Abstract Several years of weekly sampling of waters from the Shinfa River watershed in the lowlands of northwestern Ethiopia yielded 275 samples with δ D vsmow and δ 18 O vsmow values ranging from c. −10 to +100‰ and from c. −2 to +20‰, respectively. Wet season (summertime) Shinfa River water stable hydrogen and oxygen isotope values are among the lowest reported in this study, whereas the dry season (winter/spring) usually records a progressive trend towards +100 and +20‰, respectively. Overlapping with this interval of Shinfa River water sampling, air temperatures ( n = 155) also were recorded at the same time; temperatures range from c. 18 to 47°C. The coolest temperatures occur during the summer wet season, associated with the arrival of the Kiremt rains in the region, whereas the warmest temperatures occur towards the end of the dry season. In order to evaluate the extent to which this rather extreme isotope hydrology is recorded in the sediments and biota of the Shinfa River system, both hardwater calcareous deposits precipitated on basalt cobbles by evaporation in the Shinfa River channel during the dry season and aragonite from three different modern bivalve mollusc species were collected and analysed for their stable oxygen and carbon isotope compositions. Hardwater calcareous deposit δ 18 O vpdb and δ 13 C vpdb values range from c. −2 to +5‰ and c. −9 to +7‰, respectively, and preserve a trend towards progressively more positive δ 18 O vpdb and δ 13 C vpdb values through the course of the dry season. Shinfa River mollusc aragonite powders ( n = 51) were serially sampled from cf. Coelutura aegyptica , cf. Chambardia rubens and Etheria elliptica species. All species record oxygen and carbon isotopes between c. −2 and +7‰ and between c. −18 and −8‰, and each species records coherent trends between those extremes as well as a positive parametric correlation between measured oxygen and carbon isotope values. However, there does appear to be some variability of measured isotope values by species, suggesting that species-specific metabolic differences may impact the resulting range of aragonite stable carbon and oxygen values. Based upon the measured Shinfa River water δ 18 O vsmow and corresponding water temperatures at the time of sampling, a possible range of Shinfa River calcite and aragonite δ 18 O vpdb values were calculated in conjunction with well-established calcite–water and aragonite–water oxygen isotope fractionation equations. These ‘fictive’ calcite and aragonite δ 18 O vpdb values range from c. −5 to +15‰, which is a much larger range than previously documented from analyses of the hardwater calcareous deposits and mollusc aragonite samples. The narrower range of values in the natural calcite and aragonite samples may be attributed to several mechanisms, including time averaging and environmental stress. Nevertheless, the stable oxygen isotopic compositions of these natural samples offer a minimum assessment of the environmental extremes which occur in this region today, and provide a model for reconstructing the environments of the past.

ABSTRACT With an age of less than ~5000 yr and a diameter of 45 m, Kamil crater in Egypt is one of the youngest and smallest terrestrial impact craters known to date. Abundant evidence of shock-deformed sandstone has been reported from Kamil crater, including shatter cones, vesicular impact glass, high-pressure polymorphs of silica and carbon, planar deformation features (PDFs) and planar fractures (PFs) in quartz, dissociated zircon, melt veins, and intergranular melt, giving rise to a range of estimated shock pressures from ~20 to ~60 GPa. Here, we investigated shocked zircon from Kamil crater through characterization of microstructures in a centimeter-sized clast of shocked nonporous sandstone ejecta, previously described as containing quartz grains with PDFs and PFs, coesite, stishovite, diamond, and lechatelierite. Orientation analysis by electron backscatter diffraction (EBSD) showed that the quartz arenite consists of damaged detrital quartz grains surrounded by a matrix of either comminuted quartz or intergranular melt. Individual quartz grains are pervasively fractured (abundant PFs and PDFs); apparent isotropic crushing resulted in uniformly and highly dispersed orientation clusters on pole figures. Zircon grains are not abundant; however, four of 19 grains analyzed by EBSD contained {112} deformation twin lamellae, with individual lamellae ranging in length from 1 to 2 µm. Lengths of twin lamellae in Kamil zircon grains are anomalously short compared to those reported in shocked zircon from other impact structures, where individual lamellae are tens of micrometers long. Previous empirical studies have suggested that {112} twin lamellae in zircon form at ~20 GPa in non-porous target rocks, a finding supported by their coexistence, in some impactites, with high-pressure phases such as reidite. The only available experimental constraint, by diamond anvil cell, found {112} twins in zircon powder quenched at 20 GPa. The presence of coesite, stishovite, lechatelierite, and shocked quartz with PDFs in the studied sample is consistent with empirically derived pressure estimates of ~20 GPa for {112} twin formation in zircon in the ejecta sample from Kamil crater. Kamil thus represents the smallest and youngest impact structure where shock-twinned zircon has been reported. Given the apparent efficiency of {112} twin formation (21% of grains), shock-twinned zircon is here shown to provide a robust and readily identifiable record of shock deformation in a relatively common mineral at one of the smallest known terrestrial impact craters.