PP11D-01
Chemical and Isotope Compositions of Neogene Hippopotamidae Teeth From Lake Albert (Uganda): Implications for Environmental Change
The Neogene was a period of long-term global cooling and increasing climatic variability on astronomical time scales. Lake systems strongly depend on rainfall patterns and size or geographical distribution of river networks. To unravel environmental change and watershed dynamics in the western branch of the East African Rift (Lake Albert, Uganda) during the Late Neogene, we use proxy data (trace elements, O, C and Sr isotopes) from Hippopotamidae teeth. Laser ablation ICPMS profiles in enamel measured from the outside rim towards the dentin show an asymmetric trace element distribution in that the concentrations continuously decrease by up to 5 orders of magnitude within a distance of about 1 mm until a minimum is reached (<10 ppb for REE, Y, U). From thereon concentrations stay rather constant or even increase until about 100 μm in front of the dentin where concentrations rise sharply. This concentration minimum represents the least altered part of the enamel and it probably represents a primary biological fingerprint which has the potential to monitor migration pathways and palaeoenvironmental changes. On geological time scales δ13C compositions reflect a transition from pure C3 browsers (-11 per mil PDB) at 5 to 6 Ma towards C4 dominated grazers (0 per mil PDB) at 2.0 to 2.5 Ma. The oxygen stable isotope (δ18O) composition of enamel rises from 26 per mil at 5 to 6 Ma to a maximum of 32 per mil SMOW at 2.3 Ma. Increasing δ18O values suggest enhanced evaporation of the lake due to rising aridity. This is in agreement with a synchronous spread of C4 vegetation in the reach of Hippopotamid populations. The Sr isotopic composition of enamel displays a large variation and 87Sr/86Sr is 0.714 about 5 Ma ago, reaches a maximum of 0.717 at about 2.3 Ma and decreases from there on to about 0.708. Thus, Sr and O isotopic compositions correlate with each other on the geological time scale. This is plausible if the Sr isotopic composition of Hippopotamid enamel dominantly reflects the changes of the water chemistry of the lake, and is therefore a powerful tool for tracing ancient hydrological networks. The large variation of the Sr isotope composition can be explained if the lake is fed by different sources: water draining Cenozoic volcanic terrains have low 87Sr/86Sr (~ 0.704), whereas Proterozoic-Achaean terrains of the rift flanks have high 87Sr/86Sr (>0.718). Thus, the increasing 87Sr/86Sr from 5.2 to 2.3 Ma, suggests that water supply from volcanic terrains ceases and the local, Achaean run-off dominated the lake water chemistry. Consistent with the concurrent increase of ?18O, this suggests that increasing aridity and evaporation of lake water on a regional scale, interrupts the axial river network and local river discharge becomes dominant. The decrease of 87Sr/86Sr starting at about 2 Ma indicates new water supply from volcanic rock dominated terrains, which could reflect a tectonic restructuring of the rift valley or the initiation of the young Toro-Ankole igneous province. Palaeoclimate records from rift systems are governed by global climate forcing mechanisms and interacting geodynamics. Our study of the chemical and isotope record of tooth enamel from mammals permits the identification of these local and global environmental changes, in the western EARS on geological time scales. investigation.
PP11D-02
Barefoot on Hot Ground: Formation Temperatures of Plio-Pleistocene Soil Carbonates in East Africa Based on the Clumped Isotope in Carbonate (Δ47) Thermometer
We utilize the carbonate clumped isotope thermometer to investigate paleoenvironments of human evolution in the Turkana Basin of northern Kenya and southern Ethiopia. Analyses were made using a new automated online peripheral that reduces the human workload and improves the success rate of this analysis. Paleotemperatures for Plio-Pleistocene soil carbonates that formed >50cm below the paleosurface range between ~30°C and 40°C; some of this temperature variation is temporally systematic and coherent across depositional facies. Present day mean annual temperature in this region averages 29°C, and there is very little seasonal variation in average monthly temperature (<4°C), although daily maximum temperatures often exceed 40°C. These clumped isotope paleotemperatures seem unexpectedly high because soil temperatures at >50 cm depth are strongly buffered against diurnal and short-term (i.e., weeks) temperature variations. Possible explanations for these high temperatures include 1) much higher mean annual or mean seasonal air temperatures during parts of the Plio-Pleistocene, 2) a temporal bias of soil carbonate formation towards short-lived extreme temperature events, 3) nonequilibrium or diagenetic isotope effects, and 4) persistent elevation of soil temperatures relative to air temperatures. A brief deployment of remote temperature sensors near Lake Turkana revealed that soil temperatures were considerably higher than air temperatures. Temperatures at 50 cm depth were stable between 35°C and 37°C; those at 10 cm ranged diurnally between 44°C (day) and 33°C (night). Air temperatures in the shade ranged between 37°C (day) and 26°C (night). These study localities were sparsely vegetated, and the elevated soil temperatures are consistent with surface heating by solar radiation. A survey of previous data reveals that temperatures well above 45°C are common for surfaces receiving direct solar radiation. The relevant boundary condition for soil temperature is therefore surface soil temperature, and not air temperature as might be assumed. The Turkana Basin today is one of the hottest regions on Earth; our results suggest that similarly high temperatures periodically characterized the environments occupied by Australopithecus, Homo, and other human relatives over the past 4 million years.
