Kristin D. Bergmann

The Magnitude and Duration of Late Ordovician–Early Silurian Glaciation

Carbonate isotopes reveal a link between past ocean temperatures and mass extinction. Understanding ancient climate changes is hampered by the inability to disentangle trends in ocean temperature from trends in continental ice volume. We used carbonate “clumped” isotope paleothermometry to constrain ocean temperatures, and thereby estimate ice volumes, through the Late Ordovician–Early Silurian glaciation. We find tropical ocean temperatures of 32° to 37°C except for short-lived cooling by ~5°C during the final Ordovician stage. Evidence for ice sheets spans much of the study interval, but the cooling pulse coincided with a glacial maximum during which ice volumes likely equaled or exceeded those of the last (Pleistocene) glacial maximum. This cooling also coincided with a large perturbation of the carbon cycle and the Late Ordovician mass extinction.

Life: the first two billion years

Microfossils, stromatolites, preserved lipids and biologically informative isotopic ratios provide a substantial record of bacterial diversity and biogeochemical cycles in Proterozoic (2500–541 Ma) oceans that can be interpreted, at least broadly, in terms of present-day organisms and metabolic processes. Archean (more than 2500 Ma) sedimentary rocks add at least a billion years to the recorded history of life, with sedimentological and biogeochemical evidence for life at 3500 Ma, and possibly earlier; phylogenetic and functional details, however, are limited. Geochemistry provides a major constraint on early evolution, indicating that the first bacteria were shaped by anoxic environments, with distinct patterns of major and micronutrient availability. Archean rocks appear to record the Earths first iron age, with reduced Fe as the principal electron donor for photosynthesis, oxidized Fe the most abundant terminal electron acceptor for respiration, and Fe a key cofactor in proteins. With the permanent oxygenation of the atmosphere and surface ocean ca 2400 Ma, photic zone O2 limited the access of photosynthetic bacteria to electron donors other than water, while expanding the inventory of oxidants available for respiration and chemoautotrophy. Thus, halfway through Earth history, the microbial underpinnings of modern marine ecosystems began to take shape. This article is part of the themed issue ‘The new bacteriology’.

BIOLOGICAL INFLUENCES ON SEAFLOOR CARBONATE PRECIPITATION

ABSTRACT The sedimentary record reveals first-order changes in the locus of carbonate precipitation through time, documented in the decreasing abundance of carbonate precipitation on the seafloor. This pattern is most clearly recorded by the occurrence of seafloor carbonate crystal fans (bladed aragonite pseudomorphs neomorphosed to calcite or dolomite), which have a distinct temporal distribution, ubiquitous in Archean carbonate platforms, but declining through Proterozoic time and extremely rare in Phanerozoic basins. To understand better the potential influences on this pattern, we built a mathematical framework detailing the effects of organic matter delivery and microbial respiratory metabolisms on the carbonate chemistry of shallow sediments. Two nonunique end-member solutions emerge in which seafloor precipitation is favorable: enhanced anaerobic respiration of organic matter, and low organic matter delivery to the sediment-water interface. This analysis suggests that not all crystal fans reflect a unique set of circumstances; rather there may have been several different geobiological and sedimentary mechanisms that led to their deposition. We then applied this logical framework to better understand the petrogenesis of two distinct crystal fan occurrences—the Paleoproterozoic Beechey Formation, Northwest Territories, Canada, and the middle Ediacaran Rainstorm Member of the Johnnie Formation, Basin and Range, United States—using a combination of high-resolution petrography, micro X-ray fluorescence and wavelength dispersive spectroscopy, C isotopes, and sedimentary context to provide information on geobiological processes occurring at the sediment-water interface. Interestingly, both of these Proterozoic examples are associated with iron-rich secondary mineral assemblages, have elevated trace metal signatures, and sit within maximum flooding intervals, highlighting key commonalities in synsedimentary geobiological processes that led to seafloor carbonate precipitation.

Facies, stratigraphy, and evolution of a middle Ediacaran carbonate ramp: Khufai Formation, Sultanate of Oman

The Khufai Formation is the oldest carbonate platform of the Cryogenian to lowermost Cambrian Huqf Supergroup. A stratigraphic characterization of this unit includes detailed facies descriptions, a sequence-stratigraphic interpretation, and evaluation of lateral heterogeneity and overall ramp evolution. The Khufai Formation comprises one and one-half depositional sequences with a maximum flooding interval near the base of the formation and a sequence boundary within the upper peritidal facies. Most of the deposition occurred during highstand progradation of a carbonate ramp. Facies tracts include outer-ramp and midramp mudstones and wackestones, ramp-crest grainstone shoal deposits, and extensive inner-ramp, microbially dominated peritidal deposits. Outcrops in the Oman Mountains are deep-water deposits, including turbiditic grainstone and wackestone interbedded with siliciclastic-rich siltstone and crinkly laminite. Facies patterns and parasequence composition are variable both laterally across the outcrop area and vertically through time because of a combination of ramp morphology, siliciclastic supply, and possible syndepositional faulting. The lithostratigraphic boundary between the Khufai Formation and the overlying Shuram Formation is gradational and represents significant flooding of the carbonate platform. The stratigraphic characterization presented here along with the identification of key facies and diagenetic features will help further future exploration and production of hydrocarbons from the Khufai Formation.

