PP23B-1332
The mid Pliocene Warm Period: A Test-bed for Integrating Data and Models
The most recent interval of greater global warmth is the mid Pliocene warm period (3.3 to 3.0 million years ago), characterized by global surface temperatures around 3 degrees celcius higher than today. Although not a direct analogue of future global warming, this interval is a relevant natural experiment that can be used to understand processes contributing to long-term global warmth as many boundary conditions were similar to today (e.g. ocean circulation patterns, atmospheric carbon dioxide concentrations). Here we present a summary of the palaeoclimate modelling activities carried out thus far for the mid Pliocene and demonstrate how a combined data and modelling approach has led to significant advances in our understanding and ‘retrodiction' of the last great warm period of the Cenozoic Era. The development and refinement of mid Pliocene palaeoenvironmental data sets is discussed, as are the steps that have been taken to increase the sophistication of the overall modelling effort. Initially, the talk focuses on the use of atmosphere-only General Circulation Models (AGCMs) and summarises the results produced by such models. We then consider the importance of incorporating slab-ocean, dynamic ocean, mechanistic and dynamic vegetation models with AGCMs to studying the causes mid Pliocene warmth. Finally recent developments are considered and a strategy for further combined data and model investigations is presented.
PP23B-1333
Ground truthing Ordovician climate models using spatial analyses of chitinozoans and graptolites
Rapid and extreme changes in climate occurred during the middle to late Ordovician, culminating in one of the major glaciations of the Phanerozoic. The causal mechanisms and duration of the latter remain contentious and to date workers have largely relied on region-specific studies. Few climate simulation models for this time interval have been produced and new General Circulation Climate Models (GCMs) have yet to be tested. Here we examine the potential for using planktonic chitinozoans and graptolites as water mass indicators to ground truth Ordovician climate model predictions of ocean state. In the same way that Cenozoic and Mesozoic planktonic foraminifer and calcareous nannoplankton distributions can be used to track ocean surface and deep watermasses. At the core of our research strategy is the compilation of a high stratigraphical resolution biogeographical relational database of species occurrences, palaeoenvironmental and ocean-climate proxy data. This will be used to compile surface water (and where possible depth assemblage) palaeobiogeographic maps of chitinozoan and graptolite distributions. These can then be retro-tested against GCM maps of surface currents and used to identify latitudinal temperature/climate belts and gradient. We have identified the key time intervals which characterise the critical climate transitions as the N. gracilis graptolite Biozone (extreme greenhouse climate), the Boda Warm Event (Rawtheyan, "transitional") and the Hirnantian (icehouse climate). These are stratigraphically well-constrained and represented globally. In this poster we will present preliminary results from our analysis the gracilis Bizone timeslice. This will form a baseline condition for future research.
PP23B-1334
Response of the Intertropical Convergence Zone to Southern Hemisphere cooling during Ordovician glaciation
Difficulties in quantifying environmental conditions (particularly pCO2 and palaeobathymetry) and oversimplification in parameterization are major limitations on what deep time climate models can tell us about the dynamic behaviour of the Earth's climate. Research is still focussed at understanding climate proxy data and the best climate proxy datasets available for the Early Palaeozoic are those from palaeo-tropical latitudes. At the present day the tropics are the main source of the atmosphere's heat and water vapour; changes in tropical dynamics have a major influence on the Earth's climate. On the global scale, the position of the ITCZ affects pole-ward heat transport and latitudinal temperature gradients and is a critical feedback mechanism, sensitive to ice volume changes, during glaciation. Ocean water beneath the ITCZ is warm, has reduced salinity and is characterised by light δ18O(carb). Stable isotope records from low latitude Upper Ordovician limestone (from Nevada, Arctic Canada and Estonia) are interpreted as showing a pattern of predicted and coincident shifts in the position of subtropical and tropical water masses associated with the developing Hirnantian glaciation. We hypothesize that these changes reflect a shift in the position of the ITCZ that tracked the pattern of climate change during the Late Ordovician. We argue for a discrete palaeo-ITCZ, the location of which was controlled by ice volume changes. The repositioning of the ITCZ resulted in climatic belt re-organization resulting in complex feedbacks into the ocean–climate system that were a likely contributor to the Late Ordovician mass extinction.
