Search

Search Results

• 1. [Article] Anthropogenic CO₂ invasion into the northeast Pacific based on concurrent δ¹³CDIC and nutrient profiles from the California Current

  The stable isotopic signature of dissolved inorganic carbon (δ¹³CDIC) in the northeast Pacific Ocean is lower in near-surface waters by 1.1‰ relative to values predicted from global oceanic trends of ...

  Citation × Citation Citation Abstract Copy Title: Anthropogenic CO₂ invasion into the northeast Pacific based on concurrent δ¹³CDIC and nutrient profiles from the California Current Author: Wheeler, Patricia A., Key, R. M., Mix, Alan C., Ortiz, Joseph D. The stable isotopic signature of dissolved inorganic carbon (δ¹³CDIC) in the northeast Pacific Ocean is lower in near-surface waters by 1.1‰ relative to values predicted from global oceanic trends of δ¹³CDIC versus nutrients. A combination of anthropogenic carbon uptake from the atmosphere and thermodynamic, air-sea gas exchange processes in different water mass source areas account for the isotopic depletion. Here we evaluate the efficacy of using a concurrent nutrient-δ¹³C strategy to separate these two effects, with the goal of improving estimates of anthropogenic carbon uptake over the course of the Industrial Revolution. In depth profiles from the sea surface to 2500 m at four stations across the California Current (42ºN), nitrate, rather than phosphate, is best correlated to δ¹³CDIC providing the best choice for this experiment. On the basis of an assumption of no anthropogenic carbon in North Pacific Deep Waters between 1000–2500 m depth (potential densities, σθ ~ 27.3–27.7), the “anthropogenic – preanthropogenic” carbon isotope shift (∆δ¹³Ca–p) in near-surface waters of the northeast Pacific is inferred to be -0.62 ± 0.17‰, while the thermodynamic air-sea gas exchange signature is estimated at -0.48 ± 0.17‰. Values of ∆δ¹³Ca–p (similar to the regional patterns of ∆¹4C and Tritium penetration) approach zero for σθ > 26.8, indicating little penetration of anthropogenic carbon into the North pacific Intermediate Water or the upper North pacific Deep Water. Our results suggest an upper North Pacific sink of anthropogenic carbon over the past ~200 years that is ~40% greater than that estimated for the interval between ~1970 and ~1990 by Quay et al. [1992]. Our estimate of the North Pacific inventory of anthropogenic carbon, added to published estimates from the North Atlantic and Indian Ocean, is smaller than model predictions of the total carbon sink, suggesting that a significant portion of anthropogenic carbon enters the deep sea via the Southern Ocean. Title: Anthropogenic CO₂ invasion into the northeast Pacific based on concurrent δ¹³CDIC and nutrient profiles from the California Current Author: Wheeler, Patricia A., Key, R. M., Mix, Alan C., Ortiz, Joseph D. The stable isotopic signature of dissolved inorganic carbon (δ¹³CDIC) in the northeast Pacific Ocean is lower in near-surface waters by 1.1‰ relative to values predicted from global oceanic trends of δ¹³CDIC versus nutrients. A combination of anthropogenic carbon uptake from the atmosphere and thermodynamic, air-sea gas exchange processes in different water mass source areas account for the isotopic depletion. Here we evaluate the efficacy of using a concurrent nutrient-δ¹³C strategy to separate these two effects, with the goal of improving estimates of anthropogenic carbon uptake over the course of the Industrial Revolution. In depth profiles from the sea surface to 2500 m at four stations across the California Current (42ºN), nitrate, rather than phosphate, is best correlated to δ¹³CDIC providing the best choice for this experiment. On the basis of an assumption of no anthropogenic carbon in North Pacific Deep Waters between 1000–2500 m depth (potential densities, σθ ~ 27.3–27.7), the “anthropogenic – preanthropogenic” carbon isotope shift (∆δ¹³Ca–p) in near-surface waters of the northeast Pacific is inferred to be -0.62 ± 0.17‰, while the thermodynamic air-sea gas exchange signature is estimated at -0.48 ± 0.17‰. Values of ∆δ¹³Ca–p (similar to the regional patterns of ∆¹4C and Tritium penetration) approach zero for σθ > 26.8, indicating little penetration of anthropogenic carbon into the North pacific Intermediate Water or the upper North pacific Deep Water. Our results suggest an upper North Pacific sink of anthropogenic carbon over the past ~200 years that is ~40% greater than that estimated for the interval between ~1970 and ~1990 by Quay et al. [1992]. Our estimate of the North Pacific inventory of anthropogenic carbon, added to published estimates from the North Atlantic and Indian Ocean, is smaller than model predictions of the total carbon sink, suggesting that a significant portion of anthropogenic carbon enters the deep sea via the Southern Ocean. Close

• 2. [Article] Millennial-scale deep water oscillations: Reflections of the North Atlantic in the deep Pacific from 10 to 60 ka

  Copyright 1998 by the American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Millennial-scale deep water oscillations: Reflections of the North Atlantic in the deep Pacific from 10 to 60 ka Author: Lund, David C., Mix, Alan C. Copyright 1998 by the American Geophysical Union Title: Millennial-scale deep water oscillations: Reflections of the North Atlantic in the deep Pacific from 10 to 60 ka Author: Lund, David C., Mix, Alan C. Copyright 1998 by the American Geophysical Union Close

• 3. [Article] Increased dissolved terrestrial input to the coastal ocean during the last deglaciation

  Copyrighted by American Geophysical Union.

