PP41B-1439
Did Lake Elsinore's Stormy Affair End, or Just Change, in the Mid-Holocene?
High-resolution, continuous terrestrial records of Holocene climate from Southern California are scarce. Recent research on Lake Elsinore, however, has shown that the lake's sediments hold excellent potential for paleoenvironmental analysis and reconstruction. Kirby et al. (2007) use environmental magnetic susceptibility and loss-on-ignition to suggest a wet early Holocene followed by a long-term drying trend. New grain size and δ18O(calcite) data reveal a more complex Holocene climate history for Southern California than previously recognized at the site. A modern comparison between 20th century San Jacinto River discharge and various sand size classifications suggests that percent sand is a reasonable, qualitative proxy for past climate wetness, especially intervals of above-average discharge (i.e., higher lake level). This modern analysis also indicates that exceptional discharge years produce discrete sand peaks; although, these sand peaks are not visible generally to the naked eye - at least for the 20th century. From this, percent sand is also used as a qualitative measure of past "storminess". Together, the sand data indicate a stormy early-to- mid Holocene with a pronounced decrease in the frequency of large storms by ~3,300 cy BP as well as millennial-scale hydrologic cycles throughout the Holocene. A count of visual "storm" layers supports the storm interpretation, which reveals 18 events before, and only 7 after, 4,000 cy BP. The occurrence of mudcracks in a slightly shallower core (1m modern depth difference) indicates only 2 potential lowstands before, and 5 after, 4,000 cy BP as well as no clear evidence for whole-lake, Holocene desiccation until ~500 cy BP. Although still in progress, the δ18O(calcite) also indicate a shift in the region's climatic state ca. 4,000-5,000 cy BP as well as millennial scale hydrologic cycles(?). This transition out of the mid-Holocene ca. 4,000 cy BP is observed elsewhere in Southern California (e.g., Dry Lake and Big Bear Lake), and it presents an interesting climatic phenomenon in the absence of an obvious climatic forcing.
PP41B-1440
Mid-Holocene Climate Variability in the Western Sierra Nevada as Recorded in Sediments from Swamp Lake, Yosemite NP
Paleoclimatic records have revealed the widespread occurrence of drought in western North America during the middle Holocene (~8 – 3 ka), particularly at sites in the Great Basin and Southwest. In the western Sierra Nevada and Sacramento-San Joaquin River basin of California, however, the climatic signature of this interval is less clear, due in part to a shortage of high-resolution proxy records in the watershed that extend beyond the last 2 kyr. The varved Holocene section of the 20,000-yr sedimentary record from Swamp Lake, a small mid-elevation (1554 m) lake in the central Sierra Nevada, provides a rare opportunity to examine more detailed relationships among climate variability, drought, and ecosystem responses during the mid-Holocene interval. In this study we will present the results of organic geochemical and isotopic analyses of sedimentary organic matter from a 10 m core from the deep basin of Swamp Lake. We utilize carbon and nitrogen elemental abundances (TOC, TN, C/N) and isotopic compositions (δ13C, (δ15N) of bulk organic matter, biogenic silica (BSi) content and ratios of biomarker compounds to reconstruct changes in lake productivity, organic matter sources, and lake level in relation to climatic variables. In addition, we will evaluate the use of compound-specific hydrogen isotope (δD) measurements on leaf-wax n-alkanes extracted from the sediment as a means of reconstructing the hydrologic status of the lake and watershed. Our proxy records indicate dry but highly variable conditions in the Sierra Nevada during the early and mid- Holocene, and a distinct shift toward wetter conditions at the transition to the late Holocene (~3.1 ka) corresponding to warmer SSTs off northern California. Periodic, century-scale variability in lake productivity and organic matter source proxies may be linked to climatic changes influenced by solar variability or low- frequency shifts in ocean conditions.
PP41B-1441
Testing for Holocene Hydroclimatic Change in the Sierras: New Insights From Fallen Leaf Lake, Lake Tahoe Basin, California
The Sierra Nevada is the primary source of California's and northwestern Nevada's water resources. To understand the potential for severe and prolonged drought in the Sierras, it is necessary to study past events recorded by proxy data. Fallen Leaf Lake (FLL), California, is an ideal glaciolacustrine environment to examine hydroclimatic fluctuations during the last ~10,000 years. Recent studies have discovered submerged and upright trees rooted ~36 m below the surface of FLL suggesting lake levels were significantly lower during the past. Several trees have been radiocarbon dated at ~1200 AD, which is consistent with the timing of the Medieval Climatic Anomaly (MCA) observed at other locations in the Sierras. Furthermore, several studies have presented evidence for aridity and lake fluctuations during the mid- Holocene. We conducted field campaigns from 2006 - 2008 to acquire high-resolution seismic CHIRP, sidescan sonar, sediment cores and ROV video footage in FLL to search for additional submerged trees, as well as geomorphic and sedimentary evidence for hydroclimatic changes. Here we present preliminary results from those surveys. Side-scan sonar imagery provided complete coverage of the lakefloor and imaged 9 upright trees, over 80 downed trees, and what appear to be in-situ stumps. Examination of the side-scan and ROV data reveals numerous submerged paleoshorelines along the entire circumference of FLL to depths > ~60 m below present lake level. Over 40 line-km of seismic CHIRP data and 5 piston cores were collected to define the stratigraphic framework during the Holocene. Up to 50 m of lacustrine sediments rest above a hummocky basal reflector inferred to be Tioga-aged glacial material. Overlying the glacial deposits, (coarse-grained?) beds with nearly uniform thickness abruptly transition up-section into down-lapping, lenticular shaped packages that are separated by thin onlapping, divergent beds. The data suggest a change in the hydrodynamic conditions, sediment flux and/or mode of transport during the early Holocene that resulted in a combination of current-controlled and gravity-driven deposition. Sediment cores reveal an increase in turbidite deposits, coarse-grained material and magnetic susceptibility between 3.6 - 4.9 kyr BP. One scenario involves high sedimentation and discharge immediately following Tioga recession then a shift into variable conditions of the Holocene. The increased magnetic susceptibility and turbidite deposition may reflect a change from arid to pluvial conditions in the mid-early Holocene. Multiple shoreline fluctuations have allowed the preservation of in-situ ancient trees beneath the lake surface and provide strong evidence for sustained drought during medieval times. In summary, the data reveal dramatic changes in the shoreline and sedimentary processes during the Holocene and demonstrate the feasibility of high-resolution geophysical methods towards identifying potential sites to examine paleoclimate proxies as well as the potential for future studies in FLL. Pending radiocarbon dates from cores will allow us to compare more precisely the timing of depositional changes in FLL to regional climatic events.
