Bruno Malaizé

Age scale of the air in the summit ice: Implication for glacial‐interglacial temperature change

The air occluded in ice sheets and glaciers has, in general, a younger age (defined as the time after its isolation from the atmosphere) than the surrounding ice matrix because snow is first transformed into open porous firn, in which the air can exchange with the atmosphere. Only at a certain depth (firn-ice transition) the pores are pinched off and the air is definitely isolated from the atmosphere. The firn-ice transition depth is at around 70 m under present climatic conditions at Summit, central Greenland. The air at this depth is roughly 10 years old due to diffusive mixing, whereas the ice is about 220 years old. This results in an age difference between the air and the ice of 210 years. This difference depends on temperature and accumulation rate and did thus not remain constant during the past. We used a dynamic firn densification model to calculate the firn-ice transition depth and the age of the ice at this depth and an air diffusion model to determine the age of the air at the transition. Past temperatures and accumulation rates have been deduced from the δ18O record using time independent functions. We present the results of model calculations of two paleotemperature scenarios yielding a record of the age difference between the air and the ice for the Greenland Ice Core Project (GRIP) and the Greenland Ice Sheet Project Two (GISP2) ice cores for the last 100,000 years. During the Holocene, the age difference stayed rather stable around 200 years, while it reached values up to 1400 years during the last glaciation for the colder scenario. The model results are compared with age differences obtained independently by matching corresponding climate events in the methane and δ18O records assuming a very small phase lag between variations in the Greenland surface temperature and the atmospheric methane. The past firn-ice transition depths are compared with diffusive column heights obtained from δ15N of N2 measurements. The results of this study corroborate the large temperature change of 20 to 25 K from the coldest glacial to Holocene climate found by evaluating borehole temperature profiles.

CH4 and δ18O of O2 records from Antarctic and Greenland ice: A clue for stratigraphic disturbance in the bottom part of the Greenland Ice Core Project and the Greenland Ice Sheet Project 2 ice cores

The suggestion of climatic instability during the last interglacial period (Eem), based on the bottom 10% of the Greenland Ice core Project (GRIP) isotopic profile, has been questioned because the bottom record from the neighboring Greenland Ice Sheet Project 2 (GISP2) core (28 km away) is strikingly different over the same interval and because records of the δ18O of atmospheric O2 from both cores showed unexpected rapid fluctuations. Here we present detailed methane records from the Vostok (Antarctica), GRIP, and GISP2 cores over the relevant intervals. The GRIP and GISP2 data show rapid and large changes in methane concentration, which are correlative with variations of the δ18O of the ice, while the Vostok record shows no such variations. This discrepancy reinforces the suggestion that the bottom sections of the Greenland records are disturbed. By combining the methane data with measurements of δ18O of O2 in the same samples, we attempt to constrain the nature of the stratigraphic disturbance and the age of the analyzed ice samples. Our results suggest that ice layers from part of the last interglacial period exist in the lower section of both ice cores and that some of the apparent climate instabilities in the GRIP core would be the result of a mixture of ice from the last interglacial with ice from the beginning of the last glaciation or from the penultimate glaciation.

The Dole effect over the last two glacial-interglacial cycles

Detailed measurements of δ 18 O of atmospheric oxygen performed on air trapped in the Vostok ice cores (Antarctica) are used to extend the record of the Dole effect over two climatic cycles (back to 240 kyr B.P.). Except for glacial terminations I and II and for an unexpected minimum occurring around 175 kyr, the Dole effect shows small variations (ADole within ± 0.5‰). These small variations, however, show a well-marked 23 kyr precessional periodicity, thus confirming the results obtained by Bender et al. [1994a] for the first climatic cycle. To explain the minimum value reached around 175 kyr, we invoke the possibility of a peak in the oceanic productivity linked to climatic events induced at low latitudes under glacial conditions.