  Citation × Citation Citation Abstract Copy Title: Increased dissolved terrestrial input to the coastal ocean during the last deglaciation Author: Haley, B. A., Klinkhammer, Gary P., Mix, Alan C. Copyrighted by American Geophysical Union. Title: Increased dissolved terrestrial input to the coastal ocean during the last deglaciation Author: Haley, B. A., Klinkhammer, Gary P., Mix, Alan C. Copyrighted by American Geophysical Union. Close

• 4. [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

• 5. [Article] Leg 202 summary

  Shipboard scientific party : Chapter 1, Leg 202 Summary

  Citation × Citation Citation Abstract Copy Title: Leg 202 summary Author: Blum, P., Mix, Alan C., Tiedemann, R. Shipboard scientific party : Chapter 1, Leg 202 Summary Title: Leg 202 summary Author: Blum, P., Mix, Alan C., Tiedemann, R. Shipboard scientific party : Chapter 1, Leg 202 Summary Close

• 6. [Article] Paleoceanography of the Eastern Equatorial Pacific during the Neogene : synthesis of Leg 138 drilling results

  The primary objective of Leg 138 was to provide detailed information about the ocean's response to global climate change during the Neogene. Two north south transects were drilled (95° and 110°W) within ...

  Citation × Citation Citation Abstract Copy Title: Paleoceanography of the Eastern Equatorial Pacific during the Neogene : synthesis of Leg 138 drilling results Author: Mayer, Larry A., Pisias, Nicklas G., Mix, Alan C. The primary objective of Leg 138 was to provide detailed information about the ocean's response to global climate change during the Neogene. Two north south transects were drilled (95° and 110°W) within the region of equatorial divergence driven upwelling (and thus high accumulation rates and resolution) and spanning the major equatorial ocean current boundaries (and thus recording a high amplitude signal of the response of the sediment to climatically and/or tectonically driven changes in ocean circulation). The Neogene is marked by a number of well known climatic and tectonic events (the closing of the Isthmus of Panama, the onset of North Atlantic Deep Water (NADW), the rapid uplift of the Himalayas, the major intensification of Northern Hemisphere glaciation), and the response of the ocean before and after these events was a key focus of Leg 138 drilling. To address these objectives at the highest resolution possible, the Leg 138 scientific staff developed a number of new shipboard strategies and analytical procedures. These included the real time analysis of the near continuous gamma ray attenuation porosity evaluator (GRAPE) and susceptibility profiles produced by the multisensor track (MST) on unsplit cores to monitor core recovery and, if necessary, to modify the drilling strategy to ensure proper offset of coring gaps; the collection of near continuous color reflectance data on split cores; the logging of the first hole drilled at each site to optimize drilling and sampling strategies for subsequent holes; and the use of multiple continuous records to unambiguously construct complete composite sections for each site. The complete, continuous records provided by the GRAPE (with a temporal resolution of often yr), in conjunction with an excellent microfossil stratigraphy and often excellent magnetostratigraphy, allowed for astronomical tuning of the stratigraphic record and resulted in a set of internally consistent, high resolution age models that provide a secure, absolute time scale for the past 6 m.y. For the period before 6 m.y., the absolute time calibration is less secure, but it is still better than any previously offered. The high resolution stratigraphic framework of Leg 138 provided new insight into the previously ambiguous tectonic history of the region. By assuming that maximum sedimentation rates along the north south transect would be expected at the equator, the Leg 138 stratigraphy supports the 1985 work of Cox and Engerbretson, which calls for two different poles of rotation of the Pacific Plate during the interval 0-20 Ma. The Leg 138 plate reconstructions also support several previously hypothesized ridge crest jumps and a slowing of the absolute motion of the Nazca Plate at about 5 Ma. Although Leg 138 data that predates about 13 Ma is limited, the impression that one can gain from these data is that the eastern equatorial Pacific was characterized by relatively high carbonate concentrations and accumulation rates before about 11 Ma. This pattern was interrupted occasionally by rapid massive outpourings of near monospecific laminated diatom oozes that probably represent the formation of massive mats along strong surface water fronts. The laminated diatom oozes (LDO) continue to be present in the Leg 138 record (many of them being expressed as seismic reflections) until about 4.4 Ma. Carbonate accumulation rates begin to decline slowly between 11 and 9.8 Ma, when, at about 9.5 Ma, a near complete loss of carbonate (the "carbonate crash") takes place everywhere in the Leg 138 region (and beyond), except at the westernmost sites close to the equator. The "carbonate crash" was a time of fundamental change for the eastern equatorial Pacific, and perhaps for most of the ocean basins. Unlike many of the carbonate variations that precede and postdate it, this "crash" represents a major dissolution event whose effects can be traced seismically in the central and western Pacific. The changes in bottom water chemistry associated with this event (or series of events) appear to be related to the early phases of the closing of the Panama Gateway. The role of NADW initiation and intensification for controlling carbonate accumulation in the eastern equatorial Pacific is still not resolved; however, ocean modeling demonstrates that the closing of the Panama Gateway may also have a direct influence on NADW production. Therefore, the effects of changes in the Panama Gateway sill depth and the production of NADW may be manifested in the history of eastern equatorial Pacific sedimentation. The "carbonate crash" was followed by a recovery of the carbonate system (except in the Guatemala and Peru basins, which never recovered) that led up to the late Miocene/ early Pliocene sedimentation rate maxima, during which equatorial sedimentation rates are as much as five times greater than those of the late Pliocene or Pleistocene. Examination of modern productivity/ preser vation relationships implies that the sedimentation rate maximum was the result of enhanced productivity. The distribution of eolian sediments and isotopic gradients, along with an analysis of the modes of variance in carbonate deposition over the last 6 m.y., suggest a more northerly position of the Intertropical Convergence Zone (ITCZ), a stronger north south gradient across the equator, and a more zonal circulation focused along the equator during the time of maximum sedimentation. The mechanisms suggested for these changes in circulation patterns include the response of the eastern equatorial Pacific to the closing of the Isthmus of Panama, as well as a global increase in the flux of Ca and Si into the oceans, a possible response to evolution of the Himalayas and the Tibetan Plateau. In an effort to understand the response of the climate system to external (orbital) forcing, 6-m.y.