RRS Charles Darwin Cruise 129, Durban, South Africa to Port Louis, Mauritius 6 Jul - 11 Aug 2001. History of the Deep Western Boundary Current in the Madagascar and Mascarene Basins

Cruise 129 of the RRS Charles Darwin was designed to investigate the history of the Deep Western Boundary Current in the Madagascar and Mascarene Basins and make hydrographic and biological measurements in support of those objectives. Swath bathymetry (10 kHz) and 3.5 kHz sub-bottom profiles were recorded continuously. Kasten (16) and box (29) cores were recovered and one piston core. 16 CTD casts with transmissometer and fluorometer, and plankton net hauls at 9 stations (generally 3 tow depths per station) were also deployed. The topography proved to be quite rugged on the ridges to the north and south of Madagascar, as was the western flank of the Mascarene Ridge. This made it impossible to find coring targets at the depths of AAIW. Furthermore, much of the sediment apron east of Madagascar is made of turbidites and only a few contourite deposits, none forming a significant sediment drift. It appears that the deep Western boundary current is not particularly strong here, and recent work has shown that the dominant current is related to Rossby waves in the basin with a period of 60 days. Nevertheless sufficient material has been obtained in order to examine aspects of the paleoceanography and flow history in this basin, especially from the flow constriction in Amirante Passage

Long-term variations in Iceland–Scotland overflow strength during the Holocene

The overflow of deep water from the Nordic seas into the North Atlantic plays a critical role in global ocean circulation and climate. Approximately half of this overflow occurs via the Iceland–Scotland (I–S) overflow, yet the history of its strength throughout the Holocene (~ 0–11 700 yr ago, ka) is poorly constrained, with previous studies presenting apparently contradictory evidence regarding its long-term variability. Here, we provide a comprehensive reconstruction of I–S overflow strength throughout the Holocene using sediment grain size data from a depth transect of 13 cores from the Iceland Basin. Our data are consistent with the hypothesis that the main axis of the I–S overflow on the Iceland slope was shallower during the early Holocene, deepening to its present depth by ~ 7 ka. Our results also reveal weaker I–S overflow during the early and late Holocene, with maximum overflow strength occurring at ~ 7 ka, the time of a regional climate thermal maximum. Climate model simulations suggest a shoaling of deep convection in the Nordic seas during the early and late Holocene, consistent with our evidence for weaker I–S overflow during these intervals. Whereas the reduction in I–S overflow strength during the early Holocene likely resulted from melting remnant glacial ice sheets, the decline throughout the last 7000 yr was caused by an orbitally induced increase in the amount of Arctic sea ice entering the Nordic seas. Although the flux of Arctic sea ice to the Nordic seas is expected to decrease throughout the next century, model simulations predict that under high emissions scenarios, competing effects, such as warmer sea surface temperatures in the Nordic seas, will result in reduced deep convection, likely driving a weaker I–S overflow

New insights from multi-proxy data from the West Antarctic continental rise: Implications for dating and interpreting Late Quaternary palaeoenvironmental records

The Antarctic Peninsula’s Pacific margin is one of the best studied sectors of the Antarctic continental margin. Since the 1990s, several research cruises have targeted the continental rise with geophysical surveys, conventional coring and deep-sea drilling. The previous studies highlighted the potential of large sediment drifts on the rise as high-resolution palaeoenvironmental archives. However, these studies also suffered from chronological difficulties arising from the lack of calcareous microfossils, with initial results from geomagnetic relative palaeointensity (RPI) dating promising a possible solution. This paper presents data from new sediment cores recovered on cruise JR298 from seven continental rise sites west of the Antarctic Peninsula and in the Bellingshausen Sea with the objectives to (i) seek calcareous foraminifera, especially at shallow drift sites, to constrain RPI-based age models, and (ii) investigate the depositional history at these locations. We present the results of chronological and multi-proxy analyses on these cores and two cores previously collected from the study area. We establish new age models for the JR298 records and compare them with published RPI-based age models. In addition, we evaluate the reliability of different palaeoproductivity proxies and infer depositional processes. Planktic foraminifera are present in various core intervals. Although their stable oxygen isotope (δ18O) ratios, tephrochronological constraints and glacial-interglacial changes in sediment composition provide age models largely consistent with the RPI chronologies, we also observe distinct differences, predominantly in the Bellingshausen Sea cores. Enrichments of solid-phase manganese together with evidence for “burn-down” of organic carbon in late glacial and peak interglacial sediments document non-steady-state diagenesis that may have altered magnetic mineralogy and, thus, RPI proxies. This process may explain discrepancies between RPI-based age models and those derived from δ18O data combined with tephrochronology. The data also indicate that organic carbon is a much less reliable productivity proxy than biogenic barium or organically-associated bromine in the investigated sediments. In agreement with previous studies, sediment facies indicate a strong control of deposition on the rise by bottom currents that interacted with detritus supplied by meltwater plumes, gravitational down-slope transport processes and pelagic settling of iceberg-rafted debris (IRD) and planktic microfossils. Bottom-current velocities underwent only minor changes over glacial-interglacial cycles at the drift crests, with down-slope deposition only rarely affecting these shallow locations. Maximum concentrations of coarse IRD at the seafloor surfaces of the shallow sites result predominantly from upward pumping caused by extensive bioturbation. This process has to be taken into account when past changes in IRD deposition are inferred from quantifying clasts >1 mm in size

