To determine whether the ∼200 Ma central Atlantic magmatic province (CAMP) coincides with a normal polarity bias and a purported abrupt change in polar wander at the J1 cusp, we collected samples for ...paleomagnetic study from 80 sites distributed over a ∼2500‐m‐thick section of sedimentary units that are interbedded with and overlie CAMP lavas in the Hartford basin, which together represent the initial 2.4 Ma of the Jurassic according to cycle stratigraphic analysis. Characteristic directions carried by hematite were isolated by thermal demagnetization in 71 sites and define a coherent magnetostratigraphy supported by a positive reversal test and an interbasin fold test. Despite a pronounced overall normal polarity bias (only three relatively short reverse polarity intervals could be confirmed in the sampled section), normal polarity Chron H24n that encompasses the CAMP extrusive zone is no more than 1.6 Ma in duration. Elongation/inclination analysis of the 315 characteristic directions, which have a flattened distribution, produces a result in agreement with a published mean direction for the CAMP volcanic units as well as published results similarly corrected for inclination error from the Newark basin. The three data sets (CAMP volcanics, Newark corrected sediments, and Hartford corrected sediments) provide a 201 Ma reference pole for eastern North America at 67.0°N, 93.8°E, A95 = 3.2°. Paleopoles from the Moenave and Wingate formations from the Colorado Plateau that virtually define the J1 cusp can be brought into agreement with the 201 Ma reference pole with corrections for net clockwise rotation of the plateau relative to eastern North America and presumed sedimentary inclination error. The corrected data show that apparent polar wander for North America proceeds directly toward higher latitudes over the Late Triassic and Early Jurassic with no obvious change that can be associated with CAMP.
We developed a paleointensity technique to account for concave‐up Arai diagrams due to multidomain (MD) contributions to determine unbiased paleointensities for 24 trial samples from site GA‐X in ...Pleistocene lavas from Floreana Island, Galapagos Archipelago. The main magnetization carrier is fine‐grained low‐titanium magnetite of variable grain size. We used a comprehensive back‐zero‐forth (BZF) heating technique by adding an additional zero‐field heating between the Thellier two opposite in‐field heating steps in order to estimate paleointensities in various standard protocols and provide internal self‐consistency checks. After the first BZF experiment, we gave each sample a total thermal remanent magnetization (tTRM) by cooling from the Curie point in the presence of a low (15 µT) laboratory‐applied field. Then we repeated the BZF protocol, with the laboratory‐applied tTRM as a synthetic natural remanent magnetization (NRM), using the same laboratory‐applied field and temperature steps to obtain the synthetic Arai signatures, which should only represent the domain‐state dependent properties of the samples. We corrected the original Arai diagrams from the first BZF experiment by using the Arai signatures from the repeated BZF experiment, which neutralizes the typical MD concave‐up effect. Eleven samples meet the Arai diagram post‐selection criteria and provide qualified paleointensity estimates with a mean value for site GA‐X of 4.23 ± 1.29 µT, consistent with an excursional geomagnetic field direction reported for this site.
Key Points
develop a new paleointensity technique for multidomain igneous rocks
Identifying locations for secure sequestration of CO₂ in geological formations is one of our most pressing global scientific problems. Injection into basalt formations provides unique and significant ...advantages over other potential geological storage options, including large potential storage volumes and permanent fixation of carbon by mineralization. The Central Atlantic Magmatic Province basalt flows along the eastern seaboard of the United States may provide large and secure storage reservoirs both onshore and offshore. Sites in the South Georgia basin, the New York Bight basin, and the Sandy Hook basin offer promising basalt-hosted reservoirs with considerable potential for CO₂ sequestration due to their proximity to major metropolitan centers, and thus to large industrial sources for CO₂. Onshore sites are suggested for cost-effective characterization studies of these reservoirs, although offshore sites may offer larger potential capacity and additional long-term advantages for safe and secure CO₂ sequestration.
New rock magnetic results (thermal fluctuation tomography, high-resolution first-order reversal curves and low temperature measurements) for samples from the Paleocene–Eocene thermal maximum and ...carbon isotope excursion in cored sections at Ancora and Wilson Lake on the Atlantic Coastal Plain of New Jersey indicate the presence of predominantly isolated, near-equidimensional single-domain magnetic particles rather than the chain patterns observed in a cultured magnetotactic bacteria sample or magnetofossils in extracts. The various published results can be reconciled with the recognition that chain magnetosomes tend to be preferentially extracted in the magnetic separation process but, as we show, may represent only a small fraction of the overall magnetic assemblage that accounts for the greatly enhanced magnetization of the carbon isotope excursion sediment but whose origin is thus unclear.