-long, continuous records of carbonate (derived from GRAPE), δ¹⁸O and insolation were analyzed and compared. Evolutionary spectral calculations of the variance and coherence among these records indicate that the insolation record is dominated by precessional frequencies, but that the relative importance of the two precessional frequencies has changed significantly over the last 6 m.y. In general, precessional forcing is not found in the carbonate or isotopic records. In the tilt band, however, a linear response is present between solar forcing and the carbonate and isotope records over some intervals. The carbonate record appears to be tightly coupled to the tilt component of insolation before about 1.9 Ma; however, the isotope record does not begin to show sensitivity to orbital tilt until about 4.5 Ma, the time of significant changes in sedimentation patterns in the eastern equatorial Pacific. Only during the last 500,000 yr do all frequencies respond in a similar manner; we also see a marked increase in the response of the isotopic record to orbital forcing (including 100,000- and 400,000-yr periods). Title: Paleoceanography of the Eastern Equatorial Pacific during the Neogene : synthesis of Leg 138 drilling results Author: Mayer, Larry A., Pisias, Nicklas G., Mix, Alan C. The primary objective of Leg 138 was to provide detailed information about the ocean's response to global climate change during the Neogene. Two north south transects were drilled (95° and 110°W) within the region of equatorial divergence driven upwelling (and thus high accumulation rates and resolution) and spanning the major equatorial ocean current boundaries (and thus recording a high amplitude signal of the response of the sediment to climatically and/or tectonically driven changes in ocean circulation). The Neogene is marked by a number of well known climatic and tectonic events (the closing of the Isthmus of Panama, the onset of North Atlantic Deep Water (NADW), the rapid uplift of the Himalayas, the major intensification of Northern Hemisphere glaciation), and the response of the ocean before and after these events was a key focus of Leg 138 drilling. To address these objectives at the highest resolution possible, the Leg 138 scientific staff developed a number of new shipboard strategies and analytical procedures. These included the real time analysis of the near continuous gamma ray attenuation porosity evaluator (GRAPE) and susceptibility profiles produced by the multisensor track (MST) on unsplit cores to monitor core recovery and, if necessary, to modify the drilling strategy to ensure proper offset of coring gaps; the collection of near continuous color reflectance data on split cores; the logging of the first hole drilled at each site to optimize drilling and sampling strategies for subsequent holes; and the use of multiple continuous records to unambiguously construct complete composite sections for each site. The complete, continuous records provided by the GRAPE (with a temporal resolution of often yr), in conjunction with an excellent microfossil stratigraphy and often excellent magnetostratigraphy, allowed for astronomical tuning of the stratigraphic record and resulted in a set of internally consistent, high resolution age models that provide a secure, absolute time scale for the past 6 m.y. For the period before 6 m.y., the absolute time calibration is less secure, but it is still better than any previously offered. The high resolution stratigraphic framework of Leg 138 provided new insight into the previously ambiguous tectonic history of the region. By assuming that maximum sedimentation rates along the north south transect would be expected at the equator, the Leg 138 stratigraphy supports the 1985 work of Cox and Engerbretson, which calls for two different poles of rotation of the Pacific Plate during the interval 0-20 Ma. The Leg 138 plate reconstructions also support several previously hypothesized ridge crest jumps and a slowing of the absolute motion of the Nazca Plate at about 5 Ma. Although Leg 138 data that predates about 13 Ma is limited, the impression that one can gain from these data is that the eastern equatorial Pacific was characterized by relatively high carbonate concentrations and accumulation rates before about 11 Ma. This pattern was interrupted occasionally by rapid massive outpourings of near monospecific laminated diatom oozes that probably represent the formation of massive mats along strong surface water fronts. The laminated diatom oozes (LDO) continue to be present in the Leg 138 record (many of them being expressed as seismic reflections) until about 4.4 Ma. Carbonate accumulation rates begin to decline slowly between 11 and 9.8 Ma, when, at about 9.5 Ma, a near complete loss of carbonate (the "carbonate crash") takes place everywhere in the Leg 138 region (and beyond), except at the westernmost sites close to the equator. The "carbonate crash" was a time of fundamental change for the eastern equatorial Pacific, and perhaps for most of the ocean basins. Unlike many of the carbonate variations that precede and postdate it, this "crash" represents a major dissolution event whose effects can be traced seismically in the central and western Pacific. The changes in bottom water chemistry associated with this event (or series of events) appear to be related to the early phases of the closing of the Panama Gateway. The role of NADW initiation and intensification for controlling carbonate accumulation in the eastern equatorial Pacific is still not resolved; however, ocean modeling demonstrates that the closing of the Panama Gateway may also have a direct influence on NADW production. Therefore, the effects of changes in the Panama Gateway sill depth and the production of NADW may be manifested in the history of eastern equatorial Pacific sedimentation. The "carbonate crash" was followed by a recovery of the carbonate system (except in the Guatemala and Peru basins, which never recovered) that led up to the late Miocene/ early Pliocene sedimentation rate maxima, during which equatorial sedimentation rates are as much as five times greater than those of the late Pliocene or Pleistocene. Examination of modern productivity/ preser vation relationships implies that the sedimentation rate maximum was the result of enhanced productivity. The distribution of eolian sediments and isotopic gradients, along with an analysis of the modes of variance in carbonate deposition over the last 6 m.y., suggest a more northerly position of the Intertropical Convergence Zone (ITCZ), a stronger north south gradient across the equator, and a more zonal circulation focused along the equator during the time of maximum sedimentation. The mechanisms suggested for these changes in circulation patterns include the response of the eastern equatorial Pacific to the closing of the Isthmus of Panama, as well as a global increase in the flux of Ca and Si into the oceans, a possible response to evolution of the Himalayas and the Tibetan Plateau. In an effort to understand the response of the climate system to external (orbital) forcing, 6-m.y.-long, continuous records of carbonate (derived from GRAPE), δ¹⁸O and insolation were analyzed and compared. Evolutionary spectral calculations of the variance and coherence among these records indicate that the insolation record is dominated by precessional frequencies, but that the relative importance of the two precessional frequencies has changed significantly over the last 6 m.y. In general, precessional forcing is not found in the carbonate or isotopic records. In the tilt band, however, a linear response is present between solar forcing and the carbonate and isotope records over some intervals. The carbonate record appears to be tightly coupled to the tilt component of insolation before about 1.9 Ma; however, the isotope record does not begin to show sensitivity to orbital tilt until about 4.5 Ma, the time of significant changes in sedimentation patterns in the eastern equatorial Pacific. Only during the last 500,000 yr do all frequencies respond in a similar manner; we also see a marked increase in the response of the isotopic record to orbital forcing (including 100,000- and 400,000-yr periods). Close

