Eastern Mediterranean palaeoclimates from 26 to 5 ka B.P. documented by pollen and isotopic analysis of a core in the anoxic Bannock Basin

Continuous pollen and isotopic records were established for core BAN 84 09 GC retrieved from the anoxic Bannock Basin in the Eastern Mediterranean. On the basis of two ¹⁴C dates, they document the palaeoclimate between about 25.7 ka B.P. and 5.2 ka B.P. in the northern borderlands of the Ionian Basin. The upper half of the core has been redeposited.

The isotopic record displays a correlation with pollen percentages that is strong and positive for Artemisia (sage-brush) and negative for Quercus (oak). The last glacial maximum and the deglaciation are identified by these combined taxa, together with Chenopodiaceae. The glacial maximum around 18 ka B.P. (which has elsewhere been dated from 20 to 15 ka B.P.) has pollen percentages that are high for Artemisia and low for Quercus. The climate in the pollen source area was arid, cold in winter, briefly warm in summer and sustained the vegetation of a semi-desert. The onset of deglaciation after 18 ka B.P. coincides with that of the decline in Artemisia pollen percentage. However, this decline does not indicate reduced aridity, because it is accompanied by a pollen percentage rise of the even more arid herbs Chenopodiaceae and Ephedra. Throughout the deglaciation from 18 to 11 ka B.P., the aridity progressively increases, culminating at 11 ka B.P. This trend is briefly interrupted by a more humid event, shown by a peak in Artemisia pollen percentage and a smaller peak in oak; these two peaks are coeval with the Bölling-Alleröd chronozone (13-11 ka B.P.). Maximum aridity occurs during the Younger Dryas chronozone (11-10 ka B.P.). Afterwards, the oak pollen percentage begins a steady increase, and its maximum value is coeval with the lowest isotopic value, dated at 8760 ± 170 yr B.P. This period was one of high moisture, warm summers, and, according to altitude, mild to cool winters. This climate sustained forests that were Mediterranean in the lowlands and warm temperate in the uplands. A high pollen concentration is observed during this period and reveals the presence of sapropel S1, which is otherwise unrecognizable in this entirely black core. During the following period between 8760 ± 170 and 5200 yr B.P., the δ¹⁸0 reverts to slightly higher values and the Quercus pollen percentage decreases, while the pollen percentage of the wetter Ostrya, the oriental hornbeam, increases. The high pollen concentration during the deposition of sapropel S1 cannot have been caused by increased pollen input into the sea, this pollen being wind-borne, nor by increased pollen production for all taxa, both trees and herbs. We conclude that it is entirely due to increased preservation of this allochtonous organic material by the deep anoxia of the bottom water, below a thick anoxic water column. The coincidence of sapropel deposition with warm and humid local climate as well as with the second global meltwater pulse suggests that the cessation of bottom-water ventilation was due to decreased surface water density, resulting from less saline incoming Atlantic surface water, increased local runoff, and warmer winters.     

The Gulf of California: Review of ecosystem status and sustainability challenges

The Gulf of California is unique because of its geographical location and conformation. It hosts diverse ecosystems and important fisheries that support industry and provide livelihood to coastal settlements. It is also the site of interests and problems, and an intense interaction among managers, producers, and conservationists. In this report, we scrutinize the abiotic (hydrography, climate, ocean circulation, and chemistry) and biotic (phyto- and zooplankton, fish, invertebrates, marine mammals, birds, and turtles) components of the marine ecosystem, and some particular aspects of climate variability, endemisms, harmful algal blooms, oxygen minimum layer, and pollution. We also review the current conditions and conflicts around the main fisheries (shrimp, small and large pelagic fishes, squid, artisanal and sportfishing), the most important human activity in the Gulf of California. We cover some aspects of management and conservation of fisheries, especially the claimed overexploitation of fish resources and the ecosystems, and review proposals for creating networks of marine protected areas. We conclude by identifying main needs for information and research, particularly the integration of data bases, the implementation of models and paleoreconstructions, establishment of monitoring programs, and the evaluation of fishing impacts and management actions.     

