image: This image shows the Mississippi River Delta in 2001. view more
Credit: Image courtesy NASA.
Boulder, Colo., USA - River deltas, low-lying landforms that host critical and diverse ecosystems as well as high concentrations of human population, face an uncertain future. Even as some deltas experience decreased sediment supply from damming, others will see increased sediment discharge from land-use changes. Accurate estimates of the current rate of subsidence in the Mississippi Delta (southern USA) are important for planning wetland restoration and predictions of storm surge flooding.
Parts of coastal Louisiana (southern USA) are undergoing accelerated land loss due to the combined effects of sea-level rise and land subsidence. In the Mississippi Delta, where rates of land loss are especially severe, subsidence of the land surface reflects natural processes, such as sediment compaction and crustal loading, but this is exacerbated by anthropogenic withdrawal of fluids (water, oil, natural gas).
In this study for Geology, Makan Karegar and colleagues use precise Global Positioning System (GPS) data to measure subsidence rates of the Mississippi Delta. They also use tide gauge records to better understand the relationship between subsidence and sea-level rise in southern Louisiana.
The authors show that while the majority of the delta is relatively stable, parts of the delta may not be viable in the long term. The southern portion of the delta continues to experience high rates of subsidence (5 to 6 mm per year). The current rate of relative sea-level rise (the combined effect of land subsidence and sea-level rise) along parts of the coastal delta is nearly 8 to 9 mm per year.
Given stable sea level and sediment deposition, a delta will tend toward an equilibrium state where subsidence is more or less balanced by sediment deposition. In the Mississippi River system, however, a series of dams on various upstream tributaries have reduced sediment supply to the delta, while levees on the lower part of the river have artificially channelized the flow, forcing sediments to be deposited beyond the delta in the deeper Gulf of Mexico.
The data presented by Karegar and colleagues have implications for land reclamation and wetland restoration in the region. Mitigation efforts may include river diversion to encourage resedimentation, and pumping of offshore sands to restore barrier islands.
FEATURED ARTICLE
A three-dimensional surface velocity field for the Mississippi Delta: Implications for coastal restoration and flood potential
Makan A. Karegar et al., University of South Florida, Tampa, Florida, USA. Published online ahead of print on 27 Apr. 2015; http://dx.doi.org/10.1130/G36598.1.
Other recently posted GEOLOGY articles (see below) cover such topics as
- 1. Study of subsidence and sea-level rise along the coast of Java, Indonesia, and what it means for engineering the "drowning" Mississippi Delta;
2. Parrotfish as critical players in sand production in the Maldives;
3. Slushball Earth; and
4. A model of the physical processes of burial, mixing, and specimen decay and a look at Homestead Cave, Utah, USA.
GEOLOGY articles published online ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.
What makes a delta wave-dominated?
Jaap H. Nienhuis et al., Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA, and Massachusetts Institute of Technology, Cambridge, Massachusetts, USA. Published online ahead of print on 27 Apr. 2015; http://dx.doi.org/10.1130/G36518.1.
The Mississippi Delta is drowning. Efforts are in place to save parts of the delta by creating small, engineered diversions with the assumption that the diversions will deposit sediment in a shape resembling the original Mississippi Delta. However, due to continued retreat and drowning of barrier islands protecting the modern delta, future wave attack will be a lot higher than today. This new study by Jaap Nienhuis, Andrew Ashton, and Liviu Giosan proposes a simple metric to predict whether a delta will have a river-dominated shape like the Mississippi delta, with its multiple branching channels and irregular, marshy, shorelines, or a smooth, cuspate wave-dominated shape. For a series of small deltas along the north coast of Java, Indonesia, this technique successfully predicts the transition between river-dominated and wave dominated forms, suggesting that this metric has the potential to predict shape of engineered sediment diversions in the wake of future increased wave attack.
