PP24A-01 INVITED 16:00h
The Seasonal Cycle In North Pacific Sea Surface Temperature And The Glaciation Of North America 2.7 Million Years Ago
The causes for major intensification of Northern Hemisphere Glaciation 2.7 Million years ago, one of the most dramatic climate shifts in the Cenozoic, are not yet resolved. In particular, the onset of Northern Hemisphere Glaciation has proven to be inconsistent with early ideas regarding the water vapor requirement. It has been suggested that glaciation began in response to increased North Atlantic Deep Water formation and the flow of warm Gulf Stream waters into the high-latitude North Atlantic, associated with the closure of the Panama seaway. However, recent studies show that this closure and associated changes in North Atlantic circulation occurred 4.6 Ma ago, well before the onset of intense NHG. Thus, the Atlantic Ocean is unlikey to be a key factor in this climate change enigma. The role of the North Pacific has so far been overlooked, despite the fact that it is the major water vapor source upstream (in an atmospheric sense) of the North American continent. We report two independent data sets from a sediment core in the western subarctic Pacific indicating that summertime/autumn sea surface temperature in this ocean region rose in response to a freshwater-driven stratification 2.7 million years ago2, even as the wintertime surface ocean cooled, wintertime floating ice became abundant, and global climate descended into glacial conditions. This increase in the seasonal temperature variation of the subarctic Pacific reflected a reduction in the buffering of surface temperature by the ocean interior. The late summer/autumn warming 2.7 million years ago would have extended into the autumn, providing water vapor to northern North America when it could be precipitated and accumulated as snow. Thus, stratification of the subarctic Pacific at 2.7 Myr allowed for the initiation of major Northern Hemisphere Glaciation by maintaining the water vapor source to the continents even as climate cooling favored the preservation of snow and ice.
PP24A-02 16:15h
1.1 Million Years of Environmental Change in the Sea of Okhotsk
Based on sedimentary records from the central Sea of Okhotsk, we reconstruct the closely coupled glacial/interglacial changes in terrigenous flux, marine productivity, and sea ice coverage over the past 1.1 Ma. The correspondance of our sedimentary records to the China loess grain size record (China Loess Particle Timescale, CHILOPARTS) suggests that environmental changes in both the Sea of Okhotsk area and in SE Asia were closely related via the Siberian atmospheric high pressure cell. During full glacial times, our records point to a strong Siberian High causing northerly wind directions, the extension of the sea ice cover, and a reduced Amur River discharge. Deglacial maxima of terrigenous flux were succeeded by or synchronous to high-productivity events. Marine productivity was strengthened during glacial terminations due to an effective nutrient utilization at times of enhanced water column stratification and high nutrient supply from fluvial runoff and sea ice thawing. During interglacials, SE monsoonal winds prevailed analogous to the today's summer situation of a pronounced Mongolian Heat Low and a strong Hawaian High. Strong freshwater discharge induced by high precipitation rates in the Amur drainage area, and a seasonally reduced and mobile sea ice cover favored marine productivity (although being considerably lower than during the terminations), and a lowered flux of ice-rafted detritus.
PP24A-03 INVITED 16:30h
Glacial Ventilation of the North Pacific
Previous work on sediment cores from the North Pacific showed that above ~2 km d13C on the benthic foram Cibicidoides was higher during glacial time than it is today, after correcting for secular change of ~0.3 permil. This led to the suggestion that the ocean was better ventilated either through greater transport of a paleo North Pacific Intermediate Water, or transport was the same as today and preformed d13C was higher ([O2] was higher). Below ~2km, d13C was about the same as today, after correction. A new synthesis of apparent ventilation ages based on the paired benthic (BF) and planktonic foram (PF) 14C method provides general support for the scenario based on d13C. Although many 14C data are available for this synthesis, we exercised some reasonable quality control by selecting data that met the following criteria: (1) analyses based on high deposition rate cores, or laminated intervals of cores, (2) analyses conducted at peaks in BF abundance, and (3) analyses from a narrow window of glacial maximum time (~18-20 ka). The result shows that above ~2.5 km apparent ventilation ages are less than today (better ventilation), and the one sample from > 3km is the same as today (~1700 yrs). When d13C and BF-PF 14C data are compared between the North Atlantic and North Pacific Oceans, it seems likely the same water filled these basins deeper than ~3.5 km. d13C of each basin was about 0 permil, and although the average apparent ventilation age was ~1200 yrs for the North Atlantic during the LGM, the two oldest determinations are 1550 and 1450 yrs. The fly in the ointment is still the very low d13C observed in the South Atlantic sector of the Southern Ocean. Although the available data are reproducible and may not reflect low d13C in the fluff layer at the seafloor, results from a zonally averaged circulation-biogeochemistry model showed that d13C may become unlinked from nutrient content during a change of the ocean general circulation.
