PP21A-01 INVITED
Understanding Oceanic Anoxic Events: An Integrated Geochemical Approach
Discrete intervals of widespread organic carbon accumulation, termed Oceanic Anoxic Events (OAEs), occurred at a few relatively brief intervals during the Mesozoic. Recent studies have shown that these events took place at the same time as other substantial environmental changes that included global warming, ocean acidification, and unusually high levels of species extinctions. However, many factors relating to the behaviour of the Earth System during OAEs remain unclear. These include: The primary driving mechanism(s) - was there one common mechanism or were OAEs the result of different processes; the spatial and temporal extent of seawater anoxia during OAEs; the precise effects on marine and terrestrial biota; variations in atmospheric CO2 and global temperature; and the mechanism and timescale of Earth's recovery process. The records of environmental change during OAEs are best preserved in marine deposits, with continental shelf sections being particularly well studied. The combined use of geochemical, sedimentological and palaeontological observations indicates a complex interplay of factors. Significant advances in our understanding of OAEs have taken place in the last decade or so using new geochemical and isotopic proxies and a high- resolution, multidisciplinary approach. For example, Sr- and Os-isotope data indicate that rates of chemical weathering increased markedly during the Toarcian (Early Jurassic) OAE, whilst Mo-isotope data suggest that the areal extent of seawater anoxia fluctuated during the OAE despite the persistence of euxinic conditions in some regions. The pattern of Mo-isotope data for the Toarcian contrasts strongly with new Mo-isotope results from the Kimmeridge Clay Formation (Late Jurassic), when anoxic conditions were confined to European epicontinental seas and were likely to have resulted from very different primary causes. Cyclostratigraphic analysis has been used to provide a temporal framework for the timescale of OAEs at sub- Milankovitch resolution. Evidence from a number of OAEs indicates that sudden and major changes during these events sometimes occurred in no more that a few hundred years. These abrupt changes appear to correspond with the crossing of certain critical thresholds in the Earth System. The rate and magnitude of environmental change during OAEs, in parameters such as atmospheric CO2, global temperature increase, etc., have been shown to have been broadly similar to present day rates of change. The study of OAEs can thus potentially inform us about the possible longer-term consequences of anthropogenic environmental change.
PP21A-02
Biogeochemistry of Neoproterozoic Snowball Earth and its Aftermath in South China
Using a combination of lipid biomarkers, inorganic geochemical proxies (Fe speciation and trace element analyses) and stable isotopic abundances (13C, 34S, 98Mo), we are reconstructing ocean chemistry and aquatic microbial community structure around the Neoproterozoic Sturtian and Marinoan glacial events and later into the Ediacaran in South China. We have sampled outcrop sediment extensively at four locations (Yichang, Shimen, Minle and Zhaoxin), obtaining sedimentary facies ranging from shallow shelf to the deep basin. Our stratigraphic coverage spans from unweathered sediments of the pre-Sturtian Gongdong Formation up to the post-Marinoan Doushantuo Formation. This approach allows us to compile, for the first time, a highly detailed, spatial and temporal record of Neoproterozoic marine biogeochemistry in South China. To assess and augment conventional extractable biomarker hydrocarbon data, we are using the technique of catalytic hydropyrolysis (HyPy) on kerogen residues to release well-preserved kerogen-bound biomarkers that are much less susceptible to contamination since they are covalently bound within an immobile solid matrix deposited synchronously with the host sediment. Interglacial Datangpo and Ediacaran Doushantuo formations that sandwich the Marinoan diamictites are most interesting for detailed organic and inorganic examination and comparison in this study due to their distinct stratigraphic context and unique spatial preservation. A diverse range of normal, branched (2-methyl, 3-methyl and mid-chain monomethylalkanes) and polycyclic alkanes (steranes and hopanes) have been successfully isolated from Doushantuo sediments in Shimen, and a distinct correlation between host-rock lithology and molecular and isotopic characteristics of detected alkane biomarkers was observed, suggesting that the hydrocarbon signals are indigenous and syngenetic with the host rocks. Sediments with appreciable inorganic carbonate content appear to hold most promise for preserving the original biomarker content, since the acidic clay content of black shales may promote maturation and aromatization of the indigenous organic matter. Temporal trends in our geochemical and isotopic records will be discussed.
