Angelo Camerlenghi

The International Bathymetric Chart of the Arctic Ocean (IBCAO) Version 3.0

The International Bathymetric Chart of the Arctic Ocean (IBCAO) released its first gridded bathymetric compilation in 1999. The IBCAO bathymetric portrayals have since supported a wide range of Arc ...

Geological evidence for mud diapirism on the Mediterranean Ridge accretionary complex

Abstract A mud breccia has been repeatedly cored from four mud diapir fields located on the crest of the Mediterranean Ridge. This breccia is composed of subrounded clasts supported by a clay-rich matrix and ranges in age from Cretaceous to early Miocene. Sedimentological analysis allows identification of a “primitive” intrusive and a “reworked” mud breccia, the latter being formed by mud extrusion and gravitational reworking of the former. Homogeneous mud, sorted by grain size, which occurs at the top of the diapirs has been interpreted as a lake deposit. Based on composition and carbon isotopic ratios, gases present within the diapiric material suggest a partial thermogenic origin, and gas escape structures have been identified in the mud breccia and host sediments. Carbonate crusts occurring in the host sediments are not related to bacterial oxidation of methane and consequent authigenic calcite precipitation. Their carbon and oxygen isotopic ratios reveal that they are characteristic of early deep-sea lithification in the Mediterranean Sea. The presence of a marker bed composed of Mn nodules and bacterial colonies, the mud lake deposits showing lower carbonate content than primitive mud breccia, and the gas escape structures suggest the presence of active fluid vents in the vicinity of the diapiric structures. Evidence of active mud diapirism can be found from about 300,000 yrs B.P. to the present.

Giant sediment drifts on the continental rise west of the Antarctic Peninsula

Multichannel seismic reflection profiles from the continental rise west of the Antarctic Peninsula between 63° and 69°S show the growth of eight very large mound-shaped sedimentary bodies. MCS profiles and long-range side-scan sonar (GLORIA) images show the sea floor between mounds is traversed by channels originating in a dendritic pattern near the base of the continental slope. The mounds are interpreted as sediment drifts, constructed mainly from the fine-grained components of turbidity currents originating on the continental slope, entrained in a nepheloid layer within the ambient southwesterly bottom currents and redeposited downcurrent.

Holocene history of the Larsen-A Ice Shelf constrained by geomagnetic paleointensity dating

A sedimentary record collected from beneath the former Lar- sen-A Ice Shelf reveals the Holocene history of the Larsen-A re- gion. The record begins with the transition from grounded ice to a floating ice shelf, completed by 10.7 6 0.5 ka, and ends with the modern recession. The record contains several late Holocene dia- tomaceous ooze layers that suggest proximity to productive open- water events. Radiocarbon ages obtained from these sediments were complicated by the presence of detrital and reworked carbon. We have eliminated these complications and constructed a chro- nology for the Larsen-A Ice Shelf history via tuning of the geo- magnetic field paleointensity record with a reference curve. This approach provides chronological control to sediment sequences that lack appropriate material for radiocarbon dating. Geomag- netic paleointensity features with wavelengths of 2-3 k.y. can be recognized and interhemispherically correlated, illustrating the po- tential to use geomagnetic paleointensity variations as a global cor- relation and dating tool at sub-Milankovitch time scales.

Geophysical evidences of mud diapirism on the Mediterranean Ridge Accretionary Complex

Mud volcanoes, mud cones, and mud ridges have been identified on the inner portion of the crestal area, and possibly on the inner escarpment, of the Mediterranean Ridge accretionary complex. Four areas containing one or more mud diapirs have been investigated through bathymetric profiling, single channel seismic reflection profiling, heat flow measurements, and coring. A sequence of events is identified in the evolution of the mud diapirs: initially the expulsion on the seafloor of gasrich mud produces a seafloor depression outlined in the seismic record by downward dip of the host sediment reflectors towards the mud conduit; subsequent eruptions of fluid mud may create a flat topped mud volcano with step-like profile; finally, the intrusion of viscous mud produces a mud cone.The origin of the diapirs is deep within the Mediterranean Ridge. Although a minimum depth of about 400 m below the seafloor has been computed from the hydrostatic balance between the diapiric sediments and the host sediments, a maximum depth, suggested by geometric considerations, ranges between 5.3 and 7 km. The presence of thermogenic gas in the diapiric sediments suggests a better constrained origin depth of at least 2.2 km.The heat flow measured within the Olimpi mud diapir field and along a transect orthogonal to the diapiric field is low, ranging between 16 ± 5 and 41 ± 6 mW m−2. Due to the presence of gas, the thermal conductivity of the diapiric sediments is lower than that of the host hemipelagic oozes (0.6–0.9 and 1.0–1.15 W m−1 K−1 respectively).We consider the distribution of mud diapirs to be controlled by the presence of tectonic features such as reverse faults or thrusts (inner escarpment) that develop where the thickness of the Late Miocene evaporites appears to be minimum. An upward migration through time of the position of the décollement within the stratigraphic column from the Upper Oligocene (diapiric sediments) to the Upper Miocene (present position) is identified.

