PP33A-1003
Investigating the Forcing and Response in Proxy Records of Multi-Decadal Scale Climate Variability
Multi-decadal climate variability has been detected in both instrumental and proxy climate records and these climate modes have been linked to internal interactions in the climate system, changes in solar irradiance, and episodes of explosive volcanism. We are interested in investigating the links in multi-decadal scale variability between highly resolved proxy records (e.g., tree-rings, speleothems, corals, and lake sediments) and those observed in solar output records. We first investigated the ~2000 year El Malpais (New Mexico) tree-ring record [Grissino-Mayer, 1996] and the late Holocene section of the Dongge Cave (China) speleothem record [Wang et al., 2005]. The El Malpais record contains significant (95% level) concentrations of variance at four periods (~25, ~37, ~57, and ~76 yrs); however, further examination of three temporal subsets found these periodicities varied between subsets. The ~25 yr cycle, which is observed in the PDO, is present in the youngest and oldest subsets. The ~37 and ~54 yr cycles are present in a single subset; the other subsets lack spectral power at these periods. The ~76 yr cycle, which is ~Gleissberg sunspot cycle, is present in the two youngest and most replicated subsets. The chronology of the Dongge record is based on uneven time increments and interpolation was used to generate time series with a constant time increment. Spectral results using the interpolated record tended to suppress periodicities ≤decadal in comparison to the Lomb-Scargle spectrum, which evaluates the spectrum of uneven time series. Robust periodicities at ~23 and ~74 yrs were identified in the Dongge record, which are ~ the PDO and the Gleissberg cycle. Cross-spectral analysis of the El Malpais and Dongge records did not find significant coherence.
PP33A-1004
Late Pleistocene/Holocene paleoclimate reconstruction and eruptive history of Central American volcanoes from lake bottom sediments of Lake Nicaragua
A shallow coring program in Lake Nicaragua was completed in May/June 2006 by the University of Texas (UT Department of Geography and UT Institute for Geophysics). A total of 35 sediment cores with lengths ranging between 12 cm and 100 cm along with five longer cores were extracted from the lake using a gravity corer and a modified manual square rod piston corer, respectively. Analyses of lake sediments have the following objectives: 1) to correlate the geophysical results with the core data to provide a stratigraphic framework for the shallow lake sediments; 2) to constrain past climate variability in this rather poorly investigated area; and 3) to establish a time series of explosive volcanic activity based on the identification and dating of tephra layers in the cores. Initial measurements of magnetic susceptibility, dry density, loss on ignition and XRF scanning indicated a dominance of fine-grained homogeneous diatomaceous sediments cover most of the lake floor. Increasing values in magnetic susceptibility in the upper part of several short cores most likely reflect increased erosion caused by land-use changes during the Spanish colonial period (1522-1822). Results on the two longest cores from the northeastern (355 cm) and southwestern (478 cm) parts of the lake reveal complete Holocene paleoclimate records in both areas that are comparable to other terrestrial and marine records in the Central and South- American tropics (i.e. Cariaco Basin). A lithologic change from homogeneous gyttia (diatomaceous mud) to blue- grayish waxy clay at the bottom of these records marks the Late Pleistocene-Holocene transition as indicated by a radiocarbon dating on plant remains. The latter dense clay forms a distinctive stratigraphic marker in the lake basin. Tephra layers to date were detected in most gravity cores recovered west of Ometepe Island (Volcan Concepcion), and in long records in the northeastern basin (San Antonio Tephra, Masaya volcano, ca. 7,400 interpolated cal yr BP) and nearby the Solentiname Archipelago (unknown eruption, Arenal volcano, Costa Rica; 8,810 +/- 25 cal yr BP). These layers allow independent dating and correlation of sediments.
PP33A-1005
Phasing of orbital forcing and Antarctic climate over the past 470,000 years from an extended Dome Fuji O2/N2 chronology
In order to investigate the roles of orbital and greenhouse-gas forcings on climate, one needs a chronology of Antarctic ice cores with an accuracy better than ~2 kyr (~1/10 of precession cycle). Recently, Kawamura et al. (2007, Nature) established such an Antarctic chronology for the past 360 kyr using the O2/N2 ratio of trapped air in the Dome Fuji and Vostok ice cores (Dome Fuji data available from http://www.ncdc.noaa.gov/paleo/icecore/antarctica/domefuji/domefuji.html). The O2/N2 ratio in these cores is depleted relative to the atmospheric ratio through physical fractionation during bubble close-off, and the magnitude of fractionation is linked to the magnitude of original snow metamorphism by local summer insolation. Thus, an accurate chronology can be constructed by orbital tuning between O2/N2 and local summer (solstice) insolation without the need to assume a lag. The new chronology permits comparisons between Antarctic climate, insolation and atmospheric greenhouse gas variations, thus providing a possible way to separate the respective contributions. We are currently measuring the O2/N2 ratio in the period 320- 470 kyr to extend the previous chronology, using the second Dome Fuji core (which recently reached 3035 m depth or ~720 kyr; , the first core reached 2503 m or ~340 kyr). We improved the quality of the new data by storing the core at low temperature (-46 to -50°C) after transportation from Dome Fuji. The new values for 320-340 kyr agree well with the previous data corrected for O2/N2 depletion during storage. Although the measurement is still ongoing, we find a promising resemblance between the raw O2/N2 data and the local summer insolation. Complete results will be reported for the current measurements and resulting chronology for the past 470 kyr, covering Marine Isotope Stage 11 and Termination V, and climatic implications will be discussed with special focus on the timing of orbital forcing and climatic changes.
