Diversidad étnica e identidad en la llanura costera del istmo oaxaqueño

En este artículo se desarrollan una serie puntos para entender al istmo oaxaqueño como una zona de convergencia cultural diversa, múltiple y por lo tanto diferenciada tanto en el ámbito de la historia de los pueblos que en ella viven, como en el ámbito de la estructura económica que permite su reproducción no sólo como grupos étnicos, sino como comunidades. El autor plantea que las manifestaciones culturales de cada grupo se confunden, se entrelazan se influencian mutuamente y en una lógica asociada a la perdurabilidad de dichos procesos, terminan por imponerse. Para comprender las relaciones interétnicas no basta con dar cuenta de las características generales de estos grupos sino que es necesario actualizar la información que se ha obtenido de ellos, sobre todo ahora que nuevos procesos nacionales o internacionales están afectando de manera irreversible la composición y la estructura de estas culturas. Aspectos como la falta de vías de comercialización de productos agrícolas y pesqueros, la marginación sempiterna, la falta de empleo a escala regional, la migración, etc.. son entre otros, las aristas de una realidad que se revela en su irremediable avance frente a comunidades que ponen en funcionamiento estrategias de reproducción y de sobreviviencia para mantenerse como comunidad. Las manifestaciones de las relaciones interétnicas aquí se definen como asimétricas horizontales en virtud de una diferenciación entre etnias y las relaciones entre ellas, de tal forma que así como se ha analizado la relación grupos étnicos-sociedad nacional, en la que se constatan desigualdades flagrantes a nivel del acceso a la riqueza generada en el país, calidad de vida, educación, comunicación, y otras variables importantes, de la misma manera se constata que en los grupos étnicos hay unos que tienen una posición privilegiada en el acceso a vías de comunicación, redes comerciales e influencia política, entre otros aspectos, mientras que otros se encuentran excluidos de ellos, no sólo por su condición de indígenas, sino también como consecuencia de la dominación entre etnias que existe en la región. Finalmente, el autor enfatiza que en esta zona del estado de Oaxaca existe una gama considerable de recursos naturales cuya apropiación y explotación para beneficios económicos está en el centro de la discusión actual. En efecto, el sistema de lagunas del Golfo de Tehuantepec, las reservas de la biodiversidad como la selva de los Chimalapas, o la actividad ganadera agrícola en la zona mixe son, entre otras, no sólo zonas importantes de explotación económica, que han fortalecido a grupos de poder locales. También son espacios de control estratégico para el futuro desarrollo del país pues en el istmo oaxaqueño se ha ubicado una fuente importante de riqueza en agua, bosques, especies endémicas, etc... que puede tener un papel importante en la vida económica regional y nacional. La cuestión es saber qué dispositivos sociales o legales se han puesto en marcha para definir a los beneficiarios de esa riqueza natural.

Radiolarian events in the eastern Equatorial Pacific

The development of an orbitally tuned time scale for the ODP leg 138 sites provides biostratigraphers a very high resolution chronostratigraphic framework. With this framework we are better able to define which of the first and last appearances of species appear to be synchronous. In addition, the geographic distribution of sites provides the means with which the detailed spatial patterns of invasion of new species and the extinction of older species can be mapped. These maps not only provide information on the process of evolution, migration, and extinction, they can also be related to water mass distributions and near-surface circulation of the ocean. Of 39 radiolarian events studied at 11 sites in the eastern equatorial Pacific, 28 were found to have a minimum range in their estimated age that exceeded 0.15 m.y. The temporal pattern of first and last appearances of these diachronous events have coherent spatial patterns that indicate shifts in the areas of high oceanographic gradients over the past 10 Ma. These changes in the locations of high gradient regions suggest that the South Equatorial Current (SEC) was north of its present position prior to approximately 7 Ma. There was a southward shift in the northern boundary of this current between approximately 6 and 7 Ma, and the development of a relatively strong gradient between the northeastern and northwestern sites. Between approximately 3.7 and 3.4 Ma, there was a very slight northward shift in the northern boundary of the SEC and the steep gradients between the northeastern and northwestern sites may have disappeared. This change is thought to be associated with the closing of the Isthmus of Panama. The temporal-spatial patterns of diachronous events younger than 3.4 Ma are consistent with patterns of circulation in the modern ocean.

Nd isotopic composition of fossil fish teeth and debris from ODP Legs 138, 165, and 202

Changes in circulation associated with the shoaling of the Isthmus of Panama and the Caribbean carbonate crash in the Miocene were investigated using Nd isotopes from fossil fish teeth and debris from two sites in the Caribbean Basin (Ocean Drilling Program Sites 998 and 999) and two sites in the eastern equatorial Pacific (Sites 846 and 1241). The total range for e-Nd values measured from 18 to 4.5 Ma in the Caribbean is -7.3 to 0. These values are higher than Atlantic water masses (~-11) and range up to values equivalent to contemporaneous Pacific water masses, confirming that flow into the Caribbean Basin was composed of a mixture of Pacific and Atlantic waters, with an upper limit of almost pure Pacific-sourced waters. Throughout the Caribbean record, particularly during the carbonate crash (10-12 Ma), low carbonate mass accumulation rates (MARs) correlate with more radiogenic e-Nd values, indicating increased flow of corrosive Pacific intermediate water into the Caribbean Basin during intervals of dissolution. This flow pattern agrees with results from general ocean circulation models designed to study the effect of the shoaling of the Central American Seaway. Low carbonate MARs and high e-Nd values also correlate with intervals of increased Northern Component Water production and, therefore, enhanced conveyor circulation, suggesting that the conveyor may respond to changes in circulation associated with shoaling of the Central American Seaway. Reduced Pacific throughflow related to shoaling of the seaway led to a gradual increase in carbonate preservation and more Atlantic-like e-Nd values following the carbonate crash.