PP24A-04 16:45h
Oceanographic Influences on the Style and Timing of Glacial-Interglacial Change in Western Beringia
Late Cenozoic glacial/interglacial change imposed on the Bering Strait region radical changes in paleogeography unique to the Northern Hemisphere. A forested middle Pliocene Arctic gave way to the first major glaciation of the northern hemisphere. Glacial and marine deposits found interbedded along the coasts of Alaska and Chukotka record a number of critical transitions in the evolution of Northern Hemisphere climate. Glacial periods across much of Beringia during the early and middle Pleistocene were at least an order of magnitude more extensive than during the Last Glacial Maximum (LGM) for reasons that remain unclear but must be related to fundamental changes in moisture flux. Valley glaciers (not ice sheets) dominated the landscape of central and western Beringia throughout the Quaternary, though some valley glaciers reached the sea. Eurasian ice sheets to the west and perhaps extensive sea ice over parts of the deep Bering Sea prevented vapor flux to Beringia creating pervasive aridity, limiting the size of ice sheets and the distribution of deep snow cover especially during the LGM. Interglacial periods repeatedly flooded the Bering Strait, rapidly changing the configuration of the coastlines, altering regional continentality, and reinvigorating the exchange of water masses between the North Pacific, Arctic Ocean and North Atlantic. During the last interglacial (MIS 5e) the winter sea ice limit was as much as 800 km further north than now, and summer sea ice may be been periodically absent. Treeline was hundreds of kilometers further north, notably eliminating tundra across Chukotka to the Arctic Ocean. The fragmented coastal record of the Bering Strait registers clear evidence for the rapid initiation of valley glaciation and the deposition of glaciomarine sediments at or near the end of warm interglacial periods including Stage 11 (or 9), but especially the substage 5e/5d transition and the substage 5a/4 transition in parts of coastal Chukotka. The Flaxman deposits (Gubik FM) dated to MIS stage 5a on the Alaskan North Slope, record both flooding of the Bering Strait and collapse of an ice sheet that likely accumulated since 5d over some part of the western Canadian Arctic. Flooding of the Bering Strait in MIS 5a, coupled with an insolation high, may have contributed to the collapse of Canadian and Eurasian ice sheets allowing the penetration of moisture across the continent and the expansion of valley glaciers in the Bering Straits region well beyond LGM limits. Increasing continentality, sea ice cover and the expansion of the Scandinavian/Eurasian ice sheet (Siegert et al., 2001) then limited available moisture supply across most of Beringia. During MIS stage 3 climate remained relatively harsh across much of Alaska, but parts of western Beringia experienced the temporary return of interglacial vegetation to near modern conditions coincident with insolation highs. LGM glaciation across most of Beringia was restricted to local mountain ranges and dominated by valley and cirque glaciers. A mosaic of dry habitats characterized by herb and forb-tundra dominated ice -free valleys and lowlands with some evidence for mesic conditions across parts of central Beringia. A rapid rise of sea level at the end of the LGM likely caused swift migration of the shoreline as summers warmed into the early Holocene and re-established modern vegetation.
