PP51A-1327 0800h
Modeling Freshwater Impacts on Ocean Circulation During the Miocene at 20 and 14 Ma
A large modeling effort has been committed to better understand the role of freshwater impacts on the past climates, the thermohaline ocean circulation (THC) patterns, and variations of sea level. The restructuring of the THC may be caused by many factors, including ice sheets melting of the major ice sheets, sea ice melting and forming, and for long-term variability - also by changes in major gateways. The sensitivity of the past THC to freshwater fluxes could depend critically on the land-sea distribution and thus be different from the present-day. However, our understanding of the role of paleogeography in THC dynamics and climate change is not yet complete. There is a consensus that the Cenozoic cooling trend and southern glaciation during the last 25 million years was largely caused by continental drift and bathymetrical changes. In the Southern Hemisphere, the Miocene marks further widening of the ocean passage between Antarctica and Australia, the opening of the Drake Passage, and therefore the emergence of the fully developed Antarctic Circumpolar Current. We focus on two different Miocene time slices (20 and 14 Ma), which are characterized by major transitions from warm to cold polar climate. In a series of numerical experiments, we show that the Miocene THC is noticeably sensitive to both the geographical conditions and to the southern high-latitudinal freshwater pulses that might have been caused by young cryosphere evolution. Our coupled ocean-atmosphere simulation reveals changes in all major ocean circulation parameters - the ocean heat transport, meridional overturning, sea level, etc. The sensitivity to freshwater impacts appears to be a function of land-sea distribution and is different at all three points on the timeline - the two paleo time slices and the present one.
http://www.personal.psu.edu/bjh18
PP51A-1328 0800h
Duration of Pliocene Ice-Rafting Events Offshore of Prydz Bay, Antarctica, Derived From Extraterrestrial Helium-3
The sediment record of ODP Site 1165, located 400 km offshore of Prydz Bay, Antarctica, contains layers rich in ice-rafted debris (IRD) dating from the Early Miocene to the present. The IRD represents pulses of icebergs from the Lambert Glacier and the Antarctic coast west of the site. If the duration of the ice-rafting events is short, they are likely to represent relatively rapid deglaciations of the continent. Alternatively, the IRD could be concentrated in these layers by winnowing or a reduced sedimentation rate. In order to distinguish between these possibilities we have measured helium isotopes across two of the layers. Interplanetary dust particles (IDPs) are enriched in helium-3 compared to terrestrial materials and they are deposited at a relatively constant rate, thus providing a means of determining the (relative) duration of the ice-rafting event. IDPs and helium-3 are diluted one of the Pliocene IRD-rich layers, indicating relatively fast deposition and increased iceberg flux, probably during deglaciation.
PP51A-1329 0800h
The climate response to a hemispheric asymmetric change in polar ice cover
A remarkable feature of the mid-Pliocene climate is the asymmetry in polar ice cover: Antarctica was well on its way towards full glaciation, whereas the northern high latitudes was still relatively warm and ice-free. The question we ask is: given that we are interested in the fast-acting components of the climate (the atmosphere and surface ocean), what sort of climate is consistent with such a hemispheric asymmetric ice configuration? We investigate this question by modeling the climate response to the removal of northern hemisphere sea ice using the Community Climate model version 3 coupled to a slab ocean model. In addition to showing the expected strong warming in the northern hemisphere, the remarkable aspect of the simulation is a pronounced hemispheric asymmetric change to the tropical climate, characterized by northward shift of the marine ITCZ. Furthermore, humidity increases throughout the northern hemisphere and decreases throughout the southern hemisphere, indicative a changed Hadley cell regime. The observed low-latitude and southern hemisphere changes are not reproduced in a control study using fixed-SSTs, indicating that an interactive surface ocean is crucial to obtaining the tropical climate behavior. . Our model results suggest that an altered tropical climate may be a strong feature during the unipolar glaciated conditions in the late Cenozoic, in sympathy with a previously proposed hypothesis for hemispheric asymmetric climate change (Flohn 1980). To test this idea, we are analyzing various published and synthesized paleoclimate proxy data during this period. Our study may also offer some insight into global warming conditions with a much reduced if not absent Arctic sea ice cover.
