PP14A-01 INVITED
The Bioinorganic Bridge Between Life and Environment
Nearly half the Earth's surface is covered by oligotrophic oceans in which life struggles to survive. These barren ecosystems do not lack water or sunshine, nor the bulk biological elements H, C and O. Instead, they are deficient in one or more of the other elements necessary for life. Hence, the distribution of life on Earth is captive, in part, to the distribution of the 20 or so "bioessential" nutrient elements that are critical components of DNA, RNA, proteins and other biomolecules. Having substantially unraveled this relationship in today's oceans, biogeochemists are beginning to consider how it has evolved over the ~ 4 billion year history of life. A growing body of evidence suggests that major evolutionary transitions are shaped by changes in the availability of bioessential metals. The most striking example is the ecological diversification of eukaryotes. Fossil evidence suggests that this transition roughly coincides with oxygenation of the deep sea, and hence with a hypothetical increase in Cu and Zn coupled to a decrease in Fe in the oceans (Saito et al., 2003). Bioinformatic analyses of protein metal binding sites encoded in whole genomes reveal that the "metallomes" of eukaryotes are systematically enriched in Zn, and depleted in Fe, Mn and Co, compared to those of prokaryotes (DuPont et al., 2006). Arguably, then, eukaryotes rose to ecological prominence because of the advent of oceans that favored organisms with eukaryotic stoichiometry. We are exploring this concept mechanistically by investigating the biochemical responses of representative eukaryotic green algae and cyanobacteria to chronic Fe-deficiency. We observed the intracellular Cu quota of the eukaryotic green alga Chlamydomonas reinhardtii increase under chronic Fe-deficiency. This shift occurs because green algae replace key Fe proteins involved in photosynthetic electron transfer (e.g., cytochrome c6) with analogous Cu-based proteins (e.g., plastocyanin). In contrast, we found that the Cu quota of the cyanobacterium Synechocystis sp. PCC6803 did not increase under chronic Fe-deficiency. Rather, intracellular Cu decreased. We hypothesize that, like eukaryotes, cyanobacteria respond to Fe- deficiency by producing compensatory proteins, but that these proteins either do not use Cu or reallocate intracellular Cu rather than increasing Cu content. The implication of these results is that cyanobacteria are better equipped than algae to cope with combined Fe- and Cu-deficiency. Eukaryotic algae would not have encountered favorable conditions until thorough oxygenation of the oceans in the Neoproterozoic ushered in the Fe-poor but comparatively Cu-rich conditions typical of Phanerozoic oceans. DuPont et al. (2006) Proc. Nat. Acad. Sci. USA 103: 17822-17827 Saito et al. (2003) Inorg. Chim. Acta 356: 208-318
PP14A-02 INVITED
Understanding the Origin and Diagenetic History of Multiple Sulfur Isotope Signals in Late Archean Sedimentary Rocks
We have been studying the chemical taphonomy of multiple sulfur isotope ratios in a suite of drill core samples through the Transvaal Supergroup, South Africa, collected during the Agouron Drilling Project; these samples capture distal slope and basinal environments adjacent to a major Late Archean-age (~2.6-2.52 Ga) carbonate platform. Bulk rock analyses reveal sulfur isotope ratios that generally carry strong non-mass-dependent isotope fractionations, but little δ34S variability. One challenge that arises when interpreting existing data is the persistent observation that the sulfide-bearing minerals present in a given rock sample demonstrate complex origins that combine detrital, pelagic, diagenetic, and metamorphic components. More specifically, petrographic textures reveal that, much like other deposits of the same age, this rock succession has witnessed several episodes of sulfide mineralization. Rock magnetic data indicate that this later sulfide mineralization is carried by the magnetic sulfide-bearing mineral pyrrhotite. Using scanning high-resolution low-temperature superconductivity SQUID (superconducting quantum interference device) microscopy, we have imaged, at a 50 micron scale, the late magnetization in black shale samples containing diverse sulfide mineral components. When combined with petrography, these magnetic images allow both the identification of pristine unaltered sulfides and later mineralization. At the same scale, precise measurements of sulfur isotopes made by secondary ion mass spectrometry (SIMS) using a Cameca IMS 7F-GEO instrument allow us to target these different phases and resolve both non-mass-dependent isotopic signals and systematic micron-scale variations in early diagenetic pyrite nodules; these relationships provide quantitative constraints on environmental conditions and respiratory processes operating in Late Archean sedimentary environments. More broadly, the combination of these two techniques make it possible to interrogate, at a scale previously unobtainable, the origin and preservation of geobiological proxies recording the evolution of the sulfur cycle and the first appearance and rise of atmospheric oxygen.
