I. N. McCave

Evolution of Ocean Temperature and Ice Volume Through the Mid-Pleistocene Climate Transition

Cycling Down The Mid-Pleistocene Transition, which lasted from approximately 1.25 million to 700 thousand years ago, was a period during which the dominant periodicity of Earths climate cycles inexplicably changed from 41 thousand to 100 thousand years. This change is clearly apparent in the oxygen isotopic composition of many calcifying marine organisms, but changes in both ice volume and temperature affect the signal, and so exactly what the signal means has remained unclear. Elderfield et al. (p. 704; see the Perspective by Clark) separated these two effects by measuring both the oxygen isotopic makeup and the Mg/Ca (a proxy that reflects changes in temperature only) of certain benthic foraminifera. The findings reveal the contributions of ice volume and temperature to glacial cycles, suggest when and why the Mid-Pleistocene Climate Transition occurred, and clarify how carbon is lost from the ocean-atmosphere during deglaciations but also changes because of ocean circulation. The effects of changes in ice volume and ocean temperature during the mid-Pleistocene transition have now been resolved. Earth’s climate underwent a fundamental change between 1250 and 700 thousand years ago, the mid-Pleistocene transition (MPT), when the dominant periodicity of climate cycles changed from 41 thousand to 100 thousand years in the absence of substantial change in orbital forcing. Over this time, an increase occurred in the amplitude of change of deep-ocean foraminiferal oxygen isotopic ratios, traditionally interpreted as defining the main rhythm of ice ages although containing large effects of changes in deep-ocean temperature. We have separated the effects of decreasing temperature and increasing global ice volume on oxygen isotope ratios. Our results suggest that the MPT was initiated by an abrupt increase in Antarctic ice volume 900 thousand years ago. We see no evidence of a pattern of gradual cooling, but near-freezing temperatures occur at every glacial maximum.

Holocene oscillations in temperature and salinity of the surface subpolar North Atlantic

The Atlantic meridional overturning circulation (AMOC) transports warm salty surface waters to high latitudes, where they cool, sink and return southwards at depth. Through its attendant meridional heat transport, the AMOC helps maintain a warm northwestern European climate, and acts as a control on the global climate. Past climate fluctuations during the Holocene epoch (∼11,700 years ago to the present) have been linked with changes in North Atlantic Ocean circulation. The behaviour of the surface flowing salty water that helped drive overturning during past climatic changes is, however, not well known. Here we investigate the temperature and salinity changes of a substantial surface inflow to a region of deep-water formation throughout the Holocene. We find that the inflow has undergone millennial-scale variations in temperature and salinity (∼3.5 °C and ∼1.5 practical salinity units, respectively) most probably controlled by subpolar gyre dynamics. The temperature and salinity variations correlate with previously reported periods of rapid climate change. The inflow becomes more saline during enhanced freshwater flux to the subpolar North Atlantic. Model studies predict a weakening of AMOC in response to enhanced Arctic freshwater fluxes, although the inflow can compensate on decadal timescales by becoming more saline. Our data suggest that such a negative feedback mechanism may have operated during past intervals of climate change.

Size sorting in marine muds: Processes, pitfalls, and prospects for paleoflow-speed proxies

The basis for, and use of, fine grain size parameters for inference of paleoflow speeds is reviewed here. The basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field, experimental data on suspended sediment transport and deposition, and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed. In the deep sea, sorting events occur under resuspension/deposition events in benthic storms. At flow speeds below 10–15 cm s−1, size in the noncohesive “sortable silt” (10–63 μm) range is controlled by selective deposition, whereas above that range, removal of finer material by winnowing also plays a role. The best particle size instruments to measure a flow speed–related grain size employ the settling velocity method, while laser diffraction sizers can yield misleading results because of particle shape effects. Potential problems, including source effects, downslope supply on continental margins, spatial variability of flow over bedforms, and influence of ice-rafted detritus, are examined. A number of studies using the sortable silt flow speed proxy are reviewed, and inverse modeling of grain size distributions is examined. Outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed.