A tentative reconstruction of the last interglacial and glacial inception in Greenland based on new gas measurements in the Greenland Ice Core Project (GRIP) ice core

parameters. The GRIP d 18 Oice chronological sequence is obtained by comparing a new set of d 18 O of atmospheric O2 and CH4 measurements from the bottom section of the GRIP core with their counterpart in the Vostok Antarctic profiles. This comparison clearly identifies ice from the penultimate glacial maximum (MIS 6, 190–130 kyr B.P.) in the GRIP core. Further it allows rough reconstruction of the last interglacial period and of the last glacial inception in Greenland which appears to lay its Antarctic counterpart. Our data suggest that while Antarctica is already entering into a glaciation, Greenland is still experiencing a warm maximum during MIS 5e. INDEX TERMS: 1040 Geochemistry: Isotopic composition/chemistry; 1827 Hydrology: Glaciology (1863); 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; KEYWORDS: interglacial, ice cap, firn

Hurricanes and climate in the Caribbean during the past 3700 years BP

A multiproxy analysis of lacustrine sediments cored in Grand-Case Pond at Saint-Martin, north of the Lesser Antilles archipelago, reveals three distinct climatic periods for the last 3700 years. From 3700 to ~2500 yr cal. BP and from 1150 yr cal. BP to the present, carbonate mud deposition occurred in connection with pond lowstands. These periods were also punctuated by severe drought events, marked by gypsum laminae, and hurricane landfalls, leading to marine sand inputs into the pond. The intermediate time interval, from 2500 to 1150 yr cal. BP, is typified by black organic mud deposition, suggesting that hypoxic to anoxic conditions prevailed at the pond bottom. These were probably linked with a perennial pond highstand and reflect more uniform and wetter climatic conditions than today. The carbon isotopic composition of the ostracod Perissocytheridea bisulcata shows that the lowest δ13C values are recorded during the hypoxic periods, as a consequence of bacterial recycling of isotopically depleted organic matter. Such a climatic history agrees closely with that documented from other records in the Caribbean area, such as the Cariaco Basin, central coast of Belize or Barbados. By constrast, discrepancies seem to emerge from the comparison between hurricane activity recorded at Saint-Martin on the one hand and Vieques (Puerto Rico) on the other hand. We explain this apparent contradiction by a balance between two distinct storm paths in response to latitudinal shifts of the Intertropical Convergence Zone (ITCZ). Stronger storm activity over the Gulf coast and the inner Caribbean Sea is favoured by a southern position of the ITCZ in connection with dry climatic conditions. Plausible links with the North Atlantic Oscillation (NAO) are also suggested.

Agulhas leakage as a key process in the modes of Quaternary climate changes.

Heat and salt transfer from the Indian Ocean to the Atlantic Ocean (Agulhas leakage) has an important effect on the global thermohaline circulation and climate. The lack of long transfer record prevents elucidation of its role on climate changes throughout the Quaternary. Here, we present a 1,350-ka accumulation rate record of the planktic foraminiferal species Globorotalia menardii. We demonstrate that, according to previous assumptions, the presence and reseeding of this fauna in the subtropical southeast Atlantic was driven by interocean exchange south of Africa. The Agulhas transfer strengthened at glacial ice-volume maxima for every glacial-interglacial transition, with maximum reinforcements organized according to a 400-ka periodicity. The long-term dynamics of Agulhas leakage may have played a crucial role in regulating meridional overturning circulation and global climate changes during the Mid-Brunhes event and the Mid-Pleistocene transition, and could also play an important role in the near future.

