OS14A-01 16:00h
The Ubiquity of Phosphogenesis and the Rarity of Phosphorites in Marine Sediments
The role that phosphorite formation plays in the marine phosphorus (P) cycle has long been debated. A shift has occurred from early models that evoked strikingly different oceanic P cycling during times of widespread phosphorite deposition to current thinking that phosphorite deposits may be lucky survivors of a series of inter-related tectonic, geochemical, sedimentological, and oceanic conditions. This paradigm shift has been facilitated by an awareness of the widespread nature of phosphatization-the authigenic formation of authigenic P-bearing minerals that contributes to phosphorite formation. This process occurs not just in continental margin sediments, but in deep sea oozes as well, and helps to clarify the driving forces behind phosphorite formation and links to the marine P cycle. The accumulation rate of P is highest along continental margins, due to focused productivity and rapid transit of organic and biogenic material through the water column/active sediment regions. The concentrations of P, however, are nearly identical between margin and deep sea sediments due to substantial terrigenous dilution on margins. How then do these marginal deposits alone become so concentrated in P? Two processes come into play to make phosphorite deposits: physical dynamism, and chemical dynamism. Physical dynamism involves the reworking or sedimentary capping of P-rich sediments, which can either concentrate the relatively heavy and insoluble disseminated P-bearing minerals or provide an episodic change in sedimentology to concentrate chemically mobilized P. Both processes can results from along-margin current dynamics and/or sea level variations. Chemical dynamism involves the diagenetic release and subsequent concentration of P-bearing minerals in particularly horizons, controlled either by sedimentology or geochemical fronts. Interestingly, net P accumulation rates are highest (i.e., the P removal pump is most efficient) when phosphorites are NOT forming. Both physical and chemical pathways involve processes not dominant in deep sea environments, and contribute to the formation of a marginal phosphorite deposit unique on sedimentological grounds but not in terms of the marine P cycle.
OS14A-02 INVITED 16:15h
Phosphorus in the ocean and marine sediments: similarities between present and past processes
Because phosphorus (P) is an essential nutrient, geochemical research has focused over the years on understanding the different aspects of the P cycle in the oceans, from the global to microbial scale. In the last 40 years, giant phosphorite deposits were largely studied, and their episodic occurrence in the geological record was alternatively interpreted as the product of shallow water environments, high productivity, low-sedimentation rates, and/or changes in sea level. Although research has focused more recently on the oceanic burial fluxes and residence time of P, there is still a general agreement on the need for more data. Thanks to new analytical techniques, allowing the detection of small quantities of phosphate (on the order of ?mol/g), and to the increased availability of sediment cores, P-bearing sediments have been found everywhere beneath the ocean floor. This finding has changed our understanding of P behavior in the ocean, and is redefining the role of P as an important nutrient, for example, over glacial-interglacial time scales. I will present glacial-interglacial reconstructions of burial and benthic fluxes of P, with the goal of understanding to which extent the P cycle is linked to global processes. The data, averaged to the whole ocean, indicate that burial fluxes of reactive P during glacial times are not considerably lower than during interglacials. This observation could lead to the conclusion that no changes occurred in P cycle on glacial-interglacial timescales and, therefore, that C cycle and climate variations were independent of P cycle. However, when the benthic flux estimates are taken into account, a different picture arises. During low sea level periods, the redistribution of sediments from shallow to deep waters, due to the reduction of the continental margin surface, fostered P regeneration during settling of organic matter. Even if P burial fluxes remain fairly constant, the oceanic phosphate inventory of glacial bottom waters was probably higher. On a different time scale, the shift in P behavior between glacial and interglacial periods could have been promoted by conditions similar to those that led to the formation of phosphorite deposits, which are abundant in the geological past but rare today.
