PP13A-1021
Foraminiferal Mn/Ca: Uncovering its Paleoproxy Potential
College of Oceanic and Atmospheric Sciences, Oregon State University, Ocean Admin Bldg 104, Corvallis, OR 97331, United States of America. The flow-through time-resolved analysis (FT-TRA) technique for foraminiferal proxy work allows for the quantification of pristine biogenic elemental data, such as Mn/Ca, whilst disregarding overgrowth-induced signals. We used this state-of-the-art technique to carry out a core-top survey of the Mn/Ca ratios for surface dwelling foraminifera from across the world's oceans. This study revealed an oceanographically consistent pattern featuring order of magnitude variability, exactly what one would expect from the distribution of dissolved Mn in the mixed layer. Foraminifera at southern hemisphere sites have the lowest Mn/Ca ratios (~ 0.01) while those from the northern hemisphere are distinctly higher (~ 0.2). This pattern is strikingly similar in distribution and scale to variations in the atmospheric iron flux suggesting that foraminiferal Mn/Ca has potential as a paleoproxy for terrestrial input and productivity. The dust-dominated pattern at the surface together with the unique profile of Mn in the water column makes the Mn/Ca ratio of foraminiferal calcite a powerful new paleoproxy.
PP13A-1022
High Precision Low-blank Lithium Isotope Ratios in Forams.
We present a high precision (±1‰, 2σ) low blank (<500 fg/ml) method for Li isotope measurements of forams using <2 ng of Li by single collector Quad ICP-MS (Agilent 7500cs). The Li isotope ratio of seawater (δ7Li) recorded in planktonic forams has the potential to constrain the evolution of seawater chemistry and elucidate the factors driving variations of oceanic mass balances linked to the continental and sea floor/hydrothermal silica cycles. In addition a δ7Li record of seawater will complement other long-term recorders of seawater chemistry such as Sr, Os and S isotopes. Li isotope measurements of forams are limited by several factors: low Li concentrations in forams (1-2 ppm), instrument-induced fractionation and mass bias effects, matrix effects, high Li blanks and incomplete recovery of Li during column separation. Modest concentrations of alkali and alkaline earth elements in the matrix result in variable mass bias in measured Li isotope ratios. Even worse, Li strongly fractionates during chromatographic clean-up to remove Na+, Ca2+ and Mg2+, from +100‰ in the leading edge to - 100‰ in the trailing edge of elution peaks (Urey 1938). Consequently, miniscule incomplete recoveries of Li during chromatographic separations can result in large unrecognized isotope fractionation of eluents. Large mass-dependent fractionation caused by a difference of 17% in mass between 6Li and 7Li, makes Li a powerful tracer of geochemical processes, but also promotes large and difficult-to-fix isotope fractionations during laboratory chemical processing. Matrix effects of Na & Ca and of column chromatography on Li isotope ratios were investigated using artificial Li solutions representative of foram compositions (matrix matching). Li/Ca and Li/Na ratios in cleaned forams are 10 μmol/mol and 3 mmol/mol respectively. An ICP-MS tolerance limit of 20 ppb for Na and 20 μM for Ca was established, much higher tolerances than by TIMS. A single step chromatographic method to quantitatively separate Li from matrix elements using both small volume resin (3.4 meq/2ml AG50W-X8) and acid (6 ml of 0.5N HCl) was developed. Our low blank (<0.5 pg/ml) and high yield (>99.99%) column method minimizes errors in measured Li isotope ratios associated with incomplete column recovery and presence of matrix elements. High sensitivity and precision achieved with a 7500cs using cold plasma (600W), soft extraction and peak jumping coupled with very low sample to blank ratios enables high precision (±1‰, 2σ) statistically significant Li isotope measurements using very small mass of Li (0.8 ng). The development of this technique makes possible good quality Li isotope measurements from samples that are mass limited for Li, i.e., reasonable number of picked forams. This will enable us to test interferences regarding chemical cleaning and species effects in planktonic forams along the road toward creating a δ7Li record of seawater for the Cenozoic.
