The black shale submember of core shales of Pennsylvanian Kansas-type cyclothems is highly enriched in redox-sensitive trace elements (TEs) relative to the overlying gray shale submember as well as ...to many other modern and ancient organic-rich deposits. Controls on trace-element behavior in the black shale can be inferred from trace-element enrichment factors (EFs, relative to gray shale) and the relationship of TEs to total organic carbon (TOC). Most TEs conformed to one of two patterns: (1) Mo, U, V, Zn, and Pb exhibited moderate EFs and strong covariation with TOC in samples having <10 wt.% TOC but large EFs and weak covariation with TOC in samples having >10 wt.% TOC; and (2) Cu, Ni, Cr, and Co exhibited low EFs and moderate to strong covariation with TOC in all samples. These patterns are inferred to represent different responses to benthic redox conditions. The first pattern is characteristic of TEs of “strong euxinic affinity”, i.e., those taken up in solid solution by Fe-sulfide or involved in other reactions catalyzed by free H
2S, and resident mainly in authigenic phases, whereas the second pattern is characteristic of TEs of “weak euxinic affinity”, i.e., those not strongly influenced by the presence of free H
2S and resident mainly in the organic carbon or detrital fractions of the sediment. These inferences allowed development of a multiproxy technique for assessing redox facies in black shale samples: euxinic conditions were considered to have existed if at least two of four “redox-indicator” trace elements (Mo, U, V, Zn) showed euxinic levels of enrichment, and nonsulfidic anoxic conditions were inferred otherwise. The validity of the procedure is indicated by (1) agreement among all four “redox indicators” for a large majority of samples (69% of 185); (2) among samples yielding a mixed redox signal, a systematic sequence of TE enrichment (V→Zn→Mo→U); and (3) for TEs of “weak euxinic affinity”, reduced variance among samples representing each redox facies. Sequential enrichment may be a response to differential redox thresholds for accumulation (e.g., V) or postdepositional remobilization of trace elements (e.g., U) and may provide a basis for finer assessment of redox conditions in low-oxygen paleoenvironments than permitted by simple classification schemes. The multiproxy procedure for redox-facies analysis developed in this study is likely to be more reliable than widely used single-proxy indicators based on trace elements (e.g., Mo, authigenic U, or V/(V+Ni)) or C–S–Fe systematics (e.g., S/TOC or DOP).
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Long-term secular variation in the isotopic composition of seawater fixed nitrogen (N) is poorly known. Here, we document variation in the N-isotopic composition of marine sediments (δ15Nsed) since ...660 Ma (million years ago) in order to understand major changes in the marine N cycle through time and their relationship to first-order climate variation. During the Phanerozoic, greenhouse climate modes were characterized by low δ15Nsed (∼−2 to +2‰) and icehouse climate modes by high δ15Nsed (∼+4 to +8‰). Shifts toward higher δ15Nsed occurred rapidly during the early stages of icehouse modes, prior to the development of major continental glaciation, suggesting a potentially important role for the marine N cycle in long-term climate change. Reservoir box modeling of the marine N cycle demonstrates that secular variation in δ15Nsed was likely due to changes in the dominant locus of denitrification, with a shift in favor of sedimentary denitrification during greenhouse modes owing to higher eustatic (global sea-level) elevations and greater on-shelf burial of organic matter, and a shift in favor of water-column denitrification during icehouse modes owing to lower eustatic elevations, enhanced organic carbon sinking fluxes, and expanded oceanic oxygen-minimum zones. The results of this study provide new insights into operation of the marine N cycle, its relationship to the global carbon cycle, and its potential role in modulating climate change at multimillion-year timescales.