PP21A-03
Mo Isotopes Record Destabilization of a Stratified Ocean at the Precambrian-Cambrian Boundary
Here we present Mo isotope signatures in black shales from two sample sets (Ara group, Oman and Yangtze Platform, China) which were deposited at and shortly after the Precambrian-Cambrian boundary (PC-C). At the first view, the overall Mo isotopic signatures (delta98/95Mo) of the Early Cambrian black shales is 1.2 permil below recent ocean water, similar to the signature found in Mesoproterozoic shales (Arnold et al. 2004), indicating a larger proportion of Mo sedimentation under strongly euxinic conditions compared to recent oceans. A chemically stratified ocean with sulfidic deep waters and modestly oxygenated surface waters as proposed by Canfield (1998) for the Paleoproterozoic and Mesoproterozoic ocean, and Jiang et al. (2007) reported Carbon isotope data from the Ediacaran Yangtze platform (635-542 Ma) to be consistent with long-term deep ocean anoxia/euxinia. A stratified ocean therefore provides a plausible scenario to explain our new PC-C Mo isotope data. On closer inspection, a transient Mo isotopic signal following immediately after the PC-C boundary in both sample sets indicates a short but intense global non-steady state situation. In particular, a short term, drastic decrease of the Mo ocean inventory to almost zero is required to reconcile the observed Mo isotope data. Combined with the extreme Mo enrichment, found in the Chinese sulfide marker bed at the PC-C boundary, this signal has to be explained with a non-uniformitarian Mo scavenging mechanism. We put forward the hypothesis of mixing of oxidized, i.e. Mo rich surface waters with upwelling euxinic bottom water masses of the stratified ocean, as H2S is the most efficient Mo scavenging reagent. This scenario not only explains the transient isotopic signal, it can also be responsible for the sudden extinction of the Ediacaran fauna by H2S poisoning. In contrast, mass extinction scenarios like bolide impact, flood basalt eruptions or methane release, do not provide a direct explanation for the observed Mo isotope data.
PP21A-04
Tracing redox processes during paleoclimatic changes in the Neoproterozoic: Stable chromium isotopic results from the Arroyo del Soldado Group (Ediacaran, Uruguay)
Positive δ13C carbonate values, combined with the occurrence of Fe-rich cherts (oxide-facies BIF) and organic-rich black shales within the late Ediacaran (ca. 580-560 Ma) Yerbal Fm. of the Arroyo del Soldato Group (Uruguay) are compatible with paleoclimatic models which postulate that enhanced bioproductivity due to higher availability of nutrient (P, N, Fe) was essential for controlling Neoproterozoic glaciations. Tracing of associated redox processes (f.e. linked to oxygenation of bottom waters in restricted basins) that might have been responsible for the deposition of Fe-rich cherts (BIFs) is therefore an important tool to better understand the seawater changes during cold-warm periods. Besides the traditionally used Fe and Mo isotopic systems, the redox-sensitive element Cr (Cr(III); Cr(IV)) and its stable isotopes offer another complementary system to trace paleo-redox processes. We have applied Cr stable isotope systematics to a sequence of samples from a late Ediacaran sedimentary sequence in Uruguay, using a 52Cr-54Cr double spike (Schoenberg et al., Chem..Geol., subm.). The middle Yerbal Fm. is dominated by organic-rich, black shales and black dolostones (δ53Cr = -0.05‰), followed by organic-rich cherts (δ53Cr = +1.83 - +4.49 ‰) and BIF (δ53Cr = -0.31 +0.90 ‰) gradually changing into Fe-bearing, organic-rich cherts and shales (δ53Cr = -0.28 - -0.01 ‰), and another sequence with BIF and organic-rich cherts topped by carbonates of the lower Polanco Fm. (δ53Cr = -0.17 to -0.27 ‰). The strongly positively fractionated Cr isotopic signatures in organic-rich and Fe-rich cherts in the Yerbal Fm. may point to significant oxidation processes either directly in the seawater column and/or during early diagenetic processes at the sediment-water interface. While these strongly positive δ53Cr values are the first to be reported from Neoproterozoic sedimentary sequence, the exact nature of the chemical process that produced these anomalies is not yet understood. However, the occurrence of these anomalies in organic-rich and Fe-rich chemical sediments that were deposited in a period following a glacial (Gaskiers?) event is compatible with "Snowball Earth" scenarios whereby impulsive oxidation of the upper seawater was in response to ice cover retraction which allowed booming of the biosphere and concomitant oxidation of accumulated Fe2+ and subsequent precipitation of the Fe-oxyhydroxides to form the "BIF" during such epochs. Schoenberg et al. (subm.) The stable Cr isotope inventory of solid earth reservoirs determined by double-spike MC-ICP-MS. Chemical Geology