A high-resolution investigation of temperature, salinity, and upwelling activity proxies in corals

We present a high-resolution study of five geochemical components of a New Caledonia coral core. Minor and trace elements (Sr/Ca, U/Ca, Mg/Ca, Ba/Ca), together with oxygen isotopes (δ18O), were investigated at near-fortnightly resolution. Geochemical measurements were compared to sea surface temperature (SSTTSG) and salinity (SSSTSG) recorded by a thermosalinograph (TSG) located less than 10 m from the coral. Results show that Sr/Ca, Mg/Ca, and δ18O are temperature-dependent (correlation coefficient to local SSTTSG between 0.74 and 0.84). Their robustness as SST tracers is tested against a satellite-based data set for a 4 year validation period. U/Ca has a more complex behavior and appears to be linked to both SSTTSG and SSSTSG, despite relatively small local SSS variations. For the first time this salinity-related imprint is quantified in a U/Ca coralline ratio. In addition to SST and SSS tracers, Ba/Ca measurements provide a possible way to investigate upwelling activity occurrences.

25. Climate variability of the last five isotopic interglacials: Direct land-sea-ice correlation from the multiproxy analysis of North-Western Iberian margin deep-sea cores

Abstract The last five isotopic interglacials (marine isotope stages 11, 9, 7, 5 and 1) were investigated in Iberian margin deep-sea cores, using terrestrial (pollen) and marine (planktic foraminifera assemblages, benthic and planktic oxygen isotopes) climatic indicators. This work shows that the climatic variability detected on the continent is contemporaneously recorded in the ocean, but temperature changes are not in phase with ice volume variations. The comparison of the different marine isotope stages highlights a common pattern within these stages. They are characterized by three major climatic cycles, related to orbital cyclicity, on which suborbital climatic fluctuations are superimposed. Particularly, suborbital events interrupt the deglacial warming associated with Terminations IV to I and the second major warm period of each isotopic interglacial as well as the transitions towards glacial marine isotope stages. MIS 7 displays a short first warm period (8kyr) followed by a striking cold and dry period succeeded by a new strong warmth. In contrast, MIS 11 presents the longest period (31kyr) of the last 450000 years.

Occurrence of an exceptional carbonate dissolution episode during early glacial isotope stage 6 in the Southeastern Atlantic

Abstract Concentration and mass accumulation rate profiles from Southeastern Atlantic sediment cores located off Namibia show that an exceptional episode in benthic carbonate dissolution occurred during early glacial isotope stage 6 (substages 6.6 and 6.5) between about 186 000 and 170 000 yr BP. Although this episode is restricted to or is more pronounced in this region than in other areas of the Atlantic Ocean, its exceptional character with respect to older and younger climatic episodes at the same site cannot be fully explained by local factors alone, but requires a combination of local and global influences. The onset of the carbonate dissolution episode is related to a more efficient transfer of organic matter from surface eutrophic areas to the lower and is due to low sea level, while its termination relates to a change in either global ocean alkalinity or bottom water circulation. An evaluation of the magnitude of this local carbonate dissolution episode suggests that its contribution to a global alkalinity change may have been significant. Carbonate dissolution was probably amplified by stronger upwelling activity of the Benguela System linked to an exceptional northern excursion of the boreal summer ITCZ during early glacial isotope stage 6. This low latitude global linkage may explain how this carbonate dissolution event as well as other ‘anomalies’ observed for early stage 6, like an important Dole effect minimum or a ‘cold’ Mediterranean sapropel, are related.

African humid periods triggered the reactivation of a large river system in Western Sahara.

The Sahara experienced several humid episodes during the late Quaternary, associated with the development of vast fluvial networks and enhanced freshwater delivery to the surrounding ocean margins. In particular, marine sediment records off Western Sahara indicate deposition of river-borne material at those times, implying sustained fluvial discharges along the West African margin. Today, however, no major river exists in this area; therefore, the origin of these sediments remains unclear. Here, using orbital radar satellite imagery, we present geomorphological data that reveal the existence of a large buried paleodrainage network on the Mauritanian coast. On the basis of evidence from the literature, we propose that reactivation of this major paleoriver during past humid periods contributed to the delivery of sediments to the Tropical Atlantic margin. This finding provides new insights for the interpretation of terrigenous sediment records off Western Africa, with important implications for our understanding of the paleohydrological history of the Sahara.