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Soulet (2018) asserts that the calculation of the 14C reservoir age for the geochronology of the Black Sea sediments suffers from a fundamental flaw. He contends that Yanchilina et al. (2017) obtained reservoir ages by first sorting the δ18O and δ13C values of the mollusks as a function of their corresponding 14C age before aligning the mollusk isotope curves to those of the nearby Sofular Cave. Sorting by 14C age would lead to an incorrect calendar age for each δ18O and δ13C measurement because it does not take into account changing reservoir ages. We reply here that this sorting by 14C age was not the procedure taken. The radiocarbon reservoir ages were derived after tuning the δ18O and δ13C composition of the mollusks, sorted by their δ18O, δ13C, radiocarbon age and stratigraphic location in each core, to the δ18O and δ13C composition of the U/Th dated Sofular Cave stalagmites as a function of calendar age, shown to reflect Black Sea surface water δ18O and δ13C composition. After the calendar age was derived for each mollusk, it was then converted to a corresponding Northern Hemisphere atmospheric 14C age using Reimer et al. (2009). The 14C reservoir age was subsequently calculated from the difference between the measured 14C age of the mollusks and the calculated Northern Hemisphere atmospheric 14C age of the mollusks, R(t) = 14Cshell(t) − 14Catm(t) (e.g., Ryan, 2007; Kwiecien et al., 2009; Jull et al., 2013; Philippsen, 2013; Soulet, 2015; Soulet et al., 2016). The Reimer et al. (2009) calibration used in our original study differs

Our knowledge of rate and processes in which semi-enclosed environments alternate from lacustrine to marine is commonly limited because of the paucity of specific proxies for sea level and salinity. Here we investigate the timing, rate, and key mechanisms involved in the transformation of the previously isolated Black Sea-Lake to the modern partly-enclosed marine sea using a suite of geophysical, geochemical, and geochronological methods. Cores were collected in transects across shelves of Ukraine, Romania, Bulgaria, and Turkey. Biogenic carbonate from these cores was analyzed for radiocarbon and strontium, oxygen, and carbon isotopes. Strontium results indicate that the submergence of the Black Sea shelf at 9300 calendar years BP was caused by the ingress of Mediterranean water and was abrupt, taking Radiocarbon years are calibrated to calendar years by tuning the oxygen and carbon isotope composition of the mollusk record to that of the U/Th dated Sofular Cave stalagmites. The match shows a reduction of the lake's prior high reservoir age accompanying the inflow of the Mediterranean water. In 900 years the salinity reached a threshold that excluded all previous Black Sea lacustrine fauna. These results imply that any substantial postglacial submergence of the Black Sea shelves did not occur prior to entry of Mediterranean water.

The Marine Isotope Stage 3 (MIS 3) is considered a period of persistent and rapid climate and sea level variabilities during which eustatic sea level is observed to have varied by tens of meters. Constraints on local sea level during this time are critical for further estimates of these variabilities. We here present constraints on relative sea level in the Marmara and Black Sea regions in the northeastern Mediterranean, inferred from reconstructions of the history of the connections and disconnections (partial or total) of these seas together with the global ocean. We use a set of independent data from seismic imaging and core-analyses to infer that the Marmara and Black Seas remained connected persistent freshwater lakes that outflowed to the global ocean during the majority of MIS 3. Marine water intrusion during the early MIS-3 stage may have occurred into the Marmara Sea-Lake but not the Black Sea-Lake. This suggests that the relative sea level was near the paleo-elevation of the Bosporus sill and possibly slightly above the Dardanelles paleo-elevation, ~80 mbsl. The Eustatic sea level may have been even lower, considering the isostatic effects of the Eurasian ice sheet would have locally uplifted the topography of the northeastern Mediterrranean.

Deglaciation of the Eurasian Ice Sheet dispensed vast amounts of meltwater laden with red-brown clay into Black and Caspian Sea's freshwater lakes between 16,350 and 15,400 calendar years BP. The arriving sediment had a mineral composition and strontium-neodymium isotope signatures (87Sr/86Sr, and eNd) consistent with the crystalline bedrock that lay beneath the ice sheet as opposed to sources from landscapes crossed by the rivers that carried meltwater to the lakes. The magnitude and rapidity of the shifts of 87Sr/86Sr and δ18O in shells of ostracods and mollusks, an eight-fold increase in sediment accumulation rate, and a plunge of 1250 14C years in the radiocarbon reservoir age indicate a volume of meltwater sufficient to have replaced most or all of the pre-existing lake water. If the release of >25,000 km3 of water from the Caspian into the Black Sea's New Euxine Lake was abrupt by judging the magnitude of erosion in the floor of the Manych Strait spillway, this event qualifies as a megaflood. Deeply-scoured furrows, channel floor bars, and swaths of mounds 5–20 km in length and 5–20 m in height are reminiscent of scabland landscapes. The meltwater is coincident with Heinrich Stadial 1 when the Alpine and Eurasian ice sheets re-advanced and temperatures of the Black Sea's lake surface remained cold. Sand grains within the red-brown clay indicate they were transported by lake surface ice.

Chirp sub-bottom profiling, multibeam bathymetric mapping and a combination of faunal and isotopic analysis of molluscs and foraminifera in sediment cores on the northern shelf of the Sea of Marmara (SoM) provide evidence of sea-level excursions, water exchanges between the adjacent Mediterranean and Black Seas, and oscillating salinity over the last 160 ka bp. During the marine isotope stages MIS-2, MIS-3, MIS-4 and MIS-6 the SoM disconnected from the Mediterranean Sea and evolved into a lake. During MIS-1, MIS-5 and MIS-7, the SoM reconnected and became salty once again. Sapropels formed shortly after each invasion of Mediterranean saltwater observed in our cores. Concurrent suboxic–dysoxic conditions prevailed over quite shallow substrates on the shelf. Ancient shorelines are pervasive at − 85 m on the northern shelf and in the region of Prince Islands coincident with the elevation of the modern bedrock sill in the Canakkale (Dardanelles) Strait. At times when global (eustatic) sea level dropped below this sill, the surface of the SoM stabilized at its outlet and freshened. Thus this particular shoreline is interpreted as the edge of the most recent SoM lake that existed from about 75 ka bp to 12 ka bp. The freshening is observed in very light (− 6‰) values of δ 18 O measured on freshwater molluscs and the complete absence of foraminifera. Two brief lacustrine episodes during MIS-5 suggest that the level of the Canakkale outlet might have been as shallow as − 50 m in the past, a likelihood supported by submerged terraces along its margins bounding the modern central channel and the presence of an euryhaline biofacies in Unit L4.1 corresponding to MIS-5b. δ 18 O profiles and carbon-14 dating show that salinification of the SoM and the blossoming of bioherms evolved rapidly after the latest connection with the Mediterranean at 12 ka bp. However, freshening proceeded more slowly once the connection was severed.

During the late glacial, marine isotope Stage 2, the Marmara Sea transformed into a brackish lake as global sea-level fell below the sill in the Dardanelles Strait. A record of the basin's reconnection to the global ocean is preserved in its sediments permitting the extraction of the paleoceanographic and paleoclimatic history of the region. The goal of this study is to develop a high-resolution record of the lacustrine to marine transition of Marmara Sea in order to reconstruct regional and global climatic events at a millennial scale. For this purpose, we mapped the paleoshorelines of Marmara Sea along the northern, eastern, and southern shelves at Cekmece, Prince Islands, and Imrali, using data from multibeam bathymetry, high-resolution subbottom profiling (chirp) and ten sediment cores. Detailed sedimentologic, biostratigraphic (foraminifers, mollusk, diatoms), X-ray fluorescence geochemical scanning, and oxygen and carbon stable isotope analyses correlated to a calibrated radiocarbon chronology provided evidence for cold and dry conditions prior to 15 ka BP, warm conditions of the Bolling-Allerod from ∼ 15 to 13 ka BP, a rapid marine incursion at 12 ka BP, still stand of Marmara Sea and sediment reworking of the paleoshorelines during the Younger Dryas at ∼ 11.5 to 10.5 ka BP, and development of strong stratification and influx of nutrients as Black Sea waters spilled into Marmara Sea at 9.2 ka BP. Stable environmental conditions developed in Marmara Sea after 6.0 ka BP as sea-level reached its present shoreline and the basin floors filled with sediments achieving their present configuration.

