1,721,492 research outputs found
Biological and environmental effects of Late Cretaceous benthic foraminiferal stable isotope compositions
No full textStable isotope ratios of d18O and d13C measured on the calcite of foraminiferal tests are widely used as proxies to reconstruct the paleoceanographic conditions of the Cretaceous and the Cenozoic. In particular oxygen isotope ratios are one of the most important proxies to estimate paleotemperatures of marine sediments for the last 100 Ma (e.g., HUBER et al., 2002 ; LEA, 2003). Most of these applications, however, have been restricted to the Cenozoic since the availability of pristine benthic foraminiferal tests and knowledge of the ecological requirements of different species are generally limited for older time periods. Furthermore, the complexity of factors influencing the stable isotope composition of benthic foraminiferal tests (carbonate growth out of thermodynamic equilibrium, vital effects, pore water d13C gradients, and age and origin of the respective water mass ; e.g., WEFER & BERGER, 1991, MACKENSEN & BICKERT, 1999) demonstrates the necessity of studies on living specimens from ecosystems with well-known environmental parameters. For the fossil record, however, comparable information is still missing, since environmental studies on stable isotopes demand excellent preservation of foraminiferal tests. Nevertheless, stable isotope studies are commonly used to reconstruct the paleoceanographic and paleoclimatic evolution also for the Cretaceous. But until now, little information exists on the potential influence of microhabitat and vital effects on the stable isotope signal of Cretaceous benthic foraminifera. For this reason, different species of Late Cretaceous benthic foraminifera from ODP (Ocean Drilling Program) Hole 690C were investigated based on the pristine preservation of foraminiferal calcite (e.g., BARRERA & HUBER, 1990). Hole 690C was drilled during ODP Leg 113 in a water depth of 2914 m on the southwestern flank of Maud Rise in the eastern Weddell Sea, southern South Atlantic. Studied sediments are composed of calcareous chalks and oozes representing the uppermost Campanian and Maastrichtian (Shipboard Scientific Party, 1990). In total, nine samples that are decided to represent the best preserved benthic foraminiferal shells were selected for stable isotope analyses. These samples are located within a time interval with very stable environmental conditions to avoid an overprint of the original interspecific isotope signatures by strong environmental changes and isotope excursions. Sediment was disintegrated in distilled water and washed over a 63 µm mesh. Between 2 and 13 individuals of the biggest size-fraction possible were measured for stable isotopes, to make sure that the isotopic signatures of adult specimens were investigated. Stable isotopes were measured with a Finnigan MAT 251 mass spectrometer at the Leibniz-Labor (Kiel), coupled on-line to the Carbo-Kiel device I (external precision better than 0.07 ‰ and 0.04 ‰ for oxygen and carbon, respectively)
Benthic foraminifera and their role to decipher paleoenvironment during mid-Cretaceous Oceanic Anoxic Events – the “anoxic benthic foraminifera” paradox
No full textEarly Maastrichtian benthic foraminiferal assemblages from the western North Atlantic (Blake Nose) and their relation to paleoenvironmental changes
No full textBenthic foraminiferal faunas from the Lower Maastrichtian (Globotruncana falsostuarti–Gansserina gansseri Planktic Foraminiferal Zone) of DSDP Site 390 (Blake Nose, western North Atlantic) were investigated in order to characterize changes in organic matter flux and bottom-water oxygenation and their relation to paleoceanographic changes. Benthic foraminiferal faunas in the lower part of the studied succession show high abundances of Praebulimina reussi and Nuttallides truempyi and are proposed to reflect fluctuations in organic matter flux to the seafloor (meso- to eutrophic) under oxygenated bottom-water conditions. The middle interval is characterized by very low numbers of benthic foraminiferal specimens but a dominance of thick-walled species (e.g., lenticulinids, laevidentalinids), may reflect carbonate dissolution. In contrast to the lower part, benthic foraminifera from the upper part of the succession show well-oxygenated bottom waters. The most dominant species during this interval are Gavelinella beccariiformis, Reussella szajnochae, and N. truempyi. Parallel to this change in the benthic foraminiferal assemblages the number of inoceramid shells decreases, reflecting a significant increase in bottom-water oxygenation. Based on these data we speculate, that the observed changes in benthic foraminiferal assemblages towards a well-oxygenated environment within the Early Maastrichtian of DSDP Site 390 could reflect the onset of a shift from low-latitude to high-latitude deep-water sources. This speculation will predate the major reorganization of the oceanic circulation resulting in a circulation mode similar to today at the Early/Late Maastrichtian boundary by 1 Ma. <br/
Benthic foraminiferal assemblages from Demerara Rise (ODP Leg 207, western tropical Atlantic): possible evidence for a progressive opening of the Equatorial Atlantic Gateway
