Grand Canyon

Geochemical study of water and gas discharging from the deeply incised aquifer system at the Grand Canyon, Arizona, provides a paradigm for understanding complex groundwater mixing phenomena, and Quaternary travertines deposited from cool springs provide a paleohydrologic record of this mixing. Geochemical data show that springs have marked compositional variability: those associated with active travertine accumulations (deeply derived endogenic waters) are more saline, richer in CO 2 , and elevated in 87 Sr/ 86 Sr relative to springs derived dominantly from surface recharge of plateau aquifers (epigenic waters). Endogenic waters and associated travertine are preferentially located along basement-penetrating faults. We propose a model whereby deeply derived fluids are conveyed upward via both magmatism and seismicity. Our model is supported by: (1) gas analyses from spring waters with high He/Ar and He/N 2 and 3 He/ 4 He ratios indicating the presence of mantle-derived He; (2) large volumes of travertine and CO 2 -rich gases in springs recording high CO 2 fluxes; and (3) 87 Sr/ 86 Sr in these springs that indicate circulation of waters through Precambrian basement. Geochemical trends are explained by mixing of epigenic waters of the Colorado Plateau aquifers with different endogenic end-member waters in different tectonic subprovinces. Endogenic waters are volumetrically minor but have significant effects on water chemistry. They are an important and largely unrecognized component of the hydrogeochemistry and neotectonics of the southwestern United States.

Selected parts of the Galeros and Kwagunt Formations, of the Neoproterozoic Chuar Group (ca. 800-740 Ma) exposed in the eastern Grand Canyon, Arizona, yield paleomagnetic data that are consistent with a primary origin. Both formations are dominated by dark mudrock with multiple, hematite-cemented fine sandstone and mudstone horizons that yield magnetizations of west declination and shallow inclination (or antipodes). The grand mean magnetization for the older Galeros Formation, determined by giving unit weight to each of 16 sites, is Dec = 265.5 • , Inc = 5.1 • , α95 = 8.5 • , κ = 19.8 and yields a paleopole of 2 • S, 163 • E (dp = 4.3; dm = 8.5). The younger Kwagunt Formation provides a grand mean magnetization of Dec = 280.8 • , Inc = 30.6 • , α95 = 8.4 • , κ = 38.6 (three sites of normal polarity and six sites of reverse polarity), determined by giving unit weight to each of the nine sites, and yields a paleopole of 18 • N, 166 • E (dp = 5.2; dm = 9.4). The NRM of about a third of the collection, mainly from coarser-grained hematitic sandstones, is dominated by a north-directed, moderate positive inclination magnetization with laboratory unblocking temperatures up to 680 • C and is interpreted as a recent overprint. We argue that the east-west and shallow magnetization in the Chuar Group strata, although similar to early Paleozoic magnetizations for North America, is primary because both groups of data pass fold tests and because Chuar strata lie in angular unconformity below the flat-lying Middle Cambrian Tapeats sandstone. Furthermore, we infer that the Chuar Group paleopoles, together with other North American Neoproterozoic results, are fully consistent with a tight counterclockwise loop in the south Pacific in the apparent polar wander path for North America during the Neoproterozoic. This new path represents a drift history that positions the central part of the Rodinia supercontinent at or near the equator at ∼800 Ma and slightly younger time. Penecontemporaneous with Chuar deposition, extension began along the Cordilleran margin of Laurentia. This large-scale extensional event culminated in the rift-to-drift transition of Laurentia away from East Gondwana or Asian (South China, Siberia) blocks by ca. 750 Ma.

Chuar Group sediments were deposited in a marine cratonic basin synchronously with marine deposition in other basins of western North America and Australia. The top of the Chuar Group is 742 Ma; thus, this succession offers a potentially important record of global climatic and tectonic changes during the mid-Neoproterozoic Ð a critical time leading to possible global glaciation during supercontinent rifting. Chuar Group cycles and sequences may record glacioeustatic¯uctuations during the climatic transition into the Sturtian Ice Age (,750±700 Ma), as well as extension related to the dispersal of Rodinia.

[Based on field work by Spamer in 1991.  Spamer & Bogan (1993) provide revised and updated data to those presented in this report. Comparative and supporting data appear in this report that have not been published elsewhere.]

