What sediment forms directly by precipitation from seawater and is hydrogenous?

• A. Authigenic Sediment
• B. Biogenous Sediment
• C. Calcareous ooze
• D. Clay

Answer: (A)

Hydrogenous sediments form when minerals precipitate directly from seawater onto the seafloor, in place. When minerals crystallize from seawater or pore waters right where they are deposited, the resulting sediment is called authigenic sediment. This category includes minerals like manganese oxides and phosphates that precipitate from seawater, or carbonate cements that form in place.

So the best choice is authigenic sediment because it specifically describes in-situ chemical precipitation from seawater, which is the hallmark of hydrogenous sediment. Biogenous sediments come from organisms and their tests, calcareous ooze is a biogenous carbonate deposit, and clay is typically terrigenous or detrital, not formed by direct precipitation from seawater.