PP11D-03
Precipitation of ikaite crystals in Antarctic marine sediments: implications from pore water geochemistry
Ikaite is a calcium carbonate hexahydrate (CaCO3•6H20) considered to be stable only at low temperatures. It has been found in form of tufa tower at locations where alkaline water mixes with water masses enriched in calcium (e.g. Ikka Fjord, Mono Lake). Large euhedral single crystals of ikaite were also recovered in marine sediments, associated with organic matter degradation, anaerobic oxidation of methane (AOM) and sulfate reduction. The hydration water in the ikaite crystals were demonstrated to record the oxygen isotope composition of the water from which they precipitated. Such a characteristic may allow using ikaite to reconstruct the ice volume in the past. For this purpose, the controls on its precipitation in the sediment column need to be investigated which is the main goal of this study. U.S. Antarctica Program cruise NBP0703 collected two cores with ikaite crystals at Antarctica Peninsula (Bransfield Strait and Firth of Tay). We determined major cation/anion concentrations, dissolved inorganic carbon (DIC) and δ13C composition of DIC in the pore waters in these two cores. Strong organic matter degradation or AOM in both cores results in quick consumption of sulfate in shallow part of the cores (SMT at around 3m).Rapid build-up of DIC is accompanied by the sharp decrease of dissolved calcium in the top 5m. Large variations were observed in δ13CDIC values (-20‰ to +13‰). The δ13C of ikaite in two cores were distinctive from each other (-19‰ and +4‰) corresponding to the DIC pools at different depths. The down core saturation state of the ikaite was modeled in PHREEQC based on the pore water chemistry, and the results are consistent with carbon isotope data, suggesting that these large crystals very likely formed within a narrow depth interval and a short time period (given high sedimentation rates of 0.5-1 cm/yr in this area).
PP11D-04
A 280,000-Year Temperature and Productivity Reconstruction From the Great Salt Lake in the Western US Using Long-Chain Alkenones
Long-chain alkenones form a key class of biomarkers that allows for paleoclimate reconstruction in most global ocean regions. Recent studies have revealed that many continental lakes also contain alkenones. Futhermore, the distribution of lacustrine alkenones changes linearly with temperature, just as in marine settings, making them a powerful quantitative paleotemperature proxy for continental regions. We have recently found abundant alkenones in the Great Salt Lake (GSL) basin, located in Utah in the western US. The lake sediment contains the well-studied C37:4, C37:3, and C37:2 long-chain alkenones, as well as significant amounts of C38:4, C38:3, C38:2 and C39:3, C39:2. A temperature calibration for C37 alkenones was recently carried out at Brown University for Lake George in New York, further confirming the linear relationship between temperature and alkenone unsaturation index. We have obtained samples of GSL from a sediment core (~2000-yr resolution) that dates back ~280ka BP. We determined long-chain alkenone distribution and abundances to infer temperature conditions and lake productivity changes across multiple glacial/interglacial cycles. Previous research on the GSL basin is primarily qualitative, providing estimates of lake levels and salinity conditions using sedimentological and paleobiological proxies. We provide for the first time a quantitative temperature reconstruction for the GSL region. Furthermore, recent publications have suggested a link between salinity and the percentage abundance of C37:4 alkenones in lakes. By measuring alkenone distribution in hypersaline GSL, we assess the extent to which this link exists in such an arid region. Our study provides insight into the degree to which the western US has been affected by large-scale climate oscillations in the past. In addition, it provides a temperature reconstruction that has the potential to illuminate timing and periodicity of temperature oscillations throughout the GSL region.
