PP13C-1455
Glaciation and Deglaciation Dynamics of Marine Isotope Stage 6 (160 - 140 kyrs BP): Land - Ice - Atmosphere Feedbacks
During the Late Quaternary, the Northern hemisphere has been affected by several glaciations and the ice sheet extents in Eurasia during these glaciations have recently been reconstructed by the Quaternary Environment of the Eurasian North (QUEEN) project (Svendsen et al. 2004). The QUEEN reconstruction of the peak Saalian (140 kyrs BP) ice sheet during Marine Isotope Stage 6 extends much more eastward and southward than the ice sheet during the Last Glacial Maximum (21 kyrs BP). However, this difference in spatial extent can be explained neither by the orbital parameters nor by the greenhouse gases that were similar during the LGM and the Late Saalian. To better understand how this Late Saalian Eurasian ice sheet could have developed so large and remained stable, our work focuses on the sensitivity of its surface mass balance to the regional parameters: dust deposition on snow, proglacial lakes (Colleoni et al., submitted, 2008a), vegetation cover (Colleoni et al., submitted, 2008b), prescribed sea surface conditions and reduced Laurentide ice volume. These regional parameters have been derived for the Late Saalian in part by adapting geological reconstructions for the Last Glacial Maximum since few geological records exist for the Late Saalian. We use an AGCM model (LMDZ4, Hourdin et al. 2006) to perform the climate and surface mass balance calculations. An asynchronous coupling with a 1D vegetation model (BIOME4, Kaplan et al., 2004) is carried out to create steady-state vegetation covers for our simulated period. References: Colleoni F., and coauthors, Influence of dust deposition and proglacial lakes on the surface mass balance of the Eurasian ice sheet during the peak Saalian (140 kya), Global and Planetary Change, submitted. Colleoni and Krinner, Sensitivity of the Late Saalian (140 kyrs BP) and LGM (21 kyrs BP) Eurasian ice sheet surface mass balance to vegetation feedbacks, GRL, submitted F. Hourdin and coauthors, The LMDZ4 general circulation model: climate performance and sensitivity to parametrized physics with emphasis on tropical convection, Climate Dynamics 27(7-8)(2006), pp. 783–813. Kaplan and coauthors (2004), Climate change and Arctic ecosystems: 2. Modeling, paleodata-model comparisons, and future projections, J. Geophy. Res., 108(D19), Issue D19, ALT 12-1. V. Peyaud, Rôle de la dynamique des calottes glaciaires dans les grands changements climatiques des périodes glaciaires-interglaciaires (Role of the Ice Sheet Dynamics in the major climate changes.), PhD thesis, 2006, Université Grenoble I, pp. 247. J.I Svendsen and coauthors, Late Quaternary ice sheet history of Northern Eurasia, Quaternary Science Reviews, 23(2004), pp. 1229-1271.
PP13C-1456
Limnocytherid Ostracod Paleoecology and Stable Isotope Chemistry: Evidence of Climate Change Derived From Lough Monreagh Sediment, Western Ireland
Biota preserved in marl lakes of western Ireland represent an excellent record of climatic variability in Western Europe and the North Atlantic Region. Oxygen isotope values of lacustrine biota conflate source precipitation and temperature variability while carbon isotope values serve as a proxy for Late Pleistocene and Early Holocene changes in vegetation and lake carbon cycling. Blanket bog carbonate sediments from Lough Monreagh in County Clare, Ireland, contain abundant ostracoda instars of Limnocytherina sanctipatricii and Metacypris cordata that were analyzed for stable oxygen and carbon isotope values. Basal marl (extrapolated age 18.3-17.9Kyr) contains an interval of benthic, cold water (4-5°C), Linmocytherina sanctipatricii with high δ13C values (-1 to +1‰VPDB) and low δ18O values (-7‰VPDB) suggesting Viséan bedrock control of lake water carbon chemistry and a moisture source with low δ18O values. This Limnocytherid bed represents a time when lake waters warmed to ~4°C (the preferred temperature for this species). Limnocytherina sanctipatricii make a second appearance at ~15.1 Kyr, in advance of Younger Dryas cold period represented by a clay layer in the core. They persist until 12.8Kyr when temperatures presumably became too cool for this species. As before, the ostracoda exhibit high δ13C values (0 to +2‰VPDB) through this period indicating relatively little terrestrial vegetation between deglaciation and the Younger Dryas. However, δ18O values are much higher than before (0 to - 3‰VPDB) indicating that the moisture source has changed. Limnocytherids return for a third time at 11.7Kyr, immediately following the Younger Dryas cold interval, when the climate had warmed enough to allow for recolonization. Their reappearance was short lived, however, and by 11.3Kyr steadily increasing temperatures caused their decline, while at the same time allowing for colonization by warm-water Paracandona euplectella and other Candonids. Lower δ13C and higher δ18O values in the Candonids represent a transition from bedrock dominated DIC of the lake water to terrestrial vegetation dominated DIC associated with increasing temperature and humidity. Further warming and increased humidity are inferred by the sudden appearance of very high concentrations of the littoral, warm water (14-20°C), eutrophic Limnocytherid Metacypris cordata at 8.7Kyr that exhibit low δ13C (-6 to -9‰VPDB) and high δ18O values (-2 to -4‰VPDB) that persist to the base of the peat at 7.8Kyr, when the lake becomes a blanket bog that limits ostracod shell preservation. The 8.2Kyr cold event appears at 8.425Kyr, evidenced by a 4‰ increase in δ13C and a sharp reduction in total ostracoda population from ~40 to 10 specimens per sample. Lough Monreagh isotope data from Limnocytherid fossil carapaces display a 12‰ shift in δ13C values over the last ~12,000 years indicative of a major shift in DIC control from bedrock weathering to vegetation adjacent to the lake. Oxygen isotope data infer changes in moisture source from meltwater-influenced surface waters during the Late Glacial and Younger Dryas periods and normal marine water during other periods.
PP13C-1457
Climate Change During the Late Glacial and Early Holocene From Lough Monreagh, Western Ireland: Evidence From Carbon and Oxygen Isotope Values of Lacustrine Marl
Lake sediment is an archive of climate variability reconstructed using proxy data such as stable isotope values and trace element chemistry. These proxy data permit reconstruction of temperature, precipitation, and terrestrial vegetation variability through time. Stable oxygen isotope values conflate temperature and water oxygen isotope variability, while carbon isotope values of marl provide a record of variability in dissolved inorganic carbon (DIC). An 8.67m core, obtained from Lough Monreagh, Western Ireland, contains marl ranging in age from about 17,300-7,894 cal yr B.P. Marl was analyzed at 1mm resolution for stable oxygen and carbon isotope values. Variation in sedimentation rate results in samples that represent up to five years at minimum resolution and as little as four months at highest resolution. Oxygen isotope values decrease at 13,500 yr B.P. from -3.5 to -5 per mil VPDB inferring a decrease in temperature followed by a warming period consistent with previous interpretations of the Older Dryas. Another decrease in oxygen isotope values (-3.9 to -7.0 per mil VPDB) occurs from 12,800-11,500 yr B.P. indicating a significant decrease in temperature consistent with the Younger Dryas. Carbon isotope values increase between 17,200-14,000 yr B.P. from 3.6 to 6.0 per mil VPDB suggesting high productivity is forcing the carbon isotope values of DIC higher than bedrock values, contrary to ostracod population data that suggest low productivity. Carbon isotope values decrease between 12,800-11,500 from 4.0 to 3.0 per mil VPDB coincident with the Younger Dryas clay layer, a time where marl was not produced in the lake. A 6 per mil decrease in carbon isotope values between 11,262-10860 yr B.P. from 4 to -2 per mil VPDB indicates a significant increase in terrestrial vegetation. Sample to sample variability nearing the top of the core is higher due to increased sedimentation rate and concomitantly the sample resolution increases. This study represents the highest resolution lacustrine record of climate change for Western Ireland to date.
