A. Christina Ravelo

Role of Panama uplift on oceanic freshwater balance

Comparison between planktic foraminiferal oxygen isotope records from the Caribbean Sea (Ocean Drilling Program [ODP] Site 999) and the equatorial east Pacific (ODP Site 851) suggests an increase in Caribbean surface-water salinity between 4.7 and 4.2 Ma. The modern Atlantic-Pacific salinity contrast of about 1‰ became fully established at 4.2 Ma as reflected by a 0.5‰ planktic foraminifera 18O enrichment in the Caribbean Sea. This is interpreted as the result of restricted surface-water exchange between the tropical Atlantic and Pacific in response to the shoaling of the Central American seaway. As a consequence, the Atlantic and Pacific surface-ocean circulation regime changed, as did the freshwater balance between the major ocean basins. Simultaneous shifts in benthic carbon isotope records in the Caribbean Sea suggest an intensification in North Atlantic thermohaline circulation. These results indicate that the Panamanian isthmus formation caused several new ocean-atmosphere feedback mechanisms that have affected climate since the early Pliocene.

Late Miocene decoupling of oceanic warmth and atmospheric carbon dioxide forcing.

Deep-time palaeoclimate studies are vitally important for developing a complete understanding of climate responses to changes in the atmospheric carbon dioxide concentration (that is, the atmospheric partial pressure of CO2, pCO2). Although past studies have explored these responses during portions of the Cenozoic era (the most recent 65.5 million years (Myr) of Earth history), comparatively little is known about the climate of the late Miocene (∼12–5 Myr ago), an interval with pCO2 values of only 200–350 parts per million by volume but nearly ice-free conditions in the Northern Hemisphere and warmer-than-modern temperatures on the continents. Here we present quantitative geochemical sea surface temperature estimates from the Miocene mid-latitude North Pacific Ocean, and show that oceanic warmth persisted throughout the interval of low pCO2 ∼12–5 Myr ago. We also present new stable isotope measurements from the western equatorial Pacific that, in conjunction with previously published data, reveal a long-term trend of thermocline shoaling in the equatorial Pacific since ∼13 Myr ago. We propose that a relatively deep global thermocline, reductions in low-latitude gradients in sea surface temperature, and cloud and water vapour feedbacks may help to explain the warmth of the late Miocene. Additional shoaling of the thermocline after 5 Myr ago probably explains the stronger coupling between pCO2, sea surface temperatures and climate that is characteristic of the more recent Pliocene and Pleistocene epochs.

RECONSTRUCTIONS OF UPWELLING, PRODUCTIVITY, AND PHOTIC ZONE DEPTH IN THE EASTERN EQUATORIAL PACIFIC OCEAN USING PLANKTONIC FORAMINIFERAL STABLE ISOTOPES AND ABUNDANCES

In the hydrographically complex eastern equatorial Pacific Ocean (EEP), the distinction between changes in productivity and changes in upwelling is important to the study of the causes and implications of changes in paleoproductivity during the Last Glacial Maximum (LGM). We studied seven EEP coretops representing a gradient of increasing primary productivity from west to east. Comparison of the coretop data indicates calcification depth and temperature for each planktonic foraminiferal species may change depending on the vertical position of hydrographic features such as the degree of stratification of the water column, as well as associated biological parameters such as the depths of the photic zone and the chlorophyll maximum. Because these biological parameters are related to primary productivity, calcification depth and temperature patterns for each species are somewhat different for high and low productivity regions in the EEP. We use the relationship between modern surface hydrography and coretop planktonic foraminiferal abundances and isotopic composition to interpret upwelling and productivity changes in the EEP over the last 20,000 years. While data indicate higher primary productivity and lower SSTs, they do not indicate that there was greater upwelling at the location of our site during the LGM relative to present.

