Claire Waelbroeck

Sea-level and deep water temperature changes derived from benthic foraminifera isotopic records.

We show that robust regressions can be established between relative sea-level (RSL) data and benthic foraminifera oxygen isotopic ratios from the North Atlantic and Equatorial Pacific Ocean over the last climatic cycle. We then apply these regressions to long benthic isotopic records retrieved at one North Atlantic and one Equatorial Pacific site to build a composite RSL curve, as well as the associated confidence interval, over the last four climatic cycles. Our proposed reconstruction of RSL is in good agreement with the sparse RSL data available prior to the last climatic cycle. We compute bottom water temperature changes at the two sites and at one Southern Indian Ocean site, taking into account potential variations in North Atlantic local deep water δ18O. Our results indicate that a Last Glacial Maximum (LGM) enrichment of the ocean mean oxygen isotopic ratio of 0.95‰ is the lowest value compatible with unfrozen deep waters in the Southern Indian Ocean if local deep water δ18O did not increase during glacials with respect to present. Such a value of the LGM mean ocean isotopic enrichment would impose a maximum decrease in local bottom water δ18O at the North Atlantic site of 0.30‰ during glacials.

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.

The timing of the last deglaciation in North Atlantic climate records

To determine the mechanisms governing the last deglaciation and the sequence of events that lead to deglaciation, it is important to obtain a temporal framework that applies to both continental and marine climate records. Radiocarbon dating has been widely used to derive calendar dates for marine sediments, but it rests on the assumption that the ‘apparent age’ of surface water (the age of surface water relative to the atmosphere) has remained constant over time. Here we present new evidence for variation in the apparent age of surface water (or reservoir age) in the North Atlantic ocean north of 40° N over the past 20,000 years. In two cores we found apparent surface-water ages to be larger than those of today by 1,230 ± 600 and 1,940 ± 750 years at the end of the Heinrich 1 surge event (15,000 years BP) and by 820 ± 430 to 1,010 ± 340 years at the end of the Younger Dryas cold episode. During the warm Bølling–Allerød period, between these two periods of large reservoir ages, apparent surface-water ages were comparable to present values. Our results allow us to reconcile the chronologies from ice cores and the North Atlantic marine records over the entire deglaciation period. Moreover, the data imply that marine carbon dates from the North Atlantic north of 40° N will need to be corrected for these highly variable effects.

Improving past sea surface temperature estimates based on planktonic fossil faunas

A new method of past sea surface temperature (SST) reconstruction based on the modern analog technique (Prell, 1985) and on the indirect approach (Bartlein et al., 1986) has been developed: the revised analog method (RAM). Applied to planktonic foraminifera, this technique leads to significant improvements in modern SST reconstruction with respect to former methods: our estimates are characterized by much lower residuals and a better coverage of the observed SST range. Moreover, the error of RAM estimates of past SSTs is lower than that associated with former reconstructions, particularly at middle and high latitudes. In low latitudes, cold season SSTs reconstructed by RAM during glacials are 1°–3°C lower than previously estimated. Our results tend thus to reconcile paleoestimates of glacial temperatures based on planktonic microfossils and on continental data in the tropics.

Climatic interpretation of the recently extended Vostok ice records

A new ice core drilled at the Russian station of Vostok in Antarctica reached 2755 m depth in September 1993. At this depth, the glaciological time scale provides an age of 260 ky BP (±25). We refine this estimate using records of dust and deuterium in the ice and of δ18O of O2 in the entrapped air. δ18O of O2 is highly correlated with insolation over the last two climatic cycles if one assumes that the EGT chronology overestimates the increase of age with depth by 12% for ages older than 112 ky BP. This modified age-depth scale gives an age of 244 ky BP at 2755 m depth and agrees well with the age-depth scale of Walbroeck et al. (in press) derived by orbital tuning of the Vostok δD record. We discuss the temperature interpretation of this latter record accounting for the influence of the origin of the ice and using information derived from deuterium-excess data. We conclude that the warmest period of stage 7 was likely as warm as today in Antarctica. A remarkable feature of the Vostok record is the high level of similarity of proxy temperature records for the last two climatic cycles (stages 6 and 7 versus stages 1–5). This similarity has no equivalent in other paleorecords.