http://www.geo.umass.edu/faculty/jbg
PP24A-05 17:00h
Long Terrestrial Paleoenvironmental Records From Beringia
Several records of vegetation change extending from MIS 5 to the present, and others covering various parts of the last glacial-interglacial cycle, have been retrieved from both west and east Beringa (Siberia and Alaska). Recently, analogue analysis has produced quantitative estimates of past climate conditions. Records from Beringia show several consistent patterns that differ from those observed in the North Atlantic sector. The last interglaciation (MIS 5), while seeing a greater extension of forest cover than the Holocene (and possibly greater reduction of permafrost), does not appear to have exceeded Holocene temperature values to the extent recorded for the North Atlantic region. The tripartite nature of stage 5 warm periods, strong in Europe, is muted in Beringia, and records of glacial stages are not generally characterized by rapid, high-amplitude fluctuations. A climatic oscillation during the Younger Dryas chronozone is apparent at several sites: the signal is subtle in interior regions, stronger at the coast. The Holocene thermal maximum occurred early, ca 9-11 ka BP. Colder periods appear to have been characterized by extreme aridity, and moisture availability has been an important control of terrestrial ecosystem structure and function through much of the last glacial-interglacial cycle. The regional response of Beringia to global and hemispheric forcing primarily reflects its location upwind of the Laurentide ice and the dynamics of the northern Pacific and western Arctic oceans (especially those of sea ice). Events generated in the North Atlantic are generally not strongly expressed, though some are detectable.
PP24A-06 17:15h
A Continuous 3.6 Mio Year Record of Terrestrial Climatic Change in NE Siberia? Seismic Investigation of Impact Crater Lake El'gygytgyn
Long ($ > $100ka) records of climatic change from terrestrial environments in Siberia are rare but essential to improve our understanding of the Arctic's role in global climate dynamics. North-east Siberia provides a key area to study climatic teleconnections between the North Pacific oceanic system, climatic pattern over NE Russia, the Arctic Ocean and other climate forcing areas such as the North Atlantic and the Tropics. Lake El'gygytgyn, located in central Chukotka, NE Russia, was formed 3.6 million years ago by a meteorite impact and apparently escaped continental scale glaciations during the entire Quaternary. If so a full-length sediment core would yield a complete record of Arctic climate evolution, back one million years prior to the first major glaciation of the Northern Hemisphere. A 13.0 m long sediment core retrieved from the lake in 1998 revealed a basal age of approx. 250 ka, confirmed the lack of glacial erosion, and underlined the sensitivity of this lacustrine environment to reflect high resolution climatic change on Milankovitch and sub-Milankovitch time scales. Seismic investigation carried out during two expedtions in 2000 and 2003 revealed a depth-velocity model of brecciated bedrock overlain by a suevite layer overlain by two lacustrine sedimentary units up to 350 m in thickness. The upper well-stratified sediment unit appears undisturbed apart from intercalation with debris flows near the slopes. Based on extrapolation of sedimentation rates the entire Quaternary and possibly parts of the late Tertiary record are within the 170m thick unit one and the earliest history of the lake is in unit two. There is no evidence of glacial erosion in the sedimentary record. High-resolution 3.5 kHz profiles indicate sharp termination lobes of non-erosive debris flows in distal areas. Near the centre of the lake the 250ka sediment-core record exhibit a few thin distal turbidites possibly generated by debris flows. The character of the sediment fill suggests a high potential of the record for paleoclimate studies and deep drilling would offers opportunities of impact studies of the brecciated bedrock. Our study is part of international and multi-disciplinary site-survey investigation of Lake El'gygytgyn. The lake has been recognised as potential deep drilling location by the International Continental Drilling Program (ICDP).