PP41B-1442
Comparison Between Mid-Holocene and Twentieth Century Climatic Change and Alpine Treeline Fluctuations in the Western U.S. Based on Compound-Specific Hydrogen Isotopes, Tree Rings, Pollen, and Macrofossils
Independent examinations of climate and vegetation over the mid-Holocene and over the twentieth century around American Lake (3400 m) in Jackson County, Colorado are compared to evaluate the ecological response of forest extent to changes in temperature. Over the twentieth century both minimum and maximum temperatures increased by about 2° C. Tree rings show an upward expansion of forest over the period of warming with significantly younger trees growing above treeline than below treeline (p = 0.00). Holocene climate is reconstructed from American Lake sediments with hydrogen isotopes, which reflect precipitation- weighted temperature. A calibration study of compound-specific hydrogen isotopes in Jackson County, Colorado (n=10) demonstrates that behenic acid hydrogen isotopes record lake-water δD values (r = 0.88). The behenic acid hydrogen-isotopic record from American Lake shows a mid-Holocene warm period, which follows a cold excursion at the 8200-year event. Such a cold excursion is consistent with Front Range evidence of glacier advances during this period. The magnitude of isotope-inferred temperature changes in the Holocene is the same as those observed in the twentieth century. During the mid-Holocene warm period, loss-on-ignition analysis shows a higher percentage of organic material in American Lake. Pollen and macrofossil analysis, in progress, will further determine whether the warm temperatures were accompanied by upward expansion of forest. Our results suggest that future warming will exceed mid-Holocene warming and may change the extent of mountain forests. The impact of expanded forests on carbon budgets and water resources may be critical in a warmer world.
PP41B-1443
Mid-Holocene Lake Level Decline and Eutrophication in Rawson Lake (Lake 239), Experimental Lakes Area, northwestern Ontario
A paleolimnological study of lake-level changes and lake eutrophication in Rawson Lake (Lake 239), a headwater lake in the Experimental Lakes Area in northwestern Ontario indicates large fluctuations have occurred over the Holocene. Quantitative estimates of lake level from near-shore cores are based on a diatom-inferred depth model that was developed from surface sediments collected along several depth transects in Rawson Lake. Changes in lake trophic status are based on the analysis of diatom assemblages and associated quantitative inferences of total phosphorus (TP) from a deep central core, as well as changes in diatom accumulation rates. Lowering of lake level began in the early Holocene and during the mid-Holocene declined by at least 8 m. An 8-m decline would result in a 42 percent decline in surface area and 58 percent decline in the volume of Rawson Lake. Concurrent with the lake-level declines was an increase in inferred total phosphorus (TP) and an increase in diatom accumulation in the central core. Fragilaria crotonensis and Aulacoseira subarctica were two of the dominant nutrient rich taxa that contributed to an increase in diatom-inferred TP during the mid-Holocene. These results provide the first substantive evidence of large declines in both water quantity and water quality in northwestern Ontario during the mid-Holocene. Present-day conditions of higher lake levels and dominance of the oligotrophic planktonic, Cyclotella stelligera in both the near-shore and central cores was established by around 3000 years ago. The transition of this site from the aridity of the mid-Holocene to more modern conditions occurred over a protracted period. Sites further to the west in the Canadian prairies appear to exhibit more abrupt and a later transition out of the arid mid-Holocene conditions. The variation in timing of the onset of modern lake conditions likely reflects the variability in geological and hydrological setting, as well as the geographic variance in timing of climatic change at specific locations.
PP41B-1444
Abrupt onset of widespread drought at the mid-Holocene transition observed from New England through Eastern Michigan
Recent studies suggest that the mid-Holocene transition in North America was characterized by significant changes in hydrologic balance. However, the spatial coherence and abruptness of the hydrological transition are not well constrained because of the limited number of high-resolution records. To further assess both the abruptness of the change and its spatial coherence in the eastern U.S., we have produced decadally resolved records of hydrologic change across the mid-Holocene transition from two ombrotrophic peatland sites, Irwin Smith Bog in eastern Michigan, and The Great Heath in Downeast Maine. We have reconstructed hydroclimatic changes using ratios of organic biomarkers to reconstruct surface vegetation, hydrogen isotope ratios of specific biomarkers to reconstruct evaporation, and testate amoebae to reconstruct water- table depths. Our results indicate that the mid-Holocene transition was marked by an abrupt, high-amplitude drying event at both sites. To further understand continental-scale spatial patterns and assess potential ocean-atmosphere linkages, we synthesized paleohydrologic data from peatlands, lakes, and other depositional systems in North America, as well as sea surface temperature (SST) data from the Atlantic and Pacific across the mid-Holocene transition. Our synthesis suggests that the mid-Holocene transition was spatially similar to precipitation and SST anomaly patterns of the "Northeast Drought" of the historical period 1962-1965 CE. During this period, precipitation deficits occurred in the Northeastern US, and excess precipitation occurred in the Northern Great Plains and the Southwestern US. The precipitation anomalies have been attributed to semi-permanent waves in the upper atmosphere polar jet stream, which were in turn supported by SST anomalies. We argue that the changing interactions between ocean and atmosphere that produced the 1960s "Northeast Drought," are mechanistically analogous to the conditions that produced the continental hydroclimatic changes at the mid-Holocene transition. We also posit that these ocean- atmosphere dynamics may also drive the multiyear to millennial scale hydrologic changes we observe in the late Holocene, after the transition.