• 7. [Article] Atmospheric transmission of North Atlantic Heinrich events

  We model the response o f the climate system during Heinrich event 2 (H2) by employing an atmospheric general circulation model, using boundary conditions based on the concept of a "canonical" Heinrich ...

  Citation × Citation Citation Abstract Copy Title: Atmospheric transmission of North Atlantic Heinrich events Author: Hostetler, S. W., Bartlein, P. J., Clark, Peter U., 1956-, Pisias, Nicklas G., Mix, Alan C. We model the response o f the climate system during Heinrich event 2 (H2) by employing an atmospheric general circulation model, using boundary conditions based on the concept of a "canonical" Heinrich event. The canonical event is initialized with a full-height Laurentide ice sheet (LIS) and CLIMAP sea surface temperatures (SSTs), followed by lowering of the LIS, then warming of North Atlantic SSTs. Our modeled temperature and wind fields exhibit spatially variable responses over the Northern Hemisphere at each stage of the H2 event. In some regions the climatic responses are additive, whereas in other regions they cancel or are of opposite sign, suggesting that Heinrich event climatic variations may have left complex signatures in geologic records. We find variations in the tropical water balance and the mass balance of ice sheets and implications for variations in terrestrial methane production from the contraction of northern permafrost regions and the expansion of tropical wetlands. Title: Atmospheric transmission of North Atlantic Heinrich events Author: Hostetler, S. W., Bartlein, P. J., Clark, Peter U., 1956-, Pisias, Nicklas G., Mix, Alan C. We model the response o f the climate system during Heinrich event 2 (H2) by employing an atmospheric general circulation model, using boundary conditions based on the concept of a "canonical" Heinrich event. The canonical event is initialized with a full-height Laurentide ice sheet (LIS) and CLIMAP sea surface temperatures (SSTs), followed by lowering of the LIS, then warming of North Atlantic SSTs. Our modeled temperature and wind fields exhibit spatially variable responses over the Northern Hemisphere at each stage of the H2 event. In some regions the climatic responses are additive, whereas in other regions they cancel or are of opposite sign, suggesting that Heinrich event climatic variations may have left complex signatures in geologic records. We find variations in the tropical water balance and the mass balance of ice sheets and implications for variations in terrestrial methane production from the contraction of northern permafrost regions and the expansion of tropical wetlands. Close

• 8. [Article] Variations of δ¹⁸O in rainwater from southwestern Oregon

  We examine the relation between δ¹⁸O in rainwater collected in southwestern Oregon and climate variables including temperature, parcel trajectory, precipitation amount, and specific humidity. Local surface ...