PP13C-1458
Model Study on the Arctic Ocean Circulation During Early Miocene
The analysis of the sediment cores acquired during the Integrated Ocean Drilling Program Expedition 302 (2004) revealed that Arctic Ocean has been subjected to significant paleoceanographic changes during the Miocene epoch (23.03-5.33 Myr ago). The core showed that the Arctic Ocean transformed from a poorly ventilated state to a ventilated and fully oxygenated state during late early Miocene (17.5 Myr ago) (Jakobsson et al., 2007). This paleooceanographic change has been attributed to the opening of the Fram Strait, which is the major deep connection between the Arctic and other world oceans. To understand the oceanographic change and its relation to the opening of the Fram Strait, we have studied how the residence time of the water in the Arctic Ocean depends on factors such as strait geometry, freshwater supply and wind. For the present study we used the ocean general circulation model Modular Ocean Model (MOM4). The model has been integrated with both a detailed paleo-bathymetry and with a highly idealized basin. The paleo-bathymetry for the Early Miocene Arctic Ocean has been reconstructed by "morphing" the International Bathymetric Chart of the Arctic Ocean (IBCAO) gridded database using tectonic modeling and GIS techniques. The idealized model topography consists of two basins connected though a narrow strait over a sill. The model is started from rest with the initial conditions salinity 35 psu and temperature 0 degree Celsius. A uniform freshwater forcing of 0.16 Sv (1 Sv = 106 m3/s) is applied along the boundaries in the northern basin while salinity is restored to 35 psu in the southern basin. One method for computing residence time is based on flow rate obtained from meridional stream function. Another approach for estimating residence time is by integrating the model with a prognostic age tracer. In this method the age of the designated volume is fixed at zero (Atlantic Ocean/southern basin in this study), while all other grid points increase in age. The use of age tracer facilitates the detailed analysis of spatial variability and evolution of water age. The influence of strait width, freshwater and wind forcing on the residence time is investigated by different sensitivity experiments by reducing the strait width, by increasing the freshwater forcing and by forcing the model with a cyclonic wind stress. One interesting result is that the age of the water in the basin can vary significantly, both vertically and horizontally, with possible implications for the interpretation of sediment records.
PP13C-1459
Ecophenotypic differences in the genetic type of Globorotalia truncatulinoides
The shifting coiling direction of the morphospecies Globorotalia truncatulinoides has been widely used to reconstruct the environmental change in Paleoceanography. This morphospecies, however, is composed of four genetic types (1 to 4) having biogeographic distribution. The genetic type 2 has only having both of left- and right- coiling forms and limited its geographic distribution in the Atlantic Ocean. In the Sargasso Sea, all examined specimens (more than 250 specimens) belong to genetic Type 2 including the left- and right- coiling forms. These opposite coiling forms of Type 2 distributed at adjacent region, the left- coiling specimens are in the central water masses and the right- coiling in the subtropical gyre. At the boundary site between these two water masses, both forms show the different depth occurrence corresponding to the water column structure. From the worldwide view, the left- coiling Type 2 (Type2-L) is found only from the North Atlantic Ocean, while the right- coiling Type 2 (Type 2-R) is in the South Atlantic too. Type 2-L probably adapt to the hydrographic condition with a thick mixed layer in the center of gyre after diverging from another genetic Type 1. Young divergence of G. trucatulinoides genetic types could bring the situation that Type 2 has ecophenotypic variation in its coiling direction. This study supplies potentially good paleoceanographic indicator reflecting the migration of the sub-surface and intermediate water.
PP13C-1460
Stable Oxygen and Carbon Isotope Characteristics of Live Benthic Foraminifera from the Okhotsk Sea: Effects of Oceanography, Food Supply, and Microhabitat Patterns
Paleoceanographic studies use benthic foraminiferal stable isotopes as proxies for interpretations of numerous parameters such as past oceanic circulation patterns, food supply, primary productivity, etc. However, only few studies have used live (rose Bengal-stained) populations to reliably calibrate stable isotope characteristics to bottom water and sediment chemistry of the surrounding environment. We report results from a study in the Okhotsk Sea, a region characterized by extreme climatic and oceanographic settings. Not only does this marginal basin of the NW-Pacific experience the southermost extent of seasonal ice cover in the entire Northern Hemisphere, it also shows extremely high primary productivity. These boundary conditions lead many to consider the Okhotsk Sea both as a modern analog for ecological and oceanographic conditions in ocean basins during past and a sensitive recorder of potential future climate change in high latitudes. We compare results of stable oxygen and carbon isotopes from the most abundant taxa to oxygen isotopic compositions of bottom water and carbon isotopes of bottom water DIC, nutrient inventories from the water column and productivity proxy-data from sediment surface profiles (chlorines, TOC, biogenic opal). Multicorer samples from the upper 10 cm at 15 sites were taken from a variety of settings with water depths ranging from less than 100 m to more than 3200 m. Results obtained show a wide range of interspecific carbon isotope values exceeding 2 per mil variability within neighbouring samples. Minimum values occur in deep endobenthic groups like Globobulima spp., whereas species living in a relatively wide depth range like V. sadonica or U. peregrina exhibit intermediate values between –0.7 and –1 per mil. Most measurements conducted to address intraspecific variability remain within a narrow range of less than 0.4 per mil. However, we do observe vertical trends with both increasing and decreasing carbon isotope gradients within the sediment column. Obtained carbon isotope values from both living and dead specimen of widely used the Cibicides spp. group stay within the range of bottom water DIC, with no systematic negative phytodetritus- effect occuring throughout the sample set despite etremely pronounced seasonality in organic matter supply on most sites. Combined with a proxy-dataset about primary productivity, we give an evaluation of benthic–pelagic coupling and the impact on benthic species adaption to the pronounced subarctic seasonal cycle and the strongly pulsed food fluxes to the ocean floor.
