U22B-01 INVITED
Annually resolved lake and shallow marine sediment records of global climate change of the past 16,000 years
In the sediments of lake Huguang Maar in coastal southeast China, the titanium content and redox-sensitive magnetic properties record the strength of winter monsoon winds at subdecadal to annual resolution over the last 16 thousand years. The record indicates a stronger winter monsoon prior to the Bølling-Allerød warming, during the Younger Dryas, and during the middle and late Holocene, when cave stalagmite oxygen isotope data indicate a weaker summer monsoon. The anti-correlation between winter and summer monsoon strength is best explained by migrations in the ITCZ that occurred simultaneously in central America and Africa. Drought associated with southward ITCZ migration may have played a role in the termination of several Chinese dynasties. A remarkable similarity of ITCZ migration in east Asia and the Americas from 700 to 900 AD raises the possibility that the coincident declines of the important Tang Dynasty in China and the Classic Maya in Central America were catalyzed by the same ITCZ migrations. The mechanisms behind these decadal-scale ITCZ-monsoon swings can be further exoplored at major climate transitions such as the onset of Younger Dryas cooling at ~12.7 ka, one of the most abrupt climate changes observed in ice core, lake and marine records in the North Atlantic realm and much of the Northern Hemisphere. Annually laminated lake sediments ideally record the dynamics of abrupt climate changes since seasonal deposition immediately responds to climate and varve counts accurately estimate the time of change. We report new sub-annual geochemical and varve microfacies data from a lake in Western Germany, which provides one of the best-dated records currently available for this climate transition, which we compare to the Cariaco Basin and Lake Huguang Maar records. The Lake Meerfelder Maar record indicates an abrupt increase in storminess, occurring from one year to the next at 12,678 ka BP, coincident with other observed climate changes in the region. We interpret this shift of the wintertime winds to signify an abrupt change in the North Atlantic westerlies to a stronger and more zonal jet. Given that oceanic heat transport associated with North Atlantic meridional overturning, in its own right, does not appear to greatly impact European climate, the observed wind shift provides the atmospheric mechanism for the strong temporal link between North Atlantic overturning and European climate during the last deglaciation, tightly coupled to ITCZ migrations.
U22B-02
Central American rainfall variations since 100 ka and moisture delivery to Greenland
We present a rainfall history for Central America based on oxygen isotope values in Uranium-series dated stalagmites collected from the Pacific Coast of Costa Rica over parts of the Holocene and from 25 to 100 ka. The oxygen isotope values of modern rainfall in our study area within the heart of the Intertropical Convergence Zone (ITCZ) are dominated by the amount effect and moisture source, and we interpret our data as a paleorainfall proxy. Our data show substantial oxygen isotope variability on centennial to multi-millennial time scales. Further, our paleorainfall time series is strongly correlated with the deuterium excess parameter in Greenland Ice, which suggests that the strength of the tropical hydrological cycle has modulated the flow of low-latitude moisture to Greenland on millennial time scales. Our results indicate a strong coupling between tropical and high latitude paleoclimate, that was likely linked via variations in the strength of the Hadley cell and its associated export of atmospheric moisture to the high latitudes. We observe the wettest periods in Central America when the Caribbean was warmer than 26.5 degrees C and the Caribbean to Pacific (cold tongue) SST gradient was largest, suggesting a combined Atlantic and Pacific Ocean control on ITCZ rainfall.
U22B-03
The Role of Tropical Pacific SST's in Medieval Hydroclimate: A Modeling Study
The possible role that tropical Pacific SST's played in driving global hydroclimate during the Medieval period is addressed. Fossil coral records from Palmyra are used to derive tropical Pacific SST's for the period from 1320 to 1462 A.D. and shows overall colder conditions as well as extended multidecadal La Niña-like states. The reconstructed SST's are used to force a 16 member ensemble of atmosphere GCM simulations, each with different initial conditions, with the atmosphere coupled to a mixed layer ocean outside of the tropical Pacific. Model results are verified against paleoclimatic proxy data from around the world. The model results are used to argue that the tropical Pacific played an active role in driving North American megadroughts and that these megadroughts are part of a global hydroclimate regime linked to persistent La Niña-like conditions in the tropical Pacific, though it is possible that SST anomalies in other basins also played a role.