PP23B-1335
Mid-Pliocene Sea-Surface Temperature Variability in the Eastern Equatorial Pacific Ocean
A pronounced east-west sea-surface temperature (SST) gradient exists in the modern equatorial Pacific Ocean; upwelling of warmer water in the Eastern Equatorial Pacific (EEP) and hence a reduction in the zonal temperature gradient characterizes El Nino conditions. Published alkenone-derived temperature estimates as well as Mg/Ca paleothermometry provide evidence of higher than present day SST in the east during the mid-Pliocene. We present new faunal-based SST estimates from the EEP that, together with existing geochemical data, provide the basis for a new mid-Pliocene low-latitude Pacific SST reconstruction. Our reconstruction suggests the persistence of upwelling through the mid-Pliocene, albeit of warmer than modern waters. This investigation is a part of the U.S. Geological Survey‘s PRISM3 global SST reconstruction.
PP23B-1336
Multivariate Statistical Analysis of Late Mississippian Paleosol Trace Element Chemistry and Application to Interpreting Pedogenic Processes and Climate Through Time
Paleosol geochemistry has primarily focused upon bulk analyses of major elements and stable isotopes. Trace elements in bulk samples, in contrast, are rarely studied, primarily due to very low concentrations and a fundamental lack of knowledge of chemical behavior in analogous modern soil-forming environments. Late Mississippian (Chesterian) paleosols of the US Appalachian region serve as a case-study to investigate temporal relationships in paleosol trace element chemistry as a function of changing climatic conditions. Both linear (coorelation matrices) and multivariate statistical techniques (cluster, principal components and canonical variates) were employed to identify statistically significant relationships between trace elements, including Ti, Ga, Ge, Y, Zr, Nb, Cs, La, Hf, Ta, W, Ce, Th. Results indicate that trace element chemistry of this Late Mississippian suite of paleosols is controlled by either organic matter content or lessivage (clay formation and accumulation by feldspar weathering). Mean annual precipitation (MAP), estimated using the CIA-K geochemical proxy, controls soil hydrology, redox chemistry, and organic matter accumulation, in addition to clay accumulation. Soil orders through time are statistically separable, with variable success in discriminating between changes in soil drainage. Our results provide a new tool to evaluate pedogenic processes and to draw inferences regarding intensity of chemical weathering and its relationship to climate change.
PP23B-1337
The Transition From the Present-Day Climate to a Snowball Earth Simulated With a Comprehensive Climate Model
Recently, Marotzke and Botzet (2007) have shown that in a comprehensive climate model both the current climate and a completely ice-covered Earth are stable states under today's total solar irradiance (TSI) and greenhouse gas concentrations. By setting TSI to near-zero they were able to cause a transition from the present-day climate to the ice-covered state within 15 years [1]. In order to study the bifurcation point, we have repeated this experiment with the same model, i.e. the Max Planck Institute for Meteorology coupled atmosphere-ocean general circulation model ECHAM5/MPI-OM, at lower resolution (horizontally T31 in the atmosphere and 3.0 degrees in the ocean). We give estimates for the critical values of TSI and greenhouse gas concentrations needed to trigger the transition from the present-day to the ice- covered state. Starting from today's climate and setting TSI to 44% of today's value leads to a glaciation within 30 years. Furthermore, we investigate the degree of oceanic ice-cover needed for an unstoppable glaciation. We find that only an almost completely ice-covered ocean guarantees that the model does not return to the present-day climate when TSI is reset to its today's value. Our results indicate that a snowball Earth could, in principle, be triggered by a brief decrease of TSI. [1] Marotzke, J. and M. Botzet (2007), Present-day and ice-covered equilibrium states in a comprehensive climate model, Geophys. Res. Lett., Vol. 34, No. 16, L16704, doi:10.1029/2006GL028880
PP23B-1338
Can Orbital Forcing Help Explain Reconstructed Monsoon Winds for the Early Jurassic?