  Citation × Citation Citation Abstract Copy Title: Variations of δ¹⁸O in rainwater from southwestern Oregon Author: Clark, Peter U., Ersek, Vasile, Mix, Alan C. We examine the relation between δ¹⁸O in rainwater collected in southwestern Oregon and climate variables including temperature, parcel trajectory, precipitation amount, and specific humidity. Local surface air temperature at the time of sample collection explains a large proportion of δ¹⁸O variability, suggesting that paleoclimatic archives that are related to rainfall δ¹⁸O should be useful for qualitative temperature reconstructions. Models of Rayleigh distillation of air masses originating in the North Pacific can broadly constrain the observed isotopic variability in southwestern Oregon. Results from a Hybrid Single‐ Particle Lagrangian Integrated Trajectory Model suggest that recent parcel pathways have little influence on the isotopic composition of precipitation collected at our site. We also find no significant relation between rainfall d18O and precipitation amount. Changes in specific humidity along the parcel tracks, however, indicate that the water vapor exchange of the air mass with the underlying ocean influenced the isotopic composition of the most enriched samples. Title: Variations of δ¹⁸O in rainwater from southwestern Oregon Author: Clark, Peter U., Ersek, Vasile, Mix, Alan C. We examine the relation between δ¹⁸O in rainwater collected in southwestern Oregon and climate variables including temperature, parcel trajectory, precipitation amount, and specific humidity. Local surface air temperature at the time of sample collection explains a large proportion of δ¹⁸O variability, suggesting that paleoclimatic archives that are related to rainfall δ¹⁸O should be useful for qualitative temperature reconstructions. Models of Rayleigh distillation of air masses originating in the North Pacific can broadly constrain the observed isotopic variability in southwestern Oregon. Results from a Hybrid Single‐ Particle Lagrangian Integrated Trajectory Model suggest that recent parcel pathways have little influence on the isotopic composition of precipitation collected at our site. We also find no significant relation between rainfall d18O and precipitation amount. Changes in specific humidity along the parcel tracks, however, indicate that the water vapor exchange of the air mass with the underlying ocean influenced the isotopic composition of the most enriched samples. Close

• 9. [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

• 10. [Article] Sediment size fractionation and focusing in the equatorial Pacific: Effect on ²³⁰Th normalization and paleoflux measurements

  This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by the American Geophysical Union and published by John Wiley & Sons, Inc. It can ...

  Citation × Citation Citation Abstract Copy Title: Sediment size fractionation and focusing in the equatorial Pacific: Effect on ²³⁰Th normalization and paleoflux measurements Author: Huh, Chih-An, Lyle, Mitchell, Moore, Willard S., Finney, Bruce P., Murray, Richard W., Marcantonio, Franco, Mix, Alan C., Murray, David W. This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by the American Geophysical Union and published by John Wiley & Sons, Inc. It can be found at: http://agupubs.onlinelibrary.wiley.com/agu/journal/10.1002/%28ISSN%291944-9186/ Title: Sediment size fractionation and focusing in the equatorial Pacific: Effect on ²³⁰Th normalization and paleoflux measurements Author: Huh, Chih-An, Lyle, Mitchell, Moore, Willard S., Finney, Bruce P., Murray, Richard W., Marcantonio, Franco, Mix, Alan C., Murray, David W. This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by the American Geophysical Union and published by John Wiley & Sons, Inc. It can be found at: http://agupubs.onlinelibrary.wiley.com/agu/journal/10.1002/%28ISSN%291944-9186/ Close

• 11. [Article] Comparison of Imbrie-Kipp transfer function and modern analog temperature estimates using sediment trap and core top foraminiferal faunas

  Copyright 1997 by the American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Comparison of Imbrie-Kipp transfer function and modern analog temperature estimates using sediment trap and core top foraminiferal faunas Author: Ortiz, Joseph D., Mix, Alan C. Copyright 1997 by the American Geophysical Union Title: Comparison of Imbrie-Kipp transfer function and modern analog temperature estimates using sediment trap and core top foraminiferal faunas Author: Ortiz, Joseph D., Mix, Alan C. Copyright 1997 by the American Geophysical Union Close

• 12. [Article] Photosynthetic fractionation of ¹³C and concentrations of dissolved CO₂ in the central equatorial Pacific during the last 255,000 years

  Carbon isotopically based estimates of CO₂ levels have been generated from a record of the photosynthetic fractionation of ¹³C (≡εp) in a central equatorial Pacific sediment core that spans the last ~255 ...