U22B-04 INVITED
300,000 Years of Asian Monsoon History from Caves: Piecing Together the Patterns, Triggers, and Feedbacks of Abrupt and Orbital-Scale Climate Change
We present a continuous 300,000-year record of the oxygen isotopic composition of cave calcite from stalagmites from Hulu, Dongge, and Shanbao Caves, in southeastern China. In its present state, the record is substantially improved in resolution (oxygen isotope resolution of a few to several decades) and range over published results. The record is, in essence, a history of the oxygen isotopic composition of meteoric precipitation through time, which, in turn, is related to monsoonal precipitation in the region. The chronology is established with precise uranium-thorium ages. The monsoon is dominated by orbital-scale variability throughout, and millennial-scale variability during glacial periods. At orbital scales, the monsoon follows northern summer insolation with no discernable phase shift, supporting a direct link between seasonal heating and the monsoon. At millennial scales, the last glacial record correlates strikingly with that observed in Greenland, with Chinese correlatives to all 25 Greenland interstadial events, and broadly similar millennial-scale sequences observed and established for the penultimate and antepenultimate glacial periods. The monsoon exhibits remarkable relationships with the ice core atmospheric gas record. A tie to the oxygen isotopic composition of atmospheric oxygen supports the idea that shifts in the monsoon and low latitude hydrology change the Dole Effect, likely through a combination of changing terrestrial productivity and changes in the average isotopic composition of waters used by land plants. A close tie to methane confirms links among atmospheric methane, the low latitude hydrologic cycle, and the extent of low-latitude wetlands. We are able to correlate features in the monsoon record to ice core, marine, and other cave records, thereby establishing, for key periods, the timing and sequence of events recorded around the globe in different surface environments. Correlation strategies include the methane-monsoon relationship to correlate cave and ice core records, a link between Heinrich Events and the monsoon to correlate cave and North Atlantic marine records, and a link between the monsoon and positions of surface currents to correlate cave and Northwest Pacific marine records. Using these strategies, we have determined the timing and sequence of events during the terminations of the last 4 glacial cycles. Our correlations place new constraints on the causes of glacial terminations, a problem that still remains unsolved despite decades of study. The relationships that we observe suggest that terminations are triggered by small orbitally-induced changes in the seasonal distribution of insolation, coupled with multiple positive feedbacks within the climate system.
U22B-05 INVITED
An 85-ka Record of Climate Change in Lowland Central America
Drill cores obtained from Lake Peten Itza, Guatemala, contain a ~85-kyr record of terrestrial climate from lowland Central America that was used to reconstruct hydrologic changes in the northern Neotropics during the last glaciation. We found that Peten climate varied between wetter conditions during interstadials and a drier state during stadials of Marine Isotope Stage (MIS) 3. The most arid periods were associated with Heinrich Events in the North Atlantic and reductions in the strength of meridional overturning circulation. The pattern of clay-gypsum (wet-dry) oscillations during MIS 3 closely resembles the temperature record from Greenland ice cores and North Atlantic marine sediment cores and precipitation proxies from the Cariaco Basin. Contrary to previous findings, the Last Glacial Maximum (LGM) was moist in the Peten lowlands and vegetation consisted of a temperate pine- oak forest. Greater moisture availability was likely related to increased winter precipitation associated with frequent polar outbreaks (i.e., cold fronts) or "Nortes" (northerly winds,), which occasionally bring rain to northern Guatemala today during the dry season. At the end of the LGM, Peten climate switched from moist to arid conditions during the so-called "Mystery Period" from 18 to 14.7 ka. Moister conditions prevailed during the warmer Bolling-Allerod (14.7 to 12.8 ka), with the exception of a brief return to dry conditions at ~13.8 ka. The onset of the Younger Dryas at 12.8 ka marked the return of dry conditions. Pollen assemblages indicate that a mesic forest had been largely established by ~11.3 ka, but sediment properties suggest that the lake continued to precipitate gypsum until ~10.35 ka. Our results support the hypothesis that summer precipitation was controlled by migrations in the meridional position of the Atlantic ITCZ during the MIS 3 and the last deglaciation. The ITCZ was located farther south during cold periods, especially during Heinrich Events, and arid conditions prevailed in the northern hemisphere Neotropics. During the LGM, however, northern Guatemala may have received significant winter precipitation that is not recorded farther south in the marine record from the Cariaco Basin.
