Search for: All records

Superlatives—whether tallest, longest, or fastest—are more interesting than averages. This characteristic applies to many aspects of the geosciences, where scales of time and space are beyond human experience. The deepest trench, the highest mountain, and the most expansive desert are much more interesting than average ones. Interest in superlatives also applies to the oldest rocks. In this essay, we show that the oldest rocks in the United States are 3.62–3.45 billion years old (Ga) and are found in three different states. These localities define an east-west−trending belt in the upper midcontinent that stretches ~3000 km from Wyoming through Minnesota and into the Upper Peninsula of Michigan. Complex U-Pb zircon systematics are observed in the oldest rocks from all three areas, complicating efforts to distinguish zircons that crystallized in the magma(s) that made the host rock from xenocrystic zircons incorporated by assimilating older rocks. Within these uncertainties, the oldest rock in the United States is 3.62 Ga (Eoarchean to Paleoarchean), but older, 3.8 Ga zirconbearing felsic crust existed and may be identified by future investigations. 

The record of the first two billion years of Earth history (the Archean) is notoriously incomplete, yet crust of this age is present on every continent. Here we examine the Archean record of the Wyoming craton in the northern Rocky Mountains, U.S.A., which is both well-exposed and readily accessible. We identify three stages of Archean continental crust formation that are also recorded in other cratons. The youngest stage is characterized by a variety of Neoarchean rock assemblages that are indistinguishable from those produced by modern plate tectonic processes. The middle stage is typified by the trondhjemite-tonalite-granodiorite (TTG) association, which involved partial melting of older, mafic crust. This older mafic crust is not preserved but can be inferred from information in igneous and detrital zircon grains and isotopic compositions of younger rocks in Wyoming and other cratons. This sequence of crust formation characterizes all cratons, but the times of transition from one stage to the next vary from craton to craton. 

The current three sigma tension in the unitarity test of the Cabbibo-Kobayashi-Maskawa (CKM) matrix is a notable problem with the Standard Model of elementary particle physics. A long-standing goal of the study of free neutron beta decay is to better determine the CKM elementV_udthrough measurements of the neutron lifetime and a decay correlation parameter. The Nab collaboration intends to measurea, the neutrino-electron correlation, with accuracy sufficient for a competitive evaluation ofV_udbased on neutron decay data alone. This paper gives a status report and an outlook. 

Abstract The India‐Eurasia collision is a key case study for understanding the influence of plate tectonic processes on Earth's crust, atmosphere, hydrosphere, and biosphere. However, the timing of the final India‐Eurasia continental collision is debated due to significant uncertainty in the age of the collision between the Kohistan‐Ladakh arc (KLA) and Eurasia along the Shyok suture zone. Here we present paleomagnetic results that constrain the Karakoram terrane in northwest India to a paleolatitude of 19.9 ± 8.9°N between 93 and 75 million years ago (Ma). Our results show that the Karakoram terrane was situated on the southern margin of Eurasia in the Late‐Cretaceous. Our results indicate that the KLA and Eurasian continent had a not converged until <61.6 Ma, placing a Paleocene older limit on the age of final closure of the Shyok suture zone. This suggests that the India‐Eurasia collision in northwestern India likely occurred after the closure of the oceanic basin between the KLA and Eurasia. The Paleocene collision event affecting India that has been widely interpreted to represent final India‐Eurasia collision instead records the arc‐continent collision between the KLA and the northern edge of India prior to final India‐Eurasia collision. Final India‐Eurasia collision in northwest India most likely occurred after the closure of the oceanic basin between the KLA and Eurasia. 

The low-density lipoprotein receptor (LDLR) is key to cellular cholesterol uptake and is also the main receptor for the vesicular stomatitis virus glycoprotein (VSV G). Here we show that in songbirds LDLR is highly divergent and lacks domains critical for ligand binding and cellular trafficking, inconsistent with universal structure conservation and function across vertebrates. Linked to the LDLR functional domain loss, zebra finches show inefficient infectivity by lentiviruses (LVs) pseudotyped with VSV G, which can be rescued by the expression of human LDLR. Finches also show an atypical plasma lipid distribution that relies largely on high-density lipoprotein (HDL). These findings provide insights into the genetics and evolution of viral infectivity and cholesterol transport mechanisms in vertebrates.