Holocene sedimentation in the southwestern Black Sea: Interplay between riverine supply, coastal eddies of the Rim Current, surface and internal waves, and saline underflow through the Strait of Bosphorus

Abstract

An extensive grid of high-resolution Huntec deep-tow seismic reflection profiles (~9150 line-km), complemented by 72 gravity cores and 25 piston cores reveals that the sedimentary architecture of the southwestern Black Sea above a prominent shelf-crossing unconformity α is characterized by three allostratigraphic units (allounits 1b, 1c, 1d), separated by two regional unconformities and their correlative conformities, α1 and α2. A fourth allounit (1a) occurs as patchy remnants between the α unconformity and a deeper Pleistocene erosional unconformity, α0. The chronology is established using 56 radiocarbon dates in five key piston cores. The α unconformity developed as a ravinement surface during the transgression associated with the transition from the Last Glacial Maximum to Holocene, thus defines the base of the Holocene successions. Allounit 1b is bounded at its base and top by the α and α1 unconformities/correlative conformities. It was deposited between ~12.5 and ~7.4 cal ka and shows a notably patchy but widespread distribution across the southwestern shelf. Allounit 1c is bounded at its base and top by the α1 and α2 unconformities/correlative conformities. It was deposited between ~7.4 and ~5.3 cal ka. Allounit 1d accumulated above the α2 unconformity/correlative conformity and was deposited since ~5.3 cal ka.Interpretation of radiometrically-dated cores and several detailed regional maps reveal that the widespread occurrence of ~12.5–7.4 cal ka deposits (allounit 1b) across the middle and outer shelf provides no possibility for a Holocene evaporative drawdown to a lowstand elevation of −120 m at 9.4 cal ka as suggested by Yanchilina et al. (2017). Instead, the volume of detritus in allounit 1b can only be explained if base level was sufficiently high to allow major distant rivers in Bulgaria and Romania (e.g., Kamchiya, Danube) to contribute mud through long-distance marine transport along a submerged shelf. There is no evidence in any cores which sample allounit 1b that the sites were located in limans, rather than on a transgressed shelf which had been flooded by post-glacial meltwater and enhanced river runoff as the climate became more humid in the early Holocene. A prolonged stillstand at the spill depth controlled by the sill in the Strait of Bosphorus is confirmed by the elevation of a drowned recurved spit downdrift from Cape Emine, Bulgaria. The development of the subaqueous unconformities/correlative conformities α1 and α2 was facilitated by storm-induced wave erosion; transport, bypass and erosion under the Rim Current and its anticyclonic eddies, and possibly breaking internal waves. Volumetrics based on isopach maps and pre-industrial sediment discharge rates of various rivers entering the western and southwestern Black Sea suggest that the total sediment yields of Bulgarian and Turkish rivers can only account for ~13–14% of the total Holocene succession (allounits 1b–1d), and that major input from the Danube River via along-shelf long-distance transport must have occurred through the entire Holocene.