1,721,058 research outputs found
PALEOENVIRONMENTAL RECONSTRUCTION ON LATE CALABRIAN STAGE SEDIMENTS FROM THE BELICE VALLEY (SOUTHWESTERN SICILY - ITALY) BASED ON OSTRACODA
Full text linkThe Middle Pleistocene Transition is the most recent re-organization of the climate system, happened between 1.2 and 0.7 Ma. Marine sediments outcropping in Sicily have been a relevant source of information on the paleoclimatic and paleoenvironmental evolution of the Mediterranean Sea, over this time window. Among others, previous studies in Sicily identified and described the very cold phase and the marked sea level fall associated with the top MIS 22, which is lithologically expressed by a calcarenite bed colonized by the Arctica islandica northern guest. Here we investigate ostracod assemblages from marine sediments in the southwestern Sicily. Sediments, ascribed to the late Calabrian Stage, have in-depth been studied by foraminifera, coccolith and pteropod specialists. Our results support previous micropaleontological-based paleoenvironmental reconstructions, but also include original elements that better define the paleobathymetric and bottom condition evolution. By grouping infralittoral, circalittoral and bathyal taxa, ostracods describe a significant sea level fall up to the A. islandica calcarenite bed and then a rise to circalittoral and bathyal environments during the interglacial MIS 21. The abundance increases of ostracod taxa able to survive in low-oxygen environments and a significant diversity loss point to the establishment of water column stratification and seafloor dysoxia, in the lower part of MIS 21. Finally, Bythocythere turgida is the only northern guest found in our samples, associated with A. islandica specimens. However, during the interglacial period we note subtle but significant percentage variations in ostracod groups that may have been driven by productivity changes, rather than real sea level variations. These changes may be associated to Atlantic Meridional Overturning Circulation weakening phases and to millennial-scale cold spells that punctuated MIS 21 © 2022. Alpine and Mediterranean Quaternary
Calcareous nannofossil palaeoenvironmental reconstruction and preservation in sapropel S1 at the Eratosthenes Seamount (Eastern Mediterranean)
Full text linkThe most recent organic carbon-enriched layer (sapropel S1) deposited at the Eratosthenes Seamount has unique features, such as an early lithological interruption, fine light silt laminae and an exceptional vertical extent that is over 25 cm thick. Here we investigate calcareous nannofossil assemblages to reconstruct very high-resolution palaeoenvironmental and palaeoceanographic variations recorded before, during and after the perturbation episode that involved the eastern Mediterranean Sea, due to the massive freshwater discharge via Nile River. Our results show that the deep chlorophyll maximum development, observed in all micropalaeontological groups from previous studies, is a gradual process that started well before the base of sapropel S1. A high-frequency variability in the nutricline depth is evident at millennial- and/or centennial-scale throughout the sapropel deposition time interval. Also we highlight the poor-preservation of delicate tiny holococcolith crystals while anoxia was occurring at the seafloor and we suggest that such a phenomenon may be used to mark the original thickness of sapropel deposition where oxygen re-ventilation fronts were developed. Finally, calcareous nannofossil reworking peaks shed light on the nature of fine silt laminae within the sapropel S1 at the Eratosthenes Seamount, which may be ascribed to fine sediment plumes from the Nile River deposited during exceptional runoff events
Surface and deep water conditions in the Sicily Channel (central Mediterranean) at the time of sapropel S5 deposition
No full textA high-resolution record of the last deglaciation in the Sicily Channel based on foraminifera and calcareous nannofossil quantitative distribution
No full textRelative abundance fluctuations in planktic foraminiferal and calcareous nannofossil assemblages are reported on
the basis of a high-resolution study of ODP Leg 160 Hole 963D, drilled in the Sicily Strait, near Capo Rossello
(southern Sicily). With its 8 m of undisturbed sediments, the core covers the interval from 1.5 to 23 kyr, allowing a
50/100-yr sampling resolution. All the short warm and cold events and sub-events recorded in this time interval in the
GRIP Greenland ice core and at several Mediterranean sites were recognized. On this basis, a total number of nine
ecozones based on planktic foraminifera and seven ecozones based on calcareous nannofossils were identified. A short
interval of climatic amelioration, identified above the base of the Holocene, precedes the segment contemporaneous
with the deposition of sapropel S1, which has no lithological expression in the local sedimentary record. This segment
has been subdivided into the two warm S1a and S1b phases, separated by a cold intermediate event. A gradual
climatic deterioration is recorded above the top of the interval correlated with sapropel S1. It is interrupted by short
cold spells, tentatively correlated with similar short duration cold events reported for this interval in the recent
literature
Sea Surface Temperatures and Paleoenvironmental Variability in the Central Mediterranean During Historical Times Reconstructed Using Planktonic Foraminifera
Full text linkThe ongoing anthropogenic‐induced warming assessment requires a robust background from
regional sea surface temperature (SST) reconstructions. Planktonic foraminifera have yielded valuable