PP41B-1445
Holocene Vegetation and Climate Shifts from Sutherland Fen, Black Rock Forest, New York - Plant Macrofossils, Charcoal, and Carbon
Sutherland Fen formed about 12,600 C-14 years ago (15,000 calendar years), the same time as adjacent Sutherland Pond and regional deglaciation. High-resolution (2 cm) analysis of the 3.2 m fen core indicates three major macrofossils zones indicative of climate shifts. These climate shifts were defined over fifty years ago through pollen stratigraphy of the regional northeastern US, but macrofossils provide new details concerning hydrological and ecological shifts. The lowest (SUB-1) dated to the late-glacial, is indicative of a shallow pond characterized by Najas, Nuphar, and Potamogeton seeds and containing Salix (willow) buds, a Rubus (berry) seed, and Picea glauca (white spruce) needles and sterigmata from the surrounding upland. Sedimention rates are highest in this boreal environmental zone. The overlying zone (SUB-2) beginning at 11,500 years ago (Holocene) indicates a continuing pond environment with aquatics such as Najas, Nuphar, and Brasenia, but Picea disappears and Pinus strobus (white pine) dominates the lower section of the zone. A warmer, drier climate produces sustained charcoal in the record at the Holocene boundary. Pinus strobus needles and seeds subsequently disappear and are replaced from 9000 to 7500 years ago by Pinus rigida (pitch pine), Betula populifolia/papyrifera (grey/paper birch), and emergent wetland plants such as Decodon, Cladium, and Cephalanthus, as well as Dulichium, Eleocharis, and Carex, suggesting a shallowing pond and a drier climate. Chara oospores indicate probably groundwater influx into the fen. About 4000 years ago, charcoal again is present. In the subsequent late Holocene a more acidic, moist, fen environment is characterized by Sphagnum, Rubus, Hypericum, Viola, Chamaedaphne, and Carex, though Brasenia and Potamogeton (pond indicators) are occasionally present. The continued presence of Sphagnum led to high carbon accumulation because of less decomposition. This increase in Sphagnum in recent millennia with aquatics suggest a cooler, wetter climate. Charcoal re-appears briefly in the uppermost sediment.
PP41B-1446
Great Basin Archaeology During the Middle Holocene: a Reflection of Environmental Change
Varying types of proxy data in the Great Basin of the western United States suggest that the environment changed dramatically during the mid-Holocene. Lake, marsh, and spring systems dried; and dune fields, first established at the end of the Pleistocene, where again activated as sediments were swept from drying basin lowlands. Plant communities reorganized and migrated along elevation gradients to adapt to these changing conditions, and animal populations followed. However, recent data suggests that conditions during the middle Holocene were variable. Minimally, three distinct periods can be recognized, herein named: the Initial Middle Holocene (ca. 8000 to 5800 cal yr BP), the Middle Holocene Gap (ca. 5800 to 5200 cal yr BP), and the Terminal Middle Holocene (ca. 5200 to 4000 cal yr BP). Depending on location and the type of proxy data studied, these periods can vary in their character and timing, but their sequence is increasingly recognized in records of both regional and global-scale. The Initial Middle Holocene is the driest and most volatile of the three periods, with a shift from winter-to summer-dominated precipitation, often delivered by torrential storms. Conversely, the Middle Holocene Gap is a relatively mesic interval with increased winter precipitation and cooler temperatures. A shift towards drier conditions is again evidenced during the Terminal Middle Holocene; however, conditions are never again as dry or as volatile as during the Initial Middle Holocene. The archaeological signature of the Great Basin during the middle Holocene reflects adaptation to this changing environment. During the Initial Middle Holocene, archaeological sites are relatively scarce, and when present, are near water sources substantial enough to persist through the intense drought. The uplands became a focus of sustained seasonal use for the first time as increasingly diverse resources and environments are routinely exploited. It follows that milling gear is a regular and increasingly important element of the prehistoric toolkit. At the onset of the Middle Holocene Gap, rising populations across the Great Basin (and worldwide) are reflected by a surge in archaeological visibility as environmental conditions improved. These populations persist through the less volatile Terminal Middle Holocene. Cultural adaptations to lean times refined during the middle Holocene lead to a punctuated increase in prehistoric populations at the middle-to-late Holocene transition around 4000 cal yr BP, when conditions reach near modern norms. This review offers a synopsis of our current understanding of mid-Holocene environmental change in the Great Basin and how the archaeological record reflects prehistoric adaptations to this dynamic period.
PP41B-1447
Holocene glacier fluctuations on northeast Baffin Island, Arctic Canada inferred from lake sediment records
Spurred by changing climate, the accelerated melting of alpine glaciers is raising questions about the future state of the cryosphere. Reconstructions of alpine glacier behavior during past warm intervals can serve as analogs for near future warming. These reconstructions can be difficult because glaciers leave discontinuous moraine records and less extensive ice limits are often overrun by subsequent advances. However, proglacial lakes act as uninterrupted glacial sediment traps and are therefore good candidates for obtaining continuous records of glacier activity. We investigate post-Last Glacial Maximum glacier fluctuations for two neighboring valleys in northeast Baffin Island using proglacial lake sediments coupled with sediments from several non-glacial lakes. Basal radiocarbon ages on macrofossils from three lakes provide a minimum age of 10.3±0.1 cal ka for Laurentide Ice Sheet retreat from these valleys. We use sedimentology aided by magnetic susceptibility (MS) measurements to obtain information about glacigenic sediment input to the lakes. Non-glacial lakes have relatively low sedimentation rates (0.005 cm yr-1) and organic-rich sediments. In contrast, relatively high sedimentation rates (0.03 cm yr-1) and the presence of minerogenic sediment throughout the Holocene indicate that glaciers persisted in the proglacial lake catchments despite warm early Holocene conditions (~5°C warmer than today). In one proglacial lake, MS was high during the early Holocene and decreased to near-zero values from ~5.5-3.0 cal ka, when glaciers were probably more distal. MS increased after 3.0 cal ka to the highest values in the record around 1.0 cal ka. Increasing sedimentation rates (0.07 cm yr-1) and the formation of varved sediments from 1.0 cal ka to the present suggest that glaciers were more proximal in the lake Holocene. In one valley, two lakes that act as threshold indicators (i.e. they receive glacial sediment only when the glacier reaches a certain extent) contain no evidence of glacier input after 10.3 cal ka until <1.0±0.1 cal ka. Furthermore, the only Holocene moraine present in this valley is steep-sided and lichen-free. This evidence indicates that maximum Holocene glacier extent in this valley likely occurred during the Little Ice Age. The glaciers in this region have retreated from their Little Ice Age margins, and will likely retreat farther, although they will probably not melt away completely unless temperatures rise above those reached during the early Holocene.
