Publication: The Last Interglacial in the Mediterranean as a model
for the present interglacial
Full text at PDC
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Deposits of the Last Interglacial on the south and southeastern coasts of Spain are shallow marine and coastal sediments, with a warm fauna of Strombus bubonius. These units exhibit a diversity of morpho-sedimentary models controlled by the tectonic activity of the Mediterranean area, which is closely related to the approximation of Africa and Iberia during the Quaternary. There are three well-dated peaks of maximum sea level (T-I: isotopic substage 7a, T-II: isotopic substage Se, T-III: isotopic substage 5c). A younger episode, T-IV, probably corresponds to the isotopic substage 5a. Episodes T-II, T-III and T-IV were laid down during Last Interglacial age. In addition, three Holocene peaks of maximum sea level: H1 ca. 5100 yr B.P., H2 ca. 3500 yr B.P. and H3 ca. 2400 yr B.P. were found. The three main peaks of the Last Interglacial correspond to the morpho-sedimentary Tyrrhenian units T-II, T-III and T-IV, deposited during a time span of sorne 45,000 years. Several smaller oscillations can be distinguished within each of these units as subunits separated by erosional surfaces. At least three of such mapable subunits were distinguished within the peak T-II (Se); each lasted ca. 10,500 yr. As the positive oscillations of sea level (H1, H2, and H3) recorded during the present Interglacial (Holocene) are much shorter, we infer that they are smaller-scale fluctuations (2500-1100 yr cycles) within the first oscillation (duration: ca. 10,500 yr) of the first Holocene peak of sea level which has not yet been completed. In addition to changes of sea level, the vertical and lateral arrangement of morpho-sedimentary units, which can be designated as the stratigraphic architecture, depends on tectonics and oceanography, including geoidal and steric changes and coastal dynamics. The coastal dynamics factor largely depends on the exchange of waters between the Atlantic and the Mediterranean. Maximum incursions of water coincides with warm periods (highstands) when the coastal accretion increases. The tectonic factor greatly influences and modifies the effects of sea-level changes in the coastal areas of tectonically-active regions such as the Mediterranean. Areas with tectonic uplift will be characterized by a staircase of prograding gravelly beaches, whereas sinking areas will favour the deposition of vertically stacked sequences with coastal onlap of barrier island and lagoon deposits. Rates of sea-level rise for the coming years of 1 cm/yr, have been suggested by sorne authors. These gradients greatly exceed those produced by any tectonic factor in the Spanish coast during the last 100 kyr. Shoreface erosion and transgression with landward migration of barrier islands and lagoons will occur in subsiding areas (Murcia-Alicante and Valencia), even with relatively low rates of sea-Ievel rise Oess than 0.5 cm/yr). Higher rates of sea-level rise (0.5-1 cm/yr) will increase the transgressive trend. Areas with subsidence rates higher than 7.5 cm/kyr (Mar Menor and Oval of Valencia) are prone to transgression and erosion of barrier islands and lagoons both in the cases of stable and rising sea levels. Risks are smaller in areas with lower rates of subsidence (La Mata, Santa Pola and Torrevieja lagoons) when a stable sea level is considered; however, any rise of sea level will trigger coastal erosion.