U21F-01
Integrated studies of Milankovitch solar forcing and climate response: strategies for understanding climate processes
An understanding of climate dynamics and processes, particularly those that operate on long time scales, can be derived from paleoclimate data when the forcing and the climatic response is well-known. Such is the case with Milankovitch forcing - changes in the latitudinal and seasonal distribution of solar forcing at the top of the atmosphere are precisely known, and the climate response to that forcing is recorded in sediments throughout the world's oceans and on land; in between the forcing and response is a ‘black box' that represents the oceanic and atmospheric dynamics that must be understood in order to build theory and models that can be used to predict future climate change. Drilling to recover long continuous records that include many realizations of climate cycles through epochs both colder and warmer than today, is crucial in order to obtain a dynamic range of climate states comparable to the amplitude of predicted future global warming. Such drilling is currently being supported and facilitated by programs like the International Continental Drilling Program and the Integrated Ocean Drilling Program. Isolating mechanisms of climate change can be approached by first assuming that regional signals are generated by regional processes and that the remaining unexplained variance is related to teleconnections or far-field effects between regions. Tests of theory can often result from the comparison of regional signals from different time periods with different mean climate conditions, hence the need for improved spatial and temporal coverage of Milankovitch-resolving records around the globe. Examples of tractable problems, that can be attacked by coordinated research efforts amongst observationalists (geologists) and dynamicists, will be given. These include constraining the causes of glacial advance and retreats, of changes in the tropical Pacific temperature patterns, and of California margin ocean upwelling and coastal continental climate change.
U21F-02
Orbital forcing of continental climate during the Pleistocene: a complete astronomically-tuned climatic record from Lake Baikal, SE Siberia
Baikal was the first ancient rift lake where an ODP-style coring technology was used to recover a spectacular continuous paleoclimate record . Glacial/interglacial diatom productivity cycles recorded as biogenic silica content variations in sediments reveals strong power in Milankovitch frequency bands. By splicing BDP-96-1 and BDP- 96-2 drill cores, we generated a composite BDP-96 BioSi record over the entire Pleistocene. During the present interglacial, Lake Baikal biogenic silica (BioSi) proxy response is anti-phased with the early Holocene regional humidity indices. Instead, BioSi closely follows a significant rise in regional annual temperature reflected in palynological records and predicted by GCM simulations around 6 ka. In terms of orbital configuration, this time corresponds to September perihelion (SP), 4-5 ka past June insolation maximum. The Holocene-like BioSi timing is confirmed by observations during the last interglacial and around two paleomagnetic reversals constrained by independent dating methods. September perihelia timing during the Pleistocene were used as a tuning target for the BDP-96 composite record. Tuning results in a robust timescale which passes spectral analysis and magnetic reversal tests with a high degree of confidence. Power spectral analysis shows a major improvement compared with previously reported Lake Baikal timescales. Significant power is now aligned into the precession index frequencies (1/(19 kyr), 1/(22 kyr), and 1/(24kyr), testifying to the success of the tuning, as well as to the presence of consistent BioSi signal in the precession band. More importantly, the SP-tuning (which manipulates the record at precession index frequencies only) confines high signal power to a narrow band at the obliquity frequency, at 1/(41 kyr). Two major spectral peaks also occur at 1/(94 kyr) and 1/(75 kyr), which only partly coincide with the predicted orbital eccentricity band (1/(128 kyr) to 1/(95 kyr)). Cross-spectral analysis further confirms SP-tuning efficacy. At the same time, however, an anomaly is observed between the BioSi obliquity component and the obliquity variation. A phasing of -32° ± 3° points to a possible inherent time-lagged response of BioSi (productivity) and terrestrial vegetation proxies to insolation forcing. The presence of this ca. 4-kyr lag suggests that factors other than ‘slow physics of ice sheets' may have contributed to generating lagged responses to insolation in the northern hemisphere climate proxy records. Long-term changes in sedimentation rates in Lake Baikal at the hemipelagic drill site suggest a significant orbital forcing of sedimentation process (both biogenic and clastic) in this rift basin. The Middle Pleistocene Transition in the Lake Baikal record is associated with significant changes in average sedimentation rates, with a particularly high rate during glacial MIS 24 (880-900 ka) in contrast with rather low rates over MIS 23-19.We suggest the composite BDP-96 Baikal record as a new benchmark correlation target for terrestrial records in continental Eurasia as an alternative to June 65°N insolation and ODP marine oxygen isotope timescales, commonly used as targets for tuning loess/soil sections and palynological records in this part of the world.
