844 resultados para diatom
Resumo:
Studies combining sedimentological and biological evidence to reconstruct Holocene climate beyond the major changes, and especially seasonality, are rare in Europe, and are nearly completely absent in Germany. The present study tries to reconstruct changes of seasonality from evidence of annual algal successions within the framework of well-established pollen zonation and 14C-AMS dates from terrestrial plants. Laminated Holocene sediments in Lake Jues (10°20.70' E, 51°39.30' N, 241 m a.s.l.), located at the SW margin of the Harz Mountains, central Germany, were studied for sediment characteristics, pollen, diatoms and coccal green algae. An age model is based on 21 calibrated AMS radiocarbon dates from terrestrial plants. The sedimentary record covers the entire Holocene period. Trophic status and circulation/stagnation patterns of the lake were inferred from algal assemblages, the subannual structure of varves and the physico-chemical properties of the sediment. During the Holocene, mixing conditions alternated between di-, oligo- and meromictic depending on length and variability of spring and fall periods, and the stability of winter and summer weather. The trophic state was controlled by nutrient input, circulation patterns and the temperature-dependent rates of organic production and mineralization. Climate shifts, mainly in phase with those recorded from other European regions, are inferred from changing limnological conditions and terrestrial vegetation. Significant changes occurred at 11,600 cal. yr. BP (Preboreal warming), between 10,600 and 10,100 cal. yr. BP (Boreal cooling), and between 8,400 and 4,550 cal. yr. BP (warm and dry interval of the Atlantic). Since 4,550 cal. yr. BP the climate became gradually cooler, wetter and more oceanic. This trend was interrupted by warmer and dryer phases between 3,440 and 2,850 cal. yr. BP and, likely, between 2,500 and 2,250 cal. yr. BP.
Latest Oligocene through early middle Miocene diatom biostratigraphy of the eastern tropical Pacific
Resumo:
Study of DSDP Sites 71, 77, and 495 has allowed the development of a refined diatom biostratigraphy for the latest Oligocene through early middle Miocene of the eastern tropical Pacific which is well correlated to the low-latitude zonations for planktonic foraminifers, coccoliths, and radiolarians. Six zones and 7 subzones are proposed, and correlation with high-latitude diatoms zonations for the North Pacific, the Norwegian Sea, and the Southern Ocean is suggested by the discovery of selected diatoms in these tropical sediments which were previously thought to be restricted to high latitudes. Six new species and one new variety of diatoms which are stratigraphically useful are proposed : Actinocyclus hajosiae, n. sp., A. radionovae, n. sp., Coscinodiscus blysmos, n. sp., C. praenodulifer, n. sp., Craspedodiscus rydei, n. sp., Thalassiosira bukryi, n. sp., and Coscinodiscus lewisianus var. robustus n. var.
Resumo:
Thick sections of Pliocene and Pleistocene biosiliceous clay and ooze were recovered by the Hydraulic Piston Corer (I-IPC) at three northwest Pacific sites (DSDP Sites 578, 579, and 580). They contain a well-preserved paleomagnetic record which made it possible to evaluate diatom events used in low and high latitudes in the transitional region of the northwest Pacific. Equatorial Pacific events are usually isochronons between the equatorial and subarctic regions. However, species which have short ranges in low latitudes tend to have diachronous first and last appearances in higher latitudes. All subarctic North Pacific datum species are present in the sediments at three sites which lie north and south across the subarctic front, but their ranges become shorter in southern regions. They do not penetrate into the equatorial region. Spatial distributions of these events are influenced by the paleo-position of the subarctic front. The migration of species from their home-area outwards, in the form of the first appearance, is related to the fluctuations of the subarctic front. The last appearance of species is a response to the change of the surface water temperature that is beyond the limit of tolerance of the species, or an unstable oceanic environment due to major change of climate.
Resumo:
There is increasing evidence that different light intensities strongly modulate the effects of ocean acidification (OA) on marine phytoplankton. The aim of the present study was to investigate interactive effects of OA and dynamic light, mimicking natural mixing regimes. The Antarctic diatom Chaetoceros debilis was grown under two pCO2 (390 and 1000 latm) and light conditions (constant and dynamic), the latter yielding the same integrated irradiance over the day. To characterize interactive effects between treatments, growth, elemental composition, primary production and photophysiology were investigated. Dynamic light reduced growth and strongly altered the effects of OA on primary production, being unaffected by elevated pCO2 under constant light, yet significantly reduced under dynamic light. Interactive effects between OA and light were also observed for Chl production and particulate organic carbon (POC) quotas. Response patterns can be explained by changes in the cellular energetic balance. While the energy transfer efficiency from photochemistry to biomass production (Phi_e,C) was not affected by OA under constant light, it was drastically reduced under dynamic light. Contrasting responses under different light conditions need to be considered when making predictions regarding a more stratified and acidified future ocean.