PP24A-07 17:30h
Sedimentation in Lake El'gygytgyn, Northeastern Siberia, During the Past Three Climate Cycles
Lake El'gygytgyn, located in central Chukotka, NE Siberia, is a 3.6 million year old impact crater lake with a diameter of 12 km and a water depth of 170 m. Sediment cores of 13 and 16 m length from the deepest part of the lake were investigated for chronology, physical properties, sedimentology, biogeochemistry, inorganic geochemistry, mineralogy, palynology and diatom assemblages. The cores are undisturbed and complete. With basal ages of approx. 250 and 300 ka, respectively, they represent the longest continuous climate records as yet available from the Arctic continent. Besides two ash layers and a number of distal, fine-grained turbidites, four "pelagic" sediment units of different composition can be distinguished. During warm periods (units 1 and 2), summer melt of the ice cover leads to high aquatic primary production, ventilation of the entire water column by annual turnover, decomposition of most of the settling particulate organic matter, and bioturbation of the sediments. Among these periods, MIS 5.5 (Eemian) and to a smaller degree late MIS 1 (Holocene) exhibited significantly higher organic matter accumulation (unit 1) than MIS 3, 5.1, 5.3, 6.1, 6.3, 6.5, 7.1, 7.3, and 7.5 (unit 2). This was probably due to enhanced nutrient and organic matter supply from a more dense vegetation cover in the catchment. The density and composition of the regional vegetation, in turn, was not only controlled by insolation, but also by precipitation and/or temperature changes due to alterations in the atmospheric circulation pattern. During cold periods (units 3 and 4), a persistent ice cover hampers primary production and leads to a stratified water column with anoxic bottom waters, good preservation of the settling organic matter, and the formation of laminated sediments due to the absence of bioturbating organisms. Among these periods, cold and particularly dry climates lead to the widespread absence of blanketing snow on the lake ice cover, enabling the formation of sediment clasts and sufficient light penetration for significant primary production beneath the ice (unit 3). This mode prevailed during MIS 2, 5.2, 5.4, 6.2 and 6.4. The high aridity can best be explained by a predominance of westerly winds. A cold but more moist climate, in contrast, generates a blanketing snow cover on the ice, which hampers clast formation and significantly reduces the aquatic primary production (unit 4). Cold and moist climates prevailed at Lake El'gygytgyn during MIS 4, 6.6, 7.2, 7.4 and the top of MIS 8. The higher moisture supply could be due to northerly or easterly winds more frequently reaching the area in consequence of a westward migration or weakening of the Siberian High.
PP24A-08 17:45h
The Growth and Decay of the Southern Lobe of the Cordilleran Ice Sheet
During the last glacial, the relatively small, ephemeral Cordilleran Ice Sheet lay between the moisture source of the North Pacific Ocean and the extensive Laurentide Ice Sheet on North America. A number of important climate considerations have resulted from studies of this ice sheet including: the apparent asynchroneity of ice sheet advance around the rim of the northwest Pacific Ocean and the incredible speed of Cordilleran ice accumulation and retreat. Terrestrial records of glacial sediments have previously been the only means of evaluating the growth and decay of the Ice Sheet in response to global climate forcing. MD02-2496 (48\deg 58' N; 127\deg 02' W; 1190 meters water depth; 38.38m core length) cored on the slope of Vancouver Island, British Columbia is the first well-dated high resolution record of climate forcing and ice sheet response on the southern lobe of the Cordilleran Ice Sheet during the last glacial cycle. Evidence from magnetic susceptibility and color reflectance at MD02-2496 demonstrate that the continental slope is highly sensitive to glacially derived sediment delivery. At the same time the oxygen isotope record of surface dwelling planktonic foraminifera {\it G. Bulloides } resembles the temperature record of the Greenland Ice Sheet and provides a local themometer. At 20 cm resolution, many rapid climate events are identifiable including the Younger Dryas, Allerod and interstadial events. The presence of glacial-marine sediments in the core is demonstrated by large increases in magnetic susceptibility and concurrent shifts in sediment color from olive to grey. These indicators suggest glacial-marine sedimentation began to diminish around 15 Ka, with two small bursts of activity around 13.8 Ka and 12-12.5 Ka. Glacial marine sedimentation ceased at 10.5 Ka. This interval of glacial-marine sedimentation began around 31 Ka, with apparent ice-free conditions in southern British Columbia for much of Marine Isotope Stage (MIS) 3. A large increase in sedimentation rate occurred at 18 Ka suggesting maximum extent of the southern limb of the Cordilleran Ice Sheet at that time. Modern glacier distribution in the North Pacific is primarily controlled by topography and precipitation. Presently abundant moisture along the west coast of North America accompanies the dominant position of the jet stream by defining the main path of Pacific storms (low pressure systems). Potentially, the rapid growth of the Cordilleran Ice Sheet along the North Pacific rim was the product of precipitation from a storm track controlled by the jet stream. The data indicate that despite MIS 2 cooling by the Last Glacial Maximum (LGM; 21 Ka) the Cordilleran Ice Sheet had not yet achieved maximum extent in southern British Columbia. The maximum ice sheet growth in the region appears to require the Aleutian Low to be positioned far to the south of its present location. Rapid decay of the southern lobe of ice sheet probably occurred as storm tracks followed the jet stream northward with global climate warming. Changes in the position of the jet stream as it enters the North American continent would result from shifts in the positions and strengths of the Aleutian Low and North Pacific High, as well as growth of the Laurentide Ice Sheet.