation. This general decreasing trend is overprinted by structurally controlled porosity anomalies associated with Horizons A, B, and Y (Riedel et al., 2003). We calculated the decay constant L using Athy's relation for the entire sediment column, avoiding the structural anomalies of Horizons A, B, and Y (Table T2). The upper 5–10 mbsf at Sites 1251 and 1252 and, to a lesser degree, at Sites 1244 and 1245, however, show higher porosities than are predicted from the deeper trend using Athy's law. This phenomenon is absent at Sites 1246, 1247, 1248, 1249, and 1250. It should be noted that poor core recovery at Sites 1248, 1249, and 1250 may bias the observation.

Using the upper 5–10 mbsf at Sites 1244, 1245, 1251, and 1252, a different set of decay constants was calculated for these sites (Table T2). The decay constant L is about one order of magnitude smaller, reflecting the rapid compaction rate (or dewatering) of those near-surface sediments. The difference between anticline-flank Sites 1244 and 1245 and basin Sites 1251 and 1252 is mainly in the surface porosity ₀, which is ~10% higher at the basin sites. An exceptionally small decay constant (L = 26) was calculated at Site 1252; however, super-high porosities are only observed in the upper 2 mbsf. The fact that high near-surface porosities are observed at Sites 1244 and 1245 may be the reason why the AOC at these two sites is less pronounced than at Site 1246, which does not show any high near-surface porosities.

Unusually high near-surface porosities have been reported by many authors over the last few decades (e.g., Bennett et al., 1970; Booth and Dahl, 1986; Boudreau, 1998; Novosel, 2002). Physical properties of sediment are not just affected by overburden stress but are also a function of the nature of microfabric, amount of organic matter, and mineralogy, as well as grain size and shape (Bennett et al., 1999). For example, the presence of clays, especially the mineral smectite, can significantly increase the expected porosity and water content within the top few meters of the sediment (e.g., Velde and Espitalie, 1996; Bennett et al., 1999). This phenomenon, often referred to as flocculation, is attributed to the clay's active electrostatic forces that allow "edge-edge" assemblage of the clay platelets, with increased water content filling the voids (Velde, 1996). Flocculation can also result in decreased shear strength (i.e., this type of sediment would show a smaller degree of AOC, as discussed in the previous section).

Biostratigraphic analyses show that sediments at the basin Sites 1251 and 1252 are much younger in the upper 10 mbsf than at all the other Sites (Tréhu, Bohrmann, Rack, Torres, et al., 2003). The youngest microfossils, with an age of ~0.09 Ma, were identified at these two basin sites only at depths below 15 mbsf (Table T3).