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• 1. [Article] Is methane venting at the seafloor recorded by δ13C of benthic foraminifera shells?

  The isotopic composition of the dissolved inorganic carbon (DIC) collected at sites of active methane discharge on Hydrate Ridge, Oregon, reveals anaerobic methane oxidation mediated by bacteria, with ...

  Citation × Citation Citation Abstract Copy Title: Is methane venting at the seafloor recorded by δ13C of benthic foraminifera shells? Author: Haley, Brian A., Torres, Marta E., McManus, James, de Angelis, M. A., Kinports, K., Klinkhammer, Gary P., Mix, Alan C. The isotopic composition of the dissolved inorganic carbon (DIC) collected at sites of active methane discharge on Hydrate Ridge, Oregon, reveals anaerobic methane oxidation mediated by bacteria, with δ13CDIC reaching values as low as –48‰ in the upper 4 cm of the sediment. In spite of the high sulfide levels in the discharging fluids, living specimens of the benthic foraminifera Uvigerina peregrina are abundant in the vents, probably owing to the rich bacterial food source. Although pore water δ13CDIC is extremely low (–6 to –48%), the δ13C values of living (Rose Bengal stained) foraminifera shells collected from active methane seeps are not significantly lower than those observed in nonventing pelagic sediments, and are within the range expected from local organic matter decomposition (0 to –4%). The apparent δ13C disequilibrium between biogenic calcite and DIC suggests that at seep localities, foraminifera calcify mostly during periods when there is little methane discharge or during intermittent episodes of seawater flow into the sediments. The isotopic composition and Mg/Ca ratios of fossil (unstained) foraminifera recovered at carbonate-rich sites on the northern Hydrate Ridge reveals overprinting of the biogenic record by inorganic calcite with high Mg/Ca and anomalously low d13C values. Thus overprinting of the original isotopic composition of foraminifera by overgrowths or recrystallization at or below the sediment surface, rather than primary calcification in contact with 13C depleted DIC, can explain extreme 13C depletion observed in fossil foraminifera recovered from sites of active methane discharge. Title: Is methane venting at the seafloor recorded by δ13C of benthic foraminifera shells? Author: Haley, Brian A., Torres, Marta E., McManus, James, de Angelis, M. A., Kinports, K., Klinkhammer, Gary P., Mix, Alan C. The isotopic composition of the dissolved inorganic carbon (DIC) collected at sites of active methane discharge on Hydrate Ridge, Oregon, reveals anaerobic methane oxidation mediated by bacteria, with δ13CDIC reaching values as low as –48‰ in the upper 4 cm of the sediment. In spite of the high sulfide levels in the discharging fluids, living specimens of the benthic foraminifera Uvigerina peregrina are abundant in the vents, probably owing to the rich bacterial food source. Although pore water δ13CDIC is extremely low (–6 to –48%), the δ13C values of living (Rose Bengal stained) foraminifera shells collected from active methane seeps are not significantly lower than those observed in nonventing pelagic sediments, and are within the range expected from local organic matter decomposition (0 to –4%). The apparent δ13C disequilibrium between biogenic calcite and DIC suggests that at seep localities, foraminifera calcify mostly during periods when there is little methane discharge or during intermittent episodes of seawater flow into the sediments. The isotopic composition and Mg/Ca ratios of fossil (unstained) foraminifera recovered at carbonate-rich sites on the northern Hydrate Ridge reveals overprinting of the biogenic record by inorganic calcite with high Mg/Ca and anomalously low d13C values. Thus overprinting of the original isotopic composition of foraminifera by overgrowths or recrystallization at or below the sediment surface, rather than primary calcification in contact with 13C depleted DIC, can explain extreme 13C depletion observed in fossil foraminifera recovered from sites of active methane discharge. Close

• 2. [Article] δ¹³C record of Upper North Atlantic Deep Water during the past 2.6 million years

  Copyright 1995 by the American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: δ¹³C record of Upper North Atlantic Deep Water during the past 2.6 million years Author: Wright, J. D., Oppo, D. W., Prell, W. L., Raymo, M. E., Mix, Alan C., Lohmann, G. P. Copyright 1995 by the American Geophysical Union Title: δ¹³C record of Upper North Atlantic Deep Water during the past 2.6 million years Author: Wright, J. D., Oppo, D. W., Prell, W. L., Raymo, M. E., Mix, Alan C., Lohmann, G. P. Copyright 1995 by the American Geophysical Union Close

• 3. [Article] Alkenone sea-surface temperatures and carbon burial at site 846 (Eastern Equatorial Pacific Ocean) : the last 1.3 M.Y.

  We analyzed the unsaturation ratio (U k/37 ) of long-chain ketones—a molecular sea-surface temperature (SST) indicator—concentrations of carbonate and organic carbon in sediments from Site 846 in the eastern ...

