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.
“Out,
damn'd spot!” — Lady Macbeth. As we at JSR PaperClips
continue our celebration of 400th anniversary of the Bard’s demise,
we recall another famous stained contribution—that of Tony Dickson, who 50
years ago pointed out how staining of carbonate rocks with alizarin red-S and
potassium ferricyanide can aide in differentiation of orthorhombic carbonates
(such as aragonite), trigonal carbonates (such as calcite), and iron-rich
calcite and dolomite. So, with clear
conscience and citing little risk, we recommend re-reading this classic.
“Secret, and self-contained,
and solitary as an oyster,” wrote Dickens.
But he also knew that no oyster lives independent of his environment. In
this contribution, Sælen and others use this concept to attempt to
use δ18O, δ 13C, 87Sr/86Sr
and elemental analysis of oyster shells to unravel paleoenvironmental settings
of carbonates and mixed siliciclastic-carbonates from Miocene strata in the
Lorca Basin of Spain. The raw and modeling results expand on patterns derived
from contemporaneous corals and reveal novel insights into paleosalinities of
marginal marine water in this basin. More generally, the results highlight the
use of the well-preserved, low-Mg calcite shells of oysters to assess
short-term changes in sea water salinity and temperature.
Interpretation of fluvial strata in continental
basins is contingent upon recognizing stratal architectures and climate change
signals from fluvial strata, and provide insights into understanding how
fluvial sedimentation interacts with basin topography. These aspects are
difficult to assess in the pre-Devonian fluvial rock record, because
comparisons to models based on vegetated modern rivers may not be suitable. In
this paper, Lowe and Arnott carefully describe the architecture of braided and
ephemeral facies in the Cambro-Ordovician Potsdam Group in the Ottawa Embayment
and Quebec Basin in northeastern North America. The contribution reveals
pre-Devonian fluvial architectures and processes, and how they relate to global
orbitally-forced climate changes in the Late Cambrian. These aspects of
pre-Devonian fluvial sedimentology are important to the Joe Geologist because
they provide a framework to make basin-wide time-significant stratigraphic
correlations, and understand variations in stratal architectures with varying
degrees of interaction with basement topography, potentially important for
groundwater and oil migration, and discerning climate change during this
critical period.
As noted by Elvis and Ecclesiates, rivers
flow surely to the sea. Yet, exactly what happens to rivers just before they
get there is not well constrained, but has been interpreted to control avulsion
nodes, and therefore, distributary channel patterns. To test scenarios of backwater
hydraulics (especially water-surface drawdown) on sedimentology and morphology of
lower-delta plain distributary channels, Columbera et al. describe field
observations of the Cretaceous Neslen Formation (Campanian, Mesaverde Group) of
the Book Cliffs in Utah. The results reveal ribbon sand bodies with
architecture, lithofacies, and bounding surfaces and strata that are broadly
consistent with patterns that would be expected in the region of rivers where
the streambed drops below sea level. Nonetheless,
the authors conclude by noting a need for additional research on these
processes in the rock record, and the appropriate revision of sequence
stratigraphic models.
Compared to their sulfur-rich
evaporitic relatives, the origin and significance of sedimentary borates is
poorly constrained, yet these deposits can form economic resources. To better
understand lacustrine borates, Ortí et al. describe the sedimentology, petrographic characteristics, and stratigraphy
of cores from Miocene strata of three exploratory boreholes from Turkey. The
results reveal a succession rich in several scales of cyclic alternations of
primary (depositional to interstitial) glauberite (Na2Ca(SO4)2)
and probertite (NaCaB5O7(OH)4·3H2O)
(yes, those will be on the quiz). These
minerals and their varied textures are interpreted to vary according to lake
levels, chemistry, and paleogeographic setting in the paleo-saline lake. The
results emphasize the diversity of hydrogeochemistry that can influence
lacustrine evaporite depositional systems.
A half century
ago, Matthews bemoaned that fine-grained carbonate sediment had been neglected,
except in the Bahamas. To rectify this situation, he examined mineralogy of
carbonate mud offshore of present-day Belize, and found aragonite (as in the
Bahamas), but with an “unusual abundance of” high-Mg calcite, in many cases
between 30–70%. Matthews interpreted the mud to have two general sources:
production on shoals followed by transport, and in situ production. He concluded, “If this investigation has served
no other purpose, it has demonstrated that lime mud may have multiple
origins.”
Unlike fish,
carbonate platforms commonly drown.
After they do, pre-existing margins can impart large-scale inflections
in bathymetry below the subsequent shelf break that influence sediment
dispersal on the slope. Here, Hurd etal. explore the relation between a pre-existing, relict break in slope and
subsequent accumulations in Permian strata of West Texas. The data reveal that
increased slopes favor bypass and channels, which in turn favor accumulation of
downdip, onlapping organic-rich shale and carbonate mud-dominated mass
transport complexes. These results are interpreted to have general
applicability to drowned carbonate platforms. And, no fish were harmed during
this study either.
High-resolution
sequence stratigraphy can be challenging, but mixed carbonate-siliciclastic
systems offer unique challenges, but at times unique insights as well. In this paper, Schwartz et al. document
outcrop and subsurface data from a Cretaceous proximal to distal transect of
the Neuquén Basin of western Argentina, exploring controls on high-resolution
stratal patterns. The results suggest transgressive carbonate hemicycles,
overlain by storm- and wave-influenced siliciclastic shoreface deposits during
regressions. The data suggest that autogenic or eustatic controls did not
control facies patterns; rather changes in sediment supply related to more
arid–more humid shifts are interpreted to control the patterns in these
strata. These results emphasize the
potentially important role of climate shifts on stratigraphy, and highlights
its potential use for correlation.
Following up on his recent magnum opus that considered universal sequence stratigraphy, Burgess
explores the depth of “what we really know” about the fundamental controls on
sequence stratigraphy in his most recent contribution. Wasting no words,
with nary a flaw in style or content, he lucidly and simply describes his
perspective on this question, and in doing so, illuminates gaps in
understanding. His unusually succinct style carefully avoids the jargon common
in sequence stratigraphy, so even beginners can grasp his points. [Ed. Note: We
appreciate Dr. Burgess’ brief abstract, shorter than even many physics
articles, which are notoriously curt. Even though it is a
review paper, it is the early favorite to win the Outstanding Paper Award.]
