Oceanic Anoxia Research Papers - Academia.edu (original) (raw)

The Early Triassic Induan–Olenekian Stage boundary (Dienerian–Smithian sub-stage boundary) has been identified at a depth of 2719.25 m in the petroleum exploration well Senecio-1 located in the northern Perth Basin,Western Australia.... more

The Early Triassic Induan–Olenekian Stage boundary (Dienerian–Smithian sub-stage boundary) has been identified
at a depth of 2719.25 m in the petroleum exploration well Senecio-1 located in the northern Perth
Basin,Western Australia. Conodont faunas represent three conodont zones in ascending order, the Neospathodus
dieneri Zone, the Neospathodus waageni eowaageni Zone and the Neospathodus waageni waageni Zone. The
Induan–Olenekian (Dienerian–Smithian) boundary is placed at the base of the Neospathodus waageni eowaageni
Zone equivalent to the first appearance of Neospathodus ex. gr. waageni utilised elsewhere and adopted by the
IUGS ICS Triassic Subcommission to define the base of the Olenekian. Bulk kerogen δ13C carbon isotopes define
a positive peak of c. 4 per mille that essentially coincides with the Induan–Olenekian boundary as seen in proposed
Global Stratotype Sections and Points (GSSPs) in South China and Spiti, India demonstrating the global
utility of this level for correlation. An anoxic zone is recognised in the lower part of the Senecio-1 core and the
upper limit of this zone is dated as late Induan (late Dienerian). Temporal and spatialmapping of marine anoxia
and dysoxia globally demonstrates that pulses of dysoxia/anoxia affected shallow-marine zones at different
times in different locations. Dysoxia/anoxia in the shallow-marine environment appeared in the latest Permian
at the extinction level, later than in the deep-marine environment, and appears to be largely restricted to the
Induan (Griesbachian and Dienerian) and early Olenekian (Smithian). Temporally and geographically restricted
upwelling of an oxygenminimum zone into the ocean surface layer due to environmental perturbations including
extreme global warming, increased terrestrial chemical weathering intensity and continental erosion, sea
level rise, and changes in marine nutrient inventories and productivity rates, is interpreted as a likely cause of
observed variation in shallow-marine dysoxia/anoxia in the Early Triassic.

Sedimentary burial of the essential nutrient phosphorus (P) under anoxic and sulfidic conditions is incompletely understood. Here, we use chemical and micro-scale spectroscopic methods to characterize sedimentary P burial along a water... more

Sedimentary burial of the essential nutrient phosphorus (P) under anoxic and sulfidic conditions is incompletely understood. Here, we use chemical and micro-scale spectroscopic methods to characterize sedimentary P burial along a water column redox transect (six stations, 78–2107 m water depth) in the Black Sea from the shelf with its oxygenated waters to the anoxic and sulfidic deep basin. Organic P is an important P pool under all redox regimes, accounting for up to 60% of P burial. We find a general down-core increase in the relative importance of organic P, especially on the shelf where P bound to iron (Fe) and manganese (Mn) (oxyhydr)oxides is abundant in the uppermost sediment but rapidly declines in concentration with sediment depth. Our chemical and spectroscopic data indicate that the carbonate-rich sediments (Unit I, 3000years,3000 years, 3000years,0– 30 cm depth) of the sulfidic deep Black Sea contain three major P pools: calcium phosphate (apatite), organic P and P that is strongly associated with CaCO 3 and possibly clay surfaces. Apatite concentrations increase from 5% to 25% of total P in the uppermost centimeters of the deep basin sediments, highlighting the importance of apatite formation for long-term P burial. Iron(II)-associated P (ludlamite) was detected with X-ray absorption spectroscopy but was shown to be a minor P pool ($5%), indicating that lateral Fe–P transport from the shelf (''shuttling ") likely occurs but does not impact the P burial budget of the deep Black Sea. The CaCO 3 –P pool was relatively constant throughout the Unit I sediment interval and accounted for up to 55% of total P. Our results highlight that carbonate-bound P can be an important sink for P in CaCO 3-rich sediments of anoxic, sulfidic basins and should also be considered as a potential P sink (and P source in case of CaCO 3 dissolution) when reconstructing past ocean P dynamics from geological records.

The Chicxulub asteroid impact at the Cretaceous-Paleogene (K-Pg) boundary resulted in one of the most abrupt global warming events in the past 100 m.y., presenting an analogue to current global warming. Here, we present high-resolution... more

The Chicxulub asteroid impact at the Cretaceous-Paleogene (K-Pg) boundary resulted in one of the most abrupt global warming events in the past 100 m.y., presenting an analogue to current global warming. Here, we present high-resolution geochemical, micropale-ontological, and palynological records of the Brazos-1 (Texas, USA), Stevns Klint (Denmark), and Caravaca (Spain) K-Pg boundary sections to assess the rapid environmental changes during the global warming following the brief K-Pg boundary impact winter. Warming during the first millennia after the impact is associated with hypoxic bottom waters at the studied shelf sites, as indicated by molybdenum enrichments, causing major stress for benthic communities. We attribute this decline in dissolved oxygen to a combination of decreased gas solubility and ocean ventilation resulting from the warming of the sea water, and increased oxygen demand in shelf bottom waters due to increased nutrient inputs and associated high productivity.

