Isotopic and Geochemical Constraints on Paleoclimate Processes III Posters
Presiding: S R Hemming, Lamont-Doherty Earth Observatory; F Marcantonio, Department of Earth and Environmental Sciences, Tulane University
PP23A-01 1330h
Abrupt Climate Change and the Invention of Agriculture
We draw attention to the effect of abrupt climate change on the major event in human history, the invention of agriculture. Our Homo Sapiens ancestors appeared almost 1/2 million years ago but agriculture was not first invented until 11,000 thousand years before the present (ybp). Since then agriculture has been independently invented at least 5 times, which demonstrates that it is not very difficult to invent agriculture if the conditions are right. But what are the right conditions and why did they not occur until 11,000 years ago. We note that the anatomically modern species of human being (Homo Sapiens Sapiens) first appeared in East Africa 130,000 ybp. This time scale is close to that of the climate records archived in polar ice cores, marine sediments and coral cores. These data sets imply major abrupt climate changes occurring in time scales of decades with characteristic times between changes of 300-700 years. By using time -scale analysis of the data we argue that these changes were too great and too frequent to permit the development of a successful agriculture until 11,000 ybp.
PP23A-02 1330h
The Oxygen and Hydrogen Isotope Composition of Atmospheric Water Vapor at Pinery Provincial Park, Southern Great Lakes Area, Canada
More than 300 atmospheric water vapor samples were obtained between July 2004 and January 2005 at Pinery Provincial Park, southwestern Ontario, Canada. This area, which is located on the shore of Lake Huron, is variably influenced by Pacific, Tropical Atlantic and Arctic air masses, depending on the time of year. Each month, samples were collected from a sparsely vegetated dune and a nearby oak savannah site at regular intervals over a 24-hour period at 3 and 0.5m above ground level. The majority of the data describe a local water vapor line: ΔD=7.3Δ18O+1.6. The broad positive correlation between surface temperature and the isotopic composition of the water vapor follows the expected seasonal pattern. The isotopic compositions of the water vapor showed seasonally consistent differences between the 3 and 0.5m sampling heights at both locations. During the summer months, the 3m samples were depleted of oxygen-18 and hydrogen-2 by 0.3 and 3 per mil, respectively, relative to the 0.5m samples. During the summer months, transpiration is a significant contributor to atmospheric vapor at both the 3 and 0.5m levels. However, the shallow soil water tapped by the grasses is enriched in oxygen-18 and hydrogen-2 relative to the deeper soil water utilized by the trees. This difference is likely sufficient to explain the observed pattern. By late autumn, these differences in oxygen and hydrogen isotope compositions between the high and low sampling sites were much smaller, +0.1 and -1 per mil, respectively. By the winter, the pattern was completely reversed, with the 3m samples being enriched in oxygen-18 and hydrogen-2 by 1.1 and 10 per mil, respectively. We suggest that vapor contribution from ground-level sublimation of snow is largely responsible for this behavior. The diurnal variations in isotopic composition at both sites can be substantial (e.g., Δ18O = -28.3 to -19.1 per mil versus -21.4 to -20.0 per mil. The main controlling factor appears to be the meteorological conditions and air mass(es) present at the time of sampling.
PP23A-03 1330h
The Oxygen, Hydrogen and Carbon Isotope Compositions of Modern Freshwater and Terrestrial Snails, Pinery Provincial Park, Southern Great Lakes Area, Canada
We have compared the oxygen, hydrogen and carbon isotope compositions of several species of modern freshwater and terrestrial snails from a variety of ecosystems in Pinery Provincial Park, southwestern Ontario. The Park is located on the shore of Lake Huron, within a temperate-humid climatic regime. The aquatic snails have a range of oxygen and hydrogen isotope body-water compositions with a slope similar to the local evaporation line of their river and pond habitats. Body water from terrestrial snails has oxygen and hydrogen isotope compositions with a similar slope to the local soil-water line. Body-water compositions tend to be slightly enriched in oxygen-18 relative to the source water. The oxygen isotope composition of the shell aragonite most closely predicts body-water compositions for the average temperature of snail growth. Snail flesh is enriched in carbon-13 relative to diet by about 3 per mil. The aquatic shells are enriched in carbon-13 relative to flesh by an average of 11 to 20 per mil, depending on species. Shells of the two terrestrial species examined are enriched in carbon-13 relative to flesh by an average of 10 to 12 per mil. The diversity of snail microhabitats in the Park is reflected in the carbon and oxygen isotope compositions of their shells. Bottom-dwelling aquatic snails have relatively constant oxygen isotope values, which reflects limited variation in their source water composition. By comparison, shell carbon isotope values are highly variable, a consequence of compositional changes during the cycling of organic matter. Surface-dwelling aquatic snails and those living in very shallow water columns exhibit a range of positively correlated oxygen and carbon isotope values. This pattern reflects systematically changing water and carbon source compositions during the growing season. Terrestrial snail aragonite has a large range in oxygen isotope values, arising from the wide variation in composition of the slightly evaporated precipitation from which it is derived. In contrast, terrestrial shell carbon isotope values are constant, reflecting the uniform composition of the terrestrial organic carbon supply. The large range in shell oxygen and carbon isotope values obtained for this small area underscores the importance of microhabitat. The possibility of such variability needs to be considered when interpreting data for ancient snail shells formed in similar environments.