  Citation × Citation Citation Abstract Copy Title: Photosynthetic fractionation of ¹³C and concentrations of dissolved CO₂ in the central equatorial Pacific during the last 255,000 years Author: Hayes, J. M., Jasper, John P, Prahl, Fredrick, Mix, Alan C. Carbon isotopically based estimates of CO₂ levels have been generated from a record of the photosynthetic fractionation of ¹³C (≡εp) in a central equatorial Pacific sediment core that spans the last ~255 ka. Contents of ¹³C in phytoplanktonic biomass were determined by analysis of C37 alkadienones. These compounds are exclusive products of Prymnesiophyte algae which at present grow most abundantly at depths of 70-90 m in the central equatorial Pacific. A record of the isotopic composition of dissolved CO₂ was constructed from isotopic analyses of the planktonic foraminifera Neogloboquadrina dutertrei, which calcified at 70-90 m in the same region. Values of εp, derived by comparison of the organic and inorganic δ values, were transformed to yield concentrations of dissolved CO₂ (≡ce) based on a new, site-specific calibration of the relationship between εp and ce. The calibration was based on reassessment of existing εp versus ce data, which support a physiologically based model in which εp is inversely related to ce. Values of P CO₂, the partial pressure of CO₂ that would be in equilibrium with the estimated concentrations of dissolved CO₂, were calculated using Henry’s law and the temperature determined from the alkenone-unsaturation index UK′37. Uncertainties in these values arise mainly from uncertainties about the appropriateness (particularly over time) of the site-specific relationship between εp and 1/ce. These are discussed in detail and it is concluded that the observed record of εp most probably reflects significant variations in ∆p CO₂, the ocean-atmosphere disequilibrium, which appears to have ranged from ~110 μatm during interglacials. Fluxes of CO₂ to the atmosphere would thus have been significantly larger during glacial intervals. If this were characteristic of large areas of the equatorial Pacific, then greater glacial sinks for the equatorially evaded CO₂ must have existed elsewhere. Statistical analysis of air-sea p CO₂ differences and other parameters revealed significant (p<0.01) inverse correlations of ∆p CO₂, with sea surface temperature and with the mass accumulation rate of opal. The former suggests response to the strength of upwelling, the latter may indicate either drawdown of CO₂ by siliceous phytoplankton or variation of [CO₂]/[Si(OH)₄] ratios in upwelling waters. Title: Photosynthetic fractionation of ¹³C and concentrations of dissolved CO₂ in the central equatorial Pacific during the last 255,000 years Author: Hayes, J. M., Jasper, John P, Prahl, Fredrick, Mix, Alan C. Carbon isotopically based estimates of CO₂ levels have been generated from a record of the photosynthetic fractionation of ¹³C (≡εp) in a central equatorial Pacific sediment core that spans the last ~255 ka. Contents of ¹³C in phytoplanktonic biomass were determined by analysis of C37 alkadienones. These compounds are exclusive products of Prymnesiophyte algae which at present grow most abundantly at depths of 70-90 m in the central equatorial Pacific. A record of the isotopic composition of dissolved CO₂ was constructed from isotopic analyses of the planktonic foraminifera Neogloboquadrina dutertrei, which calcified at 70-90 m in the same region. Values of εp, derived by comparison of the organic and inorganic δ values, were transformed to yield concentrations of dissolved CO₂ (≡ce) based on a new, site-specific calibration of the relationship between εp and ce. The calibration was based on reassessment of existing εp versus ce data, which support a physiologically based model in which εp is inversely related to ce. Values of P CO₂, the partial pressure of CO₂ that would be in equilibrium with the estimated concentrations of dissolved CO₂, were calculated using Henry’s law and the temperature determined from the alkenone-unsaturation index UK′37. Uncertainties in these values arise mainly from uncertainties about the appropriateness (particularly over time) of the site-specific relationship between εp and 1/ce. These are discussed in detail and it is concluded that the observed record of εp most probably reflects significant variations in ∆p CO₂, the ocean-atmosphere disequilibrium, which appears to have ranged from ~110 μatm during interglacials. Fluxes of CO₂ to the atmosphere would thus have been significantly larger during glacial intervals. If this were characteristic of large areas of the equatorial Pacific, then greater glacial sinks for the equatorially evaded CO₂ must have existed elsewhere. Statistical analysis of air-sea p CO₂ differences and other parameters revealed significant (p<0.01) inverse correlations of ∆p CO₂, with sea surface temperature and with the mass accumulation rate of opal. The former suggests response to the strength of upwelling, the latter may indicate either drawdown of CO₂ by siliceous phytoplankton or variation of [CO₂]/[Si(OH)₄] ratios in upwelling waters. Close

• 13. [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

• 14. [Article] Global climate evolution during the last deglaciation

  To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by National ...

