Antarctic Peninsula Pacific Margin Research Articles

Contourite drifts and canyon-channel systems on the Northern Antarctic Peninsula Pacific margin

Contourite drifts, also known as sediment drifts, result from near-bottom currents usually flowing in deep water in response to thermohaline and wind-driven circulation (Faugeres & Stow 2008). They are present throughout the world’s oceans and can be found anywhere from the abyssal floor to outer shelf settings. Contourite drifts typically display elongate mounded shapes with sizes ranging from tens to hundreds of square kilometres. In high latitudes the development of contourite drifts is favoured by the formation of cold dense waters and the delivery of large volumes of sediment by fast-flowing ice streams on the outer shelf during glacial periods (McGinnis & Hayes 1995). Several large sedimentary mounds, interpreted as sediment drifts, are located along the Pacific margin of the Antarctic Peninsula upper continental rise (Fig. 1b). These mounds have been interpreted as contourite drifts produced by SW-flowing bottom currents redistributing the fine-grained component of channelized turbidity currents (Rebesco et al. 1996; Amblas et al. 2006). The sediment drifts contain climatic and oceanographic information valuable for the reconstruction of Neogene Antarctic …

Late Quaternary sediments from deep-sea sediment drifts on the Antarctic Peninsula Pacific margin: Climatic control on provenance of minerals

[1] We present results of detailed paleomagnetic investigations on deep-sea cores from sediment drifts located along the Pacific continental margin of the Antarctic Peninsula. High-resolution magnetic measurements on u channel samples provide detailed age models for three cores collected from drift 7, which document an age of 122 ka for the oldest sediments recovered near the drift crest at site SED-07 and a high sedimentation rate (11 cm/kyr) at site SED-12 located close to the Alexander Channel system. Low- and high-temperature magnetic measurements in conjunction with microscopic and mineralogic observations from drifts 4, 5 and 7 indicate that pseudosingle-domain detrital titanomagnetite (partially oxidized and with limited Ti substitution) is the dominant magnetic mineral in the drift sediments. The titanomagnetite occurs in two magnetic forms: (1) a low-coercivity form similar to laboratory-synthesized titanomagnetite and (2) a high-coercivity form (Bcr > 60 mT). These two forms vary in amount and stratigraphic distribution across the drifts. We did not find evidence for diagenetic magnetic iron sulfides as has been previously suggested for these drift deposits. The observed change of magnetic mineralogy in sediments deposited during Heinrich events on drift 7 appears to be related to warming periods, which temporarily modified the normal glacial transport pathways of glaciogenic detritus to and along the continental rise and thus resulted in deposition of sediments with a different provenance. Understanding this sediment provenance delivery signature at a wider spatial scale should provide information about ice sheet dynamics in West Antarctica over the last ∼100 kyr.

The overdeepening hypothesis: How erosional modification of the marine-scape during the early Pliocene altered glacial dynamics on the Antarctic Peninsula's Pacific margin

A new synthesis of diatom assemblage data from Ocean Drilling Program (ODP) Leg 178 suggests that the Pacific margin of the Antarctic Peninsula underwent a transition from a shallow shelf to an overdeepened shelf in the early Pliocene. This modification of the marine-scape was due to a relatively brief interval of erosion begun at 5.2Ma. The erosion was caused by high frequency advances of a super-inflated Antarctic Peninsula Ice Sheet (APIS). The frequent advances of the higher elevation ice sheet were a consequence of abundant moisture delivered to the region as the Polar Front migrated southward. By 5.12Ma, ice streams incised foredeepened glacial troughs into basement on the inner shelf. Sediment eroded from the inner shelf was transported through cross-shelf troughs and deposited in large trough-mouth-fan depocenters on the upper slope. Overdeepened shelf conditions became widespread as troughs widened and intra-trough banks beveled. By 4.25Ma, trough-mouth-fan construction ceased and subsequent advances of the APIS have been infrequent. We propose that the reduced frequency of grounding events signaled the transition to a modern foredeepened and overdeepened shelf. We hypothesize that a new glacial dynamic emerged in the early Pliocene because overdeepening led to accelerated heat exchange between the ocean and APIS in two ways. Firstly, the overdeepened shelf required that a larger area of the grounded ice sheet's marine terminus be in contact with the ocean. Secondly, erosional deepening of the outer shelf was equivalent to lowering a shelf edge sill that permitted frequent and voluminous intrusion of warm circumpolar deep waters that upwell in the region. The resultant accelerated melting at the APIS marine terminus, caused the super-inflated APIS to deflate on the mainland, which further decreased the possibility that grounded ice could advance on the overdeepened shelf.

