Paleoceanography and Paleoclimatology [PP]

PP33B MCC:Level 2 Wednesday

Early Holocene Climate Variability and the Timing and Extent of the Holocene Thermal Maximum (HTM): Comparisons From the Northern and Southern Hemispheres I Posters

Presiding: C Caseldine, University of Exeter; P Langdon, University of Exeter; C Turney, University of Wollongong


Millennial-scale climate cycles through the Holocene: A global, synchronous phenomenon?

* Turney, C ( , GeoQuEST Research Centre, School of Earth and Environmental Science, University of Wollongong, Northfields Ave, Wollongong, NSW 2522 Australia
McGlone, M ( , Landcare Research, PO Box 64, Lincoln, 8152 New Zealand
Wilmshurst, J ( , Landcare Research, PO Box 64, Lincoln, 8152 New Zealand

The present interglacial was until recently considered to be a period of exceptional climatic stability. Changes in the concentration of lithic grains through Holocene North Atlantic sediments, however, have now been shown to represent pervasive 900 and 500 yr cycles (loosely bundled together as a quasi-periodic '1500 yr' cycle), recording the southward advection of cold, ice bearing waters from the Labrador and Nordic Seas. Similar quasi-cycles have now been recorded throughout the Northern Hemisphere, consistent with Dansgaard Oeschger oscillations, suggesting a climate pacemaker, independent of glacial interglacial cycles and anthropogenic forcing. Antarctic ice core isotopic records provide strong evidence that this cyclicity is also present at high-latitudes in the Southern Hemisphere, supporting the contention that the changes are related to the strength of the thermohaline circulation and suggesting such events may be global in extent. To test this hypothesis, we have investigated six high-resolution Holocene sequences on a transect in the Southern Ocean that straddle important present-day climatic boundaries. Over 70 radiocarbon ages were obtained through the six sequences and calibrated using the Bayesian method option on the OxCal radiocarbon calibration program. Ages for the Subantarctic islands were complemented by changes in 'long-distance' pollen derived from the New Zealand mainland. Quasi-cyclic variations in surface moisture are recorded throughout the Holocene in many of the sequences. The wider implications of these results with regards millennial-scale climate change will be discussed.


Problems With Identifying the '8,200 Year Event' in Terrestrial Records of the Atlantic Seaboard: a Case Study From Dooagh, Achill Island, Ireland.

Head, K S ( , The Queen's University of Belfast, School of Archaeology and Palaeoecology, 42 Fitzwilliam Street , Belfast, BT7 1NN United Kingdom
* Turney, C S ( , The Queen's University of Belfast, School of Archaeology and Palaeoecology, 42 Fitzwilliam Street , Belfast, BT7 1NN United Kingdom
* Turney, C S ( , University of Wollongong, School of Earth and Environmental Sciences, Wollongong, NSW 2522 Australia
Pilcher, J R ( , The Queen's University of Belfast, School of Archaeology and Palaeoecology, 42 Fitzwilliam Street , Belfast, BT7 1NN United Kingdom
Palmer, J G , The Queen's University of Belfast, School of Archaeology and Palaeoecology, 42 Fitzwilliam Street , Belfast, BT7 1NN United Kingdom
Baillie, M G ( , The Queen's University of Belfast, School of Archaeology and Palaeoecology, 42 Fitzwilliam Street , Belfast, BT7 1NN United Kingdom

The Northern Hemisphere 8200 year cooling event is believed to represent the last known major freshwater pulse into the North Atlantic as a result of the final collapse of the North American Laurentide ice sheet. This pulse of water is generally believed to have occurred independent of orbital variations and provides an analogue for predicted increases in high-latitude precipitation and ice melt as a result of anthropogenically-driven future climate change. The precise timing, duration and magnitude of this event, however, is uncertain within the context of Ireland. Here, we investigated a high-resolution peat sequence at Dooagh, Achill Island on the west coast of Ireland to investigate whether the 8200 year event impacted on the terrestrial vegetation. We find clear evidence for an oscillation in the early Holocene using various measures of pollen, indicating a disruption to the vegetation with a sustained shift to open grassland, most probably driven largely by changes in precipitation rather than temperature. Radiocarbon dating of the event was extremely problematic, however, with bulk peat samples systematically too young suggesting significant contamination, potentially from percolating humic acids subsequent to the event. These results have major implications for high-precision dating of sequences in high rainfall areas. Nevertheless, assuming the oscillation in Dooagh is the 8200 year event, the sustained disruption to vegetation was of the order of hundreds of years, and not tens of years as suggested recently for the west coast of Ireland.


Separating climatic and human impacts in the early Holocene: biotic response around the time of the 8200 cal. yr BP event

* Edwards, K ( , University of Aberdeen, Department of Geography & Environment, Aberdeen, AB24 3UF United Kingdom
Langdon, P ( , University of Exeter, Department of Geography, Amory Building, Rennes Drive, Exeter, EX4 4RJ United Kingdom
Sugden, H ( , University of Sheffield, Department of Archaeology, Sheffield, S1 4ET United Kingdom

The early Holocene is characterised by rapid climate change events, which in the North Atlantic region are often associated with changes in thermohaline circulation. Superimposed on this in NW Europe is localised evidence for human impact on the landscape, although separating climatic and anthropogenic mechanisms for environmental change is often difficult. Biotic and sedimentological evidence from a lacustrine sequence from the Inner Hebrides, Scotland, shows a considerable reduction in inferred local woodland centred upon 8250 cal. yr BP. These data correlate precisely with a distinctive rise in the charcoal:pollen ratio and hence suggest Mesolithic human impact upon the vegetation around this time. A quantitative temperature reconstruction from chironomid analyses from the same sequence, supported by sedimentological data, indicates that the fall in arboreal pollen taxa occurred as climate warmed significantly during the early Holocene. This warming was followed by a significant cold event, with mean July temperatures reduced by 2 degrees C, that lasted for at least 320 years ca 7790-7470 cal. yr BP. Woodland recovered during this phase suggesting that the vegetation during the 8250 cal. yr BP interval was likely to have been responding to human activity, and not climate, and hence it is possible at specific sites to separate the influence of these key drivers of environmental change.


