The Marmara Sea Gateway since ~16 ky BP: non-catastrophic causes of paleoceanographic events in the Black Sea at 8.4 and 7.15 ky BP

The Late Quaternary history of connection of the Black Sea to the Eastern Mediterranean has been intensely debated. Ryan, Pitman and coworkers advocate two pulses of outflow from the Black Sea to the world ocean at ~16–14.7 ky BP and ~11–10 ky BP. From ~14.7–11 ky BP and from ~10–8.4 ky BP, they suggest that the level of the Black Sea fell to ~ -100 m. At 8.4 ky BP, they further claim that a catastrophic flood occurred in a geological instant, refilling the Black Sea with saline waters from the Mediterranean. In contrast, we continue to gather evidence from seismic profiles and dated cores in the Marmara Sea which demonstrate conclusively that the proposed flood did not occur. Instead, the Black Sea has been at or above the Bosphorus sill depth and flowing into the world ocean unabated since ~10.5 ky BP. This conclusion is based on continuous Holocene water-column stratification (leading to sapropel deposition in the Marmara Sea and the Aegean Sea), proxy indicators of sea-surface salinity, and migration of endemic species across the Bosphorus in both directions whenever appropriate hydrographic conditions existed in the strait. The two pulses of outflow documented by Ryan, Pitman and coworkers find support in our data, and we have modified our earlier interpretations so that these pulses now coincide with the development of mid-shelf deltas: \Delta 2 (16–14.7 ky BP) and \Delta 1 (10.5–9 ky BP) at the southern end of the Bosphorus Strait. However, continued Black Sea outflow after 9 ky BP prevented the northward advection of Mediterranean water and the entry of open-marine species into the Black Sea for more than 1000 years. Sufficient Mediterranean water to change the Sr-isotopic composition of slope and shelf water masses was not available until ~8.4 ky BP (along with the first arrival of many varieties of marine fauna and flora), whereas euryhaline molluscs did not successfully populate the Black Sea shelves until ~7.15 ky BP. Instead of relying on catastrophic events, we recognize a slow, progressive reconnection of the Black Sea to the world ocean, accompanied by significant time lags

Froude supercritical flow processes and sedimentary structures: new insights from experiments with a wide range of grain sizes

Recognition of Froude supercritical flow deposits in environments that range from rivers to the ocean floor has triggered a surge of interest in their flow processes, bedforms and sedimentary structures. Interpreting these supercritical flow deposits is especially important because they often represent the most powerful flows in the geological record. Insights from experiments are key to reconstruct palaeo‐flow processes from the sedimentary record. So far, all experimentally produced supercritical flow deposits are of a narrow grain‐size range (fine to medium sand), while deposits in the rock record often consist of a much wider grain‐size distribution. This paper presents results of supercritical‐flow experiments with a grain‐size distribution from clay to gravel. These experiments show that cyclic step instabilities can produce more complex and a larger variety of sedimentary structures than the previously suggested backsets and ‘scour and fill’ structures. The sedimentary structures are composed of irregular lenses, mounds and wedges with backsets and foresets, as well as undulating planar to low‐angle upstream and downstream dipping laminae. The experiments also demonstrate that the Froude number is not the only control on the sedimentary structures formed by supercritical‐flow processes. Additional controls include the size and migration rate of the hydraulic jump and the substrate cohesion. This study further demonstrates that Froude supercritical flow promotes suspension transport of all grain sizes, including gravels. Surprisingly, it was observed that all grain sizes were rapidly deposited just downstream of hydraulic jumps, including silt and clay. These results expand the range of dynamic mud deposition into supercritical‐flow conditions, where local transient shear stress reduction rather than overall flow waning conditions allow for deposition of fines. Comparison of the experimental deposits with outcrop datasets composed of conglomerates to mudstones, shows significant similarities and highlights the role of hydraulic jumps, rather than overall flow condition changes, in producing lithologically and geometrically complex stratigraphy

Bedforms and sedimentary structures related to supercritical flows in glacigenic settings

Upper-flow-regime bedforms, including upper-stage-plane beds, antidunes, chutes-and-pools and cyclic steps, are ubiquitous in glacigenic depositional environments characterized by abundant meltwater discharge and sediment supply. In this study, the depositional record of Froude near-critical and supercritical flows in glacigenic settings is reviewed, and similarities and differences between different depositional environments are discussed. Upper-flow-regime bedforms may occur in subglacial, subaerial and subaqueous environments, recording deposition by free-surface flows and submerged density flows. Although individual bedform types are generally not indicative of any specific depositional environment, some observed trends are similar to those documented in non-glacigenic settings. Important parameters for bedform evolution that differ between depositional environments include flow confinement, bed slope, aggradation rate and grain size. Cyclic-step deposits are more common in confined settings, like channels or incised valleys, or steep slopes of coarse-grained deltas. Antidune deposits prevail in unconfined settings and on more gentle slopes, like glacifluvial fans, sand-rich delta slopes or subaqueous (ice-contact) fans. At low aggradation rates, only the basal portions of bedforms are preserved, such as scour fills related to the hydraulic-jump zone of cyclic steps or antidune-wave breaking, which are common in glacifluvial systems and during glacial lake-outburst floods and (related) lake-level falls. Higher aggradation rates result in increased preservation potential, possibly leading to the preservation of complete bedforms. Such conditions are met in sediment-laden jökulhlaups and subaqueous proglacial environments characterized by expanding density flows. Coarser-grained sediment leads to steeper bedform profiles and highly scoured facies architectures, while finer-grained deposits display less steep bedform architectures. Such differences are in part related to stronger flows, faster settling of coarse clasts, and more rapid breaking of antidune waves or hydraulic-jump formation over hydraulically rough beds. © 2020 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologist

