PP34A-01 INVITED
The Importance of Anoxia in Sustaining High Productivity During Deposition of Organic- Carbon-Rich Marine Sediments During the Phanerozoic – Implications from C/N, C/P, and Nitrogen Isotopic Records of Upwellings, Mediterranean Sapropels, and Black Shales
Both improved preservation and enhanced production of organic matter are necessary for deposition of organic-carbon-rich marine sediments such as those found under modern upwelling systems, in the sapropels that accumulated episodically in the Mediterranean Sea starting in the latest Miocene, and in the multiple black shale sequences that are especially dramatic in parts of the Mesozoic and the Paleozoic. However, the high productivity that is characteristic of the well-mixed upwelling systems is constrained by wind-current fields to relatively small areas of the world's ocean. This spatial limitation is compounded by the biogeochemical limitation of the availability of N and P to primary producers, making upwelling an inadequate answer as to how the abundant organic matter preserved in sapropels and black shales was produced. A more satisfactory answer is suggested by the common existence of high C/N and C/P ratios and low del15N values in Pliocene-Pleistocene sapropels and Cretaceous black shales. The high elemental ratios indicate preferential recycling of N and P from organic matter relative to carbon, and the low nitrogen isotopic values record abundant assimilation of dissolved dinitrogen via nitrogen fixation. Both processes are favored in a strongly stratified water column. Paleoceanographic proxies indicate that the sapropels were deposited during periods of salinity stratification of the Mediterranean Sea and that the black shales were formed while wide areas of the ocean were thermally stratified; both situations led to near-surface anoxia. Although data are scant, available C/N, C/P, and nitrogen isotope values from Paleozoic black shales are similar to those of the Pliocene-Pleistocene sapropels and Cretaceous black shales, implying similar conditions of near-surface anoxia during deposition of these ancient organic-carbon-rich sequences.
PP34A-02
Influence of Anoxia on Nutrient Regeneration During Organic Matter Decomposition in the Orca Basin, Gulf of Mexico
Controls on N and P regeneration during organic matter (OM) decomposition, particularly the effects of oxygenation, remain a topic of debate. It has alternately been argued that: there is no preferential regeneration of either N or P relative to C under normal marine (i.e., oxic) conditions; N is preferentially released under oscillating redox conditions; and P is preferentially released under anoxic conditions. Upon release, remineralized nutrients can be retained in sediments or can diffuse from sediments, making them available for biological utilization and thereby increasing primary productivity and the potential delivery of OM to the sediments. To address these questions, four sediment cores were taken from the Orca Basin under different depositional regimes, oxic, oscillating, transitional, and anoxic. The Orca Basin is a small, hypersaline, anoxic/sulfidic basin on the northwest slope of the Gulf of Mexico, located 290 km south of the Mississippi delta, thus the only factors that differ between the various cores are redox state and salinity. Cores were analyzed for total organic carbon (TOC), total nitrogen (TN), organic nitrogen (TON), organic C and N isotopes, as well as a phosphorus speciation including organic, oxide-associated, authigenic, and detrital forms of P. Results indicate impacts of the oxygen regime on OM remineralization. C/N ratios indicate no preferential release of N from OM during decay. However, the N released remains sequestered in the sediments as inorganic N, and is not released to bottom waters as effectively under oscillating redox conditions. P is released preferentially from sediments relative to both C and N under anoxic conditions compared to oxic conditions. These results agree with previous studies of sedimentary systems of equivalent or older age, but studies of water column and sediment trap OM remineralization show no preferential release of P under anoxic conditions. Presumably, the timescale of study is important for the observation of preferential release of P during anoxic remineralization. It appears that decades to centuries are required for observable differences in the remineralization of C, N, and P during diagenesis.
PP34A-03 INVITED
Episodic Anoxia and Carbon Burial: a Diagenetic Modelling Perspective
Periods of anoxia lead to enhanced preservation of organic material in marine sediments. Following reoxygenation of the water column, the organic-rich layers deposited under anoxic bottom waters are subject to remineralisation, attenuating the amount of organic carbon that is ultimately buried. The present study examines the parameters that regulate organic matter preservation and remineralisation by means of a reactive-transport model. A sensitivity analysis of the model was undertaken to identify the principal parameters controlling carbon and nutrient burial under transient redox conditions. While the focus of the presentation is upon Holocene sapropels in the eastern Mediterranean Sea, preliminary results pertaining to seasonally anoxic sediments in the Baltic Sea are also discussed.