PP13A-1023
A High-Resolution Investigation of mid-Miocene Global Cooling
The mid-Miocene encompasses one of the four major cooling steps that formed part of the long-term decrease in global temperature during the Cenozoic. Four distinct mechanisms have been suggested to explain this mid- Miocene (c. 14 Ma) climatic change: drawdown of CO¬2 by burial of large amounts of marine organic matter (the "Monterey Hypothesis"); drawdown of CO¬2 by an increase in chemical weathering of fresh silicate rocks resulting from uplift of the Himalaya; changing ocean circulation patterns redistributing ocean-atmosphere heat; and the congruence of orbital parameters. However, the duration, timing and causative mechanisms of mid-Miocene cooling remain poorly constrained, largely due to incomplete sedimentary successions spanning this time period. In an attempt to resolve these issues we have conducted a high-resolution study of the Monterey Formation near Santa Barbara. Our detailed graphic logging has enabled us to identify eleven mudrock facies that are distributed cyclically (length-scales ranging from c. 0.5 to 20 m) throughout the section. Biostratigraphic dating of this section is based on diatom (this study) and calcareous nannofossil (Föllmi et al., 2005, GSA Bulletin, 117: 589-619) datums. Utilising these biostratigraphic age constraints and Fourier analysis of geochemical and physical parameters, we have produced an astronomical timescale that we have correlated with the Laskar orbital solution (Laskar et al. 2004, Astronomy and Astrophysics, 428: 261-285). High-resolution scanning electron microscopy has shown that several benthic foraminifera species are well preserved in these sedimentary deposits. The exceptional preservation of this sedimentary succession allows us to use a suite of geochemical proxies to investigate this global cooling event. The mid-Miocene carbon isotope record is characterised by a positive excursion, which commenced during global warming and ended after major expansion of the Antarctic ice sheet. We present an astronomically tuned high-resolution record (sampling on a sub-millenial timescale) of total organic carbon abundance and carbon isotope data that provides a detailed record of changes in the carbon cycle in the mid-Miocene. This new high- resolution carbon isotope record can be correlated with the distinctive carbon isotope maxima events CM5 and CM6 that have been identified previously in deep-sea sections. Our high-resolution data from the Monterey Formation provides us with a robust framework within which we can examine the dynamics and mechanisms of mid-Miocene global cooling.
PP13A-1024
Widespread formation of cherts during the early Eocene climate optimum
Radiolarian cherts in the Tethyan realm of Jurassic age were recently interpreted as resulting from high biosiliceous productivity along upwelling zones in subequatorial paleolatitudes the locations of which were confirmed by revised paleomagnetic estimates. However, the widespread occurrence of cherts in the Eocene suggests that cherts may not always be reliable proxies of latitude and upwelling zones. In a new survey of the global spatiotemporal distribution of Cenozoic cherts in Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment cores, we found that cherts occur most frequently in the Paleocene and early Eocene, with a peak in occurrences at ~50 Ma that is coincident with the time of highest bottom water temperatures of the early Eocene climatic optimum (EECO) when the global ocean was presumably characterized by reduced upwelling efficiency and biosiliceous productivity. Cherts occur less commonly during the subsequent Eocene global cooling trend. Primary paleoclimatic factors rather than secondary diagenetic processes seem therefore to control chert formation. This timing of peak Eocene chert occurrence, which is supported by detailed stratigraphic correlations, contradicts currently accepted models that involve an initial loading of large amounts of dissolved silica from enhanced weathering and/or volcanism in a supposedly sluggish ocean of the EECO, followed during the subsequent middle Eocene global cooling by more vigorous oceanic circulation and consequent upwelling that made this silica reservoir available for enhanced biosilicification, with the formation of chert as a result of biosilica transformation during diagenesis. Instead, we suggest that basin-basin fractionation by deep-sea circulation could have raised the concentration of EECO dissolved silica especially in the North Atlantic, where an alternative mode of silica burial involving widespread direct precipitation and/or absorption of silica by clay minerals could have been operative in order to maintain balance between silica input and output during the upwelling-deficient conditions of the EECO. Cherts may therefore not always be proxies of biosiliceous productivity associated with latitudinally focused upwelling zones.