PP41B-1448
Mid- To Late Holocene Environmental Change On Kamchatka, Eastern Russia - First Findings From Lake-Sediment Records
In the scope of the German-Russian research programme KALMAR (Kurile-Kamchatka and Aleutean Marginal Sea-Island Arc Systems: Geodynamics and Climate Interaction in Space and Time), the presented subproject deals with the reconstruction of late Quaternary land environments and climate dynamics, inferred from lake-sediment records and peat sections. The study follows an multi-proxy approach, using fossil bioindicators (diatoms, chironomids, pollen), geochemistry, tephra chronology, and radiocarbon dating. The overall goal is to gain insight into the nature of teleconnections between the North Atlantic, the low latitudes, and the North Pacific realm, via marine and oceanic processes. Lacustrine sediment cores were taken during an expedition to Kamchatka in September 2007. Promising Holocene records were obtained from Two-Yurts Lake, situated in a former proglacial basin of the central Kamchatka mountain chain. The last glacial advance took place during the last glacial maximum, as indicated by basal ages of peat sections on surrounding moraines. The postglacial lake sediments consist of diatomaceous oozes with interspersed ash layers, spanning the last 7.5 kyr. Fossil diatom assemblages show a wide range of species and are dominated by Aulacoseira subarctica and Stephanodiscus alpinus, both of which are planktonic cold-water forms of oligotrophic to mesotrophic lakes. These microfossil signals are consistent with modern lake hydrology, stable isotope values of diatoms, and low concentrations of organic matter with low C/N ratios. Variations in total organic carbon and biogenic opal concentrations point to maximum biological export production in the mid-Holocene between approximately 6.8 and 5.0 ka BP. This timing is consistent with a climate optimum, inferred from a pollen record in a peat section at the Pacific coast of Kamchatka. The recognition of mid- Holocene warmth gives evidence of climate affinities to eastern Siberia and Europe and suggests the prevalence of atmospheric teleconnections across northern Eurasia through the westerly wind system.
PP41B-1449
Simulating the climate of the Holocene with a coupled atmosphere-ocean GCM: A new basis for transient data-model comparisons
Two simulations with the coupled atmosphere-ocean general circulation model ECHO-G for the Holocene starting at 7 ka BP have been carried out. The first simulation is driven only by changes in orbital forcings, a second is additionally driven by reconstructions of solar activity and greenhouse gas (GHG) concentrations. Mean 2m-temperatures for the Holocene optimum at 6 ka BP agree well with other simulations carried out earlier by other groups with accelerated forcing, time slice experiments, and earth system models of intermediate complexity. This relates e.g. to increased temperatures over the northern continents during summer and autumn due to changes in orbital forcing, compared to pre-industrial conditions. During boreal winter the North Atlantic region shows a temperature see-saw with lower temperatures over southern and eastern Greenland and increased temperatures over north-eastern Europe and the Arctic. The increased temperatures over north-eastern Europe are also reflected in proxy data. The temperature see-saw pattern over the North Atlantic cannot solely be explained by changes in orbital forcing. Here changes in the mean strength of westerly winds together with changes in sea ice concentrations east of Greenland help to explain this phenomenon. Transient global mean 2m-temperatures in both simulations show a negative temperature trend until approx. 5 ka BP turning into a quasi-equillibrium state until the late Holocene. The simulation with additional solar and GHG forcing also shows an increased decadal to multi-decadal variability. A prominent feature relates to centennial-to-millennial changes in sub-Antarctic sea ice variability having an important influence on global near-surface temperatures. The spatially resolved solar-2m temperature correlation pattern shows highest correlations over tropical and sub-tropical regions. This pattern derived from the climate simulation can be used to identify regions where proxy data potentially contain information about changes in solar activity. North Atlantic ocean variability related to the thermohaline circulation (THC) in the North Atlantic shows a reduction in the mean strength for the whole time period. Transient changes in THC reflect a pronounced decrease in decadal variability. These changes are not reflected in model Eurasian temperatures. One hypothesis to explain this phenomenon involves an orbitally driven influence of the atmospheric circulation over the region on the North Atlantic-European climate, damping the effect of changes of the THC in externally forced transient climate simulations. Therefore our results help to explain climatic patterns with the externally forced influence of large-scale atmospheric and oceanic processes. This is of specific importance to interpret climatic information contained in proxy data and formulate new hypothesis about climate variability based on physically consistent assumptions.
PP41B-1450
Half-precessional climate forcing of Indian Ocean monsoon dynamics on the East African equator
The EuroCLIMATE project CHALLACEA produced a detailed multi-proxy reconstruction of the climate history of equatorial East Africa, based on the sediment record of Lake Challa, a 4.2 km2, 92-m deep crater lake on the lower East slope of Mt. Kilimanjaro (Kenya/Tanzania). Relatively stable sedimentation dynamics over the past 25,000 years resulted in a unique combination of high temporal resolution, excellent radiometric (210Pb, 14C) age control, and confidence that recording parameters of the climatic proxy signals extracted from the sediment have remained constant through time. The equatorial (3 deg. S) location of our study site in East Africa, where seasonal migration of convective activity spans the widest latitude range worldwide, produced unique information on how varying rainfall contributions from the northeasterly and southeasterly Indian Ocean monsoons shaped regional climate history. The Challa proxy records for temperature (TEX86) and moisture balance (reflection-seismic stratigraphy and the BIT index of soil bacterial input) uniquely weave together tropical climate variability at orbital and shorter time scales. The temporal pattern of reconstructed moisture balance bears the clear signature of half- precessional insolation forcing of Indian Ocean monsoon dynamics, modified by northern-latitude influence on moisture-balance variation at millennial and century time scales. During peak glacial time (but not immediately before) and the Younger Dryas, NH ice sheet influences overrode local insolation influence on monsoon intensity. After the NH ice sheets had melted and a relatively stable interglacial temperature regime developed, precession-driven summer insolation became the dominant determinant of regional moisture balance, with anti-phased patterns of Holocene hydrological change in the northern and southern (sub)tropics, and a uniquely hybrid pattern on the East African equator. In the last 2-3000 years a series of multi-century droughts with links to high latitude climate variability exerted widespread influence across the African continent. In northern and western tropical Africa these drought episodes accentuated the late- Holocene drying trend; in southern tropical Africa they mitigated or aborted the trend to increasing monsoon rainfall prescribed by SH insolation forcing.