  Citation × Citation Citation Abstract Copy Title: Global climate evolution during the last deglaciation Author: Mitrovica, Jerry X., Shakun, Jeremy D., McManus, Jerry, Moreno, Patricio I., Blois, Jessica L., Brewer, Simon, Pahnke, Katharina, Baker, Paul A., Bartlein, Patrick J., He, Feng, Morrill, Carrie, Brook, Ed, Markgraf, Vera, Carlson, Anders E., Russell, James M., Liu, Zhengyu, Johnson, Thomas C., Flower, Ben P., Otto-Bliesner, Bette L., Cheng, Hai, Kaufman, Darrell S., Adkins, Jess F., Colman, Steven M., Clark, Jorie, Marchitto, Thomas M., Whitlock, Cathy, Lynch-Stieglitz, Jean, Curry, William B., Clark, Peter U., Williams, John W., Mix, Alan C. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by National Academy of Sciences and can be found at: http://www.pnas.org/. Title: Global climate evolution during the last deglaciation Author: Mitrovica, Jerry X., Shakun, Jeremy D., McManus, Jerry, Moreno, Patricio I., Blois, Jessica L., Brewer, Simon, Pahnke, Katharina, Baker, Paul A., Bartlein, Patrick J., He, Feng, Morrill, Carrie, Brook, Ed, Markgraf, Vera, Carlson, Anders E., Russell, James M., Liu, Zhengyu, Johnson, Thomas C., Flower, Ben P., Otto-Bliesner, Bette L., Cheng, Hai, Kaufman, Darrell S., Adkins, Jess F., Colman, Steven M., Clark, Jorie, Marchitto, Thomas M., Whitlock, Cathy, Lynch-Stieglitz, Jean, Curry, William B., Clark, Peter U., Williams, John W., Mix, Alan C. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by National Academy of Sciences and can be found at: http://www.pnas.org/. Close

• 15. [Article] Leg 202 synthesis: Southeast Pacific paleoceangraphy

  Ocean Drilling Program (ODP) Leg 202 has opened a new window into understanding late Paleogene and Neogene global environmental change by providing high-quality sediment sequences from a previously unsampled ...

