ALBERT

Description: Recovery from the Late Permian mass extinction was slowed by continued environmental perturbations during the Early Triassic. Rapid fluctuations of the Early Triassic marine carbonate carbon isotope record indicate instability in the global carbon cycle, and recent 18 O apatite studies link elevated temperatures to the prolonged biotic recovery. High temperatures potentially caused enhanced continental weathering that was detrimental to marine ecosystems, but linking weathering rates to temperature has proven difficult. One proxy for weathering is the 87 Sr/ 86 Sr of marine carbonate; we present here an 87 Sr/ 86 Sr record from an upper Permian–lower Triassic succession near Zal, Iran, that is coupled to a 13 C carbonate record. An increase in the rate of 87 Sr/ 86 Sr rise from the Dienerian to the Smithian may be linked to elevated continental weathering rates caused by warming during the Smithian.

Description: We report new strain analyses of mechanically twinned calcite in veins hosted by Neogene (13.6–4.3 Ma) sedimentary and volcanic rocks recovered from the Terror Rift system in the southern Ross Sea, Antarctica, by the ANDRILL (ANtarctic geological DRILLing) McMurdo Ice Shelf (MIS) Project. Strain analyses of the ANDRILL MIS AND-1B drill core samples yield prolate and oblate ellipsoids with principal shortening and extension strains ranging from –7% to 9%, respectively. The majority of samples show ≤25% negative expected values, indicating homogeneous coaxial strain characterized predominantly by subvertical shortening. We attribute the subvertical shortening strains to mechanical twinning at relatively shallow depths in an Andersonian normal faulting stress regime induced by sedimentary and ice sheet loading of the stratigraphic sequence and characterized by low stress magnitudes. Oriented samples yield a northwest-southeast average extension direction that is subparallel to other indicators of Neogene extension. This northwest-southeast extension is consistent with strain predicted by Neogene orthogonal rifting in a north-northeast–trending rift segment, as well as models of right-lateral transtensional rifting. The overall paucity of a noncoaxial layer-parallel shortening signal in the AND-1B twin populations favors orthogonal extension in the Neogene Terror Rift system, but could also be due to spatial partitioning of strain in a transtensional rift regime.

Description: The timing and magnitude of postglacial climatic changes around the globe provide insights into the underlying drivers of natural climate change. Using geomorphologic mapping of moraines, 10 Be surface-exposure dating, snowline reconstructions, and numerical modeling, we quantified glacier behavior during Late Glacial (15–11.5 ka) and Holocene (the past ~11.5 k.y.) time in the Ben Ohau Range, New Zealand. Glaciers were more extensive during the Antarctic Cold Reversal (ACR), than subsequently, and the margins underwent a punctuated net withdrawal over the Holocene. Numerical modeling experiments that achieve the best fit to the moraines suggest that air temperature during the ACR was between 1.8 °C and 2.6 °C cooler than today, with similar (±20%) prescribed precipitation. After the ACR, a net snowline rise of ~100 m through the Younger Dryas stadial (12.9–11.7 ka) was succeeded by a further "long-term," or net, rise of ~100 m between ~11 k.y. and ~500 yr ago. Glacier snowline records in New Zealand show generally coherent Late Glacial and Holocene climate trends. However, the paleoclimate record in the southwest Pacific region shows important differences from that in the Northern Hemisphere.