Paleocurrent directions from paleomagnetic reorientation of magnetic fabrics in deep-sea sediments at the Antarctic Peninsula Pacific margin (ODP Sites 1095, 1101)

We have determined the azimuth of bottom-current flow in drift deposit sediments recovered at ODP Sites 1095 and 1101, Antarctic Peninsula, using paleomagnetic reorientation of anisotropy of magnetic susceptibility (AMS) ellipsoids. A total of 38 cores from the two ODP sites have been measured, providing spatial and directional information on the physical record of the ACC (Antarctic Circumpolar Current) in the Plio-Pleistocene. Declination and inclination of the paleomagnetic vector of each core segment were used to reorient the AMS principal axes to the geographic coordinates. The cores were reoriented using the measured direction of the characteristic remanent magnetization (ChRM) with respect to a common reference line for the core, from which we are able to determine the orientation of the paleocurrent flow for Sites 1095 (Drift 7) and 1101 (Drift 4) relative to the geographic coordinates. Both sites have paleocurrent directions trending ~ NW–SE, which in the former locality are parallel to a sediment wave field. Our study shows that a combination of magnetic fabric analysis and paleomagnetism allows deep-sea sedimentary fabric to be used as a long-term proxy of bottom-current flow history.

Miocene reversal of bottom water flow along the Pacific Margin of the Antarctic Peninsula: Stratigraphic evidence from a contourite sedimentary tail

A Fossil Mounded Sedimentary Body (MB) has been identified in the sedimentary record on the central continental rise west of Adelaide Island, on the Antarctic Peninsula Pacific margin. The growth patterns of the MB are defined through a detailed regional stratigraphic analysis using multichannel seismic reflection profiles. The MB has an elongated NE trend. It overlaps and continues to the NE of an extensive cluster of seamounts, and it developed between two non-depositional troughs. Nine seismic units have been identified: Unit 9 ( the pre-MB stage), Unit 8 ( MB growth stage), Units 7 and 6 ( MB maintenance stage), Units 5 and 4 ( transitional stage), and Units 3, 2 and 1 ( inactive stage). We interpret the MB as a patch drift plastered against the NE, lee side of an obstacle, as a long Contourite Sedimentary Tail (CST), within a deep current that flowed northeastward. This segment of the rise is, however, affected at present by a SW-flowing branch of the Lower Circumpolar Deep Water (LCDW) from the Weddell Sea. The depositional patterns of the MB growth and maintenance stages, which are attributed an early Miocene age on the basis of regional correlation of MCS profiles with DSDP Site 325 and ODP Leg 178 drill sites, provide the first evidence that bottom currents on the central continental rise flowed towards the NE at that time, probably as part of the Lower Circumpolar Deep Water (LCDW) of the Antarctic Circumpolar Current (ACC). We suggest that significant palaeocirculation and palaeoceanographic changes occurred in this area, and probably more widely, during the middle Miocene or at the Miocene/Pliocene boundary. Although these results do not modify the regional stratigraphy of the major sediment drifts found on the continental rise of the Antarctic Peninsula's Pacific margin, they do indicate that the bottom current regime controlling the development of contourite deposits may have changed over time and also that more than one water mass has probably affected their distribution.

Pliocene glacial cyclicity in a deep-sea sediment drift (Antarctic Peninsula Pacific Margin)

Giant deep-sea sediment drifts are widespread features along the continental rise of the West Antarctic Peninsula, representing the most proximal continuous sedimentary recorders for West Antarctic ice events and glacial–interglacial cyclicity. Sediment physical, geochemical records and X-ray images derived from ODP Leg 178 Site 1095 (Sediment Drift 7) show characteristics in glacial–interglacial cyclicity and associated sedimentary processes during early Pliocene. Two boundary types dividing half-cycles have been recognized: (1) interglacial-to-glacial transitions that are distinct and characterized by a sharp boundary and abrupt change in lithology, (2) glacial-to-interglacial transitions that are diffuse and characterized by a gradual decline of sediment physical and geochemical values with a marked reduction in sedimentation rates. The boundaries defined in conjunction with magneto- and biostratigraphic tie-points form the framework for the calculation of relative sedimentation and accumulation rates used for reconstructing early Pliocene paleoproductivity and terrigenous flux to the drift. Warm early Pliocene climate conditions for West Antarctic Peninsula region are indicated by open ocean conditions in glacials, short residence times of the grounded ice sheet at the shelf edge with sporadic fluctuations as well as periodic collapses of the ice sheet front. We propose a 5-phase drift sedimentation model for the early Pliocene and postulate a highly dynamic ice sheet, likely reduced in thickness, resulting in oscillations which lie closer to the shelf edge than was the case during the late Quaternary.