Assessing the Timing and Magnitude of the Holocene Thermal Maximum (HTM) Using Temperature Reconstructions from Chironomids

* Langdon, P G ( , University of Exeter, Department of Geography, Amory Building, Rennes Drive, Exeter, EX4 4RJ United Kingdom
Brooks, S J ( , Natural History Museum, Department of Entomology, Cromwell Road, London, SW7 5BD United Kingdom
Massaferro, J ( , University of Comahue, CRUB, Bariloche, 8400 Argentina
Gilchrist, S ( , University of Edinburgh, School of Geosciences, Drummond Street, Edinburgh, EH8 9XP United Kingdom

The magnitude and timing of Holocene maximum warmth has been the subject of considerable recent interest, particularly in the context of future climate change. Recent data syntheses from the Western Hemisphere of the Arctic (0-180 deg W; north of c.60 deg N) indicate that the HTM was time-transgressive over this region, with warming being particularly delayed in the vicinity of residual Laurentide ice, although the primary forcing was governed by orbital variations that scaled with latitude (Kaufman et al. 2004). Over 140 sites were used for this study, mainly relying on pollen and plant macrofossil reconstructions, with only 16 terrestrial sites providing quantitative reconstructions. Here, we present syntheses of HTM temperature reconstructions from chironomid stratigraphies using transfer functions based on modern calibration. Chironomids have been shown to be particularly sensitive indicators of summer temperatures especially during the early Holocene, where as well as indicating the timing and magnitude of peak warmth, they are also able to pick out minor climatic oscillations across a broad European transect (e.g. Caseldine et al., submitted; Brooks et al., in prep). The northern European chironomid reconstructions indicate HTM conditions from relatively high latitudes occurred around 10k cal. BP. Reconstructions from relatively lower latitudes, however, clearly demonstrate a relatively cool, unstable early Holocene that eventually leads to an HTM of conditions warmer than today after about 8k cal. yr BP that lasted for about 1.5-2.0k years, at least 2k years later than HTM conditions at higher Arctic latitudes. Some Holocene chironomid reconstructions have been undertaken from the southern hemisphere (e.g. Massaferro and Brooks, 2002), notably around central and southern Chile, although as yet no temperature-inference transfer functions exist for these regions. Preliminary comparisons will, however, be made between the different hemispheric chironomid records, relating in particular to the timing and magnitude of changes in chironomid fauna throughout the early Holocene. References Brooks, S.J., Heiri, O., Lang, B., Langdon, P.G., Velle, G. and Holmes, N. The response of Chironomidae to early Holocene summer temperature variability in northern and central Europe. In prep Quaternary Science Reviews. Caseldine, C.J., Langdon, P.G. and Holmes, N. Early Holocene climate variability and the timing and extent of the Holocene Thermal Maximum (HTM) in Northern Iceland. Submitted to Quaternary Science Reviews. Kaufmann, D.S. and 27 others. 2004. Holocene thermal maximum in the western Arctic (0-180W). Quaternary Science Reviews 23, 529-560. Massaferro, J. and Brooks, S.J. 2002. The response of chironomids to Late Quaternary climate change in the Taitao Peninsula, southern Chile. Journal of Quaternary Science 17: 101-111.


Diatoms as Proxies for a Fluctuating Ice Cap Margin, Hvitarvatn, Iceland

* Black, J L ( , INSTAAR and Geological Sciences, University of Colorado, 1560 30th St., UCB 450, Boulder, CO 80309-0450 United States
Miller, G H ( , INSTAAR and Geological Sciences, University of Colorado, 1560 30th St., UCB 450, Boulder, CO 80309-0450 United States
Geirsdottir, A ( , Department of Geosciences, Science Institute, University of Iceland, Dunhaga 3, Reykjavik, 107 Iceland

There are no complete records of terrestrial environmental change for the Holocene (11,000yrs) in Iceland and the status of Icelandic glaciers in the early Holocene remains unclear. It is not even known whether Iceland's large ice caps disappeared in the early Holocene, and if they did, when they re-grew. Icelandic lakes are particularly well suited to address these uncertainties as: 1) Glacial erosion and soft bedrock result in high lacustrine sedimentation rates, 2) Diagnostic tephras aid the geochronology, 3) Iceland's sensitivity to changes in North Atlantic circulation should produce clear signals in key environmental proxies (diatoms) preserved in lacustrine sequences, and 4) Ice-cap profiles are relatively flat so small changes in the equilibrium line altitude result in large changes in accumulation area. Hence, large changes in ice-sheet margins during the Holocene will impact sedimentation in glacier-dominated lakes and the diatom assemblages at those times. Hvitarvatn is a glacier dominated lake located on the eastern margin of Langjokull Ice Cap in central-western Iceland. The uppermost Hvitarvatn sediments reflect a glacially dominated system with planktonic, silica-demanding diatom taxa that suggest a high dissolved silica and turbid water environment consistent with high fluxes of glacial flour. Below this are Neoglacial sediments deposited when Langjokull was active, but outlet glaciers were not in contact with Hvitarvatn. The diatom assemblage here shows a small increase in abundance, but is still dominated by planktic, silica-demanding taxa. A distinct shift in lake conditions is reflected in the lowermost sediments, composed of predominantly benthic diatoms and deposited in clear water conditions with long growing seasons likely found in an environment with warmer summers than present and with no glacial erosion. Langjokull must have disappeared in the early Holocene for such a diverse, benthic dominated diatom assemblage to flourish.