PP13A-1025
Calcium isotope record of Phanerozoic oceans: Implications for chemical evolution of seawater and its causative mechanisms
Calcium isotope data (δ44/40Ca) of 280 brachiopods of Ordovician to Cretaceous age, complemented by published data from belemnites and planktonic foraminifera, are used to reconstruct the evolution of Ca isotope composition of seawater (δ44/40CaSW) over the Phanerozoic. The compiled δ44/40CaSW record shows a general increase from about 1.3‰ (NIST SRM 915a) at the beginning of the Ordovician to about 2‰ at present. Superimposed on this trend is a major long-term positive excursion from the Early Carboniferous to Early Permian as well as several short-term, mostly negative, oscillations. A numerical model of the global cycles of calcium, carbon, magnesium and strontium was used to estimate whether the recorded δ44/40CaSW variations can be explained by varying magnitudes of input and output fluxes of calcium to the oceans. The model uses the record of marine 87Sr/86Sr ratios as proxy for seafloor spreading rates, a record of oceanic Mg/Ca ratios to estimate rates of dolomite formation, and reconstructed atmospheric CO2, discharge and erosion rates to estimate continental weathering fluxes. The model results indicate that varying magnitudes of the calcium input and output fluxes cannot explain the observed δ44/40CaSW trends, suggesting that the isotope signatures of these fluxes must also have changed. As a possible mechanism we suggest variable isotope fractionation in the sedimentary output flux controlled by the dominant mineralogy in marine carbonate deposits, i.e. the oscillating 'calcite-aragonite seas'. The ultimate control of the calcium isotope budget of the Phanerozoic oceans appears to have been tectonic processes, specifically variable rates of oceanic crust production that modulated the hydrothermal calcium flux and the oceanic Mg/Ca ratio, which in turn controlled the dominant mineralogy of marine carbonates, hence the δ44/40CaSW. As to the causes of the short-term oscillations recorded in the secular δ44/40CaSW trend, we tentatively propose that these are related to variable rates of dolomite formation and/or to changing chemical composition of the riverine flux, in particular Ca/HCO3 and Ca/SO4 ratios, induced by variable proportions of silicate vs. carbonate weathering rates on the continents.
PP13A-1026
Molybdenum Behavior During Early Diagenesis: Insights from Mo Isotopes
This study presents Mo isotope data from surface sediments of the California, Mexico, and Peru continental margins. The enriched fraction of Mo in marine sediments can be represented by three primary components: Mn- controlled authigenic Mo, S-controlled authigenic Mo, and Mo associated with organic matter. These three components have distinct Mo isotope compositions and all sediments appear to reflect some mixture of these sources. The primary mechanism of Mo authigenic enrichment at depth for most of these study sites is likely Mo- sulfide precipitation; however, not all sediment profiles converge on a single authigenic isotope signature. Though Mn-associated solid-phase Mo does not typically survive early diagenesis, our data suggest that when Mn reduction is an important source of aqueous Mo within the sediments it is reflected in the isotopic composition of the authigenic Mo-sulfide fraction. While many aspects of Mo geochemical cycling remain poorly constrained, it appears that Mo isotopes may prove a useful proxy for geochemical conditions during sediment deposition.