Comparison of statistical and artificial neural network techniques for estimating past sea surface temperatures from planktonic foraminifer census data

We present the first detailed and rigorous comparison of six different computational techniques used to reconstruct sea surface temperatures (SST) from planktonic foraminifer census data. These include the Imbrie-Kipp transfer functions (IKTF), the modern analog technique (MAT), the modern analog technique with similarity index (SIMMAX), the revised analog method (RAM), and, for the first time, a set of back propagation artificial neural networks (ANN) trained on a large faunal data set, including a modification where geographical information was added among the input variables (ANND). By training the techniques on an identical database, we were able to explore the differences in SST reconstructions resulting solely from the use of different mathematical methods. The comparison indicates that while the IKTF technique consistently shows the worst performance, ANN and RAM perform slightly better than MAT and that the inclusion of the geographical information into the training database (SIMMAX and ANND) further improves the accuracy of modern SST estimates. However, when applied to an independent validation data set and an additional fossil data set, the results did not conform to this ranking. The largest differences in the reconstructed SST values occurred between groups of techniques with different approaches to SST reconstruction; that is, ANN and ANND produced SST reconstructions significantly different from those produced by RAM, SIMMAX, and MAT. The application of the various techniques to the validation data set, which allowed comparison of SST reconstructions with instrumental records, suggests that artificial neural networks might provide better paleo-SST estimates than the other techniques.

Influence of Bering Strait flow and North Atlantic circulation on glacial sea-level changes

Sea-level fluctuations of about 20-30m occurred throughout the last glacial period. These fluctuations seem to have been derived primarily from changes in the volume of Northern Hemisphere ice sheets(1-3), and cannot be attributed solely to ice melt caused by varying solar radiation(4). Here we use a fully coupled climate model to show that the transport of relatively fresh Pacific water into the North Atlantic Ocean was limited when lower sea level restricted or closed the Bering Strait, resulting in saltier North Atlantic surface waters. This invigorated deep convection in the North Atlantic Ocean, strengthening meridional overturning circulation and northward heat transport in our model, which consequently promoted melting of ice sheets in North America and Europe. Our simulations show that the associated sea-level rise led to a reopening of the Bering Strait; the flux of relatively fresh water into the North Atlantic Ocean muted meridional overturning circulation and led to cooling and ice-sheet advance in the Northern Hemisphere. We conclude that the repetition of this cycle could produce the sea-level changes that have been observed throughout the last glacial cycle.

Dating the Vostok ice core by an inverse method

Using the chronological information available in the Vostok records, we apply an inverse method to assess the quality of the Vostok glaciological timescale. The inversion procedure provides not only an optimized glaciological timescale and its confidence interval but also a reliable estimate of the duration of successive events. Our results highlight a disagreement between orbitally tuned and glaciological timescales below ∼2700 m (i.e., ∼250 kyr B.P., thousands of years before present). This disagreement could be caused by some discontinuity in the spatial variation of accumulation upstream of Vostok. Moreover, the stratigraphic datings of central Greenland ice cores (GRIP and GISP2) appear older than our optimized timescale for the late glacial. This underlines an unconsistency between the physical assumptions used to construct the Vostok glaciological timescale and the stratigraphic datings. The inverse method allows the first assessment of the evolution of the phase between Vostok climatic records and insolation. This phase significantly varies with time which gives a measure of the nonlinear character of the climatic system and suggests that the climatic response to orbital forcing is of different nature for glacial and interglacial periods. We confirm that the last interglacial, as recorded in the Vostok deuterium record, was long (16.2±2 kyr, thousands of years). However, midtransition of termination II occurred at 133.4±2.5 kyr BP, which does not support the recent claim for an earlier deglaciation. Finally, our study suggests that temperature changes are correctly estimated when using the spatial present-day deuterium-temperature relationship to interpret the Vostok deuterium record.

The impact of permafrost thawing on the carbon dynamics of tundra

There is debate on the potential release of the tundras immense carbon stocks into the atmosphere in response to global warming. We present here results obtained with a model of CO2 exchanges, coupled to a model of the soil thermal and hydrological regime in the tundra. We show that, because of the partial thawing of permafrost and subsequent increase in nutrient availability, the ecosystems response to warming may be a long-lasting increase in C accumulation, following a temporary increase in CO2 emissions. Our study also provides a consistent picture of CO2 exchanges in tundra ecosystems, reconciling the short-term experimental response to warming, recent field measurements, and Holocene C accumulation estimates.

Low-latitude hydrological cycle and rapid climate changes during the last deglaciation

Sea surface temperature and oxygen isotopic records from two well-dated Indian Ocean cores covering the last deglaciation show the occurrence of two periods of increased salinity along the route of warm surface water transport from the Indian to the Atlantic Ocean, one between 18 and 14.5 ka and the other during the Younger Dryas. Our results imply that during these periods, salt accumulated in the tropical Atlantic, creating favorable conditions for an abrupt resumption of the thermohaline circulation and abrupt northern hemisphere warming. Furthermore, we suggest that the observed pattern of millennial climate variability during the last glacial and deglaciation resulted from the interaction between the relatively slow rhythm of expansion and decay of the northern hemisphere ice sheets, and El Nino–Southern Oscillation variability, through changes in the position of the Intertropical Convergence Zone. This interaction generated an oscillator with millennial time response that operated at times of sufficient northern hemisphere ice sheets extent.