PP22A-01 INVITED
ENSO-Like Variability in the Eastern Equatorial Pacific at Orbital and Millennial Time- Scales: High-low Latitude Teleconnections
Past hydrographical conditions in the Eastern Equatorial Pacific (EEP) are analysed throughout the multi- proxy study of the long sediment core ODP Site 1240, located at the Panama Basin (0°01.311"N, 86°27.758"W, 2921mbsl). The combination of different geochemical proxies (δ18O, δ13C and Mg/Ca) in two different foraminifera species with diverse preferential dwelling depths in the water column (G. ruber and N. dutertrei) allows the parallel reconstruction for the last 275.000 years of temperature and salinity both at the surface and thermocline layers respectively. The records obtained cover the last 275,000 years at orbital but also millennial-centennial time scales. Results show the existence of recurrent events characterized by saltier thermocline waters which are associated with an intensification of the EEP upwelling system. These events coincide with Earth's orbital configurations characterized by low precession periods and were particularly strong when co-existed with high obliquity conditions, as consistently occurs at the onset of glacial Terminations. This orbital configuration strengthened the atmospheric Walker circulation in the equatorial Pacific which eventually favours the establishment of strong La Niña-like like conditions in the EEP. The thermocline proxies also support the advection of Southern Ocean waters into the equatorial thermocline during glacial Terminations. This 'tunnelling' effect could have been reinforced by the strong La Niña-like conditions through a southward displacement of the westerlies (as supported by modern satellite observations during La Niña events), therefore enhancing the Antarctic Circumpolar Current and the resumption of the Southern Ocean upwelling system. The Site 1240 records also show significant variability at millennial time-scales and demonstrate that sea surface temperatures (SST) evolved consistently with changes in atmospheric CO2. Apparent contradictory results coming from different SST proxy records based on foraminifera Mg/Ca and biomarker (Uk"37) are interpreted in terms of seasonality intensity changes which provide new insights on the understanding of the regional paleoceanography. Results also support a close linkage between oceanographic changes in the EEP and rapid changes in the intensity of the North Atlantic Meridional Overturning circulation.
PP22A-02
A 100,000 Year History of N-S and E-W Gradients of Sea Surface Temperature Across the Equatorial Pacific From Alkenone Paleothermometry.
The Eastern Equatorial Pacific, due to very dynamic ocean-atmosphere linkages, is a critical region for the understanding of past global climate changes. It is characterized by climatic asymmetries: A pool of warm sea surface temperature (SST) and intense atmospheric convective activities - The Eastern Pacific Warm Pool (EPWP) - lies North of the equator, while a prominent upwelling region - the Equatorial Cold Tongue (ECT) - is centered at 1°S. The Equatorial Front (EF) separating these two regions is currently the sharpest SST gradient in the world. Previous studies based on the oxygen isotopic and Mg/Ca compositions of planktonic foraminifera correlated changes in the cross-equatorial SST gradient during the last glacial maximum with variations in the intensity of upwelling. Here we use alkenone unsaturation to reconstruct meridional SST gradients for the last 100,000 years (kyr). Three new SST records were obtained from high-resolution sediment cores straddling the EF east of the Galapagos Islands and from the EPWP. The data reveal a relatively constant SST gradient among the cores from the ECT for the last 75 kyr. On the other hand, the end of the warm MIS 5 (75-90 kyr) reveals a highly reduced gradient. These results challenge previous suggestions of a significantly intensified Cold Tongue-Intertropical Convergence Zone complex during glacial times. This presentation will also explore E-W SST gradients across the tropical Pacific over the same time interval.
