1,721,097 research outputs found
Integrated stratigraphy of Early Aptian black shales in the Boreal Realm: calcareous nannofossil and stable isotope evidence for global and regional processes
No full textThe Early Aptian is marked by an event of widespread anoxia in the oceans, known as Oceanic Anoxic Event (OAE)1a. During this time the Lower Saxony Basin (LSB), constituting the southern extension of the Boreal-Arctic Sea, was affected by the deposition of finely laminated black shales of the Fischschiefer (FS) considered to be the product of OAE 1a. This study focuses on Upper Barremian-Lower Aptian sediments from three different localities in northern Germany encompassing the FS. The proposed integrated litho-, bio-, and chemo-stratigraphy provides an accurate time control for correlation and for detecting the timing of the processes that affected the LSB during the deposition of the FS. The paleoecological and paleoclimatic reconstructions based on calcareous nannofossils indicate that sedimentation during the Late Barremian was mostly depending on regional conditions related to the paleogeography of the LSB. The deposition of the FS was instead mainly driven by mechanisms operating on a global scale and associated to OAE 1a: a warming event, also detected at low latitudes, was accompanied by high primary productivity and influx of cosmopolitan taxa through new seaways opened to the Tethys. In the late Early Aptian local factors prevailed again on sedimentation although paralleled by a decrease in temperature documented at different latitudes which probably favoured the migration of Boreal species southwards
Stratigraphie und kalkige Nannofossilien des Unter-Apt von Alstätte NRW
No full textA new outcrop of Lower Cretaceous marine sediments (Lower Aptian) is described from Alstätte (western part of the Münsterland). The lithostratigraphy, the biostratigraphy, the calcareous nannofossil assemblages and the stable isotope signal (δ13Corg are documented. Based on a detailed lithological log the calcareous nannofossil biostratigraphy is being discussed. The studied succession can be attributed to the Flabellites oblongus nannofossil zone of early Aptian age
Biogenic carbonate paleofluxes as proxy for pCO2 during the Aptian
No full textCoccolithopores are phytoplanktonic algae which produce a calcitic exoskeleton (coccosphere) and are extremely important primary producers playing a direct role on the equilibrium of the organic and inorganic carbon cycle.
Laboratory experiments on living coccolithophores documented decreased biocalcification and partial production of deformed/malformed coccoliths in some species as a consequence to increased surface-water acidification associated with elevated CO2 concentrations. A similar response is registered during geological intervals of super-greenhouse climate and profound environmental perturbations. Specifically, malformed coccoliths were identified during the early Aptian Oceanic Anoxic Event (OAE) 1a characterized by emission of large amount of CO2, widespread organic matter burial in oxygen-depleted oceans, paralleled by a decrease in total nannofossil carbonate paleofluxes. The OAE 1a was also accompanied by a geologically rapid warming and input of biolimiting metals in the oceans associated with multiple volcanic phases of the Ontong Java Plateau (OJP). The late Aptian was instead characterized by intervals of intense sub-aerial volcanism of the Kerguelen Plateau (KP) province and resulted to be coeval with interludes of relatively cool conditions.
The aim of this study is to reconstruct the biogenic carbonate production of calcareous nannoplankton during the Aptian and detect if and how biocalcification of coccolithophores was affected by fluctuating pCO2 prior, during and after the construction of the OJP and KP. Specifically, we present quantitative analyses of nannofossil micrite in thin sections and reconstructed nannofossil calcite paleofluxes in three drill sites: the Cismon core (Northern Italy), Piobbico core (Central Italy) and DSDP Site 463 in the mid-Pacific Mountains. The data obtained revealed a drastic reduction in nannoplankton calcification starting in the latest Barremian related to a decrease in the rock-forming nannoconids (“nannoconids decline”) that culminated with the “nannoconids crisis” just prior to OAE 1a. At the end of OAE 1a, nannofossil biocalcification increased again, but it never reached pre-anoxia values. In the late Aptian, nannofossil paleofluxes reached high values only during the Nannoconus truittii acme, followed by a final collapse across the Aptian/Albian boundary interval.
The variations in nannoplankton carbonate production are interpreted as the adaptive response to perturbed surfacewater conditions that favoured small and less calcified forms and caused false extinctions among heavily calcified nannoconids. The correlation between reduced biocalcification rates and intervals of intense volcanism, suggests that mid-Cretaceous nannoplankton biocalcification and nannofossil paleofluxes were strongly controlled by excess volcanogenic CO2. Following this observation, we propose calcite paleofluxes as proxy for reconstructing past atmospheric CO2 and provide possible scenarios of paleoCO2 concentrations delivered by OJP and KP volcanism and interplay with climate changes
Dwarf coccoliths at the onset of Cretaceous oceanic anoxic events 1a and 2 : an example of calcareous nannoplankton sensitivity to excess CO2?