Integration of High and Very High-resolution Seismic Reflection Profiles to Study Upper Quaternary Deposits of a Coastal Area in the Western Gulf of Lions, SW France

High resolution (HR – sparker) and very high resolution (VHR – boomer) seismic reflection data acquired in shallow water environments of the Roussillon coastal area are integrated to provide an accurate image of the stratigraphic architecture of the Quaternary deposits. The complementary use of the two systems is shown to be of benefit for studies of shallow water environments. The HR sparker data improved the landward part of a general model of Quaternary stratigraphy previously established offshore. They document an incised valley complex interpreted as the record of successive late Quaternary relative sea-level cycles. The complex is capped by a polygenetic erosional surface developed during the last glacial period (>18 ky) and variably reworked by wave ravinement during the subsequent post-glacial transgression. The overlying transgressive systems tract is partly preserved and presents a varying configuration along the Roussillon coastal plain. The VHR boomer data provide information on the architecture of the uppermost deposits, both in the near-shore area and in the lagoon. These deposits overlie a maximum flooding surface at the top of the transgressive systems tract and constitute a highstand systems tract composed of two different architectural elements. In the near-shore area, a sandy coastal wedge is subdivided into a lower unit and an upper unit in equilibrium with present day dynamics. In the Salses-Leucate lagoon area, the sedimentary architecture is highly complex due to the closure of a former embayment and the formation of the present beach barrier.     

Bilan ecologique du detournement permanent d'un petit fleuve côtier dans l'émissaire d'eaux usées d'une grande ville

The benthic communities established on soft substrata off the outlet of the main sewer of the city of Marseilles were investigated in June 1980 and January 1981; this was about a year and a half after the permanent diversion of a polluted river into the sewer. Although the quantity of pollutants does not seem to have been appreciably modified within these communities, we note, in comparison with the former state: a decrease in the number of species and individuals of roughly 50 to 25% according to the stations (except for the one nearest the outlet); a perceptible decrease of the diversity and similarity indices; an increase in the dominance of muddy substratum species. All of these observations demonstrate degradation of the communities.     

Elemental mass-balance of material carried by major world rivers

An estimate of average river particulate matter (RPM) composition was bàsed on analyses of more than 40 elements in the Amazon, Congo, Ganges, Magdalena, Mekong, Parana and Orinoco rivers, to which were added literature data for 13 other major world rivers, covering the whole spectrum of morphoclimatic features. Geographic variations of major elements in RPM are mostly linked to weathering types and to the balance between weathering rate and river transport. As a result of chemical erosion, Al, Fe and Ti are enriched in RPM with respect to the average parent rock, while Na, Ca, Mg and Sr are strongly depleted. These figures are directly related to the relative importance of dissolved and particulate transport in rivers; this has been computed for each of 40 elements. In order to study weathering on a global scale, the total observed elemental fluxes (dissolved + particulate) have been computed and compared to theoretical ones. The latter were derived from the elemental content in the average parent rock and the total quantity of weathered material, computed from the Al ratio in RPM and in parent rock. Observed and theoretical fluxes are balanced for the less mobilized elements (rare earths, Co, Cr, Cs, Fe, Mn, Rb, Si, Th, Ti, U and V) for which no enrichment relative to Al is noted in RPM, and for B, Ba, Ca, K, Mg, Na, Sr which are relatively depleted in RPM due to their high dissolved transport. Additional fluxes have been found for Br, Sb, Pb, Cu, Mo, Zn and are possible also for Ni and P. This is reflected by marked enrichments in RPM relative to Al for the poorly or moderately dissolved transports (Pb, Cu, Zn). Several hypotheses involving either the natural origin (volcanic dust, marine aerosols, geochemical fractionation) or the artificial origin (worldwide pollution) are discussed to explain these discrepancies, assuming river transport and weathering either to be in a steady state on a global scale or not. However, none of them can fully account for these additional fluxes. It is most likely that these excesses have multiple origins, anthropogenic or natural or both. The comparison between RPM and deep-sea clay compositions emphasizes the prime influence of river input on oceanic sedimentation of Si, Al, Fe, Ti, lanthanides, Sc, Rb, V, etc. A few elements such as Zn, Sb, occur in excess in RPM as compared to deep-sea clays; in order to balance this excess, a remobilization of these elements out of the sediment can be considered. Finally, the enrichment of Co, Cu, Mn and Ni in deep-sea clays compared to RPM is discussed and attributed to several sources and processes.     