PP13A-1027
In Situ Mo Isotope Fractionation in the Water Columns of Euxinic Basins
The present study investigates for the first time the overall process of molybdenum (Mo) scavenging in modern euxinic systems using Mo concentration and stable isotope measurements. We analyzed samples from three different sites: The Black Sea, the largest permanently euxinic basin, and two anoxic basins of the Baltic Sea, the Gotland Deep and the Landsort Deep which have maximum water depths of 247 m and 459 m, respectively. Water column profiles, as well as surface sediment samples, were recovered from different water depths. Mo is a redox-sensitive trace metal which is soluble as the molybdate oxyanion in oxic seawater with a residence time of about 800 ka. The isotope signature of Mo is a relatively new proxy used to reconstruct the paleo-redox conditions of the Earth's atmosphere and the oceanic system. The Mo isotope composition in seawater is homogeneous (Siebert et al. 2003). Scavenging of Mo under euxinic conditions is related to the amount of free sulfide in the water column. Near total removal of Mo from the water column is reached at aquatic sulfide concentration of c. 11 μM (Erickson and Helz 2000). In the Black Sea this corresponds to a water depth of about 400 m. Sediment samples of the Black Sea from more then 400 m water depth show seawater isotopic composition, in line with the assumption of bulk Mo removal. However, shallower sediments deposited under lower aquatic sulfide concentrations show significant Mo isotope fractionation. The Baltic Sea oceanographic conditions, including temporary bottom water oxygenation due to sporadic North Sea water inflows, are more complex than in the Black Sea. The aquatic sulfide concentration in the water column is less than 5 μM in the two anoxic troughs. As expected from this lower sulfidity, the surface sediments show Mo fractionation similar to the oxic to slightly euxinic sediments of the Black Sea. Our new results on the Mo isotopic composition in euxinic water columns clearly indicate in situ fractionation of Mo isotopes. All euxinic water samples from the three settings are shifted towards heavier Mo isotope signatures, thus complementing the lighter values in the surface sediments (Nagler et al. 2005).
PP13A-1028
The Geochemical Figure Print of an Early Paleozoic OAE
The Paleozoic Era contains many large, commonly globally expressed positive carbon isotope excursions recorded in carbonate rocks. In younger Mesozoic rocks, similar excursions are often easily linked to organic-rich deposits formed from enhanced carbon burial under ocean-scale anoxia –i.e., oceanic anoxic events (OAEs). These events are important since voluminous organic carbon and pyrite burial in anoxic settings can be a central player in modulating the amount of oxygen and carbon dioxide in the atmosphere, and many of Earth's major extinctions are coeval with ocean-scale anoxia. In contrast, physical records of organic carbon burial tied to the carbon isotope record are scarce in the Paleozoic; leading to ambiguity in the interpretation of the isotope data. These data become less cryptic when viewed in light of coeval seawater sulfur isotope trends. For the globally expressed, Late Cambrian (SPICE) carbon isotope excursion, carbonate-C and sulfate-S records reveal parallel, positive isotope shifts suggesting enhanced organic C and pyrite S burial. Additionally, both organic carbon and pyrite sulfur isotope data from the Alum Shale of Sweden record the SPICE Event, putting to rest questions of the primary nature of the carbonate records. Comparison of the SPICE to similar isotope data from the Toarcian OAE and results from geochemical box modeling of both events lead us to conclude that the SPICE Event is a prime candidate for an early Paleozoic OAE. Additional evidence for increased ocean anoxia coincident with the SPICE also comes from the Alum Shale. Molybdenum concentrations show muted enrichment during the extent of the SPICE, despite data that show the basin was persistently euxinic before, during and after the event. Significant increases in molybdenum concentration occur only immediately after the event; suggesting a depleted seawater Mo inventory associated with a greatly expanded global anoxic Mo sink during the SPICE. An interesting result from geochemical box modeling of the SPICE and Toarcian records is the suggestion of large-scale oxidation of 34S-depleted sulfur at the end of both events. A likely source of this sulfur is the oxidation of destabilized euxinic water masses. The driver of this oxidation was most likely increasing atmospheric pO2 in conjunction with lowered pCO2-the result of the enhanced organic carbon and pyrite burial that marked the events. Lowered pCO2 allowed for lower global temperatures, invigorated ocean circulation and thus the delivery of O2 to the deep ocean, leading to the demise of each anoxic event.
