LATE QUATERNARY HISTORY OF THE MARMARA SEA AND BLACK SEA FROM HIGH-RESOLUTION SEISMIC AND GRAVITY-CORE STUDIES

Abstract

Lithologic and multi-proxy paleoenvironmental data from 21 dated cores have been used to define three allostratigraphic units (allounits) within the late Quaternary successions of the Marmara Sea and Black Sea. Allounits are bounded by unconformities and their correlative conformities. In both regions, Allounit A extends from the seafloor downward to a ∼12–11-ka sequence boundary, which is a major shelf-crossing unconformity in water depths less than ∼100–110 m. In deep basins of the Marmara Sea, the lower part of Allounit A, designated Subunit A2, is a laminated sapropel, M1. On the shelf, Subunit A2 consists of backstepping delta lobes and early-transgressive barrier islands and sand sheets. Allounit B has only been recovered in Marmara Sea cores collected at water depths greater than ∼90 m, and represents basinal or prodeltaic deposition during the 23–12-ka late Pleistocene lowstand. During the last glacial maximum, the shelves surrounding the Marmara Sea were subaerially exposed, and deltas of Allounit B accumulated along the present-day shelf edge. Following the post-glacial rise of global sea level to −75 m at ∼12 ka, the Marmara Sea quickly became inundated and thereafter rose in synchroneity with the Mediterranean. By ∼10 ka, the Black Sea rose to start spilling into the Marmara Sea, leading to establishment of a brackish-water lid that has persisted to the modern day. The strongest Black Sea outflow began at ∼10 ka and persisted to ∼6 ka, promoting the accumulation of sapropel M1 in the deep Marmara Sea, and progradation of an overflow delta just south of the exit from the Bosphorus Strait. Allounit C is a laminated sapropel (M2) in basinal cores, dated at ∼30–23 ka. Like M1, it is believed that M2 accumulated during a period of increased brackish-water input into the Marmara Sea mainly from the Black Sea. In the Black Sea, wave erosion kept the shelf stripped of unconsolidated sediments during the falling sea level associated with the last glaciation and subsequent early stages of the post-glacial Holocene transgression. This erosion created a major unconformity, α. Shelf-edge deltas of Allounit B received their sediment during the last lowstand from small rivers that likely coalesced into a single system toward the shelf edge, at modern water depths of −100 to −110 m. These deltas were active until ∼11–10.5 ka. Subsequently, sea level in the Black Sea rose to −40 m by ∼10 ka, and a set of backstepping barrier islands developed on the shelf as part of the associated transgressive systems tract. Once water level reached −40 m, continued sea-level rise stalled until ∼9 ka as the Black Sea began to spill across the Bosphorus Strait into the Marmara Sea.