In their comment Hiscott and co-authors adhere rigidly to ages and sources for sedimentary units in the subsurface of the Marmara shelf that they have previously reported in their publications from 2002 through 2007. This adherence is in spite of a superior age-depth model from our 13 m-long sediment core that penetrated deeply into the deposits under consideration and in disregard to the results of subsequent more-detailed and full-coverage mapping of the region under scrutiny. The age revisions are dismissed by the authors of the comment as representing sediments severely disturbed during coring. We rebut this criticism. The Bosporus source attributed by them to the sedimentary units sampled and dated by us appears to be driven by their conception that the Black Sea had a persistent outflow prior to its two-way connection with Marmara. Irrespective of whether the outflow was persistent, our reply shows that the drawing of the isopachs of the sedimentary units by Hiscott and co-authors was accomplished in a fashion to promote the Bosporus source hypothesis regardless of the geographic limits of their survey. The ages assigned to the units are equally indeterminate because the cores used by them have missing core tops of unknown lengths that are not discussed in their publications. Furthermore, the sub-bottom reflectors at the sites where the reflectors were calibrated to their only core without a missing top are actually hidden by the finite width of the bottom reflecting wavelet.

Two distinct series of slumps deform the upper part of the sedimentary sequence along the continental margin of the Levant. One series is found along the base of the continental slope, where it overlies the disrupted eastern edge of the Messinian evaporites. The second series of slumps transects the continental margin from the shelf break to the Levant Basin. It seemed that the two series were triggered by two unrelated, though contemporaneous, processes. The shore-parallel slumps were initiated by basinwards flow of the Messinian salt, that carried along the overlying Plio-Quaternary sediments. Seawater that percolated along the detachment faults dissolved the underlying salt to form distinctly disrupted structures. The slope-normal slumps are located on top of large canyons that cut into the pre-Messinian sedimentary rocks. A layer of salt is found in the canyons, and the Plio-Quaternary sediments were deposited on that layer. The slumps are bounded by large, NW-trending faults where post-Messinian faulted offset was measured. We presume that the flow of the salt in the canyons also drives the slope-normal slumps. Thus thin-skinned halokynetic processes generated the composite post-Tortonian structural patterns of the Levant margin. The Phoenician Structures are a prime example of the collapse of a distal continental margin due to the dissolution of a massive salt layer.

High-resolution reflection seismic profiles with core analyses of sedimentary sequence near the Sea of Marmara (SoM) entrance to the Strait of İstanbul (SoI, Bosphorus) provide detailed record of sealevel changes since the Late Glacial Maximum (LGM). The sequence is deposited over the LGM erosional surface at a maximum depth of -105 m. It includes seven seismic units that can be confidently correlated with 14C-dated sedimentary units in the cores. Unit 6 represents levee deposits of the paleo-SoI channel that was deposited by a Black Sea outflow during 11.2-10.6 kyr BP. The reflection surfaces at base of Unit 5 and 4 correspond to wave-cut terraces at -71and -63 m, respectively, and are estimated to be 10.6 and 9.8 kyr BP. The seismic Unit 2, overlaying a mud drape (Unit 3), comprises deltaic sediments that was deposited during 6.400-3200 yr 14C BP. Isopach map and forset directions of the deltaic unit indicates that the delta was sourced from Kurbağalıdere River. Depositional period of the delta is subdivided into two stages in response to changes in the 49 balance between sealevel change and sediment supply rates: a progradational and an aggradational stages. The progradational stage prevailed during 6.4-4.7 kyr BP when the sealevel rise decelerated and there was a high sediment input from the drainage area of the Kurbağalıdere River. An aggradational stage developed during 4.7-3.2 kyr BP and indicates that the delta grew during faster sealevel rise when the rate of sediment input was relatively decreased. The onset of delta deposition at 6.4 kyr BP is marked in the SoM by termination of the early Holocene Sapropel deposition and the aggradational stage overlaps with the late Holocene Sapropelic unit deposited on the shelf areas. Our finding contradicts the hypothesis of Hiscott et al. (2002) and Aksu et al. (2002) that this delta was sourced from the Black Sea and that it indicates a persistant Black Sea outflow since 10 kyr BP.

The Hudson River Benthic Mapping Project, funded by the New York State Department of Environmental Conservation, resulted in a comprehensive data set consisting of high-resolution multibeam bathymetry, sidescan sonar, and sub-bottom data, as well as over 400 sediment cores and 600 grab samples. This detailed data set made it possible to study the regional pattern and the local variations of the sediment distribution in the Hudson River Estuary. Based on these data we describe the distribution of sediment texture and process-related sedimentary environments for the whole 240-km long estuary together with along-river variations of depth, cross-sectional area, and grain size distribution. We compare these parameters with changes in surrounding geology and tributary input and, as a result, divide the Hudson River Estuary in eight sections with distinct combinations of channel morphology, bedrock type, sediment texture, and sediment dynamics. The regional sediment distribution consists of marine sand-dominated sediments near the ocean end of the estuary, a large, mud-dominated central section, and fluvial sand-dominated sediments in the freshwater section of the Hudson River Estuary. This regional trend is highly modified by small-scale variations in the sediment distribution. These local variations are controlled by changes in morphology, bedrock, and tributary input, as well as by anthropogenic modifications of the estuary. In some areas these local variations are larger than the overall trend in sediment distribution and control the actual sediment type, as well as the condition of erosion and deposition in the estuary.

High-resolution seismic data and sediment cores reveal the late Holocene subsurface stratigraphy of the broad Tappan Zee– Piermont region of the Hudson River Estuary. We identify a series of distinct, extensive horizons beneath the marginal flats, channel banks, and main channel in this area. Physical properties and lithology from sediment cores suggest that these horizons are surfaces of erosion or nondeposition. Radiocarbon dates indicate that they correspond with three distinct time horizons, with maximum ages of 3400, 2200, and 1600 yr BP. We also distinguish two sedimentary facies that occupy the marginal flats and channel banks. The deeper facies forms a deposit 2 km wide and 7 km long that accumulated at rates of 2–4 mm/yr in the vicinity of the Sparkill Creek prior to ∼1700 yr BP, overlying and onlapping the 2200 yr BP seismic surface. Based on its internal geometry, morphology, and proximity to a tributary, we interpret this facies as a delta deposit. The shallower facies accumulated more slowly (1–2 mm/yr), overlying the delta deposit to the south and dominating the marginal flats to the north. Surface sediment samples and geophysical data reveal that the modern marginal flats are no longer actively depositional, but dominated by nondeposition or erosion. Limited accommodation space in the Hudson River Estuary may be the critical factor contributing to the observed sedimentary pattern, characterized by intervals of deposition punctuated by episodes of erosion. An estuarine system that has reached a state of morphological equilibrium will be sensitive to even small fluctuations in sea-level and climate conditions, which may account for the intervals of deposition and erosion we observe. Limited accommodation space and intermittent sediment deposition in the Hudson River Estuary may be due to its evolution from a fjord filled with glacial lake sediments, which distinguishes its infilling behavior from the classic drowned river valley estuary.