A Comparison of Changes in Faunal Assemblages Over the Past 12,000 Years in Isafjardardjup, a Fjord in NW Iceland

* Quillmann, U ( , INSTAAR and Department of Geological Sciences, 1560 30th Street UCB 450, Boulder, CO 80309 United States
Andrews, J T ( , INSTAAR and Department of Geological Sciences, 1560 30th Street UCB 450, Boulder, CO 80309 United States
Jennings, A E ( , INSTAAR and Department of Geological Sciences, 1560 30th Street UCB 450, Boulder, CO 80309 United States

This study compares faunal assemblages over the past 12,000 cal yrs in the context of changes in stable isotopic composition and sedimentology at two different sites in Isafjardardjup, a fjord in NW Iceland. Isafjardardjup (66 deg N, 23 deg W) was chosen for its high sedimentation rates and for its sensitive location to both polar atmospheric and polar oceanic fronts. Core MD99-2266 was selected to represent the outer fjord environment and Core B997-339 the inner fjord environment. Faunal assemblages have changed dramatically throughout this period. {\it Elphidium excavatum} forma {\it clavata} an opportunistic species that thrives in unstable environmental conditions, was present in large numbers at both sites during the early Holocene when land-based glaciers calved and sediments accumulated rapidly. During warmer times, {\it Cassidulina laevigata} was dominant. The outer fjord site was dominated by {Cibicides lobatulus}, an indicator of strong bottom currents. {\it Cassidulina reniforme} and {\it E. excavatum}, both indicators of cooler, arctic waters and varying salinity concentrations, are prevalent at the inner fjord site, but are not so significant at the outer fjord site. Overall, infaunal species, such as {\it Nonion labradoricum}, have dominated the inner fjord site throughout the Holocene. {\it C. lobatulus} tests, found at both locations were analyzed for stable isotopic composition, which is a function of temperature, salinity, and global ice volume. The outer fjord site shows lighter oxygen isotopes overall, suggesting warmer and/or fresher bottom waters. The oxygen isotopes at the inner fjord site lighten around 8000 cal yr BP by 0.3 per mil, a shift not recorded at the outer site. The mass accumulation rates (MAR) in the outer fjord site were ten times as high as those in the inner fjord site in the early Holocene, five times as high in the middle Holocene, and roughly the same over the past 5000 years.


Variability of the Irminger current during the Holocene

* Justwan, A ( , Norwegian Polar Institute, Polar Environment Center, Tromso, 9296 Norway
* Justwan, A ( , Dept. of Earth Science, University of Tromso, Tromso, 9036 Norway
Koc, N ( , Norwegian Polar Institute, Polar Environment Center, Tromso, 9296 Norway
Jennings, A ( , INSTAAR, University of Colorado, Boulder, Co 80303 United States
Stoner, J ( , INSTAAR, University of Colorado, Boulder, Co 80303 United States

A high resolution sediment core, taken during the IMAGES cruise in 1999, MD99-2269 from the northern shelf of Iceland has been studied to assess the stability of the East Greenland - Irminger current system during the Holocene. The core has a 20m long Holocene section, which was studied at a decadal scale. The chronology of this section is established by 24 AMS dates and the Saksunarvatn tephra layer. Diatoms are utilized to reconstruct Sea Surface Temperature (SST). Three different transfer function methods have been used to reconstruct the SST at this site: WA-PLS, Maximum Likelihood and Imbrie and Kipp. The combined use of the three methods allows the SST reconstruction with a higher confidence level. Those three reconstructions generate very similar SST trend through the Holocene. The onset of the Holocene is characterized at the site by a steep temperature increase at 10 kyrs of almost 4C. The Holocene climate optimum is recorded between 7 and 4.5 ka. The SST is around 2C warmer during the major part of the Holocene compared to the SST for the last 1000 years. The cooling of the SST might be the result of a southward movement of the East Greenland-Irminger current system. The record at this site is compared to two other records, one south of the present site, LO 09-14, on the eastern branch of the North Atlantic Current, allowing a North south transect through the area. The second core MD95-2011 is located on the western branch of the North Atlantic Current and allowed the study of a east west transect through the area. The Holocene climate optimum occurs simultaneously at the studied site and at the LO09-14 site, further south. However, the Holocene climate optimum at the MD95-2011 site occurs earlier, suggesting a change in the strength of the western and eastern North Atlantic Current.