U21F-03 INVITED
Neogene climate history of Antarctica: Initial results from the ANDRILL McMurdo Ice Shelf Project
The ANDRILL (ANtarctic geological DRILLing) Program successfully recovered a 1285m-long succession of cyclic glacial, glacimarine and marine sediment with interbedded volcanic deposits from beneath the McMurdo Ice Shelf (MIS), northwest part of the Ross Ice Shelf system. The MIS drillcore represents the longest and most complete (98 percent recovery) geological record from the Antarctic continental margin to date, and provides a key reference record of climate and ice sheet variability through the Neogene. Here we present the initial outcomes from the drilling with emphasis on the potential of the record for improving our knowledge of Antarctica's influence on, and response to global climate change. Repetitive vertical successions of facies imply at least 60 fluctuations, of Milankovitch-duration, between subglacial, ice-proximal and ice-distal open marine environments. These have been grouped into 3 main types of facies cycles that corresponds to glacial-interglacial variability during climatically-distinct periods: i) Cold polar climate dominated by grounded ice, but with retreat to ice shelf conditions during interglacials (Late Miocene, ~13-10 Ma & Pleistocene, ~1-0 Ma), ii) Warmer climate dominated by ice-shelf and open water conditions (hemipelagites), with occasional periods of grounded ice. (early-Late Miocene, ~9-6 Ma). iii) Warmer climate with extended periods experiencing open ocean conditions (pelagic diatomites) with periods of sub-ice shelf and grounded ice deposition (Pliocene, ~5-2 Ma). The ~90m-thick Early Pliocene (~4 Ma) interval of diatomite shows no apparent glacial cyclicity and represents an extended period of ice-free conditions indicative of a reduced WAIS. Late Pliocene (~2.6-2.2 Ma) glacial- interglacial cycles characterised by abrupt alternations between subglacial/ice-proximal facies and open marine diatomites imply significant WAIS volume fluctuations around the time of the early Northern Hemisphere glaciations. A ~4m-thick interval of diatomaceous mudstone deposited during MIS 31 represents an open Ross Sea with SSTs warmer than the Holocene. Intriguingly, the last million years, is characterized by a relatively stable, extensive grounded ice-sheet with marine terminus. Super-interglacials following the mid-Pleistocene climate transition produced ice shelf conditions like the present day. http://www.andrill.org
U21F-04
Lake Qinghai Drilling Project: Evolution History of Lake Qinghai and East Asian Monsoon Changes since the Late Miocene
As a closed continental lake on the north-east margin of the Tibetan Plateau, Lake Qinghai is sensitive to climate variations as well as the environmental effects of Plateau growth/uplift. Supported by Chinese funding agencies and ICDP, onshore and offshore lake cores were drilled in 2005. We compare our preliminary chronostratigraphic, sedimentologic, and geochemical results with climatic records from the Loess Plateau, South China Sea, Arctic and global oceans, and we discuss the evolution of Lake Qinghai at different time scales since the late Miocene. Lake Qinghai is shown to have intimate linkages with the warm/moist East Asian summer monsoon, the cold/dry East Asian winter monsoon, and the growth/uplift of the Tibetan Plateau. Magnetostratigraphic studies of the onshore drill cores indicate that thick greenish clays were deposited during Late Miocene, suggesting the initial formation of the Qinghai Lake basin. Consistent with proxies from the Loess Plateau and the South China Sea, they imply summer-monsoon strengthening and inland intrusion. These changes may be related to a growth event of the Tibetan Plateau at 10-8 Ma, which led to the uplift of Qinghai Nanshan, formation of faulted lake basins, and enhanced summer monsoon circulation. From 6 to 4.6Ma eolian red clays in the core indicate lake basin dessication, as Loess Plateau dust flux increased with the strengthening of the winter monsoon and coincident with intense Arctic ice rafting at 6-5 Ma. From 4.6 to 3.5 Ma thick greenish clays were deposited as modern Lake Qinghai formed. Significantly increased fluxes of TOC, C/N and total sediment might be related to uplift of Qinghai Nanshan and basin subsidence at that time, and they are coeval with the increasing strength of East Asian monsoon during early Pliocene. At 3.5-2.6 Ma, continued strengthening of the East Asian summer monsoon, inland aridification, and increases in global ice volume suggest another growth event of the Tibetan Plateau. Shallow-water silty clays were deposited in the lake basin at this time. Since 2.6 Ma, deposition in the basin was characterized by shallow-water silty clays, intercalated with layers of loess- like material, eolian sand, gravel, and sand, indicating multiple lake expansion/dessication cycles, presumably at orbital frequencies, reflecting multiple migrations of the East Asian summer monsoon front driven by solar radiation and global ice volume changes over this region. Several previous studies of cores as much as 7m long from the depositional basins of Lake Qinghai have documented monsoon climate and environmental changes at the lake from the deglacial period through the Holocene, which are generally consistent with northern Hemisphere summer insolation and its seasonality changes. A wide variety of proxies have been used, and some cores have been studied at very high temporal resolution, especially for the last several hundred years. Results suggest that solar activity influences decadal regional temperatures, and that it is the East Asian summer monsoon as opposed to the Indian summer monsoon that acts as the dominate moisture source at the decadal scale within the local region. Offshore GLAD800 drill cores obtained in 2005 sampled fine-grained sediments before encountering thick units of sand. The fine-grained sections are 2-3 times longer than previous cores from similar sites. Paleolimnological proxy studies are underway on these cores to extend the young part of the paleoenvironmental record back to significantly before the last glacial maximum.