Resumo:
Most deep ocean carbon flux profiles show low and almost constant fluxes of particulate organic carbon (POC) in the deep ocean. However, the reason for the non-changing POC fluxes at depths is unknown. This study presents direct measurements of formation, degradation, and sinking velocity of diatom aggregates from laboratory studies performed at 15 °C and 4 °C during a three-week experiment. The average carbon-specific respiration rate during the experiment was 0.12 ± 0.03 at 15 °C, and decreased 3.5-fold when the temperature was lowered to 4 °C. No direct influence of temperature on aggregate sinking speed was observed. Using the remineralisation rate measured at 4 °C and an average particle sinking speed of 150 m d**-1, calculated carbon fluxes were similar to those collected in deep ocean sediment traps from a global data set, indicating that temperature plays a major role for deep ocean fluxes of POC.
Resumo:
Fragilariopsis kerguelensis, a dominant diatom species throughout the Antarctic Circumpolar Current, is coined to be one of the main drivers of the biological silicate pump. Here, we study the distribution of this important species and expected consequences of climate change upon it, using correlative species distribution modeling and publicly available presence-only data. As experience with SDM is scarce for marine phytoplankton, this also serves as a pilot study for this organism group. We used the maximum entropy method to calculate distribution models for the diatom F. kerguelensis based on yearly and monthly environmental data (sea surface temperature, salinity, nitrate and silicate concentrations). Observation data were harvested from GBIF and the Global Diatom Database, and for further analyses also from the Hustedt Diatom Collection (BRM). The models were projected on current yearly and seasonal environmental data to study current distribution and its seasonality. Furthermore, we projected the seasonal model on future environmental data obtained from climate models for the year 2100. Projected on current yearly averaged environmental data, all models showed similar distribution patterns for F. kerguelensis. The monthly model showed seasonality, for example, a shift of the southern distribution boundary toward the north in the winter. Projections on future scenarios resulted in a moderately to negligibly shrinking distribution area and a change in seasonality. We found a substantial bias in the publicly available observation datasets, which could be reduced by additional observation records we obtained from the Hustedt Diatom Collection. Present-day distribution patterns inferred from the models coincided well with background knowledge and previous reports about F. kerguelensis distribution, showing that maximum entropy-based distribution models are suitable to map distribution patterns for oceanic planktonic organisms. Our scenario projections indicate moderate effects of climate change upon the biogeography of F. kerguelensis.
Resumo:
Late Neogene biostratigraphy of diatoms has been investigated from two sites occupied during Ocean Drilling Program (ODP) Leg 186 off the coast of northeast Japan. A unique aspect of ODP Leg 186 was the installation of two permanent borehole geophysical observatories at the deep-sea terrace along the Japan Trench. The Neogene subsidence history of the forearc was documented from both Sites 1150 and 1151, and Quaternary to middle Miocene (16 Ma) sediments represent a nearly continuous stratigraphic sequence including numerous ash records, especially during the past 9 m.y. Diatoms are found in most samples in variable abundance and in a moderately well preserved state throughout the sequence. The assemblages are characterized consistently by age-diagnostic species of Denticulopsis and Neodenticula found in regions of high surface water productivity typical of middle to high latitudes. The Neogene North Pacific diatom zonation divides the Miocene to Quaternary sequences fundamentally well, except that the latest Miocene through early Pliocene Thalassiosira oestrupii Subzone is not applicable. Miocene and late Pliocene through Pleistocene diatom datum levels that have been proven to be of great stratigraphic utility in the North Pacific Ocean appear to be nearly isochronous within the level of resolution constrained by core catcher sample spacing. The taxonomy and stratigraphy of previously described species determined to be useful across the Miocene/Pliocene boundary have been investigated on the basis of the evolutionary changes within the Thalassiosira trifulta group. The biostratigraphically important forms belonging to the genus Thalassiosira have been illustrated with scanning electron micrographs.