PP13C-1461
Strontium Isotope Tracking of Pleistocene Solute Sources in Great Basin Pluvial Systems: Results From Searles Lake, Eastern California
Variations in the lithology of sediments deposited within Searles Lake (currently a dry pan in the southwestern Great Basin) are thought to represent fluctuations in past lake levels. Smith (1979, USGS Prof. Paper 1043) interpreted mud-rich lacustrine sediments (fine-grained carbonate, evaporitic salts, and clay-sized silicates) to represent deposition during wet periods, and salt-rich sediments (primarily halite and trona) to represent deposition during dry periods. The primary source of water during wet periods is thought to be overflow from the upstream Owens Lake to the north, which is fed by eastern Sierra Nevada stream water, while excess water from Searles Lake itself flowed downstream into Panamint Lake, occasionally culminating with overflow into Death Valley (Lake Manly). Previous work has demonstrated that the strontium isotope composition of Owens Lake waters has remained nearly constant over the past several hundred thousand years (87Sr/86Sr = 0.7092 ± 0.0001; Pretti and Stewart, 2002, Water Resour. Res. 38, 10.1029/2001WR000370). Thus, Sr isotopes provide a reasonably fixed endmember for tracking the contribution of Owens Lake waters (and solutes) to the downstream pluvial lakes, and may be useful for correlating highstands for all of the lakes within this pluvial chain. Here we report preliminary Sr isotope data from both mud- and salt-rich sediments sampled from Searles Lake core KM-3, with ages ranging from 10 to 900 ka (20-200 m below surface). Samples were sequentially leached with ultrapure water and acetic acid to extract salts (including carbonate), which should reflect the isotopic composition of the waters from which they precipitated. In general, the water- and acid-soluble fractions obtained from a given sample yield 87Sr/86Sr values within error of each other. Strontium isotope ratios range from 0.7085 to 0.7092, which expands the previously reported range of values for Searles Lake (0.7087-0.7089; Lin et al., Eos 74, 228). These results suggest an interplay between an Owens Lake water source and a local (groundwater?) source of solutes with a lower 87Sr/86Sr. The highest ratios were measured in mud-rich sediments, which would be expected for overflow events from Owens Lake during wet periods. However, 87Sr/86Sr ratios do not consistently correlate with sediment lithology. Diagenetic redistribution of strontium could play a role in the observed isotopic variations. Ongoing work is focused on transitions from mud-rich to salt-rich depositional environments at multiple scales within the core.
PP13C-1462
A Detailed History of the Eemian Interglacial Represented in the Ice Core Record from Mount Moulton Blue Ice Area (West Antarctica)
The blue ice area in the southern part of Mount Moulton (West Antarctica) exhibits a surface exposed stratigraphic section of ice ~ 500 kya old. We conducted high-resolution sampling (every 1 cm) on a ~ 40 meter long section of a surface trench excavated from this exposure. Using the Climate Change Institute continuous melter system we analyzed soluble major ions (Cl-, NO3-, SO42-, Na+, K+, Mg2+, Ca2+), trace elements (Zn, Pb, Hg, Cd, Cu, V, Mn, Ni, As, Al, Fe, Se, and REEs) and water isotopes from each sample yielding a temporal resolution of 6.5-7 years. The melted section covers the period from 107 to 136 ka BP based on 40 Ar/39 Ar radiometric dates from three englacial tephra layers dated by Dunbar et al. (2008), thus overlapping onset and termination of the Eemian. The resulting time-series provide a detailed record of the Eemian interglacial and match extremely well time series from other Antarctic cores covering this period of time. Some of the Mt. Moulton records shows that onset of the Eemian was very abrupt and occurred at about 135.5 ka BP, marked by a distinct drop in for example Ca concentrations. The end of the Eemain was more gradual. Interesting anomalies in the record will be discussed.
PP13C-1463
The cooling occurring during the lower Palaeozoic explained by the paleogeography
The Lower Palaeozoic climate was commonly assumed to be a long greenhouse period, the Late Ordovician glaciation taking place in a CO2 enriched atmosphere. Recent oxygen isotopic records in apatite revealed a cooling trend through the Ordovician, a climate change associated to a major radiation in the Earth Life history. If the cooling trend is established, the explanation for such a climatic change did not exist yet. By performing simulations with a climate model coupled to a geochemical model (GEOCLIM), we demonstrate that past changes in geography associated to the sea-level rise were important in bringing about cooling.
PP13C-1464
The Morphology and Distribution of Submerged Reefs in the Maui Nui Complex, Hawaii: New Insights Into Their Evolution Since the Early Pleistocene
Recent work on submerged drowned reefs in Hawaii has provided insight into reef development within the Late Pleistocene but reefs of the Early Pleistocene remain largely unexplored. The Maui-Nui Complex (MNC) provides a natural laboratory to study reef evolution throughout this time period as new data indicate the reefs grew from 1.1 – 0.5 Ma. We use new high resolution bathymetric data combined with existing regional data and field observations from ROV and submersible dives to make a detailed analysis of reef morphology and structure around the MNC. We focus specifically on the south-central region of the complex which provide the best reef exposure and find that the morphology of the reefs varies both regionally and temporally within this region. Barrier and pinnacle features dominate the steeper margins in the north of the study area while wide, shallow backstepping occurs to the south. Additionally, the central part of the study area shows karst morphology and patch and lagoonal features between the islands. We propose that this variation in the morphology and structure of the reefs has been controlled by variations in three main factors; the subsidence rates of the complex, the amplitude and period of eustatic sea-level cycles and finally the slope and continuity of the substrate. We argue that the interaction of these three factors explains the observed variations in reef morphology within the MNC and finally we present a new model of reef evolution over the last 1.5 Ma.
PP13C-1465
Carbon-Sulfur-Iron dynamics in K-G basin (Bay of Bengal)
Recent discovery of gas hydrate and exploration activities in Krishna-Godavari Basin (Bay of Bengal) has generated much interest in geochemistry of sedimentary fluid and diagenetic processes in this region. We are presently investigating the iron-sulfur-carbon coupled system in one of the cores collected on board Marian Dufresne-161 during gas hydrate exploratory cruise by the National Institute of Oceanography in May, 2007. Pore water sulfate concentrations show a 'S' shaped non-steady state pattern which may be attributed to combined effect of rapid sedimentation at the top and anaerobic methane oxidation (AMO) at the bottom of the profile (Hensen et al., 2003). At the studied core (MD161/8) the zone of AMO range from within 15 to 17 m below sea bed. Sedimentation rate ranges from 111 to 233.7 cm/kyr for the first 6 m of the sedimentary package. Total reducible sulfur (TRS = AVS + CRS + S°) concentration varies from 0.03 to 3.7%. Only negligible concentrations of AVS and S° could be recorded in few samples. TRS and FeD (dithionite reducible iron) show a smooth gradient and a negative correlation with (FeD) in the first 6 m which is followed by significant fluctuations in concentrations profiles. Below 6 m, TRS and FeD show dominantly positive correlation and few negative correlations. δ 34STRS varies from -40.6 to +15.7 permil, i.e., a fractionation (Δ δ 34Ssulf-TRS) 60.6 to 4.3 permil relative to sea water δ34Ssulfate. Sulfur isotopic fraction beyond 40-45 ‰ is attributed to disproportionation of intermediate sulfur compounds where as enriched sulfur isotopic compositions of TRS are due to Rayleigh fraction. Organic bound sulfur (OBS) concentration varies from 0.001 to 1.2%. Like TRS, OBS also show marked fluctuation with depth. OBS% does not show any correlation with total organic carbon content (TOC %) which ranges from 1.02 to 2.09%. A more detailed investigation is on to understand the influence of AMO on sulfidation, influence of iron concentrations on pyritization and mechanism of sulfur incorporation in the organic matter. A geochemical modeling will also be carried by integrating all the parameters.