PP13A-1029
Sequestration of Tellurium From Seawater by Ferromanganese Crusts: A XANES/EXAFS Perspective
Marine iron-oxyhydroxide/manganese-oxide crusts (Fe-Mn crusts) provide the richest known source of tellurium (Te). Te averages about 50 ppm in Fe-Mn crusts distributed globally, with concentrations locally up to 210 ppm. The sorption of Te onto Fe-Mn crusts likely controls the dominant redox species and concentration of Te in the global ocean (Hein et al., 2003). However, little is known about the mechanisms by which Te is sequestered by Fe-Mn crusts and Fe-Mn colloids in the water column, and then stabilized in the Fe/Mn oxyhydroxide/oxide framework. Two primary hypotheses are being tested: (a) Te(IV) is initially the predominant adsorbed species, which is subsequently oxidized on the Fe-oxyhydroxide and/or Mn oxide phases in natural systems and in sorption experiments. (b) Once oxidized, Te(VI) remains tightly bound to the Fe phase in Fe-Mn crusts as adsorbed surface complexes. These hypotheses are being examined by using the Stanford Synchrotron Radiation Laboratory's (SSRL) synchrotron-based XANES (x-ray absorption near-edge structure) spectroscopy to assess Te oxidation state in natural samples and samples in which Te(IV) and Te(VI) were sorbed onto synthetic and natural FeOOH and Mn oxides. EXAFS (extended x-ray absorption fine structure) spectroscopy is being used to resolve the local molecular-scale structure around Te in these same samples. Data have thus far been obtained for six Fe-Mn crusts from a variety of geographic locations and water depths of occurrence, with differing chemical compositions; and two model compounds, Te(IV) sorbed on FeOOH and Te(IV) sorbed on MnO2. XANES data show that for all six Fe-Mn crust samples, 85 to 100 percent of the Te occurs as Te(VI). For the model compounds, about 65 percent of the Te(IV) sorbed onto the MnO2 had oxidized to Te(VI) by the time (one week) the sample was analyzed, whereas Te sorbed onto FeOOH remained at about 100 percent Te(IV). The most striking result from the EXAFS data is that all spectra for the six Fe-Mn crust samples are virtually identical, regardless of location, depositional conditions, or chemical and mineralogical compositions. This uniformity indicates that the local structure around Te is similar for all samples and, therefore, the mode of incorporation of Te into the Fe-Mn crusts does not vary despite varying environments of formation. This implies that a single set of processes applies throughout the global ocean to the incorporation of Te into Fe-Mn crusts. Hein, J.R., Koschinsky, A., and Halliday, A.N., 2003, Geochim. Cosmochim. Acta 67: 1117-1127.
PP13A-1030
Iron Speciation using X-ray Absorption Spectroscopy of Neoproterozoic Cap Carbonates from the Pocatello Formation (Idaho)
Carbonate deposition immediately follows each of the major low latitude glacial events of the Neoproterozoic. Cap carbonates sit directly upon glacial diamictite and provide an opportunity to examine ocean chemistry immediately after glaciation. The Neoproterozoic western margins of Laurentia record at least two such cap carbonates and show unusual trace element geochemistry. Here, we use synchrotron based X-ray absorption spectroscopy (XAS) to examine the speciation of iron in Neoproterozoic rocks of the Scout Mountain Pink Dolomite member of the Pocatello Formation (ID). Speciation of redox sensitive metals such as iron can be used in conjunction with trace element data to provide a highly sensitive record of marine sediment depositional environments and diagenesis. Iron redox state is related to oceanic mixing and types and intensity of biological productivity - both of these processes should be disrupted during low latitude glaciation. The unusual and rapidly-changing climatic conditions during low latitude glaciations may lead to highly variable conditions of ocean oxidation due to decoupling between the ocean and atmosphere, a strongly stratified water column coupled with the decomposition of organic carbon, or microbially-mediated sulfate reduction. The pink dolomite shows elevated iron concentrations at its base. These concentrations return to background values over only a few centimeters of section. The elevated iron concentrations may result from a range of redox states during deposition followed by sequestration in a variety of mineral phases. Under reducing conditions Fe(II) is prevalent and will accumulate in sulfide phases. Under suboxic conditions a mixture of Fe(II) and Fe(III) may exist and the deposition of magnetite will occur. In a well oxygenated water column iron is oxidized to Fe(III) and precipitates as hematite or goethite. XAS was utilized to probe the average bonding environment of iron. Linear combination fitting of iron mineral standards was used to find bulk iron mineralogy. The resulting data suggest that absorption spectroscopy is a useful tool for unraveling rapid changes in redox state during sedimentary deposition. It provides valuable insight into the rapid and highly unusual changes in ocean chemistry at the terminus of the Sturtian glaciation.
