Luigia Manfra

Recent coastal evolution of the Doñana National Park (SW Spain)

Abstract Since the last Holocene sea level rise, about 6900 BP, a series of depositional littoral landforms has been generated at the outlet of the Guadalquivir River, with progradation along the predominant longshore drift (towards the east). The first coastal progradation occurred between 6900 and 4500 BP. The Donana and (perhaps) La Algaida spits, both associated with the oldest and highest marshland in the Donana National Park, are assumed to have been developed at an early stage. Originally, the Guadalquivir estuary was wider and deeper than now, and its environment was mainly marine. The oldest littoral formations have been dated as ca. 4735 BP. They show erosional events, and indicate the breaking-up of earlier spit-barriers to form inlets. The marine environment became increasingly dominant, with heavy erosion of cliffs and a retreating coastline. This period was followed by another sedimentary cycle (4200-2600 BP) that surrounded the earlier eroded barriers. The size of the estuary decreased due to the increasing marsh deposits, and a fluvial environment was born. About 2600 years ago, progradation gave way to a new period of intense erosion. The resulting morphology of littoral strands and erosional surfaces permitted the return to a marine environment. The shoreline again retreated. From 2300 BP, coastal progradation has prevailed, with an erosional interruption at 1000 BP. The present-day outlet of the Guadalquivir is an estuarine delta of inactive marshland (the Donana National Park), the dominant environment is fluvial.

Benzene synthesis for radiocarbon dating and study of the catalyst used for acetylen trimerization

Abstract The succession of steps undergone by the sample in the benzene synthesis line for radiocarbon dating from its original state to benzene through the intermediate formation of carbon dioxide in the Vycor tube, lithium carbide with molten lithium metal heated to at least 700°C and in absence of air, acetylene by its hydrolysis and its conversion to benzene on the catalyst, is schematically described. The optimum conditions for the above mentioned operations are defined and secondary reactions, responsible for reduced yields in the various steps, are described. Particularly, a silica-alumina catalyst activated by K2CrO4 employed for the conversion of acetylene to benzene is studied. Sixty grams of catalyst, dehydrated at a temperature of about 400°C for 1 hr and in absence of air, proved to be sufficient to convert in benzene, in 90 min time, 98–99% of a volume of acetylene averaging 4.51 at P0T0. The benzene is extracted under vacuum at 150°C for 2 hr and its purity checked by gas chromatograph analysis is about 99.0%. The efficiency of the catalyst is maximum when it is activated by Cr(VI) and it is sufficient to expose the poisoned catalyst to an air flow at a temperature of 550–600°C during 60 min to bring it to same efficiency as the fresh catalyst. No cross contamination effects are present when the regenerated catalyst is used since the values of dates are in the range of 3 σ. The reducing action of acetylene on the catalyst promotes the reduction of Cr(VI) to Cr(IV) and Cr(III) with remarkable loss of its efficiency and is responsible for reduced yield of benzene in conversion reaction with formation of by-products which are a mixture of aliphatic and aromatic polymers, the last containing a carbonyl group and an aliphatic chain. The total amount of by-products is about 5% of acetylene used. The order of reaction for conversion of acetylene to benzene, including by-products in respect of the acetylene is 1.5 in the range 20–35 mmole/1 and in the temperature range of 0–55°C. The value of the energy of activation is −4.9 ± 0.3 kcal/mole.

University of Rome Radiocarbon Dates XVII

This list includes dates from Italy, Libyan Sahara, Niger and the Dahlak Islands, obtained from 1976 to 1980, using the benzene scintillation method. Chemical processing from sample to benzene follows procedures of Alessio et al (1978a). Standard pretreatment for wood and charcoal included boiling with 5-10% HCL. Pretreatment for a-labeled samples underwent leaching with 0.2N NaOH. Since the last date list, Lecroy NIM modules have replaced the original recording electronics for the liquid scintillation detection system, described in Alessio et al (1973, 1976). Benzene sample volumes up to 4.Sml, with appropriate amounts of NE 216 scintillator (Nuclear Enterprises, Ltd) yielded the best figure-of-merit (E2/B = 2300) for 2m1 benzene and an efficiency of 72%. Age calculations comply with standard Radiocarbon protocol, as Stuiver and Polach (1977) recommended, including normalization by mass spectrometrically determined S13C (Alessio et al 1969).