Advection of Atlantic Water to the Western and Northern Svalbard Shelves Through the Last 17.5 ka cal yr BP

* Slubowska, M A ( , Department of Arctic Geology, The University Centre in Svalbard, Longyearbyen, 9171 Norway
* Slubowska, M A ( , Department of Geology, University of Tromso, Tromso, 9037 Norway
Rasmussen, T L ( , Department of Geology, University of Tromso, Tromso, 9037 Norway
Koc, N ( , Norwegian Polar Institute, Polar Environmental Centre, Tromso, 9296 Norway
Kristensen, D K ( , Norwegian Polar Institute, Polar Environmental Centre, Tromso, 9296 Norway
Nilsen, F ( , Department of Arctic Geology, The University Centre in Svalbard, Longyearbyen, 9171 Norway
Solheim, A ( , Department of Arctic Geology, The University Centre in Svalbard, Longyearbyen, 9171 Norway
Solheim, A ( , International Centre for Geohazards, Norwegian Geotechnical Institute, Ulleval Stadion, Oslo, 0806 Norway

We have studied the distribution of benthic foraminifera species together with planktonic and benthic foraminifera abundances, stable oxygen isotopes and lithology in two cores: JM02-440 from the western (77 22' N, 12 48' E, 240 m water depth) and NP94-51 from the northern (80 21' N and 16 17' E, 400 m water depth) shelf of Svalbard. The purpose of the study was to reconstruct the changes in flow and character of the relatively warm Atlantic Water through the last 17.5 ka cal yr BP. The results from these two sites were compared with previously published records from the eastern Nordic Seas in order to follow the history of the advection of Atlantic Water as it moved northwards along the Norwegian coast and into the Arctic Ocean. Our results indicate that synchronous oceanographic changes occurred at the western and northern Svalbard shelves. The benthic foraminifera and oxygen isotope records indicate almost continuous presence of the Atlantic Water at the shelf areas since the deglaciation. The Bolling-Allerod period stands out as the warmest period in our records with the highest bottom waters temperatures indicating strong inflow of Atlantic Water. However, the warm Atlantic Water was isolated below cold and probably sea ice covered surface waters in contrast to the surface waters along the Norwegian coast, which experienced enhanced temperatures. During the Younger Dryas a freshening of the bottom waters occurred and the Polar Front was located in a proximal position to both sites. The strong inflow of saline, but chilled Atlantic Water happened during the Early Holocene. A distinct cooling and freshening of the bottom water masses occurred during the Mid- and Late Holocene, and was accompanied by glacier re-advances leading to the present-day conditions. During the last millennium, the inflow of Atlantic Water appears to increase, but the conditions turned unstable. The development of the paleoceanographic conditions at the western and northern Svalbard margins correlate closely to the SST variations in the Nordic Seas region since the deglaciation, and confirms that the Svalbard area is highly sensitive to the fluctuations in the inflow of Atlantic Water. The Early Holocene warming observed in the surface waters of the Nordic Seas correlate with the strong advection of Atlantic Water to the bottom waters of the western and northern Svalbard shelves and the Arctic Ocean. The warming was not just an effect of increased solar insolation, but was also due to the enhanced flux of Atlantic Water driven by the increased wind force and/or the thermohaline circulation.


Evidence for Early-Holocene Warmth in Southwestern Alaska: Biogenic Silica, Diatom Assemblage, and Diatom-Isotope Records From Ongoke Lake.

* Clarke, G H ( , Department of Plant Biology, University of Illinois, Urbana, IL 61801 United States
Henderson, A C ( , Department of Plant Biology, University of Illinois, Urbana, IL 61801 United States
Hu, F ( , Department of Plant Biology, University of Illinois, Urbana, IL 61801 United States
Hu, F ( , Department of Geology, University of Illinois, Urbana, IL 61801 United States

Evidence of a Holocene Thermal Maximum remains equivocal in Alaska where few non-pollen proxy records exist for Holocene climatic reconstruction. We analyzed sediment cores from Ongoke Lake ($59 15'N, $159 25'W, 400 m a.s.l., 7m. max. depth), southwest Alaska, for biogenic silica content (BSi) (at contiguous < 35 yr. resolution), sedimentary diatom assemblages (< 250 yr. resolution), and diatom oxygen isotopes (< 250 resolution). Fluctuations in BSi over the past 12,000 cal. yr BP appear to reflect lake primary productivity, which is probably related to climatic changes and associated variations in nutrient availability. Spectral analysis of the BSi record reveals pronounced productivity cycles (1200, 750, and 200 years), possibly caused by centennial to millennial variations in solar irradiance as evident in the BSi record of nearby Arolik lake ($59 28'N, $161 07'W). Four periods of high BSi (> 353 mg/g) each lasting {\it c.} 1,000- 1,500 yrs, delineated by negative excursions of short duration (< 100 yrs.), occurred between 10,000 and 4,500 cal. yr BP. These periods of high BSi coincide with diatom assemblages characteristic of higher nutrient concentration and a longer ice-free season. Reduced ice cover duration, strengthening of the lake's stratification, and alteration to the timing of lake mixing probably increased the internal nutrient loading of the lake leading to increased BSi and a rise in planktonic, mesotrophic diatom species. Between 4,000 and 500 cal. yr BP, BSi fluctuates around the mean Holocene value, and rises in the relative abundance of littoral, oligotrophic diatom species and those flourishing in more turbulent conditions suggest cooler conditions with increased storminess during the late Holocene. The millennial-scale climatic inferences from BSi and diatom assemblages are supported by the diatom oxygen isotope (δ18O$_{diat}$) data from the same lake, which suggests the early Holocene was warmer and drier in comparison to the late Holocene.