No full textHigh-resolution sampling of the initial phase of both early Aptian oceanic anoxic event (OAE) 1a and latest Cenomanian OAE 2 was applied to relatively expanded sections from the western Tethys. Quantitative and morphometric investigations of calcareous nannofossil assemblages pointed out the occurrence of dwarf coccoliths. Small specimens of genera Biscutum, Zeughrabdotus, and Discorhabdus become relatively common and parallel the decrease in abundance of large nannofossils, such as the heavily calcified nannoconids and other nannoliths.
During the mid-Cretaceous the natural source of atmCO2 was Earth’s degassing, and emplacement of the Ontong Java-Manihiki and Carribean Plateau large igneous provinces (LIPs) is recognized as responsible of pCO2 as high as 2000 ppm. Coeval (and synchronous) biocalcification crises have bee documented in pelagic and neritic settings, suggesting a causal link between high concentrations of carbon dioxide in the atmosphere-ocean system and drops in benthic and planktonic calcifiers’ efficiency.
Coccolith dwarfism is here interpreted as forced by rapidly increasing pCO2 and might be the counterpart of the major decrease in nannolith abundance. Our data are consistent with works by Riebesell et al. (2000), but are apparently contradicted by recent data by Iglesias-Rodriguez et al. (2008) documenting enlarged Emiliania huxleyi coccoliths under high CO2.
We stress the fact that: (1) coccolith size alone is not a measure of calcite production (number of coccoliths must be taken into account) and of calcification rate; (2) E. huxleyi is not the best taxon for testing biocalcification: this species is super-opportunistic and might take advantage in any environment; (3)during OAEs atmCO2 concentrations was so elevated to exceed threshold values and current lab experiments might not be good analogues.
Although dwarf coccoliths might result from enhanced fertility associated to OAE1a and OAE2 regardless of ocean alkalinity, all geological data indicate a rapid, 4 to 10 times increase of atmCO2 associated with both episodes of global anoxia. We believe that the observed tiny specimens derive from difficult/reduced calcification and suspect that in order to reproduce and understand rates of Cretaceous nannoplankton mineralization, future lab experiments should be run under pCO2 as high as 1500-2000 ppm
Early Cretaceous chalks from the North Sea giving evidence for global change
No full textAmong calcareous nannofossils, important primary producers in Jurassic and Cretaceous oceans, nannoconids were carbonate rock-forming organisms. During the late Barremian and early Aptian (∼126 to 122 million years ago), nannoconids went through a crisis culminating during the Oceanic Anoxic Event 1a. Here we present nannofossil and geochemical data from a section of early Barremian-early Aptian age from the North Sea, recording the earliest chalks ever known in the Boreal Realm. These middle-late Barremian chalks were generated by blooming of endemic nannoconids under relative warm and arid conditions. A subsequent decrease of nannoconids in the latest Barremian coincides with increased nutrient and clay input. This nannoconid decline, also detected at low latitudes, was associated with the Ontong Java Plateau emplacement. We conclude that nannoconids were rock forming also at high latitudes, under clear and oligotrophic waters. Their decline was related to increased continental runoff under reinforced greenhouse conditions
Mid-Cretaceous climatic fluctuations traced by calcareous nannofossils
No full textSeveral regional to global episodes, known as Oceanic Anoxic Events (OAE)s occurred over the Aptian – early Turonian time interval and were characterized by excess CO2, intense volcanism, perturbed climate and altered oceanic chemistry. In order to either understand the dynamics leading to these “extreme” conditions and the ocean-atmosphere system recovery after the OAE, it is important to focus on a longer-term record thus to include the sequence of intervals of perturbation and the phases of stable conditions. For this purpose, calcareous nannoplankton is a useful tool, since it is extremely sensitive to changes in surface waters parameters like temperature and nutrient content and interacts with the C cycle through biological processes and production of calcareous oozes. We gathered new quantitative nannofossil data for the Tethys Ocean (Umbria Marche Basin, Italy) to derive climatic fluctuations and changes in ocean fertility during the late Albian – early Turonian. The new dataset was integrated with the nannofossil data collected for the Aptian – early Albian time interval to provide a compilation of temperature and surface water nutrient variations on the long-term through