The Black Sea was an inland lake during the last ice age and its sediments are an excellent potential source of information on Eurasian climate change, showing linkages between regionally and globally recognized millennial-scale climate events of the last deglaciation. Here, we detail changes from the last glacial maximum (LGM) through the transition to an anoxic marginal sea using isotopic (strontium and oxygen) and trace element (Sr/Ca) ratios in carbonate shells, which record changing input sources and hydrologic conditions in the basin and surrounding region. Sr isotope records show two prominent peaks between ∼18 and 16 ka BP cal, reflecting anomalous sedimentation associated with meltwater from disintegrating Eurasian ice sheets that brought Black Sea level to its spill point. Following a sharp drop in Sr isotope ratios back toward glacial values, two stages of inorganic calcite precipitation accompanied increasing oxygen isotope ratios and steady Sr isotope ratios. These calcite peaks are separated by an interval in which the geochemical proxies trend back toward glacial values. The observed changes reflect negative water balance and lake level decline during relatively warm periods (Bolling–Allerod and Preboreal) and increasing river input/less evaporation, resulting in higher lake levels, during the intervening cold period (the Younger Dryas). A final shift to marine values in Sr and oxygen isotope ratios at 9.4 ka BP cal corresponds to connection with the global ocean, and marks the onset of sedimentation on the Black Sea continental shelf. This date for the marine incursion is earlier than previously suggested based on the appearance of euryhaline fauna and the onset of sapropel formation in the deep basin.

Current interpretation of the Messinian Salinity Crisis (MSC) involves partial “desiccation” of the Mediterranean Sea coupled with the deposition of thick evaporites in the deep basins. New sets of seismic reflection profiles in the western part of the Gulf of Lions confirm the basinward extension of the Messinian erosion and enable the mapping of distinctive seismic markers indicating the Messinian Erosional Surface (or Messinian unconformity), the basin-margin detrital deposits, and the deep evaporite sequence. The geometrical relationship between these three elements and their relationship to the paleogeography of the margin during the MSC provide new information about the evolution of the study area during the Messinian. The Messinian Erosional Surface (MES), commonly correlated with the “desiccation” phase and the deposition of deep evaporites during the apogee of the event, is generally interpreted as a subaerial feature. In the Gulf of Lions, it is a complex diachronic polygenic erosional surface observed at the base of the prograding Plio-Quaternary sequence beneath the shelf and slope; it extends downslope beneath the deep basin Upper Evaporites and the Salt, and possibly correlates conformably with the base of the so-called deep Lower Evaporites. The whole morphology of the MES reflects a buried drainage pattern, supporting the interpretation of fluvial erosion driven by a substantial drop in sea level. Our results also suggest that large submarine gravity flows occurred prior to any significant accumulation of Salt in the basin and prior to the Upper Evaporites. Consequently, interbedded clastic deposits may partly account for the parallel reflectors of the Lower Evaporites. Since river erosion persisted throughout the MSC, the Salt and Upper Evaporite units may also contain a large amount of detrital sediments. The good quality of the new seismic data clearly reveals fan-shaped Messinian deposits in the downstream part of the main Messinian valleys (i.e., the Nile, Var, and Spanish rivers). The depositional scenarios generally involve a substantial sea-level fall coupled with deltaic/prodeltaic accumulations. A chaotic seismic unit (Unit D) filling Messinian lows and extending beneath the Salt within the study area is interpreted as a Messinian clastic unit. We propose a polyphase scenario of detrital fan deposition involving pre-, syn-, and post-Salt deposition in subaqueous/subaerial environments. In the Gulf of Lions, a late Miocene tectonic phase that affected the western shelf also played an important role in controlling (a) the pattern of the Messinian fluvial network, (b) the location of maximum erosion on the shelf, and (c) the location of the detrital fan depocentre downslope.

Acoustic surveying provides valuable information on the distribution of sedimentary environments in subaqueous settings. Frequently, differences in acoustic backscatter strength are used to distinguish sedimentary environments on the basis of their grain size composition. Here, we are presenting acoustic backscatter data from the Haverstraw Bay section of the Hudson River Estuary that has been mapped using sidescan, sub-bottom profiling, and multibeam bathymetry as part of the Benthic Mapping Project of the Hudson River Estuary Program, funded by New York State. In addition, hundreds of gravity cores, and grab samples provide ground truth for a classification of the river bottom into discrete substrate types. Analyses of sedimentary environments reveal patterns in backscatter strength beyond those that can be related to the sediment grain size distribution alone. An integrated interpretation of sidescan, sub-bottom profiling, and high-resolution bathymetry data indicates that the backscatter pattern can be attributed to spatial variations in the modern depositional environments which cause differences in bottom roughness and sediment compaction. Based on an integrated interpretation of the acoustic and sample data sets, we distinguished eight different sedimentary classes. Many of the classes can be linked to dynamic processes including contemporary deposition, erosion, and sediment migration in sand waves. The results provide a better understanding of the dynamic processes of the Hudson River Estuary and improve the interpretation of the acoustic backscatter data from fine grained sedimentary environments.

Geophysical mapping and sampling data provide a record of changing environmental and faunal conditions within the Hudson River estuary during the mid- to late Holocene. On the shallow, broad marginal flats of the mesohaline Hudson, fossil oyster beds (Crassostrea virginica) are found exposed on the river bottom and buried by sediment. The shallowest beds are well imaged in chirp sub-bottom and side-scan sonar data and form discrete flow-perpendicular bands, 0.6–1.0 km wide and up to 3 km long, which cover 30% of the river bottom. Radiocarbon-dated sediment cores indicate oysters thrived within two time periods from ~500–2,400 and ~5,600–6,100 cal. years b.p. Sediment and physical property data indicate a changing depositional regime consistent with the oyster chronology. Similar changes in oyster presence are found in local shell midden sites of the Lower Hudson Valley as well as elsewhere along the Atlantic coast, and may reflect climatic controls associated with warm–cool cycles during the Holocene. Oysters flourished during the mid-Holocene warm period, disappeared with the onset of cooler climate at 4,000–5,000 cal. years b.p., and returned during warmer conditions of the late Holocene. The most recent demise of oysters within the Hudson at 500–900 cal. years b.p. may have accompanied the Little Ice Age.

▪ Abstract Decades of seabed mapping, reflection profiling, and seabed sampling reveal that throughout the past two million years the Black Sea was predominantly a freshwater lake interrupted only briefly by saltwater invasions coincident with global sea level highstand. When the exterior ocean lay below the relatively shallow sill of the Bosporus outlet, the Black Sea operated in two modes. As in the neighboring Caspian Sea, a cold climate mode corresponded with an expanded lake and a warm climate mode with a shrunken lake. Thus, during much of the cold glacial Quaternary, the expanded Black Sea's lake spilled into to the Marmara Sea and from there to the Mediterranean. However, in the warm climate mode, after receiving a vast volume of ice sheet meltwater, the shoreline of the shrinking lake contracted to the outer shelf and on a few occasions even beyond the shelf edge. If the confluence of a falling interior lake and a rising global ocean persisted to the moment when the rising ocean penetrated across the dividing sill, it would set the stage for catastrophic flooding. Although recently challenged, the flood hypothesis for the connecting event best fits the full set of observations.

Overbank flow of turbidity currents sweeping through the entrenched Monterey Fan Channel has generated erosional and depositional features along the channel walls and across the adjacent levees. These features, investigated with side-scan sonar, SeaBeam bathymetry, submersibles, and a towed camera sled include fields of sediment waves, overbank channels, gullied walls and terraces, and slump scars. Overbank flow occurs where the channel path is straight, but is accentuated along sharp outside bends. Along straight channel segments sediment waves trend oblique to the channel and are commonly present on the right-hand (when looking downstream) levee adjacent to the channel. Along curved segments of the channel path, large sediment waves, with crests sub-parallel to the channel and with wavelengths reaching 3 km, are present on the outside levee as far as 15 km from the channel. Commonly, a gully is present on the lee side of the sediment wave, near the mid-point of the wave crest. Gullies are the heads of a tributary system of overbank channels, which feed a larger trunk channel on the fan surface.