Going With the Flow: Evidence for Changes in Circulation in Seneca Lake, NY During the Holocene

* Crocker, M L ( , Hobart and William Smith Colleges, Geoscience Department, Geneva, NY 14456 United States
Curtin, T M ( , Hobart and William Smith Colleges, Geoscience Department, Geneva, NY 14456 United States

Seneca Lake (4240'N, 7655'W) is one of 11 Finger Lakes located in western New York State. Holocene laminated sediment, 1.52 m thick, was recovered from 47.1 m water depth in the northern part of the lake and yields a ~14 ka record of environmental and climate variability. We used a combination of loss-on-ignition (LOI) measurements, mineralogical analysis, grain size analysis by laser diffraction, and magnetic parameters (magnetic susceptibility and anisotropy of magnetic susceptibility (AMS)) to reconstruct changes in paleocirculation patterns throughout the Holocene. Magnetic susceptibility was measured at a 1 cm interval on the unopened core. Once the core was split, it was photographed and described. Samples for LOI, mineralogy and grain size analyses were collected every 2 cm. A 2.1 x 2.1 x 2 cm plastic cube with sediment was obtained continuously downcore for anisotropy of magnetic susceptibility analysis. The MS, AMS, mineralogy, LOI, and grain size data record two significant changes in environmental conditions in the lake, one at the beginning of the Hypsithermal (~9 ka) and the other during the late Neoglacial (<~4 ka). MS values range from 0 to 5.2 x 10$^{-5}$ SI. Susceptibility values are highest during the beginning of the mid-Holocene and lower throughout the rest of the Holocene with minor fluctuations. P$'$ values, which record the strength of the magnetic fabric, range from 1.02 to 1.11. There are two peaks in P$'$ values during the Holocene, one at ~9 ka and the other at <~4 ka as mentioned above. The coarsest mean grain size are coincident with the peaks in P$'$ values, with the largest mean grain size (35μm) found at the beginning of the Hypsithermal. A concurrent increase in P$'$ and mean grain size likely reflects an increase in depositional energy from a low velocity, quiet water setting. The relatively high P$'$ values most likely do not result from a change in magnetic mineralogy. The highest P$'$ values occur at the beginning of the Hypsithermal when carbonate concentrations are highest, between 30-35%. The combination of relatively finer grain sizes and low P$'$values of sediment deposited during the late Hypsithermal and part of the Neoglacial suggests there was more extensive reworking by currents or organisms, eliminating any preferred depositional alignment of grains as a function of lake currents or low current influence during this time. Overall, variations in median grain size, MS, and P$'$ indicate varying current strengths are responsible for deposition of sediment and reflect changes in lake circulation in response to changes in air temperatures and the position of the jet stream.


Reflections on Mud: Holocene Climate Variability Recorded by Laminated Sediments in Canandaigua Lake, NY

* Morgan, C K ( , Hobart and William Smith Colleges, Geoscience Department, Geneva, NY 14456 United States
Curtin, T M ( , Hobart and William Smith Colleges, Geoscience Department, Geneva, NY 14456 United States
Darden, W ( , Amherst College, Department of Geology, Amherst, MA 01002-5000 United States
Halfman, J D ( , Hobart and William Smith Colleges, Geoscience Department, Geneva, NY 14456 United States
Woodson, H ( , Trinity University, Department of Geosciences, San Antonio, TX 78212 United States

The Holocene sedimentary deposits in Canandaigua Lake, one of 11 Finger Lakes located in central New York, record evidence for environmental and climate change over the last ~14 ka. By analyzing high-resolution (2-12kHz) seismic reflection profiles and magnetic susceptibility, loss-on-ignition (LOI), grain size, and charcoal abundance of a sediment core collected from the profundal zone, temperature and precipitation variability is reconstructed. Seismic reflection profiles provide a three-dimensional view of sedimentation in the lake throughout the Holocene. The Holocene sequence ranges from 0 to ~10 m in thickness and is thickest in the middle of the lake, likely due to sediment focusing. The sequence contains several low amplitude, laterally continuous reflections that are traceable across the lake basin. Analysis of seismic reflection profiles allowed us to select a core site in an area with a relatively continuous, undisturbed record of Holocene sediment. A 4.71 m core was collected from 4244.63' N, 7719.69'W in the middle of the lake. Magnetic susceptibility was measured at a 1 cm interval prior to splitting the core. The core was opened, photographed, and described. Samples were collected at a 2 cm interval for LOI and grain size analysis, a 10 cm interval for charcoal analysis and smear slide examination, and at select intervals for thin section analysis. Preliminary data suggest that the low amplitude reflections observed in seismic reflection profiles are due to changes in density of the sediment that result from variations in grain size and packing. The core contains alternating dark gray and olive gray laminations that are often burrowed. Terrestrial organic material and ostracode shells are present throughout the core. Bulk magnetic susceptibility values range from 5 to 52 x 10$^{-6}$ SI. Fluctuations in susceptibility are greatest in the middle of the core. Concurrent high values (40-52 x 10$^{-6}$ SI) of magnetic susceptibility and mean grain size suggest a greater influx of water and terrestrial material to the lake during mid-Holocene storm events. Temperatures were likely warmest during the early to mid-Holocene in the Canandaigua Lake basin. Although the organic matter content is relatively similar (3-6%) throughout the Holocene, the carbonate content is greatest during the early to mid-Holocene. Higher carbonate concentrations are inferred to reflect warm surface water temperatures. Concurrent with the highest carbonate content values is the presence of laterally continuous, thin, black laminae rich in organic matter that accumulated during the early to mid-Holocene Hypsithermal (~9-5 ka). The presence of the black laminae indicates a steady supply of organic matter and very low to no oxygen in lake bottom water during accumulation of the organic- rich layer. The lake may have had occasional periods of thermal stratification during the summer and less frequent overturn during some parts of the Hypsithermal.