Linking reef ecology to island building: Parrotfish identified as major producers of island-building sediment in the Maldives
C.T. Perry et al., University of Exeter, Exeter, UK. Published online ahead of print on 27 Apr. 2015; http://dx.doi.org/10.1130/G36623.1.
Coral reef islands are considered among the most vulnerable environments to climate change on Earth. However, as landforms constructed entirely of reef-derived sands, a key process that will influence future island vulnerability is the extent to which these islands are linked to the key sand-producing coral reef habitats that surround them. Using new survey methods, C.T. Perry and colleagues test this idea at Vakkaru, an atoll interior island in the Maldives. They show that the reefs around Vakkaru produce an estimated 697,000 kg (or 377 cubic meters) of new carbonate sand each year, but that most (~75%) is produced on the narrow outer reef flats. Of this total, an estimated 85% is produced entirely by parrotfish. These findings suggest that the maintenance of high parrotfish densities, and of the complex high coral cover habitats in which they thrive, will be critical for the continued generation of those sands most appropriate for island building and maintenance at this site. Furthermore, the study demonstrates that while the need to protect parrotfish populations is commonly based on the need to sustain key ecological interactions on reefs, they also play a critical role as key biogeoengineering species that can sustain local landform maintenance.
The survival of benthic macroscopic phototrophs on a Neoproterozoic snowball Earth
Qin Ye et al. China University of Geosciences, Wuhan, China. Corresponding author: Shuhai Xiao, Virginia Tech, Blacksburg, Virginia 24061, USA, xiao@vt.edu. Published online ahead of print on 27 Apr. 2015; http://dx.doi.org/10.1130/G36640.1.
Earth was a gigantic freezer during the so-called Marinoan glaciation about 650-635 million years ago. Some geologists proposed that the entire surface ocean was completely frozen to form a snowball Earth, but others suggested that the Earth was more like a slushball with open waters and an active hydrological cycle. Understanding whether there were open waters, whether these open waters were habitable, and what organisms inhabited these open waters are key questions for geobiologists who are interested in exploring the limit of climate changes and the resilience of the biosphere. Geobiologists from Virginia Tech and China University of Geosciences announce the discovery of fossil seaweeds (or benthic macroscopic phototrophs) that lived during the Marinoan glaciation in mid-latitudinal coastal environments in what is now South China. The new discovery supports the slushball scenario and suggests that open waters not only existed in mid-latitudinal coastal environments, but also provided habitable muddy substrates for these seaweeds to survive. What remains uncertain is how these seaweeds and other phototrophs impacted the global carbon cycle and paced Earth's return from a white planet to a blue planet in the subsequent Ediacaran Period.
Where does the time go?: Mixing and the depth-dependent distribution of fossil ages
Rebecca C. Terry and Mark Novak, Oregon State University, Corvallis, Oregon, USA. Published online ahead of print on 24 Apr. 2015; http://dx.doi.org/10.1130/G36483.1.
Fossil records provide a unique window into the biological dynamics of Earth's past, and are increasingly used to address issues relevant to biodiversity conservation at centennial to millennial time scales. Nevertheless, fossil records are time-averaged, incorporating the remains of multiple generations into each depositional layer. Paleobiological research hinges on knowing how time is distributed within these layers. Here, Rebecca C. Terry and Mark Novak develop a model to explain how the distribution of time captured within a layer will change with the layer's stratigraphic depth. They find support for model's predictions in age-frequency distributions reconstructed from AMS Carbon-14 dates on small mammal specimens from Homestead Cave, Utah, USA. The model highlights how the physical processes of burial, mixing and specimen decay can influence the clarity of a fossil record's biological information and drive changes in apparent biodiversity over time that are not driven by true ecological change. Accounting for time-averaging's influence is thus key for bridging between neontological and paleontological archives to reconstruct the biological dynamics of the past and detect the shifting baselines of the present.
Focal mechanism of prehistoric earthquakes deduced from pseudotachylyte fabric
Eric C. Ferré et al., Southern Illinois University, Carbondale, Illinois, USA. Published online ahead of print on 27 Apr. 2015; http://dx.doi.org/10.1130/G36587.1.