PP13A-1031
A geochemical modelling study of the evolution of the chemical composition of seawater linked to a global glaciation
The Snowball Earth theory initially proposed by Kirschvink (Kirschvink, 1992) to explain the Neoproterozoic glacial episodes, suggested that the Earth was fully ice-covered at 720 My (Sturtian episode) and 640My (Marinoan episode). In the absence of continental weathering due to the spreading of large ice sheets (Hyde et al 2000, Donnadieu 2003), the main processes changing the seawater composition are those occurring at the seawater - oceanic crust interface. Using a numerical model of carbon-alkalinity global cycles, we study oceanic crust - seawater interactions to quantify the environmental changes caused by a global glaciation wherein the atmosphere is highly enriched in CO2. We propose a scenario wherein the ocean becomes acidic (pH~6) during global glaciations, even if the carbonate dissolution occurs. During the glacial episode, the oxygen consumption by hydrothermal activity tends to enhance the anoxia of the deep ocean. The quick transition from ice-house to greenhouse conditions and the associated temperature rise causes an extended hypoxia in the surface ocean. The intense continental weathering, in the aftermath of the glaciation, deeply affects the seawater composition inducing rapid changes in terms of pH and alkalinity. According to our modelling results the short-lived and large post-glacial perturbations are more critical than glacial environmental changes. Instead of the glaciation itself, these results tend to suggest that the deglaciation might have played the role of environmental filter proposed in the classic snowball Earth theory.
PP13A-1032
Negative Sulfate O-17 Anomalies and an Extraordinary Temporal Spike at the Immediate Aftermath of Marinoan "Snowball" Earth
Understanding the composition of the atmosphere over geologic time is critical to understanding Earth system history, as the atmosphere is closely linked to the lithosphere, hydrosphere, and biosphere. While much of the history of lithosphere and hydrosphere is contained in rock and mineral records, corresponding information about the atmosphere is scarce and elusive due to the lack of direct records. Geologists have used sedimentary minerals, fossils, and geochemical models to place constraints on the concentrations of CO2, O2, or CH4 in the past. We report that the triple oxygen isotope composition of sulfate from ancient evaporites and barites exhibits variable negative 17O anomalies over the last 750 million years. We argue that these anomalies track those of atmospheric O2 and in turn reflect the partial pressure of CO2 (pCO2) in the past via a stratospheric O3-CO2-O2 photochemical reaction network. Our results suggest that pCO2 was much higher in Early Cambrian than in younger era, agreeing with previous modeling results. Most significantly, the 17O anomalies of barites in Marinoan (~ 635 million years ago) cap carbonates display a distinct negative spike (down to -0.70‰), suggesting that pCO2 was still at ~ 30 times of modern level by the time barite was precipitating in the cap carbonate sequences, which strongly supports the Neoproterozoic "snowball" Earth hypothesis. Consistently more negative 17O values for the lower barite bed than for the upper one at multiple sites at Baizhu, Hubei Province, South China attest probably a rapid CO2 drawdown. While the sulfate 17O record does not have the sensitivity to detect atmospheric pCO2 changes between glacial and interglacial times, it can be most useful in evaluating extreme atmospheric conditions (e.g. pCO2 and pO2) and their dynamic changes that have occurred in Earth history, including "snowball" Earth, impact events, or atmospheric conditions in a much earlier Earth system.