PP51A-1330 0800h
The timing difference of Last Glacial Maximum between East Antarctic Ice Sheet and northern hemisphere ice sheets: geological evidence around the Lutzow-Holm Bay region, East Antarctica
The fluctuation of both hemisphere ice sheets during the last glacial has had a great impact on global sea-level changes and climatic variations through the thermohaline circulation change. It has been well known that the age at which the northern hemisphere ice sheets was the greatest was at marine isotope stage (MIS) 2. In CLIMAP model, Antarctic ice sheet is considered to have advanced synchronously to the edge of continental shelf margin, however, the geological evidence for Antarctic ice fluctuations is sparse. We have studied the stratigraphic relationship between raised beach deposits including in situ fossil shells and glacial deposits around the Lutzow-Holm Bay region, East Antarctica. The AMS radiocarbon dating revealed that the ages of in situ fossil shells are clearly classified into two groups: the younger group is 3-8 ka, and the other is older than 30-46 ka. Any marine layers and in situ fossil shells were not disturbed by ice sheet loading or scouring. In addition, glacial deposits associated with the last greatest ice advance can be observed under marine beds including older fossll. This fact indicates that the age at which the Antarctic ice sheet was the greatest was older than 46ka not MIS 2, and the advance of the Antarctic ice sheet was not as dramatic as the advance of northern hemisphere ice sheets during the last glacial.
PP51A-1331 0800h
Holocene melting history of Antarctic ice sheet inferred from relative sea-level records around the Lutzow-Holm Bay, Antarctica.
The relative sea-level record from the last glacial period around the Antarctica is very important to infer the melting history of Antarctic ice sheet, since it is significantly dependent on crustal movements due to rebound after ice retreat. The relative sea-level variations from last glacial maximum around Antarctica are, however, more poorly defined than on any other continents. Recently, Miura et al. (2002) determined the accurate sea-level curve using in situ fossil shells and sequence stratigraphy of raised beach deposits at Skarvsnes, Lutzow-Holm Bay region, East Antarctica, and they deduced that the rapid sea-level falling event which magnitude is about 6 m for about 1000 yrs occurred in the mid-Holocene (4000 - 2700 yrBP). This event is attributed to the rapid removal of a regional ice load as a response to a warmer climatic event, which has been reported as ``mid-Holocene climatic optimum'' from various records around the Antarctica. Then we estimate the melting ice volume around Lutzow-Holm Bay region in the mid-Holocene by comparing the observations and predictions using the glacio-isostatic-adjustment model. In this study, we examine the possibility of the episodic melting of Antarctic ice sheet, and evaluate the spatial distribution, temporal change and volume of the melting ice loads by precise calculations. In our preliminary results, we can??t explain the rapid sea-level falling event unless the regional ice loads of 150 - 200 m melt around the Lutzow-Holm Bay. This result means that the volume of melting ice loads is corresponding to about 1 m for eustatic sea level (ESL) rise. Since previous authors have ascribed about 2 m for ESL to the Holocene Antarctic ice melting on the basis of relative sea-level variations at far-field, so this 1 m ESL rise is equivalent to 30 - 50 percent of the previous Antarctic melting estimate.
PP51A-1332 0800h
Deciphering Neogene Antarctic climate history is certainly important, but documenting Pleistocene events may be more relevant to prediction
This AGU session description states, "Scenarios for future climate change indicate that within the next 100 to 400 years, global annual average surface temperatures will increase by several degrees. The Neogene represents the last time in Earth history when comparable warming occurred.The aim of the session is to synthesize results from marine and terrestrial proxy data for the Miocene and Pliocene and to combine them with outputs derived from numerical climate and ice-sheet models." Yes, Miocene and Pliocene proxies are critically important to understanding Cenozoic stepwise cooling and the polar climatic isolation of Antarctica. But at least equally important is understanding Antarctic climate dynamics during Pleistocene warm interglacials. These did not dramatically affect the East Antarctic ice sheet (EAIS), but the West Antarctic ice sheet (WAIS) has a history of Pleistocene advance and retreat well beyond its current configuration. Such changes have global consequences for sea-level, bottom water formation, and ocean circulation. The WAIS is the most dynamic of the world's existing large ice sheets, and it is the source of the largest ice shelves. Global warming may already be affecting the stability of the WAIS and the ice shelves. I will review geologic evidence for Quaternary warm interglacials in the Antarctic (e.g., MIS- 5e, MIS-11, MIS-31, others), including subglacial sediments, drill cores from the Antarctic marginal seas, Southern Ocean sediments, marine deposits in outcrop, sea-level records, and Antarctic terrestrial deposits including lake cores. These will be discussed in the context of WAIS and Ross Ice Shelf history and dynamics.