The Otavi Group is a Neoproterozoic carbonate-dominated succession up to 4 km thick, which blankets the southern promontory of the Congo craton in northern Namibia. This succession was deposited ...between 770 and 580 Ma in response to north-south crustal stretching and subsequent thermal subsidence. The main shallow-water platform has a well-defined southern limit, beyond which is a distally tapered foreslope wedge of deep-water carbonate facies. The Ghaub Formation represents the younger of two Cryogenian glaciations of the platform and was deposited during the period of thermal subsidence. A deep negative δ¹³C excursion, accompanied by increased size, abundance and variety of stromatolites, occurs in the last 10 to 80 m of shallow-water carbonate on the platform beneath the Ghaub glacial erosion surface. The same phenomena are observed before the older (Sturtian) glaciation in other areas, suggesting temporal proximity of the δ¹³C excursions to glaciation. Growth of ice sheets is manifested by emergence of the platform and development of a falling-stand wedge on the foreslope, composed of upward-coarsening carbonate turbidites and debrites. Rafts of very coarse-grained, well-sorted oolite have no source on the platform. The oolite probably originated at the strandline on the foreslope and was redeposited gravitationally downslope as sea-level fell. The Ghaub Formation is a laterally continuous wedge of carbonate diamictite, limited to the distal foreslope and ca 80 m in average thickness. Tongues of massive to weakly stratified diamictite, representing proglacial rain-out and subglacial tillite, are bounded by thinner, well-bedded units consisting of hypopycnal plume fallout, ice-rafted debris, turbidites and debrites, sorted sands and gravels, and westward-directed contourites. Debris is derived from the falling-stand wedge and the top 80 m of the inner platform. The wedge rests on a laterally continuous erosion surface, presumably cut by ice, and its sedimentary makeup and stratal organization are diagnostic of an ice grounding-line wedge. The subglacial erosion surface cuts a steep-walled trough on the distal foreslope, presumably once occupied by a transverse ice-stream. In the middle of the trough stands a doubly crested moraine composed of amalgamated unstratified diamictites. Terminal deglaciation is recorded by a fining-upward, 10 m thick drape of Fe-rich carbonate debrite and turbidite, loaded with far-travelled ice-rafted debris of all sizes. If deglaciation began with the collapse of ‘sea-glacier' ice on the tropical ocean, the loss of this buttress could have triggered catastrophic ice-sheet drainage, concomitant with surface ocean ventilation. Complete deglaciation presumably was driven by ice-albedo, ice-elevation and greenhouse-gas feedbacks. The grounding-line wedge and bare upper foreslope and platform are overlain by a transgressive ‘cap dolostone', which features shallow-water sedimentary structures (sorted peloids, low-angle cross-bedding, tubestone stromatolite and giant wave ripples) from the distal foreslope to the inner platform. The structures and isotopic profiles show that it was deposited diachronously on the time scale of ice-sheet melting globally (kyr), implying very high sedimentation rates. Temperature-dependent isotope fractionation could account for the observed secular and lateral δ¹³C changes. This interpretation requires sea water pH lower than 7·3 (high pCO₂) and warming of at least 45°C in the tropics, consistent with the change in planetary albedo accompanying global deglaciation. The thickness of the highstand part of the cap-carbonate sequence on the platform implies >3 to 5 Myr of tectonic subsidence during the glacial period to create permanent accommodation. The highstand sequence prograded inward from the raised rim of the platform, inherited from karstic and glacial erosion. Sea floor cements, formerly aragonitic, are localized over palaeobathymetric highs (ice-stream moraine, platform rim and inner platform highs). Subsequent aggradation of the platform and coeval foreslope shedding were accompanied by a 0·5 km thick stratigraphic interval in which platform strata are depleted in ¹³C by up to 2·5‰, compared with coeval foreslope strata; this again suggests temperature-dependent fractionation at low pH. Above this interval, the δ¹³C gradient reverts to normal (platform strata more enriched in ¹³C than foreslope equivalents), consistent with CO₂ drawdown due to silicate weathering.
On the southwest cape of the Congo craton, a subtropical carbonate bank the size of Greenland was heavily glaciated during two Cryogenian panglacial episodes spaced 10-20 Myr apart. In NW Namibia, ...the bank underwent crustal stretching with resultant Aegean Sea-type topography during the older and longer Sturtian glaciation (717-661 Ma). This is indicated by angular discordance between glacial and preglacial strata and diamictites sourced from all older units, including crystalline basement. In contrast, the bank was flat-topped and underwent broad thermal subsidence during Marinoan glaciation (646 + or - 5-635 Ma), attested by stratal parallellism and diamictites sourced from less than or equal to 100 m stratigraphic depth. However, greater than or equal to2.0kmofreliefexistedon the Marinoan continental slope, where most glacial erosion and accumulation occurred. The average rates of Marinoan erosion (2.55-6.80 m/Myr, n = 190) and accumulation (2.65-7.07 m/Myr, n = 211) are indistinguishable, implying that the location in a continental promontory did not bias erosion over accumulation. The average accumulation rates for the Sturtian and Marinoan, scaled for different averaging times, including Marinoan uncertainty, are 3.95-4.93 m/Myr (n = 183) and 2.65-7.07 m/Myr (n = 190), respectively, suggesting that a Marinoan glacioeustatic coastal escarpment substituted for rift-related Sturtian basin-and-range topography. These slow rates, comparable to long-term pre-Quaternary accumulation rates on existing abyssal plains, reconcile glacial sedimentology with the feeble hydrologic cycle of snowball Earth.
