A glacial isostatic adjustment origin for double MIS 5.5 and Holocene marine notches in the coastline of Italy

摘要

Modern tidal notches on rocky coastlines are well developed in the Mediterranean Sea because of its microtidal regime, and occur around much of the coast of Italy. Uplifted fossil notch couplets (here called "double notches") are well developed in some sites. The two notches within a couplet have an average vertical separation of 2-4 m, and are systematically observed at an elevation of a few metres above present sea level on tectonically stable coastal sites of western Italy. The upper notch has a morphology similar to that of the notch developed at present-day mean sea level in the modern tidal regime. However, the lower notch has a smoother morphology and larger vertical dimension than the upper one, and is pervasively bored by Lithophaga activity. Although the upper notch is attributed to the MIS 5.5 stage (last interglacial, ～125 ka), the origin of the lower one has so far remained enigmatic. Based on a quantitative assessment of the glacial isostatic adjustment (GIA) expected at each single site from updated models, we argue that both notches within a couplet were formed during a single highstand of the MIS 5.5 substage, and the superposed morphology resulted from isostatic motion and tidal erosion. The lower notch of the couplet is argued to be formed during the earlier part of the highstand and its smooth vertically extended morphology is attributed to glacio-hydro-isostatic vertical movements ( = crustal subsidence) that extended tidal erosion over a large vertical distance as the crust slowly subsided. In contrast, the upper notch of the couplet is considered to develop in the later part of the highstand due to tidal action at times when eustatic and isostatic movements almost ceased. The ridge between the two notches in a couplet was the site of organic encrustation, protecting the rock there, but subsequently removed by subaerial erosion when the notch couplet was uplifted. Observations of double notches formed on the present coastline show that a similar process has been active also during the late Holocene transgression.