793 resultados para Bremen
Resumo:
The Schwalbenberg II loess-paleosol sequence (LPS) denotes a key site for Marine Isotope Stage (MIS 3) in Western Europe owing to eight succeeding cambisols, which primarily constitute the Ahrgau Subformation. Therefore, this LPS qualifies as a test candidate for the potential of temporal high-resolution geochemical data obtained X-ray fluorescence (XRF) scanning of discrete samplesproviding a fast and non-destructive tool for determining the element composition. The geochemical data is first contextualized to existing proxy data such as magnetic susceptibility (MS) and organic carbon (Corg) and then aggregated to element log ratios characteristic for weathering intensity [LOG (Ca/Sr), LOG (Rb/Sr), LOG (Ba/Sr), LOG (Rb/K)] and dust provenance [LOG (Ti/Zr), LOG (Ti/Al), LOG (Si/Al)]. Generally, an interpretation of rock magnetic particles is challenged in western Europe, where not only magnetic enhancement but also depletion plays a role. Our data indicates leaching and top-soil erosion induced MS depletion at the Schwalbenberg II LPS. Besides weathering, LOG (Ca/Sr) is susceptible for secondary calcification. Thus, also LOG (Rb/Sr) and LOG (Ba/Sr) are shown to be influenced by calcification dynamics. Consequently, LOG (Rb/K) seems to be the most suitable weathering index identifying the Sinzig Soils S1 and S2 as the most pronounced paleosols for this site. Sinzig Soil S3 is enclosed by gelic gleysols and in contrast to S1 and S2 only initially weathered pointing to colder climate conditions. Also the Remagen Soils are characterized by subtle to moderate positive excursions in the weathering indices. Comparing the Schwalbenberg II LPS with the nearby Eifel Lake Sediment Archive (ELSA) and other more distant German, Austrian and Czech LPS while discussing time and climate as limiting factors for pedogenesis, we suggest that the lithologically determined paleosols are in-situ soil formations. The provenance indices document a Zr-enrichment at the transition from the Ahrgau to the Hesbaye Subformation. This is explained by a conceptual model incorporating multiple sediment recycling and sorting effects in eolian and fluvial domains.
Resumo:
This Atlas summarises the global distribution of extant organic-walled dinoflagellate cysts in the form of 61 maps illustrated by the relative abundance of individual cyst taxa in recent marine sediments from the Atlantic Ocean and adjacent basins, the Antarctic region (South Atlantic, southwestern Pacific and southern Indian Ocean sections), the Arabian Sea and the northwestern Pacific. This synthesis is based on the integration of literature sources together with data from 835 marine surface sediments prepared on a comparable methodology and taxonomy. The relationships between distribution patterns of cyst species and the surface-water parameters (temperature, salinity, phosphate and nitrate concentrations) are documented with graphs depicting the relative abundance of species in relation to seasonal and annual values of the above mentioned parameters at the sample sites. Two ordination techniques (detrended correspondence analysis and canonical correspondence analysis) have been carried out to statistically illustrate the relationships between species distribution and sea-surface conditions. Results have been compared with previously published records and an overview of the ecological significance of each individual species is presented. Characterisations of selected environments as well as a discussion about how additional processes such as preservation and transport could have affected the present dataset are included.
Resumo:
We infer variations in paleoproductivity and eolian input at ODP Site 1082 in the Walvis Basin from stable oxygen isotope compositions of the planktonic foraminifera Globorotalia inflata, total organic carbon mass accumulation rates (TOC MAR), and X-ray fluorescence analyses of Fe content. The most pronounced paleoclimatic changes correspond to the time at about 0.9 Ma, when glacial conditions in the northern hemisphere (NH) led to the onset of pronounced 100-kyr glacial-interglacial cycles. We used Fe intensity as a proxy for eolian terrigenous input, and TOC MAR as a paleoproductivity indicator. Paleoproductivity and eolian input show generally higher-amplitude variations of glacial-interglacial cyclicity from 1.5 to 0.58 Ma, indicating pronounced variations in upwellingfavorable winds in this area. At 0.58 Ma, paleoproductivity and eolian input shifted abruptly to lower-amplitude variations with a periodicity of 100 kyr while delta18O values show a trend toward more negative isotope values for the past 0.65 Myr. Especially during glacial periods, oxygen isotope values indicate increasingly warmer sea-surface temperatures toward the end of the Pleistocene. To evaluate the relative influences of NH glaciation and southern hemisphere (SH) insolation as potential forcing mechanisms for variations of eolian input and productivity in the northern Benguela system, we filtered our proxy records at orbital frequencies. The filtered records of Fe intensity and TOC MAR indicate a strong influence of the 100-kyr and 41-kyr frequency bands, supporting our assumption that strong ice buildup in the NH is the dominant trigger for climate changes on the continent and probably in trade-wind intensity. SH insolation and low-latitude precession-related insolation changes were important for paleoproductivity variations in the northern Benguela system, modifying the nutrient supply by southern ocean intermediate waters and the zonal direction of upwelling-inducing trades by the African monsoon system, respectively.
Resumo:
In order to assess the ability of Porites corals to accurately record environmental variations, high-resolution (weekly/biweekly) coral delta18O records were obtained from four coral colonies from the northern Gulf of Aqaba, which grew at depths of 7, 19, 29, and 42 m along one transect. Adjacent to each colony, hourly temperatures, biweekly salinities, and monthly delta18O of seawater were continuously recorded over a period of 14 months (April 1999 to June 2000). Contrary to water temperature, which shows a regular and strong seasonal variation and change with depth, seawater delta18O exhibits a weak seasonality and little change with depth. Positive correlations between seawater delta18O and salinity were observed. The two parameters were related to each other by the equation delta18O Seawater (per mil, VSMOW) = 0.281 * Salinity - 9.14. The high-resolution coral delta18O records from this study show a regular pattern of seasonality and are able to capture fine details of the weekly average temperature records. They resolve more than 95% of the weekly average temperature range. On the other hand, attenuation and amplification of coral seasonal amplitudes were recorded in deep, slow-growing corals, which were not related to environmental effects (temperature and/or seawater delta18O) or sampling resolution. We propose that these result from a combined effect of subannual variations in extension rate and variable rates of spine thickening of skeletal structures within the tissue layer. However, no smoothing or distortion of the isotopic signals was observed due to calcification within the tissue layer in shallow-water, fast-growing corals. The calculations from coral delta18O calibrations against the in situ measurements show that temperature (T) is related to coral delta18O (delta c) and seawater delta18O (delta w) by the equation T (°C) = -5.38 (delta c - delta w) -1.08. Our results demonstrate that coral delta18O from the northern Gulf of Aqaba is a reliable recorder of temperature variations, and that there is a minor contribution of seawater delta18O to this proxy, which could be ignored.