Modern low-oxygen marine systems exhibit patterns of molybdenum–uranium covariation that can be linked to specific attributes and processes of the depositional system, including (1) variation in ...benthic redox conditions, (2) the operation of particulate shuttles within the water column, and (3) the evolution of watermass chemistry. The importance of these factors in each depositional system can be assessed both from the degree of enrichment of authigenic molybdenum (Mo
auth) and uranium (U
auth) and from the (Mo/U)
auth ratio of the sediment relative to the seawater Mo/U molar ratio of ∼
7.5–7.9. In open-ocean systems with suboxic bottomwaters, U
auth enrichment tends to exceed that of Mo
auth owing to onset of U
auth accumulation at the Fe(II)–Fe(III) redox boundary, resulting in sediment (Mo/U)
auth ratios less than that of seawater. As bottomwaters become increasingly reducing and at least occasionally sulfidic, the rate of accumulation of Mo
auth increases relative to that of U
auth, and sediment (Mo/U)
auth ratios equal or exceed that of seawater. In restricted marine systems with permanently sulfidic deepwaters, the relative enrichment of Mo
auth and U
auth depends on additional factors. In the Cariaco Basin, which has an aqueous Mo/U ratio similar to that of seawater, the operation of a particulate Mn–Fe-oxyhydroxide shuttle serves to accelerate the transfer of Mo to the seafloor, leading to strong enrichments in Mo
auth relative to U
auth. In the Black Sea, the chemistry of the deep watermass has evolved to the point where its aqueous Mo/U ratio is only ∼
0.04 that of seawater, as a consequence of which sediments deposited under deepwater influence are depleted in Mo
auth relative to U
auth. These Mo–U covariation patterns can be used to gain a better understanding of the watermass attributes and processes of ancient low-oxygen marine systems. Analysis of anoxic facies from two North American paleomarine systems, the Late Pennsylvanian Midcontinent Sea (LPMS) and the Late Devonian Seaway (LDS), reveals authigenic Mo–U relationships similar to those of the modern marine environments above, implying similar redox and hydrographic controls. The observed patterns are consistent with laterally unconfined circulation and strong watermass exchange within the LPMS, and with markedly restricted deepwater circulation in silled basins of the LDS. Patterns of authigenic Mo–U covariation may prove useful in analysis of other paleoceanographic systems to reveal aspects of watermass composition and environmental dynamics.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Records of the Ediacaran carbon cycle (635–541 million years ago) include the Shuram excursion (SE), the largest negative carbonate carbon isotope excursion in Earth history (down to −12‰). The ...nature of this excursion remains enigmatic given the difficulties of interpreting a perceived extreme global decrease in the δ13C of seawater dissolved inorganic carbon. Here, we present carbonate and organic carbon isotope (δ13Ccarb and δ13Corg) records from the Ediacaran Doushantuo Formation along a proximal‐to‐distal transect across the Yangtze Platform of South China as a test of the spatial variation of the SE. Contrary to expectations, our results show that the magnitude and morphology of this excursion and its relationship with coexisting δ13Corg are highly heterogeneous across the platform. Integrated geochemical, mineralogical, petrographic, and stratigraphic evidence indicates that the SE is a primary marine signature. Data compilations demonstrate that the SE was also accompanied globally by parallel negative shifts of δ34S of carbonate‐associated sulfate (CAS) and increased 87Sr/86Sr ratio and coastal CAS concentration, suggesting elevated continental weathering and coastal marine sulfate concentration during the SE. In light of these observations, we propose a heterogeneous oxidation model to explain the high spatial heterogeneity of the SE and coexisting δ13Corg records of the Doushantuo, with likely relevance to the SE in other regions. In this model, we infer continued marine redox stratification through the SE but with increased availability of oxidants (e.g., O2 and sulfate) limited to marginal near‐surface marine environments. Oxidation of limited spatiotemporal extent provides a mechanism to drive heterogeneous oxidation of subsurface reduced carbon mostly in shelf areas. Regardless of the mechanism driving the SE, future models must consider the evidence for spatial heterogeneity in δ13C presented in this study.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Patterns of uranium–molybdenum covariation in marine sediments have the potential to provide insights regarding depositional conditions and processes in paleoceanographic systems. Specifically, such ...patterns can be used to assess bottom water redox conditions, the operation of metal-oxyhydroxide particulate shuttles in the water column, and the degree of water mass restriction. The utility of this paleoenvironmental proxy is due to the differential geochemical behavior of U and Mo: (1) uptake of authigenic U by marine sediments begins at the Fe(II)–Fe(III) redox boundary (i.e., suboxic conditions), whereas authigenic Mo enrichment requires the presence of H2S (i.e., euxinic conditions), and (2) transfer of aqueous Mo to the sediment may be enhanced through particulate shuttles, whereas aqueous U is unaffected by this process. In the present study, we examine U–Mo covariation in organic-rich sediments deposited mostly in the western Tethyan region during oceanic anoxic events (OAEs) of Early Jurassic to Late Cretaceous age. Our analysis generally confirms existing interpretations of redox conditions in these formations but provides significant new insights regarding water mass restriction and the operation of particulate shuttles in depositional systems. These insights will help to address contentious issues pertaining to the character and origin of Mesozoic OAEs, such as the degree to which regional paleoceanographic factors controlled the development of the OAEs.
