Biological Evaluation of Marine ­Protected Area: Evidence of Crowding Effect on a Protected Population of Queen Conch in the Caribbean

Abstract This study provides a first evaluation of the biological impact of a marine fishery reserve on the stock of queen conch (Strombus gigas) in the Turks and Caicos Islands. The density and the shell length of the population living in the reserve are compared with those of the individuals living in the surrounding fished areas. The results show that the adult density is six times higher in the reserve than in the fished areas. The shell length analysis shows that both adults and juveniles are significantly smaller in the reserve than in the fished area. This unexpected result suggests the existence of a crowding effect (i. e. a high density‐induced reduction in growth rate) within the ­reserve. It is hypothesised that this crowding effect is due to the superimposition of two factors leading to very high density values in the reserve: (a) the reduced fishing mortality following the creation of the reserve, (b) the existence of natural barriers that impede the emigration of adults outside the reserve. These results are then discussed in relation to current considerations on marine fisheries reserves.     

Imaging and modelling the subsurface structure of volcanic calderas with high-resolution aeromagnetic data at Vulcano (Aeolian Islands,Italy)

In this paper, we present a magnetic model of the subsurface structure of Vulcano island based on high-resolution aeromagnetic data. Three profiles across the most intense magnetic anomalies over the Piano and Fossa calderas were selected for the magnetic modelling, which was constrained by structural and volcanological data, previous geophysical models, paleomagnetic data, and borehole stratigraphy obtained from two deep wells. The interpretation of the magnetic sources represents a significant contribution to the understanding of the Piano and Fossa calderas’ underlying structure, providing us with evidence of the lateral discontinuity between them at depth. We propose that the positive magnetic anomalies in the Piano caldera area are caused by: (a) the remnants of an early submarine volcano; (b) an outcropping dyke swarm related to the feeding system of the Primordial Vulcano phase (beneath Mt. Saraceno); and (c) the presence of a non-outcropping dyke system intruded along a NE–SW-oriented intra-caldera fault (beneath the eastern part of the Piano caldera). Offshore, to the west, the magnetic anomaly map suggests the presence of a submarine volcanic structure, not revealed by bathymetric data, which could represent the eruptive centre, the presence of which has been indirectly deduced from the outcrop of eastern-dipping lavas on the western seashore. Magnetic modelling of the Fossa caldera points to the presence of a highly magnetized cone-like body inside the Fossa cone, centred beneath the oldest crater rims. We interpret this body as a pile of tephritic lavas emplaced in an early phase of activity of the Fossa cone, suggesting that the volume of mafic lavas that erupted at the beginning of the construction of the Fossa edifice was more significant than has previously been deduced. Furthermore, the presence of a magnetized body inside the Fossa cone implies that high temperatures are contained in very limited spaces, do not affect its bulk inner structure, and are restricted to fumarolic conduits and vents. In addition, structures beneath the western and northern part of the Fossa caldera are revealed to have null or low magnetization, which can be ascribed to the presence of pyroclasts and hyaloclastites in this area as well as to a large volume of hydrothermally altered materials. This suggests that the hydrothermal system, with a very limited extension at present, affected a larger area in the past, especially beneath the western part of the caldera.     

Characterization of magnetic particles and magnetostratigraphic dating of shallow-water carbonates in the Ryukyu Islands, northwestern Pacific

Abstract   Characterization of magnetic particles, and magnetostratigraphic dating, of Holocene and Pleistocene shallow-water carbonates (the Ryukyu Group) in the Ryukyu Islands was carried out to infer the age of reef formation. Magnetic particles from these strata are dominated by fine-grained, single-domain magnetite/maghemite (40–140 nm in length). The magnetite crystals exhibit the size and morphologies characteristic of the magnetite formed by magnetotactic bacteria. No large multidomain grains were seen in transmission electron microscopy observations. The widespread presence of single-domain magnetite in the Ryukyu Islands suggests that bacterial magnetites carry depositional remanent magnetization, which is stable enough for magnetostratigraphic dating and thereby for elucidating the evolution of shallow-water carbonates. The polarity-reversal sequence seen in a core taken from the Ryukyu Group correlates with the timescale of the Matuyama chron, including the Jaramillo subchron, Kamikatsura or Santa Rosa Excursions, through the Brunhes chron. These magnetostratigraphic results imply that the time of reef initiation in the Ryukyu Islands of the northwestern Pacific was earlier than in the Great Barrier Reef of the southwestern Pacific, where it started after the Brunhes–Matuyama boundary.     

Ar–Ar geochronology of Santo Antão, Cape Verde Islands

Santo Antão, the northernmost island of the Cape Verde Archipelago, consists entirely of silica-undersaturated volcanic products and minor intrusions. ⁴⁰Ar–³⁹Ar incremental heating experiments have been carried out on 24 samples that cover the entire exposed chronological sequence. The oldest lavas (7.57±0.56 Ma), representing an older volcanic basement, are exposed about 620 m above mean sea level. After an interval of quiescence of up to 4.3 Ma the volcanic activity resumed and continued at low eruption rates. The older basement is unconformably overlain by a ca. 810-m-thick lava sequence that spans an age range from 2.93±0.03 to 1.18±0.01 Ma. This sequence is cut by many dykes and sills. Simultaneous volcanic activity occurred in the northeastern, central and eastern part of the island. A phonolitic pumice deposit that forms a noteworthy feature over most of the island has an estimated age of 0.20 Ma. This predates volcanic activity that formed the highest point of the island (Tope de Coroa) which has an age of 0.17±0.02 Ma. The most recent eruption on the island formed nephelinitic lavas in the Porto Novo region at 0.09±0.03 Ma. The oldest volcanism exposed on Santo Antão, which took place about 7.6 Ma ago, was simultaneous with waning activity on Maio at the eastern end of the Cape Verde Archipelago.     

南沙群岛工程地质环境分区与质量评价

南沙群岛海区自然资源丰富,但工程地质条件复杂,地质灾害频繁。随着对该海区海洋资源的开发和其它海洋工程活动的增加,调查和研究其工程地质环境与质量评价就成为非常必要的前期工作。本文在分析了南沙群岛海区地质构造、活动断裂和火山地震活动的基础上,结合海洋水动力环境要素将南沙海区工程地质环境划分为5个区:南部陆架区、西部陆坡区、南沙海槽区、礼乐滩微陆块区、珊瑚礁隆起区,并深入分析了各分区的环境特征。     

An unusual ‘blood rain’ over the Canary Islands (Spain). The storm of January 1999

An intense storm, including strong winds, a dust storm, ‘blood rain’ and heavy rains affected the Canary Archipelago between 5 and 10 January 1999, producing damage valued at 156 million euros. The present paper analyses the weather conditions and sedimentological features of the dust. The resulting data provide a possible explanation of palaeoclimatic conditions essential for the stabilization of sand dunes in the eastern parts of the Canary Islands.     

Type and evolution of landscapes of Nansha Islands

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