PP23A-04 1330h
Ar-Ar Evidence for Provenance of Ice-rafted Hornblende Grains from ODP site 984
The patterns and mechanisms of rapid climate change are important, exciting and still unresolved questions. Myriads of proxies and models have been brought to bear on the problem, and several end member scenarios have emerged. One connection that appears to be inescapable is that deep and shallow ocean circulation components are both intimately involved in the system and that ice rafted detritus (IRD) concentration varies systematically in the North Atlantic with these changes. Climate models are in general agreement about the most important factors in North Atlantic (and global) climate change. One of the important factors is the IRD and what it could potentially tells us about the role of ice sheets and/or sea ice in abrupt climate change. Core ODP-984 in the North Atlantic (61.43N, 24.08W) is located outside of the Ruddiman IRD belt, and on the Bjorn drift deposit south of Iceland. Despite its proximity to Iceland its IRD is dominantly of continental origin, and thus not from local Icelandic sources. We have measured the Ar-Ar ages of multiple individual hornblende grains from the larger than 150 micron fraction from several of the prominent IRD intervals from Marine Isotope Stages 2, 6, 8, and 10. Most of the samples have a large ca. 1 Ga population of ages, consistent with derivation from the Grenville province. The youngest sample analyzed is from approximately Termination I (close to the time of Heinrich Event H1). H1 in the IRD belt has no evidence of a substantial Grenville population, and thus it is unlikely that the Grenville province of North America (e.g., Gulf of St. Lawrence) is the source of these hornblendes. The southern part of Norway has a large Grenville province, and is the northern side of the passage leading to the North Sea trough mouth fan, an extremely large glacial-marine sediment fan. We consider it likely that the Grenville grains were derived from this province via ice streams feeding the North Sea trough mouth fan. Accordingly, this study is one example of a potentially diagnostic characteristic of IRD from an important European ice stream source.
PP23A-05 1330h
Effects of Continental Weathering and Sedimentary Sorting on the Hf, Nd, and Pb Isotopic Composition of Sediments and Implications for Dissolved Input to the Ocean
The application of radiogenic isotopes of dissolved trace metals (Hf, Pb, Nd) in the ocean to study climate induced weathering changes on the continents is a relatively new and exciting field of research. Most studies on the radiogenic isotope composition of past seawater are based on chemical sediments such as ferromanganese crusts, and suggest a strong linkage in changes in the style of weathering (chemical vs. physical/glacial) on the continents and the isotopic composition of seawater for elements such as Hf and Pb. This is evidenced by records from the northwest Atlantic, which show significant changes in the Hf and Pb isotopic composition of seawater associated with the onset of Northern Hemisphere Glaciation (NHG) over the past 3 Myr. On the other hand, the impact of glacial weathering on the Nd isotopic composition of seawater is generally thought to be negligible. In order to better understand the systematics and mechanisms of weathering, erosion, and run-off, as well as their bearing on the isotopic composition of the ocean we choose the following strategy: sediments from the Amerilik and Gothabs Fjords near Nuuk (Greenland) have been analyzed for their bulk rock and grain size specific Hf, Nd, and Pb isotopic compositions (sand, silt, mud). Both Fjords are surrounded by early Archean gneisses, among the oldest rocks on Earth, and are filled with glacially floured sediments. Precambrian rocks surround a large fraction of the circum-North Atlantic area that was covered by ice sheets during the NHG, and the source rocks in this region are the most extreme examples. Current knowledge about the impact of weathering processes on isotope systematics implies that the largest changes in the Hf (and Pb) isotopic composition of seawater are expected to arise from glacial weathering of old continental sources. Our data reveal large differences in grain size specific isotope ratios, e.g. between the mud and sand fractions, for sediments with an old provenance. Hafnium and Pb show the lowest isotopic ratios in sands and the highest ratios in clays, while Nd isotopes display the opposite trend. This grain size specific fractionation of the isotopic ratios is in line with known partitioning of Lu-Hf, Sm-Nd, and U-Th-Pb in main and trace minerals (e.g. apatite, zircon, monazite, garnet, feldspar). In Nd vs. Hf isotope space the new data describe a trend vertical to the terrestrial array with a 20 ε unit range in Hf isotopes and a 8 ε unit range in Nd isotopes, proving that sedimentary sorting could be responsible for deflection of points from the terrestrial array. This suggests that the combined effect of sedimentary sorting and incongruent weathering may be of major importance for understanding and interpreting the glacial-interglacial radiogenic isotope composition of North Atlantic deep water.