Relationship between continental rise development and palaeo-ice sheet dynamics, Northern Antarctic Peninsula Pacific margin

Acquisition of swath bathymetry data west of the North Antarctic Peninsula (NAP), between 63°S and 66°S, and its integration with the predicted seafloor topography of Smith and Sandwell [Global seafloor topography from satellite altimetry and ship depth soundings. Science 277, 1956–1962.] reveal the links between the continental rise depositional systems and the NAP palaeo-ice sheet dynamics. The NAP Pacific margin consists of a wide continental shelf dissected by several troughs, tens of kilometres wide and long. The Biscoe Trough, which has been almost entirely surveyed with multibeam sonar, shows spectacular fan-shaped streamlining sea-floor morphologies revealing the presence of ice streams during the Last Glacial Maximum. In the study area the continental rise comprises the six northernmost sediment mounds of the NAP Pacific margin and the canyon-channel systems between them. These giant sediment mounds have developed since the early Neogene by southwest flowing bottom currents, which have redistributed along the margin the fine-grained component of the turbiditic currents flowing down canyon-channel systems. The widespread evidence of shallow slope instability within the sediment mounds has been identified from both swath bathymetry and topographic parametric sonar seismic reflection profiles. Bathymetric data show that the heads of all the rise canyon-channel systems coincide geographically with the mouths of the major glacial troughs on the continental shelf edge. This suggests a close genetic link between these morphological features and allows considering a glacio-sedimentary model for the western NAP outer margin seascape development. This model considers the availability of depositional space on the continental rise as the limiting factor for mound development. The depositional space, in turn, is controlled by the spacing between glacial maxima shelf-edge reaching ice streams. This model takes into account both glacial and interglacial scenarios and gives new insights on evaluating the palaeoenvironmental record of the continental rise sediment mounds.

Biostratigraphic characterization and Quaternary microfossil palaeoecology in sediment drifts west of the Antarctic Peninsula – implications for cyclic glacial–interglacial deposition

The SEDANO Project recovered 19 gravity cores on sediment drifts from the Pacific continental margin of the Antarctic Peninsula [Camerlenghi et al. (1997a) High-resolution terrigenous sedimentary record of the sediment drifts on the Antarctic Peninsula Pacific margin. In: Ricci, C.A. (Ed.), The Antarctic region. Museo Nazionale dell’Antartide, Siena, pp. 705–710]. Fifteen cores were sampled with the aim of developing an integrated biostratigraphy and palaeoecology based on calcareous nannofossils, diatoms, planktonic and benthic foraminifera, framed in a depositional process reconstruction. Barren gray laminated and brown bioturbated hemipelagic sediments characterize glacial and interglacial cycles, respectively. Analyses from both intervals allow comparison between microfossil occurrence in glacial and interglacial cycles. The unit boundaries were drawn more accurately by means of the microfossil distribution. On the basis of micropalaeontological and sedimentological evidence, Interglacial Unit C is correlated to Oxygen Isotope Stage 5, thus dating the unit boundaries at 127 and 70 ka. Peaks in diatom abundance correlate well with interglacial units and indicate high productivity and an open ocean environment. A calcareous nannofossil cold-taxa association is present in most cores examined, and its consistent distribution within Interglacial Unit C indicates key environmental relationships. The occurrence of calcareous nannofossils has been related to temperature tolerance, sea-ice cover reduction, nutrient availability, and factors limiting primary productivity. Our results confirm that coccolithophorids occurred at southern high latitudes, in the western marginal basins of the Antarctic Peninsula, during short periods of the Late Quaternary. A foraminiferal assemblage, made up of sinistral Neogloboquadrina pachyderma and few benthics, occurs only in interglacial units. Correlation of microfossil occurrences with climatic cycles adds information on their palaeoecology and palaeoproductivity in the southern high latitudes.

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.