The Cariaco Basin is the world’s largest truly marine anoxic basin. We have conducted a comprehensive multidisciplinary investigation of the water column (42–750 m) bracketing the redox boundary (a 250-m thick ‘‘chemocline’’) of the... more

The Cariaco Basin is the world’s largest truly marine anoxic basin. We have conducted a comprehensive multidisciplinary investigation of the water column (42–750 m) bracketing the redox boundary (a 250-m thick ‘‘chemocline’’) of the Cariaco Basin to evaluate linkages between lipid biomarkers, distributions of major dissolved chemical species, and the microbial community and associated redox processes. Our multidimensional data set includes: hydrography, water column chemistry, microbial distributions and rates, and lipid biomarkers. Multivariant statistical analysis of this data set partitions the investigated water column into 5 distinct zones, each characterized by different chemistries, microbiologies and biomarker compositions. The core of this chemocline is a 25-m thick suboxic zone where both dissolved oxygen and sulfide were below detection limits, bacterial and archaeal cell numbers and the rate of chemoautotrophic (dark) carbon fixation are elevated, and dissolved chemical species and bacterial and archaeal lipid biomarkers are indicative of tightly coupled cycles of carbon, nitrogen, and sulfur through chemoautotrophy.

We show that from 1955 to 2015, the inventory of oxygen in the Black Sea has decreased by 44 % and the basin-averaged oxygen penetration depth has decreased from 140 m in 1955 to 90 m in 2015, which is the shallowest annual value recorded... more

We show that from 1955 to 2015, the inventory of oxygen in the Black Sea has decreased by 44 % and the basin-averaged oxygen penetration depth has decreased from 140 m in 1955 to 90 m in 2015, which is the shallowest annual value recorded during that period. The oxygenated Black Sea surface layer separates the world's largest reservoir of toxic hydrogen sulfide from the atmosphere. The threat of chemocline excursion events led to hot debates in the past decades arguing on the vertical stability of the Black Sea oxic/suboxic interface. In the 1970s and 1980s, when the Black Sea faced severe eutrophication , enhanced respiration rates reduced the thickness of the oxygenated layer. Re-increasing oxygen inventory in 1985– 1995 supported arguments in favor of the stability of the oxic layer. Concomitant with a reduction of nutrient loads, it also supported the perception of a Black Sea recovering from eutrophication. More recently, atmospheric warming was shown to reduce the ventilation of the lower oxic layer by lowering cold intermediate layer (CIL) formation rates. The debate on the vertical migration of the oxic interface also addressed the natural spatial variability affecting Black Sea properties when expressed in terms of depth. Here we show that using isopycnal coordinates does not overcome the significant spatial variability of oxygen penetration depth. By considering this spatial variability, the analysis of a composite historical set of oxygen profiles evidenced a significant shoaling of the oxic layer, and showed that the transient " recovery " of the 1990s was mainly a result of increased CIL formation rates during that period. As both atmospheric warming and eutrophication are expected to increase in the near future, monitoring the dynamics of the Black Sea oxic layer is urgently required to assess the threat of further shoaling.

We show that from 1955 to 2015, the inventory of oxygen in the Black Sea has decreased by 44 % and the basin-averaged oxygen penetration depth has decreased from 140 m in 1955 to 90 m in 2015, which is the shallowest annual value recorded... more

We show that from 1955 to 2015, the inventory of oxygen in the Black Sea has decreased by 44 % and the basin-averaged oxygen penetration depth has decreased from 140 m in 1955 to 90 m in 2015, which is the shallowest annual value recorded during that period.<br><br> The oxygenated Black Sea surface layer separates the world's largest reservoir of toxic hydrogen sulfide from the atmosphere. The threat of chemocline excursion events led to hot debates in the past decades arguing on the vertical stability of the Black Sea oxic/suboxic interface. In the 1970s and 1980s, when the Black Sea faced severe eutrophication, enhanced respiration rates reduced the thickness of the oxygenated layer. Re-increasing oxygen inventory in 1985–1995 supported arguments in favor of the stability of the oxic layer. Concomitant with a reduction of nutrient loads, it also supported the perception of a Black Sea recovering from eutrophication. More recently, atmospheric warming was shown to r...