Aquatic and terrestrial mollusks of the Colorado River corridor through the Grand Canyon, Arizona. occupy an uncertain position in the trophic food web of the region. In the river corridor, mollusks are probably taxonomically diverse, but their habitats are restricted to tributaries. Aquatic mollusks are known in the Colorado River itself only in the fishery between Glen Canyon Dam and Lees Ferry; limnological constraints, primarily sedimentation, preclude their colonization of the river below Lees Ferry. Aquatic mollusks of the river corridor are represented principally by Physella spp. (Physidae). In the river corridor this group may be mixed descendants of native populations and individuals introduced to the Lees Ferry fishery in the 1960s. Distribution of this group appears to be mostly by the mechanism of downstream transport in the Colorado River, whose frequent dam-controlled inundation of tributary mouths provides for the redeposition of individuals and egg masses in these habitats. Terrestrial mollusks in the river corridor are, with the singular exception of Vasey's Paradise, found only in tributaries above the high water zone of the Colorado River; they have not thus far been located along the Colorado River even in favorable habitats established in the post-dam riparian regime. Comparable habitats of Thunder River host diverse taxa in a riparian environment that provides an indication of the potential for terrestrial molluscan habitation of the riparian environment of the Colorado River. Included in the Thunder River community are mixed taxa representative both of the inner canyon and surrounding plateaus; some are significantly displaced from their normal altitudinal distributions. The effects of the river on productivity of the mollusks of the inner canyon are unknown. No previous baseline study of the mollusks is available for the river corridor, and molluscan studies of the Grand Canyon region are not substantially more advanced than they were 80 years ago. This study reports ten species as new records of occurrence in the Grand Canyon; of these, three species are new records of occurrence in the Grand Canyon region, and two are new records of occurrence for the state of Arizona. One taxon particularly, Oxyloma cf. haydeni (Binney, 1858) records a genus previously unknown alive in Arizona (rather only as fossils), and the species could be new to science; the only colony thus far known in Arizona lives at Vasey's Paradise. The Colorado River does not appear to be a barrier in the biogeographic range of any of the species found during the reconnaissance reported here, although the Grand Canyon region is the biogeographic boundary for some taxa. A significant amount of work remains to be done toward understanding the trophic position of the malacofauna in the Grand Canyon. The inner canyon provides a unique area to study the taxonomic diversity and seasonal production of native and introduced species in a protected wilderness hosting multiple life zones.

Keywords: Neoproterozoic oxygenation iron sulfur ocean chemistry paleobiology Detailed iron, sulfur and carbon chemistry through the N 742 million year old Chuar Group reveals a marine basin dominated by anoxic and ferrous iron-rich (ferruginous) bottom waters punctuated, late in the basin's development, by an intrusion of sulfide-rich (euxinic) conditions. The observation that anoxia occurred frequently in even the shallowest of Chuar environments (10s of meters or less) suggests that global atmospheric oxygen levels were significantly lower than today. In contrast, the transition from ferruginous to euxinic subsurface water is interpreted to reflect basinal control-specifically, increased export of organic carbon from surface waters. Low fluxes of organic carbon into subsurface water masses should have been insufficient to deplete oxygen via aerobic respiration, resulting in an oxic oxygen minimum zone (OMZ). Where iron was available, larger organic carbon fluxes should have depleted oxygen and facilitated anaerobic respiration using ferric iron as the oxidant, with iron carbonate as the expected mineralogical signature in basinal shale. Even higher organic fluxes would, in turn, have depleted ferric iron and upregulated anaerobic respiration by sulfate reduction, reflected in high pyrite abundances. Observations from the Chuar Group are consistent with these hypotheses, and gain further support from pyrite and sulfate sulfur isotope abundances. In general, Chuar data support the hypothesis that ferruginous subsurface waters returned to the oceans, replacing euxinia, well before the Ediacaran emergence of persistently oxygenated conditions, and even predating the Sturtian glaciation. Moreover, our data suggest that the reprise of ferruginous water masses may relate to widespread rifting during the break-up of Rodinia. This environmental transition, in turn, correlates with both microfossil and biomarker evidence for an expanding eukaryotic presence in the oceans, suggesting a physiologically mediated link among tectonics, environmental chemistry and life in the dynamic Neoproterozoic Earth system.

The malacofauna of the Grand Canyon region, Arizona (southwestern U.S.A.), has been largely unexamined since Pilsbry & Ferriss (1911) published the sole paper on the subject. The original locality data of Pilsbry & Ferriss are difficult to interpret from the publication, and they are refined here. Specimens from the early surveys and from sporadic later collections are held in the Academy of Natural Sciences of Philadelphia (ANSP). In 1991, collections were made during the first-ever survey for mollusks along the Colorado River through the Grand Canyon, where they are found usually in tributaries, not in the main stream; specimens are deposited in ANSP. The historical records, including unpublished data from ANSP collections, and new data from the 1991 survey, are tabulated with the perspective of modern systematics. Included among the species is the federally proposed endangered Kanab ambersnail, Oxyloma haydeni kanabensis Pilsbry (Gastropoda: Succineidae), the first record of this mollusk alive outside of the type locality (north of Kanab, Utah) and which extends the Recent record of the genus Oxyloma into Arizona and the Southwestern Molluscan Province. During the 1991 Grand Canyon survey, collections of mollusks were also made for the first time at Thunder River, a perennial cave pour-out 3 km north of the Colorado River in the Grand Canyon. The 1991 survey produced new records of occurrence and altitudinal distributions for the Grand Canyon, northern Arizona, and the state of Arizona. An examination is made of the distribution of Grand Canyon mollusks with respect to the four ecological Life Zones of the Grand Canyon area. No stratification of mollusks by Life Zone is detected, but corroboration of early studies is made, which indicate that species of higher (cooler, wetter) zones are more restricted to those zones, whereas species of lower (warmer, drier) zones more readily proliferate into higher zones. However, it is seen in the Grand Canyon that specialized riparian communities can promote the colonization of molluscan species normally found in higher Life Zones. Logistical constraints of exploration in this region are considered, and biases in collecting sites are acknowledged. And the first record of Pleistocene(?)-Holocene Catinella (Gastropoda: Succineidae) in the Grand Canyon is reported; the occurrence is compared to records of comparably-aged Oreohelix (Gastropoda: Oreohelicidae) in the canyon.