Seismic stratigraphic analysis of 40 in3 sleeve-gun profiles from the levees of the proximal Northwest Atlantic Mid-Ocean Channel (NAMOC) of the Labrador Sea reveals seven intervals of levee formation that are characterised by packages of low-amplitude reflections separated by a few high-amplitude reflections, except for the second last interval, which shows high-amplitude reflections throughout. The levees are strongly asymmetric with a maximum thickness of 250 m for the western and 150 m for the eastern levee. Levee asymmetry is greatest for the high-amplitude package and nil for the most recent package. The levees prograde south-eastward and overlie a seismic facies of discontinuous high-amplitude reflections that probably represents channel-fill under the channel and submarine braidplain deposits towards the south. The NAMOC levees, which are very stably positioned with

A side-scan sonar survey was conducted of Monterey Canyon and the San Gregorio fault zone, off shore of Monterey Bay. The acoustic character and morphology of the sonar images, enhanced by SeaBeam bathymetry, show the path of the San Gregorio fault zone across the shelf, upper slope, and Monterey Canyon. High backscatter linear features a few kilometers long and 100 to 200 m wide delineate the sea-floor expression of the fault zone on the shelf. Previous studies have shown that brachiopod pavements and carbonate crusts are the source of the lineations backscatter. In Monterey Canyon, the fault zone occurs where the path of the canyon makes a sharp bend from WNW to SSW (1800 m). Here, the fault is marked by NW–SE-trending, high reflectivity lineations that cross the canyon floor between 1850 m and 1900 m. The lineations can be traced to ridges on the northwestern canyon wall where they have ∼15 m of relief. Above the low-relief ridges, bowl-shaped features have been excavated on the canyon wall contributing to the widening of the canyon. We suggest that shear along the San Gregorio fault has led to the formation of the low-relief ridges near the canyon wall and that carbonate crusts, as along the shelf, may be the source of the high backscatter features on the canyon floor. The path of the fault zone across the upper slope is marked by elongated tributary canyons with high backscatter floors and `U'-shaped cross-sectional profiles. Linear features and stepped scarps suggestive of recent crustal movement and mass-wasting, occur on the walls and floors of these canyons. Three magnitude-4 earthquakes have occurred within the last 30 years in the vicinity of the canyons that may have contributed to the observed features. As shown by others, motion along the fault zone has juxtaposed diverse lithologies that outcrop on the canyon walls. Gully morphology and the canyon's drainage patterns have been influenced by the substrate into which the gullies have formed.

Side-scan sonar and seismic data from the hitherto little-known middle to lower reaches of the giant, north-south–extending Northwest Atlantic Mid-Ocean Channel (NAMOC) between lat 53° and 44°30′N reveal a low- to medium-sinuosity and partly straight channel, the course of which is increasingly basement-controlled toward south. Flow restriction by seamounts locally narrowing the channel in the south caused talweg incision by as much as 80 m below the channel floor. Asymmetric levees, resulting from southward-flowing turbidity currents spilling over the channel walls by a mechanism of continuous flow stripping, are remarkably consistent; there is a 30 to 60 m higher western levee, reflecting the effectiveness of the Coriolis force at mid-latitudes. The levees rise 10 to 30 m above the adjacent ocean floor. Consistently lower seismic penetration and more closely spaced reflectors on 3.5 kHz profiles and higher reflectivity on sleeve-gun profiles indicate a higher proportion of sand in the eastern levee. Wash-over fans on the levees formed at sharp channel bends, where deeper, coarser portions of the flows spilled out. Major portions of the turbidity currents that mostly originated off Hudson Strait, however, passed this intermediate channel segment without sedimentation. The down-channel decrease in the difference in height between the western and eastern levees, channel cross-sectional area, and levee height indicate a possible influence of the decrease in the effect of the Coriolis force, flow volume, and/or current speed; separation of the effects of individual variables, however, is currently not possible.

During latest Quaternary glaciation, the Black Sea became a giant freshwater lake. The surface of this lake drew down to levels more than 100 m below its outlet. When the Mediterranean rose to the Bosporus sill at 7,150 yr BP 1 , saltwater poured through this spillway to refill the lake and submerge, catastrophically, more than 100,000 km2 of its exposed continental shelf. The permanent drowning of a vast terrestrial landscape may possibly have accelerated the dispersal of early neolithic foragers and farmers into the interior of Europe at that time.

High-resolution mapping and reflection profiling reveal a depositional fan on the Black Sea SW shelf fed from the Strait of Istanbul (Bosporus). The fan is constructed with an initial deposit of pebbles mixed with glacial and post-glacial shell debris. The pebbles are identical in their composition to quartzite and gabbro recovered in drill cores from the Bosporus Strait. Directly above the pebble layer are mollusks and foraminifera of Mediterranean provenance dated at 6.9 ka bp (uncorrected). Synchronicity between the onset of fan construction and arrival of Mediterranean fauna suggests an origin linked to the connection of the Black Sea’s lake with the global ocean. The volume of the chaotic interior of the fan is comparable in magnitude to the volume excavated from the floor of the Bosporus Strait. We propose that when the exterior ocean breached the sill of this inlet, it transformed into an outburst of saltwater that gained energy as it enlarged the inlet. Torrents stripped the glacial and post-glacial covering from its pathway and scattered entrained debris in sheets and mounds as far away as the edge of the shelf. Even in areas where the pre-existing cover survived, its eroded surface attests to the passage of flooding water.

Drainage-extraction algorithms traditionally used for extracting river networks and watersheds from gridded land topography are applied to gridded multibeam bathymetry of the mid-California margin. The algorithms are used to automatically map two regional tributary networks of submarine canyons and deepsea channels draining Monterey Bay, the principal conduits of which are Acension and Monterey Canyons. The algorithms reliably map subaqueous drainage areas, but are prone to error in mapping the extent of submarine canyon and channel thalwegs due to operator subjectivity and algorithm limitations. A geomorphic comparison of the Acension and Monterey Canyon networks, with 12 river networks in the continental U.S., illustrates both the potential and weaknesses of using drainage extraction algorithms to analyze sediment pathways in gridded bathymetry.

Dual-frequency side-looking sonars have the potential to be used as remote sensing tools to characterize subaqueous terrains. In one case study of the carbonate-ooze-coated Blake Plateau off-shore of Georgia, U.S.A., the difference in acoustic attenuation for 50 and 20 mm wavelengths (30 and 72 kHz frequency) permits the discrimination of sub-bottom scatterers from seabed surface textural features to reveal patchy regions where a buried hard ground had been pock-marked by karst-like depressions. In a second study of the Upper Hudson River in New York, U.S.A., related to environmental contaminates, the backscatter response at 15 and 3 mm acoustic wavelengths (100 and 500 kHz frequency) serves as a useful proxy for sediment grain size with coarser detritus distinguished from finer sediments. Sand and gravel regions inferred from the backscatter were confirmed by ground truth sampling.

We present the results of a statistical study on the morphological characteristics of abyssal hills recently mapped along the two adjacent segments of the Pacific-Antarctic Ridge at 65(deg)S.