Contourite depositional system after the exit of a strait: Case study from the late Miocene South Rifian Corridor, Morocco

Idealized facies of bottom current deposits (contourites) have been established for fine-grained contourite drifts in modern deep-marine sedimentary environments. Their equivalent facies in the ancient record however are only scarcely recognized due to the weathered nature of most fine-grained deposits in outcrop. Facies related to the erosional elements (i.e. contourite channels) of contourite depositional systems have not yet been properly established and related deposits in outcrop appear non-existent. To better understand the sedimentary facies and facies sequences of contourites, the upper Miocene contourite depositional systems of the South Rifian Corridor (Morocco) is investigated. This contourite depositional system formed by the dense palaeo-Mediterranean Outflow Water. Foraminifera assemblages were used for age-constraints (7.51 to 7.35 Ma) and to determine the continental slope depositional domains. Nine sedimentary facies have been recognized based on lithology, grain-size, sedimentary structures and biogenic structures. These facies were subsequently grouped into five facies associations related to the main interpreted depositional processes (hemipelagic settling, contour currents and gravity flows). The vertical sedimentary facies succession records the tectonically induced, southward migration of the contourite depositional systems and the intermittent behaviour of the palaeo-Mediterranean Outflow Water, which is mainly driven by precession and millennial-scale climate variations. Tides substantially modulated the palaeo-Mediterranean Outflow Water on a sub-annual scale. This work shows exceptional examples of muddy and sandy contourite deposits in outcrop by which a facies distribution model from the proximal continental slope, the contourite channel to its adjacent contourite drift, is proposed. This model serves as a reference for contourite recognition both in modern environments and the ancient record. Furthermore, by establishing the hydrodynamics of overflow behaviour a framework is provided that improves process-based interpretation of deep-water bottom current deposits

Leg 181 synthesis: fronts, flows, drifts, volcanoes, and the evolution of the southwestern gateway to the Pacific Ocean, Eastern New Zealand