PP13C-1466
The response of the freshwater fauna of Antarctica to past environmental changes
Palaeolimnological studies of environmental change focusing on select species are useful, yet cannot give information on changes in whole lake community structure and dynamics. Analysis of entire fossil zooplankton communities, in addition to interpretation of diatom, pigment and sedimentological data, can provide this community information and allow for the determination of a lakes palaeoecology. Through analysis of faunal microfossils from several Eastern Antarctic freshwater lake sediment cores, we have built up a detailed picture on the effects of both large and small-scale environmental events on Antarctic freshwater faunal communities. This knowledge is important for understanding the adaptability of the Antarctic fauna. Here we present the palaeoecology of three lakes; Waterfall Lake (Vestfold Hills), Lake Reid (Larsemann Hills) and Lake Terrasovoje (Amery Oasis) whose basal sediments range in age from pre-Last Glacial Maximum through to late Holocene. These lakes also represent a cohesive biogeographical unit, which is geographically isolated from western and maritime Antarctica and can therefore also provide important information on the palaeobiogeography of the region.
PP13C-1467
Organic Matter Reponse to Orbital Rhythms at Olduvai Gorge c. 1.80 Ma
Studies of African faunal evolution suggest intensified mammalian speciation and migration during the Plio- Pleistocene (c. 1.85 Ma) was triggered by regional aridification and high-amplitude climate variability. Marine sedimentological and stable isotopic studies indicate a changing moisture balance in East Africa during this time was paced by orbital-scale variations that influence African monsoon circulation and rainfall. Coeval lithostratigraphic sections from Olduvai Gorge, Tanzania demonstrates largely precessional (c. 21 kyr) Milankovitch forcing of lake sediment facies. Pollen studies and isotopic analysis of pedogenic carbonates from the same site indicate rainfall may have increased by one-third between cycles. However, evaporative and diagenetic soil processes can significantly alter the δ 18O of precipitated carbonate. Thus, isotopic records of material less subject to post-depositional alteration are pivotal for analysis of local hydrological regimes. We hypothesize isotopic analysis of lacustrine organic material can provide evidence for moisture regime change in the paleolake Olduvai catchment at a time marked by the first migrations of Homo from Africa. Here we report bulk organic δ 13C and compound-specific n-alkane δ 13C and δD data from paleolake Olduvai. Bulk carbon isotopic analyses show ~10‰ range with values as enriched as -16.9‰ and as depleted as -26‰. Furthermore, bulk δ 13C values vary in concert with calculated solar insolation between 1.5 and 2.0 Ma. Hydrogen and carbon isotopic signatures for plant- (n-C29 and n-C31) and algae-derived (n-C17 and n- C19) straight-chain alkanes also have a significant spread in isotopic values. The wide range of bulk and molecular isotopic values from Olduvai Gorge samples and strong correlation of the data with sedimentological and geochemical results from previous studies indicate stable isotopic analysis of lacustrine organic material can record hydrologic signatures. Specifically, stable carbon isotopes indicate fluctuating ecological input to the lake that occurs in tandem with insolation cycles and correlate to moisture availability. Variability of n-alkane stable hydrogen isotopic values indicate changes in local hydrology, and potentially indicate rainfall amounts. This work implies significant variability characterized the ecological and hydrological conditions faced by mammals at the Plio-Pleistocene boundary and potentially influenced natural selection processes associated with hominin evolution.
PP13C-1468
Illawarra Reversal: the onset of the end-Permian mass extinction
The Permian magnetostratigraphic records demonstrate that a remarkable change occurred in geomagnetism in the Late Guadalupian (Middle Permian; ca. 265 Ma) from the long-term stable Kiaman Reverse Superchron (throughout the Late Carboniferous and Early-Middle Permian) to the Permian-Triassic Mixed Superchron with frequent polarity change (in the Late Permian and Triassic). This unique episode called the Illawarra Reversal probably reflects a significant mode change in geodynamo in the outer core of the planet after a 50 million year-long stable geomagnetism. The Illawarra Reversal was likely led by the appearance of thermal instability at the 2,900 km-deep core-mantle boundary in connection with mantle superplume activity. The Illawarra Reversal and the Guadalupian-Lopingian boundary event recorded the significant transition from the Paleozoic to Mesozoic-Modern world. Major global environmental changes in the Phanerozoic occurred almost simultaneously in the latest Guadalupian, e.g., 1) mass extinction, 2) ocean redox change, 3) sharp isotopic excursions (C and Sr), 4) sea-level drop, and 5) plume-related volcanism. In addition to the claimed possible link between the above-listed various environmental changes and mantle superplume activity, here I propose an extra explanation that a change in the core's geodynamo may have played another important role in determining the surface climate of the planet and the course of biotic evolution. When a superplume is launched from the core-mantle boundary, resultant thermal instability makes the geodynamo's dipole of the outer core unstable, and lowers the geomagnetic intensity. Being modulated by geo- and heliomagnetism, cosmic ray flux from the outer space into the Earth's atmosphere changes along time. The more cosmic ray penetrates through the atmosphere, the more cloud develops to increase albedo, thus enhances cooling the Earth's surface. The Illawarra Reversal, the Kamura cooling event, and other unique geologic phenomena in the Late Guadalupian were all likely caused by the superplume activity that initially triggered the breakup of Pangea. The secular change of cosmic radiation may explain not only the extinction-related global climatic changes in the end-Guadalupian but also the long- term global warming/cooling trend in Earthfs history in terms of cloud coverage over the planet.
PP13C-1469
Evidence of impact material and the extinction of the mega-fauna 12,900 years ago
Approximately 13,000 years ago the landscape of North America was very different from what exists today. The Clovis people were hunting mammoths and other megafauna that roamed the land at the close of the Pleistocene. Evidence of these large mammals and the material remains of the Clovis inhabitants no longer appear in the geological or archeological records after 12,900 years ago. This coincides with the onset of the Younger Dryas (YD) climatic event. The onset of the YD appears to have been relatively abrupt and it lasted for ~1000 yrs. There have been many theories put forth to explain the sudden changes in climate and human culture, and the Pleistocene mega-fauna extinction that appear to have coincided with the onset of the Younger Dryas (YD) climatic event. Few theories, however, have adequately explained the concurrence of all three events. Nevertheless, few researchers disagree that something major happened 12,900 years ago. In 2007, Firestone et al proposed that an extraterrestrial event (ET) (e.g., meteorite impact) caused the onset of YD climatic event, which led to the extinction of the mega-fauna and the collapse for the Clovis culture. Skeptics argue that the ET markers that have been described by Firestone et al are not sufficient evidence to prove the occurrence of an ET event because evidence for an ET event such as an impact crater, tektites, shocked quartz, high temperature minerals, and impact material are absent. Here we present for the first time chemical and textural evidence of impact material from the Clovis-age, Murray Springs "black mat" layer, Arizona, USA. The impact material contains iron oxide spherules (framboids) in a glassy iron-silica matrix, which is one indicator of a possible meteorite impact. Chemical analyses of these spherules and the glassy matrices found in the particles from the larger size fractions are compared to the chemistry of carbonaceous chondrites, impact material associated with meteorite showers and meteorite craters, and tektites. The Fe-oxide rich glass matrix has an unusually high Si concentration, consistent with the chemistry of impact material from meteorite showers. The chemistry of the matrix is not consistent with terrestrial minerals, but is similar to the composition of a eutectic point in the SiO2- Fe3O4 system with a melting point of approximately 1500°C. Such a high formation temperature is only consistent with impact or fulgaritic conditions. However, the chemistry of the glassy matrices of the MS particles is vastly different from that of terrestrial material such as fulgerites.
