Sea surface temperatures (SSTs) in the cold tongue of the eastern equatorial Pacific exert powerful controls on global atmospheric circulation patterns. We examined climate variability in this region from the Last Glacial Maximum (LGM) to the present, using a SST record reconstructed from magnesium/calcium ratios in foraminifera from sea-floor sediments near the Galápagos Islands. Cold-tongue SST varied coherently with precession-induced changes in seasonality during the past 30,000 years. Observed LGM cooling of just 1.2 degrees C implies a relaxation of tropical temperature gradients, weakened Hadley and Walker circulation, southward shift of the Intertropical Convergence Zone, and a persistent El Niño-like pattern in the tropical Pacific. This is contrasted with mid-Holocene cooling suggestive of a La Niña-like pattern with enhanced SST gradients and strengthened trade winds. Our results support a potent role for altered tropical Pacific SST gradients in global climate variations.

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[1] The El Niño-Southern Oscillation (ENSO) is the largest engine of interannual climate variability on the planet, yet its past behavior and potential for future change are poorly understood and vigorously contested. Reconstructions of past ENSO are indispensable for testing climate models tasked with predicting future ENSO activity in a warming world, but suitable geologic archives are scarce, especially for the last glacial period. Here we reconstruct mean climate and ENSO variability in the Holocene and Last Glacial Maximum (LGM) from oxygen isotopic ratios (d 18 O) of individual foraminifera retrieved from deep-sea sediments. Our results document coordinated adjustments of the tropical Pacific/ENSO system between two diametrically opposite states: an "amplified ENSO" state in the LGM associated with a reduced zonal temperature gradient, and a "damped ENSO" state in the Mid-Holocene with enhanced gradient. Orbital precession provided the switch between these states and acted as the dominant external driver of the tropical Pacific/ENSO system in the past 25,000 years. The linked response of the mean state and variability to orbital forcing provides an integrated framework for testing ENSO theory and models.

Figure 1. Map of tropical Pacific Ocean showing locations of cores used in this study. Background sea-surface temperatures (SSTs) correspond to La Niñ a conditions during August 1999. Approximate position of Intertropical Convergence Zone (ITCZ) was drawn using satellite cloud composites from 2 August 1999, available at http:// www.ssec.wisc.edu/data.

Benthic and planktonic 14C ages are presented for the last glacial termination from marine sediment core VM21‐30 from 617 m in the eastern equatorial Pacific. The benthic‐planktonic 14C age differences in the core increased to more than 6000 years between Heinrich 1 time and the end of the Younger Dryas period. Several replicated 14C ages on different benthic and planktonic species from the same samples within the deglacial section of the core indicate a minimal amount of bioturbation. Scanning electron microscopy reveals no evidence of calcite alteration or contamination. The oxygen isotope stratigraphy of planktonic and benthic foraminifera does not indicate anomalously old (glacial age) values, and there is no evidence of a large negative stable carbon isotope excursion in benthic foraminifera that would indicate input of old carbon from dissociated methane. It appears, therefore, that the benthic 14C excursion in this core is not an artifact of diagenesis, bioturbation, or a pulse of methane. A benthic Δ14C stratigraphy reconstructed from the 14C ages from the deglacial section of VM21‐30 appears to match that of Baja margin core MV99‐MC19/GC31/PC08 (705 m), but the magnitude of the low‐14C excursion is much larger in the VM21‐30 record. This would seem to imply that the VM21‐30 core was closer to the source of 14C‐depleted waters during the deglaciation, but the source of this CO2 remains elusive.

Tropical Pacific Ocean dynamics during the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA) are poorly characterized due to a lack of evidence from the eastern equatorial Pacific. We reconstructed sea surface temperature, El Niño-Southern Oscillation (ENSO) activity, and the tropical Pacific zonal gradient for the past millennium from Galápagos ocean sediments. We document a mid-millennium shift (MMS) in ocean-atmosphere circulation around 1500-1650 CE, from a state with dampened ENSO and strong zonal gradient to one with amplified ENSO and weak gradient. The MMS coincided with the deepest LIA cooling and was probably caused by a southward shift of the intertropical convergence zone. The peak of the MCA (900-1150 CE) was a warm period in the eastern Pacific, contradicting the paradigm of a persistent La Niña pattern.

[1] The equatorial cold tongue (ECT) of the eastern Pacific is the most dynamic ocean region in the world's tropics and sets the tempo for global climate anomalies arising from El Niño-Southern Oscillation (ENSO) events. This region's deglaciation history and relationship with north and south polar climates remains poorly understood, impeding integration of tropical Pacific ocean-atmosphere dynamics and ENSO variability in our understanding of glacial cycles. Here we present alkenone reconstructions of sea surface temperature (SST) across the last glacial termination from five ECT cores east of the Galapagos Islands. A composite index of SST based on these demonstrates strong temporal affinity with the two-step deglaciation of the Northern Hemisphere, composed of two distinct warming steps at the beginning of the Bölling and the end of the Younger Dryas intervals. Within dating uncertainty, warming in the ECT began in phase with the Bölling excursion, was followed by a 2-3 ka plateau, and resumed with a second pulse at the end of the Younger Dryas. On the basis of our reconstructions, about two thirds of the warming materialized at or after the end of the Younger Dryas, implying a marked delay in the region's response to deglaciation. The results challenge the prevailing paradigm that ECT deglacial history conforms to an Antarctic timing, commonly attributed to advection from the Southern Ocean through an interior oceanic link, or to synchronous response of both regions to CO 2 forcing. Our results emphasize instead the role of dynamical adjustments linked to Northern Hemisphere processes, most likely transmitted through the atmosphere.Citation: Koutavas, A., and J. P. Sachs (2008), Northern timing of deglaciation in the eastern equatorial Pacific from alkenone paleothermometry, Paleoceanography, 23, PA4205,

[1] We use planktonic oxygen isotope (d 18 O) records spanning the last 30,000 years (kyr) to constrain the magnitude and spatial pattern of glacial cooling in the upwelling environment of the eastern equatorial Pacific (EEP). Fourteen new downcore d 18O records were obtained from surface-dwelling planktonic foraminifera Globigerinoides sacculifer and Globigerinoides ruber in eight cores from the upwelling tongue of the EEP. All sites have sedimentation rates exceeding 5 cm/kyr and, with one exception, lie above the modern depth of the foraminiferal lysocline. Sites directly underlying the cool band of upwelling immediately south of the equator record mean late Holocene (LH)-Last Glacial Maximum (LGM) d18 O amplitudes ranging between 1.0 and 1.3%. We estimate that mean sea surface temperatures (SST) in this region during the LGM were on average 1.5 ± 0.5°C lower than the LH. Larger d 18O amplitudes are observed in sites north of the equator, indicating a spatial pattern of reduced meridional SST gradient across the equator during the LGM. This result is supported by comparison of Mg/Ca SST reconstructions from two sites straddling the equator. We interpret the reduction of this gradient during the LGM as evidence for a less intense cold tongue-Intertropical Convergence Zone (ITCZ) frontal system, a more southerly position of the ITCZ, and weaker southeast equatorial trades in the EEP.