Landscape evolution and depositional processes in the Miocene Amazonian Pebas lake/wetland system: Evidence from exploratory boreholes in northeastern Peru.

This study of the type and scales of depositional processes and landscape development in western Amazonia during the Miocene is based on the description and interpretation of three boreholes from the Marañon basin (Peru). The Miocene Pebas Formation, and the overlying Marañon Formation and underlying Chambira Formation are lithologically characterised. An age calculation model indicates an Oligocene age for the Chambira Formation, and an Early - early Late Miocene age for the Pebas Formation. The base of the Chambira Formation is placed at a sequence boundary and corresponds to the beginning of a regression. The succession was deposited in floodplains included in a RST and a LST under a seasonal climate with a pronounced dry season. The base of the Pebas Formation is placed at a TS. It represents TST and HST lacustrine and swamp settings at or near sealevel, formed in a tropical monsoon climate alike the present-day climate in the region. At the time, the area was a mosaic of lakes, swamps and fluvial belts, but experienced tidal influence as well. During apparently regularly recurring base level highstands, open aquatic settings (lakes at sea level) were widespread. The depositional system was driven by tectonic subsidence in the area, uplift and erosion in the Andean hinterland and the western rim of the Pebas system (the developing Subandean zone), delta lobe switching and river belt avulsions, as well as presumable Milankovitch scale precipitation/erosion cycles and eustatic sea level variation. The base of the Marañon Formation is placed at a sequence boundary. It represents the end of the Pebas lake/wetland system, and the change to permanent fluvial conditions during the Late Miocene RST and LST

The nature of aquatic landscapes in the Miocene of western Amazonia: An integrated palaeontological and geochemical approach

The Miocene Pebas Formation from the section Santa Rosa de Pichana (Loreto, Peru) was investigated using a combination of analyses of sedimentary facies, molluscan communities and taphonomy, and stable isotopes of both entire shells and growth bands in bivalves. Three sequences, comprising a succession of transgressive, maximum flooding and regressive/prograding intervals, are documented. Molluscs are most common in the transgressive/highstand intervals and are almost absent in regressive/prograding intervals. The fauna is dominated by endemic Pebasian species, such as Pachydon and Dyris spp. The nature of the deposits as well as the availability of oxygen varied in a predictable way within each of the sequences and determined the nature of the assemblages. Highest diversity was reached in the late transgressive phase before the development of dysoxia that was widespread during the late highstand and early regressive/prograding phase. The mollusc and isotope data show no indications of elevated salinities, in contrast to ichnofossils found in the section. This discrepancy is interpreted to result either from temporal separation of the ichnofossils and the mollusc fossils or from evolution beyond usual ecological tolerances of taxa that produced these ichnofossils into freshwater settings

Совершенствование инструмента с учётом особенностей бурения различными способами

In the article the special features of the main methods of slits and boreholes drilling in the mountain rocks of different strengths with the hard–alloy and artificial diamond instrument are shown. The process of instrument wearing out during rotary drilling with strokes is examined, as well as the cases of implementing of hard–alloys elements with supporting of implementing of hard–alloys elements with supporting of their self–rotating. The advantages and disadvantages of each of above – mentioned methods of drilling are shown as well as the ways of the further improvement for each method

Growing spherulitic calcite grains in saline, hyperalkaline lakes: experimental evaluation of the effects of Mg-clays and organic acids

The origin of spherical-radial calcite bodies – spherulites – in sublacustrine, hyperalkaline and saline systems is unclear, and therefore their palaeoenvironmental significance as allochems is disputed. Here, we experimentally investigate two hypotheses concerning the origin of spherulites. The first is that spherulites precipitate from solutions super-saturated with respect to magnesium-silicate clays, such as stevensite. The second is that spherulite precipitation happens in the presence of dissolved, organic acid molecules. In both cases, experiments were performed under sterile conditions using large batches of a synthetic and cell-free solution replicating waters found in hyperalkaline, saline lakes (such as Mono Lake, California). Our experimental results show that a highly alkaline and highly saline solution supersaturated with respect to calcite (control solution) will precipitate euhedral to subhedral rhombic and trigonal bladed calcite crystals. The same solution supersaturated with respect to stevensite precipitates sheet-like stevensite crystals rather than a gel, and calcite precipitation is reduced by ~ 50% compared to the control solution, producing a mixture of patchy prismatic subhedral to euhedral, and minor needle-like, calcite crystals. Enhanced magnesium concentration in solution is the likely the cause of decreased volumes of calcite precipitation, as this raised equilibrium ion activity ratio in the solution. On the other hand, when alginic acid was present then the result was widespread development of micron-size calcium carbonate spherulite bodies. With further growth time, but falling supersaturation, these spherules fused into botryoidal-topped crusts made of micron-size fibro-radial calcite crystals. We conclude that the simplest tested mechanism to deposit significant spherical-radial calcite bodies is to begin with a strongly supersaturated solution that contains specific but environmentally-common organic acids. Furthermore, we found that this morphology is not a universal consequence of having organic acids dissolved in the solution, but rather spherulite development requires specific binding behaviour. Finally, we found that the location of calcite precipitation was altered from the air:water interface to the surface of the glassware when organic acids were present, implying that attached calcite precipitates reflect precipitation via metal–organic intermediaries, rather than direct forcing via gas exchange