Deep-sea tsunami deposits in the eastern Mediterranean: New evidence and depositional models

The tsunami wave induced by the collapse of the Santorini caldera after the Bronze age (Minoan) eruption (3500 BP) produced turbidites and large volume mega-turbidites in the abyssal plains of the Ionian Sea as well as on the floor of small basins of the Mediterranean and Calabrian Ridges, characterized by the so-called ‘Cobblestone Topography’. Since the first discovery in 1978, a Holocene mud layer which has been termed ‘homogenite’ and which typically shows a graded interval at its base, has been identified and correlated in over 50 deep-sea cores recovered in the eastern Mediterranean. Four types of ‘homogenite’ can be distinguished, each related to a particular depositional setting: 1. (a) ‘Closed Cobblestone’: these are from a few decimetres to several metres thick pelagic turbidites of local provenance, exclusively found at the bottom of small-sized ponded basins of the Mediterranean and Calabrian ridges. A debris flow may be present at the base of the turbidite where the vertical relief of the basins is over 200 m. 2. (b) ‘Abyssal Plain’: a 10 to 20-m thick megaturbidite recorded in the Ionian and Sirte abyssal plains. The source area is the African continental margin, possibly the continental shelf; it can be easily identified as a transparent acoustic layer that shows recent deformation across the deformation front of the Mediterranean Ridge accretionary prism. The volume of the Minoan ‘homogenite’ in the Ionian Abyssal Plain has been calculated at a minimum of 11 km3. 3. (c) ‘Open Cobblestone’: this type is exclusively found on the outer slope of the Mediterranean Ridge, near the deformation front. Unlike types (a) and (b), the base of the turbidite here is erosional instead of depositional. This ‘homogenite’ has been deposited in small basins, and occasionally on topographic highs of the Mediterranean Ridge by the up-slope flow of turbidity currents of African provenance that formed the abyssal plain deposit (type b). 4. (d) A fourth depositional model has been identified: homogenites present in deep anoxic basins of the Mediterranean Ridge. This setting is substantially similar to type (a), but the vertical relief of the basin is much higher (up to 800 m) and the deposition occurs in high-density anoxic brines which modify the settling rate and hence the resulting sedimentological characters.

GLACIAL-INTERGLACIAL DEPOSITION ON A SEDIMENT DRIFT ON THE PACIFIC MARGIN OF THE ANTARCTIC PENINSULA

On the continental rise west of the Antarctic Peninsula there are nine large mounds interpreted as sediment drifts, separated by turbidity current channels. Drift 7 is 150 km long, 70 km wide and up to 700 m high and is asymmetric, with steep sides on the south-east (towards the continent) and south-west, and gentle slopes to north-west and north-east. Cores on the gentle sides of the drift show a cyclicity between brown, bioturbated, diatom-bearing mud with foraminifera and radiolarians, and grey, laminated, barren mud. Biostratigraphic evidence is consistent with a Late Quatermary age. Detailed lithostratigraphy and magnetic susceptibility data allow precise correlation over distances of tens of kilometres. On the basis of chemostratigraphy, the brown sediment is interpreted as interglacial (isotope stages 1 and 5) and the grey as glacial (stages 2–4 and 6). Sedimentation rates are 3.0–5.5 cm ka-1. Cores on the steep sides of the drift recovered a condensed section with thinner cycles and hiatuses. Fine grain size, very poor sorting and the absence of a mode in the silt size range indicate deposition from suspension with only weak current activity. There is little evidence for cyclic changes in bottom current strength. Supply of sediment to the benthic nepheloid layer was by entrainment of mud from turbidity currents, and by setting of pelagic material (biogenic grains, IRD, sediment suspended in meltwater plumes). Cyclic changes in sediment supply include more biogenic supply in interglacials with less sea ice cover, more terrigenous supply from turbidites in glacials with ice sheets grounded to the shelf edge, and changes in IRD content