Pseudotachylytes are rocks formed during earthquakes by friction along the seismic fault plane. The texture of these materials attests of the earthquake magnitude, direction and sense of seismic slip. Therefore pseudotachylytes host precious information on prehistoric earthquakes that occurred before seismometers existed.
Deriving rock uplift histories from data-driven inversion of river profiles
Christoph Glotzbach, Leibniz University Hannover, Hannover, Germany. Published online ahead of print on 24 Apr. 2015; http://dx.doi.org/10.1130/G36702.1.
Any reliable prediction of the future evolution of Earth's surface requires quantifying its past evolution and identifying the main driving forces. Analytical methods such as cosmogenic nuclides and thermochronology are routinely used to quantify Earth surface processes (erosion, uplift) over thousands and millions of years. A comparison of resulting rates of surface processes is, however, hampered by the large difference in their timescales. River profiles can be used to bridge these timescales and derive nearly continuous temporal/spatial information about landscape evolution. Glotzbach developed an integrative inverse modeling approach to simultaneously reconstruct river profiles, model thermochronological and cosmogenic nuclide data. The model output is a continuous and consistent estimate of Earth's surface processes, which enable a robust reconstruction of the past landscape evolution. The performance of this new integrated model was verified with synthetic datasets and a dataset from the Sila massif (Italy). The results demonstrate that even a complicated landscape evolution can be reliably retrieved by the combined interpretation of river profiles, thermochronological and cosmogenic nuclide data.
Exhumation and uplift coupled with precipitation along the western Dead Sea Rift margin
U. Ryb et al., The Fredy and Nadine Herrmann Institute of Earth Sciences, The Hebrew University, Jerusalem, Israel. Published online ahead of print on 24 Apr. 2015; http://dx.doi.org/10.1130/G36331.1.
Carbonate rocks, such as limestone or dolomite, may weather almost exclusively by dissolution, leaving no solid residue. Therefore, surface lowering (denudation) of carbonate landscapes may be controlled by precipitation. Once the relation between carbonate-rock denudation and precipitation is quantified, denudation records can be used much like long-term rain-gauges. This approach is used to constrain the average precipitation over a 107-year timescale, across the western Dead Sea Rift margin (Israel). On this time scale, precipitation-controlled -- denudation may induce tectonic uplift. Strong coupling between climate, denudation and tectonic uplift is also expected in other carbonate landscapes around the world.
Forearc hyperextension dismembered the south Tibetan ophiolites
Marco Maffione et al., Utrecht University, Budapestlaan, Utrecht, Netherlands. Published online ahead of print on 24 Apr. 2015; http://dx.doi.org/10.1130/G36472.1.
Ophiolites are relics of oceanic lithosphere emplaced over continental margins that can form virtually continuous belts, like the Jurassic and Cretaceous Neotethyan ophiolites running from the Alps to Tibet. These belts are commonly dismembered, yet tectonic causes remain poorly constrained. Paleomagnetic, structural geological and petrological evidence from the lower Cretaceous ophiolite belt exposed along the suture zone of southern Tibet show that extensional detachment faults, like those associated to oceanic core complexes along slow-spreading mid-ocean ridges, caused lithospheric attenuation and splitting, and mantle exhumation at the seafloor in the leading edge of an intra-oceanic subduction zone (forearc). Pervasive forearc faulting occurred within ~10 million years of ophiolite accretion in an extensional setting, well before its emplacement over the Indian continental margin. Marco Maffione and colleagues define this mechanism as "forearc hyperextension," and propose that it played a key role in the dismemberment of the South Tibetan ophiolites, and possible exhumation of Eurasian sub-continental mantle. Forearc hyperextension can effectively explain the condensed sequence and lateral discontinuity of the India-Asia suture zone ophiolite belt, as well as local occurrence of ultra-high pressure minerals in the ophiolitic mantle sequence.