U21F-05
Synchrony between the hemispheres from an opposite response to orbital variations.
Southern Hemisphere ice-core and marine records have been found to follow the phase of Northern Hemisphere summer insolation intensity. But such an inference of Arctic control of Southern climate is surprising, not least because the total heat transport across the equator in the modern climate is small as well as that glacial-interglacial variations in atmospheric CO2 concentrations are thought to originate with changes Southern Hemisphere climate. Here we suggest that orbital-scale changes in Southern Hemisphere climate are related not to Northern Hemisphere summer insolation intensity, but to the duration of Southern Hemisphere summer. The confusion between forcing agents arises, for example, because when Northern Hemisphere summer insolation intensity is strong, Southern Hemisphere summer is long, making it appear as if Southern Hemisphere climate follows Northern Hemisphere summer intensity. The Southern Hemisphere may respond more to changes in summer duration than intensity because of the greater fraction of ocean area or because Antarctica is constantly glaciated. We support this conjecture of local Southern Hemisphere insolation control in three ways. First, through analysis of the relationship between the seasonal cycle of insolation and its relationship with temperature and sea-ice in the modern climate. Second, through simulation of the climate response over orbital timescales using energy-balance and more sophisticated climate models. And, finally, through intercomparison of continental records of glaciation in New Zealand, Chile, and North America as well as ice-core observations from the Arctic and Antarctica. If correct, this viewpoint frees southern hemisphere climate from the inferred Northern control at orbital timescales and suggests that the differing hemispheric responses to changes in seasonal insolation together determine the global climate response to orbital variations.
U21F-06
Timing and Trends in Northern and Southern Hemisphere Atmospheric Methane During the Holocene: new Results from Antarctic and Greenlandic ice Cores
A striking feature of the ice core greenhouse gas record during the Holocene is the mid Holocene minimum in atmospheric methane, widely believed to result from changes in methane emissions from terrestrial ecosystems. High mixing ratios in the early Holocene give way to low values at about 5-6 kyr, followed by a slow rise until the rapid increase associated with the industrial revolution. The slow rise is of particular interest because of suggestions that it may have been due to early human influence. Ice cores in Greenland and Antarctica record these trends, with a small difference between the records due to dominance of northern hemisphere methane sources. This difference changes with time, and can be exploited as a tracer of methane source location. Previous efforts to do so over the Holocene have met with some success, but suffer from limitations of existing samples and problems of comparing data sets generated in different laboratories. We created completely new methane records from the GISP2 and Siple Dome ice cores, analyzing 280 samples in duplicate, with measurements of samples of the same age from both hemispheres on the same day. We used a new high precision method (Grachev and Brook, in press, Geophysical Research Letters), obtaining a mean 1 sigma precision of 1.9 ppb for individual samples, and a mean difference between the 280 duplicates of 3.1 ppb. We smoothed the records using a 500-year running mean, and examined the difference between the smoothed records to determine trends in the interpolar methane difference, expressed as ([Cn/Cs] – 1). This difference was ~7 per cent in the early Holocene, but declined to minimum values of ~ 4 per cent by 6 ka, as methane mixing ratios also declined. At ~ 5.5 ka methane mixing ratios started to rise, and this rise is associated with a dramatic increase in the interpolar difference, with values reaching greater than 10 per cent by 4.8 kyr. As methane rose in the late Holocene the gradient shrank, reaching values of ~ 5 per cent by 0.5 ka. The decrease in mixing ratio and gradient in the early Holocene is plausibly attributed to decline in emissions from both northern wetlands and tropical wetlands, with a greater decline in the north. The gradient expansion at about 5.5 ka may be related to opening up new wetland regions in North America, perhaps associated with the demise of the Laurentide ice sheet. Expansion of methane sources post 4.8 ka appears to have been driven by changes tropical emissions. Shorter-term variability is apparent and will also be discussed.