  Citation × Citation Citation Abstract Copy Title: Alkenone sea-surface temperatures and carbon burial at site 846 (Eastern Equatorial Pacific Ocean) : the last 1.3 M.Y. Author: Schulz-Bull, D., Emeis, K.-C., Doose, H., Mix, Alan C. We analyzed the unsaturation ratio (U k/37 ) of long-chain ketones—a molecular sea-surface temperature (SST) indicator—concentrations of carbonate and organic carbon in sediments from Site 846 in the eastern equatorial Pacific Ocean. Based on an isotopic age model for the composite depth section of 0-46 m below seafloor and on estimates of sediment density, accumulation rates of these biogenic compounds were calculated. Our combined temperature and biogenic flux record traces conditions at the origin of the South Equatorial Current over the last 1.3 m.y. S STs have fluctuated considerably over the interval studied. A long trend of gradual decrease from 24°C at 1.3 Ma ends between 500 and 400 Ka, when lowest values of 19°C were reached. Since this time, the temperature data indicate a warming trend to the Holocene modulated by high-amplitude variation (19° to 27°C). The inversion of the trend between 400 and 500 Ka coincides with maximal accumulation rates of carbonate, which since then have decreased. In contrast, organic carbon accumulation since then has increased in variability and in absolute values. On shorter time scales, the records show a strong link to the global climatic background. Since 1.3 Ma, carbonate (0.2-3 g/cm² k.y.¯¹) and organic carbon accumulation rates (2-30 mg/cm² k.y¯¹) were consistently high (more than twice their modern values and those of interglacials) during glacial maxima in the benthic isotope record, when temperatures were low. However, cross-spectral analyses with the δ¹⁸O record suggest that variation in organic carbon flux is not linked directly to variations in SST. Temperature maxima in our record led interglacial events by 7 k.y. in the 100-k.y. eccentricity cycle and by 5 k.y. in the 41-k.y. obliquity cycle. In contrast, maxima in organic carbon accumulation lag behind glacial maxima and low temperatures by 14 k.y. in the eccentricity cycle. On glacial/interglacial time scales, a prominent influence on SST—but not on organic carbon burial—at Site 846 appears to be the advection of cold water into the South Equatorial Current. Title: Alkenone sea-surface temperatures and carbon burial at site 846 (Eastern Equatorial Pacific Ocean) : the last 1.3 M.Y. Author: Schulz-Bull, D., Emeis, K.-C., Doose, H., Mix, Alan C. We analyzed the unsaturation ratio (U k/37 ) of long-chain ketones—a molecular sea-surface temperature (SST) indicator—concentrations of carbonate and organic carbon in sediments from Site 846 in the eastern equatorial Pacific Ocean. Based on an isotopic age model for the composite depth section of 0-46 m below seafloor and on estimates of sediment density, accumulation rates of these biogenic compounds were calculated. Our combined temperature and biogenic flux record traces conditions at the origin of the South Equatorial Current over the last 1.3 m.y. S STs have fluctuated considerably over the interval studied. A long trend of gradual decrease from 24°C at 1.3 Ma ends between 500 and 400 Ka, when lowest values of 19°C were reached. Since this time, the temperature data indicate a warming trend to the Holocene modulated by high-amplitude variation (19° to 27°C). The inversion of the trend between 400 and 500 Ka coincides with maximal accumulation rates of carbonate, which since then have decreased. In contrast, organic carbon accumulation since then has increased in variability and in absolute values. On shorter time scales, the records show a strong link to the global climatic background. Since 1.3 Ma, carbonate (0.2-3 g/cm² k.y.¯¹) and organic carbon accumulation rates (2-30 mg/cm² k.y¯¹) were consistently high (more than twice their modern values and those of interglacials) during glacial maxima in the benthic isotope record, when temperatures were low. However, cross-spectral analyses with the δ¹⁸O record suggest that variation in organic carbon flux is not linked directly to variations in SST. Temperature maxima in our record led interglacial events by 7 k.y. in the 100-k.y. eccentricity cycle and by 5 k.y. in the 41-k.y. obliquity cycle. In contrast, maxima in organic carbon accumulation lag behind glacial maxima and low temperatures by 14 k.y. in the eccentricity cycle. On glacial/interglacial time scales, a prominent influence on SST—but not on organic carbon burial—at Site 846 appears to be the advection of cold water into the South Equatorial Current. Close

• 4. [Article] Climate Sensitivity Estimated From Temperature Reconstructions of the Last Glacial Maximum

  This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by the American Association for the Advancement of Science and can be found at: h...

