ABSTRACT
Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0-3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.
Subject(s)
Altitude , Calcium Carbonate/analysis , Carbon Cycle , Seawater/chemistry , Atmosphere/chemistry , Carbon Dioxide/analysis , Diatoms/metabolism , Foraminifera/metabolism , Geologic Sediments/chemistry , Global Warming/history , Global Warming/statistics & numerical data , History, 21st Century , History, Ancient , Marine Biology , Oxygen/metabolism , Pacific Ocean , TemperatureABSTRACT
OBJECTIVE: The purpose of this study was to determine the effect of various thermal treatments on the fatigue resistance of a nickel-titanium (NiTi) engine-driven endodontic file. STUDY DESIGN: Fifteen groups of 5 files each of ISO 30 and taper .04 were tested in this study. The cutting tip (5 mm from the end) of files from 14 groups were heat treated for 30 minutes in temperatures 250 degrees C, 300 degrees C, 350 degrees C, 375 degrees C, 400 degrees C, 410 degrees C, 420 degrees C, 425 degrees C, 430 degrees C, 440 degrees C, 450 degrees C, 475 degrees C, 500 degrees C, and 550 degrees C, respectively, while 1 group was used as reference. The files were placed in a device that allowed the instruments to be tested for rotating bending fatigue inside an artificial root canal. The number of rotations to breakage was recorded for each file. The mean values of all groups were statistically analyzed using 1-way analysis of variance and Student Newman Keuls multiple comparison test at alpha = .05. RESULTS: The 430 degrees C and 440 degrees C groups showed the highest values, with fatigue resistance decreasing for thermal treatment at lower and higher temperatures. This may be the result of metallurgical changes during annealing. CONCLUSION: Within the limitations of the low sample size and the specific instrument size tested, it appears that the appropriate thermal treatment may significantly increase the fatigue resistance of the NiTi file tested.