Sediment drifts and deep-sea channel systems, Antarctic Peninsula Pacific Margin

Abstract Twelve sedimentary mounds are identified on the upper continental rise of the Pacific Margin of the Antarctic Peninsula. All these mounds are produced by a varying degree of interaction of along-slope bottom water flow with down-slope turbidity currents. These mounds provide a complete range of intermediates between two end members: the sediment drift and the channel levee. Surface sediments on drift 7 suggest that the mechanisms for the supply and transport of sediment include entrainment of material from turbidity currents within ambient bottom currents, and pelagic settling from the sea surface, including biogenic and glacially derived material. The long-lasting activity of these mechanisms is documented by the data provided by four DSDP and ODP drill sites. Bathymetric and seismic data, both at a large, comprehensive scale and at a small, detailed scale, show the geometry of the sedimentary mounds and their relationships with the adjacent turbidity current channel systems. These data allow the determination of some diagnostic criteria to identify the sediment drifts.

A sedimentary paleomagnetic record of the Matuyama chron from the Western Antarctic margin (ODP Site 1101)

A high-resolution paleomagnetic record for part of the Matuyama chron (0.7–2.1 Ma) is reported for Ocean Drilling Program Site 1101 (Leg 178), off the Antarctic Peninsula Pacific margin. A rock-magnetic investigation of 62 discrete samples revealed that the natural remanent magnetization (NRM) is carried by pseudo-single domain magnetite. Progressive alternating field demagnetization of 83 m of U-channels provided a polarity stratigraphy down to the Olduvai subchron. Two geomagnetic events preceding the Jaramillo subchron were identified, including the Cobb Mountain polarity interval. The bulk magnetic parameters vary by more than a factor of 20 over the entire time interval, but by less than a factor of 6 over the 0.7–1.1 Ma interval. This latter interval was selected for paleointensity determinations, which were carried out by normalizing the NRM by the anhysteretic remanent magnetization (ARM). Direct comparison of the Site 1101 paleointensity record with other curves available for the same time interval suggests a geomagnetic origin for features present in the record. A more quantitative comparison was achieved by means of a jackknife test performed on nine records of relative paleointensity over the 0.95–1.1 Ma interval. This test yielded no outlier for the period considered, confirming the geomagnetic character of the records. We have constructed a low-resolution stack revealing some of the characteristic paleointensity features of the Jaramillo subchron.

Seismic stratigraphy of Palmer Deep: a fault-bounded late Quaternary sediment trap on the inner continental shelf, Antarctic Peninsula Pacific margin

The Palmer Deep is an enclosed bathymetric depression on the inner portion of the Antarctic Peninsula continental shelf about 30 km southwest of Anvers Island. Three sub-basins, separated by bathymetric sills, comprise the Palmer Deep: Basin I, Basin II, and Basin III. Deep-tow boomer seismic reflection data reveal thick (>50 m) sediment sections in each basin consisting of Holocene diatomaceous mud. The boomer records proved fine-scale resolution of decimetre thick sediment layers within the uppermost (Holocene) seismic unit. Deeper penetration GI and small airgun records obtained in 1997 provide insight into the structural and depositional history of the basins which extends clearly back in time before the Holocene (unit imaged by the boomer records). The Palmer Deep contains a sediment infill estimated at about 270 m thick arranged in a complex (five unit) internal stratigraphy unusual for the inner continental shelf of Antarctica. Combined use of the boomer and airgun sources allows complementary resolution of both deep and shallow stratigraphy with some reflectors common in both records, such as the Middle Basin Reflector at 45 ms twt below seafloor. The Middle Basin Reflector most likely is of latest Pleistocene age (isotopic Stage 2) and therefore 80% of the basin fill pre-dates the classic Last Glacial Maximum. The Palmer Deep is bounded by active extensional faults as evidenced by offset and stratigraphic growth within Holocene sections. To accommodate shelf-wide glaciation on the Pacific margin of the Antarctic Peninsula we suggest a subglacial and subaqueous origin for much of the Palmer Deep basin fill. Hence, the Palmer Deep basins were the locus of subglacial `lakes' beneath the ice sheet at times of glacial maximum.

Large Amplitude Magnetic Anomalies in the Northern Sector of the Powell Basin, NE Antarctic Peninsula

