Frank A. Corsetti

Stratigraphic investigations of carbon isotope anomalies and Neoproterozoic ice ages in Death Valley, California

An unusual richness of biogeochemical events is recorded in Neoproterozoic‐ Cambrian strata of the Death Valley region, California, United States. Eight negative carbon isotope (d13C) excursions are found in carbonate units between 1.08 Ga and the Precambrian/Cambrian boundary; four of these excursions occur in carbonates that contain textural features similar to those found globally in postglacial ‘‘cap carbonates’’ (including one or more of the following: laminite with rollup structures, apparent ‘‘tube rocks,’’ seafloor precipitates, and sheet-crack cements). However, only two of these units, the Sourdough limestone member of the Kingston Peak Formation and the Noonday Dolomite, rest directly upon glacial strata. The basal Beck Spring Dolomite and the Rainstorm Member of the Johnnie Formation each contain negative excursions and cap-carbonate‐like lithofacies, but do not rest on known glacial deposits. If the negative d13C excursions are assumed to record depositional processes, two equally interesting hypotheses are possible: (1) The Death Valley succession records four glacial pulses in Neoproterozoic time, but glacial units are not preserved at two stratigraphic levels. (2) Alternatively, other global oceanographic processes can cause negative excursions and cap-carbonate‐ like facies in addition to, or independent of, glaciation.

Evidence of giant sulphur bacteria in Neoproterozoic phosphorites

In situ phosphatization and reductive cell division have recently been discovered within the vacuolate sulphur-oxidizing bacteria. Here we show that certain Neoproterozoic Doushantuo Formation (about 600 million years bp) microfossils, including structures previously interpreted as the oldest known metazoan eggs and embryos, can be interpreted as giant vacuolate sulphur bacteria. Sulphur bacteria of the genus Thiomargarita have sizes and morphologies similar to those of many Doushantuo microfossils, including symmetrical cell clusters that result from multiple stages of reductive division in three planes. We also propose that Doushantuo phosphorite precipitation was mediated by these bacteria, as shown in modern Thiomargarita-associated phosphogenic sites, thus providing the taphonomic conditions that preserved other fossils known from the Doushantuo Formation.

Precambrian-Cambrian transition: Death Valley, United States

The Death Valley region contains one of the best exposed and often visited Precambrian-Cambrian successions in the world, but the chronostratigraphic framework necessary for understanding the critical biologic and geologic events recorded in these sections has been inadequate. The recent discovery of Treptichnus (Phycodes) pedum within the uppermost parasequence of the lower member of the Wood Canyon Formation allows correlation of the Precambrian-Cambrian boundary to this region and provides a necessary global tie point for the Death Valley section. New carbon isotope chemostratigraphic profiles bracket this biostratigraphic datum and record the classic negative carbon isotope excursion at the boundary. For the first time, biostratigraphic, chemostratigraphic, and lithostratigraphic information from pretrilobite strata in this region can be directly compared with similar data from other key sections that record the precursors of the Cambrian explosion. Few Precambrian-Cambrian boundary sections contain both the facies-restricted boundary fossil T. pedum and carbon isotope data, as found in Death Valley. Thus, the Death Valley succession provides a critical link toward our understanding of the correlation between siliciclastic-dominated and carbonate-dominated Precambrian-Cambrian transition sections.

Origin and Significance of Tube Structures in Neoproterozoic Post-glacial Cap Carbonates: Example from Noonday Dolomite, Death Valley, United States

