The intriguing hypothesis of a modern “Rosengarten” in the subsurface of the deep Ionian Basin

From 2016 to 2020, Judy McKenzie joined the community of COST Action CA15103 - Uncovering the Mediterranean salt giant (MEDSALT) to verify her hypothesis that an ongoing dolomitization front exists in the pelagic sediments overlying Messinian evaporites below the Messina Abyssal Plain, in ~4000 water depth in the Ionian Sea, central Mediterranean. Legacy scientific ocean drilling data from DSDP Leg 42A, Site 374 reveal a 25 m-thick latest Miocene dolomitic mudstone capped by 8.5 m of earliest Pliocene dolomite above gypsum/dolomitic mudstone cycles and anhydrite and salts. The Pliocene dolomite is made of dolomicrite with an unusual crystal morphology, suggesting diagenetic replacement of the original pelagic calcite ooze. The underlying latest Messinian dolomitic mudstone with minor gypsum layers contains Ca-rich dolomite with white spherules of lüneburgite. DSDP Site 374 shipboard interstitial water geochemical profiles further indicate that saline brine is diffusing upwards into the early Pliocene dolomicrite. A significant decrease in sulfate concentration suggests ongoing bacterial sulfate reduction, whereas the chloride profile remains constant.

Following discussion and brainstorming with Judy, a geophysical site investigation cruise on the RV Meteor was organized by the University of Hamburg. Cruise M-144 was conducted in 2018 with a multi-channel reflection seismic survey centered at DSDP Leg 42A, Site 374 using a 6 kjoule sparker source and a digital 144-channel streamer with an active length of 600 m. The objectives of the cruise were to:

• Obtain a detailed seismic stratigraphy in the surrounding of DSDP Leg 42A, Site 374, which was targeted as re-occupation Site within IODP Proposal 857C - The demise of a salt giant: climatic-environmental transition during the terminal Messinian Salinity Crisis (Claudia Bertoni and co-workers);
• Estimate the lateral dimensions of the combined dolomite/evaporite lithologic units in the lonian Sea.

Objective 1 was achieved, and the drilling proposal supported by the R/V Meteor Site Survey was forwarded by the Science Evaluation Panel to the JR Facility Board for scheduling. Unfortunately scheduling could not happen before the end of IODP.

Objective 2 was based on the assumption that the acoustic impedance contrasts induced by the dolomitization front could be detected in relatively high-resolution seismic reflection data. In the M-144 data, the uppermost Messinian dolomite- and gypsum-bearing sediments are characterized by a package of strong and positive reflection amplitudes (High Amplitude Reflection Package, HARP). The lateral continuity of the reflections is very low and the upper boundary is quite irregular. Based on the seismic data, the areal extent of the dolomite deposit beneath the lonian abyssal plain can be estimated in a few tens of thousands km². This would be the Rosengarten of the Ionian Sea that Judy was looking for.

This abstract will present previous and new seismic data, collected with Meteor Cruise M-199 in February-March 2024 with similar acquisition parameters to those of M-144, to further address objective 2. However, crucial sedimentological and geochemical data to validate Judy’s fascinating hypothesis can only be derived from new scientific drilling.