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Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
3.
Big Time Hoffman, Paul F
Annual review of earth and planetary sciences,
05/2019, Letnik:
47, Številka:
1
Journal Article
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The Proterozoic Eon was once regarded as the neglected middle half of Earth history. The name refers to early animals, but they did not appear until the eon (2.5-0.54 Ga) was nearly over. Eukaryotic ...cells and sexual reproduction evolved much earlier in the eon, as did chloroplasts. Molecular dioxygen, the presence of which altered the geochemical behavior of nearly every element essential to life, rose from negligible to near-modern levels, and then plummeted before rising fitfully again. Plate tectonics took on a modern form, and two supercontinents, Nuna and Rodinia, successively congregated and later dispersed. Climate regulatory failures, i.e., Snowball Earth, appear to be a uniquely Proterozoic phenomenon, having occurred twice in rapid succession near the end of the eon (from 717 to 660 Ma and from 650 to 635 Ma) and arguably once near its beginning (ca. 2.43 Ga). Dynamic sea glaciers covered Snowball Earth oceans from pole to pole, and equatorial sublimation drove slow-moving ice sheets on land. Ultimately, the gradual accumulation of CO
2
triggered rapid deglaciation and transient greenhouse aftermaths. Physically based and geologically tested, Neoproterozoic Snowball Earth appears to have molecular legacies in ancient bitumens and modern organisms. This is the story of my love affair with an eon that is now a little less neglected.
Carbonate sediments of nonglacial Cryogenian (659 to 649 Ma) and early Ediacaran (635 to 590 Ma) age exhibit large positive and negative δ13Ccarb excursions in a shallow-water marine platform in ...northern Namibia. The same excursions are recorded in fringing deep-sea fans and in carbonate platforms on other paleocontinents. However, coeval carbonates in the upper foreslope of the Namibian platform, and to a lesser extent in the outermost platform, have relatively uniform δ13Ccarb compositions compatible with dissolved inorganic carbon (DIC) in the modern ocean. We attribute the uniform values to fluid-buffered diagenesis that occurred where seawater invaded the sediment in response to geothermal porewater convection. This attribution, which is testable with paired Ca and Mg isotopes, implies that large δ13Ccarb excursions observed in Neoproterozoic platforms, while sedimentary in origin, do not reflect the composition of ancient open-ocean DIC.
Constraints on Paleoproterozoic atmospheric oxygen levels Bellefroid, Eric J.; Hood, Ashleigh v. S.; Hoffman, Paul F. ...
Proceedings of the National Academy of Sciences - PNAS,
08/2018, Letnik:
115, Številka:
32
Journal Article
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The oxygenation of Earth’s surface environment dramatically altered key biological and geochemical cycles and ultimately ushered in the rise of an ecologically diverse biosphere. However, atmospheric ...oxygen partial pressures (pO₂) estimates for large swaths of the Precambrian remain intensely debated. Here we evaluate and explore the use of carbonate cerium(Ce) anomalies (Ce/Ce*) as a quantitative atmospheric pO₂ proxy and provide estimates of Proterozoic pO₂ using marine carbonates from a unique Precambrian carbonate succession—the Paleoproterozoic Pethei Group. In contrast to most previous work, wemeasure Ce/Ce* on marine carbonate precipitates that formed in situ across a depth gradient, building on previous detailed sedimentology and stratigraphy to constrain the paleo-depth of each sample. Measuring Ce/Ce* across a full platform to basin depth gradient, we found only minor depleted Ce anomalies restricted to the platform and upper slope facies. We combine these results with a Ce oxidation model to provide a quantitative constraint on atmospheric pO₂ 1.87 billion years ago (Ga). Our results suggest Paleoproterozoic atmospheric oxygen concentrations were low, near 0.1% of the present atmospheric level. This work provides another crucial line of empirical evidence that atmospheric oxygen levels returned to low concentrations following the Lomagundi Event, and remained low enough for large portions of the Proterozoic to have impacted the ecology of the earliest complex organisms.
Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and ≥5 My) glaciations. Combined uranium-lead and rhenium-osmium dating ...suggests that the older (Sturtian) glacial onset and both terminations were globally synchronous. Geochemical data imply that CO
was 10
PAL (present atmospheric level) at the younger termination, consistent with a global ice cover. Sturtian glaciation followed breakup of a tropical supercontinent, and its onset coincided with the equatorial emplacement of a large igneous province. Modeling shows that the small thermal inertia of a globally frozen surface reverses the annual mean tropical atmospheric circulation, producing an equatorial desert and net snow and frost accumulation elsewhere. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. Tropical ice sheets flow faster as CO
rises but lose mass and become sensitive to orbital changes. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Meltwater flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. The subglacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the ice cover, increasing with latitude. Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification. Whole-ocean warming and collapsing peripheral bulges allow marine coastal flooding to continue long after ice-sheet disappearance. The evolutionary legacy of Snowball Earth is perceptible in fossils and living organisms.
We have measured the iron isotope compositions and trace element concentrations of a suite of iron formation (IF) samples from the Neoproterozoic Rapitan Group, which was deposited during the older ...of two glacial episodes recorded in the Windermere Supergroup of the northern Canadian Cordillera. Like most other Neoproterozoic examples, iron in the Rapitan IF resides almost exclusively as hematite. This mineralogical simplicity compared to Archean and Paleoproterozoic banded iron formations is attributed to a limited supply of organic carbon to the Rapitan glacial ocean that inhibited diagenetic production of reduced iron phases. Sedimentological considerations indicate that the Rapitan IF was deposited during a rise in relative sea level related to a period of glacial advance and isostatic subsidence. Trace element data, including rare earth element plus yttrium (REE
+
Y) patterns, suggest an anoxic deep ocean dominated by low-temperature hydrothermal input and capped by a weakly oxic surface ocean. The iron isotope data show a trend of increasing δ
57Fe (versus IRMM-14) up-section from ~−0.7‰ to 1.2‰, corresponding to a shift from a muddy IF facies to a dominantly jaspilitic IF facies. This distinct isotopic pattern likely records a steep isotopic gradient across the iron chemocline in Rapitan seawater.
► We present trace element and iron isotope data on the Rapitan iron formation. ► The iron isotope data show a large rise in iron isotope values up-section. ► This isotopic trend is coupled to increasing water depth. ► The trend likely records a vertical iron isotope gradient in Rapitan seawater. ► We propose a new model for iron isotopic variability in ancient iron formations.
Although crizotinib demonstrates robust efficacy in anaplastic lymphoma kinase (ALK)-positive non-small-cell lung carcinoma patients, progression during treatment eventually develops. Resistant ...patient samples revealed a variety of point mutations in the kinase domain of ALK, including the L1196M gatekeeper mutation. In addition, some patients progress due to cancer metastasis in the brain. Using structure-based drug design, lipophilic efficiency, and physical-property-based optimization, highly potent macrocyclic ALK inhibitors were prepared with good absorption, distribution, metabolism, and excretion (ADME), low propensity for p-glycoprotein 1-mediated efflux, and good passive permeability. These structurally unusual macrocyclic inhibitors were potent against wild-type ALK and clinically reported ALK kinase domain mutations. Significant synthetic challenges were overcome, utilizing novel transformations to enable the use of these macrocycles in drug discovery paradigms. This work led to the discovery of 8k (PF-06463922), combining broad-spectrum potency, central nervous system ADME, and a high degree of kinase selectivity.
On the coevolution of Ediacaran oceans and animals Shen, Yanan; Zhang, Tonggang; Hoffman, Paul F
Proceedings of the National Academy of Sciences - PNAS,
05/2008, Letnik:
105, Številka:
21
Journal Article
Recenzirano
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Fe speciation and S-isotope of pyrite data from the terminal Proterozoic Sheepbed Formation in Canada and Doushantuo Formation in China reveal that ocean deep waters were anoxic after the global ...glaciations (snowball Earth) ending 635 million years ago, but that marine sulfate concentrations and inferentially atmospheric oxygen levels were higher than before the glaciations. This supports a long-postulated link between oxygen levels and the emergence of eumetazoa. Subsequent ventilation of the deep ocean, inferred from shifts in Fe speciation in Newfoundland (previously published data) and western Canada (this report), paved the way for Ediacaran macrobiota to colonize the deep seafloors.
Crizotinib (1), an anaplastic lymphoma kinase (ALK) receptor tyrosine kinase inhibitor approved by the U.S. Food and Drug Administration in 2011, is efficacious in ALK and ROS positive patients. ...Under pressure of crizotinib treatment, point mutations arise in the kinase domain of ALK, resulting in resistance and progressive disease. The successful application of both structure-based and lipophilic-efficiency-focused drug design resulted in aminopyridine 8e, which was potent across a broad panel of engineered ALK mutant cell lines and showed suitable preclinical pharmacokinetics and robust tumor growth inhibition in a crizotinib-resistant cell line (H3122-L1196M).
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