The Holocene Thermal Optimum in the Barents Sea

Murdmaa, I ( , Shirshov Institute of Oceanology, Russian Academy of Sciences , 36 Nakhimovsky Prosp., Moscow, 117997 Russian Federation
* Ivanova, E , Shirshov Institute of Oceanology, Russian Academy of Sciences , 36 Nakhimovsky Prosp., Moscow, 117997 Russian Federation
Risebrobakken, B , Bjerknes Centre for Climate Research, University of Bergen , Allégaten 55, Bergen, 5007 Norway
Akhrimenko, N , Geology Department, Lomonosov Moscow State University, Vorobyevy Gory, Moscow, 119992 Russian Federation
Yamskova, E , Geology Department, Lomonosov Moscow State University, Vorobyevy Gory, Moscow, 119992 Russian Federation

AMS 14C-dated oxygen isotope records on planktic and benthic foraminifer in our Core ASV 880 from the eastern branch of the Franz Victoria Trough (Northern Barents Sea) shows the Holocene thermal optimum at 7.8 - 6.9 cal ka BP (Duplessy et al., 2001) manifested by low oxygen isotope values as a result of warming of subsurface to bottom waters related to the increased Atlantic water input. Benthic foraminiferal assemblages during this optimum are enriched in the species indicating rather long ice-free season. We test here a hypothesis that the optimum can also be identified by foraminiferal assemblages in other parts of the sea with present water column structure indicating subsurface Atlantic water. We studied the Holocene benthic and planktic foraminiferal assemblages at several locations within the passage of the Atlantic water in surface and subsurface layers,and tried to identify the timing of assumed optimum by the interpolation between AMS 14C-dates when available, and using a level of polychaetas appearance in the Barents Sea at about 5 cal ka BP. The base of the Holocene layer is legibly distinguished in our cores by pronounced changes in sediment color, grain size, sedimentary structures, and an appearance of numerous foraminiferal tests. These two levels provide us with the tick-points for the age control in undated cores. An increase in abundance of species related to Atlantic-derived water is found at the end of Early Holocene in the south-western branch of the Franz Victoria Trough, whereas any faunal indication of the optimum are absent in other cores. On the contrary, our new oxygen isotope data clearly demonstrate the optimum at some locations. The maximum content of so-called "atlantic" benthic species occurs at different time levels in specific cores, and the same is true for planktic foraminifers generally considered to be related to the Atlantic water in the Barents Sea. Therefore, the above hypothesis is not confirmed by our data. It means that besides the water mass properties foraminifer distribution in the region is strongly controlled by food supply.


Holocene Climate Variability in the Mississippi Catchment as Inferred From Gulf of Mexico Sediments

* Meckler, A N ( , ETH Zurich, Geological Institute, Zurich, 8092 Switzerland
Haug, G H ( , GFZ Potsdam, Telegrafenberg, Potsdam, 14473 Germany
Poore, R Z ( , USGS, 600 4th Street South, St. Petersburg, FL 33701 United States
Roehl, U ( , MARUM, Leobener Strasse, Bremen, 28359 Germany
Thierstein, H R ( , ETH Zurich, Geological Institute, Zurich, 8092 Switzerland

For this study, we measured the elemental composition in a sediment core from Pigmy Basin in the Gulf of Mexico (GOM) in high resolution. Pigmy Basin is located on the Louisiana continental slope about 300 km southwest of the Mississippi River mouth. Since the Mississippi River system drains almost half of the conterminous US, we expect to see an integrated signal of North American climate in the GOM sediments. We analyzed the relative abundance of 8 major and minor elements (Al, Si, S, K, Ca, Ti, Mn, and Fe) by X-ray fluorescence core scanning. Thereby, we obtained data in 1cm as well as 2mm resolution, which correspond to 25 and 5 years, respectively. Most of these elements are derived from terrestrial sources and co-vary strongly throughout the core. Exceptions are Ca, which shows clear anticorrelation due to dilution effects, as well as S and Mn. The relative contribution of terrigenous elements shows large and periodic variations throughout the Holocene, which we interpret as changes in Mississippi transport. Interestingly, a peak in terrigenous element abundance occurs just before 8.2 kyr before present, a time of large but yet unexplained changes in many climate records. Poore et al. (2004, 2005) reconstructed the Holocene variability of surface circulation in the GOM at the same core. The observed periodic fluctuations were linked to changes in the mean position of the Intertropical Convergence Zone (ITCZ) and seem to covary with the strength of the North American monsoon. Overall, we find only a weak correlation of our elemental composition record with the ITCZ record of Poore et al. (2004, 2005), however. The same is true for other ITCZ recorders, such as the Ti record from Cariaco Basin off Venezuela (Haug et al. 2001). Hence, the first comparisons point towards an additional northern source for the observed signal in element composition.