PP13C-1470
A New Method for Producing Nanodiamonds Based on Research Into the Younger Dryas Extraterrestrial Impact
Research into a proposed extraterrestrial (ET) event 12.9 ka ago at the onset of the Younger Dryas revealed that for impact sediments (the YDB) that were tested across N America and NW Europe, all contain impact- related nanodiamonds ranging in size from 1 to 1700 nm. They appear in bulk sediment, but mostly occur inside carbon spherules and glass-like carbon, which are the charred, melted, amorphous-carbon byproducts of intense, impact-related wildfires. No diamonds were found stratigraphically above or below the impact layers. Selected area electron diffraction (SAED) by transmission electron microscopy (TEM) produced reflections of 2.06, 1.26, 1.07, and 0.89 A, which correspond to the lattice planar d-spacings of cubic diamonds. So-called "forbidden" reflections were also apparent at 1.78, 1.04, and 0.796 A, and these spacings are characteristic of a metastable cubic diamond polymorph called "n-diamond," the dominant form of diamond found in the YDB. N-diamonds have been produced under lab conditions and have been identified in meteorites, but they have never been found associated with mantle-derived diamonds. We have been able to reverse-engineer the process by which the impact-related n-diamonds form. First, various carbon-rich materials (coal, coconut shells, and wood) were charred at about 500°C under low- oxygen conditions. Next, the char was heated to more than 1000°C in a partially sealed vessel, while adding steam or nitrogen at near-atmospheric pressure. As a result, rounded, nanometer-sized domains of graphite formed in the char, to be then transformed into n-diamonds. To prevent combustion of the n- diamonds, the char was quenched under low-oxygen conditions. As it happens, this procedure is identical to the commercial process for producing activated charcoal, and in fact, samples of commercially available activated carbon manufactured by both Calgon Carbon Corporation and Norit Americas, Inc. were found to be enriched with n-diamonds. The process of forming n-diamonds requires conditions unlike any that are normal to the Earth's surface. However, the requirements match the extreme conditions that exist during an ET impact or airburst: (1) transient high temperatures; (2) an oxygen-poor (or steam-rich) atmosphere within the fireball and behind the shock front; and (3) the opportunity for quenching in a low-oxygen environment. Our research confirms that the YD nanodiamonds could not have formed under normal terrestrial conditions, and instead, required conditions consistent with an ET impact. Our research group has filed a provisional patent application for this previously unknown method of producing diamonds.
PP13C-1471
Soot as Evidence for Widespread Fires at the Younger Dryas Onset (YDB; 12.9 ka)
Evidence continues to grow in support of a major extraterrestrial (ET) impact, which was the primary trigger for the late Pleistocene megafaunal extinction in North America at the onset of the Younger Dryas (12.9 ka). Sediment at the base of a carbon-rich, dark layer (the YDB) is marked by peaks in magnetic microspherules and grains, iridium, nanodiamonds, and other materials consistent with a major ET event (Firestone, 2007; Kennett, 2008). This layer also exhibits above-background levels in charcoal, carbon spherules, glass-like carbon, and polycyclic aromatic hydrocarbons indicative of continent-wide biomass burning, which is coeval with evidence for major abrupt increase in biomass burning in the Greenland Ice sheet (Mayewski, 1993; Legrand, 1997). The synchronous, widespread presence of soot in high abundances is well-accepted as a marker for extensive, intense impact-related wildfires. By conducting a soot analysis, we tested the possibility that the explosion of the impactor triggered combustion of terrestrial biomass, as hypothesized for the Cretaceous- Tertiary (K/T) 65 Ma ago (Wolbach, 1985). Previous analyses of samples from North America, Germany, and Belgium yielded YDB soot at two sites in North America containing other impact markers: Murray Springs, AZ, with a soot content of 20 +/- 2 ppm; and at Blackville, SC, with a soot content of 2000 +/- 200 ppm (Wolbach, 2007). In this study, we report results from analyzing twenty-two additional samples from six sites in North America and Europe: Arlington Canyon, and nearby Arlington Springs, CA; Bull Creek, OK; Hall's Cave, TX; Murray Springs, AZ (new sampling); and Lommel, Belgium. Dissolution and analysis procedures were based on those used successfully for detecting soot from impact-produced wildfires at the K/T boundary, as previously described (Wolbach, 1985, 1988, 1989, 1990). The YDB layer at four of these sites (nine total samples) contains significant quantities of soot: Arlington Canyon, CA, at 2000 +/- 200 ppm; Bull Creek, OK, at 500 +/- 50 ppm; Hall's Cave, TX, at 2000 +/- 200 ppm; and Murray Springs, AZ, at 6000 +/- 600 ppm, thirty times higher than previously observed at this location. Samples from the other two sites (Arlington Springs; Lommel) lack detectable soot in the YDB layer, in spite of independent evidence for biomass burning, such as charcoal and carbon spherules, probably because local depositional conditions were too oxidizing for soot preservation. Samples from above and below the YDB layer at all sites displayed no detectable soot. The presence of significant YDB soot at five separate locations up to 3500 km apart across North America (CA, AZ, OK, TX, and SC), in combination with other pervasive wildfire evidence, suggests major, widespread burning and aeolian transport of soot across North America ~12.9 ka ago. These results support a size and energy of impact sufficient to ignite continental-scale fires.
PP13C-1472
Elemental Analysis of the Sediment, Magnetic Grains and Microspherules from the Younger Dryas Impact Layer
We have analyzed the elemental content of sediments, magnetic grains and microspherules found in the 12.9
ka Younger Dryas boundary layer (YDB) at eight North American Clovis-age sites, a site in Belgium, and 16
Carolina Bays using Prompt Gamma-ray Activation Analysis (PGAA), Instrumental Neutron Activation Analysis
(INAA), X-ray fluorescence (XRF), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and low-
background gamma-ray counting techniques. Magnetic grains, which sharply peak in the YDB, have typical
terrestrial composition at the Gainey, MI site but are highly enriched in titanium averaging 22 wt.% TiO2
at other sites with an average ratio TiO2/FeO=0.73. Magnetic grains, which were distributed
throughout the rims of the Carolina Bays, averaged 15 wt.% TiO2 with an average ratio
TiO2/FeO=0.79. Magnetic microspherules, found only in the YDB at all sites including Gainey,
averaged 34 wt.% TiO2 with an average ratio TiO2/FeO=0.77. The nearly identical
TiO2/FeO ratios for magnetic grains and microspherules suggest that they have a common origin.