International audience; This paper presents geophysical and core data obtained from several marine geology surveys carried out in the western Black Sea. These data provide a solid record of water-level fluctuation during the Last Glacial Maximum in the Black Sea. A Last Glacial Maximum lowstand wedge evidenced at the shelf edge in Romania, Bulgaria, and Turkey represents the starting point of this record. Then, a first transgressive system is identified as the Danube prodelta built under ~40 m of water depth. The related rise in water level is interpreted to have been caused by an increase in water provided to the Black Sea by the melting of the ice after 18,000 yr B.P., drained by the largest European rivers (Danube, Dnieper, Dniester). Subsequently, the Black Sea lacustrine shelf deposits formed a significant basinward-prograding wedge system, interpreted as forced regression system tracts. On top of these prograding sequences, there is a set of sand dunes that delineates a wave-cut terrace-like feature around the isobath −100 m. The upper part of the last prograding sequence is incised by anastomosed channels that end in the Danube (Viteaz) canyon, which are also built on the lacustrine prograding wedge. Overlying this succession, there is a shelfwide unconformity visible in very high-resolution seismic-reflection profiles and present all over the shelf. A uniform drape of marine sediment above the unconformity is present all over the continental shelf with practically the same thickness over nearby elevations and depressions. This mud drape represents the last stage of the Black Sea water-level fluctuation and is set after the reconnection of this basin with the Mediterranean Sea.

A study of the Monterey Canyon and fan was conducted to investigate contemporary sedimentary activity using a camera sled, Alvin dives and Sea-Beam bathymetry. Physical characteristics and plan view morphology of the walls, terraces, thalweg floor and levees of the Monterey Canyon and fan, as well as the gullied and non-gullied regions of the adjacent continental slope have been studied. The canyon floor and fan valley thalweg channel from 2900 to 3500 m exhibited properties indicative of moderate to low energy conditions (weak currents, smoothed sea-bed, deposits of disintegrated kelp, bioturbated mud substrate, fecal pellets, absence of scour). Chemosynthetic communities consisting of clams and a pogonophoran, were found in the fan valley from 3000 to 3600 m. Local areas of rockfalls and slumps from the canyon and fan valley walls were not fresh (sedimented, abundantly colonized by benthic biota). Terraces and levee crests were mud-draped and bedrock exposures on canyon walls were encrusted by benthic organisms. Freshest disturbances were found in gullies on the adjacent continental slope where bedrock was scoured clean of sediment and loose debris. A major submarine slide detached from escarpments on the lower slope, and extends across the fan. The slide surface was mud-draped, hummocky, and contained bedforms with wavelengths up to 150 m and up to 10 m relief. The survey was conducted shortly before the Loma Prieta earthquake (October 17, 1989) that caused substantial ground motion in on-shore regions of Monterey Bay. Any substantial sub-sea disturbances generated by the ensuing earthquake should be discernable from the pre-earthquake state by comparison with the deep-sea photographs and observations.

Alvin dives and camera tows within the “meander area” of the Monterey and Ascension Fan Valleys have located nine chemosynthetic communities over depths ranging from 3000 to 3600 m over a distance of 55 km. Most of the observed communities consist largely of Calyptogena phaseoliformis, but Solemya (species unknown) and a pogonophoran (genus Polybrachia), have also been identified. The δ13C values (-35.0 to -33.6 per mil) and the presence of APS reductase and ATP sulfurylase in the C. phaseoliformis tissue is consistent with sulfur chemoautotrophy. Two reduced organic matter sources for the H2S are proposed: (1) older beds exposed by the deep erosion (up to 400 m) of the fan valleys and (2) concentrations of anaerobically decomposd organic matter buried in the valley floor.

Due to the increasing demand for clean water, recreation areas, and healthy ecosystems the management of watersheds has become an issue of increasing importance. This demand has increased the need for detailed inventories of the present state of watersheds and the understanding of related processes. In 1996, the New York State Department of Environmental Conservation (DEC) initiated an effort to map the benthic habitat of the Hudson River Estuary as part of a larger Hudson River Action Plan. This project includes extensive mapping using sidescan sonar, subbottom profiling, single and multi-beam bathymetry, as well as collecting ground truth data with sediment cores, grab samples, and sediment profiling imagery (SPI). The goal of the project is the creation of a comprehensive data set that includes detailed interpretive maps of sediment distribution, grain size, bed forms, and benthic habitats (Ladd et al., 2002).

The Antarctic Multibeam Bathymetry and Geophysical Data Synthesis (AMBS) is a web-accessible data resource for marine geoscience research in the Southern Ocean. The primary focus is to preserve and provide public access to multibeam bathymetry acquired during expeditions of research vessels supported by the U.S. National Science Foundation. Since its inception in 2003, our primary goal has been to facilitate visualization and exploration of the subsea landscape to the full detail of the original data by both specialists and non-specialists. Visualization across a widerange of map scales at high latitudes is made possible by dynamic access to a gridded synthesis in both Polar and Mercator projections. A second goal is to support multi-disciplinary research needs by offering data discovery and visualization of numerous complementary geoscience datasets. In this report, we describe the design objectives and architecture of the AMBS, as well as recent developments regarding data submission and delivery via Web Services. Citation: Carbotte, S.M., W.B.F. Ryan, S. O’Hara, R. Arko, A. Goodwillie, A. Melkonian, R.A. Weissel, and V.L. Ferrini (2007), Antarctic Multibeam Bathymetry and Geophysical Data Synthesis: An On-Line Digital Data Resource for Marine Geoscience Research in the Southern Ocean, in Antarctica: A Keystone in a Changing WorldOnline Proceedings of the 10 ISAES, edited by A.K. Cooper and C.R.Raymond et al., USGS OpenFile Report 2007-1047, Short Research Paper 002,4 p.; doi:10.3133/of2007-1047.srp002.

Exploration of the Black Sea shelf reveals two major shelf-crossing unconformities. The older unconformity separates mostly-barren deposits of late glacial age from overlying Neoeuxinian sediment containing fresh to brackish fauna. This unconformity can be traced over the shelf edge to depths beyond –140 m. The substrate below is dry, firm, and contains unchallenged evidence of subaerial exposure at least to depths of –110 m. The Neoeuxinian cover is present on the outer shelf and is preserved, though incompletely, in depressions on the middle and inner shelf. It is even found as subsurface valley fill in the coastal limans and Sea of Azov. The Neoeuxinian on the shelf represents a transgression leading to a highstand at ~ –20 m below today’s sea surface, which was reached by 10,000 BP (uncorrected). Sediments with marine fauna lie above the Neoeuxinian and are separated from it by a sand to gravel layer that represents a younger unconformity. In the limans, the hiatus between the Neoeuxinian and overlying Bugazian is called “peririf” and on the shelf a “washout.” Dune fields between –65 and –80 m and wave-truncated terraces with beach-like berms at –90 to –100 m contain shell material dated between 9500 and 8500 BP, suggesting that the younger unconformity represents a post-Younger Dryas regression that took the surface of the Black Sea’s lake below the level of the global ocean. Strontium isotopes document the first arrival of saltwater at 8400 BP. Objections to the rapid flooding hypothesis in which Mediterranean water initially poured into a low-lying enclosed lake are centered on the interpretation of the younger unconformity as evidence of either (1) subaerial erosion (and thus a major early Holocene regression) or (2) underwater erosion that does not require a regression. When examined, specific criticisms appear to be based on different interpretations of observations but do not as yet present a concrete refutation of a lowstand of the lake prior to the Mediterranean connection. The flooding hypothesis is today just as vulnerable as when it was first formulated. It serves to best account for the ubiquitous nature of the younger unconformity that not only appears in sediment cores but is also widely mapped by high-resolution reflection profiling. Greater attention needs to be paid in the future to a more comprehensive investigation to find the cause of the younger unconformity.