U21F-07
Millennial climatic changes in US and European loess deposits: Links between continental, North Atlantic and Greenland records.
Loess sequences are key deposits at mid-latitudes in N Hemisphere, where no other complete records of past climatic changes are available. US and European sequences are thus ideally located to contribute testing the impact of abrupt climatic changes, described from North Atlantic and Greenland, as modeled by Ganopolski and Rahmstorf (2001). We present a synthesis of our high-resolution investigations in both continents by focusing on MIS 3 and 2. We show that the dust sedimentation, which lead to the loess formation, did not happen regularly, but better followed the dust deposition in Greenland corresponding to strong variations in the atmospheric circulation. Indeed all studied sequences show the alternation of pure or laminated loess with paleosols corresponding to artic brown soils, tundra gleys or embryonic gleys. Thus using i) grain size studies, ii) OSL and AMS dates, iii) d13C and mollusk analyses, and iv) the stratigraphical schemes and ongoing modeling experiments, we show that N Hemisphere loess sequences recorded millennial climatic variations even though with differences from one side to the other of the North Atlantic: while the general climatic history is recorded, the magnitude of the eolian events indicates differences. For example the strong N Atlantic coolings events (HE), expressed in the grain size studies by coarser material in Europe, cannot be identified in the US Great Plains. On the contrary, DO events are recorded by paleosols corresponding to finer sedimentation or stops/reductions in the dust deposition as also observed in the Greenland ice-cores. We conclude that climate variability in Western Europe appears strongly correlated with that in the North Atlantic area, at timescales at least as fine as centuries, while partly in North America.
U21F-08
Persistent Atlantic cold-water spells into the Mediterranean caused abrupt aridities in the late Quaternary Levant
The late Quaternary Levant paleohydrology and paleoclimate were recorded in the sedimentary and level history of lakes that occupied the tectonic depressions along the Dead Sea rift. The region was characterized by cold – wet climate conditions during glacials and warm-dry conditions during interglacials. This pattern was punctuated by abrupt arid events (< 200 y) that are correlated with intrusions of cold Atlantic-water into the east Mediterranean. Important examples are the abrupt falls of Lake Lisan during the Heinrich events, the catastrophic falls of Lake Lisan at the 14 and 11th millennium BP that were linked to "melt water pulses" MWP1-A and B. The Allerod fall marked the severest catastrophic aridity that prevailed in the late Quaternary Levant where the intruding cold waters enhanced the post-glacial warming - aridification trend. Subsequently, during the YD, the North Atlantic-cooling imposed a strong deviation from the post-Glacial warming-aridification trend of the Levant leading to enhanced-rain precipitation (return to the "glacial mode"). Bartov et al. (2003) proposed that the intruding cold water stopped the cyclonic uptake of vapor from the Mediterranean to the atmosphere, shutting the Levant rains. It seems that the YD cooling was associated with atmospheric changes, probably stronger effects of the Polar fronts and Westerlies that brought more rains to the Levant. Similar effects of cold seawater intrusions on the regional climate can be detected throughout the Holocene causing possibly the significant aridities of ca. 8.1, 3.5 and possibly the Medieval warming. The rapidity of the response of the regional hydrological systems to the global climate changes and the sensitivity of past human cultures to these changes (e.g. the collapse of the Natufian culture during the Allerod aridity) are certainly important lessons and alarming signals for our human society.