We use a method based on a statistical geomagnetic field model to recognize and correct for inclination error in sedimentary rocks from early Mesozoic rift basins in North America, Greenland, and ...Europe. The congruence of the corrected sedimentary results and independent data from igneous rocks on a regional scale indicates that a geocentric axial dipole field operated in the Late Triassic. The corrected paleolatitudes indicate a faster poleward drift of approximately0.6 degrees per million years for this part of Pangea and suggest that the equatorial humid belt in the Late Triassic was about as wide as it is today.
Although continents were coalesced into the single landmass Pangea, Late Triassic terrestrial tetrapod assemblages are surprisingly provincial. In eastern North America, we show that assemblages ...dominated by traversodont cynodonts are restricted to a humid 6° equatorial swath that persisted for over 20 million years characterized by "semiprecessional" (approximately 10,000-y) climatic fluctuations reflected in stable carbon isotopes and sedimentary facies in lacustrine strata. More arid regions from 5-20°N preserve procolophonid-dominated faunal assemblages associated with a much stronger expression of approximately 20,000-y climatic cycles. In the absence of geographic barriers, we hypothesize that these variations in the climatic expression of astronomical forcing produced latitudinal climatic zones that sorted terrestrial vertebrate taxa, perhaps by excretory physiology, into distinct biogeographic provinces tracking latitude, not geographic position, as the proto-North American plate translated northward. Although the early Mesozoic is usually assumed to be characterized by globally distributed land animal communities due to of a lack of geographic barriers, strong provinciality was actually the norm, and nearly global communities were present only after times of massive ecological disruptions.
South Georgia exists as a microcontinent along the North Scotia Ridge ∼1,700 km east of Cape Horn. The tectonostratigraphic units of South Georgia have long been correlated with those of the Fuegian ...Andes of southernmost South America. Accordingly, South Georgia has been regarded as a continuation of the Late Jurassic–Early Cretaceous Rocas Verdes marginal basin system, formerly situated south of Burdwood Bank and east of Cape Horn. To test this, paleomagnetic analysis of samples from the Larsen Harbour Complex, Drygalski Fjord Complex, and Annenkov Island Formation of South Georgia showed that 21 sites yield a mean direction of D = 328.5°, I = −62.1° (a95 = 3.5°) and a paleomagnetic pole at 068.2°E, 67.2°N, A95 = 4.7°. The consistency of directions and strong polarity bias, plus indications of a negative differential tilt test, point to a secondary magnetization acquired in the Late Cretaceous. Comparison of predicted versus observed directions for South Georgia relative to stable South America indicate 27.2 ± 11.2° of counter‐clockwise rotation (and 10.5° ± 4.5° of northward tilting) since the acquisition of magnetization. These results are consistent with paleomagnetic studies from the Fuegian Andes and support a paleoposition of the South Georgia microcontinent south of Burdwood Bank as strongly indicated by the geologic evidence. Partitioning this rotation between oroclinal bending during the Rocas Verdes basin collapse in the Late Cretaceous and left‐lateral translation along the North Scotia Ridge is not possible on paleomagnetic grounds, but the co‐linearity of Andean structures between the restored microcontinent and Tierra del Fuego indicates the former.
Plain Language Summary
The South Atlantic island of South Georgia forms the mountainous, glaciated, core of a microcontinent. Located some 1,700 km east of Cape Horn at the southern tip of South America, it is one of the most isolated fragments of continental crust on Earth. For many decades geological evidence has been accumulating that points to the island and hence the entire microcontinent being a displaced fragment of the Andean Cordillera of Tierra del Fuego. To test this hypothesis we collected oriented samples from igneous rocks on the main and offshore islands. The results are directly comparable to those obtained from samples from Tierra del Fuego and therefore support the geological evidence that the microcontinent originated as a continuation of the Andean Cordillera and was displaced eastward relative to South America during the opening of the South Atlantic Ocean basin.