[1] The catastrophic rupture of the North Anatolian Fault east of the Marmara Sea on 17 August 1999 highlighted a need for mapping the underwater extension of that continental transform. A new bathymetric map of Izmit Gulf indicates that the fault follows the axis of the gulf with a few minor bends. Submerged shorelines and shelf breaks that formed during the Last Glacial Maximum provide markers to quantify vertical deformation. Variable tilting of these horizons reveals that vertical deformation is highest just south of the fault. A correlation between vertical deformation of the southern fault block and distance to fault bends can be accounted for by a fault dipping steeply to the south. Hence subsidence (uplift) of the southern, hanging wall block would be expected where the fault strikes at a slightly transtensional (transpressional) orientation to relative plate motion. Subsidence reaches about 8 mm/yr west of the town of Golcuk and might be accommodated in 1–2 m subsidence events during large earthquakes. That scenario is compatible with the tsunami runups and the coseismic subsidence of the southern shore that occurred in 1999. Seafloor morphology also suggests that earthquakes are accompanied by widespread gas and fluid release. The periphery of the deepest basin displays a hummocky texture diagnostic of sediment fluidization, and mud volcanoes occur west of Hersek peninsula that might be activated by earthquakes. Finally, the backscatter imagery reveals a series of lineaments midway through the gulf that are interpreted as products of the 1999 surface rupture. The seafloor is undisturbed farther west, suggesting that surface slip decreased to an insignificant level beyond Hersek. Possibly, the stress shadow from the 10 July 1894 earthquake, which was felt strongly along the western Izmit Gulf, contributed to arrest the 1999 surface rupture.

Rivers and estuaries around the world are the focus of human settlements and activities. Needs for clean water, ecosystem preservation, commercial navigation, industrial development, and recreational access compete for the use of estuaries, and management of these resources requires a detailed understanding of estuarine morphology and sediment dynamics. This article presents an overview of the first estuary-wide study of a heavily used estuary the Hudson River, based on high-resolution acoustic mapping of the river bottom. The integration of three high-resolution acoustic methods with extensive sampling reveals an unexpected complexity of bottom features and allows detailed classification of the benthic environment in terms of riverbed morphology, sediment type, and sedimentary processes.

In contrast to the much-studied onshore and deep offshore post-Messinian sedimentary history of the Gulf of Lion, the continental shelf had been poorly explored until recently. New seismic data, acquired by ELF Oil Company on the Languedoc-Roussillon shelf (Western Mediterranean Sea), from Cap Creus in the SW to Cap d'Agde in the NE, together with data from previously drilled exploratory wells, allow us to propose a scenario for margin reconstruction following the Messinian Salinity Crisis. The seismic data display a complex pattern of prograding clinoforms and buried submarine canyons. Following the crisis that eroded the previous margin, clinoforms developed as the new Pliocene margin prograded. Three major periods characterize the evolution of the post-Messinian margin. During the first period, after the filling of the Early Pliocene rias which are now above sea level, the prograding sedimentary prisms rapidly migrated seaward and filled the underlying Messinian topographic lows. The second period consists of a transitional interval, which began with a pronounced fall in sea level that probably corresponded with the end of the Lower Pliocene. Deposits were disturbed by large slumped structures. The third period is characterized by the appearance and development of submarine canyons near the subsequent shelf edges, maybe as a result of the increased glaciations and related sea-level changes. From that time onward, most of the Late Pliocene and Quaternary sediments were directly transferred down to the deep basin.

fissure eruption at 1728 0 S. Similar lows are observed at three other drained lava lake troughs, including one which is at least 1800 years old, residing 400 m away from the present-day axis. We attribute these lows to the presence of shallow dike swarms. The degree to which other geologic features may contribute to the lows is constrained using geologic, geophysical, and geochemical observations and forward modeling. Compositional analyses of Alvin samples at 1728 0 S do not support Fe or Ti variations as a primary source. Hypotheses requiring hydrothermal alteration and porosity variations are both inconsistent with geologic observations and near-bottom gravity data analysis from similar areas. Previous mappings between paleointensity variations and the observed magnetic field over distances of several kilometers from the axis suggest that such variations do not create the field low. The dominant source of the magnetization low is most likely the presence of a 100–200 m wide region of shallow dikes which are poorly magnetized relative to extrusives, or a region heated above magnetic blocking or Curie temperatures by intrusions during the most recent eruption (though the latter interpretation cannot explain the low at the fossil trough). In the first case, this extrusive thinning implies a change in eruptive behavior over the last 750–1500 years given the local spreading rate. For the latter case, thermal models suggest the anomaly had to have been created by a dike swarms totaling at least 45 m width during the most recent eruption(s), corresponding to � 300 years of plate spreading. Models indicate that the source of the low is centered slightly east of the axial trough. This offset suggests that the axis has been progressively migrating westward over the past millennium, consistent with other studies covering greater length and timescales. Westward migration provides an explanation for the preferential emplacement of recent lavas flows west of the axis, evident in ABE bathymetry and submersible observations. INDEX TERMS: 3035 Marine Geology and Geophysics: Midocean ridge processes; 1517 Geomagnetism and Paleomagnetism: Magnetic anomaly modeling; 8499 Volcanology: General or miscellaneous; 3045 Marine Geology and Geophysics: Seafloor morphology and bottom photography; 3005 Marine Geology and Geophysics: Geomagnetism (1550)

New cores from the upper continental slope off Romania in the western Black Sea provide a continuous, high-resolution record of sedimentation rates, clay mineralogy, calcium carbonate content, and stable isotopes of oxygen and carbon over the last 20000 yr in the western Black Sea. These records all indicate major changes occurring at 15 000, 12 800, 8400, and 7 100 yr before present. These results are interpreted to reflect an evolving balance between water supplied by melting glacial ice and other river runoff and water removed by evaporation and outflow. The marked retreat of the Fennoscandian and Alpine ice between 15 000 and 14 000 yr is recorded by an increase in clays indicative of northern provenance in Black Sea sediments. A short return toward glacial values in all the measured series occurs during the Younger Dryas cold period. The timing of the first marine inflow to the Black Sea is dependent on the sill depths of the Bosporus and Dardanelles channels. The depth of the latter is known to be - 80 +/- 5 m, which is consistent with first evidence of marine inundation in the Sea of Marmara around 12 000 yr. The bedrock gorge of the Bosporus reaches depths in excess of -100 m (relative to present sea level), though it is now filled with sediments to depths as shallow as -32 m. Two scenarios are developed for the connection of the Black Sea with the Sea of Marmara. One is based on a deep Bosporus sill depth (effectively equivalent to the Dardanelles), and the other is based on a shallow Bosporus sill (less than -35 m). In the deep sill scenario the Black Sea's surface rises in tandem with the Sea of Marmara once the latter connected with the Aegean Sea, and Black Sea outflow remains continuous with inflowing marine water gradually displacing the freshwater in the deep basin. The increase in the delta(18)O of mollusk shells at 12 800 yr and the simultaneous appearance of inorganic calcite with low delta(18)O is compatible with such an early marine water influx causing periodic weak stratification of the water column. In the shallow sill scenario the Black Sea level is decoupled from world sea level and experiences rise and fall depending on the regional water budget until water from the rising Sea of Marmara breaches the shallow sill. In this case the oxygen isotope trend and the inorganic calcite precipitation is caused by increased evaporation in the basin, and the other changes in sediment properties reflect climate-driven river runoff variations within the Black Sea watershed. The presence of saline ponds on the Black Sea shelf circa 9600 yr support such evaporative draw-down, but a sensitive geochemical indicator of marine water, one that is not subject to temperature, salinity, or biological fractionation, is required to resolve whether the sill was deep or shallow.

The continental Pleistocene Laurentide Ice Sheet (LIS) had far-reaching marine influence in shaping the ocean-floor adjacent to ice margin. The basinwide submarine-canyon and deep-sea channel system of the Northwest Atlantic Mid-Ocean Channel (NAMOC) of the Labrador Sea is the submarine continuation of the drainge system of the LIS on land, forming an interconnected land/sea drainage system 6,000 km long, one of the word’s longest drainage systems of Pleistocene age. The submarine portion forms a dual system, consisting of the mud-dominated NAMOC with its tributaries and a submarine sandy braid-plain.