Abstract The global deposition of organic rich-mudrocks, a mass extinction, and marked geochemical changes in the sedimentary rocks and fossils deposited during the early Toarcian indicates a period of extreme environmental change and an... more

Abstract The global deposition of organic rich-mudrocks, a mass extinction, and marked geochemical changes in the sedimentary rocks and fossils deposited during the early Toarcian indicates a period of extreme environmental change and an oceanic anoxic event. This study investigates at a high-resolution the environmental and biotic changes that occurred during the event in a shallow-marine area using rocks deposited on the East Midlands Shelf, UK. We present a new graphic log, geochemical data (δ13Corg, total organic carbon, CaCO3, total sulphur and total nitrogen), and benthic macroinvertebrate ranges from North Quarry, Holwell, Leicestershire, UK. Comparison of the ammonite ranges between the Cleveland Basin (Yorkshire) and East Midland Shelf successions in the UK shows that there is a hiatus on the shelf across the Dactylioceras semicelatum–Cleviceras exaratum Subzone boundary. The new high-resolution geochemical data show that an ~− 5‰ δ13Corg excursion occurs similar to other lower Toarcian sections in the world, δ13Corg shifts ‘A’ to ‘C’ are missing in the hiatus, but that δ13Corg shift ‘D’ is present. Similar to the Cleveland Basin, the C. exaratum Subzone of the East Midlands Shelf succession is dominated by three epifaunal bivalve species, but their ranges differ significantly between the successions. An increase in the faunal diversity occurs within the upper C. exaratum Subzone at Holwell and other UK sections. The biotic data indicate that conditions on the East Midlands Shelf were more hospitable than in the Cleveland Basin. The bivalve populations on the shelf may have provided a source of new recruits for the basins.

The response of shallow-water sequences to oceanic anoxic event 2 and mid-Cenomanian events 1a and 1b was investigated along the west African margin of Morocco north of Agadir (Azazoul) and correlated with the deep-water sequence of the... more

The response of shallow-water sequences to oceanic anoxic event 2 and mid-Cenomanian events 1a and 1b was investigated along the west African margin of Morocco north of Agadir (Azazoul) and correlated with the deep-water sequence of the Tarfaya Basin (Mohammed Beach) based on biostratigraphy, mineralogy, phosphorus and stable isotopes. In the deeper Mohammed Beach section results show double peaks in δ13Corg for mid-Cenomanian events 1a and 1b (Rotalipora reicheli biozone, lower CC10a biozone), the characteristic oceanic anoxic event 2 δ13C excursion (Rotalipora cushmani extinction, top of CC10a biozone) and laminated (anoxic) black shale. In the shallow environment north of Agadir, a fluctuating sea-level associated with dysoxic, brackish and mesotrophic conditions prevailed during the middle to late Cenomanian, as indicated by oyster biostromes, nannofossils, planktonic and benthonic foraminiferal assemblages. Anoxic conditions characteristic of oceanic anoxic event 2 (for example, laminated black shales) did not reach into shallow-water environments until the maximum transgression of the early Turonian. Climate conditions decoupled along the western margin of Morocco between mid-Cenomanian event 1b and the Cenomanian–Turonian boundary, as also observed in eastern Tethys. North of Agadir alternating humid and dry seasonal conditions prevailed, whereas in the Tarfaya Basin the climate was dry and seasonal. This climatic decoupling can be attributed to variations in the Intertropical Convergence Zone and in the intensity of the north-east trade winds in tropical areas.

Major advances in our understanding of paleoclimate change derive from a precise reconstruction of the periods, amplitudes and phases of the ‘Milankovitch cycles’ of precession, obliquity and eccentricity. While numerous quantiative... more

Major advances in our understanding of paleoclimate change derive from a precise reconstruction of the periods, amplitudes and phases of the ‘Milankovitch cycles’ of precession, obliquity and eccentricity. While numerous quantiative approaches exist for the identification of these astronomical cycles in stratigraphic data, limitations in radioisotopic dating, and instability of the theoretical astronomical solutions beyond ~50 Myr ago, can challenge identification of the phase relationships needed to constrain climate response and anchor floating astrochronologies. Here we demonstrate that interference patterns accompanying frequency modulation (FM) of short eccentricity provide a robust basis for identifying the phase of long eccentricity forcing in stratigraphic data. One- and two-dimensional models of sedimentary distortion of the astronomical signal are used to evaluate the veracity of the FM method, and indicate that pristine eccentricity FM can be readily distingushed in paleo-records. Apart from paleoclimatic implications, the FM approach provides a quantitative technique for testing and calibrating theoretical astronomical solutions, and for refining chronologies for the deep past. We present two case studies that use the FM approach to evaluate major carbon-cycle perturbations of the Eocene and Late Cretaceous. Interference patterns in the short-eccentricity band reveal that Eocene hyperthermals ETM2 (‘Elmo‘), H2, I1 and ETM3 (X; ~52–54 Myr ago) were associated with maxima in the 405-kyr cycle of orbital eccentricity. The same eccentricity configuration favored regional anoxic episodes in the Mediterranean during the Middle and Late Cenomanian (~94.5–97 Myr ago). The initial phase of the global Oceanic Anoxic Event II (OAE II; ~93.9–94.5 Myr ago) coincides with maximum and falling 405-kyr eccentricity, and the recovery phase occurs during minimum and rising 405-kyr eccentricity. On a Myr scale, the event overlaps with a node in eccentricity amplitudes. Both studies underscore the importance of seasonality in pacing major climatic perturbations during greenhouse times.