Late Holocene Relative Sea-level Changes Research Articles

An earthquake history derived from stratigraphic and microfossil evidence of relative sea-level change at Coos Bay, southern coastal Oregon

Much of the uncertainty in determining the number and magnitude of past great earthquakes in the Cascadia subduction zone of western North America stems from difficulties in using estuarine stratigraphy to infer the size and rate of late Holocene relative sea-level changes. A sequence of interbedded peaty and muddy intertidal sediment beneath a small, protected tidal marsh in a narrow inlet of Coos Bay, Oregon, records ten rapid to instantaneous rises in relative sea level. Each rise is marked by a contact that records an upward transition from peaty to muddy sediment. But only two contacts, dating from about 1700 and 2300 yr ago, show the site-wide extent and abrupt changes in lithology and foraminiferal and diatom assemblages that can be used to infer at least half a meter of sudden coseismic subsidence. Although the characteristics of a third, gradual contact do not differ from those of some contacts produced by nonseismic processes, regional correlation with other similar sequences and high-precision 14 C dating suggest that the third contact records a great plate-boundary earthquake about 300 yr ago. A fourth contact formed too slowly to have been caused by coseismic subsidence. Because lithologic and microfossil data are not sufficient to distinguish a coseismic from a nonseismic origin for the other six peat-mud contacts, we cannot determine earthquake recurrence intervals at this site. Similar uncertainties in great earthquake recurrence and magnitude prevail at similar sites elsewhere in the Cascadia subduction zone, except those with sequences showing changes in fossils indicative of >1 m of sudden subsidence, sand sheets deposited by tsunamis, or liquefaction features.

Late Holocene relative sea-level changes and reconstruction of paleoenvironment by diatom fossil assemblage analysis in Akkeshi Moor, Hokkaido, northern Japan

Late Holocene relative sea-level changes and reconstruction of paleoenvironment by diatom fossil assemblage analysis in Akkeshi Moor, Hokkaido, northern Japan

Late-Holocene shoreline changes related to palaeoseismic events in the Ionian Islands, Greece

A systematic multidisciplinary survey on the coasts of the Ionian Islands, founded on 26 new radiocarbon accelerator mass spectrography (AMS) dates, has revealed the occurrence of sectors which were uplifted (seismo)tectonically during the late Holocene. In Corfu, two uplift movements of about 0.8m each took place in the central part of the island: one of them around 790-400 cal. BC, and also at a more recent, undetermined date. In Levkas, up-and-down tectonic movements of metric order governed late-Holocene relative sea-level changes, at least in the northern part of the island; submergence of at least 2.5 m occurred since 2400 years ago, with a possible spasmodic subsidence around cal. AD 500-700. In Cephalonia, two vertical displacements affected most of the island, around cal. AD 350-710 and in AD 1953; both movements resulted in uplift in the southeastern part of the island. Submergence seems to predominate round Ithaca. Lastly, in Zante, clear evidence of a Holocene crustal block movement is limited to the southeastern peninsula, where an uplift of 0.95 ± 0.15 m took place, probably coseism ically, around the period cal. AD 200-500. The time ranges of uplift in Cephalonia and Zante, and of possible subsidence in Levkas, correspond with a period of regional tectonic paroxysm, between the middle of the fourth century AD and the middle of the sixth century AD; this has already been documented in several areas of the eastern Mediterranean, notably in the Gulf of Corinth, the southern Hellenic arc, Turkey, Cyprus, the Lebanon and Syria.

Erratum: Postglacial relative sea-level change, Port au Port area, west Newfoundland

We first report pollen and foraminifera analyses and radiocarbon dates from two cores taken from salt-marsh deposits bordering Port au Port Bay, southwestern Newfoundland. Results show that relative sea level (RSL) stood at 2.8 m below present higher high-water level (HHWL) at 2770 ± 300 years BP and at −1.8 m at 2365 ± 175 years BP at the core sites. They permit calculation of a rate of late Holocene RSL change from western Newfoundland. We then report other available dates bearing on the earlier RSL record of this area.A date of 5800 ± 200 years BP fixes the age of minimum RSL in Port au Port Bay at 11–14 m below present. A date of 9350 ± 120 years BP from St. George's provides a minimum age for the passage of sea level below present there. A date of 12 600 ± 140 years BP from Stephenville fixes a sea level at 29 m above present, whereas one of 13 600 ± 110 years BP from Abrahams Cove dates the marine limit at 44 m. These geographically restricted data closely constrain a curve of postglacial RSL change...