► We examine U–Mo covariation in Tethyan, organic-rich sediments of Mesozoic OAEs. ► We provide new insights regarding water mass restriction in some depositional systems. ► We provide new insights regarding the operation of particulate shuttles. ► These insights regard issues pertaining to the character and origin of Mesozoic OAEs.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
atterns of uranium-molybdenum covariation in marine sediments have the potential to provide insights re- garding depositional conditions and processes in paleoceanographic systems. Specifically, such ...patterns can be used to assess bottom water redox conditions, the operation of metal-oxyhydroxide particulate shuttles in the water column, and the degree of water mass restriction. The utility of this paleoenvironmental proxy is due to the differential geochemical behavior of U and Mo: (1) uptake of authigenic U by marine sediments begins at the Fe(II)-Fe(III) redox boundary (i.e., suboxic conditions), whereas authigenic Mo enrichment re- quires the presence of H2S (i.e., euxinic conditions), and (2) transfer of aqueous Mo to the sediment may be enhanced through particulate shuttles, whereas aqueous U is unaffected by this process. In the present study, we examine U-Mo covariation in organic-rich sediments deposited mostly in the western Tethyan region during oceanic anoxic events (OAEs) of Early Jurassic to Late Cretaceous age. Our analysis generally confirms existing interpretations of redox conditions in these formations but provides significant new insights regard- ing water mass restriction and the operation of particulate shuttles in depositional systems. These insights will help to address contentious issues pertaining to the character and origin of Mesozoic OAEs, such as the degree to which regional paleoceanographic factors controlled the development of the OAEs.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The famous Permian-Triassic boundary section at Guryul Ravine in Kashmir shows repeated strong disturbances in the uppermost 3 m of the section below the main end-Permian mass extinction horizon. Two ...one-meter-thick disturbed beds, with convoluted bedding and fluid escape structures, are interpreted as seismites. Immediately above, three lenticular, fining-upward, bioclastic grainstone beds, interbedded with argillites, are interpreted as tsunamites. In these beds, hummocky cross-stratification and grading indicate deposition by waning irregular waves at a minimum water depth of 100 m based on physical processes and faunas. Bed grain sizes indicate that the waves needed to move even the coarse sand of the matrix, let alone associated large pebbles up to 20 cm in diameter, range from amplitudes of ∼40 m for wave periods of 10 s (the upper limit for storm waves) to amplitudes of ∼3 m for wave periods of 50 to 1000 s (typical of large open-ocean tsunamis). Fossil and sedimentary evidence suggests lengthy intervals between successive tsunami events, which, together with a lack of geochemical evidence for impact, favors terrestrial causes. Geochemical proxies show that the Guryul Ravine environment remained oxic or suboxic throughout the P–Tr transition, but that anoxia developed regionally at the time of the boundary crisis. This paper is the first to propose seismites and tsunamites at the P-Tr boundary, so the geographic extent of these deposits is unknown, although analogous deposits occur in many sections worldwide from published reports.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
The Bächental bituminous marls (Bächentaler Bitumenmergel) belonging to the Sachrang Member of the Lower Jurassic Middle Allgäu Formation were investigated using a multidisciplinary approach to ...determine environmental controls on the formation of organic-rich deposits in a semi-restricted basin of the NW Tethys during the Early Jurassic. The marls are subdivided into three units on the basis of mineralogical composition, source-rock parameters, redox conditions, salinity variations, and diagenetic processes. Redox proxies (e.g., pristane/phytane ratio; aryl isoprenoids; bioturbation; ternary plot of iron, total organic carbon, and sulphur) indicate varying suboxic to euxinic conditions during deposition of the Bächental section. Redox variations were mainly controlled by sea-level fluctuations with the tectonically complex bathymetry of the Bächental basin determining watermass exchange with the