Distribution of Th and Pa in the water column in relation to the ventilation of the deep Arctic basins

Distribution of ²³⁰Th and ²³¹Pa in the water column was measured in the Eurasian basins of the Arctic Ocean, i.e. the Makarov, Amundsen and Nansen basins. The profiles obtained were compared to one previously published from the Alpha Ridge (Bacon et al., Earth and Planetary Science Letters, 95 (1989) 15–22. The Amundsen and Nansen basins are similar in their ^23OTh and ²³¹Pa distribution, whereas in the Makarov Basin and at the Alpha Ridge these radionuclides are comparatively enriched in the deep basin. This may be due to low particle flux in conjunction with low water mass renewal rates. The ²³⁰Th concentration in water depth < 1000 m in the Makarov Basin is relatively low compared with the Alpha Ridge, which may be caused by an exchange of water masses between the Amundsen and Makarov basins.In the Eurasian Basin Deep Water, ²³⁰Th profiles can be explained both by a scavenging model and by a mixing-scavenging model. Enhanced particle load caused by resuspension may explain the increased scavenging in the Eurasian Basin Bottom Water. The differences in scavenging between the Norwegian-Greenland Sea and the Eurasian basins at present are not large enough to cause significant fractionation between ²³⁰Th and ²³¹Pa.     

The Sample Size Necessary to Assess Changes in Fish Biomass – A Reply

Abstract. In this article, we compare biomass results for three fish species (Symphodus ocellatus, Serranus scriba and Diplodus annularis) between May 1993 and May 2000 in the Bonifacio Strait Marine Reserve. The data were obtained from seagrass meadows using the visual census method. A randomised procedure was used to test the null hypothesis of biomass equality. To this end, decreasing sample sizes were used for the May 2000 data to determine the sample size necessary to reject the null hypothesis (5 % level). For Symphodus ocellatus, 10 samples (fixed points) were necessary to reject the null hypothesis, whereas 15 samples were required for Diplodus annularis. The null hypothesis was never rejected for Serranus scriba regardless of the sample size considered.
These results are discussed, as is the power statistical analysis proposed by Ortiz (2002; this issue, pp. 1 – 9) , to determine the optimal sample size necessary to detect perturbation effects in the marine environment. Although the power analysis is certainly a useful method to determine the sampling design in assessment the effect of marine reserves on fish biomass, the magnitude of the marine reserve effect remains problematic in ecology.     

Magnetic properties of some young basalts from the East Pacific Rise

We report the results of a study of the magnetic properties of basalts recovered from the axis and from 0.7 m.y. old crust at 21° N and 19°30 S on the East Pacific Rise as well as from the 9°03 N overlapping spreading centers. The natural remanent magnetization of the samples from 21° N and 19°30 S decreases from the axis to 0.7 m.y. old crust as a result of low-temperature oxidation. In addition, the magnetic properties of the samples from the 21° N sites indicate that: (1) the magnetic susceptibility and the Koenigsberger ratio decrease with low-temperature alteration, (2) the Curie temperature, the median demagnetizing field and the remanent coercivity increase with maghemitization, (3) the saturation magnetization measured at room temperature does not change significantly with age. The magnetic properties of the basalt samples from the 9°03 N overlapping spreading centers indicate the presence of a high magnetization zone at the tip of the eastern spreading center. This high magnetization zone is the result of the high percentage of unaltered, fine-grained titanomagnetites present in the samples. These measurements are consistent with the results of the three-dimensional inversion of the magnetic field over the 9°03 N overlapping system [Sempere et al., 1984] as well as with detailed tectonic and geochemical investigations of overlapping spreading centers (Sempere and Macdonald, 1986a; Langmuir et al., 1986; Natland et al., 1986). The high magnetization zone appears to be the result of the eruption of highly fractionated basalts enriched in iron associated with the propagation of one of the limbs of the overlapping system into older lithosphere and not just to rapid decay, due to low-temperature oxidation, of the initially high magnetization of pillows extruded in the neovolcanic zone.