Wind-blown sediments provide direct evidence of atmospheric circulation, which is otherwise very difficult to obtain from the geologic record. Eolian sandstones cover large portions of the Colorado Plateau of the southwestern US. These Early Permian through Early Jurassic sandstones reflect dunes that migrated under a distinctive wind regime that apparently varied little in direction through their 100 Myr span. To the north, the dominant winds came from the NE, curving to NW over the southern portion of the outcrops. Basic concepts of climate would suggest that the NE winds are consistent with low latitude trade winds in the northern hemisphere, and the NW winds consistent with cross-equatorial flow induced by a strong summer monsoonal circulation in the southern hemisphere of Pangea. The dunes in the south also reflect a seasonal wind reversal; during much of the year, slightly weaker winds were from the SE. Southernmost outcrops of the sandstones contain abundant trace fossils and slump structures, suggesting they were deposited at the relatively wet, southern edge of an extensive desert. Conventional wisdom, largely based on paleomagnetic evidence, would place the Colorado Plateau along or just north of the equator during the Early Permian, then moving north through the Triassic to lie near 20°N by the Early Jurassic. Yet the constancy of the wind regime indicated by the dip directions of the cross-strata suggests that the dunes and the Plateau stayed within the same climatic zone, despite the hypothesized large northward movement of Pangea. The wind regimes recorded by the dunes are consistent with paleogeographic reconstructions of the late Permian, with the Colorado Plateau straddling the equator, but are very much at odds with a northward movement to about 20° N by the early Jurassic. Previous modeling work, including our own, has produced nearly symmetric monsoon circulations (i.e., monsoons of nearly equal strength in each hemisphere) in part due to the use of present-day orbital parameters. These simulations, therefore, represent an orbital state that does not represent the extremal states that may have affected dune migration. To help resolve the discrepancy between the paleomag and paleowind Pangean positions, climate simulations with the NCAR CCSM3 were made using end members for orbital parameters.
PP23B-1339
Changing Low-Latitude Paleoenvironments During the Onset of the Late Paleozoic Ice Age.
The Carboniferous is one of the critical stepwise transitions in the evolution of the Earth System when rapid changes in climate and atmospheric composition (rise in oxygen/carbon dioxide) coincided with key events in biological evolution (the proliferation of land plants) and increased rates of tectonic plate reorganisation. The paleoequatorial Carboniferous sedimentary successions of Britain and Ireland were deposited in a shallow epicontinental seaway and record high magnitude and high frequency eustatic changes as ice sheets waxed and waned on Gondwana during the onset of the Late Paleozoic ice age. In the Asbian and Brigantian (ca.334-326 Ma) widespread shallow and marginal marine conditions are recorded by mixed carbonate and siliciclastic deposition and carbonate successions on platforms and shelves. In the mid-Brigantian, a marine transgression reduced deposition on the carbonate platforms and connected previously separate sub-basins in central Scotland. Data used to interpret global climate for this time slice are contradictory. A warmer global climate is suggested by the widespread extent of a Gondwanan macrofloral realm that requires frost-free conditions whereas isotope data suggest a cold Asbian followed by a warm, ice-free Brigantian. In the Pendleian to Yeadonian (ca.326-317 Ma), increasingly light oxygen isotope data indicate a return to cold conditions. Well- constrained periods of Gondwanan glacial sedimentation (e.g. in SE Australia), of 1 and 3 Myr duration, coincide with major paleoevironmental changes across Britain and Ireland, including the end of significant carbonate production and the earliest examples of large-scale river paleovalleys (with 20-80 m of erosional relief). Depositional environments (fluvial, deltaic) and basin bathymetry (distinct shelf edges) provided optimal conditions for the effects of sea-level change to be recorded in sedimentary successions of this time slice. These late Mississippian and early Pennsylvanian siliciclastic and carbonate successions generally reflect shifts in global climate, including glacial periods, but temporal changes in paleoenvironments and local climate overprint the record. These data provide useful constraints for modelling climate change in deep time.
PP23B-1340
The Rise of Nannoplankton Calcification Correlates With Tectonically-driven Massive Fall of Atmospheric CO2
On the basis of a global carbon-climate numerical model, we reconstructed atmospheric carbon dioxide concentrations over the first 60 million years of the Mesozoic. Our analysis indicates that PCO2 declined from more than 3000 ppmv to less than 1000 ppmv, with a drastic drop during the Late Triassic. The fast northward drift of Pangaea exposed a large continental surface to warm and humid equatorial climate, thus promoting CO2 consumption through weathering. This massive drawdown of atmospheric CO2 is consistent with sedimentological and geochemical data of the rock record and correlates with the primary radiation of calcareous nannoplankton, a biological revolution shifting the global carbonate sink from shallow water environments to the open oceans. Our numerical model shows that at time, tectonics, via weathering, increased the pH of the oceanic surface waters by 0.3 units, corresponding to a 50 % decrease in H+ concentration. This may have provided the ultimate environmental trigger which unlocked the newly oxidized Mesozoic open oceans to pelagic nannobiocalcification.