Old continental zircons from a young oceanic arc, eastern Taiwan: Implications for Luzon subduction initiation and Asian accretionary orogeny
Wen-Yu Shao et al. National Taiwan University, Taipei, Taiwan. Published online ahead of print on 24 Apr. 2015; http://dx.doi.org/10.1130/G36499.1.
This study reports zircon U-Pb and Hf isotope data from the northern Luzon arc that has accreted onto eastern Taiwan, the Eurasian continental margin since about five million years ago (5 Ma). Most zircons show inherited U-Pb ages of ca. 14 Ma (n = 9) and ca. 220 Ma (n = 56, the largest age peak), along with much older ages of ca. 0.7 billion years ago (Ga), 1.9 Ga and 2.5 Ga. Whereas the ca. 14-Ma zircons may have crystallized from the earliest magmatism of the northern Luzon arc, the Indosinian and older zircons suggest Cathaysia-type sources that we attribute to a continental fragment split off from the Eurasian margin by opening of the South China Sea and then drifted/accreted to the western Philippine Sea plate before the Luzon subduction initiation. Consequently, magmas derived from the depleted mantle wedge evolved and picked up the old continental zircons during ascent. This study not only better integrates regional tectonics with magmatic records in SE Asia, but also signifies a modern example from eastern Taiwan that highlights the importance of ribbon continents in Asian orogenesis over time and space.
Age of Matuyama-Brunhes boundary constrained by U-Pb zircon dating of a widespread tephra
Yusuke Suganuma et al., National Institute of Polar Research, Tokyo, and The Graduate University for Advanced Studies (SOKENDAI), Tokyo, Japan. Published online ahead of print on 24 Apr. 2015; http://dx.doi.org/10.1130/G36625.1.
Earth's latest magnetic field reversal event, the Matuyama-Brunhes boundary (MBB), is an important calibration point on the geological time scale, linking sediments, Antarctic ice cores, and volcanic lavas. The frequently cited age of 780 ka for the MBB is based on an astronomically calibrated age for marine sediment, and is supported by radiometric (40Ar/39Ar) dating of Hawaiian lavas. Challenging this age, however, are reports of younger ages for the MBB based on high-resolution data from marine sediments, and spikes of cosmogenic nuclides (corresponding to a significant drop of the Earth's magnetic field intensity during the reversal) in marine sediments and an Antarctic ice core. Here, we present a new radiometric age (U-Pb zircon age) of 772.7 plus or minus 7.2 ka from a volcanic ash layer just below the MBB in the Chiba section (very rapidly deposited marine sediment) in Japan. This U-Pb zircon age, coupled with a newly obtained astronomical age for the marine sediment, yields an MBB age of 770.2 plus or minus 7.3 ka. We provide the first direct comparison between astronomical age calibration, U-Pb dating, and geomagnetic reversal chronology for the MBB, fulfilling a key requirement in calibrating the geological time scale.
Cyclical processes in the North American Cordilleran orogenic system
P.G. DeCelles, University of Arizona, Tucson, Arizona, USA; and S.A. Graham, Stanford University, Stanford, California, USA. Published online ahead of print on 27 Apr. 2015; http://dx.doi.org/10.1130/G36482.1.
Mountain ranges in the western U.S. developed between 160 and 50 million years (Ma) ago as the oceanic Farallon tectonic plate subducted beneath the North American plate. This type of mountain belt features a magmatic arc -- a chain of volcanoes and underlying magma chambers in the continental crust. The Sierra Nevada batholith is the exhumed core of this magmatic arc. This batholith experienced two high flux events (HFEs) when magma production more than doubled. HFEs were dominated by melting of continental North American rocks, and dense residues formed beneath the arc while lighter, intermediate magmas formed the upper parts of the arc. Immediately following the HFEs, deformation of the upper continental crust east of the arc propagated farther eastward. Because the HFEs were fed by lower crust from the eastward region, crustal deformation and arc magmatism were coupled. Foundering of dense residues beneath the arc into the mantle created more space for crustal shortening. This cycle recurred at least twice in the western U.S. between 160 and 50 Ma. The region west of the arc was characterized by strong deformation and sediment accumulation along the plate boundary, but responded less predictably to the cycle described above. This paper suggests that this Cordilleran cycle may operate in other, similar mountain systems worldwide.