PP41B-1451
Mid- to late-Holocene NAO-type variability and the impacts of climate change on Paleo- Eskimo and Norse occupation in West Greenland based on lacustrine alkenones and compound-specific hydrogen isotopes
Mid- to late-Holocene climate in the North Atlantic region has been highly variable over millennial and centennial timescales and reveals global teleconnections. To understand the forcing and feedback mechanisms that control centennial-scale climate variability will require decadally resolved quantitative climate reconstructions from throughout the North Atlantic Sector. Here we use the alkenone unsaturation index (UK37) to quantitatively reconstruct lake water temperature for the Kangerlussuaq region of Southwestern Greenland at decadal timescales over the past 6,000 yrs. We apply a temperature calibration based on new in situ measurements and previously published data to convert UK37 to degrees C. Major reductions in Kangerlussuaq lake water temperature ca. 6-5 and 3-2.7 kyrs BP correspond to periods of decreased Meridional Overturning Circulation, expansion of the North Polar Vortex and advection of cold waters from north of Iceland into the North Atlantic Ocean. Beginning ca. 4.2 kyrs BP the alkenone record reveals centennial-scale temperature variability opposite in sign to Southwestern Ireland, suggesting centennial-scale variability in the dominant mode of the North Atlantic Oscillation (NAO) and the position of the Baffin Trough. This mid- to late-Holocene transition is likely related to the waning climatological influence of the shrinking northern ice sheets and reorganization of oceanographic conditions in the North Atlantic. The timing and severity of temperature changes depicted in the Kangerlussuaq alkenone record ca. 3.0-2.7, 2.0- 1.8, and 0.85-0.55 kyrs BP support hypotheses that climate and environmental change played a fundamental role in the final abandonment of settlements by the Saqqaq and Dorset Paleo-Eskimo cultures, and the Iron Age Norse culture. D/H ratios from sedimentary short-chain and long-chain n-alkanoic acids, reflecting lake water and precipitation D/H, respectively, provide additional insight into Holocene temperature and precipitation variability in Southwestern Greenland for the past 8,000 years.
PP41B-1452
Abrupt changes in deep-sea ecosystem structure and biodiversity during the last deglaciation and Holocene
Recent research on deep-sea sediment cores suggests that the structure and diversity of deep-sea ecosystems exhibit greater instability over millennial and centennial timescales than previously realized. Centennial scale ecosystem shifts during the last deglaciation (Termination 1, 18-11.5 ka) and the Holocene (11.5 ka to recent) have been discovered using several well-dated deep-sea microfossil records. In the northwestern Atlantic ODP site 1055, weakening of North Atlantic Deep Water production around 10 ka appears to have caused the collapse of the deep-sea benthic ecosystem and reduced diversity at ca. 1800 meters water depth. During this and other Holocene events, diversity as measured by the Shannon Index was reduced by as much as 50%. Several lower resolution records also suggest rapid ecosystem change during Termination 1 in the central and northern North Atlantic region. For example, sites 82-24-4PC (mid-Atlantic Ridge) and M23414 (Rockall Plateau) reveal abrupt diversity shifts probably associated with bottom-water temperature and surface productivity changes. This presentation will assemble data on these and other fossil ostracod records from North Atlantic deep-sea sites to discuss possible causes of abrupt ecosystem changes and the application of Ostracoda to paleoceanography.
PP41B-1453
Mid-Holocene Transition in the Eastern North Atlantic; Foraminiferal Stable Isotope Evidence from Western Norwegian Fjord Sediments
Instrumental data and stratigraphic studies suggest that basin water in fjords with deep thresholds are influenced by incoming Atlantic waters. These water masses are transported by the Norwegian-Atlantic Current on the adjacent shelf areas. Benthic foraminiferal stable isotopes have been measured with a time resolution of c. 17 year through the time interval of 6300 to 2400 cal yr BP from the western Norwegian fjord; Voldafjorden. The core investigated, HM102-04SC, was raised in 1994 from the deepest basin of the fjord system at 700 m depth and the shallowest threshold to the open ocean is c. 200 m. The stable isotope analyses were performed on the two benthic species Uvigerina mediterranea and Cassidulina laevigata. The oxygen isotope records from both species show high frequency variations superimposed on a millennial scale trend. Between 5000 and 4200 cal yr BP the oxygen isotopes show a general increase to higher mean δ18O values; equivalent to c. 1° C cooling. After c. 4400 cal yr BP the high frequency amplitude in the δ18O records increase in both species. Assuming that oxygen isotopes primarily reflects temperature, the data suggest a change in amplitude from 1-2° C before 4400 cal yr BP to amplitudes comparable to 1.5-4° C after. Comparison of the two oxygen isotope records suggest that not all the variability can be explained by temperature alone, i.e. the δ18O record from C. laevigata generally has larger high frequency amplitude than the δ18O record from U. mediterranea. The discrepancy becomes more apparent after 4400 cal yr BP. Our data do however indicate increased high frequency climate variability following the 800 year Mid-Holocene transition, reflected by a change in oxygen isotope composition corresponding to about 1° C cooling. The isotope records from Voldafjorden show a similar type of high frequency variability to oxygen isotopes from planktonic foraminifera in the Norwegian Sea. However some planktonic records in the region do not record a clear Mid-Holocene transition towards heavier oxygen isotopes. Possible explanations and mechanisms for this will be presented.