Remote forcing at the Last Glacial Maximum in the tropical Pacific Ocean

We present results of a Last Glacial Maximum (LGM) wind stress sensitivity experiment using a high-resolution ocean general circulation model of the tropical Pacific Ocean. LGM wind stress, used to drive the ocean model, was generated using an atmospheric general circulation model simulation forced by LGM boundary conditions as part of the Paleoclimate Modeling Intercomparison Project (PMIP) [Broccoli, 2000]. LGM wind stress anomalies were large in the western half of the basin, yet there was a significant hydrographic response in the eastern half. This ocean model experiment hind casts changes that are in close agreement with paleoceanographic data from the entire region, even without the explicit modeling of the air-sea interactions. Data and model both predict that the annual average thermocline tilt across the basin was enhanced. Data and model are consistent with a stronger equatorial undercurrent which shoaled to the west of where it does today, and stronger advection of water from the Peru Current into the east equatorial Pacific and across the equator. Paleoproductivity and sea surface temperature (SST) data are interpreted in light of the modeling results, indicating that paleoproductivity changes were related to wind-forced dynamical changes resulting from LGM boundary conditions, while SST changes were related to independent, possibly radiative, forcing. Overall, our results imply that much of the dynamic response of the tropical Pacific during the LGM can be explained by wind field changes resulting from global LGM boundary conditions.

Millennial-scale climate change and oceanic processes in the Late Pliocene and Early Pleistocene

We generated 200–500 year resolution records of oceanic processes in the North Atlantic (Ocean Drilling Program Site 983, 60°24′N, 23°38′W, 1983 meters water depth) for intervals in the latest Pliocene (1.86–1.93 Ma) and the earliest Pleistocene (1.75–1.83 Ma) in order to examine the linkages between millennial-scale variations in the ocean and background glacial-interglacial climate change. Within glacial intervals we find evidence for variations similar to those observed in the late Pleistocene. We find discrete ice-rafted debris (IRD) events that reoccur every 2–5 kyr. These events are preceded by a short cooling and accompanied by a reorganization of glacial deep waters. The timing of IRD events in the late Pliocene and early Pleistocene intervals is similar to that of Dansgaard-Oeschger cycles, but we find no IRD events comparable in timing to late Pleistocene Heinrich events. Although interglacial intervals are much more stable, we do find evidence for low-amplitude variations in deep water properties that reoccur every ∼2 kyr within interglacial intervals. The similarity between our late Pliocene—early Pleistocene records and late Pleistocene records implies that the mechanism driving millennial-scale variations cannot be uniquely attributed to the strongly nonlinear linkage between climate and insolation and the large ice sheets of the late Pleistocene.

Evolution of marine sedimentation in the Bering Sea since the Pliocene

Sediment of the Bering Sea, derived mainly from biogenic, glaciomarine, and, secondarily, riverine sources, reflects the history of oceanographic changes within the basin and climatic changes on the adjacent continents. Integrated Ocean Drilling Program (IODP) Expedition 323 recovered cores that reveal the evolution of sedimentation in the Bering Sea over the past 5 m.y., a period that includes globally significant events such as the early Pliocene warm period, the onset of extensive Northern Hemisphere glaciation, and the Pleistocene glacial-interglacial and millennial-scale climate cycles. To begin to understand the Bering Sea regional response to and role in these global climate change events, we examined the sedimentary constituents of Expedition 323 sites U1339, U1343, and U1344 on the Bering Slope, and U1340 and U1341 on Bowers Ridge. New particle size and petrographic analyses, combined with shipboard lithostratigraphic and physical property data, are used to characterize sediment types and texture and its distribution through space and time. The sediment comprises mainly two components, opaline diatom valves and siliciclastic grains (mainly clay and fine silt size). Approximately 40% of the variance in particle size can be explained by the abundance and preservation of diatom valves, a rough indicator of biogenic opal productivity. Particle size data indicate that productivity was generally higher during interglacials compared to glacials, and higher during the Pliocene warm period, decreasing as Northern Hemisphere glaciation intensified ∼3 m.y. ago. Although the abundance of diatoms in the sediment varied, diatom ooze and diatom mud are the dominant lithologies at Bowers Ridge, indicating that there was a persistent supply of diatoms to the sediment in the open Bering Sea during the past 5 m.y. This study provides a comprehensive view of sediment types and sedimentation processes; future work should be aimed at validating our interpretations of past changes in productivity and siliciclastic sedimentation mechanisms with multiple additional proxies.

Reduced El Niño-Southern Oscillation during the Last Glacial Maximum.

