The climatic significance of Late Ordovician-early Silurian black

The Ordovician-Silurian transition (455-430 Ma) is characterized by repeated climatic perturbations, concomitant with major changes in the global oceanic redox state best exemplified by the periodic deposition of black shales. The relationship between the climatic evolution and the oceanic redox cycles, however, remains largely debated. Here, using an ocean-atmosphere general circulation model accounting for ocean biogeochemistry (MITgcm), we investigate the mechanisms responsible for the burial of organic carbon immediately before, during and right after the latest Ordovician Hirnantian (445-444 Ma) glacial peak. Our results are compared with recent sedimentological and geochemical data. We show that the late Katian time slice (445 Ma), typified by the deposition of black shales at tropical latitudes, represents an unperturbed oceanic state, with regional organic carbon burial driven by the surface primary productivity. During the Hirnantian, our experiments predict a global oxygenation event, in agreement with the disappearance of the black shales in the sedimentary record. This suggests that deep-water burial of organic matter may not be a tenable triggering factor for the positive carbon excursion reported at that time. Our simulations indicate that the perturbation of the ocean circulation induced by the release of freshwater, in the context of the post-Hirnantian deglaciation, does not sustain over sufficiently long geological periods to cause the Rhuddanian (444 Ma) oceanic anoxic event. Input of nutrients to the ocean, through increased continental weathering and the leaching of newly-exposed glaciogenic sediments, may instead constitute the dominant control on the spread of anoxia in the early Silurian

Back to the Grindstone? The Archaeological Potential of Grinding-Stone Studies in Africa with Reference to Contemporary Grinding Practices in Marakwet, Northwest Kenya

This article presents observations on grinding-stone implements and their uses in Elgeyo-Marakwet County, northwest Kenya. Tool use in Marakwet is contextualized with a select overview of literature on grinding-stones in Africa. Grinding-stones in Marakwet are incorporated not only into quotidian but also into more performative and ritual aspects of life. These tools have distinct local traditions laden with social as well as functional importance. It is argued that regionally and temporally specific studies of grinding-stone tool assemblages can be informative on the processing of various substances. Despite being common occurrences, grinding-stone tools are an under-discussed component of many African archaeological assemblages. Yet the significance of grinding-stones must be reevaluated, as they hold the potential to inform on landscapes of past food and material processing

Ecological and evolutionary response of Tethyan planktonic foraminifera to the middle Eocene climatic optimum (MECO) from the Alano section (NE Italy)

The enigmatic middle Eocene climatic optimum (MECO) is a transient (~500kyr) warming event that significantly interrupted at ~40 Ma the long-term cooling through the middle and late Eocene, eventually resulting in establishment of permanent Antarctic ice-sheet. This event is still poorly known and data on the biotic response are so far scarce. Here we present a detailed planktonic foraminiferal analysis of the MECO interval from a marginal basin of the central-western Tethys (Alano section, northeastern Italy). The expanded and continuous Alano section provides an excellent record of this event and offers an appealing opportunity to better understand the role of climate upon calcareous plankton evolution. A sapropel-like interval, characterized by excursions in both the carbon and oxygen bulk-carbonate isotope records, represents the lithological expression of the post-MECO event in the study area and follows the δ18O negative shift, interpreted as representing the MECO warming.High-resolution quantitative analysis performed on both >38 μm and >63 μm fractions reveals pronounced and complex changes in planktonic foraminiferal assemblages indicating a strong environmental perturbation that parallels the variations of the stable isotope curves corresponding to the MECO and post-MECO intervals. These changes consist primarily in a marked increase in abundance of the relatively eutrophic subbotinids and of the small, low-oxygen tolerant Streptochilus, Chiloguembelina and Pseudohas-tigerina. At the same time, the arrival of the abundant opportunist eutrophic Jenkinsina and Pseudoglobigerinella bolivariana, typical species of very high-productivity areas, also occurs. The pronounced shift from oligotrophic to more eutrophic, opportunist, low-oxygen tolerant planktonic foraminiferal assemblages suggests increased nutrient input and surface ocean productivity in response to the environmental perturbation associated with the MECO. Particularly critical environmental conditions have been reached during the deposition of the sapropel-like beds as testified by the presence of common giant and/or odd morphotypes. This is interpreted as evidence of transient alteration in the ocean chemistry.The enhanced surface water productivity inferred by planktonic foraminiferal assemblages at the onset of the event should have resulted in heavier δ13C values. The recorded lightening of the carbon stable isotope preceding the maximum warmth therefore represents a robust indication that it derives principally by a conspicuous increase of pCO2. The increased productivity of surface waters, also supported by geochemical data, may have acted as mechanism for pCO2 reduction and returned the climate system to the general Eocene cooling trend. The oxygen-depleted deep waters and the organic carbon burial following the peak of the MECO event represent the local response to the MECO warming and suggest that high sequestration of organic matter, if representing a widespread response to this event, might have contributed to the decrease of pCO2 as well. Though the true mechanisms are still obscure, several lines of evidence indicate a potential pressure on planktonic foraminiferal evolution during the MECO event including permanent changes besides transient and ecologically controlled variations

Major palaeoenvironmental perturbation in an Early Aptian carbonate platform: Prelude of the Oceanic Anoxic Event 1a?