Intra‐clinothem variability in sedimentary texture and process regime recorded down slope profiles

Shelf‐margin clinothem successions can archive process interactions at the shelf to slope transition, and their architecture provides constraints on the interplay of factors that control basin‐margin evolution. However, detailed textural analysis and facies distributions from shelf to slope transitions remain poorly documented. This study uses quantitative grain‐size and sorting data from coeval shelf and slope deposits of a single clinothem that crops out along a 5 km long, dip‐parallel transect of the Eocene Sobrarbe Deltaic Complex (Ainsa Basin, south‐central Pyrenees, Spain). Systematic sampling of sandstone beds tied to measured sections has captured vertical and basinward changes in sedimentary texture and facies distributions at an intra‐clinothem scale. Two types of hyperpycnal flow‐related slope deposits, both rich in mica and terrestrial organic matter, are differentiated according to grain size, sorting and bed geometry: (i) sustained hyperpycnal flow deposits, which are physically linked to coarse channelized sediments in the shelf setting and which deposit sand down the complete slope profile; (ii) episodic hyperpycnal flow deposits, which are disconnected from, and incise into, shelf sands and which are associated with sediment bypass of the proximal slope and coarse‐grained sand deposition on the medial and distal slope. Both types of hyperpycnites are interbedded with relatively homogenous, organic‐free and mica‐free, well‐sorted, very fine‐grained sandstones, which are interpreted to be remobilized from wave‐dominated shelf environments; these wave‐dominated deposits are found only on the proximal and medial slope. Coarse‐grained sediment bypass into the deeper‐water slope settings is therefore dominated by episodic hyperpycnal flows, whilst sustained hyperpycnal flows and turbidity currents remobilizing wave‐dominated shelf deposits are responsible for the full range of grain sizes in the proximal and medial slope, thus facilitating clinoform progradation. This novel dataset highlights previously undocumented intra‐clinothem variability related to updip changes in the shelf process‐regime, which is therefore a key factor controlling downdip architecture and resulting sedimentary texture

Persistent Holocene outflow from the Black Sea to the eastern Mediterranean Sea still contradicts the Noah's Flood Hypothesis: A review of 1997–2021 evidence and a regional paleoceanographic synthesis for the latest Pleistocene–Holocene

This review and synthesis weaves various multiproxy data into a single coherent narrative for the latest Pleistocene–Holocene paleoclimatic and paleoceanographic evolution of the Black Sea, Marmara Sea and the Aegean Sea. This narrative, referred to as the “Outflow Hypothesis” rests on several key observations and interpretations which are incompatible with the suggestion that the post-LGM reconnection of the Black Sea basin to the global ocean occurred as a catastrophic flood. The widespread occurrence of sub-storm-wavebase uppermost Pleistocene to lower Holocene sediments across the southwestern Black Sea shelf at elevations as shallow as −78 m shows that the level of the Neoeuxine Lake (today's Black Sea) between 12.3 cal ka and 9.5 cal ka was high enough to spill outward into the Marmara Sea over the shallow sill in the southern Strait of Bosphorus (−37 m today). Southwest-prograded clinoforms immediately south of the strait in the northeastern Marmara Sea record the development of an early Holocene (11.1–10.2 cal ka) mid-shelf delta (Δ1) showing ~3.3 km of aggressive progradation while its topset-to-foreset break climbed 8–9 m into a rising Marmara Sea. A streamlined south-prograded barform in the throat of the strait and giant megaflutes along its thalweg confirm the vigorous outflow from the early Holocene Neoeuxine Lake required to explain the climbing Δ1 lobe. Multiproxy data from the northeastern Marmara Sea and southwestern Black Sea shelves indicate that the post-Last Glacial Maximum (LGM) reconnection of the Black Sea with the eastern Mediterranean occurred in a gradual fashion: first, at ~10.2 cal ka, a salt wedge lifted the brackish outflow off the floor of the Strait of Bosphorus terminating Δ1 progradation; second, a more persistent density underflow introduced enough seawater strontium into the Black Sea to be taken up in mollusc shells by ~9.5 cal ka, and finally a range of euryhaline marine organisms replaced lacustrine faunas when salinity levels became favourable by ~7.5 cal ka. The onset of sapropel M1 deposition across the Marmara Sea followed the breach of the Strait of Dardanelles at 13.8 cal ka when, as originally suggested by other researchers, nutrient-rich highly saline Mediterranean waters forced lower density relict lacustrine waters to the surface and then out through the Strait of Dardanelles, initiating water-column stratification. Once the low-salinity cap was expelled, the deep waters of the fully saline Marmara Sea remained stagnant and sapropel accumulation continued. The onset of outflow from the Neoeuxine Lake at 11.1 cal ka re-established water-column stratification, induced effective deep circulation across the Marmara Sea, and created a low salinity lid across the northern Aegean Sea, initiating sapropel S1 deposition in that area