TiO2/FeO=0.12 in Earth's crust, and no terrestrial or extraterrestrial sources have such high ratios
except for lunar KREEP with TiO2/FeO=0.63 in Apollo 17 basalt. REE concentrations in magnetic
grains have negative Eu signatures characteristic of KREEP and the relative concentrations of Sc, V, Mn, Co,
Ni, As, Zr, Sb, Ce, REE, Hf, W, Ta, Th, U, and other trace elements in magnetic grains are consistent with
KREEP ratios at all sites except Gainey. Potassium is significantly enriched in 40K in YDB sediments
and Clovis chert which is consistent with known Fe-rich meteorites and 40K/K ratios inferred from a
comparison of Lunar Prospector Gamma-ray Spectrometer (GRS) and ground truth data. The high
concentration of magnetic grains with terrestrial composition at Gainey suggests that they are local ejecta
from a nearby impact site. High water content was found in the magnetic grains at all sites, e.g. 18 at.% H at
Gainey, which is consistent with their formation in a steam explosion following an impact into the nearby
Laurentian Ice Sheet. Although no craters have been identified with the YDB impact, four deep holes
extending to 193-723 feet below sea level in Lakes Superior, Michigan, Huron, and Ontario are candidates.
Also in Lake Ontario, a proposed Pleistocene-aged 1-km impact crater has been identified at Charity Shoal
where a negative magnetic anomaly also exists. Similar titanium-rich compositions of magnetic grains and
microspherules at numerous sites far from Gainey suggest that they are ejecta emitted directly from the
airburst of the impacting object. The major axes of the Carolina Bays are oriented towards the Great Lakes
and the KREEP-like composition of magnetic grains found mixed throughout the Bays establishes their origin
in the YDB impact, but the process that formed the Bays is not yet known. The similarity between YDB
magnetic grains and microspherules to lunar KREEP is a mystery but it is interestingly that SAU-169, a
meteorite ascribed to lunar Procellarum KREEP Terrane origin, landed in Oman near the time of the YDB
impact.
http://ie.lbl.gov/Mammoth/Impact.html
PP13C-1473
Nanodiamonds in a Stratigraphic Datum Layer Correlated with the Continent-Wide Younger Dryas Impact Stratum (YDB) at 12.9 ka
Firestone et al. (2007) presented evidence for a latest Pleistocene extraterrestrial (ET) impact over North America at 12.9 ka (11,000 RCYBP), coinciding with the onset of the Younger Dryas Climatic Episode. This hypothesis was based on the identification of multiple markers of ET origin found in an impact layer (YDB) at many locations across North America and NW Europe. We present new evidence in support of a continent- wide stratigraphic datum layer based on widely distributed and abundant nanodiamonds formed during the ET impact and preserved in the YDB. Of several hundred samples analyzed from 16 sites on two continents, we have yet to find YDB sediments lacking nanodiamonds, and we have yet to find non-YDB late Pleistocene sediments containing nanodiamonds. This widespread distribution of nanodiamonds is currently most clearly demonstrated in carbon spherules (mean diameter ~100 microns) found in the YDB layer at many locations across the continent and in NW Europe. The n-diamonds in these spherules are the most abundant diamond allotrope found. All three allotropes of diamond have been consistently identified using high-resolution scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with confirmation by selected area diffraction (SAED). We also report a large abundance of nanodiamonds in bulk sediments from the YDB layer at sites in Michigan, North Carolina, South Carolina, Arizona, and Oklahoma. Established protocols for acid-digestion were used to isolate the diamonds, except for eliminating the phosphoric and perchloric acid steps, which were found to oxidize the diamonds. Labor-intensive studies are underway on bulk sediments collected from stratigraphic transects at 11 additional sites across North America and NW Europe. We have found that residues resulting from the acid-digestion are significantly larger (i.e., heavier) in YDB samples, regardless of diamond abundance. The increased mass largely results from high concentrations of acid-resistant amorphous carbon, which itself appears to be an identifying characteristic of the YDB impact layer. Nanodiamonds in YDB sediments are widespread probably as a result of atmospheric transport of these nano-sized particles during the impact event. The YDB layer thus appears to be an increasingly valuable and easily discernable datum layer of unusual continent-wide extent, as identified by an increasing array of ET proxies including nanodiamonds. Its value is further strengthened by its frequent stratigraphic position at the base of a widely dispersed and conspicuous, carbon-rich sedimentary layer. If confirmed, the presence of such a widespread, well-dated, and readily identifiable datum should assist with better understanding of human, biotic, climatic, and other environmental and ecological processes of the last deglacial episode on continental scales.
PP13C-1474
Bull Creek Environment and the ET Event
A recent hypothesis states that an extra-terrestrial (ET) event, in this case a comet explosion over NE North America, initiated the Younger Dryas chronozone--~11,000-10,000 radiocarbon years before present (RCYBP)--and left event specific markers including magnetic grains with iridium, magnetic microspherules, charcoal, soot, carbon spherules, glass-like carbon containing nanodiamonds, and fullerenes with ET helium. This hypothesis results in several predictions, including shuffling of plant and animal communities, disruption of human cultural adaptations, and megafaunal extinctions immediately following the ET event. This poster investigates the question: Does evidence exist for a disruptive event centered at ~11,000 RCYBP in the paleoenvironmental proxy records within the Bull Creek project area, Oklahoma Panhandle, USA? Evidence from multiple proxies, including particle size distribution, stable carbon isotopes, pollen, and phytoliths, is employed to reconstruct the paleoenvironment and search for discontinuities in the data sets.
