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.
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.
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.
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.
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.]
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.