  Citation × Citation Citation Abstract Copy Title: Climate Sensitivity Estimated From Temperature Reconstructions of the Last Glacial Maximum Author: Urban, Nathan M., Shakun, Jeremy D., Mahowald, Natalie M., Bartlein, Patrick J., Schmittner, Andreas, Clark, Peter U., Rosell-Mele, Antoni, Mix, Alan C. This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by the American Association for the Advancement of Science and can be found at: http://www.sciencemag.org/. Title: Climate Sensitivity Estimated From Temperature Reconstructions of the Last Glacial Maximum Author: Urban, Nathan M., Shakun, Jeremy D., Mahowald, Natalie M., Bartlein, Patrick J., Schmittner, Andreas, Clark, Peter U., Rosell-Mele, Antoni, Mix, Alan C. This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by the American Association for the Advancement of Science and can be found at: http://www.sciencemag.org/. Close

• 5. [Article] Simulating the global distribution of nitrogen isotopes in the ocean

  We present a new nitrogen isotope model incorporated into the three-dimensional ocean component of a global Earth system climate model designed for millennial timescale simulations. The model includes ...

  Citation × Citation Citation Abstract Copy Title: Simulating the global distribution of nitrogen isotopes in the ocean Author: Somes, Christopher J., Lehmann, Moritz F., Bourbonnais, Annie, Eby, Michael, Schmittner, Andreas, Altabet, Mark A., Galbraith, Eric D., Mix, Alan C., Montoya, Joseph P., Letelier, Ricardo M. We present a new nitrogen isotope model incorporated into the three-dimensional ocean component of a global Earth system climate model designed for millennial timescale simulations. The model includes prognostic tracers for the two stable nitrogen isotopes, 14N and 15N, in the nitrate (NO3−), phytoplankton, zooplankton, and detritus variables of the marine ecosystem model. The isotope effects of algal NO3− uptake, nitrogen fixation, water column denitrification, and zooplankton excretion are considered as well as the removal of NO3− by sedimentary denitrification. A global database of δ15NO3− observations is compiled from previous studies and compared to the model results on a regional basis where sufficient observations exist. The model is able to qualitatively and quantitatively reproduce many of the observed patterns such as high subsurface values in water column denitrification zones and the meridional and vertical gradients in the Southern Ocean. The observed pronounced subsurface minimum in the Atlantic is underestimated by the model presumably owing to too little simulated nitrogen fixation there. Sensitivity experiments reveal that algal NO3− uptake, nitrogen fixation, and water column denitrification have the strongest effects on the simulated distribution of nitrogen isotopes, whereas the effect from zooplankton excretion is weaker. Both water column and sedimentary denitrification also have important indirect effects on the nitrogen isotope distribution by reducing the fixed nitrogen inventory, which creates an ecological niche for nitrogen fixers and, thus, stimulates additional N2 fixation in the model. Important model deficiencies are identified, and strategies for future improvement and possibilities for model application are outlined. Title: Simulating the global distribution of nitrogen isotopes in the ocean Author: Somes, Christopher J., Lehmann, Moritz F., Bourbonnais, Annie, Eby, Michael, Schmittner, Andreas, Altabet, Mark A., Galbraith, Eric D., Mix, Alan C., Montoya, Joseph P., Letelier, Ricardo M. We present a new nitrogen isotope model incorporated into the three-dimensional ocean component of a global Earth system climate model designed for millennial timescale simulations. The model includes prognostic tracers for the two stable nitrogen isotopes, 14N and 15N, in the nitrate (NO3−), phytoplankton, zooplankton, and detritus variables of the marine ecosystem model. The isotope effects of algal NO3− uptake, nitrogen fixation, water column denitrification, and zooplankton excretion are considered as well as the removal of NO3− by sedimentary denitrification. A global database of δ15NO3− observations is compiled from previous studies and compared to the model results on a regional basis where sufficient observations exist. The model is able to qualitatively and quantitatively reproduce many of the observed patterns such as high subsurface values in water column denitrification zones and the meridional and vertical gradients in the Southern Ocean. The observed pronounced subsurface minimum in the Atlantic is underestimated by the model presumably owing to too little simulated nitrogen fixation there. Sensitivity experiments reveal that algal NO3− uptake, nitrogen fixation, and water column denitrification have the strongest effects on the simulated distribution of nitrogen isotopes, whereas the effect from zooplankton excretion is weaker. Both water column and sedimentary denitrification also have important indirect effects on the nitrogen isotope distribution by reducing the fixed nitrogen inventory, which creates an ecological niche for nitrogen fixers and, thus, stimulates additional N2 fixation in the model. Important model deficiencies are identified, and strategies for future improvement and possibilities for model application are outlined. Close

• 6. [Article] Evaluation of automated flow-through time-resolved analysis of foraminifera for Mg/Ca paleothermometry

  Copyrighted by American Geophysical Union.

  Citation × Citation Citation Abstract Copy Title: Evaluation of automated flow-through time-resolved analysis of foraminifera for Mg/Ca paleothermometry Author: Haley, Brian A., Cheseby, M., Benway, H. M., Klinkhammer, Gary P., Mix, Alan C. Copyrighted by American Geophysical Union. Title: Evaluation of automated flow-through time-resolved analysis of foraminifera for Mg/Ca paleothermometry Author: Haley, Brian A., Cheseby, M., Benway, H. M., Klinkhammer, Gary P., Mix, Alan C. Copyrighted by American Geophysical Union. Close