PP34A-04
Geochemistry of Barium in Sediments of the Western Black Sea
During two cruises with RV POSEIDON and RV METEOR to the western Black Sea (west of Crimean Peninsula) in 2004 and 2007, respectively, sediment cores were taken along a transect from 500 to 1700 m water depth. Pore water and solid phase investigations aimed at unraveling the geochemical behaviour of barium in this anoxic water column and sedimentary environment. In particular, our study - which represents the first investigation on barium geochemistry in Black Sea sediments - focussed on the following questions: (1) Does barite form in the anoxic water column and - if so - is the amount of barite reaching the sediment surface dependent on water depth as reported for oxic marine depositional environments?, (2) Can barium/barite be used as a sedimentary tracer of past productivity in the Black Sea?, (3) How is barium redistributed post-depositionally at the sulfate/methane transition (SMT) which is typically located around 2 m sediment depth?, (4) Can we use diagenetic barite to trace the downward migration of the sulfate/methane transition which occurred in the Black Sea deposits since the transition from the last glacial freshwater lake situation to the Holocene anoxic marine period? Understanding the geochemistry of barium in sediments underlying anoxic water bodies – like is the case in the Black Sea – will significantly contribute to improving interpretations of fossil sedimentary records laid down under oxygen-depleted water column conditions, like below or within oxygen minimum zones or during Cretaceous Oceanic Anoxic Events.
PP34A-05
Los Molles Formation, Neuquén Basin Argentina: A Southern Hemisphere perspective on the Toarcian Oceanic Anoxic Event
In the Early Jurassic, the Toarcian, Oceanic Anoxic Event (~180 m.a.) was characterized by high sea surface temperatures (~6°C warmer than present), mass extinction, large igneous province emplacement, and the deposition of sediments rich in organic carbon. However, most studies of the event have focused on the northern hemisphere sections, leading some researchers to question its global nature. Here we report first results from a combined sedimentological, biostratigraphic and geochemical study of a southern hemisphere Pliensbachian-Toarcian black shale succession, the Los Molles Formation of the Neuquén Basin, Argentina, in order to assess the global extent of the Oceanic Anoxic Event. Three sections along a N-S transect have been investigated: Arroyo Lapa, Chacay Melehue, and Arroyo Serrucho. The sediments comprise interbedded fine-grained sandstone and laminated dark shales, with localized abundant reworked volcaniclastic material, and sporadic carbonate nodules. An abundance of fossils includes hildoceratid and dactylioceratid ammonites and pectinoid bivalves. Carbon-isotope analyses of terrestrial organic material from Arroyo Lapa reveals fluctuations in values comparable to those previously reported from Western Europe. The preliminary results support the hypothesis that Toarcian oceanic anoxia and attendant carbon-cycle perturbation was a phenomenon of global significance.
PP34A-06
The OAE1a Negative Carbon Isotope Spike: a Stepwise Transition Revealed by a High- Resolution Biomarker Carbon Isotope Record
Oceanic anoxic events (OAEs) are time envelopes in the Cretaceous when ocean conditions favoured the episodic deposition of organic-rich black shale, associated to a positive carbon isotope (δ13C) excursion, on a global scale. The OAE1a (Early Aptian, ~120 Myr ago) is characterized by an enigmatic negative δ13C spike of up to 3‰ in marine carbonates and of 4 to 5‰ in the organic carbon at its base. This carbon-cycle perturbation is believed to reflect a massive release of 13C depleted carbon into the ocean and the atmosphere. This light carbon has been proposed to derive from the dissociation of methane hydrates. However the estimates for the sedimentary methane hydrates budget in the Cretaceous are too low to explain such a C-anomaly. The aim of this study is to elucidate the initiation of the OAE1a and thus understand the transition of the Earth systems to OAE conditions. Because exchanges between ocean and atmosphere pools are faster than the resolution of the carbon isotope records available so far for the OAE1a, a higher resolution carbon isotope study on specific biomarkers is essential. Bulk and biomarkers δ13C were measured at sampling intervals of 2000 to 6000 years on samples from the Cismon core (Southern Alps, Italy). The new carbon isotope record, including carbonates, bulk organic carbon, marine and terrestrial lipid biomarkers, shows a stepwise initiation of OAE1a which was divided into five intervals, covering ~60 kyr. Each interval is characterized by specific δ13C behaviours of marine and terrestrial markers over less than 10 kyr. This record successively describes a massive outgassing of mantle CO2, a temperature rise, an increase in carbon-isotope fractionation and partial methane hydrates dissolution. Thus, the negative δ13C spike recorded at the onset of the OAE1a was caused by a combination of different processes affecting the carbon-cycle and primarily triggered by an intense volcanic activity on the Ontong-Java Plateau.