PP11D-05
Towards a Stacked 1 Ma-year Radiometrically Dated Palaeoclimate Record From Italian Speleothems
Stable isotope patterns from precisely dated Late Pleistocene speleothems from Corchia Cave (Italy) record orbital- and millennial-scale variations in North Atlantic circulation. Recent developments in uranium-lead (U- Pb) dating make it possible to extend the speleothem record from this cave beyond 0.5 Ma. The cave's speleothems are ideal for U-Pb dating because of their low detrital content, high uranium concentrations and well-behaved uranium isotope systematics. We present stable isotope data from a 23-cm core recovered from an actively growing subaqueous calcite mound sampled from the floor of a pool in Corchia Cave. The stable isotope patterns, anchored by a preliminary U-Th/U-U/U-Pb chronology, preserve every glacial-interglacial cycle back to about 1 Ma. To supplement this record, we are stacking an isotope sequence derived from stalagmites from the same cave, which have less continuous records but offer much greater sampling resolution and dating potential. The first of these stalagmites has been dated to between 0.96 and 0.80 Ma, and its isotopic pattern shows excellent agreement with that of the calcite core, and provides a precisely dated, well-resolved sequence through several glacial terminations at the time of the Mid-Pleistocene Transition.
PP11D-06
Physical Driver for the 62-Myr Cycle in Fossil Diversity
The observation that fossil diversity is strongly modulated by two cycles, 62 Myr and 140 Myr, is one of the
strangest discoveries of the last few years in geology and paleontology. (1) The cycles show strongest
among those genera that are short-lived, i.e. endured for 45 Myr or less. In an attempt to identify patterns,
we have subdivided the data into 170 groups consisting of various phyla, orders and classes, to examine the
strength of the cycles; these results will be presented. As examples, the 62 Myr cycle is absent in the
segmented worms (annelida), strong in the porifera, strong in the echinoderms, moderate in the bryosoa,
moderate in the older anthropoda, very weak in the foraminifera, insignificant in the bivalves, very strong in
the spiriferids, strong in trilobites, and very weak in fishes. No clear pattern has emerged from analysis of
these taxonomic units. Several models will be discussed that could account for the cycles. One is the
possibility of a helium-3 instability in the sun. Helium-3 is produced in the inner regions of the sun, but burned
and destroyed in the deepest part of the core; only a layer outside the core remains. A regular overturn of
the layer mixing it into the deeper core would result in a cycle of solar activity with a few percent modulation
and a period that could be between 50 to 200 Myr.(2) This idea once received a great deal of attention when
it was thought to be an explanation for the low flux of neutrinos from the sun. A second model to explain the
cycle is the passage of the sun through variations in the mass density of the Milky Way Galaxy. Higher
density puts an increased torque on comets in the Oort clould; that diminishes the angular momentum of half
of them, and increases the eccentricity of the orbits. Doubling of the density (e.g. caused by passage
through a giant molecular cloud or an arm of the galaxy) would result in a doubling of the number of comets
hitting the earth. The passage through the arms of the galaxy has too long a period for the 62 Myr cycle, but
could account for the 140 Myr cycle. We know that cometary impacts have major consequences for the
diversity of life. We will discuss the possibility of such structure in the Milky Way. We will also present a
description of other proposed explanations for the cycles, most of which can be ruled out. References: 1.
Cycles in fossil diversity, Robert A. Rohde and Richard A. Muller, Nature vol. 454, pp. 208-210 (March 10,
2005). 2. The Solar Spoon, F. W. W. Dilke and D. O. Gough, Nature vol. 240, pp. 262-264; 293-294 (Dec. 1,
1972)
http://www.muller.lbl.gov
PP11D-07
Did tropical rainforest vegetation exist during the Late Cretaceous? New data from the late Campanian to early Maastrichtian Olmos Formation, Coahuila, Mexico.
