Say 1700 ish! See this article  (http://onlinelibrary.wiley.com/doi/10.1029/2004JB003259/full)

The truth is that near-seafloor sediment with plenty of gas and fluid can have a Vp lower than seawater. This sounds impressive, but ODP/DSDP data have shown reductions in Vp - compared to seawater - until the sediment starts compacting proper, a few meters below the seafloor. I remember having to tackle this change in Vp for work offshore the Azores in Alves et al. (2003) Marine Geology - Insights on the evolution of mid-ocean basins: the Atlantis Basin of southern Azores

Vp and Vs velocities can be very low in uncosolidated sediments, especially in case of partial saturation. Dispersion and strong attenuation can also be noticed at ultrasonic and seismic frequenciesespecially in case of patchy saturation (in the sense of White). Some effective models based on double porosity theory allow to handle this kind of mechanisms (see Pride et al, 2004). In very extreme cases, anelastic effects can also be observed.