PP13C-1475
The Effect of the Younger Dryas on Paleoindian Occupations in Eastern North America Evidence from Artifactual, Pollen, and Radiocarbon Records
The Younger Dryas appears to have been a period when human populations in Eastern North America were undergoing significant stress. Attribute and locational data on Paleoindian materials from across the continent is available from the Paleoindian Database of the Americas or PIDBA, available on-line at http://pidba.utk.edu. Tallying the diagnostic projectile point sample from PIDBA in the on-line "entire sample" dated 24 April 2008, suggests that a population decline followed by a rebound may have occurred, particularly in southeastern North America. Following Clovis, fluted points with deeply indented bases and short to full flutes occur in many areas, such as the Redstone, Barnes, Cumberland, and Folsom types. In southeastern North America, these are thought to have been replaced by unfluted lanceolate and waisted forms, including the Beaver Lake, Suwannee/Simpson, and Quad types, which are in turn replaced by Dalton forms. Within the Southeast, a significant decline occurs between Clovis (N=1993 points) and presumably immediate post-Clovis full fluted forms (N=947 points). This may correspond to a similar decline in population, assuming the point types occurred for comparable periods of time, and were used in a similar fashion. Goodyear noted a similar pattern between Clovis (n=179) and presumed immediate post-Clovis Redstone (n=40) forms in South Carolina. Comparable declines have also been observed in North Carolina by Daniel and Goodyear ( and in Virginia by MacAvoy. In the PIDBA database across the Southeast, projectile point numbers increase following the immediate post Clovis decline, from 947 full fluted to 1717 unfluted and then 2594 Dalton points. The increase in the latter part of the Younger Dryas may actually be even more pronounced, since Dalton points, which are quite widespread, are only systematically recorded in a few states. Radiocarbon dates from Paleoindian and Early Archaic assemblages reveal a similar pattern. Dates falling in the initial centuries of the Younger Dryas are decidedly uncommon in the Southeast and over the larger region. In a sample of 218 dates from the Southeast and adjoining areas, only seven fall between 10,900 and 10,570 14C yr BP or between ca. 12,850 and 12,600 cal yr BP, and all of these are at the recent end of this range, between 10,570 and 10,710 14C yr BP. In the Southeast, at least, there appears to be a ca. 250-300 year 'gap' in the distribution of radiocarbon dates, with few reported from ca. 12,900-12,600 cal yr BP. Finally, analyses of fossil pollen records from the southeastern United States, although admittedly few in number, indicate that major vegetational shifts, both abrupt and characterized by oscillations, were occurring across the region during the Pleistocene/Holocene transition and that these shifts appear to be synchronic or quasi-synchronic with the Younger Dryas. The results suggests that the earliest inhabitants in the southeastern United States were faced with an environment that was anything but stable.
PP13C-1476
Hexagonal Diamonds (Lonsdaleite) Discovered in the K/T Impact Layer in Spain and New Zealand
We present the first evidence from Cretaceous-Tertiary (K/T) boundary clay and rock for shocked hexagonal nanodiamonds (lonsdaleite), these being found in concentrations greater than 50 ppm at Needles Point, New Zealand, and Caravaca, Spain. This is also the first evidence for K/T diamonds of any kind outside of North America. No diamonds were detected immediately above or below the impact layer. Cubic diamonds have been reported earlier from North American K/T sediments by Carlisle and Braman (1991; 45 ppm) and Hough et al. (1997; 18 ppm), but lonsdaleite was not detected. Carlisle and Braman suggested that the cubic diamonds arrived already formed within the impactor, but Hough argued that they were shock-produced by the impact with Earth. Hence, it is not yet clear that K/T cubic diamonds were formed through shock. Lonsdaleite does not co-occur with terrestrial diamonds but is found with cubic diamonds in ET impact craters (e.g., Popigai, Sudbury). Both also have been reported in the impact layer of the proposed Younger Dryas impact event at 12.9 ka. Lonsdaleite is formed by shocking carbonaceous material, e. g., graphite, under extreme conditions of pressure and temperature (more than 15 GPa at more than 1000° C), thus making this mineral an excellent impact-shock indicator (DeCarli, 2002). Although lonsdaleite is also contained in meteorites, such as ureilites, there appears to be a consensus of opinion that crater-related lonsdaleite formed during ET impact. K/T sediment samples were acquired from the boundary layer, as well as above and below. To extract the diamonds from the sediments, we utilized the protocol from Amari (1994) and Huss and Lewis (1995), but modified their methodology after determining that phosphoric and perchloric acids oxidize metastable lonsdaleite. We extracted the diamonds successfully after eliminating those acids, which may explain why lonsdaleite was not apparent in extractions by others. The extracted lonsdaleite was analyzed by transmission electron microscopy (TEM) and by selected area diffraction (SAED), which displayed characteristic reflections corresponding to lattice planar spacings of 2.18, 1.26, 1.09, and 0.82 A. A scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS) confirmed their carbon composition. With exposure to long-wave ultraviolet (365 nm) radiation, clusters of lonsdaleite crystals exhibited a blue fluorescence that is characteristic of many diamonds. Individual crystals were angular to sub-rounded in shape and ranged in size from 20 to 1000 nm, with a mean size of about 50 nm. This discovery represents (1) the strongest available evidence for K/T diamond formation during the impact; (2) the first discovery of K/T diamonds outside North America; and (3) the first occurrence of any form of K/T diamonds in the Southern Hemisphere, about 12,000 km from the Chicxulub Crater in Mexico.
PP13C-1477
End-Pleistocene Soil Constituents from Selected Sites on the Mid-Atlantic Coastal Plain: First Results
Stratigraphic analyses of soil samples taken from dated and undated sites located along the mid-Atlantic Coastal Plain have yielded evidence of increased contemporary biomass burning, compared to under and overlying strata. Host strata ages are known or projected to bracket the onset of the Younger Dryas cooling episode at 12.9 cal ka. This ongoing investigation includes samples from: 1) a late-Pleistocene aged periglacial feature located within the Pine Barrens of southern New Jersey; 2) an artifact dated stratum (~ 12.9 ka) in an embankment on the Chesapeake Bay in Maryland; and 3) an as yet undated (C14 test results pending) embankment of the Perquimans River in northeastern North Carolina projected to be age-appropriate. Sample analysis of scanning electron (SEM) micrographs from the Chesapeake Bay site revealed charred fragments of late-Wisconsinan Krummholz birch (Betula) and species of spruce (Picea) and fir (Abies), which are not extant on the modern-day, temperate Coastal Plain. In addition, organic faunal material is found in association with ancient charred boreal wood, including hollow hair and skin fragments that are as yet unidentified, perhaps from cold climate adapted animals as inferred from host sediment age. Charred wood fragments are found to be attracted to a neodymium magnet. Some aggregates of organic matter appear to contain magnetic spherule-like grains whose composition is awaiting geochemical analysis. Photomicrographs of all specimens and a stratigraphic breakdown in the relative amount of burned carbon associated with each site and strata will be presented, along with the results of various analyses that are currently underway.
PP13C-1478
Discovery of Lower Pleistocene Shallow-marine Deposits on Mayaguana Island, Bahamas. Implications for Eustatic Sea-Level Curves Derived From Deep-Sea Oxygen-Isotope Records
87Sr/86Sr-dated marine and beach sediments exposed along the north shore of Mayaguana Island (Bahamas) provide new estimates of the elevation of high sea stands during the Early Pleistocene that will contribute to better calibrate eustatic sea-level curves derived from deep-sea oxygen-isotope records. A newly investigated sea cliff located to the west of Mount Misery Point on the northern coast of Mayaguana, in the SE part of the Bahamian archipelago, includes two vertically stacked sequences of shallow-marine carbonates separated and capped by paleosols and eolianites. The lower unit, reaching up to 5.5 m above modern sea level, consists of coarse laminated calcarenites containing numerous mollusk and red-algal fragments, and large in-situ coral specimens (Diploria strigosa). The second unit, exposed between 7.3 and 10 m, includes bioturbated, coral-rich limestones, overlain by thinly bedded calcarenites characterized by an early generation of fibrous rim cement. 87Sr/86Sr ratios measured from these carbonates range from 0.709123 at the base of the section to 0.709142 at its top. The first unit can be interpreted as a peri-reefal facies deposited when relative sea level was at least 5.5 m above present. The second unit corresponds to one shallowing-upward sequence of subtidal and beach deposits generated when sea level was around 9 m above its actual stand. Sr-isotope ratios indicate that both units were formed during the Early Pleistocene, likely between 1.6 and 1.0 Ma BP. Comparison with existing oxygen-isotope records from deep- sea sediments suggests that the identified sea-level highstands could correspond to negative δ18O peaks estimated at 1.45 and 1.50 Ma BP. Based on the elevation of fossil reefs dating from the last interglacial (Marine Isotope Stage 5e) and the occurrence of Upper Miocene shallow-marine deposits close to modern sea level, Mayaguana can be considered as tectonically stable. The elevation values obtained for these Early Pleistocene highstands are thus potential candidates to calibrate Late Cenozoic eustatic sea-level curves derived from deep-sea oxygen-isotope records.