PP22A-03
Surface and Subsurface Changes in the Indonesian Throughflow Region From the LGM to Present Inferred From G. ruber and P. obliquiloculata Mg/Ca and del 18O
The major climate phenomena in the Indo-Pacific Warm Pool, the El Niño-Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the monsoons, are all accompanied by significant changes in thermocline structure and temperature. Variations in ENSO, the IOD and the monsoons can influence the volume of the Indonesian Throughflow (ITF), the surface and thermocline water transported from the Pacific to the Indian Ocean through Indonesian gateways. These variations influence the heat and salinity budgets of both ocean basins. On short time scales, the temperature and depth of the thermocline in the Makassar Strait (Indonesia) correlate with the volume of ITF transported through the strait. In order to reconstruct past thermocline structure, we have measured the δ18O and Mg/Ca of surface mixed layer (G. ruber) and thermocline dwelling (P. obliquiloculata) foraminifera in several cores from the Makassar Strait and south of Java, near the exit of the ITF to the Indian Ocean. The records of G. ruber and P. obliquiloculata indicate changes in thermocline structure during both the deglaciation and Holocene. These changes occur simultaneously in several cores and indicate changes in regional climate that may have impacted the Throughflow transport.
PP22A-04
Indonesian Throughflow variability over the last 140 kyr: Temperature and precipitation proxy records from the Timor Sea
Centennial- to millennial-scale sea surface and thermocline δ18O and Mg/Ca derived temperatures from the outflow area of the Indonesian Throughflow (ITF) indicate that thermocline temperatures were approximately 2°C cooler in the Timor Strait than in the E Indian Ocean during glacial intervals, implying a substantial decrease in ITF thermocline flow. Exposure of the Sunda Shelf during extreme sea level lowstands probably prevented cool, low salinity water from the Java Sea to flow into the southern Makassar Strait during boreal winter monsoon, thus promoting warm surface water flow through the Makassar Strait. Therefore, the modern thermocline dominated ITF during boreal winter cannot be considered a realistic analogue for glacial times. XRF-Scanner derived terrigenous flux records of eolian dust and riverine sediment from NW Australia and Timor indicate changes in the Australasian monsoon associated with the migration of hydrological fronts between Indian Ocean and ITF water masses. Riverine and dust fluxes in the Timor Sea exhibit a seesaw pattern during Dansgaard-Oeschger and Heinrich events, which reflects shifts in the position of the ITCZ over boreal winter. During Dansgaard-Oeschger events, dry trade winds dominate over NW Australia, whereas monsoonal rainfall increase during Heinrich cold phases, when tropical SE Asia remain dry. However, on longer timescales (glacial-interglacial and precessional) fundamental changes in land/sea distribution driven by sea-level fluctuations mainly controlled evaporation/precipitation and circulation patterns in the Australasian region.
PP22A-05
Hydrographic Variability in the Indo-Pacific Warm Pool During the Past 25,000 Years
The Indo-Pacific Warm Pool is one of the warmest regions in the modern oceans, and as such is important to global atmospheric heat and water vapour budgets. Previous studies indicate sea surface temperatures within the warm pool were about 3°C cooler during the last glacial and that SSTs began to warm at the last deglaciation prior to the major retreat of continental ice volume (Visser et al., 2003). There is also evidence that Pacific intermediate waters that originate in the southern Ocean began to warm before deglacial warming began in the tropics (Stott et al., 2007). Here we present δ18O and Mg/Ca records from surface and thermocline dwelling foraminifera from two sites in the Makassar Strait for the past 25,000 years. We find that the thermocline began to warm approximately 1,000 years earlier than surface waters, in good agreement with other records from nearby regions. The reconstructed δ18O of the seawater reached minimum values during the early Holocene, indicating a salinity minimum at that time in association with rising Northern Hemisphere summer solar radiation and increased convective activity in this region (Partin et al., 2007). Finally, we find a gradual trend towards cooler and fresher waters in the mid to early Holocene, which may reflect the southward shift of the Intertropical Convergence Zone during this time.