New findings of Bronze Age homogenites in the Ionian Sea: Geodynamic implications for the Mediterranean

Abstract Holocene homogenites similar in character and stratigraphic position to those previously recorded in the Ionian Basin, and related to the gigantic tsunami originating from Santorini (Bronze Age eruption) are described from a core transect crossing the deformation front of the Mediterranean Ridge east of Victor Hensen Seahill. Investigations on grain sizes and carbonate content carried out on three cores from three different areas of the Ionian Basin, encompassing the Bronze Age homogenite, revealed important similarities and some differences. The most obvious difference concerns the carbonate content, which is minimum at the base of the unit in a core close to the Messina Abyssal Plain, whereas it is maximum in cores from the Western Mediterranean and Calabrian Ridges, characterized by the typical “cobblestone topography”. A terrigenous versus pelagic nature of the components is documented, and a distal versus local provenance is inferred. Homogenites are critically compared with unifites, a type of re-sediment recently described from the eastern Mediterranean. Comparison includes in particular: (a) source and sediment transport; and (b) triggering mechanism. A substantial difference exists between the sedimentation patterns of the Ionian and Levantine Basins seaward of the Calabrian and Hellenic Arcs in the latest Quaternary, and the difference contains a clear geodynamic message: sedimentation rates increase from the Late Pleistocene to the Holocene in the Ionian Basin, whereas they decrease in the Levantine Basin. The glacio-eustatic signal is well expressed in the Levantine Basin, where the source areas are situated consistently in the south, or on a passive-type, aseismic continental margin (north African margin). In the Ionian Basin on the contrary the glacioeustatic signal is totally obscured by the strong tectonic signal from the active northern continental margin of the Mediterranean.

Margin architecture reveals the transition to the modern Antarctic ice sheet ca. 3 Ma

Seismic reflection data collected primarily on the continental slope of the Pacific margin of the Antarctic Peninsula after drilling Ocean Drilling Program (ODP) Leg 178 allow us to date a regional change in the style of margin accretion to ca. 3 Ma. Late Pliocene deep erosion of continental shelf and slope, which likely included the emplacement of a megadebris-flow deposit, was followed by prominent growth of steep, relatively stable continental slope prograding wedges, while the deep distal margin sedimentation rate was significantly reduced. A review of the available stratigraphy from Deep Sea Drilling Project–ODP drilling, and correlation with available seismic stratigraphic information, allowed us to recognize comparable changes in terms of age and trends in continental margin evolution in several places around Antarctica. We argue that this late Pliocene architectural change of the Antarctic margin reflects a change in the texture and water content of the sediment delivered by the Antarctic ice sheet following the transition from wet- to dry-based ice regimes. This circum-Antarctic change coincides with the late Pliocene (ca. 3 Ma) global cooling and is proposed as the marker event of the transition to the modern cold polar dry-based Antarctic ice sheet.

Mid-late Pleistocene glacimarine sedimentary processes of a high-latitude, deep-sea sediment drift (Antarctic Peninsula Pacific margin)

The effects of glaciation on sediment drifts is recognised from marked sedimentary facies variation in deep sea cores taken from the continental rise of the Antarctic Peninsula Pacific margin. Nineteen sediment cores were visually described, logged for magnetic susceptibility, and X-radiographed. About 1000 analyses were performed for grain size, clay minerals and biostratigraphy (foraminifera, nannofossils and diatoms). Four sediment types associated with distinct sedimentary processes are recognised based on textural/compositional analysis. (1) Hemipelagic mud forms the bulk of the interglacial sediment, and accumulated from the pelagic settling of bioclasts and ice-rafted/wind-transported detritus. (2) Terrigenous mud forms the bulk of the glacial sediment, and accumulated from a combination of sedimentary processes including turbidity currents, turbid plumes, and bottom current reworking of nepheloid layers. (3) Silty deposits occurring as laminated layers and lenses, represent the lateral spillout of low-density turbidity currents. (4) Lastly, glacial/interglacial gravelly mud layers derive from settling of ice-rafted detritus. Five depositional settings are interpreted within sediment Drift 7, each characterised by the dominance/interaction of one or several depositional processes. The repetitive succession of typical sedimentary facies is inferred to reflect a sequence of four climatic stages (glaciation, glacial, deglaciation, and interglacial), each one characterised by a distinctive clay mineral assemblage and bioclastic content. Variations in clay mineral assemblage within interglacial stage 5 (core SED-06) suggest minor colder climatic fluctuations, possibly correlatable with substages 5a to 5e.