PP13C-1479
Past High sea Stands, Evidence From Conch Bar Cave, Middle Caicos
Flank margin caves form at sea level through a combination of enhanced dissolution by greater groundwater flux in the distal part of the freshwater lens and undersaturation associated with mixing of saline and freshwater discharging at the island margin (Smart et al., 1988). Measurement of the elevation of distinctive horizontally extensive and vertically restricted phreatic passages relative to the present sea level, allows for the position of former high sea stands to be established. Distinct horizons of cave development have been determined in Conch Bar Cave (21 49' N, 71 47' W), Middle Caicos, Turks and Caicos Islands. Passages were observed between -15 to -12, -1 to +3, +2 to +6, +7 to +10 and +16 to +22 m above sea level (asl). The highest level present is more than 16 m above the present sea level, which suggests either that there has been uplift of the platform or that past sea levels have been above this elevation. Assuming a stable platform, the elevations of passage development are in agreement with other records of high sea stands from the Bahamas region. Evidence for a MIS 11 sea level of up to 20 m asl has been observed in the Bahamas and Bermuda (Kindler and Hearty, 2000) which may have been responsible for forming the high elevation passage at +16 m asl. Additionally, high stands of +6 m asl are well documented in the Bahamas (Chen et al., 1991; Hearty and Neumann, 2001) and may have formed the +2 to +6 m cave passage. The antiquity of the passages has been determined through U-series dating of speleothems collected from a variety of elevations. The majority of samples were deposited post MIS 5a, as would be expected to confirm the opinion that emergent Bahamian flank margin caves are restricted to forming during MIS 5e (Mylroie and Carew, 1990). Speleothem from the high elevation +16 m passage therefore constrains the timing of passage formation to MIS 5 or earlier only. However, flowstone from present sea level began deposition at 260 ± 6.0 ka placing the initiation of growth within MIS 8, and the origin of the cave void prior to that. Continuous growth of the sample until 236 ± 6.6 ka indicates that sea level could not have risen above the present datum during this time. This is in agreement with deposition of the DWBAH flowstone from Grand Bahama that grew until 235 ka at -8.5 m below sea level (Lundberg and Ford, 1994), however, this is in contrast to the results of Thompson and Goldstein (2005) that suggest based on open system U-series ages for corals from Barbados, that sea level reached the present datum around 239 ka.
PP13C-1480
New Constraints on Dolomitization Models and Neogene Tectono-eustatic Events in the SE Bahamas From Exposed Messinian and Upper Pliocene Dolostones on Mayaguana Island
For the first time, massive dolostone units of Messinian and Late Pliocene age have been observed on the surface of the Bahamas archipelago, providing new constraints on dolomitization models and the Late Cenozoic sea-level and tectonic history in this area. In the Bahamas, massive dolostone units have so far only been identified from cores at depths ranging from 9 m in Mayaguana to 44 m on San Salvador. We report here on an extensive, one km-long sea-cliff exposure near Little Bay, on the north coast of Mayaguana, displaying two vertically stacked dolostone units separated by a karstic surface, and reaching up to 3 m above sea level. The lower crypto-crystalline dolostone is very hard, greenish-brown in colour (white when broken) and displays well-preserved cross bedding and ripple laminations, suggesting a tide- influenced depositional setting. The upper coarse-crystalline mimetic dolostone is also very hard, brown in colour, poorly stratified and contains numerous dm-sized fossils (corals, red algae, mollusks) indicating a high-energy peri-reefal depositional environment. The lower crypto-crystalline dolostone yielded an average 87Sr/ 86Sr ratio of 0.708988, and δ18O and δ13C values of +3.12 and +1.94 per mil, respectively, whereas the upper coarse-crystalline mimetic dolostone gave a 87Sr/ 86Sr ratio of 0.709083, and δ18O and δ13C values of +1.46 and -1.10 per mil, respectively. The oldest possible age of dolomitization, and thus the youngest possible age of deposition, can be constrained between 5.4 and 5.6 Ma (Messinian) for the lower unit, and between 2.2 and 2.3 Ma (Late Pliocene) for the upper one. Preliminary stable-isotope data suggest that the dolomitizing fluid could have been seawater in both cases. The negative δ13C value measured from the upper unit is likely related to a late phase of freshwater diagenesis. These rocks represent the oldest stratigraphic units observed so far at the surface of the Bahamas archipelago. If confirmed, the Messinian age of the lower crypto-crystalline dolostone would signify a yet undetected phase of dolomitization in the Caribbean region. Finally, further processing of field data and expected new geochemical results will certainly provide new constraints on dolomitization models and the Late Cenozoic eustatic and tectonic history in the southeastern Bahamas.
PP13C-1481
1,340 year long Late Holocene stalagmite stable isotope records from the Yucatan Peninsula, Mexico
We present stalagmite d18O and d13C stable isotope records from the Yucatan Peninsula, Mexico, with 3 year resolutions, extending from 650 C.E. to present. The chronology of these records is based on 12 U/Th radiometric ages. Two circa 400 year long cycles of a 4 per mil magnitude are observed in the d13C record, with values ranging from -8 to -11 per mil. The d18O record, in contrast, shows higher dominance of decadal and multi-decadal scale variability with values ranging from -4 to -6 per mil. The most negative stable isotopic values are observed during the 8th century and between the years 1760 and 1840, whereas the most positive values occur during the 17th century, end of the 10th and beginning of the 9th. Modeling and instrumental data indicate that local rainfall and stalagmite d18O decrease with increasing rainfall amount. More than 70% of total rainfall in the Yucatan occurs during summer when the ITCZ, and its associated convective activity, has its northern most position over this region. The observed stalagmite d18O and d13C variability suggests that rapid changes in the position and strength of the ITCZ have occurred over the last 1,300 years, affecting the regional hydrological cycle and biological conditions. As suggested by these records, wetter conditions than present prevailed during the 8th and at the end of the 18th centuries whereas several severe short term droughts (circa 10 years long) punctuated the 9th and 10th centuries, during the Late and Post-Classic Maya periods.