Pliocene and Pleistocene eastern equatorial Pacific Mg/Ca subsurface temperature

During the early Pliocene warm period (~4.6-4.2 Ma) in the Eastern Equatorial Pacific upwelling region, sea surface temperatures were warm in comparison to modern conditions. Warm upwelling regions have global effects on the heat budget and atmospheric circulation, and are argued to have contributed to Pliocene warmth. Though warm upwelling regions could be explained by weak winds and/or a deep thermocline, the temporal and spatial evolution of the equatorial thermocline is poorly understood. Here we reconstruct temporal and spatial changes in subsurface temperature to monitor thermocline depth and show the thermocline was deeper during the early Pliocene warm period than it is today. We measured subsurface temperature records from Eastern Equatorial Pacific ODP transect Sites 848, 849, and 853 using Mg/Ca records from Globorotalia tumida, which has a depth habitat of ~50-100 m. In the early Pliocene, subsurface temperatures were ~4-5°C warmer than modern temperatures, indicating the thermocline was relatively deep. Subsurface temperatures steeply cooled ~2-3°C from 4.8 to 4.0 Ma and continued to cool an additional 2-3°C from 4.0 Ma to present. Compared to records from other regions, the data suggests the pronounced subsurface cooling between 4.8 and 4.0 Ma was a regional signal related to restriction of the Isthmus of Panama, while continued cooling from 4.0 Ma to present was likely related to global processes that changed global thermocline structure. Additionally, the spatial evolution of the equatorial thermocline along a N-S transect across ODP Sites 853, 849 and 848 suggests an intensification of the southeast trades from the Pliocene to present. Large-scale atmospheric and oceanographic circulation processes link high and low latitude climate through their influence on equatorial thermocline source water regions and consequently the equatorial thermocline. Through these low latitude/high latitude linkages, changes in the equatorial thermocline and thermocline source water played an important role in the transition from the warm Pliocene to the cold Pleistocene.

Nannofossil and planktic foraminiferal events of ODP Site 166-1006 sediments

Detailed biostratigraphy in Site 1006 based on planktonic foraminifers and nannofossils shows large-scale sedimentation rate variability in the Florida Strait west of the Great Bahama Bank. A 'floating' cyclostratigraphy based mainly on resistivity logs and magnetic susceptibility data has been fixed to the biostratigraphy in the absence of magnetostratigraphy. The strongest orbital cycle present is the precessional beat, which is present in the borehole logs throughout the record. Counting the cycles resulted in an accurate time scale and thus a sedimentation rate time series. Spectral analysis of the sedimentation rate time series shows that the short-term cycle of eccentricity (~125 k.y.) and the long term cycle of eccentricity (~400 k.y.) are pervasive throughout the Miocene record, together with the long-term ~2-m.y. eccentricity cycle. The Great Bahama Bank produced pulses of shallow carbonate input once every precessional (sea level) cycle during the Miocene and perhaps two pulses per cycle in the early Pliocene. The amount of sediment exported in these pulses appears to be controlled by eccentricity modulation of the precessional amplitude and therefore the amplitude of the sea-level rise. Finally, an increase in sedimentation rate just after the Miocene/Pliocene boundary is attributed to a change in the location and strength of sediment drift currents in the Florida Strait due to reorganization of the currents following the closure of the Panama Isthmus.

Benthic foraminifera accumulation rate and benthic foraminiferal productivity proxies at ODP Site 165-999 in the Caribbean (Appendix A)

This study tests the hypothesis that the late Miocene to early Pliocene constriction and closure of the Central American Seaway (CAS), connecting the tropical Atlantic and East quatorial Pacific (EEP), caused a decrease in productivity in the Caribbean, due to decreased coastal upwelling and an end to the connection with high-productivity tropical Pacific waters. The present study compared paleoceanographic proxies for the interval between 8.3 and 2.5 Ma in 47 samples from south Caribbean ODP Site 999 with published data on EEP DSDP Site 503. Proxies for Site 999 include the relative abundance of benthic foraminiferal species representing bottom current velocity and the flux of organic matter to the sea floor, the ratio of infaunal/epifaunal benthic foraminiferal species and benthic foraminifer accumulation rates (BFARs). In addition, we calculated % resistant planktic foraminifers species and used the previously published % sand fraction and benthic carbon isotope values from Site 999. During early shoaling of the Isthmus (8.3-7.9 Ma) the Caribbean was under mesotrophic conditions, with little ventilation of bottom waters and low current velocity. The pre-closure interval (7.6-4.2 Ma) saw enhanced seasonal input of phytodetritus with even more reduced ventilation, and enhanced dissolution between 6.8 and 4.8 Ma. During the post-closure interval (4.2-2.5 Ma) in the Caribbean, paleoproductivity decreased, current velocity was reduced, and ventilation improved, while the seasonality of phytodetrital input was reduced dramatically, coinciding with the establishment of the Atlantic-Pacific salinity contrast at 4.2 Ma. Our data support the hypothesis that late Miocene constriction of the CAS at 7.9 Ma and its closure at 4.2 Ma caused a gradual decrease in paleoproductivity in the Caribbean, consistent with decreased current velocity and seasonality of the phytodetrital input.