We investigate changes in calcareous nannofossil and ostracod communities, which reflect surface and bottom water conditions, respectively, across the Cretaceous-Paleogene transition (K-Pg) at the ...Cerro Azul Section, Jagüel Formation, Neuquén Basin, Argentina. The K-Pg transition at the Cerro Azul Section is characterized by calcareous nannofossil and ostracod turnovers, from assemblages dominated by Cretaceous forms to assemblages composed by incoming Danian taxa and several survivor species. These assemblage changes were associated with a collapse in carbonate production at the K-Pg boundary, probably related to a drop of surface water productivity, and a subsequent recovery in the early Danian, as suggested by carbonate content, log(Ba/Fe) and log(Ba/Ti) trends. During the Late Maastrichtian, peak relative abundances of Eiffellithus spp., just before the K-Pg transition, were probably related to enhanced surface water productivity. High abundances of Cervisiella operculata suggest decreased surface productivity during the earliest Danian. This surface productivity collapse may have reduced food availability in the benthic habitat, which probably led to the stablishment of Citherudiae-dominated ostracod assemblages. Upward in the section, increased relative abundances of Braarudosphaera bigelowii, are probably related to overall intensified weathering conditions and, consequently, increased continental runoff and fresh water input.
•Report of a continuous K-Pg record of the Atlantic incursion into the Neuquén Basin;•Comparison of calcareous nannofossils, ostracod, geochemical and magnetic data;•Eiffellithus spp. increased in relative abundances when surface productivity was high;•Cervisiella operculata and Cytheruridae abundances increase when productivity drops;•Braarudosphaera bigelowii abundances correlate with enhanced weathering intensity.
The tectonic evolution of the Equatorial Atlantic Gateway (EAG) throughout the early Cretaceous had global implications. This break-up of western Gondwana has played a significant role in causing, or ...amplifying, environmental changes due to its influence on ocean chemistry, nutrient distribution, water mass circulation, and bottom waters ventilation. However, the timing of its establishment and the areal extent of this oceanic gateway is still poorly constrained. Marine settings were recorded by the black shales of the Romualdo Formation (Late Aptian), Araripe Basin - Northeastern Brazil, which are known worldwide for their fossil content and importance for paleogeographic reconstructions of Gondwana. In this contribution, we present multiproxy analyses, combining macro- and micropaleontological, as well as sediment compositional data to evaluate paleoenvironmental conditions during the deposition of sedimentary strata assigned to the Romualdo Formation. We investigate the ~100-m-thick succession of the Sobradinho Section, which is the most complete exposure of the Romualdo Formation. Increased abundances of ostracods, the occurrence of agglutinated benthic foraminifera, bakevelliid bivalves, and cassiopid gastropods coincide with levels of relatively high paleosalinity estimates based on the Sr/Ba (strontium/barium) and S/TOC (sulfur/total organic carbon) ratios. These levels correspond to a transgressive system tract and the lower interval of a highstand systems tract. Throughout the section, black shales occur intercalated with sparse sandstone and calcarenite levels, and the deposition of these black shales is associated with dysoxic to anoxic bottom water conditions, as suggested by the V/Cr (vanadium/chrome) ratio. The occurrence of stagnant bottom water conditions may explain the mass mortality of ostracods in the lower interval of the succession. The upper part of the highstand system tract in the studied section was deposited in coastal environments and is characterized by coarser-grained siliciclastic-dominated facies (high log(Zr/Rb) (zirconium/rubidium) ratio), as well as increased phytoclasts and terrigenous (high magnetic susceptibility) contents.
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•Environmental changes recorded in the Aptian succession of the Araripe Basin.•Changes in paleosalinity are identified based on distinct proxies.•Marine influence was stronger in three distinct stratigraphic intervals.•Bottom water conditions varied from dysoxic to anoxic.•Ostracod abundance peaked when salinity increased.
