PRESSURE PUMPING, CHAMBER FILLING, AND AMMONITE PRESERVATION, TALKEETNA MOUNTAINS, ALASKA

Well-preserved ammonites are common in parts of the Late Cretaceous Matanuska Formation in the Talkeetna Mountains of south-central Alaska. Ammonites range up to about 1m in diameter, often have preserved pearly shell material and typically exhibit shell chambers completely filled with sediment.

For several reasons, it is unlikely that sediment fills the shell chambers by passive diffusion through the siphuncle. 1. The observed siphuncle diameter is small (<~0.55 cm) relative to shell diameter, and siphuncle diameter narrows toward the inner whorls. 2. The siphuncle may be very long. An ammonite with a measured shell diameter of 35 cm has a siphuncle length of about 206 cm. 3. The shell chambers are closed, except for the siphuncle, with no fluid through-flow. 4. Finally, passive diffusion, if it occurs, would cease at about 50% chamber filling when the siphuncle becomes covered with sediment in the chambers. In this case, incompletely filled and partially crushed chambers should characterize most specimens. Instead, most individuals show complete filling.

Pressure pumping related to diurnal tides is proposed to actively move water in and out of gas-filled ammonite chambers and eventually fill them with sediment. Rhythmic tidal flows change the depth of the water column, thereby changing the pressure on the sea floor. Increasing pressure during rising tide compresses the gas in unfilled ammonite chambers, reduces gas volume, and forces entry of water carrying suspended sediment through the siphuncle into the chambers. The sediment settles in the chamber and during the falling tide decompression forces water out of the chamber. Pressure pumping is an active mechanism which would allow complete filling of the shell chambers even if the siphuncle is very narrow or very long. Active pressure pumping may also explain the lack of laminations in filled chambers.

Application of Boyles Law shows a tidal change of 3 meters causes a percentage change in volume of a gas-filled chamber of ~23% at the surface, ~2.5% at 100 meters, ~1% at 300 meters, and ~0.5% at 700 meters, a depth near the crush strength of modern Nautilus. For water carrying 1% sediment, complete filling requires ~10 years at 50 m depth and ~25 years at 150 m depth. The abundance of completely filled ammonites indicates the filling process commonly goes to completion.