Ice and climate modeling: An editorial essay

The growth and decay of ice sheets are driven by forces affecting the seasonal cycles of snowfall and snowmelt. The external forces are likely to be variations in the earth's orbit which cause differences in the solar radiation received. Radiational control of snowmelt is modulated by the seasonal cycles of snow albedo and cloud cover. The effects of orbital changes can be magnified by feedbacks involving atmospheric CO₂ content, ocean temperatures and desert areas. Climate modeling of the causes of the Pleistocene ice ages involves modeling the interactions of all components of the climate system; snow, sea ice, glacier ice, the ocean, the atmosphere, and the solid earth. Such modeling is also necessary for interpreting oxygen isotope records from ice and ocean as paleoclimatic evidence.     

Effects of mesoscale atmospheric convection cells on the waters of the East China Sea

Currents and atmospheric parameters were measured in the East China Sea in February, 1975, as part of the AMTEX'75 program. These data were used to describe outbreaks of cold continental air over this warm and shallow sea. Particular emphasis was placed on describing the structure of mesoscale atmospheric cells embedded in the outbreaks and the effects of these cells on the water column.Two cold air outbreaks were recorded. Heat fluxes (latent plus sensible) as high as 1270 cal/cm² day were calculated. Evidence of mesoscale atmospheric cells was found during outbreaks in satellite imagery and in solarimeter data. The development of mesoscale cells was described by correlating fluctuations in the air temperature and absolute humidity records. The cells were found to be best developed when satellite imagery showed that they were of the closed variety. The data suggest that cellular activity matures from open to closed cell types.During the period of greatest development, a representative closed cell was 24–30 km in diameter, moved at 8 m s^-1 over the spar buoy, had a temperature fluctuation of 0.4 °C, an absolute humidity fluctuation of 0.4 g kg^-1, and wind speed and heat flux fluctuations of - 12.5%.A non-dimensional index, formed from the fluctuations of the air temperature and absolute humidity records, was used to indicate the passage of mesoscale atmospheric cells over the measuring site. Using this index as input and the fluctuations in the oceanic parameters at a depth of 20 m as output, it was found that the passage of mature cells was significantly correlated with temperature fluctuations and current fluctuations aligned 25 ° to the right of the wind about 45 min later.     

Late Holocene paleoecology of the Southern Plains

Analyses of pollen and land snails from rocksheter sites in the Osage Hills of northeastern Oklahoma indicate that the period 2000-1000 yr B.P. was moister than today. During that time, colonies of the prairie vole Microtus ochrogaster were present in the Texas Panhandle. About 1000 yr B.P. the climate changed to dry conditions that have persisted to the present. Disjunct colonies of small mammals in Texas became extinct at the beginning of the dry episode, thereby establishing the composition of the modern fauna. The climatic model for the origin of the Panhandle Aspect (A.D. 1200–1500) is questioned on the grounds that the Southern Plains experienced a long period of dry climate commencing A.D. 950.     

Late Pleistocene paleo-oceanography of the Norwegian-Greenland Sea: Benthic foraminiferal evidence

Fluctuations in benthic foraminiferal faunas over the last 130,000 yr in four piston cores from the Norwegian Sea are correlated with the standard worldwide oxygen-isotope stratigraphy. One species, Cibicides wuellerstorfi, dominates in the Holocene section of each core, but alternates downcore with Oridorsalis tener, a species dominant today only in the deepest part of the basin. O. tener is the most abundant species throughout the entire basin during periods of particularly cold climate when the Norwegian Sea presumably was ice covered year round and surface productivity lowered. Portions of isotope Stages 6, 3, and 2 are barren of benthic foraminifera; this is probably due to lowered benthic productivity, perhaps combined with dilution by ice-rafted sediment; there is no evidence that the Norwegian Sea became azoic. The Holocene and Substage 5e (the last interglacial) are similar faunally. This similarity, combined with other evidence, supports the presumption that the Norwegian Sea was a source of dense overflows into the North Atlantic during Substage 5e as it is today. Oxygen-isotope analyses of benthic foraminifera indicate that Norwegian Sea bottom waters warmer than they are today from Substage 5d to Stage 2, with the possible exception of Substage 5a. These data show that the glacial Norwegian Sea was not a sink for dense surface water, as it is now, and thus it was not a source of deep-water overflows. The benthic foraminiferal populations of the deep Norwegian Sea seem at least as responsive to near-surface conditions, such as sea-ice cover, as they are to fluctuations in the hydrography of the deep water. Benthic foraminiferal evidence from the Norwegian Sea is insufficient in itself to establish whether or not the basin was a source of overflows into the North Atlantic at any time between the Substage 5e/5d boundary at 115,000 yr B.P. and the Holocene.     

An experimental investigation of volatile exsolution in evolving magma chambers

Previous laboratory experiments investigating the fluid dynamics of replenished magma chambers have been extended to model effects resulting from the release of gas. Turbulent transfer of heat between a layer of dense, hot and volatile-rich mafic magma overlying cooler more evolved magma can lead to crystallization and exsolution of volatiles in the lower layer. Small gas bubbles can cause the bulk density to decrease to that of the upper layer and thus produce sudden overturning and initiate mixing, followed by further exsolution of gas and explosive eruption. These processes have been modelled in the laboratory using a chemical reaction between sodium or potassium carbonate and nitric acid to release small bubbles of CO₂. We have investigated both the initial overturning produced by gas release in the lower layer, and the subsequent evolution of gas due to intimate mixing of the two layers. The latter experiments, in which the reactants remained isolated in the two layers until overturning occurred, demonstrated unambiguously that the fluxes of chemical components across the interfaces between convecting layers are very slow compared to the flux of heat. This shows that the evolution of layers of magma of different origins and composition can take place nearly independently of each other. The magmas can coexist in the same stratified chamber, until their bulk densities become equal and they mix together. The processes illustrated in these experiments could occur in H₂O-bearing magmas such as in the calcalkaline association and in CO₂-bearing mafic magmas such as in silica undersaturated suites.