Analysis and modelling of gravity- and piston coring based on soil mechanics

The effects of gravity- and piston corers on the dimensional accuracy of marine sediment cores is analysed using principles of soil mechanics. A model for the coring process is built around the feedback that arises and develops between the core barrel and the sampled sediment. This model for sediment response is applied to different hypothetical coring scenarios, which are then compared to real examples, providing insights into the specific effects of each sampling method and the development of these effects down-core. Four cores from a single location on the Iberian Margin are found to contain stratigraphically intact successions that differ in length by a factor of up to 2.7, due solely to the different effects of each coring method. These dimensional discrepancies are attributed to the combined effects of ‘over-sampling’ in the upper portions of the piston cores (due to cable rebound causing upward piston acceleration), and ‘under-sampling’ dominant in the basal portions of the open-barrel gravity-type cores. It is suggested that heavier piston corers, deployed on longer, lighter cables, are prone to greater over-sampling ratios over longer stratigraphic intervals, due to the increased likelihood and extent of cable rebound. Cable rebound may also give rise to double penetration of gravity corers, resulting in repeated stratigraphic intervals. Knowledge of the dimensional accuracy of marine sediment cores is essential to an evaluation of past sedimentation rates, and hence interpretations of past depositional processes. It is therefore essential that we recognise the sampling effects of each coring method, and their variability down-core, lest coring artefacts be interpreted as sedimentary signals. Different core types may be more suited to different palaeoceanographic investigations. Hence, failing the development of a practical cable-deployed recoilless piston corer, a combination of a variety of core types will permit the best acquisition of the in situ stratigraphic truth. Our results suggest that a large-diameter (Dc∼20–30 cm) square-barrel gravity corer for the top 10–12 m combined with a cylindrical piston corer below ∼10 m may provide the least deformed material.

Distribution, composition and flux of particulate material over the European margin at 47°–50°N

In the framework of the Ocean Margin Exchange project, a multi-disciplinary study has been conducted at the shelf edge and slope of the Goban Spur in order to determine the spatial distribution, quantity and quality of particle flux, and delineate the transport mechanisms of the major organic and inorganic components. We present here a synthesis view of the major transport modes of both biogenic and lithogenic material being delivered to the open slope of the Goban Spur. We attempt to differentiate between the direct biogenic flux from the surface mixed layer and the advective component, both biogenic and lithogenic. Long-term moorings, instrumented with sediment traps, current meters and transmissometers have yielded samples and near-continuous recordings of hydrographic variables (current direction and speed, temperature and salinity) and light transmission for a period of 2.5 years. Numerous stations have been occupied for CTD casts with light transmission and collection of water samples. The sedimenting material has been analysed for a variety of marker compounds including phytoplankton pigments, isotopic, biomineral and trace metal composition and microscopical analyses. These samples are augmented by seasonal information on the distribution and composition of fine particles and marine snow in the water column. The slope shows well-developed bottom nepheloid layers always present and intermediate nepheloid layers intermittently present. Concentrations are mainly in the range 50–130 mg m−3 in nepheloid layers and 6–25 mg m−3 in clear water. A seasonal variability in the concentration at the clear water minimum is argued to be related to seasonal variations in vertical flux and aggregate break-up in transit during summer months. It is suggested that the winter sink for this seasonal change in particulate matter involves some re-aggregation and scavenging, and some conversion of particulate to dissolved organic matter. This may provide a slow seasonal pump of dissolved organic carbon to the deep ocean interior. Differences in trapped quantities at different water depths are interpreted as due to lateral flux from the continental margin. There is a major lateral input between 600 and 1050 m at an inner station and between 600 and 1440 m at an outer one. The transport is thought to be related to intermediate nepheloid layers, but those measured are too dilute to be able to supply the flux. Observed bottom nepheloid layers are highly concentrated very close to the bed (up to 5 g m−3), with a population of large aggregates. Some of these are capable of delivering the flux seen offshore during intermittent detachment of nepheloid layers into mid-water. Concentrated bottom nepheloid layers are also able to deliver large particles with unstable phytoplankton pigments to the deep sea floor in a few tens of days. Calculated CaCO3 fluxes are adjusted for dissolution, which is inferred from Ca/Al ratios to be occurring in the CaCO3-saturated upper water column where up to 80% of the CaCO3 resulting from primary production is dissolved.

Recent sedimentation beneath the Deep Western Boundary Current off northern New Zealand