PP34A-07 INVITED
Paleoceanographic implications of the osmium isotope record during the mid-Cretaceous Oceanic Anoxic Event 2
The mid-Cretaceous Earth was characterised by elevated surface temperatures, high atmospheric CO2 contents, the generalised absence of polar ice caps, and reduced latitudinal heat gradients resulting in sluggish atmospheric and oceanic circulation compared to present-day conditions. At this time, several intervals of widespread marine anoxia are recorded in the sedimentary record. The Cenomanian-Turonian Boundary Event (CTBE; ca. 93.5 Ma) is the most widespread and arguably best studied of these global Oceanic Anoxic Events occurring during the Mesozoic. OAE2 resulted in worldwide deposition of organic-rich (black) shales and is evidenced by an abrupt positive δ13C excursion in both bulk organic matter (up to 6‰) and carbonates (2-3‰). This isotopic shift, which can be correlated at several distant sites, is widely thought to result from an increase in sedimentary burial of 13C- depleted organic carbon in response to the oxygen-depletion at this time. In addition to a selective extinction, this episode of carbon sequestration appears to have led to a ephemeral but significant reduction in the partial pressure of atmospheric CO2 and cooling of surface temperatures, making this interval of particular interest for studies of the effects of climate change. Recent osmium isotope measurements indicate that large-scale magmatic activity occurred during the deposition of the black shales: indeed, seawater 187Os/188Os isotope ratios in organic-rich sediments show that the marine osmium isotope record changes abruptly just at or prior to the onset of OAE2. A two-component mixing equation indicates that over 97% of the total osmium content in contemporaneous seawater is magmatic in origin, a ca. 30-50 fold increase relative to pre-OAE conditions. Although general consensus on a causal link to black shale deposition has yet to be achieved, the time-lag of up to 23 kyrs between magmatism – as evidenced by Os isotopes – and the deposition of significant amounts of organic carbon in the global ocean – as shown by the carbon isotopes – most likely indicates that the oceans were stratified just prior to the magmatic episode.
PP34A-08
Perturbation of the Global Fe Cycle during OAE II
The mid-Cretaceous (~125-88 Mya) was a time of elevated global temperatures with little to no latitudinal temperature gradient. During this period, the global carbon cycle had major perturbations reflecting increased carbon burial via either increased productivity or enhanced organic matter preservation, or both. In these times of elevated temperatures and increased productivity, the environmental conditions of the basinal Proto Atlantic were anoxic to euxinic (i.e., ocean anoxic events or OAE's.) These conditions led to the deposition of organic-rich laminated black shales in the deep basin and in some cases across continental shelves. A sharp increase in organic matter burial is reflected in positive carbon isotope excursion across the basin. Enrichments of redox-sensitive transition trace metals, including Fe and Mo, indicate major changes in the biogeochemical cycling of these metals under euxinic conditions. Here we use Fe abundance and isotope compositions to reconstruct the extent of euxinia in the Proto North Atlantic. By comparing several sites across the basin, representing euxinic, intermittently euxinic or anoxic, and oxic settings, we will examine the expression of a shelf-to-basin reactive Fe shuttle in the geological record and construct an isotope mass balance for the basin. Analogous to the Black Sea model (Severmann et al. 2008), euxinic sites show significant Fe enrichments and are expected to have decreased total Fe δ56Fe values (reactive Fe sink). In contrast, a predominantly oxic site from Eastbourne (UK) has decreased Fe values, indicating that it may have been a shelf source of reactive Fe with concomitant increased δ56Fe values. In addition, Fe isotopes may help us to discern whether the dominant reactive Fe source during OAE II is continental shelf Fe shuttle or volcanic activity related to the large igneous provinces.