OS14A-03 16:30h
The Peru Margin as an Authigenic Mineral Factory, Evidence From Surface Sediments and Oceanography
Characteristics of sediments deposited within an intense oxygen-minimum zone (OMZ) on the Peru continental margin were mapped by submersible, and studied in samples collected in deck-deployed box cores and submersible push cores on two east-west transects over water depths of 75 to 1000 m at 12 degrees and 13.5 degrees S. On the basis of sampling of the top 1-2 cm of available cores, three main belts of sediments were identified in each transect with increasing depth: 1) organic-carbon (OC)-rich muds; 2) authigenic phosphatic mineral crusts; and 3) glaucony facies. These facies patterns are primarily controlled by redox conditions and strength of bottom currents. OC-rich sediments on the 12-degree transect were mainly located on the outer shelf and upper slope (150-350 m), but they occurred in much shallower water (ca. 100 m) on the 13.5-degree transect. The organic matter is almost entirely marine, resulting from very high primary productivity. The OC concentrations are highest (up to 18%) in sediments where intermediate water masses with low dissolved oxygen concentrations (less than 5 micromoles/kg) impinge on the slope at water depths between 75 and 450 m. The region between 175 and 350 m depth is characterized by bedforms stabilized by bacterial mats, extensive authigenic mineral crusts, and (or) thick organic flocs. Currents as high as 30 cm/sec were measured over that depth interval. Current-resuspension of surficial organic matter, activity of organisms, and transport to and from more oxygenated zones contribute to greater oxidation and poorer preservation of organic matter than occur under oxygen-deficient conditions. Phosphate-rich sediments occurred at depths of about 300 to 550 m on both transects. Nodular crusts cemented by carbonate-fluorapatite (CFA; phosphorite) or dolomite form within the OMZ. The crusts start by cementation of sediment near the sediment-water interface forming stiff but friable phosphatizes claystone "protocrusts". The protocrusts evolve into dense, dark phosphorite crusts, cemented breccias, and pavements. The degree of phosphatization and thickness of the phosphorite crusts depends on rates of sediment supply and strength and frequency of currents that re-expose crusts on the seafloor. Glaucony-rich surficial sediments, relatively undiluted by other components, mainly were found in deeper water on the 13.5 degree transect (750 m to at least 1067 m). These sediments consist almost entirely of sand-size glaucony pellets (aggregates of clay minerals with pelletoid shapes). These widespread glaucony sands possibly formed in situ and were then concentrated and reworked by strong currents that winnowed away the fine-grained matrix. Overall, sedimentation rate must be slow in order for the glaucony minerals to remain in contact with seawater, which is the source of cations during growth. The close association of glaucony and phosphorite indicates that there is a delicate balance between slightly oxidizing and slightly reducing conditions at the base of the OMZ- slightly reducing to mobilize iron and phosphate, and slightly oxidizing to form glaucony.
OS14A-04 INVITED 16:45h
Tracing Tethyan Phosphogenesis From Temporal Variations of $^{44}$Ca/$^{42}$Ca and $^{143}$Nd/$^{144}$Nd Isotope Ratios in Francolites and P Accumulation
Measurements of $^{44}$Ca/$^{42}$Ca and $^{143}$Nd/$^{144}$ isotope ratios in carbonate fluorapatite (CFA) through the Cretaceous-Eocene of the Negev (Israel) and of other sites in the Tethys margins, together with quantified rates of P and Ca accumulation and bulk sedimentation, allow us to understand variations of Tethyan phosphogenesis in time and space. The data provide a $\sim$ 90 m.y. (Hauterivian-Eocene) record of the Ca and Nd isotopic composition in 72 CFA samples representing 25 time-stratigraphic phosphate levels. Similar temporal changes are displayed by $\delta$$^{44}$Ca and $\epsilon$Nd$_{T}$. $\delta$$^{44}$Ca is much lighter in the Hauterivian-Albian ($\delta$$^{44}$Ca = - 0. 