Key Points
South Georgia is a microcontinent on the Scotia Ridge regarded as a continuation of the Andes. To test this, oriented samples were collected
The paleomagnetic results agree with studies from the Andes supporting the paleoposition of the South Georgia indicated by the geology
Partitioning this rotation between oroclinal bending and translation along the North Scotia Ridge is not possible on paleomagnetic grounds
It has been almost 60 years since the first results from the Early Permian Bolzano Quartz Porphyries from the Trento Plateau of northern Italy (Southern Alps) showed paleomagnetic inclinations ...steeper than inclinations from broadly coeval units from central Europe. This experimental discrepancy, confirmed ever since at varying levels of magnitude and certitude, implied that northern Italy had paleolatitudes too northerly relative to Europe to be considered part of the European continent. On the other hand, it became progressively more apparent that paleomagnetic data from northern Italy were more compatible with data from Africa than with data from Europe, and this observation revived and complemented Argand’s original concept of Adria as a promontory of Africa. But if Adria was part of Africa, then the paleolatitude anomaly of Adria relative to Europe translated into a huge crustal misfit of Gondwana relative to Laurasia when these landmasses were forced into a classic Wegenerian Pangea as typified by the Bullard fit of the circum-Atlantic continents. This crustal misfit between Gondwana and Laurasia was shown to persist in the ever-growing paleomagnetic database even when data from Adria were provisionally excluded as non-cratonic in nature. Various solutions were offered that ultimately involved placing Gondwana to the east (allowing it to be more northerly) relative to Laurasia and envisaging a dextral shear occurring in the Tethys (Mediterranean) realm between these supercontinental landmasses. This shear or transformation was initially thought to occur as a continuum over the course of the Mesozoic–Cenozoic (the so-called ‘Tethys Twist’) but soon afterwards when plate tectonics came into play and limited the younger extent, as a discrete event during the post-Triassic, Triassic or most probably – as in the latest and preferred reconstructions – the Permian between a configuration of Pangea termed B – with the northwestern margin of Africa against southern Europe – to a configuration termed Pangea A-2, with the northwestern margin of Africa against eastern North America, that is more proximal in shape to the classic Pangea A-1 that started fragmenting in the Jurassic with the opening of the Atlantic Ocean. The Permian timing and presumed locus of the ~2300 km dextral shear is supported by rotated tectonic domains in Sardinia and elsewhere along the interface between Lauarasia and Gondwana. The concept of Pangea B and its transformation into Pangea A developed therefore in close conjunction with the concept and paleomagnetic support of Adria as a promontory of Africa, and has ramifications to many aspects of tectonics, climate change and biogeography yet to be explored.
We studied the stratigraphy, composition, and paleomagnetic properties of lateritic weathering profiles of Permian age from northern Iran and western Karakoram, Pakistan. A limited set of samples ...deemed representative yielded stable low-inclination paleomagnetic components carried essentially by hematite of chemical origin isolated in massive, fine-grained, and homogeneous ferricrete facies. These laterites originated at equatorial paleolatitudes characterized by intense weathering processes under warm and humid climatic conditions. Paleomagnetic estimates of paleolatitude from Iran, Karakoram, and north Tibet from this study and the literature, albeit sparse, provide testable constraints on the motion of the Cimmerian terranes as the result of the opening of the Neo-Tethys Ocean along the eastern margin of Gondwana during the Permian. We confirm and help refine previous suggestions that the Cimmerian terranes migrated from southern Gondwanan paleolatitudes in the Early Permian to subequatorial paleolatitudes by the Middle Permian-Early Triassic. As a novel conclusion, we find that timing, rates, and geometry of Cimmerian tectonics are broadly compatible with the transformation of Pangea from an Irvingian B to a Wegenerian A-type configuration with Neo-Tethyan opening taking place contemporaneously essentially in the Permian.