PP41B-1454
Magnetite and Silicate Spherules from the GISP2 Core at the 536 A.D. Horizon
We examined the solid fraction of melt water from a depth of 361.45-361.55 meters in the GISP2 ice core. This subbottom depth covers the time from 536.15 to 536.66 A.D. roughly from February 24th to August 28th, 536 A.D.[1,2]. Earlier work measured a Cl value of 64 ppb from 535.9 to 536.2 A.D. roughly from November 24th, 535 A.D. to March 14th, 536 A.D. [3]. This is the highest nonvolcanic Cl value in the last 2000 years. The Cl value of 48 ppb from 536.2 to 536.5 A.D. or roughly from March 14th to July 1st, 536 A.D. is the third highest nonvolcanic Cl value in the last 2000 years. These high Cl values are at the same time as the beginning of an 18-month long period starting in March of 536 A.D. when ' the sun gave no more light than the moon' [4]. Because there is no evidence for a volcanic eruption during this 18-month time period, some authors have proposed a cosmogenic origin for the dust veil [5]. Our data supports the latter hypothesis. We have found 5 perfectly round, smooth spherules at the depth corresponding to early 536 A.D. Three of the spherules are pure iron oxide. They range in size from 0.3 to 1.3 micrometers in diameter. One spherule is mixed silicate plus iron oxide, with a diameter of 0.5 micrometers. One spherule is Na-K aluminum silicate, with a diameter of 2.6 micrometers. The spherules occur in association with crystals of titanium oxide and zircon, and partially crystalline Ca-Na feldspar. Associated K-Na feldspar and quartz grains have sharp edges. The largest grains in the sample are translucent C-O spheroids of volatile material that is sensitive to the electron beam. The translucent C-O spheroids often contain small amounts (0.2 to 2 percent) of Na and Cl. We also found some calcium carbonate precipitate. We interpret the perfectly round FeO and silicate spherules as impact spherules. The associated crystalline and sharp edged grains are also impact ejecta. The mixed chemistry of the spherules and the lack of Ni are inconsistent with their origin as ablation products from meteorites (i.e. cosmic spherules). Instead they must have originated from a terrestrial impact event. We have found an impact ejecta layer in the Gulf of Carpentaria, Australia that contains abundant magnetite spherules with quench textures and ablated surfaces. Using existing carbon-14 ages, the spherule horizon is from an impact that occurred prior to 900 A.D. and after 70 A.D [6]. The most likely source craters in the Gulf formed from an impactor that was about 640 meters in diameter, in line with the 600-meter estimate of [5] for the diameter of the impactor needed to produce the dust veil event of 536-537 A.D. Because the age and impactor size are both a good match within the errors of our data, the Carpentaria impact event is our best candidate for the source of the impact spherules in the GISP2 ice core. However, much more work remains before we can be certain that the impact into the Gulf of Carpentaria was the source of the dust veil event that began in March of 536 A.D. [1]. Meese et al., 1994. Science, 266:1680-1682. [2]. Sowers et al., 1993. Paleoceanography 8:737-766. [3]. Mayewski et al., 1997. J. Geophys. Res., 102:26345-26366. [4]. Stothers, 1984. Nature 307:344-345. [5] Rigby et al., 2004. Astron. Geophys. 45:1.23-1.26. [6] Abbott et al., submitted, Geophys. Res. Lett.
PP41B-1455
Holocene Climatic and Hydrologic Variability as Recorded in the Benthic Foraminifera Ammonia Beccarii From Tampa Bay, Florida
The objective of this study is to reconstruct the climate history of Tampa Bay, Florida over the Holocene epoch using the benthic foraminifera Ammonia beccarii from five sediment cores. Here we present a reconstruction based on oxygen isotopic ratios and Mg/Ca data that provides critical information on the history of climate changes in southwest Florida. Oxygen isotopes and Mg/Ca data from samples of A. beccarii taken from a series of five sediment cores provide records of temperature and salinity changes in Tampa Bay over the last 10,000 years. Sample age was constrained using a total of 21 AMS radiocarbon dates, 11 measured on A. beccarii and 10 measured on other material in the sediment (shell, bulk organic sediment, mollusk, organic sediment, and plant). The temperature reconstruction we present provides evidence of significant variability in the climate of Tampa Bay throughout the Holocene epoch, as indicated by a relative temperature range of 6° C. The highest reconstructed temperatures within this record are found from 1000-700 yr BP, which correlates with the commonly accepted timing of the Medieval Warm Period. The lowest temperatures reflected in this record occur from 500-150 yr BP, correlating with the timing of the Little Ice Age. This record also shows that relative temperatures have increased by approximately 3-4° C from 500 yr BP to present. The signal for δ18Osw was determined from δ18Ocalcite and relative temperatures reconstructed from Mg/Ca; changes in both δ18Osw and temperature are relative since the temperature calibration is not species specific. The results would be improved if a Mg/Ca temperature calibration for the species A. beccarii was developed and used. Values of δ18Osw fall within a range of 2.0 permil VPDB over the last 10,000 years, indicating significant changes to the hydrology of Tampa Bay. These results support evidence from the Gulf of Mexico for substantial hydrologic variability on the sub-centennial-scale. These initial results provide the first reconstruction of Holocene climate in Tampa Bay, FL.
PP41B-1456
Transitioning out of the middle Holocene - enhancement of ENSO cycles between 4 and 3 ka along the Pacific coast of North America
A synthesis of high-resolution marine and terrestrial proxy records between the latitudes of Mexico and Alaska reveals that modern atmospheric-oceanic conditions were established along the Pacific coast of North America between 4 and 3 ka. Along the northern and central California margin diatom assemblages, percent opal, and oxygen isotopes of planktonic foraminifers give evidence that spring upwelling increased abruptly between 4 and 3 ka in response to strengthening of the North Pacific High and intensification of upwelling-favorable, northwest winds. Alkenones and diatoms document a coincident rise in SST that is interpreted to represent a decline in fall upwelling. These results are supported by regional modeling studies by Diffenbaugh and Ashfaq (2007; Paleoceanography 22, PA3101). Wet-dry cycles become more pronounced along the California coast between ~ 4 and 3 ka, indicating a more prominent expression of ENSO cycles. Where marine records lie in offshore areas with reduced SST gradients (off Baja California, southern California, Vancouver Island, and in the central Gulf of Alaska), there is little evidence of a 4 to 3 ka change in SST. Nevertheless, proxy records of precipitation and upwelling in more coastal records of these regions support the conclusion that ENSO cycles were enhanced between ~ 4 and 3 ka. After ~4 ka, a more regular delivery of winter precipitation from North Pacific sources effectively ended the persistent drought that typified the middle part of the Holocene in paleoclimate records of the western interior of the US.