PP23B-1341
Regional water cycle change during the PETM from new records in central Utah and CCSM
Isotopic and lithological data from stratigraphic sections in central Utah provide new insight into the evolution of climate in this region during the Paleocene-Eocene thermal maximum, a ~200 kyr pulse of carbon buildup in Earth's atmosphere and global warming. Within the pedogenically modified floodplain clastic rocks of the North Horn Fm., this event is marked by an abrupt negative carbon isotope excursion and a notable positive shift in oxygen isotope ratios of paleosol carbonate nodules. The magnitude of the changes in central Utah carbonate δ13C and δ18O are relatively small and large, respectively, in comparison with equivalent records from the Bighorn Basin (WY) some 500 km to the NNE. These patterns are evaluated in the context of simulations of early Paleogene warm climates made using the NCAR CCSM1.4 fully coupled climate model. They are found to be consistent with the model's predicted N-S gradient in hydroclimatological response to greenhouse forcing, which would have driven a decrease in plant photosynthetic 13C fractionation due to changes in water availability and an increase in evaporative enrichment of soil water δ18O relative to changes at the Wyoming site. The termination of the PETM in the Utah section, as defined by carbonate δ13C values, is marked by several meters of braided channel sandstones, perhaps signifying the renewed penetration of moisture into the region and enhanced erosion on a partially de-vegitated landscape. While coupled climate model simulations appear to be consistent with the spatial gradients in PETM hydroclimatology documented by the proxy data, our comparison highlights the need for transient climate model simulations of deep-time paleoclimate events to further investigate model performance relative to records of the temporal component of climate change.
PP23B-1342
Kimberlite eruptive frequency and major low latitude glaciation through the Phanerozoic
An examination of the temporal distribution of kimberlites throughout the Phanerozoic shows a strong anti- correlation between kimberlite eruption frequency and major glaciation. A cumulative frequency plot of kimberlite eruption ages shows a kimberlite-free window in the Permo-Carboniferous and no significant kimberlite eruption since c. 48 Ma. Both of these intervals coincide with geochemically modelled episodes of low atmospheric CO2 concentration (<600 ppmv). A dip in kimberlite eruptive frequency is also seen in the Late Ordovician-Early Silurian. Earlier kimberlite hiatuses are hinted at for the Neoproterozoic although the dataset is incomplete for this time interval and low confidence can be placed in these. The period around 45 Ma is recognised as a time of global plate reorganisation, and has recently been identified as a time of cooling in the equatorial Pacific and Antarctic. Although kimberlite eruptions are volumetrically small, their frequency during kimberlitic periods of the Phanerozoic is high (at least 8000 are known) and the magma likely possessed a high carbon dioxide content (up to 20 % by volume). The temporal relationships suggest that kimberlites are either a proxy for an unrecognized mantle source of CO2 or are themselves directly responsible for buffering of atmospheric CO2 levels against the long-term steady draw-down by tectonic and depositional processes, maintaining them at >600-1000ppmv. A conceptual model is presented suggesting that a progressive drop in atmospheric pCO2, following the loss of this mantle-derived CO2 buffer in the early Cenozoic, early Pennsylvanian, and possibly Late Ordovician, made the global climate system more sensitive to other perturbations resulting in prolonged (> 2 million years) global glaciation.
PP23B-1343
TIME CONTINUOUS CLIMATE MODELS FOR THE CRETACEOUS
Conventional paleoclimate studies use a "timeslice" approach in which climate simulations are run for a particular time interval (e.g. K/T, 66 Ma). This standard approach, however, has limited applicability for understanding the climatic transitions that take place between time intervals and the long-term pattern of climatic change. We present a new approach called "Time Continuous" Climate Modeling (TCCM), which is designed to evaluate global and regional climatic changes as continents slowly move, mountains ranges are built and eroded, and mid-ocean ridges and subduction zones evolve. To evaluate the impacts of these slow paleogeographic changes, simulations are run over closely spaced time intervals (1-5 million years). At such high-resolution time steps, we will be able to detect climatic tipping points, the importance of subtle changes in geography, the changes in global climatic as continents move across the globe, and the pattern of climate change that takes place over tens of millions of years. The TCCM approach is made possible by the use of: 1) high-performance Linux clusters 2) more efficient climates models (i.e., FOAM) and, 3) the availability of paleogeographic and paleobathymetric maps at one million year intervals (PALEOMAP Project). The initial simulation, in this case for the Late Cretaceous (66 Ma), is a standard 100-year coupled run. Each subsequent simulation, however, substitutes a new paleogeography (68 Ma), and uses the previous climatic end-results as the initial conditions for the subsequent simulation. Since the paleogeography only changes slightly between runs, the new simulation quickly stabilizes (~10 model years). Should instabilities develop, a full simulation, or ocean-only simulations can be run to resolve the problems. TCCM provides a new look at climatic evolution over longer time scales while minimizing the cost in terms of computing time and expense.
