Alan C Mix

The last interglacial ocean

The final effort of the CLIMAP project was a study of the last interglaciation, a time of minimum ice volume some 122,000 yr ago coincident with the Substage 5e oxygen isotopic minimum. Based on detailed oxygen isotope analyses and biotic census counts in 52 cores across the world ocean, last interglacial sea-surface temperatures (SST) were compared with those today. There are small SST departures in the mid-latitude North Atlantic (warmer) and the Gulf of Mexico (cooler). The eastern boundary currents of the South Atlantic and Pacific oceans are marked by large SST anomalies in individual cores, but their interpretations are precluded by no-analog problems and by discordancies among estimates from different biotic groups. In general, the last interglacial ocean was not significantly different from the modern ocean. The relative sequencing of ice decay versus oceanic warming on the Stage 6/5 oxygen isotopic transition and of ice growth versus oceanic cooling on the Stage 5e/5d transition was also studied. In most of the Southern Hemisphere, the oceanic response marked by the biotic census counts preceded (led) the global ice-volume response marked by the oxygen-isotope signal by several thousand years. The reverse pattern is evident in the North Atlantic Ocean and the Gulf of Mexico, where the oceanic response lagged that of global ice volume by several thousand years. As a result, the very warm temperatures associated with the last interglaciation were regionally diachronous by several thousand years. These regional lead-lag relationships agree with those observed on other transitions and in long-term phase relationships; they cannot be explained simply as artifacts of bioturbational translations of the original signals.

North Atlantic surface-ocean control of Pleistocene deep-ocean circulation

Deep basins of the North Atlantic were occupied by a cold 13 C-depleted (nutrient-rich) water mass during glaciations, and a warmer 13C-enriched (nutrient-poor) water mass (modern NADW) during interglaciations. This mode of deep-water variability is related directly to migration of the North Atlantic Polar Front. Down-core stable isotope records from the Atlantic and Antarctic suggest that 13C depletion of the glacial North Atlantic may reflect higher preformed nutrients (due to bottom-water formation under sea and/or shelf ice) and increased residence time of northern-source water, rather than replacement of NADW with AABW.

Oxygen-isotope analyses and Pleistocene ice volumes

Abstract The oxygen-isotope record from fossil foraminifera in deep-sea sediments is commonly used as a proxy for global ice volume. The linkage between δ 18 O and ice volume, however, is probably nonlinear. We have developed a simple numerical model of the isotopic response of the oceans to ice-volume change. The major features it simulates are (1) the changing mean isotopic composition of snow as a function of ice volume (colder snow temperatures forced by climate change and higher-elevation accumulation areas imply more negative mean δ 18 O); (2) the nonequilibrium isotopic composition of ice sheets (the past history of an ice sheet is integrated into its mean isotopic composition, which introduces a lag of isotopic “ice volume,” i.e. , the measured δ 18 O record, scaled to ice-volume units, behind true ice volume); (3) selective preservation of isotopically more negative (colder, higher-latitude) ice (this geographic effect can selectively amplify or dampen the isotopic response to the ice-volume signal). We illustrate the response of our model to simple hypothetical ice-volume transitions of ice growth and ice decay. Sensitivity tests are illustrated for all model parameters. The results suggest that oxygen-isotope records reproduce the general patterns of ice-volume change fairly accurately. The foraminiferal isotope record, however, may misrepresent the true amplitude of the ice-volume signal and lag true ice volume by 1000 to 3000 yr.

Eolian Evidence for Spatial Variability of Late Quaternary Climates in Tropical Africa

Abstract Study of the eolian fraction of late Quaternary sediments from the tropical Atlantic reveals that two modes of long-term climate variability have existed in tropical Africa during the last 150,000 yr. Tropical northwest Africa (i.e., the southwestern Sahara and Sahel) was driest during glaciations and stades, but wetter than at present during interglaciations and interstades. This may be a response to ice sheets at higher latitudes, via equatorward displacement of the westerlies and the subtropical high. In contrast, central equatorial Africa (southeast of the Sahara) was most arid during interstades and times of ice growth, and most humid during deglaciation. Wet periods in this area correspond to insolation maxima in northern hemisphere summer. A 23,000-yr precessional rhythm is suggested, supporting a direct link between African Monsoon intensity and orbitally modulated insolation. The late Holocene is the only time observed when both areas are arid during an interglacial episode. This may reflect, in part, anthropogenic disturbance of late Holocene climates.

Structure and timing of the last deglaciation: Oxygen-isotope evidence☆

Foraminiferal oxygen-isotope data from 24 tropical Atlantic sediment cores, constrained by 77 14C dates, are stacked to form a composite record of isotopic Termination 1.. This record indicates that most of the isotopic transition at the end of the last ice age occurred between 14 ka BP and 6 ka BP. Minor isotopic expression of deglaciation is permitted as early as 16 ka BP, but the most rapid rate of change occurred between 14 ka BP and 12 ka BP. Three ‘steps’ of maximum change are present. Although they are close to the statistical limits of detection in the composite record, the clear presence of the steps in individual records suggests that they are real. We estimate their timing at 14-12 ka BP (Termination 1-a), 10-9 ka BP (Termination 1-b), and 8-6 ka BP (Termination 1-c). Centering of the termination near 11 ka BP is consistent with the ‘Milankovitch’ hypothesis that high summer insolation caused deglaciation. In detail, however, maximum rates of change prior to the 11 ka BP insolation extreme, and the inferred steps require additional mechanisms controlling the tempo of glacial-interglacial climate change. Steps equivalent to those in δ18O have not been detected in ice-margin retreat data. Steps in the isotopic transition, if real, may record thinning of the ice sheets not accompanied by loss of area. Alternation between near-equilibrium and near-stagnant ice-sheet profiles during deglaciation is hypothesized, perhaps due to calving and unstable ‘down-draw’ of the ice sheets followed by partial re-equilibration. Significant problems remain. The effects of temperature on the isotope record are only partially constrained. Presently available data allow only semi-quantitative intercalibration of ice volume, sea level, and isotopic estimates of glaciation.

Radiocarbon measurements on coexisting benthic and planktic foraminifera shells: potential for reconstructing ocean ventilation times over the past 20 000 years

Abstract In this paper the potential of AMS 14C dating of shells handpicked from deep sea sediments is explored. We show that while the age difference between planktonic (surface dwelling) and benthic (bottom dwelling) shells must carry information regarding paleocirculation rates, this message is likely obscured by effects associated with the coupling between bioturbation and dissolution and between bioturbation and abundance change. It is also possible that the 14C/12C ratio in planktonic shells was initially not identical to that in surface water and that the 14C/12C ratio in benthic shells was initially not identical to that in bottom water. These and other biases will plague all attempts to extract the desired information regarding circulation rate changes over the last 20000 years. However in sorting them out, much will be learned about the origin and history of the calcite particles found in deep sea sediments.

14C measurements on foraminifera of deep sea core V28-238 and their preliminary interpretation

Abstract In this paper first results obtained by AMS dating foraminifera are presented. The amount of material used for these studies was 7 to 10 mg calcium carbonate. A preliminary interpretation is given.