Sarah E. Greene

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.

A subseafloor carbonate factory across the Triassic-Jurassic transition

Triassic-Jurassic (T-J) boundary successions record a paucity of carbonate in association with the mass extinction. Here we demonstrate that three globally disparate T-J sections contain volumetrically important early diagenetic carbonate, i.e., carbonate formed soon after deposition of the sediment but commonly ignored as secondary, that contains information about the extinction and may constitute a previously unrecognized pathway in the carbon cycle. Petrographic analyses of unusual carbonate fans from three sites reveal that they grew just below the sediment-water interface, nearly concomitant with primary sediment deposition. Thus, the shallow subseafloor can be a carbonate sink of unknown size, and may be a predictable consequence of ocean acidification where carbonate precipitation first returns within the sediment before recovering in the water column.

MICROFACIES OF THE COTHAM MARBLE: A TUBESTONE CARBONATE MICROBIALITE FROM THE UPPER TRIASSIC, SOUTHWESTERN U.K.

ABSTRACT A remarkably aerially extensive (∼2,000 km2) unit of carbonate microbialites occurs in many Triassic–Jurassic boundary interval outcrops of the southwestern United Kingdom and captures petrographic evidence that could link them to the end-Triassic extinction event. The bioherms—known regionally as the Cotham Marble—occur as discrete ∼20-cm-thick, decimeter- to meter-scale mounds, and display at least five growth phases that alternate between laminated and dendritic mesofabrics. Cross sections parallel to bedding through the dendritic phases expose a reticulate dendritic framework separated by polygonal spaces (∼1–3 cm diameter), characteristic of “tubestone” microbialites. Microscopically, the dendrolites contain evenly distributed rod to filamentous putative microfossils (∼2 µm diameter and ∼10 µm in length) in a matrix of micrite and contain higher total organic carbon than the surrounding matrix. Round to ellipsoidal spar-filled regions (∼200 µm in diameter) within the dendrolites (previously interpreted as serpulid worm tubes) likely resulted from the production of gas bubbles within rapidly lithifying mats or are a two-dimensional artifact of evenly spaced three-dimensional branching within the mats. The fill between the dendrolites of the first layer contains abundant phycoma clusters of the green algal prasinophyte Tasmanites, commonly considered a “disaster taxon.” The cyclic phases represent episodic and laterally extensive environmental change within shallow water coastal environments during a marine transgression. Collectively, the presence of microbial micrite in a shallow marine setting, the marked lateral extent of the bioherms, and the abundance of Tasmanites suggest the Cotham Marble microbialites formed during the high pCO2 and relatively warmer conditions associated with the events of the end-Triassic mass extinction.

A microbial carbonate response in synchrony with the end-Triassic mass extinction across the SW UK.

The eruption of the Central Atlantic Magmatic Province (CAMP)—the largest igneous province known—has been linked to the end-Triassic mass extinction event, however reconciling the response of the biosphere (at local and nonlocal scales) to potential CAMP-induced geochemical excursions has remained challenging. Here we present a combined sedimentary and biological response to an ecosystem collapse in Triassic-Jurassic strata of the southwest United Kingdom (SW UK) expressed as widely distributed carbonate microbialites and associated biogeochemical facies. The microbialites (1) occur at the same stratigraphic level as the mass extinction extinction, (2) host a negative isotope excursion in δ13Corg found in other successions around the world, and (3) co-occur with an acme of prasinophyte algae ‘disaster taxa’ also dominant in Triassic-Jurassic boundary strata of other European sections. Although the duration of microbialite deposition is uncertain, it is likely that they formed rapidly (perhaps fewer than ten thousand years), thus providing a high-resolution glimpse into the initial carbon isotopic perturbation coincident with the end-Triassic mass extinction. These findings indicate microbialites from the SW UK capture a nonlocal biosedimentary response to the cascading effects of massive volcanism and add to the current understanding of paleoecology in the aftermath of the end-Triassic extinction.

Mixed Siliciclastic/Carbonate Systems

Abstract The ichnology of mixed siliciclastic/carbonate successions, in which sediment admixture has had a clear effect on infaunal populations, is discussed. Fundamental influences that allow for the development of unique ichnological signatures in mixed systems include grain size, grain shape, and early diagenetic alteration. Shell debris within dominantly siliciclastic successions introduces complexities into the infaunal habitat that are clearly reflected in trace-fossil assemblages. Some differences are preservational, with trace fossils inherently more difficult to recognize in coarser bioclastic intervals than in fine-grained siliciclastic intervals. Faunal level differences include those where some taxa are excluded by admixed bioclastic detritus or by reduced ecospace availability. Diagenetic processes in mixed systems also operate at different scales and temporal intervals, leading to fundamental differences in preservation and in substrate consistency. In mixed systems, quintessential carbonate ichnotextures co-occur with prototypical siliciclastic ichnotextures, such as successions with interstratified or coplanar firmground and hardground assemblages.

LINGULIDE RESPONSE TO SEVERE STORMS RECORDED IN MIDDLE TRIASSIC STRATA OF NORTHEASTERN BRITISH COLUMBIA

Abstract The distribution and significance of lingulide brachiopods (Lingularia selwyni Whiteaves) and the trace fossil Lingulichnus in storm-generated, subtidal sandstone beds in the upper Toad and Liard formations of northeastern British Columbia is summarized. Storm-generated sandstone beds from two depositional environments (proximal offshore to offshore transition and lower shoreface) were analyzed. Lingulide material is present in only a small proportion of the storm-generated sandstone beds in the study interval. This distribution is interpreted to reflect the inherent patchiness of infaunal communities both in pre- and post-event communities. Middle Triassic lingulides, like their extant relatives, were capable of (1) surviving storm-induced burial by extending their burrows upward and reconnecting with the sediment-water interface; (2) surviving storm-induced exhumation and transport; and (3) reinhabiting the sediment and reestablishing themselves in new settings after relocation by storms. Depositional environment played a pivotal role in the likelihood that individual lingulides survived storm events. Lingulide faunas deposited in the proximal offshore to offshore transition experienced up to 15%–25% mortality during storm events with most storm survivors (90%–95%) able to reestablish themselves in feeding position at the sediment-water interface. Lingulide faunas deposited in lower shoreface settings experienced considerably higher mortality, with ∼60%–75% of the animals dying prior to the end of the storm and a further ∼45% surviving the storm events but failing to reestablish themselves at the sediment-water interface in life position. Escape burrowing and reburrowing following exhumation, entrainment, and transport are fundamental adaptations for infauna occupying such dynamic depositional settings as the subtidal littoral zone (neritic zone). This ability was likely integral in the long-term success of lingulides in Phanerozoic marine successions.

MICROFACIES OF THE COTHAM MARBLE: A TUBESTONE CARBONATE MICROBIALITE FROM THE UPPER TRIASSIC SOUTHWESTERN U.K.: A REPLYREPLY TO MAYALL AND WRIGHT COMMENT

Mayall and Wright question interpretations in our microfacies analysis of the Cotham Marble microbialites (Ibarra et al. 2014) in which we primarily highlight previously overlooked aspects of Cotham Marble microbialite formation. They are specifically unconvinced about the Cotham Marble’s potential relevance to the end-Triassic mass extinction and our interpretation that Microtubus is not integral to the formation of the dendrolitic microbialite phases. Here we address Mayall and Wright’s comments under the same headings in which they present them.