PP23B-1344
Simulation of Modern and Cretaceous δ18O With a Global Ocean-Atmosphere General Circulation Model
The δ18O content of marine carbonates has been the most prolific and powerful proxy for inferring past climate. However, this climate proxy requires knowledge of the 18O content of past marine waters. In the absence of such information, the 18O content of past waters is often assumed to be similar or systematically offset from modern. We test this assumption using an ocean-atmosphere general circulation model (GENESIS version 3.0 coupled to MOM2) to simulate modern and middle Cretaceous climates. Our model includes full water isotopic capabilities, including transport and fractionation in the atmospheric physics and passive transport in the ocean. The modern simulation of precipitation and marine δ18O in the model compares well with modern δ18O except in the Arctic Ocean where seawater δ18O is too low due to insufficient mixing with the global ocean. The Cretaceous zonal sea-surface δ18O predicted by the model is slightly (< 0.5‰) more depleted than modern in low latitudes and much heavier (by up to 8.5‰) in the Arctic Ocean. These differences are mainly due to the enhanced and unrealistic partitioning of light δ18O into the Arctic Ocean in the modern simulation. And the atmospheric transport of Cretaceous is similar to that of modern in our simulation, too. Our model results indicate that Cretaceous isotopic zonal gradients would have been similar to the modern unless isotopic partitioning between the global ocean and isolated basins were very different. Based on our model ƒÔδ18O, Cretaceous high latitude sea-surface temperature could be 4°C lower than estimates calculated using a global mean surface seawater δ18O. Our estimates reduce the equator- to-pole temperature gradient difference between model and proxy data without increasing model heat transport.
PP23B-1345
Comparing lithologic and rock magnetic cyclicity in the Upper Cretaceous Fort Hayes Limestone, southern Colorado-northern New Mexico
The Upper Cretaceous (Turonian) Ft. Hayes Limestone is composed of well-developed limestone-shale couplets (0.2-2 m) deposited in response to orbitally controlled climate changes in the Western Interior seaway. Individual precession/obliquity driven limestone-shale couplets are bundled into eccentricity cycles; these bundles have been correlated with the aid of distinct bentonite beds over distances of 800 km. Previous interpretations of the couplets suggest they are the result of alternating wet and dry climatic periods. We are examining the rock magnetic characteristics of the Ft. Hayes Limestone at several localities to determine the relationship between lithologically defined cyclicity and specific rock magnetic data. Previous rock magnetic studies of other Mesozoic and Cenozoic marine deposits have recognized orbitally controlled dust and/or fluvial-derived signals. Samples collected every 5-10 cm through three stratigraphic sections in southern Colorado and northeast New Mexico show considerable variation in intensity of anhysteretic remanent magnetization (ARM) and susceptibility (MS). Limestone and shale ARM values range from about 0.3 to about 9.0 x 10 -6 Am 2 /kg); notably the available data set shows no distinct difference between limestone and shale ARM intentities. Samples distributed laterally at constant position in the same limestone bed yield statistically indistinguishable ARM intensities, as well as multiple specimens from the same sample. Variations in ARM values do not correlate with the lithologic variations of individual limestone-shale couplets. However, we see well-developed longer term ARM variability, with low values (<2 x 10 -6 Am 2 /kg) in the lower Ft. Hayes rising to higher (2 to 6 x 10 -6 Am 2 /kg) values with variations that do not correlate with lithology or couplet bundles (describe trend) in the middle and upper part of the member. These trends can be correlated between each of the sections despite differences in limestone vs. shale abundance. ARM values in the conformably overlying Smokey Hill Shale decrease significantly (<1.5 x 10 -6 Am 2 /kg) and show little or no variability over the basal 10 meters. We interpret this longer term ARM variability as representing regional changes in fine-grained eolian and/or fluvial-derived magnetic mineral assemblages, likely dominated by a moderate coercivity cubic phase (i.e. low Ti magnetite and possibly maghemite). There appears to be little or no relationship between the orbitally controlled climatic changes that produced individual limestone-shale couplets, which sheds light on their previously interpreted wet-dry climatic origins.