U22B-06
Orbital- versus glacial-mode forcing of tropical African climate: Results of scientific drilling in Lake Malawi, East Africa
Lake Malawi extends from 9-14 degrees S within the East African Rift Valley, and at 700 m deep, contains more than 20 percent of the surface water on the African continent. In 2005 the Lake Malawi Scientific Drilling Project drilled 7 holes at two sites in the lake, recovering a continuous sediment record that samples much of the Quaternary. Detailed studies completed to date on sediments deposited during the past 145 ka indicate periods of severe aridity at precessional frequency between 135 and 75 ka, when the lake's water volume was periodically reduced by at least 95 percent. These dramatic drops in lake level (more than 550 m), signifying markedly arid conditions in the catchment, are documented in sediment lithology (decreased organic carbon content and increased authigenic carbonate content during severe lowstands), aquatic microfossils (appearance of a littoral ostracode fauna, and saline/alkaline lake diatom flora during extreme low lake stages), as well as in dramatic reductions in catchment pollen production. These intervals of pronounced tropical African aridity in the early late-Pleistocene were much more severe than the Last Glacial Maximum, and are consistent with sediment records from Lakes Tanganyika (East Africa) and Bosumtwi (West Africa). In all three lakes a major rise in water levels and a shift to more humid conditions is observed after ~70 ka. The transition to wetter, more stable conditions coincides with the relaxation of orbital eccentricity and a reduction in the amplitude of precession. The observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical Africa, but these new drill cores provide evidence for dramatically drier conditions prior to 70 ka that have not as yet been detected in marine sediment records. Such climate change may have stimulated the expansion and migrations of early modern human populations. http://malawidrilling.syr.edu
U22B-07
A Tropical Template for D-O Events During MIS 3? Results From the Lake Malawi Drilling Project
As part of the Lake Malawi Drilling Project we have undertaken a 1 cm-scale resolution (ca. 20 yr) XRF scanning study of core from site 2A in the Northern Basin of the lake. We have focused on certain parameters, including Fe, Zr:Ti, and Si:Ti; these may be interpreted to represent changing input of terrigenous sediments (delivered to the lake by rivers), soils developed from weathered volcanic ash (delivered more effectively under drier windier conditions), and biogenic silica, respectively. We present here a record of East African climate since 55 ka that indicates rapid shifts between discrete climate modes that appear to be related to abrupt warming (Dansgaard- Oeschger) and cooling (Heinrich) events observed in Greenland. The events in Malawi appear to correspond to episodes of dry conditions and increased northerly winds. Such conditions may occur at times that southward excursion of the Intertropical Convergence Zone brings the tropical rain belt to the south of the study area. Although the timing of the Malawi events cannot be determined exactly, our independent age model, based on 25 radiocarbon dates and stratigraphic correlations to events dated in a nearby at 63 and 75 ka, implies that they occur prior to their Greenland counterparts, consistent with southward excursions of the ITCZ during Greenland stadials. The magnitude and form of each of the events recorded in Malawi sediments corresponds to the scale of the subsequent Greenland warming. This suggests that a tropical component of climate sets a template for abrupt high northern latitude climate fluctuations associated with the bipolar seesaw.
U22B-08
The 1 Ma Lake Bosumtwi (West Africa) Paleoclimate Record: Comparisons to Marine and Polar Records
Lake Bosumtwi is a hydrologically closed lake occupying a 1.07 Ma impact crater in Ghana, West Africa. The lake lies beneath the path of the seasonal migration of the ITCZ and therefore can provide a sedimentary record of monsoon variability in West Africa. Scientific drilling recovered a 291-m long sediment section that spans the full 1 Ma history of the lake. This long continental record is ideal for comparison to long marine and ice-core records at both glacial-interglacial and abrupt-change timescales. Oxygen-isotope stratigraphy, derived from calcareous fossils, often provides age control and a way to place individual marine sediment cores into a global stratigraphic framework. Lacking a direct tie-in to the marine oxygen-isotope stratigraphy, individual lacustrine basins can present challenges for global correlation. Through radiocarbon, optically stimulated luminescence and paleomagnetic dating, limited age control has been established for the 1 Ma Lake Bosumtwi sediment sequence. Within a Bosumtwi sediment sequence that is mostly laminated occur intervals of non-laminated sediment having increased density, decreased organic content and a high-coercivity magnetic mineral assemblage. Some of these massive layers contain slump-folding and intraformational clasts. These lithologies are interpreted to represent lake-level lowstands when a diminished West African summer monsoon resulted in decreased moisture balance and lake-level regression. Some Bosumtwi lake-level lowstands match intervals of increased sea surface salinity in the Gulf of Guinea resulting from reduced river discharge (Weldeab et al. 2007, Science, 316, 1303-1307). However, during other intervals (MIS2) there are differences between the two records. Corresponding to glacial stages and stadials, increased amounts of high-coercivity magnetic minerals are present in the Lake Bosumtwi sediment. Elevated aerosol dust export from arid Sahel sources, possibly accompanied by enhanced magnetic-mineral diagenesis during lake- level lowstands, is interpreted to have produced this magnetic signature. The Bosumtwi dust proxy record displays variability similar to that of published dust records from marine and polar settings. Comparing this continental dust record to a marine dust record off the coast of West Africa provides additional constraints on identifying glacial-interglacial variability in the 1 Ma long lacustrine sediment sequence.