Grazing by microzooplankton has been shown to significantly impact freshwater cyanobacteria blooms; however, the contribution of rotifers to the overall effect of microzooplankton grazing is not well understood. We conducted monthly microzooplankton community grazing (dilution) experiments June-October 2019, concurrent with incubations of field-collected rotifers feeding upon the natural assemblage of microplankton prey < 75 mu m in Vancouver Lake (Washington State, USA), a lake annually affected by cyanobacteria blooms. Our results showed that just days after a large bloom, the microzooplankton community grazing impact on phytoplankton biomass was exceptionally high (> 1000% d(-1)), yet the impact by rotifers was low (< 1% d(-1)). As the bloom diminished in September and October, the grazing impact of rotifers increased dramatically, specifically consuming substantial dinoflagellate (<= 574%) and ciliate (<= 382%) biomass daily. Analysis of rotifers in Vancouver Lake during these months showed the presence of large, carnivorous Asplanchna spp., which indicates multi-trophic grazing dynamics within the rotifer assemblage. We conclude that non-rotifer micro-grazers (i.e., ciliates) were likely responsible for the initial dissipation of cyanobacteria just after the bloom peak, while rotifers primarily removed micro-grazers later in autumn. This study highlights the trophic roles of micro-grazers in controlling harmful cyanobacteria blooms and quantifies the specific grazing contributions of rotifers.

Mountain lakes experience interannual variability in spring snowpack and ice cover that can lead to differences in physical, chemical, and biological properties in the succeeding summer. Lake studies that capture extreme years of snow and ice would be useful to understand and anticipate effects of climate change, but such data are rare for remote mountain lakes. Monitoring of lakes in Olympic, North Cascades, and Mount Rainier National Parks from 2007 to 2018 allowed us to examine limnological differences along interannual and elevation-driven climate gradients that included unusually high (2011-2012) and 100-yr record low (2015) snowpack years. Years with lower spring snowpack had earlier ice-out. Across lakes, our analysis suggested an average of 0.075 degrees C lake warming per day of lost ice duration (0.525 degrees C per week), giving rise to other ecosystem changes linked to temperature such as lower dissolved oxygen, higher total dissolved N, higher chlorophyll, and higher abundance of cladoceran zooplankton. Conversely, in years with higher snowpack and a shorter ice-free season, lakes were colder and clearer (1 m deeper Secchi depth for every 1 m May snow water equivalent), with more dilute ions as well as lower algal biomass and zooplankton abundance. These results add to evidence that changes in snowpack or ice-out dates alter mountain lake ecology through multiple processes associated with hydrology, terrestrial-aquatic connection, water temperature, productivity, ion composition, and plankton communities.

Major and trace element and radiogenic isotopic characteristics of primitive mafic Pleistocene and Holocene lavas from Newberry Volcano, Oregon, define two groups. The first consists of dry tholeiitic high-alumina olivine basalts that are slightly enriched in highly incompatible elements. The second group consists of calc-alkaline basalts that contained 2-4 wt % H2O prior to eruption and shows strong enrichment in the light rare earth elements, Ba, and Sr, and deficits in Nb, Ta, Hf, and Zr. The tholeiitic basalts reflect 6-11% anhydrous adiabatic decompression melting of spinel peridotite. The calc-alkaline basalts derived from compositionally distinct sources with strong LIL enrichment and relative depletion in HFSE, but with Sr, Nd, Hf, and Pb isotopic composition only slightly distinct from the sources of the tholeiitic magmas. Radiogenic Os correlates with LREE enrichment in the calc-alkaline magmas, which indicates that their source materials include a contribution from a mafic component that was melted in the garnet stability field. The calc-alkaline magmas were derived by melting of peridotite metasomatized by a fluid/melt that originated by melting of a mixture of the sediment plus MORB basalt/mantle in the underlying subducting oceanic plate. While the trace element characteristics of the calc-alkaline magmas were determined by the subduction component, their isotopic characteristics were modified during transit through the mantle by interaction with the highly magmatically processed mantle wedge beneath Newberry Volcano that, without the slab component, serves as the source of the tholeiitic magmas.

