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

Abstract

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.

The end of the Great Khersonian Drying of Eurasia: Magnetostratigraphic dating of the Maeotian transgression in the Eastern Paratethys

“…Among the three studied drill cores, the magnetic polarity pattern of Hole 380B covers the Khersonian-Maeotian time interval best (Figure 11f). In this core, unit IVb has been correlated with the Pontian, while units IVc and IVd, containing a normal-reversed-normal polarity triad, are correlated with chron C3A (Van Baak et al, 2016). Unit IVe, containing dominantly reversed polarities and only two short normal polarity intervals, has been correlated to the C3Ar-C3Br.2r interval while unit V is too poorly constrained for a clear correlation.…”

“…The DSDP Site 380 is one of the most important sources of information but initially the presence of greigite (Fe 3 S 4 ) severely complicated establishing a robust polarity zonation (Giovanoli, 1979). Only recently it became clear that greigite could be a reliable carrier of magnetization (Vasiliev et al, , 2008 and a greigite-based magnetostratigraphic time frame was established for these old DSDP cores (Van Baak et al, 2016). Among the three studied drill cores, the magnetic polarity pattern of Hole 380B covers the Khersonian-Maeotian time interval best (Figure 11f).…”

“…These sections consist of upper Bessarabian-Maeotian alternating shallow sandstones and shales (Saulea et al, 1969) and in the upper part (Pontian to Romanian) of shales, siltstones, sandstones, and coals (Pană, 1966;Grasu, Catana, & Bobos, 1999;Panaiotu, Vasiliev, Panaiotu, Krijgsman, & Langereis, 2007). Previous studies indicate that the late Miocene to Pliocene sedimentary successions are rather continuous without major hiatuses and unconformities (Tărăpoancă et al, 2003;Vasiliev et al, 2004;Van Baak, Vasiliev, Palcu, Dekkers, & Krijgsman, 2016).…”

“…rugosa, Euxinocythere praebacuana, Amnicythere sp., A. crebra as well as Mediocytherideis apatoica and M. sp. This typical upper Maeotian ostracod assemblage is replaced by Pontian ostracod fauna, mainly of Pannonian type, after the Pontian flooding event of the Eastern Paratethys at 6.1 Ma (e.g., Floroiu, Stoica, Vasiliev, & Krijgsman, 2011;Krijgsman et al, 2010;Stoica et al, 2013;Van Baak et al, 2016.…”

Timing of arrival of the Danube to the Black Sea: Provenance of sediments from DSDP site 380/380A

“…assemblages (Figure 2) are indicative of great environmental turnovers, but are not unequivocally age indicative.Recent magnetostratigraphic, 40 Ar/ 39 Ar and biostratigraphic results provide new age constraints for the sediments recovered at DSDP Site 380/380A(van Baak et al, 2015b(van Baak et al, , 2016Grothe et al, 2014), although many uncertainties remain, mainly due to the two large (100s of m) MTCs in the drilled interval(Tari et al, 2015).Three important tie points can be established. (1U R E 4 Provenance-sensitive heavy mineral ratio plots for silt-grade (30-63 lm) material from the DSDP 380/380A core and some of the present-day and palaeo-rivers of the western Black Sea area.…”