The uppermost Pleistocene–Holocene mud drape across the Marmara Sea: quantification of detrital supply from southern Marmara rivers

The Marmara Sea (area 11,350 km2; volume 3,378 km3; central basins >1100 m deep) straddles the North Anatolian Transform Fault separating the Eurasian and Aegean-Anatolian tectonic plates. Along with the shallow straits of Dardanelles and Bosphorus (depths ~63 m and ~40 m, respectively), the Marmara Sea forms the only marine connection between the Black Sea and the eastern Mediterranean. During Pleistocene glacial stages, the modern straits were subaerial valleys and the modern Marmara basin was occupied by the landlocked Propontis Lake. Previous researchers attributed major portions of a widely distributed uppermost Pleistocene–Holocene mud blanket (locally >10–25 m thick; volume 43–47 km3) to transport of suspended load through one or both of the straits, as either the Aegean Sea (at ~13.8 cal ka) or the Neoeuxine Lake (today's Black Sea, at ~11.1 cal ka) began to spill into the Marmara basin. To test these suggestions, the thicknesses and volume of the mud blanket were determined from >5000 line-km of airgun, sparker and boomer profiles and >100 cores, and compared with the contemporary supply from local rivers to decide, by difference, if the straits might have had a significant role. Volume calculations for the detrital supply from rivers rely on (1) decades of daily water- and sediment-discharge data from gauging stations, acquired before 20th century dam construction and, independently, (2) the BQART model which uses a variety of hydrological, geomorphic, geological and climate data. These calculations demonstrate that >85–90% of the detritus in the offshore mud blanket was supplied by steep rivers (Kocasu River and its tributaries) and mountainous streams draining the highlands of the southern Marmara region. Geochemistry of the <38 μm fraction supports this source. Any input through the Dardanelles has been sporadic and limited to perhaps ~5 Gt of suspended load (equivalent to ~5.2 km3 of porous mud when deposited) because of changing directions and rates of flow since the Last Glacial Maximum. Resedimentation through mass wasting and transgressive shoreface erosion appear to be minor compared with river supply. The isolated nature of the Marmara basin and its supply from mostly a single watershed afford an opportunity to verify the reliability of this type of hindcast analysis, based upon sediment-discharge data and catchment models – analysis which cannot be completed with a comparable level of certainty along open marine coastlines elsewhere

Anatomy of a submarine pyroclastic flow and associated turbidity current: July 2003 dome collapse, Soufrière Hills volcano, Montserrat, West Indies

The 12 to 13 July 2003 andesite lava dome collapse at the Soufrière Hills volcano, Montserrat, provides the first opportunity to document comprehensively both the sub-aerial and submarine sequence of events for an eruption. Numerous pyroclastic flows entered the ocean during the collapse, depositing approximately 90% of the total material into the submarine environment. During peak collapse conditions, as the main flow penetrated the air–ocean interface, phreatic explosions were observed and a surge cloud decoupled from the main flow body to travel 2 to 3 km over the ocean surface before settling. The bulk of the flow was submerged and rapidly mixed with sea water forming a water-saturated mass flow. Efficient sorting and physical differentiation occurred within the flow before initial deposition at 500 m water depth. The coarsest components (?60% of the total volume) were deposited proximally from a dense granular flow, while the finer components (?40%) were efficiently elutriated into the overlying part of the flow, which evolved into a far-reaching turbidity current.<br/

Quantitative estimation of Holocene surface salinity variation in the Black Sea using dinoflagellate cyst process length

Reconstruction of salinity in the Holocene Black Sea has been an ongoing debate over the past four decades. Here we calibrate summer surface water salinity in the Black Sea, Sea of Azov and Caspian Sea with the process length of the dinoflagellate cyst Lingulodinium machaerophorum. We then apply this calibration to make a regional reconstruction of paleosalinity in the Black Sea, calculated by averaging out process length variation observed at four core sites from the Black Sea with high sedimentation rates and dated by multiple mollusk shell ages. Results show a very gradual change of salinity from ~14 ± 0.91 psu around 9.9 cal ka BP to a minimum ~12.3 ± 0.91 psu around 8.5 cal ka BP, reaching current salinities of ~17.1 ± 0.91 psu around 4.1 cal ka BP. The resolution of our sampling is about 250 years, and it fails to reveal a catastrophic salinization event at ~9.14 cal ka BP advocated by other researchers. The dinoflagellate cyst salinity-proxy does not record large Holocene salinity fluctuations, and after early Holocene freshening, it shows correspondence to the regional sea-level curve of Brückner et al. (2010) derived from Balabanov (2007)