A deep-towed, side-looking sonar survey of an outer fan, distributary channel near DSDP Sites 614 and 615 on the Mississippi Fan defined three distinct morphologic zones: (1) a relatively “straight” channel zone (sinuosity of 1.1), (2) a channel termination zone, and (3) a splay and “finger-shaped distributary” zone. Boundaries between zones are gradual with no direct connection observed between this channel and the leveed channel that was mapped and drilled farther upfan during DSDP Leg 96.

A synthesis is presented of published statistics on the lengths, scarp heights, and spacing of normal faults on the flanks of the East Pacific Rise (EPR) between 19°S and 21°N. Nine data sets are included which were collected at different times employing one of four different sonar mapping systems (the Scripps deep-tow instrument, SeaMARC I, SeaMARC II, and GLORIA) with widely ranging data collection capabilities. The EPR in this region is morphologically quite similar, and in some cases the data sets were obtained from the same area of the seafloor. It is assumed, therefore, that data collection procedure and system resolution have had a greater influence on the fault statistics observed than varying tectonic processes. We try to account for these sampling effects such that quantitative comparisons of fault development at midocean ridges may then be made. We discuss the problem of identifying faults in sonar images and the errors associated with measurements of the relevant parameters. We also investigate the statistical properties of the fault populations and derive expressions for correcting observed values of the population means at finite resolution to obtain the corresponding values at infinite resolution. The theoretical predictions compare well with the results of a decimation procedure applied to real data sets to simulate diminishing resolution. We also discuss the effects that surveying geometry can have on the interpretation of fault lengths. The data synthesis is augmented by an analysis of the correlation between scarp height and fault length for areas near 3.5°S and 12°N on the EPR. The correlation, which we find to be linear, is used to estimate (1) the minimum operational fault target size for each of the four mapping systems, and (2) the topography due to faulting in an area where only fault lengths are known.

SeaMARC II backscatter data for the East Pacific Rise are used to create structural maps of ridge-parallel fabric on the crest and flanks of the rise from 12°50′N to 15°10′N. The structural data are statistically analyzed to describe the spacing, density, azimuth, facing direction, and length of faults. Results of the statistical studies are compared with predictions for the width of the zone of active fault formation, models for the generation of abyssal hills, plate kinematic predictions, models of along-strike segmentation, and observations of the asymmetric subsidence of oceanic crust on the Cocos and Pacific plates. Comparisons of the number of faults with distance from the rise crest and examination of the stratigraphic relationship between seamounts and the ridge-parallel tectonic fabric illustrate that the zone where new faults are created is located within a few kilometers of the ridge crest. Fault density data reveal that fault distributions do not resemble periodic processes. Examination of fault density and spacing data along-strike indicates that there is a high probability that the mean number of faults per crustal block differs significantly from north to south. Changes in along-strike statistics correlate well with the occurrence of three overlapping spreading centers located within the survey area. Additionally, fault spacing s determined from SeaMARC I and SeaMARC II backscatter imagery demonstrate that quantitative analyses of seafloor fabric are dependent upon the instrument used to collect the data. Analysis of inward and outward facing faults indicates that especially in the northern portion of the survey area, half-graben models are better predictors of abyssal hill morphology than full-graben models. Within the survey area, inward facing faults are more abundant and affect more of the oceanic crust than outward facing faults. Although the Cocos and Pacific plates subside at different rates, this asymmetry is not reflected in the tectonic component of morphology. Fault azimuth is observed to vary as a function of crustal age. Although the overall trend of the change in azimuthal values agrees with the trend predicted by relative poles of opening for the Pacific and Cocos plates, the variability in azimuthal data suggests that other processes contribute to the orientation of lineations formed near the axis of the East Pacific Rise. Finally, the statistical analyses demonstrate that all fault parameters are better correlated about the ridge axis than along-strike of the axis. The differences between the northern and southern portions of the survey area are reflected in the SeaMARC II bathymetric data which depict a continuous narrow ridge crest in the southern region, and an irregularly shaped ridge crest in the northern region that shoals and deepens every 30 to 40 km. This along-strike variability, observed over distances of less than 100 km, suggests that large-scale plate stresses are not the only processes responsible for generating the tectonic fabric observed on the flanks of the East Pacific Rise.

Sonar reflectance is used to quantify the relative ages of young volcanic terrains on a seamount chain and a nearby spreading center in the north equatorial Pacific. The reflectance is a measure both of the burial of lava by pelagic sedimentation and of the microrelief of the lava. The extent of sediment cover and the lava type distribution are calibrated with thousands of bottom photographs. The relative ages indicate that the locus of volcanic eruptions has migrated toward the East Pacific Rise axis. Zero-age activity lies just to the west of the East Pacific Rise south of the Clipperton Transform and is probably the cause of an anomalously shallow neovolcanic zone. The evidence fits a scenario in which eruptive centers appear, evolve, and become extinct in succession, with little or no overlap in time. The eruptive centers are fed in sequence through conduits from buoyant, tightly focused magma plumes. The near identical heights of many of the centers at the time of their latest eruptions suggest that the seamounts tap the same mantle depth range. Broad summit plateaus with calderas are formed if the plume volume is such that the hydraulic head is reached and magma is trapped within the volcano in ephemeral magma chambers or conduits. If the plume volume is small and the hydraulic head is not reached, the seamount remains small with only a small crater. The plumes ascend asynchronously from a common region of nucleation in the asthenosphere. Plumes that follow in close succession build a composite seamount with a string of summit calderas. Plumes that rise less often result in a line of discrete circular volcanoes.

A northwest trending group of seamounts located just west of the East Pacific Rise (EPR) axis has been mapped using high-resolution Sea MARC I side-scan sonar and Sea Beam multibeam bathymetry. These seamounts have 9–13 km basal diameters, and the shapes of the volcanos change from conical domes near the EPR axis to truncated cones and larger, elliptical truncated cones farther away from the rise axis. The detailed resolution of constructional volcanic features on these seamounts and the progressive development of summit depressions (craters to large complex calderas) and summit plateaus suggest to us a succession of evolutionary stages through which seamounts grow. Our evolutionary scheme for constructing these volcanos calls for the development of satellite extrusive centers around the base of a growing seamount which serve to drain off magmas and precipitate summit collapse. The creation of a summit depression on a seamount will structurally constrain the style of eruptive volcanism to be principally along ring fractures which localize vents around the margin of a crater or caldera. The concentration and channelization of volcanic eruptions around the top of the volcano lead to a progradation of the summit area and the establishment of a summit plateau. The steep sides of a seamount reflect growth of the summit area out over the upper flanks of the volcano caused by continued eruption of lavas around the margin of the caldera. The steep constructional slopes are sites for vigorous mass wasting that shapes and modifies flank terrain. As a seamount evolves, the crestal topography is the product of many volcanic cycles characterized by filling and subsequent draining of the caldera and expansion of caldera area through progressive collapse and migration of primary magmatic conduits. The final phase of volcanism may be a calderafilling episode that builds a rough conical mount over the summit plateau.

8 pages, 6 figures, The Valencia Fan developed as the distal fill of a deep-sea valley, detached from the continental slope and the main sedimentary source. A survey of side-scan sonar, Sea Beam and reflection seismics shows that the sediment is largely fed through the Valencia Valley. The upper fan comprises large channels with low-relief levees, and the middle fan has sinuous distributary channels. Depositional bedforms predominate on the valley floor and levees, and erosional bedforms are common in the valley walls. A change to slope on the fan apex and the presence of volcanoes on the upper fan are the main factors influencing fan-growth pattern. © 1985 Springer-Verlag New York Inc.