Abstract The Neoproterozoic Noonday Dolomite (Death Valley, USA), a post-glacial cap carbonate, contains closely packed, meter-long, cm-wide, tube-like structures that define the vertical accretion direction. Similar tubestones are known from post-glacial cap carbonates in Namibia and Brazil. In vertical cross section, the tubes average 2 cm in diameter, pinch and swell greatly along their length, may bifurcate and coalesce, and are filled with brown laminated micrite/microspar where best preserved. The tubes do not root or terminate in a particular layer and are randomly distributed where present. The laminated host rock is composed of an early lithified, microclotted fabric with framework void space filled with sparry dolomite cement. The contact between the tube fill and the host rock is diffuse and feathered; commonly, wisps of laminated host rock cross the tube fill and bridge between adjacent stromatolitic structures, compartmentalizing the tubes. The tubes likely result from the contemporaneous interplay between microbialite growth and sedimentation/cementation, rather than fluid or gas escape, as demonstrated by the compartmentalization by bridging laminae. Vertical cross sections resemble inter-column depressions that form between columnar stromatolites. Bed-parallel sections, however, reveal that the tube structures represent isolated, sediment-filled depressions within a continuous layer of stromatolite. The genesis of this unusual stromatolite morphology is likely related to highly supersaturated seawater in the aftermath of low-latitude glaciation in Neoproterozoic time. Similar tube-forming microbialites are known from alkaline lake systems such as Lake Turkana, Pavilion Lake, and paleo-Lake Gosuite (Green River Formation). The tubestones are interpreted to represent a rarely attained end-member in stromatolite morphospace, likely associated with anomalously high carbonate supersaturation.

A complex microbiota from snowball Earth times: Microfossils from the Neoproterozoic Kingston Peak Formation, Death Valley, USA

A thin carbonate unit associated with a Sturtian-age (≈750–700 million years ago) glaciogenic diamictite of the Neoproterozoic Kingston Peak Formation, eastern California, contains microfossil evidence of a once-thriving prokaryotic and eukaryotic microbial community (preserved in chert and carbonate). Stratiform stromatolites, oncoids, and rare columnar stromatolites also occur. The microbial fossils, which include putative autotrophic and heterotrophic eukaryotes, are similar to those found in chert in the underlying preglacial units. They indicate that microbial life adapted to shallow-water carbonate environments did not suffer the significant extinction postulated for this phase of low-latitude glaciation and that trophic complexity survived through snowball Earth times.

Filamentous sulfur bacteria preserved in modern and ancient phosphatic sediments: implications for the role of oxygen and bacteria in phosphogenesis

Marine phosphate-rich sedimentary deposits (phosphorites) are important geological reservoirs for the biologically essential nutrient phosphorous. Phosphorites first appear in abundance approximately 600 million years ago, but their proliferation at that time is poorly understood. Recent marine phosphorites spatially correlate with the habitats of vacuolated sulfide-oxidizing bacteria that store polyphosphates under oxic conditions to be utilized under sulfidic conditions. Hydrolysis of the stored polyphosphate results in the rapid precipitation of the phosphate-rich mineral apatite-providing a mechanism to explain the association between modern phosphorites and these bacteria. Whether sulfur bacteria were important to the formation of ancient phosphorites has been unresolved. Here, we present the remains of modern sulfide-oxidizing bacteria that are partially encrusted in apatite, providing evidence that bacterially mediated phosphogenesis can rapidly permineralize sulfide-oxidizing bacteria and perhaps other types of organic remains. We also describe filamentous microfossils that resemble modern sulfide-oxidizing bacteria from two major phosphogenic episodes in the geologic record. These microfossils contain sulfur-rich inclusions that may represent relict sulfur globules, a diagnostic feature of modern sulfide-oxidizing bacteria. These findings suggest that sulfur bacteria, which are known to mediate the precipitation of apatite in modern sediments, were also present in certain phosphogenic settings for at least the last 600 million years. If polyphosphate-utilizing sulfide-oxidizing bacteria also played a role in the formation of ancient phosphorites, their requirements for oxygen, or oxygen-requiring metabolites such as nitrate, might explain the temporal correlation between the first appearance of globally distributed marine phosphorites and increasing oxygenation of Neoproterozoic oceans.