the Aptian – early Turonian interval in the western Tethys. The nannofossil Temperature Index (TI) outlines warm conditions for OAE 1a, followed by a cooling trend culminating soon after the N. truittii acme interval. Progressively increasing temperature (T) characterized instead the latest phases of the Aptian and warmer conditions were reached in the Albian – Cenomanian, although interrupted by relatively cooler phases, as for example in correspondence of the MCE. The highest paleo-T were reached across OAE 2. Surface water fertility (F) resulted instead to be relatively high during most of the early-middle Aptian, exception made for the N. truittii acme interval when a significant decrease occurred. The latest Aptian – early Albian interval was marked by intermediate trophism in surface waters, with intervals of higher F during black shales deposition. During the interval comprised between the OAE 1b and the OAE 1d, F was relatively high while it decreased in the Cenomanian. A distinct F pulse was detected prior to OAE 2. We correlated the TI with available paleo-T proxies: a good correlation exists with the ð18O records through the Aptian - upper Albian but it is not possible to provide a comparison for the lower-middle Albian since there are no ð18O data available. During the Cenomanian, the trends of the two paleo-T records are instead the opposite, with the TI shifted towards cooler T and displaying oscillations not evidenced by the ð18O. Further paleo-T reconstructions are available across the upper Aptian - lower Albian from TEX86 data from the proto-North Atlantic Ocean and show very good correspondence with the TI. We therefore calibrated the TI (which does not provide absolute T values) on the basis of TEX86 paleo-T and estimated the extent of T-variations
Nannofossil carbonate paleofluxes as proxy for pCO2 during the Aptian
No full textThe Aptian (~121 to ~113 Ma) has been characterized by super-greenhouse climate and profound environmental
perturbations, including the early Aptian Oceanic Anoxic Event (OAE 1) a, an episode of widespread organic matter
burial in oxygen-depleted oceans. The OAE 1a is thought to be related to the emplacement of the Ontong Java Plateau
which probably introduced in the atmosphere a large amount of CO2 with consequent impact on biota, climate and
ocean chemistry. The major perturbation of the carbon cycle is mirrored by the carbon isotopic record which shows a
negative shift at the beginning of OAE 1a followed by a positive excursion which persisted also after the event. The aim
of this study is to detect if and how the biogenic carbonate production of calcareous nannofossil during the Aptian was
affected by high pCO2. Calcareous nannoplankton is in fact extremely sensitive to changes in physical and chemical
conditions of the oceans including CO2 concentration. Laboratory experiments on living coccolithophores indicate that
coccolith type, abundance and degree of mineralization depend on chemical-physical-trophic conditions of water as
well as on pCO2. The geological record has revealed the presence of dwarf/malformed coccoliths during OAE 1a
interpreted to be the response of some coccolithophore species to increased surface-water acidification. Here, we intend
to focus on the Aptian interval and present quantitative analyses of nannofossil micrite in thin sections and, specifically,
we reconstructed nannofossil absolute abundances and calcite paleofluxes in three drill sites: the Cismon core (Northern
Italy), Piobbico core (Central Italy) and DSDP Site 463 in the mid-Pacific Mountains. The data obtained revealed a
drastic reduction in nannoplankton calcification starting in the latest Barremian related to a decrease in the rock-forming
nannoconids, “nannoconids decline”, that culminates with the “nannoconids crisis” just prior to OAE 1a. At the end of
OAE 1a, nannofossil biocalcification increased again, but it never reached pre-anoxia values. In the late Aptian,
nannofossil paleofluxes reached high values during the Nannoconus truittii acme, followed by a final decrease through
the Aptian/Albian boundary interval. These variations in carbonate production can be interpreted as the adaptive
response of calcareous nan noplankton to perturbed surface-water conditions that favoured small and less calcified
forms and caused false extinction among heavily calcified nannoconids (Lazarus effect). We conclude that, despite
metabolic processes, CO2 concentrations influence the ocean chemistry and the carbonate system. In particular, the
correlation between reduced biocalcification rates and intervals of intense volcanism, suggest that mid-Cretaceous
nannoplankton biocalcification and nannofossil paleofluxes were strongly controlled by excess volcanogenic CO2.
