Presence of biogenic magnetite in ferromanganese nodules.

Transmission electron microscopy (TEM) and rock magnetic study of ferromanganese nodule sample JC120-104B collected from Clarion-Clipperton zone (CCZ) in the eastern Pacific Ocean indicate the presence of biogenic magnetite (magnetofossils). First-order reversal curves (FORCs) and decomposition of isothermal remanent magnetization (IRM) curves were used as the main tool for the characterization of magnetic properties of the bulk magnetic minerals present in the sample. TEM was performed for the direct identification of biogenic magnetic minerals (magnetofossils). The nodule sample has distinctive alternating Mn and Fe-rich layers per micro-X-ray fluorescence data. While diagenetic precipitation of Mn is known for the less oxygenated environment, the presence of biogenic magnetite is also common in the environments where the supply of oxygen is limited. Moreover, the increase in magnetic properties is consistent with the increase in Mn-content, which is related to favourable conditions for Mn precipitation as well as magnetite biomineralization in oxic-suboxic transition zone. Investigations on magnetofossil fingerprints lead to a better understanding of paleoenvironmental conditions involved in the formation and growth of deep-sea ferromanganese nodules.

Carbon cycle instability and orbital forcing during the Middle Eocene Climatic Optimum.

The Middle Eocene Climatic Optimum (MECO) is a global warming event that occurred at about 40 Ma. In comparison to the most known global warming events of the Paleogene, the MECO has some peculiar features that make its interpretation controversial. The main peculiarities of the MECO are a duration of ~500 kyr and a carbon isotope signature that varies from site to site. Here we present new carbon and oxygen stable isotopes records (δ13C and δ18O) from three foraminiferal genera dwelling at different depths throughout the water column and the sea bottom during the middle Eocene, from eastern Turkey. We document that the MECO is related to major oceanographic and climatic changes in the Neo-Tethys and also in other oceanic basins. The carbon isotope signature of the MECO is difficult to interpret because it is highly variable from site to site. We hypothesize that such δ13C signature indicates highly unstable oceanographic and carbon cycle conditions, which may have been forced by the coincidence between a 400 kyr and a 2.4 Myr orbital eccentricity minimum. Such forcing has been also suggested for the Cretaceous Oceanic Anoxic Events, which resemble the MECO event more than the Cenozoic hyperthermals.

Culture-independent characterization of novel psychrophilic magnetotactic cocci from Antarctic marine sediments.

Magnetotactic bacteria (MTB) are a heterogeneous group of ubiquitous aquatic microorganisms capable of biomineralizing nano-sized, membrane-bound, magnetic iron-rich mineral particles called magnetosomes. MTB are found in chemically-stratified aquatic sediments and/or water columns with a wide range of salinities, moderate to high temperatures, and pH varying from neutral to strongly alkaline. MTB from very cold environments have not been investigated to any great degree and here we characterize MTB from the low temperature Antarctic maritime region. Sediment samples were collected at nine sampling sites within Admiralty Bay, King George Island (62°23'S 58°27'W) from 2009 to 2013. Samples from five sites contained MTB and those from two of these sites contained large number of magnetotactic cocci that were studied using electron microscopy and molecular techniques. The magnetotactic cocci contained magnetosomes either arranged as two or four chains or as a disorganized cluster. The crystalline habit and composition of all magnetosomes analyzed with high-resolution transmission electron microscopy and energy dispersive X-ray microanalysis were consistent with elongated prismatic crystals of magnetite (Fe3 O4 ). The retrieved 16S rRNA gene sequences from magnetically-enriched magnetotactic cocci clustered into three distinct groups affiliated with the Alphaproteobacteria class of the Proteobacteria. Novel sequences of each phylogenetic cluster were confirmed using fluorescent in situ hybridization. Metagenomic data analysis of magnetically-enriched magnetotactic cocci revealed the presence of mam genes and MTB-specific hypothetical protein coding genes. Sequence homology and phylogenetic analysis indicated that predicted proteins are related to those of cultivated alphaproteobacterial MTB. The consistent and continuous low temperature of the sediment where the magnetotactic cocci are present (always below 1°C) suggests that these MTB from maritime Antarctica are psychrophiles. Moreover, similar morphotypes and 16S gene sequences were retrieved from samples collected from different sites from maritime Antarctica for several years suggesting that these new strains of MTB are indigenous members of Antarctic microbiota.