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The deposition of aeolian dust has profound effects on biogeochemical cycling, soil development, and hydrology in alpine ecosystems. In the western United States, it has been proposed that much of the dust reaching the Rocky Mountains is derived from arid regions located to the west-southwest. Because these areas are vulnerable to drought-related changes in vegetation, grazing impacts, mining, off-road vehicle travel, urban development, and other factors that affect the availability of fine-grained sediment for wind deflation, it is imperative that source areas for this dust be better constrained to guide future management decisions. Here we collect modern dust in the Uinta Mountains (Utah) and compare its 87Sr/86Sr and 143Nd/144Nd “fingerprint” with surficial sediments in possible source areas. Our results confirm that this dust is exotic to the Uinta Mountains, and demonstrate its similarity to numerous dune fields in western Utah, as well as surficial sediments in parts of the Colorado Plateau, southern Basin and Range, and the Mojave Desert. The seasonal sampling we employed also reveals that considerable dust is deposited during the shorter alpine summer. Recognition that specific areas in the southwestern US are sources of dust to the Rocky Mountains, particularly during summer months, should be considered in future land management planning.

The chemistry of the oceans in part reflects a balance between inputs from the continents and mantle. Traditionally, it has been thought that Archean ocean chemistry was dominated by mantle sources, but recent work has suggested that continental weathering during the Archean provided a much higher flux to the oceans than previously recognized. Here, we present new Rb-Sr and Sm-Nd isotope compositions on carbonate (dolomite and limestone) from the 2.94 Ga Red Lake and 2.80 Ga Steep Rock groups in the Superior Province, Canada to assess the potential impact continental weathering had on ocean chemistry during the Mesoarchean, a time when initiation of modern-style plate tectonics has been proposed to have occurred. The low Rb contents of all carbonate samples suggest that clastic contamination does not affect the Sr isotope compositions. Using O and Sr isotope modeling, we identified unaltered samples and estimate a 87Sr/86Sr ratio of 0.70173 for seawater at 2.94 Ga and 0.70182 at 2.80 Ga. Strontium isotope compositions from both Red Lake and Steep Rock indicate that seawater was significantly more radiogenic than contemporaneous mantle, and suggests that weathering of evolved continental crust was an important input to seawater. Continental weathering likely affected seawater chemistry through uplift of continental lithosphere during the initiation of modern-style plate tectonics at 3.2 Ga, a model that is contrary to those that suggest the Archean continents were small in extent and largely submerged. Initiation of modern-style plate tectonics and associated continental weathering had an important effect on the biosphere, including increased nutrient delivery, as well as creation of ecological niches that allowed development of the first biologically produced shallow marine redox gradients.

Controls on Archean seawater chemistry remain controversial. Many studies have suggested that it was largely controlled by oceanic hydrothermal fluid circulation. Recent work, however, from clastic sequences, Hf–O isotope data from detrital zircons, and models for the Rb/Sr evolution of the continental crust suggest that intense continental weathering and low-temperature surface alteration were more important than previously thought during the early Archean. This is consistent with biogeochemical studies that suggest the Archean had a diverse microbial ecology, which would, in part, need to be sustained by nutrients (e.g., phosphorus) that were derived from continental weathering. To further quantify continental weathering during the early Archean, we analyzed 3.26 Ga barite from the Fig Tree Group, South Africa for strontium, oxygen, and sulfur isotope compositions. We propose that the seawater component of the barite is characterized by 87Sr/86Sr ratios >0.701, which is significantly more radiogenic than contemporaneous mantle (∼0.7007–0.7008). The radiogenic nature of seawater at this time suggests that the continental weathering flux at 3.26 Ga had a large impact on ocean chemistry 400 million years earlier than previously suggested.