PP14A-03
Assessing the Redox Structure of the Proterozoic 'Sulfidic' Ocean
Over recent years, there has been considerable interest in the suggestion that after about 1.84 Ga the Proterozoic ocean was characterised by widespread sulfidic conditions for a period of up to one billion years. Much of this interest stems from the ensuing implications for oxygen production, climate and biological evolution. However, both the spatial and temporal extent of sulfidic conditions over this period have been questioned. In particular, suboxic conditions have been proposed for the deeper ocean, while other studies have suggested a global sulfidic state. All of these previous studies are limited either by considering relatively few samples with little stratigraphic context, or by utilising poorly-constrained models. To provide a robust assessment of the redox structure of the ocean during the transition from ferruginous conditions at 1.84 Ga, we have investigated over 400 samples from the 1.9-1.8 Ga Animikie Basin, Ontario, Canada. Samples come from 6 cores spread over a distance of 400 km. Based on a detailed stratigraphic model, we can track sulfidic conditions beneath oxic surface waters over 250 km from the paleo-shoreline. However, the deeper ocean remained ferruginous, confirming continued ocean anoxia, rather than oxic or suboxic conditions, and demonstrating that global sulfidic conditions did not occur at this time. This redox structure likely arose due to a balance between rates of upwelling dissolved Fe(II), an enhanced riverine sulfate flux closer to the shoreline, and the availability of nutrients to promote organic matter production and bacterial sulfate reduction.
PP14A-04 INVITED
Temporal Trends in Syngenetic Lipid Biomarker Signals from Proterozoic Sedimentary Organic Matter
The development of continuous-flow catalytic hydropyrolysis (HyPy) for reproducible recovery of biomarker lipid skeletons covalently-bound within kerogen has proved to be an important analytical breakthrough for ancient lipid biomarker research. The parallel analyses of free (solvent-extractable) and kerogen-bound biomarkers affords more confidence that we have correctly identified syngenetic compounds. Combining HyPy with detailed biomarker product analyzes using metastable reaction monitoring-gas chromatography mass spectrometry (MRM-GC-MS) allows detection of a large suite of biomarker compounds which are usually too low in abundance to be analyzed in detail using conventional GC-MS. Here we compare free and bound lipid biomarker records generated from Paleoproterozoic (ca. 1.7 Ga) to Late Ediacaran age (ca. 542 Ma) strata from marine basins from North and South China, Australia and Oman. Fundamental changes in eukaryotic community structure are evident after the Sturtian glaciation (ca. 713 Ma) from distinctive sterane distributions. In particular, radiations in basal animals (sponges) and chlorophyte microalgae are first apparent in Huqf sedimentary rocks from South Oman Salt Basin. Marine microbial communities were not globally homogenous in contemporaneous Proterozoic settings from comparison of biomarker profiles and this could reflect differences in ocean chemistry, affecting nutrient supply, from basin to basin.
PP14A-05
Early Neoproterozoic Sulfur Isotopes and the Rise (and Fall) of Oxygen
Several lines of evidence suggest that an increase in the oxidation state of the ocean-atmosphere system in the terminal Neoproterozoic led to elevated seawater sulfate concentrations. This evidence includes, but is not limited to, a shift in the abundance of redox-sensitive elements and a substantial rise in the sulfur isotope difference between seawater sulfate and contemporaneously deposited sedimentary pyrite (Δ34Ssulfate-pyrite) several millions of years prior to the appearance of the Ediacaran Fuana. Importantly, the extent of fractonation between seawater sulfate and co-occuring sedimentary pyrite is controlled in large part by sulfate availability, local redox conditions and the nature of microbial sulfur cycling. The amount of oxygen in the ocean-atmosphere system is believed to be a control on marine sulfate levels because the primary source of seawater sulfate is riverine delivery resulting in part from the oxidative weathering of pyrite. However, very little work has been done to characterize Δ34Ssulfate- pyrite, marine sulfate levels and Earth-surface redox in the early to mid Neoproterozoic (Tonian). Here, we present a high-resolution δ34S (sulfate and pyrite) and δ13Ccarbonate record as generated from two time-equivalent sections of the Neoproterozoic Bitter Springs Formation (~800 Ma), Australia, that include an extended (106 yrs.) >5‰ negative δ13Ccarbonate excursion that has been recognized in multiple localities around the world (i.e., Bitter Springs Stage). Sulfur and carbon isotope profiles from each section are nearly identical and share the following characteristics: (1) δ34Ssulfate increases upsection with values ranging from 10 to 40‰; (2) δ13Ccarbonate variations are not coupled to changes in δ34Ssulfate; (3) δ34Spyrite varies systematically through this interval and is positively coupled to changes in δ13Ccarbonate, particularly through the Bitter Springs Stage; and (4) Δ34Ssulfate-pyrite values range from ~10 to >50‰. Together, these results suggest that seawater sulfate concentrations, and inferentially atmospheric O2 concentrations, fluctuated considerably through the Neoproterozoic with a substantial increase occurring prior to the Sturtian glacial event.