Tethys Ocean. Accordingly, strongest anoxia and highest total organic carbon content (up to 13%) occur in the middle part of the profile (upper tenuicostatum and lower falciferum zones), coincident with an increase in surface-water productivity during a period of relative sea-level lowstand that induced salinity stratification in a stagnant basin setting. This level corresponds to the time interval of the lower Toarcian oceanic anoxic event (T-OAE). However, the absence of the widely observed lower Toarcian negative carbon isotope excursion in the study section questions its unrestricted use as a global chemostratigraphic marker. Stratigraphic correlation of the thermally immature Bächental bituminous marls with the Posidonia Shale of SW Germany on the basis of C27/C29 sterane ratio profiles and ammonite data suggests that deposition of organic matter-rich sediments in isolated basins in the Alpine realm commenced earlier (late Pliensbachian margaritatus Zone) than in regionally proximal epicontinental seas (early Toarcian tenuicostatum Zone). The late Pliensbachian onset of reducing conditions in the Bächental basin coincided with an influx of volcaniclastic detritus that was possibly connected to complex rifting processes of the Alpine Tethys and with a globally observed eruption-induced extinction event. The level of maximum organic matter accumulation in the Bächental basin corresponds to the main eruptive phase of the Karoo-Ferrar large igneous province (LIP), confirming its massive impact on global climate and oceanic conditions during the Early Jurassic. The Bächental marl succession is thus a record of the complex interaction of global (i.e., LIP) and local (e.g., redox and salinity variations, basin morphology) factors that caused reducing conditions and organic matter enrichment in the Bächental basin. These developments resulted in highly inhomogeneous environmental conditions in semi-restricted basins of the NW Tethyan domain during late Pliensbachian and early Toarcian time.
•Bächental bituminous marls accumulated in a semi-restricted basin of the NW Tethys.•Early Toarcian sea-level fall increased restriction and initiated deepwater anoxia.•Increased humidity and weathering stimulated nutrient fluxes and productivity.•Oceanographic changes were induced by Pliensbachian/Toarcian magmatism.•Applicability of the Toarcian CIE as a global stratigraphic marker is questioned.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The Devonian Period was characterized by major changes in both the terrestrial biosphere, e.g. the evolution of trees and seed plants and the appearance of multi-storied forests, and in the marine ...biosphere, e.g. an extended biotic crisis that decimated tropical marine benthos, especially the stromatoporoid-tabulate coral reef community. Teleconnections between these terrestrial and marine events are poorly understood, but a key may lie in the role of soils as a geochemical interface between the lithosphere and atmosphere/hydrosphere, and the role of land plants in mediating weathering processes at this interface. The effectiveness of terrestrial floras in weathering was significantly enhanced as a consequence of increases in the size and geographic extent of vascular land plants during the Devonian. In this regard, the most important palaeobotanical innovations were (1) arborescence (tree stature), which increased maximum depths of root penetration and rhizoturbation, and (2) the seed habit, which freed land plants from reproductive dependence on moist lowland habitats and allowed colonization of drier upland and primary successional areas. These developments resulted in a transient intensification of pedogenesis (soil formation) and to large increases in the thickness and areal extent of soils. Enhanced chemical weathering may have led to increased riverine nutrient fluxes that promoted development of eutrophic conditions in epicontinental seaways, resulting in algal blooms, widespread bottomwater anoxia, and high sedimentary organic carbon fluxes. Long-term effects included drawdown of atmospheric pCO2 and global cooling, leading to a brief Late Devonian glaciation, which set the stage for icehouse conditions during the Permo-Carboniferous. This model provides a framework for understanding links between early land plant evolution and coeval marine anoxic and biotic events, but further testing of Devonian terrestrial-marine teleconnections is needed.