19 to - 0.06 $\permil$; n = 9) than in the Campanian-Eocene ($\delta$$^{44}$Ca = + 0.29 to + 0.40 $\permil$; n = 41), whereas $\epsilon$Nd$_{T}$ increases from continental crust-like values in the Hauterivian-Albian ($\epsilon$Nd$_{T}$ = -12.8 to - 10.9; n = 8) to more radiogenic Pacific-like values ($\epsilon$Nd$_{T}$ = - 7.5 to - 6.2; n = 27) in the Campanian. Both $\delta$$^{44}$Ca and $\epsilon$Nd$_{T}$ peaks in the Campanian coincide with the peak of Tethyan phosphogenesis in the Negev, marked by a sharp rise in P accumulation rates (from $<$ 200 $\mu$mole.cm$^{-2}$ kyr$^{-1}$ in pre-Campanian times to $\sim$ 1700 $<$ 200 $\mu$mole.cm$^{-2}$ kyr$^{-1}$ in the Campanian) and by a decrease in the rates of Ca accumulation and bulk sedimentation. The coincident increases of $\delta$$^{44}$Ca, $\epsilon$Nd$_{T}$, and P accumulation in the Negev during the Campanian is interpreted as the combined effect of the Late Cretaceous global sea level rise, the development of a long-transit, westward-flowing circumglobal Tethyan current enhanced by widening of the Caribbean threshold at those times, and a favorable paleolatitude ($8\deg$ -$15\deg$) of the south Tethys shelf in the path of easterly winds. Extensive flooding of continental platforms, induced by the Late Cretaceous global sea level rise, probably reduced the influx of riverine Ca to oceans and increased carbonate deposition on shelves both causing seawater enrichment with heavy Ca. Similarly, intensification of ocean circulation, causing more incursion of Pacific water masses into the Tethys, probably also led to increase both the input of radiogenic Nd into the Tethys and the availability of dissolved P in photic waters. The much lower $\epsilon$Nd$_{T}$ values recorded in N. Africa CFA during Eocene times ($\epsilon$Nd$_{T}$ = - 10.0 to - 9.4) suggests Atlantic waters as main source of dissolved P for fuelling phosphate deposition in this area.
OS14A-05 17:05h
Marcasite in Sediments and Sedimentary Rocks - Conundrum and Opportunity
For many years marcasite in ancient sedimentary rocks was treated mainly as a curiosity. In the literature, the occurrence and distribution of pyrite, its much more abundant polymorph, dominates discussions of early sulfide mineral diagenesis in sediments. Curiously though, while marcasite has been detected in sedimentary rocks of all ages, it has yet to be reported from modern sediments. This unusual situation may present us with one of those odd instances when "the past becomes the key to the present". While both marcasite and pyrite require reducing pore-waters for their formation, marcasite is unique in that it additionally requires a low pH. In experiments, marcasite occurs below pH 6 and is predominant below pH 4, apparently due to higher growth rates of marcasite vs. pyrite under those conditions. In marine surface sediments, pH values around 8 in the overlying seawater (due to the carbonate-bicarbonate buffer) make it very difficult for pH values of pore-waters to drop low enough for predominant marcasite formation. In nature, the low pH values needed for abundant marcasite formation are for example encountered in salt marsh sediments when previously formed sedimentary sulfides are oxidized. During sequence stratigraphic studies of Devonian black shales, iron sulfides with morphological characteristics of marcasite (confirmed by EBSD) were encountered as cements in lag deposits covering erosion surfaces. By analogy with the low pH values in oxidizing salt marsh sediments, one can speculate that concentration of previously formed iron sulfides in these lags (concretions, burrow linings, framboids) laid the foundation for subsequent marcasite formation. The lags in question formed as a consequence of intermittent sea level drops, subjecting previously deposited black shales to wave erosion, and promoting formation of pyritiferous lags. Oxidation of pyrite grains should have led to abundant acid production, temporary lowering of pH in lag pore spaces, and pore waters rich in dissolved iron. Under those conditions, hydrogen sulfide production from underlying black shales should have enabled rapid marcasite formation. Rapid marcasite growth is indicated by radial fibrous aggregates that fill the pore spaces of sandy to granular lags. Thus, observations from the rock record support the hypothesis that marcasite forms where sediment layers with abundant iron sulfides undergo intermittent oxidation. Rather than indicating a true deficiency, the dearth of marcasite in modern sediments may simply be a matter of methodology. In ancient rocks at least, iron sulfide aggregates that XRD analysis identifies as pyritic, often reveal traces of residual marcasite when examined with electron backscatter diffraction (EBSD) in polished thin sections. Preliminary analyses suggest that marcasite is more widespread in ancient strata, and may have (until now) escaped detection in modern sediment because of small grain size and low abundance. Systematic study may reveal it to be a useful indicator of rapid oxidation events in sedimentary successions.