Sub-Millennial Scale Climatic and Hydrologic Variability in the Gulf of Mexico during the Early Holocene

* LoDico, J M ( , University of South Florida, 140 7th Avenue South, St. Petersburg, FL 33701 United States
Flower, B P ( , University of South Florida, 140 7th Avenue South, St. Petersburg, FL 33701 United States
Quinn, T M ( , University of South Florida, 140 7th Avenue South, St. Petersburg, FL 33701 United States

Sediment core MD02-2550 from Orca Basin located in the northern Gulf of Mexico (GOM) provides a high-resolution early Holocene record of climatic and hydrologic changes from ~10.5 to 7 thousand calendar years before present (ka). Paired analyses of Mg/Ca and δ18O on the planktonic foraminifer {\it Globigerinoides ruber} (white variety, 250-355 μm) sampled at ~ 20 year resolution were used to generate proxy records of sea surface temperature (SST) and the δ18O of seawater in the GOM (δ18O$_{GOM}$). The Mg/Ca-SST record contains an overall ~(~1.5 C warming trend from 10.5 to 7 ka that appears to track the intensity of the annual insolation cycle and six temperature oscillations (~0.5-2 C), the frequency of which are consistent with those found in records of solar variability. The δ18O$_{GOM}$ record contains five ~0.5 ‰ oscillations from 10.5 to 7 ka that bear some resemblance to regional hydrologic records from Haiti and the Cariaco Basin, plus a ~ -0.8 ‰ excursion that may be associated with the "8.2 ka event" recorded in Greenland air temperatures. The δ18O$_{GOM}$ record, if interpreted as a salinity proxy, suggest large salinity fluctuations (> 2 ‰) reflecting changes in evaporation-precipitation (E-P) and Mississippi River input to the GOM. Percent {\it Globigerinoides sacculifer} records from three cores in the GOM exhibit remarkably coherent changes, suggesting episodic centennial-scale incursions of Caribbean waters. Spectral analysis of the Mg/Ca-SST and the δ18O$_{GOM}$ time series indicate that surface water conditions may be influenced by solar variations because they share significant periods of variability with atmospheric Δ14C near 700, 200, and 80-70 years. Our results add to the growing body of evidence that the sub-tropics were characterized by significant decadal to centennial-scale climatic and hydrologic variability during the early Holocene.


Multi-Decadal to Millennial Scale Holocene Hydrologic Variation in the Southern Hemisphere Tropics of South America

* Ekdahl, E J ( , Department of Geosciences, University of Nebraska-Lincoln 214 Bessey Hall, Lincoln, NE 68588-0340 United States
Fritz, S C ( , Department of Geosciences, University of Nebraska-Lincoln 214 Bessey Hall, Lincoln, NE 68588-0340 United States
Baker, P A ( , Division of Earth and Ocean Sciences, Duke University, Durham, NC 27708 United States
Burns, S J ( , Department of Geosciences, University of Massachusetts, Amherst, MA 01002 United States
Coley, K ( , Department of Geography, Royal Holloway College, Egham, Surrey, TW20 0EX United Kingdom
Rigsby, C A ( , Department of Geology, East Carolina University, Greenville, NC 27858 United States

Numerous sites in the Northern Hemisphere show multi-decadal to millennial scale climate variation during the Holocene, many of which have been correlated with changes in atmospheric radiocarbon production or with changes in North Atlantic oceanic circulation. The manifestation of such climate variability in the hydrology of the Southern Hemisphere tropics of South America is unclear, because of the limited number of records at suitably high resolution. In the Lake Titicaca drainage basin of Bolivia and Peru, high-resolution lacustrine records reveal the overall pattern of Holocene lake-level change, the influence of precessional forcing of the South American Summer Monsoon, and the effects of high-frequency climate variability in records of lake productivity and lake ecology. Precessional forcing of regional precipitation is evident in the Lake Titicaca basin as a massive (ca. 85 m) mid-Holocene decline in lake level beginning about 7800 cal yr BP and a subsequent rise in lake level after 4000 cal yr BP. Here we show that multi-decadal to millennial-scale climate variability, superimposed upon the envelope of change at orbital time scales, is similar in timing and pattern to the ice-rafted debris record of Holocene Bond events in the North Atlantic. A high-resolution carbon isotopic record from Lake Titicaca that spans the entire Holocene suggests that cold intervals of Holocene Bond events are periods of increased precipitation, thus indicating an anti-phasing of precipitation variation on the Altiplano relative to the Northern Hemisphere tropics. A similar pattern of variation is also evident in high-resolution (2-30 yr spacing) diatom and geochemical records that span the last 7000 yr from two smaller lakes, Lagos Umayo and Lagunillas, in the Lake Titicaca drainage basin.


Strengthening of PDO-like multi-decadal drought recurrence during the early Holocene: a high-resolution diatom record from the Northern Rocky Mountains

* Stone, J R ( , University of Nebraska-Lincoln, 214 Bessey Hall Department of Geosciences, Lincoln, NE 68566-0340 United States
Fritz, S C ( , University of Nebraska-Lincoln, 214 Bessey Hall Department of Geosciences, Lincoln, NE 68566-0340 United States

The diatom stratigraphy of Foy Lake, Montana is one of a very small number of continental records of sufficient resolution and length to detect patterns of change in severe multi-decadal drought recurrence throughout the entire Holocene. Time-series analysis of a decadally-resolved 13,000-year record suggests that the Holocene Thermal Maximum is a period of persistent drought cyclicity, inferred from regular (50-70yr) and pronounced oscillations between two dominant diatom groups representing deeper and shallower lake conditions. The onset of persistent multi-decadal drought cyclicity in the record is coincident with major changes in the boundary conditions at the end of the Last Glacial Maximum as a cooler, more stable climate is replaced by recurrent drought with frequencies similar to 20th century multi-decadal phase changes of the Pacific Decadal Oscillation. This multi-decadal variability is overprinted on millennial-scale oscillations, inferred to represent broad changes in the effective moisture of the Northern Rocky Mountains throughout the mid-Holocene. As solar insolation continued to wane, the pattern of multi-decadal drought became less stable, attenuating at approximately 4,500 years B.P.