Building on an earlier study that confirmed the stability of the 405‐kyr eccentricity climate cycle and the timing of the Newark‐Hartford astrochronostratigraphic polarity time scale back to 215 Ma, ...we extend the magnetochronology of the Late Triassic Chinle Formation to its basal unconformity in scientific drill core PFNP‐1A from Petrified Forest National Park (Arizona, USA). The 335‐m‐thick Chinle section is imprinted with paleomagnetic polarity zones PF1r to PF10n, which we correlate to chrons E17r to E9n (~209 to 224 Ma) of the Newark‐Hartford astrochronostratigraphic polarity time scale. A sediment accumulation rate of ~34 m/Myr can be extended down to ~270 m, close to the base of the Sonsela Member and the base of magnetozone PF5n, which we correlate to chron E14n that onsets at 216.16 Ma. Magnetozones PF5r to PF10n in the underlying 65‐m‐thick section of the mudstone‐dominated Blue Mesa and Mesa Redondo members plausibly correlate to chrons E13r to E9n, indicating a sediment accumulation rate of only ~10 m/Myr. Published high‐precision U‐Pb detrital zircon dates from the lower Chinle tend to be several million years older than the magnetochronological age model. The source of this discrepancy is unclear but may be due to sporadic introduction of juvenile zircons that get recycled. The new magnetochronological constraint on the base of the Sonsela Member brings the apparent timing of the included Adamanian‐Revueltian land vertebrate faunal zone boundary and the Zone II to Zone III palynofloral transition closer to the temporal range of the ~215 Ma Manicouagan impact structure in Canada.
Key Points
Chinle Formation as recovered extends from ~224 to 209 Ma (Chrons E9n to E17r)
Base of Sonsela Member at ~216 Ma (Chron E14n) places older age limit on Adamanian‐Revueltian faunal transition
Published high‐precision U‐Pb detrital zircon ages from the lower Sonsela and Blue Mesa members tend to be anomalously old by comparison
Paleomagnetic inclinations in sedimentary formations are frequently suspected of being too shallow. Recognition and correction of shallow bias is therefore critical for paleogeographical ...reconstructions. This paper tests the reliability of the elongation/inclination (
E/
I) correction method in several ways. First we consider the
E/
I trends predicted by various PSV models. We explored the role of sample size on the reliability of the
E/
I estimates and found that for data sets smaller than
∼
100–150, the results were less reliable. The Giant Gaussian Process-type paleosecular variation models were all constrained by paleomagnetic data from lava flows of the last five million years. Therefore, to test whether the method can be used in more ancient times, we compare model predictions of
E/
I trends with observations from five Large Igneous Provinces since the early Cretaceous (Yemen, Kerguelen, Faroe Islands, Deccan and Paraná basalts). All data are consistent at the 95% level of confidence with the
E/
I trends predicted by the paleosecular variation models. The Paraná data set also illustrated the effect of unrecognized tilting and combining data over a large latitudinal spread on the
E/
I estimates underscoring the necessity of adhering to the two principle assumptions of the method. Then we discuss the geological implications of various applications of the
E/
I method. In general the
E/
I corrected data are more consistent with data from contemporaneous lavas, with predictions from the well constrained synthetic apparent polar wander paths, and other geological constraints. Finally, we compare the
E/
I corrections with corrections from an entirely different method of inclination correction: the anisotropy of remanence method of Jackson et al. Jackson, M.J., Banerjee, S.K., Marvin, J.A., Lu, R., Gruber, W., 1991. Detrital remanence, inclination errors and anhysteretic remanence anisotropy: quantitative model and experimental results. Geophys. J. Int. 104, 95–103 which relies on measurement of remanence and particle anisotropies of the sediments. In the two cases where a direct comparison can be made, the two methods give corrections that are consistent within error. In summary, it appears that the
E/
I method for recognizing and corrected the effects of sedimentary flattening is reasonably robust for at least the Mesozoic and Cenozoic when the source of scatter is geomagnetic and sedimentary flattening in origin.
Recent evidence from the ~201.5Ma Central Atlantic Magmatic Province (CAMP) in the Newark rift basin demonstrates that this Large Igneous Province produced a transient doubling of atmospheric pCO2, ...followed by a falloff to pre-eruptive concentrations over ~300kyr. This paper confirms the short-term findings from the Newark basin, and tests the million-year effects of the CAMP volcanism on Early Jurassic pCO2 from strata in the corollary Hartford basin of Eastern North America (ENA) also using the pedogenic carbonate paleobarometer. We find pCO2 levels for pre-CAMP background of 2000±700ppm (at S(z)=3000±1000ppm), increasing to ~5000±1700ppm immediately above the first lava flow unit, consistent with observations from the Newark. The longer post-extrusive Portland Formation of the Hartford basin records a fourth pulse of pCO2 to ~4500±1200ppm, about 240kyr after the last lava recorded in the ENA section. We interpret this fourth increase as due to a major episode of volcanism, and revise the main CAMP duration to 840±60kyr. The Portland also records a post-eruptive decrease in pCO2 reaching pre-eruptive background concentrations of ~2000ppm in only ~300kyr, and continuing to levels below pre-CAMP background over the subsequent 1.5Myr following the final episode of eruptions. Geochemical modeling (using modified COPSE code) demonstrates that the rapidity of the pCO2 decreases, and fall to concentrations below background can be accounted for by a 1.5-fold amplification of the continental silicate weathering response due to the presence of the CAMP basalts themselves. These results demonstrate that a continental flood basalt capable of producing a short-term perturbation of the carbon system may actually have an overall net-cooling effect on global climates due to a long-term net-decrease in pCO2 to below pre-eruptive levels, as previous models have suggested followed the emplacement of the Deccan Traps.