The Early Aptian Oceanic Anoxic Event (OAE 1a) was characterized by intensified greenhouse climate conditions, widespread accumulation of organic deposits in open-marine settings, major perturbations in the C cycle and a generalized increase in terrestrial runoff. Sedimentological, diagenetic and chemostratigraphic analyses of Lower Aptian platform carbonates from the North Cantabrian basin (N Spain) illustrate the regional impact and effects of those global conditions on shallow marine environments. The studied interval outlines four stages of platform evolution. Stage 1 (earliest Bedoulian) is defined by an initial rapid marine transgression that led to deposition of shallow water oligotrophic photozoan skeletal assemblages, and by a later interval of subaerial exposure. Stage 2 (early Bedoulian) starts with a rapid transgression followed by deposition of grainstones that yield heterozoan assemblages, more typical of mesotrophic conditions, along with ferruginized oolites. Stage 3 (early Bedoulian) is defined by the drowning of the carbonate platform and subsequent deposition of open-marine marls, which are thought to represent the local expression of the OAE 1a. Finally, stage 4 shows the return of shallow water photozoan carbonate sedimentation. The carbonate O and C stable isotope records have revealed prominent negative excursions during deposition of the marly interval of the stage 3, which may be associated with the important global changes that occurred at the onset of the OAE 1a. The change in skeletal assemblages that preceded the isotopic excursions and the platform drowning documents conditions of environmental stress caused by a combination of local and global factors. The global change, coupled with increased basin subsidence, triggered the drowning of the platform by progressive reduction of the growth potential of the carbonate factory

Major palaeoenvironmental perturbation in an Early Aptian carbonate platform: Prelude of the Oceanic Anoxic Event 1a?

The Early Aptian Oceanic Anoxic Event (OAE 1a) was characterized by intensified greenhouse climate conditions, widespread accumulation of organic deposits in open-marine settings, major perturbations in the C cycle and a generalized increase in terrestrial runoff. Sedimentological, diagenetic and chemostratigraphic analyses of Lower Aptian platform carbonates from the North Cantabrian basin (N Spain) illustrate the regional impact and effects of those global conditions on shallow marine environments. The studied interval outlines four stages of platform evolution. Stage 1 (earliest Bedoulian) is defined by an initial rapid marine transgression that led to deposition of shallow water oligotrophic photozoan skeletal assemblages, and by a later interval of subaerial exposure. Stage 2 (early Bedoulian) starts with a rapid transgression followed by deposition of grainstones that yield heterozoan assemblages, more typical of mesotrophic conditions, along with ferruginized oolites. Stage 3 (early Bedoulian) is defined by the drowning of the carbonate platform and subsequent deposition of open-marine marls, which are thought to represent the local expression of the OAE 1a. Finally, stage 4 shows the return of shallow water photozoan carbonate sedimentation. The carbonate O and C stable isotope records have revealed prominent negative excursions during deposition of the marly interval of the stage 3, which may be associated with the important global changes that occurred at the onset of the OAE 1a. The change in skeletal assemblages that preceded the isotopic excursions and the platform drowning documents conditions of environmental stress caused by a combination of local and global factors. The global change, coupled with increased basin subsidence, triggered the drowning of the platform by progressive reduction of the growth potential of the carbonate factory

Modos e modas da doença e do corpo

O presente artigo busca compreender o processo da "Construção Social da Saúde" levando-se em consideração que a saúde, no decorrer da historia dos homens, foi sempre considerada um "bem" e, por isso, mereceu constante preocupação, no sentido de tornar-se geradora de modas, de modos de fazer e de existir, de conflitos, dualidades e controle social. No decorrer desse tempo, modelos de saúde foram sendo criados, interpretados e recriados, quando necessário, provocando igual processo de transformação nas maneiras de sentir, pensar e agir da população usuária dos mais variados recursos de saúde disponíveis, segundo as relações entre o "mágico e o necessário" , estabelecendo, entre os que serviam e os que eram servidos, uma relação também tão mágica quanto necessária, intermediada pelo corpo, destes sujeitos, depositário do estado de saúde ou de doença. Além do processo de transformação das mentalidades, são ainda levados em consideração os processos de construção, desconstrução e de evolução do imaginário e das representações sociais vivenciados pelos sujeitos e seus corpos. A evolução dos conhecimentos e o avanço científico-tecnológico são enfocados também como fontes modelares e comunicativas no sentido de ditar regras ao corpo que a humanidade porta socialmente neste século.The present article aims at understanding the process of "Social Construction in Health" bearing in mind that health has, along the history of mankind, been always considered a good. It has thus been of constant concern in that it becomes the generator of trends, modes of doing and existing, conflicts, dualities and social control. Along that time health models were designed, interpreted and recreated when necessary, leading to a similar transformation process in the forms of feeling and acting of the users of the most varied health resources available. This is done within the relation between the "magic and the necessary" establishing between those who serve and who were served a relation also as magic as necessary which is intermediated by the body of those subjects, depositary of health and illness states. Besides the transformation process of mentalities also taken into consideration are the processes of construction, deconstruction and evolution of the imaginary and the social representations experienced by the subjects and their bodies. The evolution of knowledge and technical and scientific advances are also focused as model and communicative sources in the sense of dictating rules to the body that humanity socially bears in this century