The deep-basin desiccation model for the origin of Messinian-age Mediterranean evaporites implies two basic assumptions: 1. (1) The evaporites were in part laid down under shallow-water conditions, with intermittent subaerial episodes brought about by evaporitic drawdown. 2. (2) Some of the depressions in which the evaporites were deposited were already several kilometers deep during Messinian time, and hence, the general configuration of these basins predated, and was essentially independent of the salinity crisis. The first assumption has been more or less accepted by the scientific community as a consequence of the discovery of supratidal to intertidal features in the evaporites, whereas there has been less acceptance of the second assumption. The arguments on which the second assumption was first formulated, were multiple and included: (a) the presence in seismic reflection profiles of large pre-salinity-crisis depocenters with sediment-distribution patterns not substantially different from those of the post-salinity-crisis strata; (b) systematic changes in the evaporitic facies along basin-slope transects indicative of existing paleorelief; and (c) the occurrence of continental-rise and abyssal-plain-type deposits in both pre- and post-salinity-crisis successions. New attention is herein focused on Messinian-age erosional surfaces created by evaporitic drawdown of an isolated Mediterranean Sea. These erosional surfaces are detected as discordances in seismic reflection profiles. The discordances can be traced from the subsurface of the present coastal plains and continental shelves to the subsurface of the modern abyssal plains and show entrenchment into the underlying strata. On the passive-type continental margins, like those of the Balearic Sea and those in the southeastern corner of the Levantine Sea, the seaward gradients of the erosion surfaces, corrected for post-Messinian isostatic sediment loading and compaction and adjusted for regional subsidence, permit the calculation of a former relief between the pre-salinity-crisis shore-line and basin center of more than 2.5 km for the western Mediterranean and more than 3.0 km for the eastern Mediterranean.

The morphology of a seamount in the Valencia Trough of the Western Mediterranean has been investigated with side-looking sonar (Sea MARC I) and swath-mapping sonar (Sea Beam). The seamount is characterized by deep reentrants on its flanks, radiating spurs and a broad summit plateau. Presently its summit lies 1020 m below sea level. Avalanche scars and debris flow deposits are detected on re-entrant floors and slopes. Fragments of aphanitic basalt and andesitic tuff, sampled at DSDP Site 122 attest to the volcanic origin of the Valencia Seamount. First constructed as a subsea volcano about 20–25 m.y. ago, its flanks became almost entirely emerged 6 m.y. ago at the time of the evaporitic drawdown of the Mediterranean. Valencia Seamount has some features in common with young and old seamounts on the floor of the Pacific Ocean that have never emerged above sea level and other features which may be attributed to subaerial exposure. Two working hypotheses, erosional and constructional, are presented to explain the observed morphology. The erosional hypothesis better accounts for the known paleogeographic and paleoceanographic history of the Mediterranean. The mapped spurs are interpreted as flank rift zones which give many seamounts a starfish-like appearance. Fountain eruptions of hyaloclastites from vents could explain the broadly arched summit plateau. Peaks might be secondary parasitic cones or remanent erosional features of stocks and dykes which are more resistent to mass-wasting than an extrusive cover of vesicular lava flows and deposits of tuff and ash. Gullies are most abundant on the middle and lower flanks. Whereas the gullies might be breached calderas, it is more reasonable to expect that they are dominantly subaerial erosional in origin, considering the emergence history of this particular seamount.

Substantial subaerial denudation of the North African continental margin took place during the Late Miocene salinity crisis. The Nile drainage was deeply entrenched as far as 1200 km inland, upto the Aswan cataract. Beneath the modern birdfoot delta the thalwegs of ancient Messinian river valleys lie as much as 2.5 km below present sea level. The relief between the top of the incised Late Miocene coastal plain and the axis of excavated channels exceeds 1.2 km. In the subsurface of the lower reaches of the modern Nile Cone a network of bifurcating entrenched channels descends to 4.6 km below sea level to become covered by the southern limit of the Messinian Evaporite Formation. A paleodrainage system is also present beneath the Levantine margin. Seismic reflection profiles document an extremely rough and steeply gullied morphology which is analogous to the badlands of South Dakota. In addition, reflection profiles reveal uniform and gentle seaward gradients of the preerosion (Jurassic to Late Miocene) and posterosion (Pliocene and Quaternary) sedimentary successions beneath the coastal plain and continental shelf. The closely comformable nature of all these depositional surfaces strongly indicates that the badland topography does not owe its origin to an epiorogenic uplift and tilting of the continental edge, but, instead, to a lowering of base level brought about by an unprecedented eustatic drop in water level within isolated Mediterranean basins. Calculations which consider the progressive isostatic loading of both the onshore and offshore sedimentary prism indicate a fall of the base level of at least 3.5 km. This accounts for the observed depth in the Levantine Basin at which Messinian-age shallow-water evaporites lap against the contemporaneous North African subaerial erosional surface.

Episodes of basin-wide abyssal stagnation have occurred in the Mediterranean Sea during the “glacial” Pleistocene and on a much larger scale in the Atlantic and Indian Oceans during the Cretaceous Period. The sedimentary products of euxinification are organic-rich sapropels which have accumulated an order of magnitude more carbon during the Cretaceous than that which is present in all the known world reserves of coal and petroleum. The storage in the Cretaceous strata of excess carbon and sulfur in the form of fossilized photosynthetic substances and pyrite is thought to have led to a significant global increase in atmospheric oxygen and to an unusual sequence of calcium-rich evaporitic salts in the South Atlantic during the Aptian. Stagnant episodes in the Mediterranean and Atlantic effectively destroyed all benthic life formerly existing on substratums underlying hydrogen-sulfide bearing anoxic bottom waters. Locally thin organic-rich strata on topographic highs near the paleo-equator of the Pacific Ocean during the Cretaceous owe their origin to an intermittently expanding oxygen-minimum zone as contrasted to total euxinification within the deep basins of the Atlantic and Indian Oceans. In the context of both land and sea areas, the late Mesozoic may have conceivably been the major “carboniferous” period of the earth since the end of the Precambrian.

High-resolution Sea Beam bathymetry and Sea MARC I side scan sonar data have been obtained in the MARK area, a 100-km-long portion of the Mid-Atlantic Ridge rift valley south of the Kane Fracture Zone. These data reveal a surprisingly complex rift valley structure that is composed of two distinct spreading cells which overlap to create a small, zero-offset transform or discordant zone. The northern spreading cell consists of a magmatically robust, active ridge segment 40–50 km in length that extends from the eastern Kane ridge-transform intersection south to about 23°12′ N. The rift valley in this area is dominated by a large constructional volcanic ridge that creates 200–500 m of relief and is associated with high-temperature hydrothermal activity. The southern spreading cell is characterized by a NNE-trending band of small (50–200 m high), conical volcanos that are built upon relatively old, fissured and sediment-covered lavas, and which in some cases are themselves fissured and faulted. This cell appears to be in a predominantly extensional phase with only small, isolated eruptions. These two spreading cells overlap in an anomalous zone between 23°05′ N and 23°17′ N that lacks a well-developed rift valley or neovolcanic zone, and may represent a slow-spreading ridge analogue to the overlapping spreading centers found at the East Pacific Rise. Despite the complexity of the MARK area, volcanic and tectonic activity appears to be confined to the 10–17 km wide rift valley floor. Block faulting along near-vertical, small-offset normal faults, accompanied by minor amounts of back-tilting (generally less than 5°), begins within a few km of the ridge axis and is largely completed by the time the crust is transported up into the rift valley walls. Features that appear to be constructional volcanic ridges formed in the median valley are preserved largely intact in the rift mountains. Mass-wasting and gullying of scarp faces, and sedimentation which buries low-relief seafloor features, are the major geological processes occurring outside of the rift valley. The morphological and structural heterogeneity within the MARK rift valley and in the flanking rift mountains documented in this study are largely the product of two spreading cells that evolve independently to the interplay between extensional tectonism and episodic variations in magma production rates.