Abstract: We determined interstellar cosmic ray exposure ages of 40 large presolar silicon carbide grains extracted from the Murchison CM2 meteorite. Our ages, based on cosmogenic Ne-21, range from 3.9 ± 1.6 Ma ∼ ∼ to 3 ± 2 Ga before the start of the Solar System 4.6 Ga ago. A majority of the grains have interstellar lifetimes of <300 Ma, which is shorter than theoretical estimates for large grains. These grains condensed in outflows of asymptotic giant branch stars <4.9 Ga ago that possibly formed during an episode of enhanced star ∼ formation 7 Ga ago. A minority of the grains have ages >1 Ga. Longer lifetimes are expected for large grains. We determined that at least 12 of the analyzed grains were parts of aggregates in the interstellar medium: The large difference in nuclear recoil loss of cosmic ray spallation products 3He and 21Ne enabled us to estimate that the irradiated objects in the interstellar medium were up to 30 times larger than the analyzed grains. Furthermore, we estimate that the majority of the grains acquired the bulk of their cosmogenic nuclides in the interstellar medium and not by exposure to an enhanced particle flux of the early active sun.

Herber, J., Klotz, F., Frommeyer, B., Weis, S., Straile, D., Kolar, A., Sikorski, J., Egert, M., Dannenmann, M., Thaumarchaeon Pester, M., 2020. A single drives nitrification in deep oligotrophic Lake Constance. Environmental Microbiology 22, 212-228.

https://doi.org/10.1111/1462-2920.14840 Ammonia released during organic matter mineralization is converted during nitrification to nitrate. We followed spatiotemporal dynamics of the nitrifying microbial community in deep oligotrophic Lake Constance. Depth- dependent decrease of total ammonium (0.01 -0.84 μM) indicated the hypolimnion as the major place of 15 nitrification with N-isotope dilution measurements indicating a threefold daily turnover of hypolimnetic total Thaumarchaeota ammonium. This was mirrored by a strong increase of ammonia-oxidizing towards the hypolimnion (13% - 21% of bacterioplankton) throughout spring to autumn as revealed by amplicon sequencing and quantitative polymerase chain reaction. Ammonia-oxidizing bacteria were typically two orders of magnitude less abundant and completely ammonia-oxidizing (comammox) bacteria were not detected. Both, amoA 16S rRNA gene and (encoding ammonia monooxygenase subunit B) analyses identified only one major Thaumarchaeota species-level operational taxonomic unit (OTU) of (99% of all ammonia oxidizers in the Nitrosopumilus hypolimnion), which was affiliated to spp. The relative abundance distribution of the single Thaumarchaeon Chloroflexi Nitrospira strongly correlated to an equally abundant clade CL500-11 OTU and a OTU that was one order of magnitude less abundant. The latter dominated among recognized nitrite oxidizers. This extremely low diversity of nitrifiers shows how vulnerable the ecosystem process of nitrification may be in Lake Constance as Central Europe's third largest lake.

Hiatt, E.E., Pufahl, P.K., Guimarães da Silva, L., 2020. Iron and phosphorus biochemical systems and the Cryogenian-Ediacaran transition, Jacadigo basin, Brazil: Implications for the Neoproterozoic oxygenation event. Precambrian Research 337, 105533.

http://www.sciencedirect.com/science/article/pii/S0301926819304000 Termination of the Marinoan Snowball Earth ice age (635 Ma) marked the transition to a greenhouse world. This climate change forever modified the biogeochemical cycling of iron and phosphorus, ended large-scale iron formation deposition, began accumulation of phosphorus on marine shelves, and led to the Ediacaran radiation of eukaryotes. The Jacadigo Basin, Brazil, contains a nearly complete record of this critical transition. Glaciomarine diamictites and iron formation accumulated during two Marinoan ice advances with hydrothermal input of iron delivered via ice-margin upwelling. Biochemically precipitated rhythmites of siderite, sedimentary apatite, and hematite represent microbially mediated, sub-sea ice precipitation. Siderite laminae preserve 13 microbial textures and have a mean ∂ C = -8.80‰, PDB (+/-0.86‰) reflecting degradation of organic matter at the seafloor. These millimeter-scale rhythmites are a sensitive record of sub-ice dynamics because they formed in response to short-term fluctuations of O due to seasonal sub-ice photosynthesis. They demonstrate 2 the connection between ice cover, O , and cycling of iron and nutrient elements such as phosphorus. 2 These biochemical rhythmites suggest that Cryogenian sea ice limited oxygen production prior to the onset of the Neoproterozoic Oxygenation Event. O increased enough to concentrate bioavailable phosphorus at the 2