insights into late Quaternary SST dynamics, but the techniques to estimate SST from fossil assemblages have only rarely been used in very recent sedimentary records (the last 2,000 years). Here we use two transfer function methods, modern analog technique and artificial neural networks, to reconstruct SST variability in two cores from the Central Mediterranean Sea that span the last five centuries. Both cores show similar and considerable changes in the planktonic foraminifera assemblages. However, the inferred mean annual SSTs only varied in a narrow range, in agreement with instrumental data that go back to 1850 CE. Our
reconstructions extend this time frame and indicate that SST variability did not exceed 1.5 °C over the past
three centuries. Rather than temperature, we suggest that the changes in the assemblages reflect switches
between sea surface winter/spring productivity and a deep winter mixed layer, due to the atmosphere/ocean
interplay that governs different productivity modes in neighboring mesoscale gyres
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Reventilation Episodes During the Sapropel S1 Deposition in the Eastern Mediterranean Based on Holococcolith Preservation
No full textOrganic-rich layers (sapropels), preserved in eastern Mediterranean marine sediment records, represent pronounced perturbations to thermohaline circulation and environmental conditions in the basin, in response to enhanced African monsoon activity and subsequent massive freshwater discharge. During the most recent event, Sapropel S1 formed between 10.8 and 6.1 ka, when freshwater-driven stratification caused seafloor anoxia below ~1,800-m depth, as a result of both failure of deep water formation and enhanced productivity. Here we analyze coccolith assemblages from the open eastern Mediterranean that form a west-east transect across the basin and provide insights on past environmental changes. We focus on holococcoliths, which are specifically produced by coccolithophores as part of their life cycle during the haploid phase. Since holococcolith calcification is characterized by nanocrystals highly susceptible to dissolution, we are testing their potential preservation under different bottom environmental conditions, including the effect of postdepositional oxidation. A comparison with benthic foraminifera assemblages in a core recovered close to Lybia reveals that holococcolith preservation is enhanced during seafloor reventilation and benthic foraminiferal repopulation in the middle to upper part of the record, before the actual sapropel termination. There are two such events of improved deep-water oxygenation in the Aegean and Adriatic Seas at 8.2 and 7.4 ka. The latter episode marks the onset of the transition to restored circulation in the eastern Mediterranean Sea, due to resumption of deep-water formation in the southern Aegean Sea and the conclusion of enhanced biogenic productivity
High-frequency modification of the central Mediterranean seafloor environment over the last 74 ka
No full textHere we present a high-resolution record of benthic foraminiferal assemblages for the last 74 kyr from the Sicily Channel Ocean Drilling Program Leg 160 Site 963. Benthic foraminiferal results are compared with geochemical (benthic and planktic δ18O and δ13C) and calcareous plankton data, previously acquired on the same marine core sediments. Within the succession, three benthic foraminifera compositional zones were defined. Temporal changes in the assemblages are interpreted in the context of the modification of subtropical and temperate climate systems that affected the Mediterranean thermohaline circulation. A close connection between bottom conditions in the Sicily Channel and eastern Mediterranean Sea is evident in two intervals, characterized in the ODP Site 963 by reduced oxygen conditions. The first one, around 53–51 ka, is tentatively attributed to the ‘missing’ sapropel S2 while the second, between 35 and 29 ka, is marked by short and recurrent episodes of bottom-water oxygen decrease. Both are related to a weakening of the intermediate circulation in the Sicily Channel connected with relatively high northern hemisphere summer insolation and increase in Nile River discharge, which inhibited vertical mixing and intermediate water ventilation in Eastern Mediterranean Sea. Over the last deglaciation, the African humid period (AHP) and the sea level rise, also influenced the water mass structure in the Sicily Channel and a reduction of the bottom ventilation is suggested by a strong reduction of deep-water Miliolids. Decreased bottom oxygen levels, which testifies for a weakening of intermediate circulation in Sicily Strait, also characterized the interval corresponding to S1 deposition. Yet, the strong decrease of benthic foraminiferal abundance related to a low surface water trophic level, appears to be conditioned by the reduction of trophic levels in the western Mediterranean. The very high relative abundance of U. mediterranea recorded during this interval is explained by the availability of organic matter during a limited short year period and/or the availability of more degraded organic matter from river runoff. In general, compositional data analysis highlighted a quite complex response of benthic foraminifera to paleoclimatic changes. However, changes in the benthic foraminiferal assemblages recorded during the last glacial are coherent with surface paleoproductivity dynamics connected with D-O oscillation, and support oligotrophic, meso-eutrophic and oligo-mesotrophic conditions during the early interstadials, late interstadials and stadials, respectively
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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