The Late Cretaceous-Cenozoic geology of New Zealand represents the evolution of a post-Gondwana, Pacific-facing passive margin which interacted, first, with the mid-Cenozoic development of the Australian/Antarctic and Australian/Pacific plate boundaries and, second, with the subsequent development of the oceanic thermohaline circulation system. Situated between the Tasmanian and southwest Pacific oceanic current gateways, the stratigraphy of the New Zealand region provides our best record of the evolution of the Pacific Ocean's largest deep cold-water inflow, the Deep Western Boundary Current (DWBC), and also possesses an important record of Antarctic Intermediate Water flow. Prior to Leg 181, our knowledge of southwest Pacific Ocean history and, in particular, the development of the DWBC and its local partner, the Antarctic Circumpolar Current (ACC), was poor. Seven holes were therefore drilled east of New Zealand to determine the stratigraphy, sedimentary systems, and paleoceanography of the DWBC, ACC, and related water masses and fronts. The sites comprised a transect of water depths from 396 to 4488 m and spanned a latitudinal range from 39° to 51°S. Leg 181 drilling provided the data needed to study a wide range of problems in the Southern Ocean Neogene.\ud \ud Driven by rifting and a new cycle of seafloor spreading along the Mid-Pacific Rise, New Zealand's youngest (Kaikoura) stratigraphic cycle begins with Late Cretaceous rift fill followed by subsidence and marine transgression until the late Eocene. Biopelagic oozes accumulated throughout as an abyssal apron around the Pacific perimeter of the New Zealand Plateau, seen as Paleocene siliceous nannofossil chalk, chert, and clay at Site 1121 (water depth = 4488 m) and nannofossil chalk at Site 1124 (water depth = 3967 m). At the Eocene/Oligocene boundary (~33.7 Ma), the spreading ridge between Australia and Antarctica broke through south of the Tasman Rise, linking for the first time the Indian and Pacific Oceans into a continuous Southern Ocean. Powerful wind-forced currents, predecessors to the modern ACC, were funneled through the Tasmanian Gateway and into the Pacific, where their path, combined with that of the thermohaline DWBC, was impeded by the shallowly submergent New Zealand Plateau, centered then at latitude ~55°S. All drill sites within or east of the Tasmanian Gateway and all onland sections in New Zealand record this event as a regional unconformity, the Marshall Paraconformity, across which a there is a time gap of ~3-10 m.y., a result of a combination of corrosion, erosion, and nondeposition. Above the paraconformity, sedimentation in both shallow and deep water resumed as late Oligocene (~27-29 Ma) sediment drifts (Site 1124; water depth = 3967 m). Younger deepwater drifts at Sites 1123 (water depth = 3290 m) and 1124 comprise alternating nannofossil chalks containing greater or lesser amounts of terrigenous clay. At Site 1123 on the North Chatham Drift, sediment accumulated essentially continuously from ~20.5 Ma onward. Analysis of this record shows that the stratigraphic rhythms there correspond to 41-k.y. Milankovitch climatic cycles, with faster DWBC flow during colder or glacial intervals. Site 1123 is globally unique. It provides an essentially complete, richly microfossiliferous Miocene to Quaternary record of uniform ~4-cm/k.y. sedimentation that has been astronomically tuned. It also contains an almost complete paleomagnetic record since Chron C6r at 20.5 Ma, including the first record of new magnetic subchron C5ADn1r. Shallower-water Sites 1125 (water depth = 1366 m), 1120 (water depth = 546 m), and 1119 (water depth = 396 m) reveal, respectively, a major productivity bloom between 5.6 and 4.8 Ma on the north side of the Subtropical Front (STF) (Site 1125), foraminiferal nannofossil chalk accumulation punctuated by paraconformities at 16.7-15.8, 5.6-1.9, and 0.9-0.24 Ma (Site 1120), and enhanced frontal flows along a seaward-relocated STF during glaciations (Site 1119). The late Quaternary climatic record at Site 1119 also closely matches that of air temperature in the Vostok ice core, indicating close links between climate change in southern middle and polar latitudes.\ud \ud From ~24 Ma onward, abundant terrigenous material was shed into the southwest Pacific from rising mountains along the South Island Alpine Fault plate boundary. Gradually changing clay mineral assemblages in DWBC drifts, with chlorite + illite replacing smectite + kaolinite, reflect the increasing influence of newly unroofed basement (Rangitata) graywackes and schists through the Miocene-Quaternary. From 12 Ma onward, sediments were augmented by an influx of mainly rhyolitic tephra from the North Island volcanic arc. Site 1122 (water depth = 4432 m), on the left bank levee of the abyssal Bounty Fan, records a marked increase in the input of terrigenous turbidites and fan building starting at ~1.7 Ma and peaking at average rates >50 cm/k.y. after 0.7 Ma. Site 1124, on the Rekohu Drift near the Hikurangi Channel, records the start of overbank turbidite deposition, and therefore avulsion of the Hikurangi Channel from the Hikurangi Trough following channel deflection by a large submarine landslide from the North Island continental margin at ~1.65 Ma. Geological and oceanographic events that have occurred in the southwest Pacific since the Eocene/Oligocene boundary (~33.7 Ma) together compose the Eastern New Zealand Sedimentary System (ENZOSS), studies of which are contributing to our understanding of the history of global ocean circulation and climate change

A ~240 ka record of Ice Sheet and Ocean interactions on the Snorri Drift, SW of Iceland

Core MD99-2323 was extracted from the Snorri Drift at a water depth of 1062 m, just south of the Denmark Strait, and ~120 km from the Last Glacial Maximum (LGM) margins of the Iceland and East Greenland Ice Sheets. The core chronology (~7.5 to 240 cal ka) is derived from radiocarbon dates, marker tephra, paleomagnetic excursion, and correlation with North Atlantic δ18O records on Neogloboquadrina pachyderma (δ18ONp). Sedimentation averaged ~7.5 cm/kyr. Records of proxy flow speed, ice rafted debris (IRD) and oxygen isotopes show that many IRD abundance peaks represent winnowing of the fine fraction by faster flows rather than pulses of increased IRD flux. The overall pattern of flow speed does not resemble the classic fast interglacial/slow glacial pattern seen in records of Nordic Sea overflow, rather the current record is suggested to be partly controlled by the production of brine-driven gravity flows from adjacent ice fronts, especially during cold periods. On a smaller scale the usual glacial/slow – interglacial/fast pattern appears to be the case during ~5 kyr oscillations during Marine Isotope Stage (MIS) 6 where periodic low flow speed is matched by high values of planktonic oxygen isotope ratios. Eight peaks in quartz wt% reflect increased contributions from glacial erosion of Precambrian and Caledonian bedrock from E and NE Greenland; peaks in dolomite may reflect glacial-marine transport from the Laurentide Ice Sheet. Cross wavelet analysis of the δ18ONp versus sortable silt and quartz records indicate significant precession and obliquity periodicities, but with little temporal correlations due to leads and lags in responses