Showing papers by "David W. Lea published in 2000"
TL;DR: Comparison of SST estimates from eastern and western sites indicates that the equatorial Pacific zonal SST gradient was similar or somewhat larger during glacial episodes, and extraction of a salinity proxy from the magnesium/calcium and oxygen isotope data indicates that transport of water vapor into the western Pacific was enhanced duringglacial episodes.
Abstract: Magnesium/calcium data from planktonic foraminifera in equatorial Pacific sediment cores demonstrate that tropical Pacific sea surface temperatures (SSTs) were 2.8° ± 0.7°C colder than the present at the last glacial maximum. Glacial-interglacial temperature differences as great as 5°C are observed over the last 450 thousand years. Changes in SST coincide with changes in Antarctic air temperature and precede changes in continental ice volume by about 3 thousand years, suggesting that tropical cooling played a major role in driving ice-age climate. Comparison of SST estimates from eastern and western sites indicates that the equatorial Pacific zonal SST gradient was similar or somewhat larger during glacial episodes. Extraction of a salinity proxy from the magnesium/calcium and oxygen isotope data indicates that transport of water vapor into the western Pacific was enhanced during glacial episodes.
1,012 citations
TL;DR: In this article, a model of ocean and sediment geochemistry is used to evaluate the current proposals for explaining the glacial/interglacial pCO2 cycles within the context of the ocean carbon cycle.
Abstract: Fifteen years after the discovery of major glacial/interglacial cycles in the CO2 concentration of the atmosphere, it seems that all of the simple mechanisms for lowering pCO2 have been eliminated. We use a model of ocean and sediment geochemistry, which in- cludes new developments of iron limitation of biological production at the sea surface and anoxic diagenesis and its effect on CaCO3 preservation in the sediments, to evaluate the current proposals for explaining the glacial/ interglacial pCO2 cycles within the context of the ocean carbon cycle. After equilibration with CaCO3 the model is unable to generate glacial pCO2 by increasing ocean NO3 but predicts that a doubling of ocean H4SiO4 might suffice. However, the model is unable to generate a doubling of ocean H4SiO4 by any reasonable changes in SiO2 weathering or production. Our conclusions force us to challenge one or more of the assumptions at the foundations of chemical oceanography. We can abandon the stability of the "Redfield ratio" of nitrogen to phos- phorus in living marine phytoplankton and the ultimate limitation of marine photosynthesis by phosphorus. We can challenge the idea that the pH of the deep ocean is held relatively invariant by equilibrium with CaCO3 .A third possibility, which challenges physical oceanogra- phers, is that diapycnal mixing in ocean circulation mod- els exceeds the rate of mixing in the real ocean, dimin- ishing the model pCO2 sensitivity to biological carbon uptake.
450 citations
TL;DR: Laboratory experiments with the planktonic foraminifera Orbulina universa and Globigerina bulloides were used to examine the effects of temperature, irradiance, [CO32-], [HPO42-], and ontogeny on shell d13C values.
129 citations
TL;DR: Foraminifera records from diverse hydrographic settings reveal coherent variability (5 ± 1%) between ocean basins and between surface and deep waters over the past 300 kyr as discussed by the authors.
Abstract: [1] Records of Sr/Ca changes in planktonic and benthic foraminifera from diverse hydrographic settings reveal coherent variability (5 ± 1%) between ocean basins and between surface and deep waters over the past 300 kyr. There is a general increase in foraminiferal Sr/Ca over the penultimate glaciation declining to minimum values during stage 5 and an increase in Sr/Ca from stage 5 through stage 2. Coincident changes in benthic foraminifera records from the Atlantic and Pacific basins imply that Sr/Ca variations are not dominated by dissolution. Planktonic culturing data provide evidence that the downcore Sr/Ca variations are not controlled by temperature changes and suggest only a small influence of salinity and pH. Variation common to the records is most readily explained by changes in mean ocean Sr/Ca. If fossil foraminifera reliably record higher glacial seawater Sr/Ca, coral Sr paleothermometry would underestimate sea surface temperature during glacialepisodes.
117 citations