Following this observation, we propose calcite paleofluxes as proxy for reconstructing past atmospheric CO2 in the
oceans and provide possible scenarios of CO2 concentrations
The effects of paleoenvironmental changes on nannoplankton biocalcification: mid-Cretaceous size variations of Biscutum constans
No full textThe investigation of the geological record allows to decipher the response of organisms to paleoenvironmental changes occurring at a time scale larger than human observation. One open issue regards the effects of stressing factors on coccolithophore algae calcification process. These planktonic organisms are important primary producers and largely contribute to the organic and inorganic carbon cycle. In the last decades, evidence for a direct response of coccolithophores to stressing factors were provided by experiments on living forms and by studies of extreme Cretaceous events, such as the early Aptian Oceanic Anoxic Event (OAE) 1a and latest Cenomanian OAE 2. These OAEs were marked by altered carbon cycle, trace elements anomalies, rapid and intense warming and peaks in surface water fertility. The studies evidenced fluctuations in the mean size of some selected nannofossil species, among which, Biscutum constans resulted to be the most sensitive. B. constans reached the smallest mean size under the climax of the OAEs coinciding with super greenhouse climate, excess CO2, accelerated nutrient recycling and trace metals peaks. In order to better understand the role of these stressing factor/s on B. constans calcification, we decided to investigate B. constans size variations through a longer time interval (ca. 27 Myrs) spanning the Aptian to the Cenomanian. This time interval includes either periods of stability and episodes of global environmental perturbations such as OAE 1a, OAE 1b, OAE 1d, the Mid-Cenomanian Event and OAE 2. Prior to this study, it was unknown if B. constans was affected by size changes during interludes of “stable” paleoenvironmental conditions. We focused on the Umbria-Marche Basin (central Italy) by investigating the Piobbico core and the Monte Petrano section which are stratigraphically well constrained and a complete characterization of paleotemperature and paleofertility is available. By performing morphometrics analyses we intended to better understand which factor (or combination of factors) was directly altering the biocalcification process in this species. The results revealed indeed changes in the mean size of B. constans. A relatively prolonged interval of smaller specimens was detected after OAE 1a. A recovery in B. constans average size was identified in the Albian although the largest specimens were found in the middle Albian. A relative decrease in size is detected just prior to OAE 1d. The subsequent main shift coincides with OAE 2 marked by dwarf specimens. Statistical analyses were performed to detect any possible dependence from temperature or nutrient variations suggesting no direct connection between these parameters and size, whilst trace elements peaks and phases of most intense volcanism resulted to correlate with minimum B. constans size
Environmental consequences of Ontong Java Plateau and Kerguelen Plateau volcanism : Climate and ocean variability under excess CO2
No full textPaleoclimate and paleoecology of the mid Cretaceous traced by calcareous nannofossils
No full textThe Aptian – early Turonian time interval was marked by major environmental changes at regional to global scale. Specifically, it was a time of super-greenhouse conditions and the climate–ocean system experienced phases of stability perturbed by transient, sometimes prolonged, anomalies of the global carbon cycle. Several regional to global episodes occurred over this time interval: the early Aptian Oceanic Anoxic Event (OAE) 1a, the early Albian OAE 1b, the latest Albian OAE 1d, the Mid-Cenomanian Event (MCE I) and the Cenomanian – Turonian OAE 2. Decades of multidisciplinary research focused on OAEs since they constitute ideal case-histories for the understanding of our planet functioning during perturbations of the C cycle. They were, in fact, characterized by
excess CO2, intense volcanism, and altered climate and oceanic chemistry. A useful tool for reconstructing the marine ecosystem dynamics of the past, is calcareous nannoplankton, since it is extremely sensitive to changes in surface waters parameters like temperature and nutrient content and interacts with the C cycle through biological processes and production of calcareous oozes.
Here, we gathered new quantitative nannofossil data for the Tethys Ocean (Umbria Marche Basin, Italy) to derive climatic fluctuations and changes in ocean fertility during the late Albian – early Turonian. Over this time interval, the Tethys Ocean was characterized by phases of rhythmic black shale deposition controlled by orbital forcing. The Pialli Level is the Tethyan sedimentary expression of the latest Albian OAE 1d, characterized by large-scale occurrence of black shales and a 13C positive excursion recognized in several deep-marine settings. The other prominent 13C anomaly coincides with the OAE 2 represented, in Italy, by the Bonarelli Level. Between these two main C-isotopic excursions, a double-spiked minor anomaly identifies the MCE I, lithologically represented
by a shift to black shales and black chert bands alternating with whitish limestones. The new dataset has been integrated with the nannofossil data previously collected for the Aptian – early Albian
time interval to provide a compilation of temperature and surface water variations on the long-term throughout the Aptian – early Turonian interval. The nannofossil Temperature and Nutrient Indices outline warm conditions for the OAE 1a which was followed by a cooling trend culminating soon after the N. truittii acme interval. Progressively increasing temperature characterized instead the latest phases of the Aptian and warmer conditions were reached in the Albian – Cenomanian, although interrupted by relatively cooler phases, as for example in correspondence of the MCE. Highest nannofossil-derived paleotemperatures were reached across OAE 2. Surface water fertility resulted instead to be relatively high during most of the early-middle Aptian, exception made for the N. truittii acme interval when a significant decrease occurred. The latest Aptian – early Albian interval was marked by intermediate trophism in surface waters, with intervals of higher fertility during black shales deposition. During the interval comprised between the OAE 1b and the OAE 1d, fertility was relatively high while it decreased in the Cenomanian. A distinct fertility pulse was detected prior to OAE 2
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