In order to apply the vanadium (V) stable isotope system for studies of planetary accretion and evolution in the solar system and redox variations in terrestrial magmatic processes, the V isotope composition of the Bulk Silicate Earth (BSE) needs to be precisely constrained. Previous studies have shown that fertile peridotites have systematically higher V-51/V-50 ratios than MORB. This, however, is in conflict with the theoretical prediction that mantle melting residues should be enriched in V-50 rather than V-51. To address these issues, a more precise estimate of the V isotope composition of the BSE is required. This study presents delta V-51 data for eleven peridotite xenoliths from two late Cenozoic eruption centers at Tariat in central Mongolia, ten komatiites from five localities ranging in age between 3.48 and 2.41 Ga, and four 1.98 Ga picrites from the Onega Plateau in Fennoscandia. The mean delta V-51 for fertile spinel lherzolites is -0.91 +/- 0.06 parts per thousand (2SD, n = 8). They show no resolvable difference in V isotope compositions compared to three moderately to highly refractory peridotite xenoliths analyzed, with a mean 5 51 V of -0.93 +/- 0.01 parts per thousand (2SD, n = 3). The mean delta V-51 for the komatiites is -0.91 +/- 0.05 parts per thousand (2SD, n = 10), which is identical to that for the fertile peridotites. Based on the V isotope compositions of the peridotites and komatiites analyzed in this study, the mean delta V-51 of the BSE is estimated to be -0.91 +/- 0.09 parts per thousand (2SD, n = 18).

The analysis of lunar samples returned to Earth by the Apollo and Luna missions changed our view of the processes involved in planet formation. The data obtained on lunar samples brought to light the importance during planet growth of highly energetic collisions that lead to global-scale melting. This violent birth determines the initial structure and long-term evolution of planets. Once past its formative era, the lunar surface has served as a recorder of more than 4 billion years of interaction with the space environment. The chronologic record of lunar cratering determined from the returned samples underpins age estimates for planetary surfaces throughout the inner Solar System and provides evidence of the dynamic nature of the Solar System during the planet-forming era.

Substantial quantities of sediments are known to enter the deep lithosphere at subduction zones, but the extent to which sediments melt and the process involved in sediment contribution to the deep lithosphere are inadequately understood. Vigorous debate continues about whether the subducted sediment component is terrigenous or pelagic and transported as a hydrous melt, an aqueous fluid, or bulk sediment. In this contribution, we conduct an integrated study on a variety of deep-seated xenoliths in the Neogene Hannuoba basalts from the northern margin of the North China Craton. Among these xenoliths, clinopyroxenite xenoliths are compositionally and isotopically distinct. Mineral chemistry shows that the clinopyroxenite xenoliths come from a depth near the MOHO, rather than from the mantle as suggested previously. The clinopyroxenite xenoliths have extremely evolved Sr-Nd-Hf isotopic compositions and are interpreted to have a late Archean protolith age. The extremely low contents of Cr and Ni for the clinopyroxenite xenoliths preclude a magmatic origin. Instead, a metasomatic origin is suggested, which is strongly supported for the clinopyroxenites by the occurrence of hydrous minerals and high contents of large-ion-lithophile elements (K, Rb, Ba, Th and Sr) and light rare earth elements, as well as elevated delta O-18 (9.9-11.3 parts per thousand) and light delta Mg-26 (-1.04 parts per thousand to 1.42 parts per thousand) isotopic compositions. Furthermore, their high high-field-strength element (Nb, Ta, Zr and Hf) contents indicate that the metasomatic agent is a hydrous melt, rather than an aqueous fluid. The metasomatic melts are considered to be derived from a mixed source of sedimentary carbonates and ancient, felsic continental materials. A combination of zircon ages and oxygen isotope data for the clinopyroxenite xenoliths further restricts the timing of metasomatism to the late Paleozoic. Considering the regional tectonic setting, the sediments most likely came from the subducted Paleo-Asian oceanic slab. Thus the Hannuoba clinopyroxenite xenoliths provide direct evidence for melting of the subducted Paleo-Asian oceanic slab sediment and its interaction with the deep lithosphere. The data show that melting of subducted sediments can take place at a much shallower depth than commonly thought and place an independent constraint on future models of slab geotherms. (C) 2019 Elsevier Ltd. All rights reserved.