PP14A-06
Large base-level and glacioeustatic changes during the pan-glacial episode ending in 635 Ma
The pan-glacial episode ending in 635 Ma occurred while the Otavi Group in northern Namibia was a flat-
topped carbonate platform with a well-defined, south-facing, submarine foreslope undergoing thermal
subsidence. Glacial erosion produced <80 m of relief between a bowl-shaped inner platform veneered by
lodgement tillite and a raised outer platform devoid of glacial deposits. The upper foreslope also lacks glacial
debris but the distal foreslope, >5 km seaward of the platform edge, bears a laterally-continuous prism of
structureless diamictites and well-stratified proglacial deposits, collectively interpreted as a submarine ice
grounding-line wedge. The prism disconformably overlies an upward-coarsening stack of carbonate turbidites
and debris flows interpreted as a low-stand wedge. The δ13C composition of diamictite clasts
shows derivation from the top 85 m of preglacial strata on the platform and from the low-stand wedge.
Shallow-water structures (sorted peloids, low-angle cross-lamination, giant wave ripples) in the cap dolostone
on the distal foreslope above the grounding-line wedge, combined with deep-water facies directly above the
cap dolostone on the outer platform, indicate that the magnitude of base-level rise attending deglaciation was
greater than the depth of the distal foreslope below the rim of the platform. We estimate this depth to be
>0.6 km, based on comparison with the bathymetry of the leeward side of the Great Bahama Bank and on
geoplumb indicators (tubestone stromatolites) in the foreslope cap dolostone itself. Factoring in the isostatic
rebound of the platform after ice-sheet unloading, sinking of the sea-floor due to meltwater addition, loss of
gravitational 'pull' on the ocean by the ice-sheet itself, and thermal subsidence of the platform over the
course of the glacial period, the inferred base-level change of >0.6 km translates into a glacioeustatic rise
of ~1.5 km. This is equivalent to a global average ice-sheet thickness of ~3.0 km on all continents.
Our estimate is insensitive to the extent of ocean ice-cover, but is consistent with steady-state ice-sheet
thickness in climate models with fully ice-covered oceans.
http://www.snowballearth.org
PP14A-07
A Modern Analogue for Proterozoic Inverse Carbon Isotope Signatures
The carbon isotope distribution preserved in sedimentary lipids changes near the Neoproterozoic-Cambrian boundary. In older samples, n-alkyl lipids contain more 13C than both isoprenoid lipids and kerogen [1]. In younger samples, the opposite prevails. Although extreme heterotrophy has been invoked as a mechanism to explain the enrichment in 13C [2], here we suggest another explanation. The switch may reflect a fundamental transition from an oligotrophic ocean dominated by prokaryotic biomass, to an ocean in which carbon fixation is more intensive and burial is dominated by eukaryotic biomass. An analogue for Proterozoic ordering is found in the modern, oligotrophic Pacific Ocean, where n-alkyl lipids of picoplankton (0.2-0.5 μm particulate matter) contain excess 13C relative to the same lipids found in larger size classes (> 0.5 μm). Picoplanktonic lipids are heavier isotopically (-18 ‰) than both the sterols of eukaryotes (-23 ‰ to -26 ‰) and the total organic matter (-20 ‰; TOM). The 0.2-0.5 μm size class also has a distinct chain-length abundance profile. Although large particles must be the vehicle for total carbon export, paradoxically the lipid component of export production appears to be dominated by the 0.2-0.5 μm source. The picoplanktonic chain lengths and isotopic composition dominate lipids of TOM at 670 meters. When the ratio of prokaryotic to eukaryotic production is high, as in the modern central Pacific Ocean, it appears that exported material has an inverse carbon isotope signature similar to that preserved in Precambrian samples. [1] Logan, G. A. et al., Nature 376:53-56 (1995). [2] Rothman, D. H. et al., PNAS 100:8124-8129 (2003).
PP14A-08 INVITED
Telling time in shallow water carbonates - from the Cambrian of Morocco to the Holocene of the Bahamas
Virtually all pre-Mesozoic records of changing ocean-atmosphere chemistry and temperature are derived from chemical sediments deposited on continental shelves. Paired isotope studies of these carbonate rocks have placed the evolution of life in the context of a variable and, at times, non-uniformitarian carbon-sulfur cycle. Furthermore, paired δ13C and 87Sr/86Sr have allowed for global correlation of these stratigraphic successions and have built a framework for studying the history of global change. However, in the absence of quantitative constraints on the rate and duration of geochemical variability, paired stable isotope data tell us little about the processes most important to the variable carbon-sulfur cycle and the co-evolution of life and the surface environment. We discuss two methods of telling time in shelfal carbonate successions. First, we illustrate the use of interbedded volcanic ashes in determining the rate of isotopic change during the Cambrian radiation of animals in Morocco. Next, we present a study of Holocene stratigraphy in the Bahamas that sheds light on the characteristic timescales for the dominant processes controlling carbonate production on continental shelves, and discuss the promise and risk of determining an absolute timescale in the absence of geochronology.