Greigite-based magnetostratigraphic framework for the Late Miocene to recent DSDP Leg 42B cores from the Black Sea: A new time frame for old cores

Abstract:

In 1975, DSDP Leg 42B to the Black Sea, three sites were drilled with a total of 2318 m cored and a recovery of 55%. While to modern standards this may not be very impressive, these sites still represent the best record of sedimentation in the basinal part of the Black Sea. The main stratigraphic objectives of DSDP Leg 42B were to 1) obtain a complete Pleistocene litho- and biostratigraphic section and 2) study interactions between the Black Sea and Mediterranean Sea, focusing on glacio-eustatic sea level change, periods of lacustrine sedimentation, periods of stagnation, and to establish a paleoclimatic record. Major problems establishing a timescale emerged after drilling due to a) the general shortage of definitive paleontological age markers and b) the general lack of agreement on correlation and time zonation of sedimentary units.

Magnetostratigraphic dating could have solved these timescale problems but was hindered by the presence of the little understood authigenic iron sulphide mineral greigite (Fe₃S₄) as main magnetic carrier. In recent years, the understanding of greigite has significantly improved and is considered a reliable magnetic carrier. Especially in the circum-Black Sea region, many Miocene to recent, land-based sections are magnetostratigraphically dated with greigite as magnetic carrier. We therefore resample the cores of DSDP Leg 42B to see whether after 40 years of storage any of the original signal is preserved.

Our results show these cores are surprisingly useful for magnetostratigraphic dating. We create an integrated bio-magnetostratigraphic framework for the sites of Leg 42B, focusing on the Latest Miocene and Plio-Pleistocene. Ar/Ar dating of an ash-layer at site 380A gives additional age constraints. Our age model gives important new insights into the response of the Black Sea to major paleoenvironmental and climatic changes like e.g. the Messinian salinity crisis, the mid-Pliocene warm period and Pleistocene glaciations on the Northern Hemisphere.

A future IODP expedition to the Black Sea is long overdue and important for paleoclimatic and paleoenvironmental processes in central and eastern Europe and also for an array of geochemical studies. Given the chronological constraints are no longer problematic, a repeat of the confusion after the 1970’s expedition seems unlikely.