The triangular shaped region defined by Chatham Rise in the south, the Louisville Seamount Chain in the east and the Pacific/Australian plate boundary in the west is one of the most active sedimentary regions of the world ocean. The SW Pacific Deep Western Boundary Current (DWBC), with a transport of 20 Sv, flows through the area producing regions of scour, nepheloid layers and typical depositional bedforms. Sediment is supplied to the region by turbidity currents directly via Hikurangi Channel and by the DWBC, which removes material from Bounty Fan south of Chatham Rise, as well as by fallout of volcanic ash and pelagic biogenic material. The CaCO3 content of the sediments is very much controlled by terrigenous dilution, with Chatham Rise having ∼ 60%, but Hikurangi Plateau (near the Channel), sited well above the 4750 m-deep CCD, having only ∼ 20%. Although there is much evidence of scour around pinnacles and in scoured, possibly furrowed, mud deposits seen on 3.5 kHz profiles, dilute nepheloid layers, slow geostrophic and measured velocities, photographic and sediment grain-size evidence do not indicate fast flows. Mudwaves are of the irregular, vertically migrating type (rather than progressive antidunes), suggesting flows < ∼ 0.10 m s−1. In common with other Southern Ocean areas, flows may have been strongest during glacial times. Hikurangi Channel delivers its turbidity currents into the path of the DWBC, which sweeps them along to create a deep-sea fan with the characteristics of a contourite drift, here termed a fan-drift. This is mantled with migrating, climbing mudwaves, which are ascribed to deposition from episodic turbidity currents travelling at more than 0.1 m s−1. The fan-drift forms on the right-hand side of a boundary channel along the foot of the 1000 m-high Rapuhia Scarp. It is interpreted as a large right-bank levee formed by turbidity currents overspilling the Channel, which also owes its origin to scour by the DWBC. The turbidite deposits are dominantly mud and the evidence of scour/deposition patterns around a swath-mapped seamount indicate the depositing flows to be consistent with the DWBC flow direction, demonstrating entrainment of the turbidity currents by the deep geostrophic flow. Patterns of sedimentation around the Louisville Seamount Chain indicate action of a strong filament of the DWBC on the east side of the seamounts at least as far north as 37°S, and a relative intensification of the current on the west side of the seamounts north of 38°S.

Evidence for Heinrich layers off Portugal (Tore Seamount: 39 °N, 12 °W)

Abstract The Tore Seamount is a circular, volcano-like feature 100 km in diameter with its summit at 2200 m water depth and a small, 5000 m deep basin in its interior. It is situated approximately 300 km west of Lisbon and is surrounded by deep abyssal plains. This site with a standard pelagic stratigraphy is the southernmost point where the so-called Heinrich events have so far been recorded. A succession of alternating interglacial/glacial periods reveals a stratigraphic record back to the beginning of isotopic stage 7 (225 kyr). Climatic changes are identifiable by coherent variations in colour, carbonate content and distribution of ice-rafted detritus in the carbonate-free fraction. Inputs of ice-rafted quartz are well defined. Characteristics in common with other sites showing Heinrich layers include a high terrigenous to biogenic ratio, a dramatic decrease in the accumulation rate of foraminifera shells, an increase in dolomite abundance and the occurrence of polar foraminiferal species indicating southwards penetration of cold waters which lead us to consider a wider southeastern extent of the North Atlantic ice-rafted detritus belt than hitherto. If the presently accepted position of the Polar Front is maintained, icebergs must have been swept southwards from the southern boundary of the pack ice in a current merging into the ancestral Canary Current, bringing ice-rafted material to the Tore Seamount. The coincidence of reddish-feldspar, probably derived from the northern Appalachian Triassic red facies, with the transparent quartz suggests at least a partial Labrador source for all the Heinrich layers here, including HL 3. In comparison to other sites in the entire North Atlantic, two exceptions stand out: the absence of HL 5 and the low detritus to biogenics ratio for HL 3. The simultaneous occurrence of these two types of ice-rafted minerals is a new piece in the puzzle of the origin of Heinrich layers.

Development of sediment drifts approaching an active plate margin under the SW Pacific Deep Western Boundary Current