PP22A-06
Hydrological Cycle in the Western Equatorial Warm Pool over the Past 220 k years
The Western Pacific Warm Pool is a major source of heat and moisture to extra-tropical regions, and its condition could have great impact on global climate response to various forcing factors. We reconstructed the rainfall pattern over Papua New Guinea (PNG) for the past 220 kyr using terrigenous elemental contents (Ti, Fe, K and Si) and calcareous productivity (Ca) recorded in a marine sediment core MD05-2920 (2°51.48S, 144°32.04E) from 100 km off the Sepik River mouth in Northern PNG. The core chronostratigraphy is established by 14C dating and benthic foraminiferal oxygen isotopes. The Sepik and Ramu river system forms one of the highest sediment discharge zones in the world because of high rainfall rates, warm and humid climate, steep topography and erodible volcanic rocks in the draining basin. At present, the rainfall over this area is under the influence of both Asia-Australian monsoon and El Niño Southern Oscillation (ENSO). The results obtained by an XRF core scanner indicate that for the whole record major sediment components are of terrigenous river-born nature and biogenic CaCO3. Spectral analysis reveals that dominant peaks for Ti are precession and obliquity periods whereas Ca variability is rather dominated by obliquity. The wet periods appear during maximum local insolation, which is in phase with minimum East Asian summer monsoon strength recorded by Chinese speleothems. Modeled past ENSO activity cannot explain the reconstructed rainfall and productivity patterns. Taken together, the fresh water cycle over New Guinea is better explained by latitudinal shifts of the Intertropical Convergence Zone rather than ENSO-type variability on orbital time scales. The variability of calcareous productivity is likely related to general changes in nutricline depth of the tropical Pacific band.
PP22A-07 INVITED
Antarctic and Deep Pacific Temperature Changes Lead Tropical Pacific SSTs During Rapid Climate Change
The role that tropical Pacific ocean/atmospheric variability has had in abrupt climatic changes throughout the Quaternary continues to be debated. The debate centers in part, on how representative modern ocean/atmospheric variability in the tropical Pacific is for characterizing past variability, particularly protracted climatic changes such as the Dansgaard/Oeschger climatic events and even the glacial terminations. Among the strategies needed to assess a tropical Pacific influence on these longer time scales is a robust estimate of the phasing of tropical vs extratropical climate change. This is a difficult challenge to address with paleo- archives such as marine sediment cores because our ability to quantify timing and rates of change from these archives can be as large as or larger than the phasing timescale itself. In the case of the last glacial termination we present a new look at old proxy data from marine cores in the tropical Pacific in an attempt to place in temporal relationship the relative timing of high vs low latitude climatic warming. We show that deep sea temperatures within the Pacific that are indicative of circum-Antarctic climate began to warm in advance of the tropical surface ocean by approximately 1000 years. Deep waters between 1000 and 3500 meters appear to have warmed by 2oC between 19 and 14.6 kyBP and therefore, well ahead of the meltwater pulse 1A. In fact, it appears that virtually all of the deglacial warming in the deep Pacific Ocean occurred before the major onset of deglaciation in the northern hemisphere. Consequently, from a southern hemisphere perspective, much of the warming associated with the glacial termination was not associated with the rise in atmospheric CO2 concentrations. We further show that a similar lead-lag relationship existed between tropical SSTs and Pacific Deep Water temperatures during the abrupt climatic changes in Marine Stage III. It appears therefore that there has been a persistent and recurrent pattern of southern-first, tropics-next during periods of large abrupt climatic change.
PP22A-08 INVITED
Mechanisms for deglacial temperature rise in the tropical Pacific
Trying to explain deglacial temperature rise in the tropical Pacific, previous hypotheses have emphasized the importance of CO2 forcing. Using a series of CGCM simulations and analysing transient glacial-interglacial modeling experiments recently conducted with an earth system model of intermediate complexity, we demonstrate that tropical Atlantic-Pacific connections on millennial timescales, orbital forcing as well as remote atmospheric forcing due the retreating Laurentide ice-sheet played a key role in shaping the surface temperature evolution in the tropical Pacific during the last 21,000 years. Furthermore, we address the apparent mistmatch between alkenone and Mg/Ca-based SST reconstructions in the tropical Pacific. Assuming different seasonal biases, both proxy timeseries can be well reproduced by our transient glacial-interglacial simulations. The simulated annual mean temperature evolution, however, does not match any of SST reconstructions. The seasonally stratified modeling results provide a framework to reconcile the apparent mismatches between alkenone and Mg/Ca-based SST reconstructions. The results discussed here question the validity of recent efforts to derive global climate sensitivities from tropical Pacific SST reconstructions.