PP23B-1346
Climate and Ocean Circulation at the Paleocene/Eocene Boundary and Their Sensitivity to Atmospheric CO2
We study the climate at the Paleocene/Eocene (PE) boundary (55 Ma) using the coupled AO-GCM ECHAM5/MPI- OM. Applying simple PE boundary conditions and a CO2 concentration of 560ppm already yields a very warm, sea-ice free climate. Still, the Pole-Equator SST gradient is higher than suggested by proxy data. The simulated ocean circulation in the Atlantic is characterized by deepwater formation in the Proto-Labrador Sea. This North Atlantic Deepwater (NADW) flows southward as a western boundary current below the depth of the open Panama Strait. For a CO2 concentration of only 280ppm, the NADW formation becomes even stronger and deeper. By contrast, Nunes and Norris (2006) inferred a northward Atlantic bottom water track for the period before the Paleocene/Eocene Thermal Maximum from d13C gradients. The Pacific Meridional Overturning Circulation for a CO2 concentration of 560ppm is quite weak. Yet for the reduced atmospheric CO2 concentration of 280ppm, strong southern deepwater formation occurs.
PP23B-1347
Stable isotopes in pedogenic calcite: Can the positive linear covariant trends be used to quantify paleo-evaporation rates?
Paleoclimatological models suggest enhanced evaporation rates in subtropical regions during greenhouse- world conditions. Laboratory evaporation experiments show that calcites precipitated from variably saturated solutions yield a positive linear covariant trend (PLCT) in δ18O vs δ13C values. This investigation experimentally quantifies calcite PLCT so that δ13C of subtropical paleosol calcretes may be used as a regional proxy of paleo-evaporation rates. A series of powdered CaCO3 samples with δ18O and δ13C values of -19.6‰ and -37.2‰ VPDB respectively were dissolved in deionized water in a pressure sealed container; it also contained separate vials of calcite reacted with HCl to generate a range of pCO2 environments, thus simulating a soil atmosphere. The variable pCO2 conditions simulate expected soil atmosphere pCO2 conditions in a calcrete horizon during alternative phases of calcite dissolution and precipitation. After 24 hrs, the solutions were placed in an open beaker in an incubator at 36°C and allowed to evaporate. Aliquots of 100 μL were removed at 24 hr intervals and the time of calcite crystal nucleation was also noted. Water analyses yielded δ18O enrichments ranging from an initial value of -4.8‰ VSMOW to a range of +10.0‰ to +14.8‰ VSMOW after an evaporation period of 75 hrs. The most enriched water values were attained from the solutions formed under lower pCO2 conditions (more enriched calcite δ18O, δ13C). The array of calcite δ18O vs δ13C values fall upon a PLCT that projects from a theoretical meteoric calcite line (MCL) calculated from the incubation temperature and deionized water δ18O and δ13C values. The precipitated calcite δ18O values range from the MCL value of -8.8‰ VPDB to +0.5‰ VPDB. The higher pCO2 waters precipitated calcite very early during evaporation, and thus the δ18O and δ13C calcite values are slightly enriched relative to the theoretical MCL. The lower pCO2 conditions precipitated calcite late in the evaporation of the fluids, and thus yield more enriched calcite δ18O and δ13C values. Ongoing experiments under warmer and cooler evaporation temperatures will aid in the development of a quantitative model for paleo-evaporation rates from paleosol calcretes.
PP23B-1348
Using Apatitic Conodont Oxygen Isotopic Values to Determine the Cause of Late Ordovician Third-Order Stratigraphic Sequences
Recent work suggests a link between third-order (~1-5 Myr) sea-level fluctuations and climate change, even in greenhouse periods. Upper Ordovician third-order transgressive-regressive sequences are pervasive in the stratigraphic record, can be correlated worldwide (i.e. North America, Baltica, China), and ambiguous in cause. We are evaluating climate's role in third-order sea-level change by analyzing the δ18O of conodont apatite, which is a proxy for both glacio-eustasy and paleotemperature. Conodont phosphatic oxygen is a more robust repository of primary oxygen isotope values than more extensively-studied calcareous fossils, which have been extensively studied. If sea-level change is climatically-driven (glacio-eustasy and thermo-eustasy), δ18O values will decrease with sea-level rise and increase with sea-level fall. We report preliminary results from Upper Ordovician sequences in the Monitor Range of central Nevada. The six stratigraphic sequences (30m-95m thick) preserve basinal-to-outer-shelf carbonates with the youngest sequence representing the Hirnantian glaciation. We collected conodont samples at a 2-10m resolution and also determined bulk carbonate δ13C for additional chemostratigraphy. With this information, we hope to determine if glacio-eustasy has a role within the five pre-Hirnantian sequences.