Description: Tomographic studies of the mantle of southern California (USA) commonly found evidence for seismically high speed material, known as the Isabella anomaly, extending from near the base of the crust of the southwestern Sierra Nevada foothills into the asthenosphere. This anomaly has been interpreted to mark downwelling lithospheric material that had been removed from the southern Sierra Nevada. Using data from the Sierra Nevada EarthScope Project (SNEP) array, we investigate the lithosphere of the Sierra Nevada and surrounding region to better understand the process by which batholiths form dense lithospheric roots that become unstable and founder. Inverting phase velocities of fundamental mode Rayleigh waves for shear wave speeds provides observations of the distribution of high and low wave-speed anomalies, which correspond to portions of the batholith that formed an intact lithospheric root, and where seismically slower shallow asthenosphere marks areas where lithosphere has been removed. Our results corroborate previous observations that the southern Sierra Nevada has thin crust underlain by shallow asthenosphere. High shear wave velocity (Vs) material in the mantle beneath the southwestern foothills marks the location of the Isabella anomaly, to the east of which is a region of low Vs mantle where asthenosphere has risen to replace the delaminating root. Farther north, near the latitude of Long Valley, low velocities at shallow depths beneath the high elevations of the eastern Sierra indicate the presence of asthenosphere close to the base of the crust. Thicker high-speed material, however, underlies the western foothills of the Sierra Nevada at this latitude and dips to the east where it extends to depths of ~100 km or more, giving it the appearance of a portion of lithosphere that has detached from the east but remains attached to the west as it is currently peeling off. The structure of the Sierra Nevada changes near the latitude of Lake Tahoe, where thinner lithosphere extends between depths of 40 and 80 km, but does not reach greater depths. It appears that the lithospheric material of the Sierra Nevada from latitudes close to Lake Tahoe, and continuing to the north, is not being removed, indicating a change between the structure and evolution of the southern and northern Sierra Nevada.

Description: The Afar Depression, at the northern end of the East African Rift, is the only place on land where the transition from a plume-induced continental breakup to seafloor spreading is active today. New images of seismic velocity structure, based on exceptional new data sets, show that the mantle plume that initiated rifting in Africa is absent beneath Afar today. The images are dominated by a major low-velocity feature at ~75 km depth closely mimicking the abrupt changes in rift axis orientation seen at the surface. This is likely associated with passive upwelling beneath the rift. Additional focused low-velocity anomalies show that small diapiric upwellings are present beneath major off-axis volcanoes. These multiple melting sources can explain the wide range of geochemical signatures seen in Afar. These images suggest that passive upwelling beneath Afar marks the initiation of rift segmentation as continental breakup progresses to seafloor spreading.

Description: Teleseismic P-wave tomography using the Sierra Nevada Earthscope Project (SNEP) deployment, older temporary deployments in the Sierra, and broadband stations from permanent and USArray Transportable Array (TA) stations was derived from starting models either lacking lateral variation (one dimensional [1-D]) or created from three-dimensional (3-D) surface-wave models. The use of multiple starting models permits examination of the robustness of different features while limiting the inherent ambiguities of teleseismic body-wave tomography. Our results confirm that mafic residuum of the Mesozoic Sierran batholith has been removed from the eastern Sierra north to at least 39°N. Low-wavespeed material near the Moho under the eastern Sierra is probably silicic lower crust and warm and possible melt-laden upper mantle. If the residuum remains in the upper mantle, there are three possible locations for it: a high-wavespeed (+~5%) body extending down to ~250 km near 36°N, 119.3°W termed the Isabella anomaly, an unusually high-wavespeed (to +10%) and shallow (top ~40–50 km) anomaly at the south end of the Gorda slab near 40.5°N, 122.25°W termed the Redding anomaly, and a more ill-defined region of high wavespeeds in the crust to uppermost mantle (〈~70 km) along the western foothills of the Sierra termed the Foothills anomaly. The Foothills anomaly is most pronounced from 36.5° to 38°N and is distinguished from high-wavespeed lithosphere to the west by an unusually deep Moho. We infer that this body could either reflect in situ lower lithospheric material from the Mesozoic arc or could be material displaced from the east. The Isabella anomaly is nearly equant in plan view with a diameter ~100 km and a plunge to the east of ~60°–70°. This anomaly contains more than enough material to account for the lower lithosphere thought to have been under the eastern Sierra south of ~38°N until ca. 10 Ma. The Redding anomaly cannot be trivially separated from the Gorda slab and could either represent imbrication of the downgoing oceanic lithosphere or entrained continental lithosphere, possibly in part from the Sierra. Both the Redding and Isabella anomalies appear to have connections to the Foothills anomaly, but these connections are at the limit of resolution of this data set and could be artifacts from adjacent but unrelated high-wavespeed bodies. Our preferred inference that the Isabella anomaly has been derived from the eastern Sierra suggests that foundering of lithospheric material is not simply described as a two-dimensional (2-D) delamination and/or Rayleigh-Taylor instability as modeled in the literature to date.

