Kimmeridgian Foraminiferal Faunas of Northern Eurasia: Significance for Interregional Correlations and Palaeobiogeography

Foraminifer is a very useful microorganism to perform biostratigraphical zonation of the Upper Jurassic. Foraminiferal biozones are calibrated by the ammonite standard zones and can be used for intra- and interregional correlations. Furthermore, the fossil record of foraminiferal faunas is well known among basins of the Northern Eurasia and is also used for accurate palaeoenvironmental, palaeobiogeographical, or biofacial reconstructions. It allows identifying a complex set of biotic and abiotic events which may be used to propose a more general palaeoecoloecological and palaeoceanographical reconstruction of the Subboreal, Boreal, and Arctic seas. Then, the late Kimmeridgian Northern Eurasian seas formed a network of well-connected palaeobasins during the sea-level rise and resulted in rather similar palaeoenvironmental conditions.

Spatio-temporal variability in microfossil and geochemical records of Cenomanian-Turonian oceanic anoxic event-2:a review

The Cenomanian-Turonian boundary oceanic anoxic event(OAE-2;Bonarelli event;~94 Ma)is associated with a species turnover in the marine calcareous microfossil record,widespread marine anoxia,black shale deposition and positiveδ13C excursions.This study reviews 141 CTBE sites globally,aiming to understand micropaleontological,geochemical,and sedimentological expressions of OAE-2.There is a clear palaeogeographic and palaeobathymetric heterogeneity in the development of OAE-2 marine anoxia.A majority of the documented OAE-2 sites are from deep marine environments.The calcareous nannoplankton and benthic foraminifera record a diversity decline,while planktic foraminifera shows community level shifts and no major mass extinction.The variability of total organic carbon in OAE-2 sediments across sites(<1 to>10 wt.%)and theδ13C profiles(diachronous)have been attributed to different mechanisms of anoxia development.The increased primary“productivity model”gains support from productivity proxies(e.g.,Ba,P,Cu,Ni),redox-sensitive elements(e.g.,Mn,Mo,U,V,As),and eutrophic genera(e.g.,benthic foraminifera Gabonita spp.,calcareous nannofossils Biscutum spp.and Zeugrhabdotus erectus).The enhanced organic carbon“preservation model”in stratified(semi)restricted basins gains support from sites with lower enrichment of redox-sensitive elements and oligotrophic flora and fauna in the OAE-2 records.Geochronology of the Caribbean and the High Atlantic Large Igneous Province events(CLIP and HALIP)mark them as likely triggers of the OAE-2 related global perturbation of marine biogeochemistry.Volcanic triggers may have caused climate warming,altered hydrological cycles,enhanced continental weathering,shifts in ocean circulation,and nutrient flows.Addressing knowledge gaps,further research is urged,utilizing innovative proxies,and exploring underrepresented depositional systems to comprehensively understand OAE-2 onset and biotic crisis.

Cleaning of Marine Sediment Samples for Large Diatom Stable Isotope Analysis

Diatom stable isotope analysis offers considerable potential in palaeoceanography, par-ticularly where carbonate material is scarce or absent. However, extracting pure diatom frustules free of external labile organic matter from marine sediments is an essential requirement for their applica-tions as paleoenvironmental proxies. Here, based largely on previous work, we developed a method in-cluding physical separation and chemical oxidation steps to concentrate and clean pure large diatoms from laminated diatom mat and diatomaceous clay sediment samples for their stable isotope analysis. Using the physical separation techniques consisting of the removal of carbonate and excess organic matter, sieving, differential settling, and heavy liquid floatation, pure diatoms can be successfully iso-lated from the sediment samples with opal concentration more than 10%. Subsequent time oxidation experiment shows that labile organic matter coating pure diatom valves can be effectively removed with 30% H2O2 at 65 ℃ for 2 h. Measurements of δ13C after every step of physical separation demonstrate that contaminants and lost diatoms can influence the original diatom stable isotope signal, highlighting the importance of a visual check for dominant diatom size in the initial sample and purity in the final sample. Although the protocol described here was only applied to diatom mats or diatom oozes con-taining large diatoms （Ethmodiscus rex）, we believe that this method can be adapted to common dia-toms of general marine sediment samples.