► Eruption of the CAMP resulted in rapid, discrete increases of atmospheric pCO2. ► Each pCO2 pulse is followed by a <300kyr decrease below pre-eruptive concentrations. ► A 50% amplified global weathering response accounts for a decrease below background. ► Weathering is amplified by highly reactive CAMP basalts straddling the humid tropics. ► LIPs can be net sinks for CO2 and therefore long-term net coolers of climate.
We obtained bulk-sediment δ18O and δ13C data from biostratigraphically-constrained Tethyan marine sections at Aghia Marina (Greece), Guri Zi (Albania), and Brumano and Italcementi Quarry (Italy), and ...revised the published chemostratigraphy of the Pizzo Mondello section (Italy). We migrated these records from the depth to the time domain using available chronostratigraphic tie points, generating Ladinian–Rhaetian δ13C and δ18O records spanning from ~242 to ~201Ma. The δ18O record seems to be affected by diagenesis, whereas the δ13C record appears to preserve a primary signal and shows values increasing by ~1‰ in the Ladinian followed by an ~0.6‰ decrease across the Ladinian–Carnian boundary, followed by relatively constant (but oscillatory) Carnian values punctuated by a negative excursion at ~233Ma in the early Carnian, a second negative excursion at ~229.5Ma across the early–late Carnian boundary, and a positive excursion at ~227Ma across the Carnian–Norian boundary. The Norian record is characterized by a long-term decreasing trend and a negative excursion at ~216Ma. Rapid increases and decreases in δ13C have been observed in the Rhaetian, but these may be at least in part due to mixing of different sources of carbonate carbon with different δ13C values. Our Triassic δ13C record has been compared to data from the literature, and a composite δ13C record spanning the last ~242Myr of Earth's history has been generated. This composite record shows a sequence of dated δ13C trends and events that can be used for stratigraphic correlation as well as for a better understanding of the global carbon cycle in the Mesozoic–Cenozoic.
•We obtained δ18O and δ13C data from Late Triassic marine limestone sections.•We obtained conodont biostratigraphic data for these sections.•We generated a composite δ13C and δ18O record spanning from 242Ma to 201Ma.•The δ13C record preserves a primary signal characterized by trends and excursions.•A Triassic–Recent δ13C record has also been constructed using literature data.
A magnetostratigraphy for ∼60 m of Koobi Fora Formation sediment in Area 104 was derived from 46 oriented samples that produced well-resolved characteristic magnetizations from progressive thermal ...demagnetization. Approximately 59 m below the Morte Tuff, previously dated to ∼1.51 Ma (millions of years ago), the Olduvai-Matuyama boundary (∼1.78 Ma) was found to be at the level of marker bed A2—inconsistent with the Area 102 type section and thus contrary to fossil dating schemes that utilize temporal equivalence between A2 104 and A2 102. The magnetostratigraphic data, coupled with the Morte Tuff, provide a means to interpolate new ages for marker beds A2 104 and the White Tuff, as well as multiple Area 104 hominin fossils. Noteworthy is the new date of ∼1.63 Ma for KNM-ER 3733, which now implicates KNM-ER 2598 as the sole early African Homo erectus fossil demonstrably older than Dmanisi and Java Homo specimens. Re-dating KNM-ER 3733 creates a ∼300-kyr gap at 1.9 to 1.6 Ma in the African fossil record of H. erectus, which might be partially spanned by hand axes recently dated at ∼1.76 Ma, if the Acheulian is indeed proprietary to this species.
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