PP41B-1457
An Extreme Expression of the California Current during the Holocene: New Evidence from the Washington Margin
A multi-proxy bulk sedimentary geochemical and foraminiferal faunal study of a sediment core (TTN131-4 GC36; 46°30'N, 125°30'W, 1951m) from the continental slope off Washington State reveals millennial-scale variability in ocean circulation over the past 8000 years and offers a novel marine perspective on regional Holocene climate variability. Foraminiferal isotopes, faunal census data, and bulk sedimentary Corg, CaCO3 and trace metal concentrations were determined for GC36. Planktonic foraminiferal faunal records suggest cooler than present SSTs between 7 and 4.5 ka. Between 4.5 and 3 ky, benthic foraminifer δ18O and δ13C records display shifts towards more positive values (0.39 and 0.9‰, respectively), Corg increases, and sedimentary Re and U suggest a shift from less well oxygenated sedimentary conditions at 4 ka to more oxygenated sediments by 3.5 ka, an interpretation supported by benthic foraminifer assemblages. The Washington margin paleoceanographic data generally compliments previous studies from more southerly margin sites that suggest insolation-driven strengthening of the North Pacific High, resulting in a stronger California Current and increased regional coastal upwelling. This is expressed off the Oregon Margin as stronger upwelling and cooler SSTs as gyral waters are forced off shore. A reduction in the southward flow of the California Current and decreased upwelling has been proposed between 4.8 and 3.6 ka. Prior to ~ 4 ka, foraminiferal data and bulk sedimentary data support a stronger California Current System and coastal upwelling. Corg, however, displays an opposing interpretation that may be related to a change in preservation history. After ~ 4 ka planktonic foraminiferal assemblages and benthic foraminiferal data display increased variability, likely related to the weakening of the North Pacific high pressure system and the development of modern oceanographic and atmospheric circulation regimes.
PP41B-1458
High-Resolution Records of Mid-Holocene Paleoceanographic Change From the Subarctic Northeast Pacific Ocean
We present data from four new ultra-high resolution (SAR >2 mm/yr) marine sediment cores recovered along the margin of the Gulf of Alaska region in the Subarctic Northeast Pacific Ocean (SNEPO) recording fluctuations in detrital, biogenic, & authigenic sedimentary components. These regional records of climate change indicate four major regimes since the onset of the Holocene along the SNEPO margin. One of the most distinctive environmental shifts was the change from the relatively warm & moist conditions of the early Holocene Thermal Maximum (HTM) into colder & drier conditions that occurred between approximately 3000 - 7000 cal yrs BP in this high-latitude region. There is a key shift in both paleoproductivity proxies and redox- sensitive trace metal accumulation rates associated with this climatic transition. Based on observations of modern atmosphere-ocean-ecosystem interactions operating in the SNEPO, we interpret these biogeochemical shifts to reflect a change in the baseline mechanics of the atmospheric Aleutian Low (AL) pressure cell. The AL is the principal driving force that leads to nutrient upwelling in the Alaska Gyre, as well as the mechanism that controls coastal stratification via precipitation input & associated fluvial runoff. The measured changes in productivity and trace metals imply a millennial-scale oscillation in upwelling intensity and concomitant horizontal advection towards the more stratified waters of the coastal SNEPO. This oscillatory behavior lasts only 3000 years and terminates during a widespread glacial advance, when paleoproductivity indicators increase monotonically into the late Holocene. Both the magnitude and the millennial-scale frequency are statistically different from observational data of modern conditions in the SNEPO, suggesting that different mechanisms controlled the atmosphere-ocean-ecosystem linkage over this earlier time interval. Changes in high-latitude Northern Hemisphere summer insolation coupled to complex ocean-atmosphere feedbacks may be responsible.
PP41B-1459
DIATOM TRANSFER FUNCTIONS FOR SEA SURFACE TEMPERATURE AND PRIMARY PRODUCTIVITY IN UPWELLING AREAS: THE CUPEX PROJECT
Marine productivity plays an important role in natural carbon dioxide (CO2) variations through the "biological pump". This biological pump (mainly driven by diatom activity) is focused in specific regions, like the upwelling areas (less that 1% of the world's oceans total area), which support high export production. In these areas, atmospheric CO2 can be transported into the ocean. Therefore, to understand natural variations in atmospheric CO2 and other oceanic properties, we must understand the long-term history of these regional upwelling systems. Because of the interrelation between wind-driven coastal upwelling and diatom domination in phytoplankton blooms, diatom time-series from such areas can provide important information on climate change related variability in upwelling and nutrient supply. Therefore, upwelling areas represent key regions for oceanic properties reconstructions, which can be approached by the use of diatom Transfer Functions (TFs). Since the early 80's, TFs development became a major issue due to the extreme importance of understanding past ocean properties. However, the major focus has been on open ocean and calcareous microorganisms. Besides, the existing data for sea surface properties was a problem (data was cease and irregular), as such, some properties were more investigated than others. Today, with satellite data and better instrumental technologies, more and uniformly distributed information is available. Furthermore, the modern computer technology and power enable us to apply stronger and faster statistical tools and models. The development of TFs for coastal upwelling areas is one of the main goals of the CUPEX project: Coastal Upwelling Natural Variability: the last two Climate Extremes (21,000 ± 2,000 Cal-yr BP and 8,000 ± 1,500 Cal- yr BP), so that quantitative reconstructions of the environmental conditions of the last two past climate extremes: the Last Glacial Maximum and the Holocene Optimum are possible. Our environmental reconstructions comprise past nutrients content, sea surface temperature (SST) and primary productivity (PP), and the first results refer to the American coastal upwelling systems (the California and the Humboldt systems) and the Arabian Sea monsoon related upwelling system.