Microbially-Mediated Environmental Influences on Metazoan Colonization of Matground Ecosystems: Evidence from the Lower Cambrian Harkless Formation

Abstract Prior to the advent of widespread bioturbation during Cambro–Ordovician times, microbial mats may have covered large expanses of the continental shelf. Evidence of matgrounds in shallow-marine settings is provided by abundant wrinkle structures in Lower Cambrian strata of the Great Basin, United States. Wrinkle structures from the Lower Cambrian Harkless Formation commonly co-occur with a distinctive assemblage of invertebrate fossils, providing evidence for the possibility of selective metazoan colonization of matground substrates. Molds of linguliform brachiopods are abundant on many wrinkle surfaces. The agglutinated problematicum, Volborthella tenuis, also is found on wrinkle surfaces and in laminations beneath wrinkle-structure surfaces. Bedding-parallel trace fossils, such as Planolites, Diplichnites, and Taphrhelminthopsis, commonly crosscut wrinkle structures, while vertically oriented trace fossils are absent. Microbial mats containing layered microbial communities would have considerably compressed stratified redox zones beneath the sediment-water interface in marine-shelf settings. Sulfidic and anoxic conditions within and beneath microbial mats would have precluded habitation by many metazoans, while those that adapted to such conditions may have found matgrounds a unique, though temporally fleeting, ecological niche. The distinctive, low-diversity fossil assemblage found in association with the wrinkle structures in the Great Basin suggests that some early animals may have been adapted to hypoxic and sulfidic conditions found in matground substrates, while others may have been physiologically excluded from these environments.

Chemostratigraphy of Neoproterozoic-Cambrian units, White-Inyo Region, eastern California and western Nevada; implications for global correlation and faunal distribution

Comparison of secular variations in carbon-isotopic compositions of carbonates from the NeoproterozoicCambrian succession in the Whiteinyo region of eastern California with similarly-aged sequences worldwide suggests that, in California, a hiatus potentially encompassing the latest Neoproterozoic and much of the Tommotian may be present. The hiatus is found between the carbonate units in the Lower Deep Spring Formation, suggesting that the lowermost Deep Spring, Reed, and Wyman formations are Neoproterozoic in age, and the remaining Deep Spring Formation is uppermost Tommotian or Atdabanian in age. It is unclear where the Neoproterozoic-Cambrian boundary may lie, since chemostratigraphic data is not available for the boundary in New foundland type section, but it is most-likely encompassed within the inferred hiatus

Mercury anomalies and the timing of biotic recovery following the end-Triassic mass extinction.

The end-Triassic mass extinction overlapped with the eruption of the Central Atlantic Magmatic Province (CAMP), and release of CO2 and other volcanic volatiles has been implicated in the extinction. However, the timing of marine biotic recovery versus CAMP eruptions remains uncertain. Here we use Hg concentrations and isotopes as indicators of CAMP volcanism in continental shelf sediments, the primary archive of faunal data. In Triassic–Jurassic strata, Muller Canyon, Nevada, Hg levels rise in the extinction interval, peak before the appearance of the first Jurassic ammonite, remain above background in association with a depauperate fauna, and fall to pre-extinction levels during significant pelagic and benthic faunal recovery. Hg isotopes display no significant mass independent fractionation within the extinction and depauperate intervals, consistent with a volcanic origin for the Hg. The Hg and palaeontological evidence from the same archive indicate that significant biotic recovery did not begin until CAMP eruptions ceased.

Chemotrophic microbial mats and their potential for preservation in the rock record.

Putative microbialites are commonly regarded to have formed in association with photosynthetic microorganisms, such as cyanobacteria. However, many modern microbial mat ecosystems are dominated by chemotrophic bacteria and archaea. Like phototrophs, filamentous sulfur-oxidizing bacteria form large mats at the sediment/water interface that can act to stabilize sediments, and their metabolic activities may mediate the formation of marine phosphorites. Similarly, bacteria and archaea associated with the anaerobic oxidation of methane (AOM) catalyze the precipitation of seafloor authigenic carbonates. When preserved, lipid biomarkers, isotopic signatures, body fossils, and lithological indicators of the local depositional environment may be used to identify chemotrophic mats in the rock record. The recognition of chemotrophic communities in the rock record has the potential to transform our understanding of ancient microbial ecologies, evolution, and geochemical conditions. Chemotrophic microbes on Earth occupy naturally occurring interfaces between oxidized and reduced chemical species and thus may provide a new set of search criteria to target life-detection efforts on other planets.