Paleoceanographic reconstructions are usually based on the chemical composition of foraminiferal tests, and pristine calcite preservation is crucial to support reliable interpretations. Therefore, ...prior knowledge about the structure and chemical composition of microfossils calcite is important to assess whether geochemical results reflect environmental and/or post-burial signals. Here, we present the first microstructural study of early Danian benthic foraminiferal calcite, and evaluate environmental signals and post depositional biases in benthic and planktonic foraminiferal and bulk carbon (δ13C) and oxygen (δ18O) isotope records at Ocean Drilling Program (ODP) Site 1262 (South Atlantic Ocean). We focused our study on the first ∼350 kyr that followed the Cretaceous-Paleogene (K-Pg) boundary, encompassing the Dan-C2 carbon cycle perturbation. Benthic and planktonic δ13C records show the expected surface-to-bottom gradient, suggesting the preservation of original environmental signals. However, benthic and planktonic δ18O records show strongly overlapping and scattered values, suggesting diagenetic alteration of isotopic signals. We identified the occurrence of authigenic carbonates, such as girvasite and siderite, in benthic foraminiferal tests, which increased Mg, Mn, Ba and Fe contents of tests calcite. Our results support that thermodynamic changes occurred in tandem with the decrease of bottom and pore water oxygenation to favor precipitation of authigenic carbonate phases, which altered isotopic fractionation of δ18O. Nevertheless, bulk sediments clearly depict the negative δ13C and δ18O excursions characteristic of the DanC2, with remarkably low values, characterizing the record of a surface ocean environmental signal. This low δ13C and δ18O were mostly recorded by highly-abundant calcispheres (calcareous dinocysts), suggesting that this group of microfossils has a high potential for surface ocean paleoceanographic reconstructions.
•Authigenic carbonate phases were identified in Danian benthic foraminifera.•Crystallization of these phases affected environmental δ18O signals.•After the Dan-C2 event, foraminiferal calcite preservation improved.•Bulk δ18O records, dominated by micrite/dinocysts, recorded environmental signals.
One of the major environmental and biotic turnovers of the Phanerozoic occurred at the Cretaceous-Paleogene (K-Pg) boundary, being the focus of countless scientific studies that addressed the timing, ...causes and consequences of this global event. Paleoenvironmental conditions that preceded the K-Pg boundary, however, have been less studied, especially at low latitudes. In order to help reconstructing late Maastrichtian marine environments along the equatorial South American margin, we apply micropaleontological and geochemical techniques to a stratigraphic core that recovered the Colón Formation in northern Colombia. Low abundance foraminiferal assemblages were overall dominated by benthic taxa (Maastrichtian local biozones Siphogenerinoides bramletti and Ammobaculites colombiana), and the planktonic species Guembelitria cretacea. Ostracod assemblages were mainly composed of the genera Veenia, Cytherella, Paracypris, and Cythereis. Calcareous nannofossils were typical of Upper Cretaceous low-latitude successions, with low abundance and diversity assemblages dominated by Micula staurophora, Kamptnerius magnificus and Cervisiella operculata. Sediment elemental ratios (Zr/Rb, Fe/Ca and Sr/Ba), as well foraminiferal, ostracod and nannofossils assemblages, indicate a shallowing upward trend, characterized by a transition from inner platform settings to sublittoral conditions. Input of terrigenous sediments and weathering intensity also increased upward in the section, probably related to the uplift pulses of the Andean orogeny. Proportions of benthic infauna, as well the V/Cr ratio and the distribution of redox-sensitive trace metals (Ni, Cu) along the core, mostly indicate suboxic bottom water conditions in the Colón Formation.
•The upper part of Colon Formation comprises Maastrichtian foraminiferal assemblages (Regional zones Siphogenerinoides bramletti and Ammobaculites colombiana).•The upper part of Colón Formation recorded a transition from inner platform settings to sublittoral conditions.•First report of ostracod and calcareous nannofossil assemblages for the Maastrichtian of northern South America.•Possible distal record of Deccan volcanic activity.
Early Danian environmental perturbations related to carbon isotope negative excursions (CIEs) were recognized at Deep Sea Drilling Project (DSDP) Hole 516F, Rio Grande Rise. Within the first ~160 kyr ...of the Danian, environmental stability is evidenced by well‑oxygenated bottom waters, good carbonate preservation, and relatively high primary production. The preservation of carbonates and the oxygenation of the deep water decreased progressively, leading to the formation of red layers in the sediments between 65.86 Ma and 65.71 Ma. The onset of carbonate dissolution and the reduction in dissolved oxygen concentrations occurred approximately 50 kyr before the onset of the Dan-C2 event, and was probably related to the third volcanic event of the Deccan Traps (DT). Our results support the link between enhanced emissions of volcanic gasses and deep ocean carbonate dissolution and deoxygenation during the Early Danian. The CIEs at the Rio Grande Rise are positioned within the planktonic foraminiferal biozone P1b, suggesting that the first appearance of Subbotina triloculinoides within Chron C29r (at 65.89 Ma) is a diachronous datum.