Early Holocene Change in Atmospheric Circulation in the North-Central USA

* Dean, W E ( , USGS, MS980 Federal Center, Denver, CO 80225

Numerous proxies in cores from Elk Lake, northwestern Minnesota, have provided a record of climatic and environmental change with annual resolution for the last 10,000 years. The proxies that allow reconstruction of the lake's physical and chemical paleolimnology (diatoms, redox-sensitive trace metals, and 18O values) show that that prior to about 8.2 cal ka the lake was a stable, dimictic lake that was strongly stratified. The same proxies show that after 8.2 cal. ka the lake was turbulent, well-mixed and shallower. The proxies that are related to climate factors external to the lake (dust as % Al and % Si, varve thickness, and pollen) show that prior to 8.2 cal. ka the lake was receiving relatively little dust, implying little wind activity. After 8.2 cal ka, there was a marked increase in the influx of dust indicating an increase in westerly winds. Lastly, the ostracode faunal assemblages, which provide information about the limnology and watershed characteristics, indicate that, for 1000 years prior to 8.2 cal. ka, the lake was stable and dilute with characteristics typical of lakes in boreal forests. At 8.2 cal. ka, the ostracode assemblage abruptly shifted to an assemblage typical of Canadian prairie lakes that exhibit large seasonal variability in physical characteristics. This marks the northward displacement of the polar front and beginning of westerlies. The Elk Lake record further shows that the so-called 8.2 cal. yr cold event, recognized in ice-core and other records from the circum-North Atlantic, and thought by some to be caused by catastrophic drainage of freshwater from proglacial lakes Agassiz and Ojibway, was but a brief manifestation of a more fundamental and lasting change in the climate of North America. This fundamental climate change was the result of changes in atmospheric circulation in response to marked changes in the relative proportions of land, water, and, especially, glacial ice in North America during the early Holocene, the beginning of the altithermal or prairie period in Minnesota. Substantial changes in salinity also occurred at many lacustrine sites in the Northern Great Plains around 8.2 cal. ka so that such changes are not unique to Elk Lake, and thus the driver of these changes must be regional or global in extent. Bear Lake, Utah and Idaho, is a mesosaline-alkaline lake that historically was bipassed by the Bear River to the east of the lake. However, during the last glacial interval the lake and river were connected until about 16 cal. ka. As the salinity of the lake increased without Bear River influx, carbonate began to precipitate, first as calcite and then as aragonite, the dominant carbonate mineral deposited during most of the Holocene. C- O- and Sr-isotope data indicate that at about 9.5 cal ka the lake abruptly freshened suggesting that Bear River was again reconnected to the lake. The cause of the reconnection of Bear River with Bear Lake may have been tectonic, geomorphic, or climatic. However, the dominance of a boreal ostracode assemblage prior to 8.2 cal ka in Elk Lake, indicating that the polar front was far south of its present position, may have permitted the temporary establishment of a polar low over Utah, Idaho, and Wyoming in winter, increasing the snow pack in the Uinta Mountains and the Bear River Range to the west of the lake thereby increasing the surface- and ground-water flow to the lake. By 8.5 cal ka the salinity of the lake had decreased sufficiently to trigger another pulse of calcite precipitation that lasted about 1000 years. At 8.2 cal ka, when the Elk Lake ostracode record indicates that the polar front had retreated to the north, the Bear River was disconnected from the lake, and by 7.5 cal ka aragonite was again forming.


Geomorphic and sedimentary responses to strengthened early-Holocene monsoons: Evidence for contemporaneous 'events' in South Asia and Australia

* Goodbred, S L ( , Vanderbilt University, Earth and Environmental Sciences VU Station B 351805, Nashville, TN 37235-1805 United States

Monsoon strength is especially sensitive to regional insolation and thus generally varies with orbital cycles. However, the magnitude of this variability is affected by still poorly understood linkages with global boundary conditions (e.g., ice cover) and/or oceanographic feedbacks. Over the past 125,000 years, one of the most notable monsoon excursions was a dramatic strengthening during the early Holocene hypsithermal. In South Asia, the impact of this enhanced monsoon is recognized from alpine valley glaciers, bedrock valley alluviation, floodplain incision, and continental margin sedimentation, all indicating a system wide response to increased monsoon precipitation. No other similar geomorphic/sedimentologic episode has been recognized from the late Quaternary. Age control on these responses is not ideal, but encompass dates from 4-12 ka with most distributed 6-10 ka. More work is needed to constrain these ages, although they are clearly linked with the monsoon strengthening. Switching to the southern hemisphere, research in northeast Australia has documented very similar geomorphic/sedimentary behavior, such as floodplain incision and rapid continental margin sedimentation. The timing of these responses are effectively contemporaneous with those in South Asia, concentrating from 6-10 ka. Taken together, these findings support: 1) the occurrence of a brief (?) but dramatic strengthening of South Asian and Australian monsoons; 2) that this excursion was largely contemporaneous in both the northern and southern hemispheres; and 3) that erosional, transport, depositional systems responded acutely to these changes in climate.