A major problem in paleobotany and paleoclimatology is the origin of modern tropical and paratropical rainforests. Studies of leaf macrofossils, beginning with those of Wolfe and Upchurch, have suggested that tropical and paratropical (i.e., megathermal) rainforests with dominant angiosperms are of Cenozoic origin, and that comparable vegetation was either absent or greatly restricted during the Late Cretaceous. Earth System modeling studies, in contrast, predict the existence of megathermal rainforest vegetation during the mid- and Late Cretaceous, though with less areal extent than during the Late Cenozoic and Recent. Megathermal climate with year-round precipitation is simulated along the paleoequator and along the northern margin of the Tethys Ocean, and tends to occur in highly focused regions, in contrast to the more latitudinally zoned pattern of the Recent. Low-resolution climatic indicators, such as the distribution of coals and tree fern spores, are consistent with evidence from climate modeling for megathermal wet climates during the Late Cretaceous, and by extension megathermal rainforest vegetation. However, corroborative data from plant macrofossil assemblages is needed, because the physiognomy of leaves and woods directly reflects plant adaptation to the environment and can estimate climate independently of the generic and familial affinities of the paleoflora. Newly collected plant macrofossil assemblages from the late Campian to early Maastrichtian Olmos Formation of Coahuila, Mexico, provide evidence for megathermal rainforest vegetation on the northern margin of the Tethys Ocean at approximately 35 degrees paleolatitude. The newly collected leaf flora is 72 percent entire- margined and has abundant palms, features typical of modern megathermal rainforests. Thirty percent of the species have large leaves, and 50 percent of the species have drip tips, features indicative of wet conditions. Simple and multiple regression functions based on the physiognomy of modern vegetation indicate the presence of paratropical rainforest, with a mean annual temperature of 20-23°C and mean annual precipitation of 1.5-3 m. The wood flora contains 5 species of palms and at least 9 species of dicots, none of which have growth rings, and is most similar in hydraulic architecture to woods from wet tropical forests. The Olmos macrofossil assemblages occur in stratigraphic sequences that contain thick (>1m) coal beds and in a region where Earth System models predict high biomass vegetation that grew under paratropical to warm subtropical wet conditions. Our results imply that earlier plant macrofossil studies are in error and that better sampling of low-latitude floras will provide additional macrofossil evidence for megathermal rainforest vegetation during the mid- and Late Cretaceous.
PP11D-08
Sea level forcing of mid-ocean ridge magmatism on Milankovitch timescales
It is well-documented that Iceland experienced a pulse of elevated volcanism immediately following the last deglaciation (Maclennan et al., 2002). Modeling results suggest ice sheet retreat depressurized the mantle thus enhancing melt production and the supply of magma to the surface (Jull and McKenzie, 1996). Here we take a similar approach, but instead model the effect of glacial-interglacial changes in sea level on mantle melting at mid-ocean ridges. Loading rates reaching ±2 cm/year of water are comparable to the tectonic unloading rate of ~2 cm/year of mantle rock that drives magmatic activity at a slow-spreading ridge. Although the magnitude of sea level forcing is smaller than subglacial forcing, the sea level effect is globally distributed and could have significant consequences for ocean crust architecture and geothermal heat delivery to the deep ocean. We use a model of melt production based on analytical corner flow velocities coupled to the pMELTS model (Ghiorso et al. 2002; Asimow et al. 2004) of melting of the Workman and Hart (2006) depleted upper mantle source composition. For simplicity we assume that the hydrostatic pressure signal from sea-level variation is felt instantaneously by the entire melting regime, and that melts migrate from source to ridge axis at a constant rate. We neglect crustal magmatic and hydrothermal processes that might damp or delay the signal. We examined mid-ocean ridge systems with half-spreading rates from 30 mm/yr to 100 mm/yr and melt migration rates from 2.5 to 50 m/yr. For the case of 30 mm/yr half-spreading rate and 10 m/yr melt migration, we find that the rate of melt delivery to the crust varies ±30% relative to steady state conditions when the model is driven by a record of sea-level variability for the last 140 kyr. Notably, we simulate that melt delivery increased by ~30% beginning at 75 kyr BP, coincident with a rapid decrease in sea level of approximately 60 m. We also estimate a ~30% increase in melt delivery from 30 kyr BP to 20 kyr BP, driven by a 50 m decrease in sea level. Melt delivery decreased more than 50% from the Last Glacial Maximum to 5 kyr BP, driven by the ~120 m rise in sea level during the deglaciation. Our results suggest that modest changes in hydrostatic pressure driven by ice sheet growth and decay yield substantial alterations in magma flux at mid-ocean ridges relative to steady state conditions. These simulations raise the possibility that mantle melting may act as a negative feedback on ice sheet size by modulating deep ocean temperature. We estimate that enhanced melt production during sea level low-stands increases deep ocean temperature by order 0.1 °C. We speculate this modest warming may contribute to deglaciations by reducing sea-ice extent in the Southern Ocean, which may in turn promote ventilation of the abyssal ocean and the release of sequestered carbon dioxide to the atmosphere. References: Asimow, P.D., et al., 2004. G-cubed 5(1): 10.1029/2003GC000568. Ghiorso, M.S., et al., 2002. G-cubed 3(5): 10.1029/2001GC000217. Jull, M., and McKenzie, D., 1996. JGR 101(B10): 21815-21828. Maclennan, J., et al., 2002. G-cubed 3(11): 10.1029/2001GC000282 Workman, R.K. and Hart, S.R., 2005. EPSL 231(1-2): 53-72.