Magnetic profiles obtained during the Hesant 92/93 cruise with the R/V Hesperides show large amplitude anomalies (up to 1000 nT) along a 100 km wide band in the northern margin of the Powell Basin. The anomalies, which are also locally identified in the eastern and western margins, are attributed to the continuation of the two branches of the Antarctic Peninsula Pacific Margin Anomaly (PMA). Interactive modelling of two-dimensional bodies in four profiles oriented NNW-SSE allows us to determine the main features of the magnetic source bodies within the continental crust. These are elongated in a N60/degE trend, and their base is located at a depth exceeding 15 km. Equivalent magnetic susceptibilities mostly between 0.07 and 0.1 (SI) are obtained. These values are consistent with the hypothesis that remanent magnetisation of the magnetic source bodies is sub-parallel to the present geomagnetic field (norImally magnetised). The general trends of the bathymetry a nd the geometry of the acoustic basement on multichannel seismic profiles are consistent with the upper surface of magnetic bodies. In order to match the observed anomalies it is also necessary to consider a second tabular shaped body with induced magnetisation in almost all the profiles, which could represent layers 2 and 3 of the oceanic crust of the Powell Basin. Three different geometries of connection between the anomalies in the Powell Basin margins and the PMA branches are discussed. The most plausible one is the occurrence of two branches, although they are closer together than in the Bransfield Strait. The northern branch would continue along the fragments of continental crust of the South Scotia Ridge located at the northern boundary of the Powell Basin, whereas the southern branch would be located only in the eastern and western passive margins of the Powell Basin. The apparent splitting of the southern branch of the anomalous body indicates that it was emplaced before Oligo cene times, when the opening of this basin occurred, and that it was subsequently fragmented during the Cenozoic. A possible time of formation of the PMA body would be during the long Cretaceous normal polarity interval, which also coincides with a peak in magmatic activity along the Antarctic Peninsula.

Ice sheet retreat from the Antarctic Peninsula shelf

Side-scan sonar and sub-bottom acoustic profiler data and sediment cores reveal the processes that controlled sediment transport and deposition on the continental shelf of the Antarctic Peninsula Pacific margin off Anvers Island, during deglaciation over the last 11,000 years or more. Glacial flutes and striations mark the flow of low-profile ice streams draining the interior, across the middle and outer shelf. Most probably, ice sheets were grounded to the continental shelf edge along this margin during the last glacial maximum. Iceberg furrows overwrite the ice sheet record in areas between 500 and 350 m water depth, and reflect calving from a retreating ice shelf front. Cores show open marine sedimentation replacing diamicton deposition close to the grounding line during this retreat, which rapidly cleared the outer and middle shelf shortly before 11,000 years BP (from AMS 14C dates on organic carbon). The shallower, scoured and largely sediment-free inner shelf cleared later, probably before 6000 years BP. Open marine sediments on the middle and outer shelf include a pelagic biogenic component and suspended sediment from modern glacier tongues, supplemented by resuspension of older sediment in shallow shelf regions (by currents and by grounded icebergs). Sedimentation is too slow to be able to fill in the concave-up profile of the continental shelf during a full interglacial, confirming the intense glacial-interglacial cyclicity of sedimentation on the continental slope inferred from seismic reflection profiles. The observed rapid deglaciation of the middle and outer shelf supports published numerical model results that the Antarctic Peninsula's narrow interior and broad continental shelf make the ice sheet sensitive to imposed eustatic sea-level change. A low-profile marine-based ice sheet over the continental shelf during glacial maximum would have made a major contribution to that sensitivity, in the early stages of deglaciation. It follows that the Antarctic Peninsula ice sheet, and probably most others, are not so sensitive today.

The depositional pattern and distribution of glacial-interglacial sequences on the Antarctic Peninsula Pacific margin

The outer shelf on the Antarctic Peninsula Pacific margin south of 63°30′S is underlain by Pliocene-Pleistocene prograding sequences which have been produced mainly by the action of ice sheets grounded out to the shelf edge at times of glacial maximum. Most sediment in these sequences has probably been transported to the margin in a deforming basal till, which implies deposition on a broad front: a “line source”. A representative prograding sequence mapped across an extensive network of multichannel seismic reflection lines has an elongate depocentre on the upper palaeoslope, which is consistent with the grounded ice sheet model. However, it is likely that each sequence recognized on existing multichannel seismic data represents several ice advances. Depth-to-surface maps reveal a broad variation along the margin in the amount of progradation, reflecting differences in sediment supply. The pattern of progradation and the bathymetry of the outer shelf suggest that the main depocentre in the area studied was fed by an ice stream at times of glacial maximum. Seismic lines across the margin farther to the southwest indicate the existence of other depocentres. Several broad depositional lobes have probably coalesced to form the extensive outer shelf. The present continental slope is smooth and steep, and is not cut by major canyons. A downslope change in seismic facies and scouring on the uppermost rise probably reflect downslope transitions from slumps to debris flows to turbidity currents. These processes are likely to be most active at times of glacial maximum. Deep drilling data indicate that the rise sediments consist mainly of terrigenous turbidite and ice-rafted detritus. A marked upward change in seismic facies on the continental rise indicates a change to a higher energy sedimentary regime and appears to correlate with the start of glacial progradation on the shelf.