Measuring Strata Thicknesses in Cores to Assess Recent Sediment Compaction and Subsidence of Egypt's Nile Delta Coastal Margin

Abstract

This study develops a method to determine compaction and subsidence of Holocene fluviomarine sections along Egypt's northern Nile Delta coastal margin based on variations of strata thickness with depth. Thicknesses of 3183 oxidized layers in 85 long cores (10 to 45 m) are examined to determine the nature of down-section and spatial patterns. The thickest layers are at depths of 1 to 2 m (dating to <1000 y BP) and become significantly reduced within the next meter due to (1) rapid expulsion of interstitial pore water from overburden compression of sediment and (2) evaporation in near-surface deposits in this hyperarid setting. Thicknesses decrease more irregularly to depths of 5 to 6 m and then more gently to the base of the sections. The more gradual compaction of strata at mid- and lower-core depths accounts for more than 50% of total Holocene compaction. The derivatives of regression curves determine the strata thickness reduction rate, which is treated as a proxy for compaction rate. Average compaction rates for Holocene sections vary along the approximately 225-km-long coastal margin: about 8.4 mm/y in the NE, about 7.7 mm/y in the NC, and about 3.7 mm/y in the NW sectors. These rates, somewhat higher than those previously proposed for this delta, are within the range for land subsidence obtained from recent satellite surveys. The interaction of natural factors, such as moderate to high compaction rates and rising global sea level, with human activities, especially those reducing sediment supplied to the lower delta, presents serious concerns for the delta's future. Without emplacement of a continuous network of protective structures along the coast, a marked landward retreat of about 30 km by the Mediterranean coast is projected in little more than a century, resulting in submergence and loss of agricultural and wetland terrains vital to Egypt's rapidly expanding population.