The magnesium isotope record of cave carbonate archives

Here we explore the potential of magnesium (δ<sup>26</sup>Mg) isotope time-series data as continental climate proxies in speleothem calcite archives. For this purpose, a total of six Pleistocene and Holocene stalagmites from caves in Germany, Morocco and Peru and two flowstones from a cave in Austria were investigated. These caves represent the semi-arid to arid (Morocco), the warm-temperate (Germany), the equatorial-humid (Peru) and the cold-humid (Austria) climate zones. Changes in the calcite magnesium isotope signature with time are compared against carbon and oxygen isotope records from these speleothems. Similar to other proxies, the non-trivial interaction of a number of environmental, equilibrium and disequilibrium processes governs the δ<sup>26</sup>Mg fractionation in continental settings. These include the different sources of magnesium isotopes such as rainwater or snow as well as soil and host rock, soil zone biogenic activity, shifts in silicate versus carbonate weathering ratios and residence time of water in the soil and karst zone. Pleistocene stalagmites from Morocco show the lowest mean δ<sup>26</sup>Mg values (GDA: −4.26 ± 0.07‰ and HK3: −4.17 ± 0.15‰), and the data are well explained in terms of changes in aridity over time. The Pleistocene to Holocene stalagmites from Peru show the highest mean value of all stalagmites (NC-A and NC-B δ<sup>26</sup>Mg: −3.96 ± 0.04‰) but only minor variations in Mg-isotope composition, which is consistent with the rather stable equatorial climate at this site. Holocene stalagmites from Germany (AH-1 mean δ<sup>26</sup>Mg: −4.01 ± 0.07‰; BU 4 mean δ<sup>26</sup>Mg: −4.20 ± 0.10‰) suggest changes in outside air temperature was the principal driver rather than rainfall amount. The alpine Pleistocene flowstones from Austria (SPA 52: −3.00 ± 0.73‰; SPA 59: −3.70 ± 0.43‰) are affected by glacial versus interglacial climate change with outside air temperature affecting soil zone activity and weathering balance. Several δ<sup>26</sup>Mg values of the Austrian and two δ<sup>26</sup>Mg values of the German speleothems are shifted to higher values due to sampling in detrital layers (Mg-bearing clay minerals) of the speleothems. The data and their interpretation shown here highlight the potential but also the limitations of the magnesium isotope proxy applied in continental climate research. An obvious potential lies in its sensitivity for even subtle changes in soil-zone parameters, a hitherto rather poorly understood but extremely important component in cave archive research. Limitations are most obvious in the low resolution and high sample amount needed for analysis. Future research should focus on experimental and conceptual aspects including quantitative and well-calibrated leaching and precipitation experiments

Hominin homelands of East Java: revised stratigraphy and landscape reconstructions for Plio-Pleistocene Trinil

Bioarchaeolog

Neogene Amazonia: Introduction to the special issue Journal of South American Earth Sciences, New contributions on Neogene geography and depositional environments in Amazonia

The paleontological data presented in this special issue provide a new insight into species migration and radiation in Amazonia during the Miocene. At the time, Amazonia was characterized by a very extensive, long-lived, semi-isolated, freshwater wetland ecosystem that was supplied with water and sediment from the Andes and repeatedly influenced by marine conditions. As a result, the wetland had a fauna of characteristic and unusual species composition, containing marine lineages that adapted to freshwater as well as obligate freshwater species. The marine imprint is particularly well reflected in fish and ostracod faunas. Floodplain and isolated lakes in this Miocene wetland also harbored an abundant and highly diverse endemic mollusc fauna. Of these, only Anticorbula (extant in the heart of Amazonia and coastal Guyana) adapted from marine to freshwater conditions (Vermeij and Wesselingh, 2002). However, the Miocene mollusc fauna was decimated somewhere between the Neogene and Quaternary, though it is not clear why this occurred. As Lundberg et al. (1998) point out, it is likely related to the onset of the Amazon River, because the molluscs were highly specialized to a life in shallow lakes and poorly equipped for channel and floodplain lake environments. This explains why only a very small percentage of Miocene molluscs survived until the Present, specifically, those that inhabited the floodplain lake environment. The lacustrine taxa could not handle the fully fluvial conditions that resulted from a full-blown Amazon River and probably perished in the Late Miocene. Evidence from mollusc and fish migration patterns suggest that during the Early-Middle Miocene, a biogeographic connection existed between Amazonia and areas across the present Eastern Cordillera and Caribbean, as Albert et al. and Wesselingh and Mactosay, respectively, note in this issue. In contrast, the findings by Hulka et al. presented in this issue reveal Middle-Late Miocene marine deposits in Bolivia which further underline the regional extent of these incursions. Although the relation between the Bolivian marine deposits and northwest Amazonia remains uncertain, they illustrate that, whether from the north, west, or south, marine incursions reached central South America. Late Miocene sedimentary sequences in Brazil and Peru (Räsänen et al., 1995; Latrubesse et al., 1997; Campbell et al., 2001) are still debated. Rebata et al. give us insight into sediments along the Marañon River (Peru), which they interpret as marine. Ongoing work by other research groups might in the near future provide further insight into the details of the depositional environments and paleogeography during this time interval. The implications of the 'adaptation by marine lineages to freshwater' scenario for aquatic and terrestrial flora and fauna developments are far reaching, as already has been shown for fishes. A recent paper by Nores (2004) also suggests that the marine incursions may be responsible for dispersion patterns in birds. In addition, Wilkinson et al. provide new models on speciation of freshwater organisms in megafan/river systems. Together with these models, they offer new clues about how diversification may have intensified, along with megafan activation, during the Neogene Andean paroxysm. © 2005 Elsevier Ltd. All rights reserved