PP21A-05 INVITED
Sulfur and Oxygen isotope variability across Cretaceous Ocean Anoxic Events
The marine carbon and sulfur cycles are linked through bacterial sulfate reduction and organic matter oxidation in organic-rich sediments. Therefore, reconstructing temporal variability in the sulfur cycle remains an important goal for understanding changes in the carbon cycle, paleoalkalinity and paleoclimate. Traditionally the sulfur isotopic composition (δ34S) of sulfur minerals has been used to explore changes in the biogeochemical sulfur cycle through Earth history. The δ34S varies largely as a function of pyrite burial, its isotopic composition, and river input. Recent work has advanced the use of the oxygen isotopic composition of sulfate (δ18OSO4) as a separate mechanism to probe changes in the sulfur cycle over time. The δ18OSO4 varies with changes in the pathways of sulfate reduction and sulfide reoxidation in organic rich sediments. Thus the measurement of both the sulfur and oxygen isotope composition of sulfate minerals affords the possibility to explore variations in the biogeochemical sulfur cycle in precisely the location – organic rich sediments – where it is coupled to the carbon cycle. We will present data comparing the δ34S and the δ18OSO4) in marine barite from the middle Cretaceous. We will use this coupled isotope data to explore causes of temporal variability in the Cretaceous sulfur cycle. Unlike the δ34S of marine barite across this time interval, the δ18OSO4) shows rapid and large isotopic excursions associated with Ocean Anoxic Events (OAE). The δ18OSO4) remains constant across OAE1b, increases after OAE 1c, decreases after OAE 1d, and then increases both during the Mid-Cenomanian event and OAE 2. These results suggest that there were distinct differences in the carbon cycle during different OAEs. We suggest that these differences may be due to euxinic (i.e. presence of H2S in the water column) versus anoxic oceans, which would impact the pathways of sulfide oxidation and therefore impact the δ18OSO4). The δ34S, on the other hand, is likely responding to a longer term changes in the amount or isotopic composition of pyrite being buried and does not respond to the rapid variability in the carbon cycle over this time.
PP21A-06
A Very Weakly Reducing Atmosphere Prior to the Rise in Atmospheric Oxygen
We describe the photochemistry of stable anoxic atmospheres in response to declining fluxes of reducing gases. The goal is to understand the atmospheric consequences of increases in the oxidation state of the surface ocean prior to the Great Oxidation Event, when Earth's atmosphere first transitioned from weakly reducing to weakly oxidizing on a global scale approximately 2.4 billion years ago (Ga). Drill cores of the 2.5 Ga Mt. McRae shale reveal evidence for free oxygen availability in Earth's surface ocean, and perhaps require the presence of a trace amount of surficial free oxygen to induce oxidative weathering of the continents (Anbar et al., 2007 ; Kaufman et al., 2007). Simultaneous co-detection of a mass-independent sulfur isotope (MIF S) signal indicates generally reducing conditions must have existed throughout the troposphere (Zahnle et al., 2006) at this time. Using a simple box model, we had previously predicted (Claire et al., 2006) that such an atmosphere might exist as a result of likely evolutionary models of planetary fluxes. This work will use detailed photochemical models to investigate the vertical dependence of methane photolysis, OH production, SO2 photolysis, and S8 deposition in very weakly reducing atmospheres that may have proceeded the Great Oxidation Event. Anbar et al."A Whiff of Oxygen Before the Great Oxidation Event?" Science, 2007, in press Claire et al."Biogeochemical modelling of the rise in atmospheric oxygen." Geobiology, 4, 239-269, 2006. Kaufman et al."Late Archean biospheric oxygenation and atmospheric evolution." Science, 2007, in press. Zahnle et al."The loss of mass-independent fractionation in sulfur due to a Paleoproterozoic collapse of atmospheric methane." Geobiology, 4, 271-283, 2006.