A new tilt on predicting future ENSO variability A new finding should improve the ability of climate models to predict the behavior of the El Niño–Southern Oscillation (ENSO) in a warmer future. Ford et al. looked at the distribution of surface and subsurface temperatures in the eastern and western equatorial Pacific 19,000 years ago and between 3000 and 6000 years ago. Temperatures fluctuated over a greater range during the older period. ENSO thus depended more on the tilt of the equatorial Pacific thermocline than on the east-to-west temperature gradient, as previously thought. Science, this issue p. 255 El Niño–Southern Oscillation was less variable during the Last Glacial Maximum than it is now. El Niño–Southern Oscillation (ENSO) is a major source of global interannual variability, but its response to climate change is uncertain. Paleoclimate records from the Last Glacial Maximum (LGM) provide insight into ENSO behavior when global boundary conditions (ice sheet extent, atmospheric partial pressure of CO2) were different from those today. In this work, we reconstruct LGM temperature variability at equatorial Pacific sites using measurements of individual planktonic foraminifera shells. A deep equatorial thermocline altered the dynamics in the eastern equatorial cold tongue, resulting in reduced ENSO variability during the LGM compared to the Late Holocene. These results suggest that ENSO was not tied directly to the east-west temperature gradient, as previously suggested. Rather, the thermocline of the eastern equatorial Pacific played a decisive role in the ENSO response to LGM climate.

The evolution of the equatorial thermocline and the early Pliocene El Padre mean state

The tropical Pacific thermocline strength, depth, and tilt are critical to tropical mean state and variability. During the early Pliocene (~3.5 to 4.5 Ma), the Eastern Equatorial Pacific (EEP) thermocline was deeper and the cold tongue was warmer than today, which resulted in a mean state with a reduced zonal sea surface temperature gradient or El Padre. However, it is unclear whether the deep thermocline was a local feature of the EEP or a basin-wide condition with global implications. Our measurements of Mg/Ca of Globorotalia tumida in a western equatorial Pacific site indicate Pliocene subsurface temperatures warmer than today; thus, El Padre included a basin-wide thermocline that was relatively warm, deep, and weakly tilted. At ~4 Ma, thermocline steepening was coupled to cooling of the cold tongue. Since ~4 Ma, the basin-wide thermocline cooled/shoaled gradually, with implications for thermocline feedbacks in tropical dynamics and the interpretation of TEX86-derived temperatures.

Late Pleistocene tropical Pacific temperature sensitivity to radiative greenhouse gas forcing

Understanding how global temperature changes with increasing atmospheric greenhouse gas concentrations, or climate sensitivity, is of central importance to climate change research. Climate models provide sensitivity estimates that may not fully incorporate slow, long-term feedbacks such as those involving ice sheets and vegetation. Geological studies, on the other hand, can provide estimates that integrate long- and short-term climate feedbacks to radiative forcing. Because high latitudes are thought to be most sensitive to greenhouse gas forcing owing to, for example, ice-albedo feedbacks, we focus on the tropical Pacific Ocean to derive a minimum value for long-term climate sensitivity. Using Mg/Ca paleothermometry from the planktonic foraminifera Globigerinoides ruber from the past 500 k.y. at Ocean Drilling Program (ODP) Site 871 in the western Pacific warm pool, we estimate the tropical Pacific climate sensitivity parameter (λ) to be 0.94–1.06 °C (W m −2 ) −1 , higher than that predicted by model simulations of the Last Glacial Maximum or by models of doubled greenhouse gas concentration forcing. This result suggests that models may not yet adequately represent the long-term feedbacks related to ocean circulation, vegetation and associated dust, or the cryosphere, and/or may underestimate the effects of tropical clouds or other short-term feedback processes.

Glacial-interglacial changes in central tropical Pacific surface seawater property gradients

Much uncertainty exists about the state of the oceanic and atmospheric circulation in the tropical Pacific over the last glacial cycle. Studies have been hampered by the fact that sediment cores suitable for study were concentrated in the western and eastern parts of the tropical Pacific, with little information from the central tropical Pacific. Here we present information from a suite of sediment cores collected from the Line Islands Ridge in the central tropical Pacific, which show sedimentation rates and stratigraphies suitable for paleoceanographic investigations. Based on the radiocarbon and oxygen isotope measurements on the planktonic foraminifera Globigerinoides ruber, we construct preliminary age models for selected cores and show that the gradient in the oxygen isotope ratio of G. ruber between the equator and 8°N is enhanced during glacial stages relative to interglacial stages. This stronger gradient could reflect enhanced equatorial cooling (perhaps reflecting a stronger Walker circulation) or an enhanced salinity gradient (perhaps reflecting increased rainfall in the central tropical Pacific).