No full textThis paper is based on Santonian–Campanian sediments of Ocean Drilling Program Sites 1257 (2951 mbsl) and 1259 (2353 mbsl) from Demerara Rise (Leg 207, western tropical Atlantic, off Surinam). According to its position, Demerara Rise should have been influenced by the early opening of the Equatorial Atlantic Gateway and the establishment of a bottom-water connection between the central and South Atlantic Oceans during the Late Cretaceous. The investigated benthic foraminiferal faunas demonstrate strong fluctuations in bottom-water oxygenation and organic-matter flux to the sea-floor. The Santonian–earliest Campanian interval is characterised by laminated black shales without benthic foraminifera in the lowermost part, followed by an increasing number of benthic foraminifera. These are indicative of anoxic to dysoxic bottom waters, high organic-matter fluxes and a position within the oxygen minimum zone. At the shallower Site 1259, benthic foraminifera occurred earlier (Santonian) than at the deeper Site 1257 (Early Campanian). This suggests that the shallower site was characterised by fluctuations in the oxygen minimum zone and that a re-oxygenation of the sea-floor started considerably earlier at shallower water-depths. We speculate that this re-oxygenation was related to the ongoing opening of the Equatorial Atlantic Gateway. A condensed glauconitic chalk interval of Early Campanian age (Nannofossil Zone CC18 of Sissingh) overlies the laminated shales at both sites. This interval contains benthic foraminiferal faunas reflecting increasing bottom-water oxygenation and reduced organic-matter flux. This glauconitic chalk is strongly condensed and contains most of the Lower and mid-Campanian. Benthic foraminiferal species indicative of well-oxygenated and more oligotrophic environments characterise the overlying mid- to Upper Campanian nannofossil chalk. During deposition of the nannofossil chalk, a permanent deep-water connection between the central and South Atlantic Oceans is proposed, leading to ventilated and well-oxygenated bottom waters. If this speculation is true, the establishment of a permanent deep-water connection between the central and South Atlantic Oceans terminated Oceanic Anoxic Event 3 “black shale” formation in the central and South Atlantic marginal basins during the Early Campanian (Nannofossil Zone CC18) and led to well-oxygenated bottom waters in the entire Atlantic Ocean during the Late Campanian (at least from Nannofossil Zone CC22 onwards). <br/
Stable isotopic indication for the cyst formation depth of Campanian/Maastrichtian calcareous dinoflagellates
No full textSuitability of stable oxygen and carbon isotopes of calcareous dinoflagellate cysts for paleoceanographic studies: evidence from the Campanian/Maastrichtian cooling phase
No full textorder to determine the possible usefulness of stable isotope measurements on calcareous dinoflagellate cysts for paleoceanographic and paleoclimatic studies, we have produced oxygen and carbon isotope records of the species Pirumella krasheninnikovii (Bolli, H.M., 1974. Jurassic and Cretaceous Calcisphaerulidae from DSDP Leg 27, eastern Indian Ocean. Initial Reports of the Deep Sea Drilling Project 27, 843–907.) for the latest Campanian to earliest Maastrichtian (~73–68 Ma) of high-latitude Ocean Drilling Program Hole 690C in the southern South Atlantic (eastern Weddell Sea). Foraminiferal stable isotope values characterize this time interval as a phase of continuous cooling, superimposed by a short-term, strong cooling event between 71 and 70 Ma that was detected at various sites in the mid to high latitudes. This event is interpreted to reflect short-term surface-water cooling, leading to changes in the mode and direction of deep-water formation and possibly the growth of continental ice sheets. Our δ18O values of calcareous cysts verify this event as well as the general cooling of surface waters during the latest Campanian and early Maastrichtian. Prior to this cooling event, between ~72 and 71 Ma, a negative excursion in accompanying δ13C records of planktic and benthic foraminifera was observed. This negative excursion and the subsequent positive one were proposed to reflect sea-level fluctuations, changes in productivity, and/or changes in the ratio of organic to inorganic carbon input to the ocean. The δ13C values of calcareous cysts show the positive excursion between 71 and 70 Ma, but don't show the expected negative excursion before. In addition, they yield extremely light carbon isotope values that probably resulted from photosynthetic processes.As the stable oxygen isotope data of the calcareous dinoflagellate species P. krasheninnikovii show isotopic shifts comparable to planktic foraminifera from the same locality and from various other sites in the Pacific and South Atlantic oceans, we suggest that calcareous cyst oxygen isotopes can provide information for paleoceanographic reconstructions of sea-surface water temperatures and paleoceanographic changes. However, the application of carbon isotope data from cysts of P. krasheninnikovii appears to be questionable, and investigations of different species are needed to further evaluate the reconstruction potential based on stable isotopes and to assess species-specific vital effects
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Density split statistics: Joint model of counts and lensing in cells
No full textWe present density split statistics, a framework that studies lensing and counts-in-cells as a function of foreground galaxy density, thereby providing a large-scale measurement of both 2-point and 3-point statistics. Our method extends our earlier work on trough lensing and is summarized as follows: given a foreground (low redshift) population of galaxies, we divide the sky into subareas of equal size but distinct galaxy density. We then measure lensing around uniformly spaced points separately in each of these subareas, as well as counts-in-cells statistics (CiC). The lensing signals trace the matter density contrast around regions of fixed galaxy density. Through the CiC measurements this can be related to the density profile around regions of fixed matter density. Together, these measurements constitute a powerful probe of cosmology, the skewness of the density field and the connection of galaxies and matter. In this paper we show how to model both the density split lensing signal and CiC from basic ingredients: a non-linear power spectrum, clustering hierarchy coefficients from perturbation theory and a parametric model for galaxy bias and shot-noise. Using N-body simulations, we demonstrate that this model is sufficiently accurate for a cosmological analysis on year 1 data from the Dark Energy Survey
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