One or two oroclines in the Variscan orogen of Iberia? Implications for Pangea amalgamation
Daniel Pastor-Galán et al., Paleomagnetic Laboratory "Fort Hoofddijk," University of Utrecht, Budapestlaan 17, 3584CD Utrecht, Netherlands. Published online ahead of print on 27 Apr. 2015; http://dx.doi.org/10.1130/G36701.1.
The Iberian peninsula in southwest Europe contains a very special segment of the mountain belt that formed about 320 million years ago during amalgamation of Pangea, the last of the supercontinents. In particular, the Iberian segment shows a striking and contorted "S" shape geometry, the Cantabrian bend to North and the Central Iberian bend to the south. The Cantabrian bend is well studied and formed at the latest stages of the formation of Pangea. On the contrary, the Central Iberian bend is poorly understood -- it is not well known when and how it formed. Despite that fact, researchers have interpreted the Central-Iberian bend as a late-collisional effect as well. This bends represent a large plate tectonic problem as their formation is not predicted by any global reconstructions of Pangea. In this research paper we have investigated the southern bend of the orogen to constrain its geometry and kinematics by means of paleomagnetic analyses. Our results show that this southern bend, if it existed, could not have formed at the same time as the northern one, and therefore it is not late orogenic. We suggest a global reconstruction that could lead to the formation of the mountain belt as we observe it now.
Compaction-driven melt segregation in migmatites
Roberto F. Weinberg et al., Monash University, Clayton, Victoria, Australia. Published online ahead of print on 24 Apr. 2015; http://dx.doi.org/10.1130/G36562.1.
The melting of rocks at depth is one of the main processes of mass transfer on Earth and controls the origin and evolution of both the continental and oceanic crusts. Melt is extracted from rock pores and just how this happens controls all processes that follow, from volcanism to mineralization. Here, Roberto F. Weinberg and colleagues use the newly developed theory of compaction instability to show that melt extraction from rocks at depth should be at high angles to the maximum compression stress, in harmony with field observations and in contrast to most previously published experiments that predict extraction in structures nearly perpendicular to these. These findings explain how folds and melt extraction interact in the deep continental crust to remove melt from the source and give rise to magma accumulations in the upper continental crust.
Dating shallow thrusts with zircon (U-Th)/He thermochronometry -- The shear heating connection
Matteo Maino et al., Università di Pavia, Pavia, Italy. Published online ahead of print on 24 Apr. 2015; http://dx.doi.org/10.1130/G36492.1.
This research presents a new method for dating the motion of a thrust developed into the shallow crust (less than 6 to 7 km). Generally, the rocks at this depth have temperatures of less than 180 degrees C, which is the temperature of closure of the (U-Th)/He thermochronometer on zircons (ZHe). It is known that faulting generates overheating within the fault zone because of rock fracturing and pulverization and plastic deformation. Therefore we performed ZHe analysis into the fault zone and surrounding wall rocks to test the possibility that only within the fault zone the zircons were reset, thus providing a confirmation of the shear heating effectiveness and a possible way for fault dating. We applied this approach to a segment of the Penninic Front, which is a major Alpine thrust that drove the nappe stacking during the Early Oligocene. The results indicate that only the zircons collected from the fault zone are reset (~29 to ~34 million years ago), while the wall rocks samples are older (~93 to ~312 million years ago). Our results underscore the validity of the ZHe technique for dating faults. Furthermore, thermal modeling demonstrates that the shear heating associated with the fault motion is an efficient mechanism for generating temperature increases during important thrusting.
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Journal
Geology