PP33A-1006
Be-10 Variations in Dome Fuji Ice Core During the Last Deglaciation
Cosmogenic Be-10 is produced in the atmosphere by an interaction of cosmic ray particles and nitrogen and oxygen atoms. The Be-10 is then quickly attached to aerosols and falls on the surface of the land and sea. Assuming Be-10 in an ice core reflects the variations of the production rate of the nuclide, it is possible to know the history of the cosmic ray flux from an ice core record of the nuclide. Several Be-10 records in the Holocene epoch have been obtained from ice cores from Greenland (Beer et al., 1988, 1990; Finkel and Nishiizumi, 1997) and Antarctica (Raisbeck et al., 1990; Steig et al., 1996, Bard et al., 1997; Horiuchi et al., 2007). The records show Be-10 variations relevant to the known periodicities and amplitudes of the solar activity, which, as well as the intensity of the earthfs magnetic field, controls the cosmic ray flux in the atmosphere (Lal and Peters, 1967; Masarik and Beer, 1999). However, the detailed Be-10 records of the last deglaciation are very few (Finkel and Nishiizumi, 1997) and much less investigated than those of the Holocene epoch. In this paper, we present a Be-10 record of the last deglaciation, which is obtained from an ice core retrieved from Dome Fuji station, Eastern Antarctica (77° 19E#338; S, 39° 42E#338; E). The Be-10 was analyzed by using an accelerator mass spectrometry (AMS) of MALT at the University of Tokyo, Japan. One of the problems that should be considered when we investigate the past changes in the cosmic rays by using the Be-10 variations is contamination of local meteorological signals. The last deglaciation was marked by large, hemispheric, millennial-scale climate variations: the Bølling-Allerød and Younger Dryas periods in the north and the Antarctic Cold Reversal in the south. Because these events were not coincident in the north and south (Blunier et al., 1998; Blunier and Brook, 2001), the nature of the Be-10 flux variations may be different in Greenland and Antarctica if climatic changes really influenced the Be-10 variations. However, the Be-10 flux variations in the Dome Fuji ice core show a fairly similar profile to those in the GISP2 (Greenland summit) ice core (Finkel and Nishiizumi, 1997). This fact strongly suggests that local effects are not significant for inland Antarctica (and Greenland summit), and shows a clear possibility of inter-hemispheric correlation between Greenland and Antarctica ice cores via Be-10 flux, even in drastic climatic transitions.
PP33A-1007
Circulation Changes in the Southern Ocean During the Last Glacial Inception, Induced by an Atmospheric Coupling with the Northern Hemisphere
The changes in the oceanic circulation at the initiation of the last glacial period, and its role in the chain of events linking the decrease of Northern summer insolation with the establishment of large ice continental sheets and global cooling are still poorly known. We present here new constraints on the deep-water circulation changes in the Southern Ocean during the last glacial inception (130 60 ky), in relationship to surface hydrology and global climatology. We use high-resolution oxygen isotope composition (δ18O ) records measured on planktic and benthic foraminifera, and carbon isotope composition of the benthic species Cibicides as a tracer of deep- water ventilation. From the comparison of three marine sediment cores, we consider the three major water masses of the Southern Ocean: Antarctic Intermediate Waters (AAIW) from core MD97-2120 (45°32S 174°56E 1210m) ( Pahnke et al. 2003; Pahnke and Zahn 2005) in the southwest Pacific sector; Circumpolar Deep Waters (CDW) from the newly analyzed core MD02-2488 (46°29S, 88°01E, 3420 m) (this study) in the Indian sector of the Southern Ocean; and Antarctic Bottom Waters (AABW) from ODP site 1089 (41°S, 10°E, 4620m) ( Gersonde et al 1999; Hodell et al 2003; Mortyn et al 2003) in the South Atlantic. The records are compared on a common timescale that we defined, based on the assumption that the major temperatures changes occur simultaneously above Antarctica ( EPICA community members, 2004) and in the subantarctic zone of the Southern Ocean. The data show that surface coolings associated with Marine Isotope Stage (MIS) 5.5-5.4 and 5.1-4 transitions in the high southern latitudes follow the 65°N summer insolation decrease and the northern ice-sheets growth. Simultaneously, we observe two strong reductions of deep-water ventilation and one shift in intermediate waters charcteristics in the Southern Ocean. This suggests a rapid coupling between the Southern Ocean deep waters changes and the Northern Hemisphere climatology. Comparison with North Atlantic core (SU90-08, 43°03 N, 30°02W, 3080 m) ( Grousset et al., 1993), which shows active NADW formation throughout MIS 5, allows to discard the hypothesis of a deep ocean coupling via NADW. The study of four additional cores from the Indian sector of the Southern Ocean suggests that an atmospheric coupling between the Northern and Southern hemispheres could induce deep-water circulation changes in the Southern Ocean.
