Luke C Skinner

Ventilation of the Deep Southern Ocean and Deglacial CO2 Rise

Telling Up from Down It is generally believed that carbon dioxide accumulates in the deep ocean during cold periods and that it is released rapidly and in huge quantities during deglaciation, but evidence of deep ocean carbon dioxide storage has been elusive. Now Skinner et al. (p. 1147; see the Perspective by Anderson and Carr) present radiocarbon data from the Southern Ocean that indicate that the deep water circulating around Antarctica was about twice as old relative to the atmosphere as it is today, a condition considered indicative of carbon dioxide accumulation and storage. Radiocarbon analyses show that old, deep water existed around Antarctica at the end of the last glacial period. Past glacial-interglacial increases in the concentration of atmospheric carbon dioxide (CO2) are thought to arise from the rapid release of CO2 sequestered in the deep sea, primarily via the Southern Ocean. Here, we present radiocarbon evidence from the Atlantic sector of the Southern Ocean that strongly supports this hypothesis. We show that during the last glacial period, deep water circulating around Antarctica was more than two times older than today relative to the atmosphere. During deglaciation, the dissipation of this old and presumably CO2-enriched deep water played an important role in the pulsed rise of atmospheric CO2 through its variable influence on the upwelling branch of the Antarctic overturning circulation.

800,000 Years of Abrupt Climate Variability

Greenland climate variability for the past 800,000 years was inferred from the Antarctic ice-core temperature record. We constructed an 800,000-year synthetic record of Greenland climate variability based on the thermal bipolar seesaw model. Our Greenland analog reproduces much of the variability seen in the Greenland ice cores over the past 100,000 years. The synthetic record shows strong similarity with the absolutely dated speleothem record from China, allowing us to place ice core records within an absolute timeframe for the past 400,000 years. Hence, it provides both a stratigraphic reference and a conceptual basis for assessing the long-term evolution of millennial-scale variability and its potential role in climate change at longer time scales. Indeed, we provide evidence for a ubiquitous association between bipolar seesaw oscillations and glacial terminations throughout the Middle to Late Pleistocene.

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.

Interglacials of the last 800,000 years

Interglacials, including the present (Holocene) period, are warm, low land ice extent (high sea level), end-members of glacial cycles. Based on a sea level definition, we identify eleven interglacials in the last 800,000 years, a result that is robust to alternative definitions. Data compilations suggest that despite spatial heterogeneity, Marine Isotope Stages (MIS) 5e (last interglacial) and 11c (~400 ka ago) were globally strong (warm), while MIS 13a (~500 ka ago) was cool at many locations. A step change in strength of interglacials at 450 ka is apparent only in atmospheric CO2 and in Antarctic and deep ocean temperature. The onset of an interglacial (glacial termination) seems to require a reducing precession parameter (increasing Northern Hemisphere summer insolation), but this condition alone is insufficient. Terminations involve rapid, nonlinear, reactions of ice volume, CO2, and temperature to external astronomical forcing. The precise timing of events may be modulated by millennial-scale climate change that can lead to a contrasting timing of maximum interglacial intensity in each hemisphere. A variety of temporal trends is observed, such that maxima in the main records are observed either early or late in different interglacials. The end of an interglacial (glacial inception) is a slower process involving a global sequence of changes. Interglacials have been typically 10–30 ka long. The combination of minimal reduction in northern summer insolation over the next few orbital cycles, owing to low eccentricity, and high atmospheric greenhouse gas concentrations implies that the next glacial inception is many tens of millennia in the future.

Radiocarbon evidence for alternating northern and southern sources of ventilation of the deep Atlantic carbon pool during the last deglaciation

Significance This study sheds light on the mechanisms of deglacial atmospheric CO2 rise and, more specifically, on the hypothesized role of a “bipolar seesaw” in deep Atlantic ventilation. Comparing new high-resolution radiocarbon reconstructions from the Northeast Atlantic with existing data from the Southern Ocean, we show that a bipolar ventilation seesaw did indeed operate during the last deglaciation. Whereas today the deep Atlantic’s carbon pool is “flushed” from the north by North Atlantic Deep Water export, it was flushed instead from the south during Heinrich Stadial 1 and the Younger Dryas, in time with sustained atmospheric CO2 rise. Recent theories for glacial–interglacial climate transitions call on millennial climate perturbations that purged the deep sea of sequestered carbon dioxide via a “bipolar ventilation seesaw.” However, the viability of this hypothesis has been contested, and robust evidence in its support is lacking. Here we present a record of North Atlantic deep-water radiocarbon ventilation, which we compare with similar data from the Southern Ocean. A striking coherence in ventilation changes is found, with extremely high ventilation ages prevailing across the deep Atlantic during the last glacial period. The data also reveal two reversals in the ventilation gradient between the deep North Atlantic and Southern Ocean during Heinrich Stadial 1 and the Younger Dryas. These coincided with periods of sustained atmospheric CO2 rise and appear to have been driven by enhanced ocean–atmosphere exchange, primarily in the Southern Ocean. These results confirm the operation of a bipolar ventilation seesaw during deglaciation and underline the contribution of abrupt regional climate anomalies to longer-term global climate transitions.

