Tracing the Messinian Salinity Crisis at IODP Site U1611 in the Western Alboran Basin: Data from Calcareous Nannofossil and Palynological Assemblages

One of the objectives of IODP Expedition 401 was to document the evolution of the Alborán Basin (Western Mediterranean), before, during, and after the Messinian Salinity Crisis (MSC). To address this objective, two holes ~1.3 km apart were drilled at Site U1611: Hole A (36°18.7537′N; 4°34.2717′W) and Hole B (36°19.3779′N, 4°34.7520′W) at 810.07 and 784.00 m water depths, respectively. At this site we carried out calcareous nannofossil assemblage and palynological analyses to refine the biostratigraphy (produced onboard) and investigate assemblage evolution across the MSC interval. Preliminary data from both holes show that calcareous nannofossil assemblages are not continuously present, varying from abundant to barren and preservation varies between good and poor. In both holes, assemblages are dominated by different species of Reticulofenestra, particularly small Reticulofenestra (< 3 µm, differentiated in open and closed morphology), which are considered opportunistic species that can adapt to a wide range of salinity and nutrients. A succession of Sphenolithus spp., Helicosphaera carteri, Umbilicosphaera spp. and Rhabdosphaera spp. high-abundance peaks, was also identified in both holes. These calcareous nannofossil high abundance peaks have been considered as regional “bioevents” and have been shown to be useful to approximate the MSC onset in other Mediterranean sub-basins in previous published literature. A sharp decrease in coccolith abundances above these abundance peaks in both holes, supports the interpretation of the MSC onset at these levels. The interval containing these peaks is in Hole U1611A and it is constrained between cores 38R and 34R (~992 and ~952 m CSF-A) while in Hole U1611B it is between cores 48R and 42R (~978 and ~951 m CSF-A). Concerning the palynological content, while there is not a clear change in Hole A at these intervals, the same interval in Hole B is marked by a clear increase in pollen (P: terrigenous input) vs. dinoflagellate cysts (D: marine palynomorphs) with a relative abundance (P/P+D), shifting from 81% to 98%. The bottom depth of this interval is consistent with ~16 ± 3 m depth difference of correlative sedimentary facies between the two holes around this depth. By comparison, the top depth of this interval seems more difficult to correlate, probably because of the poor recovery in U1611A (just 20% recovery in Core 35R) and a consequent relatively lower sampling resolution. Conversely, the end of the MSC has been recognized in both holes for a sharp increase in calcareous nannofossil and dinoflagellate cyst abundances (U1611A, core 18R, ~820 m CSF-A; U1611B, core 14R, ~815 m CSF-A), confirming a return of open ocean connectivity (i.e., Atlantic-Mediterranean gateway).