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The Hushpuckney Shale Member of the Swope Formation (Missourian Stage, eastern Kansas) is the core shale of a Kansas-type cyclothem, formed during the late transgressive to early regressive phases of ...a Late Pennsylvanian glacio-eustatic cycle. Coeval high-frequency climato-environmental dynamics of the Midcontinent Seaway are preserved in the KGS Orville Edmonds No. 1A study core as centimeter-scale variation in major components (organic carbon, authigenic sulfides and phosphate, detrital siliciclastics), organic macerals, trace-element redox proxies (Mo, U, V, Zn), and ichnofabric features. Benthic O
2 levels declined sharply from the base of the black shale (0 cm), went sulfidic at ∼4 cm, and reached a redox minimum at ∼21 cm; above this level, redox potential gradually rose, fluctuating between sulfidic and nonsulfidic conditions from ∼35 cm to the black shale/gray shale contact at 52 cm. Onset of dysoxic conditions at that contact allowed establishment of a benthic community of soft-bodied organisms comprised of deep-tiered tracemakers tolerant of low-O
2 conditions (
Trichichnus), and shallow-tiered tracemakers favoring “soupground” (
Helminthopsis) or “firmground” substrates (the
Zoophycos–
Phycosiphon–
Schaubcylindrichnus–
Planolites association). Most geochemical records exhibit a “low-order” cycle spanning the full 52 cm thickness of the black shale submember, reflecting a dominant glacio-eustatic control. The base and top of the black shale submember record lateral migration of the pycnocline across the Midcontinent Shelf during the transgressive and regressive phases, respectively. The maximum flooding surface (MFS) is at ∼17–23 cm, an interval containing the euxinic peak and characterized by high concentrations of illite (representing a cratonic siliciclastic flux) and terrestrial organic macerals, the product of transient increases in humidity, weathering rates, and the export of coal-swamp vegetation associated with the interglacial highstand of the Swope cyclothem. Although a transgressive “surface of maximum starvation” (SMS) cannot be confidently identified, a phosphate-rich “regressive condensation surface” at ∼28–34 cm records pycnoclinal weakening due to increased aridity associated with renewed southern hemisphere icesheet growth; a correlative shift in dominance from terrestrial- to marine-derived organic macerals reflects increased upwelling of nutrient-rich deepwaters and enhanced primary productivity. Penecontemporaneous paleogeographic and -climatic factors (e.g., semirestricted circulation, monsoonal precipitation) predisposed the Midcontinent Seaway toward sensitivity to high-frequency climato-environmental fluctuations, which are preserved as 2- to 7-cm-thick cycles in the study core. The 52-cm-thick black shale submember contains ∼12 such “high-order” cycles, which have an estimated average duration of ∼2 to 9 kyr, implying a sub-Milankovitch-band (i.e., millennial scale) climatic control. The results of this study are relevant to the sequence stratigraphic interpretation of Kansas-type cyclothems, the dynamics of Gondwanan Ice Age glacio-eustatic cycles, and the development of boundary conditions for Late Pennsylvanian paleoclimate models.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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