The Paleocene–Eocene boundary (∼55.8 million years ago) is marked by an abrupt negative carbon isotope excursion (CIE) that coincides with an oxygen isotope decrease interpreted as the ...Paleocene–Eocene thermal maximum. Biogenic magnetite (Fe₃O₄) in the form of giant (micron-sized) spearhead-like and spindle-like magnetofossils, as well as nano-sized magnetotactic bacteria magnetosome chains, have been reported in clay-rich sediments in the New Jersey Atlantic Coastal Plain and were thought to account for the distinctive single-domain magnetic properties of these sediments. Uncalibrated strong field magnet extraction techniques have been typically used to provide material for scanning and transmission electron microscopic imaging of these magnetic particles, whose concentration in the natural sediment is thus difficult to quantify. In this study, we use a recently developed ultrahighresolution, synchrotron-based, full-field transmission X-ray microscope to study the iron-rich minerals within the clay sediment in their bulk state. We are able to estimate the total magnetization concentration of the giant biogenic magnetofossils to be only ∼10% of whole sediment. Along with previous rock magnetic studies on the CIE clay, we suggest that most of the magnetite in the clay occurs as isolated, near-equidimensional nanoparticles, a suggestion that points to a nonbiogenic origin, such as comet impact plume condensates in what may be very rapidly deposited CIE clays.
Theoretical first‐order reversal curves (FORCs) were generated by numerically solving a thermally activated Stoner‐Wohlfarth model for assemblages of randomly oriented magnetic particles. The ...thermally activated Stoner‐Wohlfarth model extends previous models based on Preisach theory. The new numerical simulations show that the shapes of reversal curves and the FORC distributions are significantly modified by the effect of the thermal energy only if superparamagnetic particles are predominant. However, most assemblages containing moderate amounts of superparamagnetic particles are hardly distinguishable from stable single‐domain assemblages. The most relevant thermal effect is a reduction of coercivity that translates in a shift of the FORC distribution toward the origin. Not all of the distinctive characteristics previously predicted for superparamagnetic grain assemblages were confirmed by our calculations, and most of the observed modifications due to thermal effects can be considered minor. A direct comparison with hysteresis parameters shows that these simpler experiments can be equally effective in characterizing viscous and superparamagnetic particles.
Key Points
Numerical model provides a solid base for interpretation of FORC diagrams of SP/SD particles
Results from thermally activated Stoner‐Wohlfarth approach improve previous models based on Preisach theory
Different versions of a composite apparent polar wander (APW) path of variably selected global poles assembled and averaged in North American coordinates using plate reconstructions show either a ...smooth progression or a large (∼30°) gap in mean paleopoles in the Late Jurassic, between about 160 and 145 Ma. In an effort to further examine this issue, we sampled accessible outcrops/subcrops of kimberlites associated with high‐precision U‐Pb perovskite ages in the Timiskaming area of Ontario, Canada. The 154.9 ± 1.1 Ma Peddie kimberlite yields a stable normal polarity magnetization that is coaxial within less than 5° of the reverse polarity magnetization of the 157.5 ± 1.2 Ma Triple B kimberlite. The combined ∼156 Ma Triple B and Peddie pole (75.5°N, 189.5°E, A95 = 2.8°) lies about midway between igneous poles from North America nearest in age (169 Ma Moat volcanics and the 146 Ma Ithaca kimberlites), showing that the polar motion was at a relatively steady yet rapid (∼1.5°/Myr) pace. A similar large rapid polar swing has been recognized in the Middle to Late Jurassic APW path for Adria‐Africa and Iran‐Eurasia, suggesting a major mass redistribution. One possibility is that slab breakoff and subduction reversal along the western margin of the Americas triggered an episode of true polar wander.
Key Points:
U‐Pb‐dated kimberlites provide reliable paleomagnetic poles for North America
Kimberlite paleopoles confirm 30° polar shift in Late Jurassic
Polar shift may represent an episode of true polar wander at 1.5°/Myr
Radiolarian cherts in the Tethyan realm of Jurassic age were recently interpreted as resulting from high biosiliceous productivity along upwelling zones in subequatorial paleolatitudes the locations ...of which were confirmed by revised paleomagnetic estimates. However, the widespread occurrence of cherts in the Eocene suggests that cherts may not always be reliable proxies of latitude and upwelling zones. In a new survey of the global spatio-temporal distribution of Cenozoic cherts in Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment cores, we found that cherts occur most frequently in the Paleocene and early Eocene, with a peak in occurrences at ∼
50 Ma that is coincident with the time of highest bottom water temperatures of the early Eocene climatic optimum (EECO) when the global ocean was presumably characterized by reduced upwelling efficiency and biosiliceous productivity. Cherts occur less commonly during the subsequent Eocene global cooling trend. Primary paleoclimatic factors rather than secondary diagenetic processes seem therefore to control chert formation. This timing of peak Eocene chert occurrence, which is supported by detailed stratigraphic correlations, contradicts currently accepted models that involve an initial loading of large amounts of dissolved silica from enhanced weathering and/or volcanism in a supposedly sluggish ocean of the EECO, followed during the subsequent middle Eocene global cooling by more vigorous oceanic circulation and consequent upwelling that made this silica reservoir available for enhanced biosilicification, with the formation of chert as a result of biosilica transformation during diagenesis. Instead, we suggest that basin–basin fractionation by deep-sea circulation could have raised the concentration of EECO dissolved silica especially in the North Atlantic, where an alternative mode of silica burial involving widespread direct precipitation and/or absorption of silica by clay minerals could have been operative in order to maintain balance between silica input and output during the upwelling-deficient conditions of the EECO. Cherts may therefore not always be proxies of biosiliceous productivity associated with latitudinally focused upwelling zones.
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