•Drops in carbonate content and deep water oxygenation precede in ~50 kyr the CIEs (Dan-C2 interval).•Dan-C2 event is likely correlated to the third event of the Deccan Trap (Albenali Formation).•Subbotina triloculinoides is probably a diachronous biostratigraphic marker.
The Cretaceous/Paleogene boundary (K/Pg) event, associated with a meteorite impact at Chicxulub, Mexico, is globally recognized as one of the largest mass extinctions in natural history, marking the ...end of the Mesozoic Era. However, most of the outcrops with records and geochemical evidence of this boundary are distributed in the Northern Hemisphere and at mid-latitudes. Here, aiming to contribute to the knowledge of this event at high southern latitudes, we characterize a single iridium anomaly correlated with the K/Pg boundary, present within a carbonaceous mudstone level in a continental depositional environment in the Río de las Chinas Valley, Chilean Patagonia. High-resolution geochemical and palynofacies analyses were performed on a stratigraphic section from the top of the Dorotea Formation. Results showed that the iridium enrichment coincides with an anomaly of other platinum group elements. In addition, the palynofacies analysis showed a disturbance in the depositional environment, marked by an abrupt change from non-degraded phytoclasts (e.g., cuticles) to pseudoamorphous and degraded cuticles, and by an increase in the abundance of spores at the same level. The Río de las Chinas Valley locality provides new evidence for the comprehensive study of the end-Cretaceous event, from the poorly represented continental environments of high southern latitudes.
•The site with the southernmost K/Pg boundary in the Americas is analyzed.•Ir and other platinoid elements anomalies confirm the K/Pg boundary at high latitudes.•A spore spike and palynological alteration are coeval with the Ir-enrichment level.•The abrupt palynological change shows a disturbance of the depositional environment.
The Cerro Azul Section provides a continuous record of the Cretaceous-Paleogene (K-Pg) transition in a shallow marine context of the South Atlantic Ocean in the Neuquén Basin. Ostracod assemblages ...were severely affected by environmental changes across the event. Excellent ostracod preservation at the Cerro Azul Section allows to infer paleoecological preferences of four Danian species based on carbon and oxygen stable isotopes. The studied species were Paracypris bertelsae Ceolin and Whatley, 2015, Cytherella spp., Togoina argentinensis and Henryhowella (Wichmannella) meridionalis (Bertels, 1974). To assess the reliability of ostracod δ13C and δ18O values as paleoenvironmental proxies, we pre-characterized valves with micro x-ray microdiffraction (μXRD) and microRaman (μRaman spectroscopy). Togoina argentinensis and Henryhowella (Wichmannella) meridionalis present in their major calcitic compositions, with small differences of crystallinity within intervals of environmental stress, but no authigenic phases formed during diagenesis. δ13C and δ18O values depict clear interspecific differences between smooth specimens and ornamented specimens. These differences in stable isotope values were likely controlled by microhabitat preferences, which would be comparable with patterns described for benthic foraminifera as well as paleonutrients variations, suing Ba/Ti and P/Ti ratios.
•Exceptional preservation of Danian ostracods to evaluate paleoproductivity.•Valves thickness changed according to paleosalinity.•Kerogen was found in Henryhowella valve.•P/Ti and Ba/Ti associated to δ13C.
The usual methods applied to recover calcareous nannofossils from claystones are extremely simple, cost-effective and fast. Some samples, however, exhibit heterogeneous mixtures of biogenic ...materials, detrital particles and other aggregates that visually mask the coccoliths under the microscope. For instance, a high content of organic matter in association with calcareous nannofossil specimens was observed in outcrop samples of the Vaca Muerta Formation (Neuquén Basin, Argentina). To solve this problem, traditional preparation techniques are herein improved to aid better visualization of calcareous nannofossils. Sixteen tests were qualitatively and quantitatively evaluated, testing the efficiency of some chemicals and physicals methods such as the addition of a solvent (oxidizing agent) in the smear slide technique, as well as the use of ultrasound in the random decantation technique.
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Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NMLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