A warm and poorly ventilated deep Arctic Mediterranean during the last glacial period

Slow circulation in the cold Arctic The Arctic Ocean and Nordic Seas together supply dense, sinking water to the Atlantic Meridional Overturning Circulation (AMOC). The redistribution of heat by the AMOC, in turn, exerts a major influence on climate in the Northern Hemisphere. Thornalley et al. report that during the last glacial period, those regions were nearly stagnant and supplied almost none of the water that they presently contribute to the AMOC. This low rate of flow into the Atlantic was probably due to an absence of vigorous deep-water formation in the Arctic Mediterranean as a consequence of the extensive ice cover there at that time. Science, this issue p. 706 Deep-water formation in some Arctic seas nearly ceased during the peak of the last glacial period. Changes in the formation of dense water in the Arctic Ocean and Nordic Seas [the “Arctic Mediterranean” (AM)] probably contributed to the altered climate of the last glacial period. We examined past changes in AM circulation by reconstructing radiocarbon ventilation ages of the deep Nordic Seas over the past 30,000 years. Our results show that the glacial deep AM was extremely poorly ventilated (ventilation ages of up to 10,000 years). Subsequent episodic overflow of aged water into the mid-depth North Atlantic occurred during deglaciation. Proxy data also suggest that the deep glacial AM was ~2° to 3°C warmer than modern temperatures; deglacial mixing of the deep AM with the upper ocean thus potentially contributed to the melting of sea ice, icebergs, and terminal ice-sheet margins.

North Atlantic versus Southern Ocean contributions to a deglacial surge in deep ocean ventilation

Past glacial-interglacial climate transitions were accompanied by millennial-scale pulses in atmospheric CO2 that are widely thought to have resulted from the release of CO2 via the Southern Ocean. However, direct proxy evidence for a Southern Ocean role in regulating past ocean-atmosphere CO2 exchange is scarce. Here we use combined radiocarbon and neodymium isotope measurements from the last deglaciation to confirm greatly enhanced overturning and/or air-sea exchange rates relative to today, in particular during the Bolling-Allerod warm interval. We show that this deglacial pulse in ocean ventilation was not driven by the North Atlantic overturning alone, and must have involved an increase in the ventilation of southern-sourced deep waters. Our results thus confirm the removal of a physical and/or dynamical barrier to effective air-sea (CO2) exchange in the Southern Ocean during deglaciation, and highlight the Antarctic region as a key locus for global climate/carbon-cycle feedbacks.

Increased reservoir ages and poorly ventilated deep waters inferred in the glacial Eastern Equatorial Pacific

Consistent evidence for a poorly ventilated deep Pacific Ocean that could have released its radiocarbon-depleted carbon stock to the atmosphere during the last deglaciation has long been sought. Such evidence remains lacking, in part due to a paucity of surface reservoir age reconstructions required for accurate deep-ocean ventilation age estimates. Here we combine new radiocarbon data from the Eastern Equatorial Pacific (EEP) with chronostratigraphic calendar age constraints to estimate shallow sub-surface reservoir age variability, and thus provide estimates of deep-ocean ventilation ages. Both shallow- and deep-water ventilation ages drop across the last deglaciation, consistent with similar reconstructions from the South Pacific and Southern Ocean. The observed regional fingerprint linking the Southern Ocean and the EEP is consistent with a dominant southern source for EEP thermocline waters and suggests relatively invariant ocean interior transport pathways but significantly reduced air–sea gas exchange in the glacial southern high latitudes.

Land-ocean changes on orbital and millennial time scales and the penultimate glaciation

Past glacials can be thought of as natural experiments in which variations in boundary conditions influenced the character of climate change. However, beyond the last glacial, an integrated view of orbital- and millennial-scale changes and their relation to the record of glaciation has been lacking. Here, we present a detailed record of variations in the land-ocean system from the Portuguese margin during the penultimate glacial and place it within the framework of ice-volume changes, with particular reference to European ice-sheet dynamics. The interaction of orbital- and millennial-scale variability divides the glacial into an early part with warmer and wetter overall conditions and prominent climate oscillations, a transitional mid-part, and a late part with more subdued changes as the system entered a maximum glacial state. The most extreme event occurred in the mid-part and was associated with melting of the extensive European ice sheet and maximum discharge from the Fleuve Manche river. This led to disruption of the meridional overturning circulation, but not a major activation of the bipolar seesaw. In addition to stadial duration, magnitude of freshwater forcing, and background climate, the evidence also points to the influence of the location of freshwater discharges on the extent of interhemispheric heat transport.

Phasing of Millennial Climate Events and Northeast Atlantic Deep‐Water Temperature Change Since 50 Ka Bp

remains lacking. Nevertheless, a growing body of proxy and modelling evidence tends to support the hypothesis that changes in the Atlantic meridional overturning circulation (MOC) have at least been implicated in, if not indeed responsible for orchestrating, the asymmetrical pattern of climate change observed at high latitudes [e.g. Ganopolski and Rahmstorf, 2001; McManus et al., 2004; Piotrowski et al., 2004; Schmittner et al., 2003; Skinner and Shackleton, 2004]. The observations of Shackleton et al. [2000] (reproduced and expanded in Figure 1) are of particular interest in this regard, for they strongly suggest that robust clues as to the roles of the ocean circulation and the cryosphere in mediating (or responding to) inter-hemispheric climate change are encrypted in the Phasing of Millennial Climate Events and Northeast Atlantic Deep-Water Temperature Change Since 50 ka BP