ABSTRACT We reconstructed a record of the micrometeorite flux in the Late Cretaceous using the distribution of extraterrestrial spinel grains across an ~2 m.y. interval of elevated 3 He in the Turonian Stage (ca. 92–90 Ma). From ~30 m of the limestone succession in the Bottaccione section, Italy, a total of 979 kg of rock from levels below and within the 3 He excursion yielded 603 spinel grains (32–355 μm size). Of those, 115 represent equilibrated ordinary chondritic chromite (EC). Within the 3 He excursion, there is no change in the number of EC grains per kilogram of sediment, but H-chondritic grains dominate over L and LL grains (70%, 27%, and 3%), contrary to the interval before the excursion, where the relation between the three groups (50%, 44%, and 6%) is similar to today and to the Early Cretaceous. Intriguingly, within the 3 He anomaly, there is also a factor-of-five increase of vanadium-rich chrome spinels likely originating from achondritic and unequilibrated ordinary chondritic meteorites. The 3 He anomaly has an unusually spiky and temporal progression not readily explained by present models for delivery of extraterrestrial dust to Earth. Previous suggestions of a relation to a comet or asteroid shower possibly associated with dust-producing lunar impacts are not supported by our data. Instead, the spinel data preliminary indicate a more general disturbance of the asteroid belt, where different parent bodies or source regions of micrometeorites were affected at the same time. More spinel grains need to be recovered and more oxygen isotopic analyses of grains are required to resolve the origin of the 3 He anomaly.

ABSTRACT The present-day ocean-climate system configuration took shape during the Miocene Epoch. Toward the end of the epoch, in the late Tortonian at ca. 8.5 Ma, there was an exceptional event: collisional disruption of an >150-km-diameter asteroid, which created the Veritas family of asteroids in the asteroid belt. This event increased the flux of interplanetary dust particles rich in 3 He to Earth and probably caused a period of increased dust in the atmosphere, with consequent alteration of global and local environmental conditions. A late Miocene 3 He anomaly likely related to the Veritas event has been registered in deep-sea sediments from Ocean Drilling Program (ODP) Site 926 (Atlantic Ocean), ODP Site 757 (Indian Ocean), and in the late Tortonian–early Messinian Monte dei Corvi section near Ancona, Italy. Here, we report the results of a study in the Monte dei Corvi section aimed to recover extraterrestrial chrome-spinel grains across the 3 He anomaly interval, as has been done for the similar late Eocene 3 He anomaly in the nearby Massignano section. In this study, three ~100 kg samples were collected from the Monte dei Corvi section: two within the 3 He peak interval and one outside the anomaly interval as a background reference sample. In total, 1151 chrome-spinel grains (>63 µm) were recovered, but based on chemical composition, none of the grains has a clear extraterrestrial origin. This supports the inference that the 3 He anomaly is indeed related to the Veritas event and not to an approximately coeval breakup of a smaller H-chondritic body in the asteroid belt, an event registered in meteoritic cosmic-ray exposure ages. Spectral studies of the Veritas asteroids indicate that they are made up of carbonaceous chondritic material. Such meteorites generally have very low chrome-spinel concentrations in the grain-size range considered here, contrary to the very chromite-rich ordinary chondrites. The terrestrial grains recovered were classified, and their composition showed that all the grains have an ophiolitic origin with no substantial compositional and distributional change through the section. The source area of the terrestrial grains was probably the Dinarides orogen.

From Pliocene to middle Pleistocene time, a large lake occupied most of the San Luis Valley above 2300 m elevation (7550 ft) in southern Colorado. This ancient lake accumulated sediments of the Alamosa Formation (Siebenthal, 1910), for which the lake is herein named. The existence of this lake was first postulated in 1822 and proven in 1910 from well logs. At its maximum extent of nearly 4000 km 2 , it was one of the largest high-altitude lakes in North America, similar to but larger than Lake Texcoco in the Valley of Mexico. Lake Alamosa persisted for ~3 m.y., expanding and contracting and filling the valley with sediment until ca. 430 ka, when it overtopped a low sill and cut a deep gorge through Oligocene volcanic rocks in the San Luis Hills and drained to the south. As the lake drained, nearly 100 km 3 (81 × 10 6 acre-ft or more) of water coursed southward and flowed into the Rio Grande, entering at what is now the mouth of the Red River. The key to this new interpretation is the discovery of ancient shoreline deposits, including spits, barrier bars, and lagoon deposits nestled among bays and in backwater positions on the northern margin of the San Luis Hills, southeast of Alamosa, Colorado. Alluvial and lacustrine sediment nearly filled the basin prior to the lake's overflow, which occurred ca. 430 ka as estimated from 3 He surface-exposure ages of 431 ± 6 ka and 439 ± 6 ka on a shoreline basalt boulder, and from strongly developed relict calcic soils on barrier bars and spits at 2330–2340 m (7645–7676 ft), which is the lake's highest shoreline elevation. Overtopping of the lake's hydrologic sill was probably driven by high lake levels at the close of marine oxygen-isotope stage (OIS) 12 (452–427 ka), one of the most extensive middle Pleistocene glacial episodes on the North American continent. Hydrologic modeling of stream inflow during full-glacial-maximum conditions suggests that Lake Alamosa could fill at modern precipitation amounts if the mean annual temperature were just 5 °C (10 °F) cooler, or could fill at modern temperatures with 1.5 times current mean annual precipitation. Thus, during pluvial epochs the lake would rise to successively higher levels owing to sedimentation; finally during OIS 12, the lake overflowed and spilled to the south. The integration of the upper (Colorado) and lower (New Mexico) reaches of the Rio Grande expanded the river's drainage basin by nearly 18,000 km 2 and added recharge areas in the high-altitude, glaciated San Juan Mountains, southern Sawatch Range, and northern Sangre de Cristo Mountains. This large increase in mountainous drainage influenced the river's dynamics downstream in New Mexico through down-cutting and lowering of water tables in the southern part of the San Luis Valley.