PP13A-1033
Orbital Tuning as an Inverse Problem: Dating the Aptian Selli OAE in the Cismon borehole (Southern Alps, Italy)
The Selli Level is a black shale interval that records in Italy an Aptian global oceanic anoxic event. We studied about 30 m of the Cismon APTICORE borehole, where the Selli Level is about 5 m thick and is composed of alternating limestone and black shale. A thin nodular interval (about 2 cm) immediately below the base of the Selli suggests condensed sedimentation or a hiatus. The goal of our study is to date by orbital tuning the duration of the Selli Level and of the hiatus/condensation at is base. Orbital tuning is the process of matching measurements of climate-related sediment properties (eg., CaCO3 content) with Milankovitch periodicities. Orbital tuning can be cast as an inverse problem: find sedimentation rate models that result in measurement time series with spectral peaks at the Milankovitch frequencies. To minimize unrealistic variations in sedimentation rate, we use as a model a sequence of layers with constant sedimentation rates. This model allows for abrupt changes of sedimentation rate (eg., at the boundaries of the Selli) and for hiatuses (eg., at the base of the Selli). We quantify the final uncertainty of the sedimentation rates and account for uncertainty in the tuning frequencies with Monte Carlo modeling. For tuning, we use the orbital periods of short eccentricity (fixed at 100 kyr) and obliquity (allowed to vary in the 35-41 kyr interval). Our measurement series is a downhole log of electrical resistivity (which is well correlated to CaCO3 content) with a vertical resolution of about 1 cm. The inversion results give sedimentation rates of 3-5 m/Myr in the Selli Level and higher rates of 5-9.5 m/Myr above and below. The most likely duration for the Selli Level is about 1.2 Myr. If ages are calibrated to the magnetic stratigraphy with a base M0 at 121 Ma, the Selli Level was deposited from about 120.1 to 119 Ma. The inversion gives a duration of about 19 kyr for the hiatus at the base of the Selli. Nannofossil abundance and composition in the nodular interval suggest a 12-14 kyr condensation, resulting from carbonate dissolution under rapidly increasing pCO2. Finally, we determine a best-fit obliquity period of about 38.4 kyr at this time (around 120 Ma) compared to the present-day value of 41 kyr.
PP13A-1034
Stable Isotope Constraints on the Ocean from Hydrothermally-altered Igneous Rocks
The 18O/16O ratio of the ocean provides an important constraint on the global geochemical cycles in the Precambrian Earth. The oxygen isotope ratio of the ocean is most likely buffered near its present day value as long as plate tectonics is operative. A quasi-steady state value for oxygen isotopes is reached on a 100 Myr timescale after the onset of plate tectonics. Hydrothermally-altered igneous rocks constrain the oxygen and hydrogen isotope value of the hydrosphere back through time. Whereas, the oxygen isotope composition of seawater owes its value to the competition between low temperature chemical weathering and mid-ocean ridge hydrothermal exchange, there is no such process for hydrogen isotopes. Changes in the oxygen isotope ratio of seawater should be reflected in hydrothermally altered rocks by the presence of low or high 18O exchanged igneous rocks with normal δD values. The distribution of D and 18O in hydrothermally rocks is used to infer the position of the meteoric water line back through time. Results from the Phanerozoic, the Proterozoic, and the Archean fail to confirm the hypothesis that the global oceans were ever strongly 18O-depleted. The meteoric water line is anchored to the isotopic composition of seawater, the isotope standard for both oxygen and hydrogen isotopes. The ability to use sedimentary rocks or other proxies for climate depend upon the variation in the stable isotopic composition of seawater. Thus far, the hydrothermal record does not support the existence of low 18O oceans. This suggests that low 18O values observed in carbonates and cherts result from either precipitation from oceans with higher temperature or from bodies of water isolated from the open ocean.