  Citation × Citation Citation Abstract Copy Title: Leg 202 synthesis: Southeast Pacific paleoceangraphy Author: Tiedemann, Ralf, Mix, Alan C. Ocean Drilling Program (ODP) Leg 202 has opened a new window into understanding late Paleogene and Neogene global environmental change by providing high-quality sediment sequences from a previously unsampled region, the eastern South Pacific. Eleven sites (1232– 1242) that record variations on timescales ranging from decades to tens of millions of years were drilled and investigated on transects of both depth (489–4072 m) and latitude (41°S–8°N). Building on the shipboard results presented in the Leg 202 Initial Reports volume, postcruise research has significantly improved the stratigraphic framework and provided new insights into climate-related processes, which operate on different timescales and are relevant to hypotheses concerning the bipolar “see-saw” climate mechanism, orbitally driven changes in the continent-ocean-ice-atmosphere system and tectonic processes associated with the uplift of the Andes, closure of the Central American Seaway, and major expansions of polar ice sheets. Stable isotope records and refinements in bio- and magnetostratigraphy in combination with orbitally tuned cyclostratigraphy significantly improved the Pleistocene stratigraphy at Sites 1233 and 1234, the Miocene– Pliocene stratigraphy at Sites 1236, 1237, 1239, and 1241, and the Oligocene stratigraphy around the late Oligocene climate optimum at Site 1237. Site 1233 filled a crucial gap in the stratigraphic and paleoceanographic archive of the South Pacific sector of the Southern Ocean by providing an outstanding reproducible accelerator mass spectrometry (AMS) ¹⁴C-dated paleomagnetic record of centennial- and millennial- scale variability, representing regional variations in environmental/climatic conditions for the past 70,000 yr. With regard to the Cenozoic timetable, the continuous and complete sedimentary sequence of Site 1237, spanning the last ~31 m.y., has the potential of becoming a stratigraphic reference section for the South Pacific. Postcruise work demonstrated the excellence of this record for producing a stratigraphic framework that combines the biostratigraphy and terrific magnetostratigraphy with orbitally tuned stable isotope records. With respect to the bipolar see-saw hypotheses, Leg 202 studies on high-resolution records changed the view of the global distribution of millennial-scale climate change. These studies clearly demonstrate that millennial-scale climate and biogeochemical systems of the southeast Pacific and Chile closely align with those recorded in Antarctica and the southern oceans and that these climate patterns extend to the equatorial Pacific, either transmitted directly by the eastern boundary current or indirectly by “the oceanic tunnel” (subsurface transport via Antarctic Mode or Intermediate Water, Equatorial Undercurrent) injecting Southern Hemisphere extratropical water masses into the equatorial upwelling system. On orbital timescales, most spectacular was the finding that Earth’s final transition into an “icehouse” climate ~13.9 m.y. ago, the middle Miocene intensification of Antarctic glaciation, coincided with a striking transition from obliquity to eccentricity as the drivers of climate change. Thus, the late Pleistocene 100-k.y. climate cycles are not unique in Earth’s history, and although the examples from the Miocene and Pleistocene are both associated with climate cooling, they occur under significantly different global boundary conditions. This important contribution from Leg 202 issues a challenge to climatologists to understand multiple origins of 100-k.y. climate cycles that are now well documented in the geologic record. On timescales of millions of years, late Neogene upper ocean temperature reconstructions in combination with salinity assessments at selected sites from Leg 202 provide further insights into the reorganization of ocean-atmosphere couplings that are linked to the shoaling of the Central American Seaway (CAS), uplift of the Andes, and Pliocene amplification of polar ice sheet expansion. Regional shoaling of the thermocline in the low-latitude eastern Pacific from 5.3 to 4.0 Ma most likely resulted from shoaling of the CAS, as suggested by model experiments. Mixed-layer cooling and freshening in the tropical northeast Pacific warm pool as well as declining sea-surface temperature (SST) and increasing biological productivity off Chile parallel intensification of Northern Hemisphere glaciation from 3.6 to 2.4 Ma. The similarity of temporal changes in SST, upwelling, and dust flux between the Benguela and Chile upwelling systems suggests that uplift of the Andes was probably of secondary importance for generating the observed changes in the southeast Pacific within the last 6 m.y., as the Benguela upwelling system was not affected by mountain uplift. The expected atmosphere- oceanic response of the southeast Pacific to uplift of the Andes probably played a larger role during the late Miocene, prior to 6 Ma. A particular scientific challenge of future Leg 202 paleoclimate research is to better understand the transitions between these different timescales with respect to couplings between global, regional, and local processes. Title: Leg 202 synthesis: Southeast Pacific paleoceangraphy Author: Tiedemann, Ralf, Mix, Alan C. Ocean Drilling Program (ODP) Leg 202 has opened a new window into understanding late Paleogene and Neogene global environmental change by providing high-quality sediment sequences from a previously unsampled region, the eastern South Pacific. Eleven sites (1232– 1242) that record variations on timescales ranging from decades to tens of millions of years were drilled and investigated on transects of both depth (489–4072 m) and latitude (41°S–8°N). Building on the shipboard results presented in the Leg 202 Initial Reports volume, postcruise research has significantly improved the stratigraphic framework and provided new insights into climate-related processes, which operate on different timescales and are relevant to hypotheses concerning the bipolar “see-saw” climate mechanism, orbitally driven changes in the continent-ocean-ice-atmosphere system and tectonic processes associated with the uplift of the Andes, closure of the Central American Seaway, and major expansions of polar ice sheets. Stable isotope records and refinements in bio- and magnetostratigraphy in combination with orbitally tuned cyclostratigraphy significantly improved the Pleistocene stratigraphy at Sites 1233 and 1234, the Miocene– Pliocene stratigraphy at Sites 1236, 1237, 1239, and 1241, and the Oligocene stratigraphy around the late Oligocene climate optimum at Site 1237. Site 1233 filled a crucial gap in the stratigraphic and paleoceanographic archive of the South Pacific sector of the Southern Ocean by providing an outstanding reproducible accelerator mass spectrometry (AMS) ¹⁴C-dated paleomagnetic record of centennial- and millennial- scale variability, representing regional variations in environmental/climatic conditions for the past 70,000 yr. With regard to the Cenozoic timetable, the continuous and complete sedimentary sequence of Site 1237, spanning the last ~31 m.y., has the potential of becoming a stratigraphic reference section for the South Pacific. Postcruise work demonstrated the excellence of this record for producing a stratigraphic framework that combines the biostratigraphy and terrific magnetostratigraphy with orbitally tuned stable isotope records. With respect to the bipolar see-saw hypotheses, Leg 202 studies on high-resolution records changed the view of the global distribution of millennial-scale climate change. These studies clearly demonstrate that millennial-scale climate and biogeochemical systems of the southeast Pacific and Chile closely align with those recorded in Antarctica and the southern oceans and that these climate patterns extend to the equatorial Pacific, either transmitted directly by the eastern boundary current or indirectly by “the oceanic tunnel” (subsurface transport via Antarctic Mode or Intermediate Water, Equatorial Undercurrent) injecting Southern Hemisphere extratropical water masses into the equatorial upwelling system. On orbital timescales, most spectacular was the finding that Earth’s final transition into an “icehouse” climate ~13.9 m.y. ago, the middle Miocene intensification of Antarctic glaciation, coincided with a striking transition from obliquity to eccentricity as the drivers of climate change. Thus, the late Pleistocene 100-k.y. climate cycles are not unique in Earth’s history, and although the examples from the Miocene and Pleistocene are both associated with climate cooling, they occur under significantly different global boundary conditions. This important contribution from Leg 202 issues a challenge to climatologists to understand multiple origins of 100-k.y. climate cycles that are now well documented in the geologic record. On timescales of millions of years, late Neogene upper ocean temperature reconstructions in combination with salinity assessments at selected sites from Leg 202 provide further insights into the reorganization of ocean-atmosphere couplings that are linked to the shoaling of the Central American Seaway (CAS), uplift of the Andes, and Pliocene amplification of polar ice sheet expansion. Regional shoaling of the thermocline in the low-latitude eastern Pacific from 5.3 to 4.0 Ma most likely resulted from shoaling of the CAS, as suggested by model experiments. Mixed-layer cooling and freshening in the tropical northeast Pacific warm pool as well as declining sea-surface temperature (SST) and increasing biological productivity off Chile parallel intensification of Northern Hemisphere glaciation from 3.6 to 2.4 Ma. The similarity of temporal changes in SST, upwelling, and dust flux between the Benguela and Chile upwelling systems suggests that uplift of the Andes was probably of secondary importance for generating the observed changes in the southeast Pacific within the last 6 m.y., as the Benguela upwelling system was not affected by mountain uplift. The expected atmosphere- oceanic response of the southeast Pacific to uplift of the Andes probably played a larger role during the late Miocene, prior to 6 Ma. A particular scientific challenge of future Leg 202 paleoclimate research is to better understand the transitions between these different timescales with respect to couplings between global, regional, and local processes. Close