Monday, October 31, 2016

Highlights—Paleoclimate and pCO2

Estimates of pCO2 in deep time provide fundamental constraints on ancient atmospheric conditions, yet can include considerable uncertainty because of variability related to distinct carbon sources. To better constrain the role of different OM sources on pCO2 estimates derived from pedogenic carbonate, Myers et al. use a range of sources of organic matter to estimate the concentration of carbon dioxide in the atmosphere in both modern and Jurassic soils. The results indicate that the pedogenic carbonate CO2 paleobarometer is sensitive to variation in stable carbon isotope composition of soil-respired CO2. Additionally, organic matter occluded in pedogenic calcite produces reasonable pCO2 estimates. 

Effects of different organic-matter sources on estimates of atmospheric and soil pCO2 using pedogenic carbonate by Timothy S. Myers, Neil J. Tabor, Louis L. Jacobs, and Robert Bussert

Thursday, October 27, 2016

Highlights—Quartz (hamster?) Transport and Surface Microtextures

Although provenance plays a fundamental role in establishing sandstone mineralogy, transport processes impact the sediment as well. To explore the potential utility of transport-associated microtextures, Sweet and Brannan analyze the abundance of glacially and fluvially induced microtextures to assess the role of fluvial overprint on glacially modified grains along ~188 km of the proglacial Chitina River, SE Alaska.  The analysis of SEM observations documents the relative proportions of glacially induced stylus microtextures (i.e., grooves, troughs, and gouges) and fluvial percussion-induced microtextures (i.e., v-shaped cracks and edge rounding) on quartz grain surfaces, and compares how this ratio changes with distance downstream. The results indicate that the glacially induced microtextures input to the river persist downstream, but are modified progressively by traction and saltation. This type of analysis could be applied to ancient fluvial strata provide insights to 1) differentiate between proglacial and nonglacial braided rivers and 2) reconstruct proglacial paleogeography from ancient strata. Furthermore, the results clearly show that it makes a difference how you roll.

Monday, October 24, 2016

Highlights—Imaging Cement Zonation

Many diagenetic events can be recorded in rocks as only subtle cement zones or corrosion surfaces on mineral faces. To better observe and characterize these features, Buckman and others illustrate the application of Charge Contrast Imaging (CCI) to examine diagenetic features including zoning, twinning, fractures, and geochemical changes. Applying the technique to a complex Cretaceous coquina carbonate and Devonian reef carbonate sample illustrates results comparable to SEM-Cathodoluminescence (and optical-CL), but with several advantages. 1) CCI does not suffer from image smearing, due to latency of the red light phase within CL, a different type of charge. 2) CCI typically is faster than SEM-CL. 3) CCI appears to highlight fabric and cement details within regions of carbonate that appear nonluminescent under CL. Nonetheless, as with cathodoluminescence, the specific character of CCI varies as a function of elemental composition and lattice dislocations.  

Charge contrast imaging (CCI): revealing enhanced diagenetic features of a coquina limestone by James O. Buckman, Patrick W.M. Corbett, and Lauren Mitchell

Wednesday, October 12, 2016

Highlights—Fans Off the Shelf

On planet Earth, most sediment moves downslope; unfortunately, how and when it does so can be complicated. To explore linkages and develop predictive conceptual models of relations between shelf and basinal sedimentary systems, Koo and others describe shelf-edge architecture and evolution of coeval basin-floor fans in Maastrichtian strata (Washakie Basin, Wyoming), a high-sediment-supply basin. The results of examining 630 wireline logs show that the progradation and aggradation style of submarine fans is strongly coupled with the behavior of correlative the shelf edge and shelf-edge deltas, a link interpreted to be driven by relative changes in sea level. This conceptual model is interpreted to be applicable to sediment supply-dominated margins, even as sea level changes, because of limited space on the shelf. In contrast, this linkage might not be appropriate in accommodation-dominated margins with plenty of storage space on the shelf.

Coupling between shelf-edge architecture and submarine-fan growth style in a supply-dominated margin by Woong Mo Koo, Cornel Olariu, Ronald J. Steel, Mariana I. Olariu, Cristian R. Carvajal, and Wonsuck Kim

Friday, October 7, 2016

Highlight—Microbial Mat Morphology—Nature or Nurture?

Microbial mat descriptions and classifications in modern peritidal systems focused on morphological descriptions, leaving the origins to be debated among beer-drinking geologists. To understand the roles of microbial communities and environmental conditions in the genesis and morphology of tidal flat microbialites, Trembath-Reichert and others applied optical microscopy and gene sequencing methods to study the microbial composition of mats from a tidal algal marsh in Caicos, B.W.I. Microscopy results are consistent with an interpretation that Cyanobacteria colonizing the initial mat surface are responsible its structure.  Although genetic data show the same relative abundances of the same Cyanobacteria in both “flat” and “biscuit” type mats, gene sequencing reveals that total diversity and community composition is significantly greater in the biscuit mat morphotype. These results suggest that mat morphology reflects time-integrated microbial response to various environmental factors, and in which the community diversity increases with time after environmental disturbance.  Morphology is not a consistent signature of component microorganisms. [Ed. Note: The importance of Genes cannot be understated.]

Gene sequencing-based analysis of microbial mat morphotypes: Caicos Platform, British West Indies by Elizabeth Trembath-Reichert, Lewis M. Ward, Sarah P. Slotznick, Steven L. Bachtel, Charles Kerans, John P. Grotzinger, and Woodward W. Fischer  

Tuesday, October 4, 2016

Highlights—The Bottom Drops Out in Mexico

Although potential central to understanding the evolution of both the southern part of North American Cordillera and the opening of the Gulf of Mexico, Early Cretaceous strata of Mexico have remained enigmatic. To better understand tectonic and stratigraphic evolution in this area, Sierra-Rojas and co-authors describe Lower Cretaceous strata of the Tentzo basin of southwestern Mexico. Integrating outcrop observations, petrography, and detrital-zircon geochronology, the study reveals a thick succession of red beds (conglomerate, sandstone, siltstone) overlain by carbonates, collectively interpreted to represent alluvial, lacustrine, fluvial, tidal, and marine environments. The thick succession, characterized by rapid accumulation (3.6 mm/yr), is interpreted to represent a backarc basin in an extensional setting on the paleo-western margin of Mexico, but that in time became part of the broad, stable platform facing the Gulf of Mexico. The results suggest that these basins in southwestern Mexico are more closely related to a Pacific volcanic arc, and are not related directly to the opening of the Gulf of Mexico, helping to constrain tectonodynamics.