PP21A-07
Combining Radiogenic Strontium (87Sr/86Sr) and Stable Strontium Isotope (δ88/86Sr) Fractionation to Balance the Ocean Strontium Budget
The stable 88Sr/86Sr-ratio has traditionally been considered to be a constant value in order to correct instrumental mass fractionation during measurement of the radiogenic strontium isotope ratio (87Sr/86Sr). However, recent high precision measurements (Fietzke and Eisenhauer, 2006) showed that the 88Sr/86Sr ratio of marine carbonates and certain silicates (deSouza et al., 2007) are not constant, but rather variable and temperature dependent. Variations of the 88Sr/86Sr-ratios are expressed in the usual δ-notation: δ88/86Sr=((88Sr/86Sr)sample/(88Sr/86Sr)NBS987-1)*1000, where δ88/86SrNBS987=0. The recent measurements also showed that the presently accepted (88Sr/86Sr)-value for seawater differs by about ~0.4 ‰ from the 88Sr/86Sr)NBS987 value. This in turn implies that the 87Sr/86Sr isotope ratio in seawater not corrected for natural isotope fractionation is also significantly different from its normalized value (87Sr/86Sr=0.70916). Correcting the 87Sr/86Sr ratio of seawater results in a 87Sr/86Sr value of about ~0.70930. First results from continental rocks and minerals indicate that they are significantly lighter than seawater by about 0.3 ‰ and presumably become fractionated due to dissolution, precipitation of secondary mineral phases and biological utilization ((deSouza et al., 2007), (Halicz et al., 2007)). The combination of δ88/86Sr and 87Sr/86Sr data provides a unique solution for three-component mixing processes and a way of distinguishing the different sources that supply Sr to seawater. Latter mixing has to account for the characteristic radiogenic ingrowth of the 87Sr/86Sr ratio as well for the natural fractionation related to chemical weathering, temperature and other yet not known effects. This may help to elucidate the link between continental weathering and atmospheric pCO2 in more detail on geological time scales. deSouza G., Reynolds B., and Bourdon B. (2007) Evidence for Stable Strontium Isotope Fractionation during Chemical Weathering. 17th Goldschmidt Conference. Fietzke J. and Eisenhauer A. (2006) Determination of temperature-dependent stable strontium isotope (δ88/86Sr) fractionation via bracketing standard MC-ICP-MS. Geochmistry, Geophysics, Geosystems 7(8), doi:10.1029/2006GC001243. Halicz L., I. Segal, N. Fruchter, B. Lazar, and Stein M. (2007) 86Sr/88Sr Ratio by ICP-MS-MC as a New Tracer of Terrestrial Geochemical Processes. 17th Goldschmidt Conference 2007.
PP21A-08
Systematic temporal thallium isotope variations in a 72 Ma old Fe-Mn crust: A proxy for changes in ocean chemistry?
A large equilibrium isotope fractionation of about 2 per mil occurs between thallium (Tl) in seawater and modern Fe-Mn crusts [1]. We measured the Tl isotope composition through the entire thickness of the Pacific Fe-Mn crust CD29-2, which has recently been dated using Os isotope stratigraphy [2]. The samples have a spatial resolution of between 0.2 and 1 mm which equates to about 0.1 to 0.5 Myrs. The most prominent variations occur in the time interval between 72 Ma (the base of the crust) and 50 Ma. The Tl isotope composition decreases from ε205Tl = +7.5 (ε205Tl represents the deviation of the 205Tl/203Tl isotope ratio of a sample from NIST SRM 997 Tl in parts per 104) at 72 Ma to a minimum of ε205Tl = +5.0 at 67 Ma and immediately rebounds to ε205Tl = +6.0. No change is thereafter recorded until 59 Ma, where ε205Tl increases smoothly from +6.0 at 59 Ma to +10.5 at 50 Ma. The remainder of the crust displays only limited Tl isotope variations. Five previous analyses at various depths in CD29-2 [3] are fully consistent with our more detailed investigation. The variations most likely reflect either changes in the Tl isotope composition of seawater over time [3] or could represent a change in the Tl isotope fractionation factor between Fe-Mn crusts and seawater. Changing the Tl isotope composition of seawater requires that the fluxes of ocean inputs or outputs change over time. Based on previous studies, it appears that only a change in the relative outputs of Tl associated with Fe-Mn oxyhydroxides adsorbed onto pelagic sediments and Tl uptake into oceanic crust altered at low temperatures has the necessary isotopic leverage to drive the observed variations. Recent theoretical calculations of the mechanism controlling Tl isotope fractionation have predicted that there is a large equilibrium isotope fractionation between Tl1+ and Tl3+ [4]. Published Tl isotope compositions of modern Fe-Mn crusts [1] and altered mid-ocean ridge basalts [5] are consistent with these two reservoirs exclusively incorporating Tl3+ and Tl1+, respectively. Thus, changes in the speciation of Tl in seawater may also be responsible for the observed Tl isotope variation recorded in Fe-Mn crusts. However, under normal aqueous conditions only Tl1+ should be thermodynamically stable. We are therefore conducting further investigations to establish if the oxidized form of Tl is stable in seawater. [1] Rehk\¨{a}mper et al., EPSL, 2002, [2] Klemm et al., EPSL, 2005, [3] Rehk\¨{a}mper et al., EPSL, 2004, [4] Schauble, GCA, 2007, [5] Nielsen et al., EPSL, 2006