APPARENT SEA-LEVEL RISE CAUSED BY SEDIMENT COMPACTION

Apparent rates of sea-level rise from tide gages on the US Atlantic coast are unusually high. These tide gages are embedded in clastic sediment - typically in sand on ocean coasts and in muds on estuarine shores. Mud and clay have initial porosities higher than surficial sand. Clean sand, just deposited, has a nominal porosity, within wide limits, of p=0.42. Clean sand buried at shallow depth has an appproximate intermediate p=0.37. Clean sand, more deeply buried and without chemical deposition in the pores, could have p=0.30 or less. Here, p=vol voids/vol(voids+solids). All sands, no matter how deep and compacted now, were once recent deposits on the earth's surface with the nominal p=0.42. By compacting from 0.42 to 0.37, a sand layer loses 8% of its thickness. A tide gage embedded above compacting sediment loses elevation, producing a water level record that indicates apparent sea-level rise equal to the loss in gage elevation. The USGS monitors subsidence in the marsh behind Atlantic City using an extensometer that penetrates 320m of section. Between 1980 and 2000, this extensometer measured 29mm of vertical compaction (1.5mm/yr) at this site. The Atlantic City tide gage rests above the same sedimentary section, plus surface sands deposited in historic time. The compaction of buried and surficial clastic sediment is an explanation for high apparent rates of sea-level rise (about 4.0mm/yr) at Atlantic City, Sandy Hook, and Hampton. On the Atlantic side of Florida, apparent rates of sea-level rise are generally lower than those from the mid-Atlantic coast, probably because historically recent calcium carbonate deposition in clastic pore space strengthens the Florida sediment, reducing compaction. The empirical necessity for compaction requires (as a minimum) reevaluation of theoretically-derived, post-glacial isostatic explanations of apparent US Atlantic coast sea-level rise.