PP33A-1008
Climatic Forcings of the Last Major Glacial Inception: A GCM Simulation of 115.5 Ka
The onset of Northern Hemisphere glaciation at around 115.5 Ka is thought to have been caused by a number of factors. Two of the most important of these are a reduction in atmospheric CO2 from approximately 380 ppm to 180 ppm, and changes in the earth's eccentricity, precession, and obliquity due to Milankovitch orbital cycles. We used the NCAR CCSM3 GCM in fully coupled mode to simulate the climate at 115.5 ka B.P. The fully-coupled mode includes dynamical atmospheric and oceanic components, as well as sophisticated land surface and sea ice schemes. Sea level and the distribution of the continents were held at present-day values, since they changed little between 0 Ka and 115.5 Ka. Thus, our model simulation can also be thought of as examining the roles of lowered CO2 and orbital configuration in driving glacial inception. In particular, we hypothesize that these climatic forcings will lead to a succession of cool summers and warm wet winters in key regions of the high latitude Northern Hemisphere. In turn, we expect this will be conducive to building the perennial snow pack that is an essential precursor to the Laurentide and Fenno-Scandinavian ice sheets. Though the simulation is still underway at this writing, preliminary results from the first 100 years of the run suggest that this does indeed take place. Key results from the completed run will be presented at the meeting, along with an assessment of how they differ from a present-day CCSM3 control run. Furthermore, in glacial inception regions the performance of both the control run and an existing CCSM3 preindustrial simulation are being compared to ERA40 reanalyses as an additional test of model fidelity. The model simulation is also being verified using all available data from geologic record for the time around 115.5 Ka.
PP33A-1009
Modelling Andes Uplift Impact on Atmospheric Circulation: Consequences for Neogene Faunal and Floral Evolution ?
Tectonics in South America is marked by the uplift of the Andes during the Cenozoic. The Andes are approximately 7000 km long, oriented north-south, with some peak elevations in excess of 6000 m. Such a topographic structure has potentially a strong impact on atmospheric circulation. Climate model studies have showed that the Andes, as a topographic barrier, influence eastern Pacific Ocean climate and also meridional moisture transport above the south American continent. However, most studies have been done at the regional scale, and no quantification of rainfall changes due to a lower topography has been done. Here we use the high resolution Atmospheric General Circulation Model LMDz4 to quantify the impact of the Andes topography on the rainfall regime over the whole South American continent. Interpreting sensitivity experiments, we discuss about the tectonics history from 55 Ma to present-day and how to apply this method for Neogene paleoclimate, in a different continental configuration.
PP33A-1010
A New Look at the Early Eocene Arctic Climate in CCSM3: Sensitivity to pCO2 and Basin Geography
We present results from new Eocene climate modeling experiments that support the role of high pCO2 in maintaining Arctic warmth during the early Cenozoic. The fully-coupled NCAR Community Climate System Model (CCSM) (v.3) was employed with early Eocene geography in two experiments to test the sensitivity of climate to a large increase in pCO2 (from 2x to 8x pre-industrial pCO2). In a third experiment, we open an ocean passageway from the Eocene Arctic to the Pacific to establish the maximum sensitivity of Arctic climate to neighboring oceans, as periodic connections with adjacent ocean basins may have influenced Arctic climate at this time. To compare with the 8xCO2 scenario, we also run this open Arctic experiment at 8xCO2. In the CO2 sensitivity experiments, annual average global mean temperature rose ~3.4C with a quadrupling of pCO2, consistent with previous modeling studies. The greatest warming occurred in the Arctic Polar region, and is due in part to reduced sea ice formation in the high pCO2 experiment. Arctic surface temperatures from the high pCO2 scenario (8xCO2) agree most closely with new proxy data from the early and middle Eocene Arctic. Mean annual temperature estimates from various proxies range from ~10- 15C. Modeled Arctic temperatures range from 2-8C, and begin to approach 10C along the North American coast. While still somewhat lower than that estimated from proxies, it is important to note that in this 8xCO2 experiment, wintertime Arctic air temperatures remain mostly above freezing. Opening the Arctic to the influence of the Pacific in the third experiment has the effect of warming the average sea surface temperature in the Arctic basin by ~4C. Central Arctic ocean temperatures warm by as much as 5C to 7C. Temperatures are as high as 10C along northern Europe in the open Arctic scenario.