Description: Here we make the first quantitative estimates of the maximum total number of large terrestrial impact craters that can survive until the current era on Earth. Our estimates of crater survival are made using the age of Earth’s crust and the expected flux of large extraterrestrial bodies striking Earth. This analysis neglects effects such as weathering and burial, which tend to preferentially erase smaller craters. Thus, our results represent the maximum number of craters we expect to exist on Earth today. Although the constant recycling of crustal material creates a strong bias toward younger ages, our model suggests that the terrestrial cratering record can be used to determine the size frequency distribution of the impactors that bombarded Earth over the past 3.5 G.y. However, our model suggests that the cratering record cannot be used to distinguish between scenarios with a constant flux of impactors over time and scenarios where the bombardment rate was considerably higher in Earth’s ancient history. Luckily, layers of distal impact ejecta may act as records of impacts even when the source craters have been tectonically recycled. We suggest that searches for these distal impact ejecta layers will be more fruitful for constraining the bombardment history of Earth than searches for large craters.

Description: Integration of petrologic, chronologic and petrophysical xenolith data with geophysical observations can offer fundamental insights into understanding the evolution of continental crust. We present the results of a deep crustal xenolith study from the northern Rocky Mountain region of the western U.S., where seismic experiments reveal an anomalously thick (10–30 km), high seismic velocity (compressional body wave, Vp 〉 7.0 km/s) lower crustal layer, herein referred to as the 7.x layer. Xenoliths exhumed by Eocene minettes from the Bearpaw Mountains of central Montana, within the Great Falls tectonic zone, include mafic and intermediate garnet granulites, mafic hornblende eclogite, and felsic granulites. Calculated pressures of 0.6–1.5 GPa are consistent with derivation from 23–54 km depths. Samples record diverse and commonly polymetamorphic pressure-temperature histories including prograde burial and episodes of decompression. Samples with barometrically determined depths consistent with residence within the seismically defined 7.x layer have calculated bulk P-wave velocities of 6.9–7.8 km/s, indicating heterogeneity in the layer. Shallower samples have markedly slower velocities consistent with seismic models. New monazite total U-Th-Pb data and a variety of additional published geochronology indicate a prolonged and episodic metamorphic history, beginning with protolith ages as old as Archean and followed by metamorphic and deep crustal fluid-flow events ca. 2.1 Ga, 1.8–1.7 Ga, and 1.5–1.3 Ga. We suggest that the 7.x layer in this region owes its character to a variety of processes, including magmatic underplating and intraplating, associated with multiple tectonic events from the Neoarchean to the Mesoproterozoic.

Description: Two Permian-Triassic boundary (PTB) sections from south China provide insights regarding the origin of negative excursions in carbonate and organic carbon isotope records associated with the largest mass extinction in Earth history. Xiakou, a carbonate ramp section, exhibits 13 C carb excursions of up to –2, and Xinmin, a deep-shelf section, exhibits 13 C org excursions of up to –6. In both sections, these excursions are associated with volcanic ash layers, and excursion size scales with ash layer thickness. These relationships document the direct influence of volcanism on the Earth-surface carbon cycle during the PTB crisis. Previous studies of ash layers in south China PTB sections have invoked a source in regional subduction-zone volcanism in the eastern Tethys, but our analysis suggests that these ash layers may represent distal deposits from large-scale explosive eruptions of the Siberian Traps. If confirmed by further investigation, this hypothesis would have important implications both for kill mechanisms during the PTB crisis as well as for refinement of the eruption history of the Siberian Traps.