OS14A-06 INVITED 17:20h
Non-paragenesis of Authigenic Sulfide Minerals: Mackinawite and Greigite are Not Precursors of Sedimentary Pyrite
Sedimentary sulfide minerals have traditionally been "operationally" divided into pyrite and acid volatile sulfide (AVS) minerals. AVS minerals have generally, with very scant direct evidence, been held to be comprised of mackinawite (tetragonal FeS) and greigite (cubic Fe$^{}_{3}$S$^{}_{4}$). They are also often referred to in the literature generically as FeS or iron monosulfides. Based largely on experimental studies at elevated temperatures over almost a third of a century and their metastability relative to pyrite, it has become almost dogma among sedimentary geochemists that mackinawite and or griegite are necessary precursors for pyrite formation in sediments. Being precursors to pyrite necessarily implies that they must be formed before pyrite and the normal paragenesis is supposed to be mackinawite to greigite to pyrite. The implication is that in their absence no authigenic sedimentary pyrite can be produced. This is not supported by many observations of pyrite formation in sediments where, during early diagenesis, abundant pyrite is commonly produced in the absence of any detectable AVS. Reactions for pyrite formation such as FeS$^{}_{(s)}$ + S$^{0}_{(s)}$ = FeS$^{}_{2(s)}$ or FeS$^{}_{(s)}$ + H$^{}_{2}$S = FeS$^{}_{2(s)}$ + H$^{}_{2}$, for example, represent the net mass balances and do not describe the process. More recently, two reaction mechanisms for pyrite formation, the "polysulfide" and "H$^{}_{2}$S" pathways, have gained wide acceptance. The reaction mechanisms involve dissolved species.If present at all, mackinawite and greigite contribute to pyrite formation via their dissolution which provides reactive components to solution. However, these dissolved components do not necessarily require mackinawite or greigite as their source. For the polysulfide pathway for pyrite formation the net reaction is better expressed as Fe$^{2+}$ + S$^{2-}_{n}$ = FeS$^{}_{2(s)}$ + S$^{2-}_{n-2}$ and for the H$^{}_{2}$S pathway the reaction is FeS$^{}_{(aq)}$ + H$^{}_{2}$S = FeS$^{}_{2(s)}$ + H$^{}_{2}$. FeS$^{}_{(aq)}$ represents aqueous FeS clusters that are true solution components formed by the reaction between Fe(II) and S(-II) and not simply mackinawite nanoparticles or colloids. Pyrite and, possibly, mackinawite and greigite are all formed at the same time or in no particular order. Non-paragenesis in the sedimentary iron-sulfur system is enhanced by continual S(-II) production through microbial activity in the deep biosphere which results in production of any or all of the three minerals at any time during the evolution of the sediment.
OS14A-07 INVITED 17:40h
Marine Barite Formation - Pathways, Processes, and Product
The seawater is generally under-saturated with respect to barite. The mineral barite however, is observed in sinking oceanic particulate matter worldwide. It has been suggested that marine barite forms authigenically within microenvironments in association with organic matter and other biogenic debris. Barite can also precipitate diagenetically within sulfate reducing sediment at the oxic anoxic front or from hydrothermal or other Ba rich solutions when they encounter sulfate rich seawater. Barite is widely used to reconstruct ocean chemistry (87Sr/86Sr, del34S, Sr/Ba) and export production. To reliably record seawater chemistry and water column processes it is essential that the authigenic mineral will precipitate in seawater. Moreover, parameters affecting the chemistry (trace element substitution, isotope incorporation) of the mineral such as temperature, pressure, salinity and precipitation rate should be well constraint. Some of these requirements are met for marine barite but not all. A discussion of what controls the chemistry of marine barite and how this can affect potential use of this mineral for paleoceanographic reconstruction will be presented.