Paleoarchean to Mesoarchean gneisses are exposed in the Minnesota River Valley (MRV) and in northern Michigan, USA. The MRV rocks include a complex of granitic migmatites, schistose to gneissic amphibolite, metagabbro, and paragneisses. The best-known units are the Morton gneiss and the Montevideo gneiss. U–Pb geochronology for the MRV rocks shows that they have a long and complex history, with initial crystallization at c.3500 Ma and other igneous events at c.3420 Ma, c.3385 Ma, and c.3140 Ma. Many gneisses are clearly now mixtures of material formed, probably by both tectonic and magmatic processes, during at least the first three of these events. Hf and Nd isotopic systematics suggest derivation from precursor material that had resided in the crust for at least 100 m.y. prior to 3500 Ma and further demonstrate closed-system melting and remelting for about 1000 m.y. The ancient gneisses were intruded by synkinematic granite at 2603 Ma and by syenogranite and aplite at 2590 Ma, igneous events likely related to granulite facies metamorphism recorded in paragneisses and events related to suturing of the MRV terrane to the Superior craton. Rocks exposed in northern Michigan include tonalitic gneiss and granite. Although they are less well-studied, the tonalitic gneiss is c.3600 Ma and a late-granitic intrusion is c.2650 Ma. Thus, the two terranes are similar and were probably once continuous, constituting a major feature on the southern margin of the Superior craton. Similarly ancient rocks are known on the northern margin of, and within, the Superior craton. Thus, the MRV rocks were likely a crustal component that was sutured to the Superior Province during the 2.6 Ga Sacred Heart Event.

Before the Great Oxidation Event (GOE) 2.4–2.2 billion years ago it has been traditionally thought that oceanic water columns were uniformly anoxic due to a lack of oxygen-producing microorganisms. Recently, however, it has been proposed that transient oxygenation of shallow seawater occurred between 2.8 and 3.0 billion years ago. Here, we present a novel combination of stable Fe and radiogenic U–Th–Pb isotope data that demonstrate significant oxygen contents in the shallow oceans at 3.2 Ga, based on analysis of the Manzimnyama Banded Iron Formation (BIF), Fig Tree Group, South Africa. This unit is exceptional in that proximal, shallow-water and distal, deep-water facies are preserved. When compared to the distal, deep-water facies, the proximal samples show elevated U concentrations and moderately positive δ 56 Fe values, indicating vertical stratification in dissolved oxygen contents. Confirmation of oxidizing conditions using U abundances is robustly constrained using samples that have been closed to U and Pb mobility using U–Th–Pb geochronology. Although redox-sensitive elements have been commonly used in ancient rocks to infer redox conditions, post-depositional element mobility has been rarely tested, and U–Th–Pb geochronology can constrain open- or closed-system behavior. The U abundances and δ 56 Fe values of the Manzimnyama BIF suggest the proximal, shallow-water samples record precipitation under stronger oxidizing conditions compared to the distal deeper-water facies, which in turn indicates the existence of a discrete redox boundary between deep and shallow ocean waters at this time; this work, therefore, documents the oldest known preserved marine redox gradient in the rock record. The relative enrichment of O 2 in the upper water column is likely due to the existence of oxygen-producing microorganisms such as cyanobacteria. These results provide a new approach for identifying free oxygen in Earth's ancient oceans, including confirming the age of redox proxies, and indicate that cyanobacteria evolved prior to 3.2 Ga.

Martian meteorites from a single Mars ejection site record 2 billion years of magmatic activity., The timing and nature of igneous activity recorded at a single Mars ejection site can be determined from the isotope analyses of Martian meteorites. Northwest Africa (NWA) 7635 has an Sm-Nd crystallization age of 2.403 ± 0.140 billion years, and isotope data indicate that it is derived from an incompatible trace element–depleted mantle source similar to that which produced a geochemically distinct group of 327- to 574-million-year-old “depleted” shergottites. Cosmogenic nuclide data demonstrate that NWA 7635 was ejected from Mars 1.1 million years ago (Ma), as were at least 10 other depleted shergottites. The shared ejection age is consistent with a common ejection site for these meteorites. The spatial association of 327- to 2403-Ma depleted shergottites indicates >2 billion years of magmatism from a long-lived and geochemically distinct volcanic center near the ejection site.