The abyssal Pacific Ocean is fed by a 1000 km wide, deep western boundary current (DWBC) that flows northward along the continental margin, east of New Zealand. Between the passive margin of Chatham Rise and the subduction zone of Kermadec Trench, a distance of 1200 km, the DWBC has formed a suite of sediment drifts over a depth range of 2200–5700 m. Airgun and 3.5-kHz profiles record a variety of drift types that reflect regional variations in bathymetry, sediment supply, and the tectonic/volcanic framework. On Chatham Rise the DWBC has deposited a sinuous, linear body along the south flank (3000 m), an extensive apronlike drift on the north flank (2200–4500 m), and a ridgelike drift about the rise base (4500–5200 m). The flow has also deposited a body of sediment over 400 km long within a moat at the base of the nearby Louisville Seamount Chain. Further downcurrent, the 250 km long Rekohu Drift (3600–4190 m) has developed northward to 39°S. South of this latitude, drifts comprise mainly reworked pelagic/hemipelagic material and sediment transported from distant southerly sources. In contrast, drifts north of 39°S have received a major injection of terrigenous sediment from Hikurangi Channel which runs 1400 km from New Zealand, eastward across the Hikurangi Plateau to disgorge on to the abyssal floor at the plateau edge. En route, turbidity current overspill from the channel has moved north under the influence of the shallow DWBC to contribute to a series of small ridge and patch drifts among the numerous seamounts on the plateau at 3500–4200 m. Off Hikurangi Channel mouth, a large fan has accumulated. The DWBC has extended the fan into a drift running over 250 km along the base of Hikurangi Plateau (5150–5770 m) toward Kermadec Trench. Here drift sediment becomes increasingly disrupted by mass wasting associated with the active subduction in this area. The seismic stratigraphy reveals the drifts to rest mainly on a widespread erosional surface that is interpreted to mark the inception of the DWBC in the region with the late Oligocene opening of the Australian-Antarctic seaway. Drift construction commenced during the Miocene but was punctuated in the late Miocene by another period of erosion that coincided with increased bottom water production in Antarctica. Deposition resumed in Plio-Pleistocene times when large quantities of sediment from the rapidly rising landmass of New Zealand were injected into the boundary current. The modern flow continues to affect drift deposition as manifest by an active boundary channel along the foot of Hikurangi Plateau and widespread scour zones and sediment wave fields.

Regional sediment recycling in the abyssal Southwest Pacific Ocean

An active plate boundary with a high sediment output, three major submarine channels, and the world9s largest deep western boundary current (DWBC) make up an extensive recycling system along the 4500 km continental margin, east of New Zealand. Seismic reflection, sedimentary, and oceanographic data demonstrate that detritus from the rising mountains of the New Zealand plate boundary is transferred to the Southwest Pacific abyssal floor by turbidity currents flowing along Solander (>450 km long), Bounty (950 km), and Hikurangi (1400 km) channels. These conduits discharge directly into the DWBC, which transports material north to form a series of sediment drifts. The northernmost drift, containing sediment from Hikurangi Channel and eroded drifts to the south, is now subducting into Kermadec Trench. Geochemical data suggest that sediment is recycled through the mantle to re-emerge in the arc volcanic rocks. Thus one cycle is completed and a new one begins.

Evolution of the sedimentary system beneath the deep Pacific inflow off eastern New Zealand

Results from Ocean Drilling Program sites 1121–1124 show the Eastern New Zealand Oceanic Sedimentary System (ENZOSS) evolved in response to: (1) the inception of the circum-Antarctic circulation, (2) orbital and non-orbital regulation of the global thermohaline flow, and (3) development of the New Zealand plate boundary. ENZOSS began in the early Oligocene following opening of the Tasmanian gateway and inception of the ancestral Antarctic Circumpolar Current (ACC) and SW Pacific Deep Western Boundary Current (DWBC). Widespread erosion, marked by the Marshall Paraconformity, was followed by extensive drift formation in the late Oligocene–early Miocene. Alternating nannofossil chalk and nannofossil-rich mud deposited in response to 41-kyr orbital regulation of the abyssal circulation, with the mudstones representing times of increased inflow of corrosive southern-source waters. Drift deposition at the deepest sites was interrupted by bouts of erosion coincident with Mi1–5 isotopic events signifying expansions of the East Antarctic Ice Sheet and enhanced bottom water formation. By late Miocene times, the basic ENZOSS was established. South of Bounty Trough, the energetic ACC instigated an erosional/low depositional regime. To the north, where the DWBC prevailed, orbitally regulated drift deposition continued. Increased convergence at the New Zealand plate boundary enhanced the terrigenous supply, but little of this sediment reached the deep ENZOSS as the three main sediment conduits – Solander, Bounty and Hikurangi channels – had not fully developed. The Plio–Pleistocene heralded a change from a carbonate- to terrigenous-dominant supply caused by interception of the DWBC by the three channels (∼1.6 Ma for Bounty and Hikurangi, time of Solander interception unknown). The Solander and Bounty fans, and Hikurangi Fan-drift systems formed, and drifts downstream of those systems, received terrigenous detritus. Supply increased with accelerating uplift along the plate boundary, but delivery to the DWBC was regulated by eustatic fluctuations of sea level. Times of maximum supply to all three channels was during glacial lowstands whereas the supply either ceased (Bounty, Solander), or reduced (Hikurangi) in highstands. In glacial times, sediment was entrained by a DWBC invigorated by an increased input of Antarctic bottom water. The ACC also accelerated under strengthened glacial winds. Thus, glacials were times of optimum sediment supply to ENZOSS depocentres where depositional rates were 2–3 times more than interglacial rates.