Iuliana Vasiliev

Identification and environmental interpretation of diagenetic and biogenic greigite in sediments: A lesson from the Messinian Black Sea

Greigite (Fe3S4) 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 (Fe3O4) 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. Key Points: We provide evidence for the presence of biogenic greigite in ancient sediments Diagenetic greigite enhancements are climatically controlled Greigite is a paleoenvironmental indicator in anoxic environments

Black Sea desiccation during the Messinian Salinity Crisis: Fact or fiction?

The late Miocene Messinian Salinity Crisis (MSC) was an extraordinary geologic event in the Mediterranean Basin marked by massive salt accumulation and presumably basin desiccation as a consequence of the reduced water exchange with the Atlantic Ocean. The discovery of a desiccation deposit in the Black Sea, the so-called Pebbly Breccia unit, was used to claim that the Black Sea also became desiccated during the MSC. Erosional features interpreted from seismic profiles of the Black Sea margin, correlated by some to the Pebbly Breccia unit, were used to support this hypothesis. However, the age of the Pebbly Breccia is poorly constrained, and its origin and relevance to the MSC subject to controversy. Here we present new biostratigraphic (dinoflagellate cyst) data from two key sedimentary successions located in a deep and a marginal setting of the Black Sea Basin. These records demonstrate that the Pebbly Breccia predates the Mediterranean water-level drop during the MSC. We argue that the presumed erosional features in the Black Sea Basin are not related to the MSC and likely represent an older Miocene event.

Micropaleontological response to the changing paleoenvironment across the Sarmatian-Pannonian boundary in the Transylvanian Basin (Miocene, Oarba de Mureş section, Romania)

Micropaleontological response to the changing paleoenvironment across the Sarmatian-Pannonian boundary in the Transylvanian Basin (Miocene, Oarba de Mureş section, Romania) The Sarmatian-Pannonian transition has been investigated in Section A of Oarba de Mureş in the central Transylvanian Basin. Micropaleontological assemblages are diagnostic for different environmental settings and demonstrate a clear zonation, which was used to reconstruct the genetic units. Five stratigraphic sequences were described and subdivided based on the microfossil assemblages. Transgressive intervals were documented by five-chambered and biserial planktonic foraminifera, normal regressions by assemblages with abundant mysid, dasyclads, diatoms, and benthic rotaliid foraminifera, while the forced regressions are characterized by reworking. The Sarmatian-Pannonian boundary (11.3 Ma) is clearly documented by microfossils and is calibrated with radiometric and magnetostratigraphic data. A new interpretation for the interbasinal correlation is proposed by synchronizing the top of the Central Paratethyan Sarmatian with the top of the Eastern Paratethyan Bessarabian.

A Greigite-Based Magnetostratigraphic Time Frame for the Late Miocene to Recent DSDP Leg 42B Cores from the Black Sea

Throughout the Late Neogene, the Black Sea experienced large paleoenvironmental changes, switching between (anoxic) marine conditions when connected to the Mediterranean Sea and (oxic) freshwater conditions at times of isolation. We create a magnetostratigraphic time frame for three sites drilled during Deep Sea Drilling Project (DSDP) Leg 42B to the Black Sea (drilled in 1975). At the time, magnetostratigraphic dating was impossible because of the presence of the little understood iron sulfide mineral greigite (in sediments a precursor to pyrite) as magnetic carrier. Our rock-magnetic results indicate that only anoxic conditions result in poor magnetic signal, likely as a result of pyrite formation in the water column rather than in the sediment. The magnetostratigraphic results indicate that Hole 379A, drilled in the basin center, has a continuous sedimentary record dating back to 1.3 Ma. Hole 380/380A is subdivided into three consistent intervals, 0-700 mbsf, 700-860 mbsf and 860-1075 mbsf. The top unit covers the Pleistocene but the magnetostratigraphy is likely compromised by the presence of mass transport deposits. The middle unit spans between 4.3 and 6.1 Ma and records continuous deposition at ~10 cm/kyr. The lower unit lacks the independent age constraints to correlate the obtained magnetostratigraphy. Hole 381 is drilled on the Bosporus slope and as a result, hiatuses are common. A correlation to the nearby Hole 380/380A is proposed, but indicates deposits cannot straightforwardly be traced across the slope. Our improved age model does not support the original interpretation based on these cores of a desiccation of the Black Sea during the Messinian salinity crisis.

Palaeomagnetic Results from the Sarmatian/Pannonian Boundary in North-Eastern Croatia (Vranovi? Section, Našice Quarry)

The Sarmatian/Pannonian boundary in the Central Paratethys basin is marked by a major regressive event, which isolated the basin from the open sea and resulted in a palaeoenvironmental change from restricted marine to brackish water ecosystems. The exact age of this environmental change is still ambiguous since direct age control on the boundary interval is lacking, mainly due to the scarcity of suitable sections. The Vranovi? section in the Nasice Quarry in Croatia, however, is relatively long and continuously exposed. A detailed sedimentological and biostratigraphic study indicates that it contains the Sarmatian/Pannonian boundary and that it reflects the same palaeoenvironmental trend as other Paratethyan sequences. Here, we present palaeomagnetic and rock magnetic results from the Vranovi? section, based on 183 sampled levels distributed along 55 m of cyclically bedded limestones and marls. Rock magnetic data indicate the presence of maghemite or haematite in the Sarmatian deposits and low contents of magnetite in the Pannonian rocks. Thermal demagnetization results indicate dominantly normal polarities, and the mean direction closely coincides with the present-day field direction at Nasice. We conclude that magnetostratigraphic age control cannot be derived for the Vranovi? section because of a dominant secondary (post-tilt) magnetization. Consequently, a firm numerical age based on magnetostratigraphy cannot be assigned to Sarmatian/Pannonian boundary events from this section.