The Colorado River ecosystem in Grand Canyon is strongly impacted by operation of Glen Canyon Dam. Fluctuating releases of cold, hypolimnetic water from Lake Powell (~8ºC) for peak hydroelectric power generation has had a profound impact on the formerly warm, silty Colorado River. Clear, cold stenothermal conditions preclude successful reproduction and recruitment of most native fishes in the mainstem and introduced trout have become dominant species in the fish community. To assess the effects of dam operations on the native fish assemblage, the distribution and abundance of native and non-native fishes of the Colorado River ecosystem in Grand Canyon were assessed in the first year of a two-year research and monitoring program funded by the Grand Canyon Monitoring and Research Center. Study sites were located along 159 miles of the Colorado River from the Paria River to below Havasu Creek. An intensive area of study was established from just above the confluence of the Little Colorado River (LCR) at river mile (rm) 60 to Tanner Rapid at rm 68.6 (LCR inflow reach). Additional sampling was conducted in major tributaries (LCR, Bright Angel, Shinumo, Kanab, Havasu). Mini-hoopnets, minnow traps, electrofishing, trammel nets, and seines were used to sample fish during 120 days of fieldwork spread over four seasons between March 1998 and February 1999. Some 4,618 fish were captured in the Colorado River mainstem and tributaries. Non-native species dominated the catch in the mainstem Colorado River (1,515/2,565, 59%) and rainbow trout Oncorhychus mykiss was the most common non-native species (n=955, 37%). Of 1,050 native fish captured in the mainstem, speckled dace Rhinichthys osculus (n=463, 18%) and humpback chub Gila cypha (n=395, 15%) were the most abundant. The use of mini-hoopnets to sample fish in mainstem habitats revealed a relative abundance of humpback chub in the 100-200 mm TL size class, a population feature not observed in previous studies. In the smaller tributaries (Bright Angel, Shinumo, Kanab, Havasu), native species dominated the catch (739/847, 87%). Speckled dace (n=370, 44%) and humpback chub (n=133, 16%) were the most abundant native fishes. The LCR assemblage was dominated by native species (963/1206, 80%) with humpback chub (n=515, 43%) and speckled dace (n=308, 19%) dominating. The pattern of appearance, distribution, and abundance of young-of-year (YOY) humpback chub in the mainstem Colorado River observed in our study was consistent with a "source-sink" model of downstream dispersal from the LCR coupled with high mortality from predation in the mainstem Colorado River. The abundance of adult rainbow trout in the LCR inflow reach of the Colorado River coupled with their complementary distribution with juvenile humpback chub in January 1999 strongly suggests that predation by rainbow trout may have been responsible for the near disappearance of YOY humpback chub between late summer and early winter. Mortality rates for small fish are likely exacerbated by fluctuating flows, which disrupt habitat associations and increase the probability of downstream movement and dispersal. Fluctuating flows also reduce the suitability of river edge habitats and food resources needed for rearing small fish. The rarity of small humpback chub downstream of the LCR inflow coupled with high abundance of trout and cold fluctuating flows suggest that conditions for recruitment of YOY humpback chub in the Colorado River in Grand Canyon are extremely poor. In order to provide opportunities for increased survivorship of small humpback chub in Grand Canyon, resource managers should consider ways to stabilize flows and increase temperature in the mainstem Colorado River. Implementation of more natural flow and temperature conditions will increase the stability of shoreline habitat and cover, and likely result in increased food resources and growth rates in small fishes, and decrease mortality associated with picivory.

1] The incision and aggradation of the Colorado River in eastern Grand Canyon through middle to late Quaternary time can be traced in detail using well-exposed fill terraces dated by a combination of optically stimulated luminescence, uranium series, and cosmogenic nuclide dating. This fluvial history provides the best bedrock incision rate for this important landscape and highlights the complications and advantages of fill terrace records for understanding river long-profile evolution and incision. The use of fill terraces, as distinct from strath terraces, for calculating incision rates is complicated by the cyclic alluviation and incision they record. In the example of the Grand Canyon this has led to various rates being reported by different workers and rates that tend to be overestimates in shorter records. We illustrate that a meaningful long-term bedrock incision rate of $140 m/m.y. can be extracted from the Grand Canyon record by linking episodes when the Colorado River is floored on bedrock. Variable incision rates reported in the greater region may be, to some degree, due to inconsistent calculations. Our data also highlight that the Colorado River has been a mixed alluvial-bedrock river through both time and space and has been a bedrock river for less than half of its Pleistocene history. This strong temporal variation, combined with the varying bedrock the river encounters on its path, heightens the challenge of understanding the tectonic, climatic, and drainage integration controls on the form and evolution of the Colorado River's long profile.