PP41B-1460
Mid- to Late Holocene Climate Variability in South-Central Chile on Millennial-to- Centennial Time Scales: Evidence From Sediments of the Coastal Lakes Lanalhue and Lleu Lleu (38°S)
The Holocene climate variability in south-central Chile is most notably regulated by changes in the latitudinal position and in the intensity of the Southern Westerly Winds (SWW). While a more southward position of the SWW during the early Holocene implied dry and warm conditions, a northward shift of the wind belt led to a general cooling trend and a parallel increase in precipitation towards the late Holocene. Here, we present new data from a multi-proxy study of Holocene lake sediments from Lago Lanalhue and Lago Lleu Lleu (38°S), including sedimentology, radiocarbon dating, mineralogy and geochemistry. Both of these coastal lakes developed in ancient river valleys that once drained to the Pacific Ocean. Due to localized tectonic uplift, the former rivers were dammed by rising sills during the early Holocene. Ever since, lacustrine sediments accumulated in the formed lake basins, preserving continuous records of Holocene climate variability. The sediment records of Lago Lanalhue and Lago Lleu Lleu indicate a long-term climatic trend with a significant shift from more arid conditions during the middle Holocene (8000 to 4200 cal yr BP) to more humid conditions during the late Holocene (4200 cal yr BP to present). In both lakes, the transition out of the mid-Holocene period is marked by a gradual increase in the terrigenous sediment supply. Coevally, the sediments of Lago Lanalhue record a decline in diatom occurrence, pointing to a decrease in lake productivity. On the other hand, numerous intercalated clastic layers characterize the late Holocene sediments in Lago Lleu Lleu, most likely suggesting an increase in the El Nino activity. During the last 2000 years, pronounced variations in the terrigenous sediment input suggest important hydrological changes also on the centennial time-scale. The sediment supply to the lakes was low during the Medieval Warm Period (1200 to 800 cal yr BP), indicating relatively dry conditions. In contrast, wet conditions prevailed during the Little Ice Age (800 to 100 cal yr BP) as inferred from enhanced terrigenous input.
PP41B-1461
Mid-late Holocene Sea Surface Temperature and Productivity Changes in the Continental Shelf of the East China Sea
Although the mid-late Holocene climate event about 4000 years ago (the 4 ka event) has been observed almost globally, this event was most prominent in terrestrial climate proxies obtained in the lower latitudes. Here we report Holocene sea surface temperature (SST) and productivity records reconstructed using biomarkers for Core B3 from the distal mud area on the continental shelf of the East China Sea (ECS), to evaluate the oceanic response to this event. The record reveals a large temperature drop of 5.5oC from the Holocene optimum (24.7oC at 5.7 ka) to the 4 ka event (19.2oC at 3.8 ka). This mid-late Holocene cooling period in Core B3 is correlated, within age uncertainties, with decreases in the East Asia summer monsoon intensity, and with the transition period to increased ENSO activities in the Equatorial Pacific. Our SST record provides oceanic evidence for the global nature of the mid-late Holocene climate change, most likely caused by the southward migration of the ITCZ in response to the solar insolation decrease. However, the large SST drop in B3 indicates that the cooling event was amplified in the continental shelf region of the ECS, most likely by the initiation/strengthening of location eddy circulation which caused upwelling and resulted in additional SST decrease. Upwelling induced by eddy circulation also increased surface productivity in the late Holocene, which was reflected in higher marine phytoplankton biomarker contents in Core B3. Significant productivity increases and phytoplankton community structure changes during the last few hundred years could be related to anthropogenic activities.
PP41B-1462
Hydrographic changes of the Kuroshio Current in the upper reach area during the past 6000 years
A 6000 year record of planktonic foraminiferal (G. ruber) Mg/Ca ratio and δ18O of Site ORI715-21 (22.7°N, 121.5°E ; 760 m water depth) at centennial resolution off the east coast of Taiwan was obtained. Age model was constructed by NN AMS radiocarbon dating points. The planktonic foraminifera δ18O profile shows a generally depleted trend from -2.4 to -2.8 permil, but Mg/Ca ratio shows a relative stable pattern with an average of ~28 degree C. δ18O values of sea- water were then calculated and show a decreasing trend from ~0.4 to ~0.0 permil during the last 6,000 years. These records indicate a stable sea surface temperature but decreasing salinities in the upper reach area of the Kuroshio current. The results are comparable to that of the Southern Okinawa Trough (MD01-2403) in showing similar pattern. The decreasing trend of SSS appears to be coincident to the tropical and temperate records of the Pacific Ocean but in difference from the East Asian Monsoon (EAM) records indicated by the Chinese speleothem δ18O records, which shows a weakening trend for the last 6,000 years.
PP41B-1463
Holocene History of the Western Entrance of the Strait of Magellan
Southwestern Patagonia (south of 50°S) remains one of the most poorly studied regions in the Southern Hemisphere despite its special interest and suitability for palaeoenvironmental studies. This region lies on the windward side of the Andes and is therefore particularly sensitive to the influence of the Southern Hemisphere westerly-winds and the strength of the Antarctic Circumpolar Current that reach South American coasts around 45°S. Its variety of different marine and terrestrial environments provides modern analogues to calibrate palaeoenvironmental studies, and post-glacial volcanic activity offers the potential for tephrochronology through the Holocene. To date, published palaeoenvironmental reconstructions for the Strait of Magellan sector mainly focus on its central part with only a few works referring to areas close to its western/Pacific entrance. Moreover, micropaleontological studies of marine sediments are still restricted to the deep-ocean off central and northern Chile. The existing ice caps in the region, in the Cordillera Darwin, Isla Santa Ines, and Gran Campo Nevado, joined the Patagonian Icefields during the last glaciation to form the Patagonian ice sheet that flowed into the Strait of Magellan to reach the Segunda Angostura area. The ice retreat from that point has left a series of deep basins (500-600m deep) along the Strait, ending in a shallower sill (currently ca. 60m deep) at its Pacific entrance. The timing and process of the opening of the Western entrance of the Strait of Magellan to Pacific waters is still poorly understood, although a recent work suggests the marine transgression occurred between 14,500 and 13,500 cal. yr BP. The present work implies that fully marine conditions were already established at approximately 9,000 14C yr BP. In this context, this work present the first results of an ongoing research project, focused on the study of shallow marine sediments from an area close to the Pacific entrance of the Strait of Magellan.(52°- 53°S), Chile. Sedimentological, biostratigraphical and tephrochronological techniques are applied to produce the first foraminiferal profile for the Holocene in the area, and minimum radiocarbon dates for the establishment of marine conditions on both sides of the Strait.