No abstract

Greigite (Fe 3 S 4 ) is a widespread authigenic magnetic mineral in anoxic sediments and is also commonly biosynthesized by magnetotactic bacteria in aqueous environments. While the presence of fossilized bacterial magnetite (Fe 3 O 4 ) has now been widely demonstrated, the preservation of greigite magnetofossils in the geological record is only poorly constrained. Here we investigate Mio-Pliocene sediments of the former Black Sea to test whether we can detect greigite magnetofossils and to unravel potential environmental controls on greigite formation. Our magnetic analyses and transmission electron microscope (TEM) observations indicate the presence of both diagenetic and bacterial greigite, and suggest a potentially widespread preservation of greigite magnetofossils in ancient sediments, which has important implications for assessing the reliability of paleomagnetic records carried by greigite. TEM-based chemical and structural analyses also indicate the common presence of nickel-substituted diagenetic iron sulfide crystals with a ferrimagnetic greigite structure. In addition, our cyclostratigraphic framework allows correlation of magnetic properties of Messinian Black Sea sediments (Taman Peninsula, Russia) to global climate records. Diagenetic greigite enhancements appear to be climatically controlled, with greigite mainly occurring in warm/wet periods. Diagenetic greigite formation can be explained by variations in terrigenous inputs and dissolved pore water sulfate concentrations in different sedimentary environments. Our analysis demonstrates the usefulness of greigite for studying long-term climate variability in anoxic environments.

The controversial Younger Dryas impact hypothesis suggests that at the onset of the Younger Dryas an extraterrestrial impact over North America caused a global catastrophe. The main evidence for this impact-after the other markers proved to be neither reproducible nor consistent with an impact-is the alleged occurrence of several nanodiamond polymorphs, including the proposed presence of lonsdaleite, a shock polymorph of diamond. We examined the Usselo soil horizon at Geldrop-Aalsterhut (The Netherlands), which formed during the Allerød/Early Younger Dryas and would have captured such impact material. Our accelerator mass spectrometry radiocarbon dates of 14 individual charcoal particles are internally consistent and show that wildfires occurred well after the proposed impact. In addition we present evidence for the occurrence of cubic diamond in glass-like carbon. No lonsdaleite was found. The relation of the cubic nanodiamonds to glass-like carbon, which is produced during wildfires, suggests that these nanodiamonds might have formed after, rather than at the onset of, the Younger Dryas. Our analysis thus provides no support for the Younger Dryas impact hypothesis.radiocarbon dating | carbon spherules | wildfire temperature | electron microscopy

[1] Two gravity cores retrieved off NW Africa at the border of arid and subtropical environments (GeoB 13602-1 and GeoB 13601-4) were analyzed to extract records of Late Quaternary climate change and sediment export. We apply end-member (EM) unmixing to 350 acquisition curves of isothermal remanent magnetization (IRM). Our approach enables to discriminate rock magnetic signatures of aeolian and fluvial material, to determine biomineralization and reductive diagenesis. Based on the occurrence of pedogenically formed magnetic minerals in the fluvial and aeolian EMs, we can infer that goethite formed in favor to hematite in more humid climate zones. The diagenetic EM dominates in the lower parts of the cores and within a thin near-surface layer probably representing the modern Fe 2+ /Fe 3+ redox boundary. Up to 60% of the IRM signal is allocated to a biogenic EM underlining the importance of bacterial magnetite even in siliciclastic sediments. Magnetosomes are found well preserved over most of the record, indicating suboxic conditions. Temporal variations of the aeolian and fluvial EMs appear to faithfully reproduce and support trends of dry and humid conditions on the continent. The proportion of aeolian to fluvial material was dramatically higher during Heinrich Stadials, especially during Heinrich Stadial 1. Dust export from the Arabian-Asian corridor appears to vary contemporaneous to increased dust fluxes at the continental margin of NW Africa emphasizing that meltwater discharge in the North Atlantic had an enormous impact on atmospheric dynamics.