Cap carbonate platform facies model, Noonday Formation, SE California

The Neoproterozoic outcrop belt of the Death Valley region, California, preserves an oblique cross section of the Noonday Formation, a mixed carbonate-siliciclastic platform that hosts distinctive basal Ediacaran cap carbonate−affiliated sedimentary structures, stromatolite textures, and δ^(13)C_(carb) values. The Noonday platform encompasses two depositional sequences that reveal two cycles of relative sea-level change within strata conventionally considered to record a single, rapid, postglacial sea-level rise. In updip localities, facies of the first depositional sequence record the transition from a carbonate ramp to a stromatolite-bearing, “tubestone”-textured, reef-rimmed platform; downdip, localities seaward of the reefal escarpment variably preserve a thin and condensed onlapping foreslope wedge. Base-level fall exposed the reef crest to karstic dissolution and propagated submarine incised valleys into the seaward margin of the reef. Overlying strata record the backfilling of a submarine incised valley and reestablishment of a back-stepping, carbonate-dominated ramp prior to a second subaerial exposure event that defines the contact between the Noonday and Johnnie formations. We address the relative contributions of syndepositional tectonism and recovery from low-latitude deglaciation in dictating Noonday platform architecture and the intra−Noonday Formation sequence boundary. Noonday Formation deposition coincided with extension of the Laurentian margin during disaggregation of the Rodinian supercontinent. Within this framework, previous work has suggested that the intra−Noonday Formation sequence boundary records growth faulting that reinforced differential topography, uplifting reef-rimmed horsts—exposing the reef crest to karstic dissolution—and downdropping grabens. However, we trace the intra−Noonday Formation sequence boundary seaward of the reef crest and demonstrate that, for a time, wave base was situated downdip of the reef escarpment on putatively downdropped fault blocks. Thus, if the Noonday margin were undergoing extension, then the creation of the intra−Noonday Formation sequence boundary required a concomitant decrease in accommodation due, perhaps, to postglacial isostatic uplift attendant with low-latitude deglaciation. We speculate that Noonday Formation sequence architecture records (1) immediate deglacial flooding, (2) shoaling and exposure due to isostatic rebound induced by either a hiatus in meltwater flux or rapid ice-sheet collapse against a background of global deglaciation, and (3) resumed flooding following complete deglaciation. As rift-related tectonism could amplify or counter glacial isostasy, inferences of the amplitude of local postglacial sea-level change will require robust estimates of syndepositional extension across the Noonday margin.

Pyrite-walled tube structures in a Mesoproterozoic sediment-hosted metal sulfide deposit

Unusual decimeter-scale structures occur in the sediment-hosted Black Butte Copper Mine Project deposit within lower Mesoproterozoic strata of the Belt Supergroup, Montana. These low domal and stratiform lenses are made up of millimeter-scale, hollow or mineral-filled tubes bounded by pyrite walls. X-ray micro−computed tomography (micro-CT) shows that the tube structures are similar to the porous fabric of modern diffuse hydrothermal vents, and they do not resemble textures associated with the mineralization of known microbial communities. We determined the sulfur isotopic composition of sulfide minerals with in situ secondary ion mass spectrometry (SIMS) and of texture-specific sulfate phases with multicollector−inductively coupled plasma−mass spectrometry (MC-ICP-MS). The sedimentological setting, ore paragenesis, sulfur isotope systematics, and porosity structure of these porous precipitates constrain the site of their formation to above the sediment-water interface where metalliferous hydrothermal fluids vented into the overlying water column. These data constrain the geochemistry of the Mesoproterozoic sediment-water interface and the site of deposition for copper-cobalt-silver mineralization. Metals in the hydrothermal fluids titrated sulfide in seawater to create tortuous fluid-flow conduits. Pyrite that precipitated at the vent sites exhibits large sulfur isotope fractionation (>50‰), which indicates a close association between the vents and sulfate-reducing microbiota. In the subsurface, base metal sulfides precipitated from sulfide formed during the reduction of early diagenetic barite, also ultimately derived from seawater. This model suggests dynamic bottom-water redox conditions at the vent site driven by the interplay between sulfate-reducing organisms and metalliferous fluid effluence.