Geosciences

1] Many orogenic belts experience alternations in shortening and extension (tectonic mode switches) during continuous plate convergence. The geodynamics of such alternations are not well understood. We present a record of Late Cretaceous to Eocene alternations of shortening and extension from the interior of the retroarc Sevier-Laramide orogen of the western United States. We integrate new Lu-Hf garnet geochronometry with revised PT paths utilizing differential thermobarometry combined with isochemical G-minimization plots, and monazite Th-Pb inclusion geochronometry to produce a well-constrained "M" shaped PTt path. Two burial events (86 and 65 Ma) are separated by $3 kbar of decompression. The first burial episode is Late Cretaceous, records a 2 kbar pressure increase at $515-550°C and is dated by a Lu-Hf garnet isochron age of 85.5 AE 1.9 Ma (2s); the second burial episode records $1 kbar of pressure increase at $585-615°C, and is dated by radially decreasing Th-Pb ages of monazite inclusions in garnet between $65 and 45 Ma. We propose a synconvergent lithospheric delamination cycle, superimposed on a dynamic orogenic wedge, as a viable mechanism. Wedge tapers may evolve from critical to subcritical (amplification), to supercritical (separation), and back to subcritical (re-equilibration) owing to elevation changes resulting from isostatic adjustments during the amplification and separation of Rayleigh-Taylor instabilities, and post-separation thermal and rheological re-equilibration. For the Sevier-Laramide hinterland, the sequence of Late Cretaceous delamination, low-angle subduction, and slab rollback/foundering during continued plate convergence explains the burial-exhumation-burial-exhumation record and the "M-shaped" PTt path. Citation: Wells, M. L., T. D. Hoisch, A. M. Cruz-Uribe, and J. D. Vervoort (2012), Geodynamics of synconvergent extension and tectonic mode switching:

1] Many orogenic belts experience alternations in shortening and extension (tectonic mode switches) during continuous plate convergence. The geodynamics of such alternations are not well understood. We present a record of Late Cretaceous to Eocene alternations of shortening and extension from the interior of the retroarc Sevier-Laramide orogen of the western United States. We integrate new Lu-Hf garnet geochronometry with revised PT paths utilizing differential thermobarometry combined with isochemical G-minimization plots, and monazite Th-Pb inclusion geochronometry to produce a well-constrained "M" shaped PTt path. Two burial events (86 and 65 Ma) are separated by $3 kbar of decompression. The first burial episode is Late Cretaceous, records a 2 kbar pressure increase at $515-550°C and is dated by a Lu-Hf garnet isochron age of 85.5 AE 1.9 Ma (2s); the second burial episode records $1 kbar of pressure increase at $585-615°C, and is dated by radially decreasing Th-Pb ages of monazite inclusions in garnet between $65 and 45 Ma. We propose a synconvergent lithospheric delamination cycle, superimposed on a dynamic orogenic wedge, as a viable mechanism. Wedge tapers may evolve from critical to subcritical (amplification), to supercritical (separation), and back to subcritical (re-equilibration) owing to elevation changes resulting from isostatic adjustments during the amplification and separation of Rayleigh-Taylor instabilities, and post-separation thermal and rheological re-equilibration. For the Sevier-Laramide hinterland, the sequence of Late Cretaceous delamination, low-angle subduction, and slab rollback/foundering during continued plate convergence explains the burial-exhumation-burial-exhumation record and the "M-shaped" PTt path. Citation: Wells, M. L., T. D. Hoisch, A. M. Cruz-Uribe, and J. D. Vervoort (2012), Geodynamics of synconvergent extension and tectonic mode switching:

: The objective of this project was to develop an ecohydrologically-based stream type classification and methodology for four military reservations in the southwestern United States representing the four Level III Eco regions that occur here: Fort Irwin (Mojave Basin and Range), Yuma Proving Ground (YPG; Sonoran Desert), Fort Huachuca (Madrean Archipelago), and Fort Bliss (Chihuahuan Desert). The goals of this project were to: (1) characterize ephemeral and intermittent streams using vegetation, hydrologic, and geomorphic attributes and develop a stream type classification based on those attributes, (2) associate wildlife habitat with the variables and stream type classification to improve management of TER-S and species of concern, and (3) provide a tool that allows Department of Defense (DoD) managers to evaluate the impacts of perturbations (e.g. climate change, military activities) on the hydrologic regimes of these systems and the species that depend on them.

Two new long sediment cores (EC1 and EC3), recovered from different locations within the infilled basin at Les Echets, France, provide a new high-resolution record of terrestrial and lacustrine responses to climatic changes during Marine Isotope Stages (MIS) 3 and 2. The lithologies of the cores are described in detail and correlated with each other by stratigraphic marker horizons, fluctuations in organic matter and AMS radiocarbon ages. The tentative correlation of the new cores to those described and analysed by de Beaulieu et al. (1980) and de Beaulieu & Reille (1984a) provides a preliminary chronostratigraphic framework. Sedimentation during MIS3 started with accumulation of sands and silts and was followed by alternating gyttja and clayey gyttja silts. Exceptionally high sedimentation rates during MIS2 led to the infilling of the basin. Alternating organic-rich and minerogenicrich sediments appear to coincide with changes in pollen assemblages (de Beaulieu & Reille 1984a) and suggest that millennial-scale climatic changes controlled lake productivity and catchment stability during most of MIS3.

The stable isotope composition of benthic ostracods from a deep-lake sediment core (JO2004-1) recovered from Lake Ohrid (Albania-Macedonia) was studied to investigate regional responses to climate change at the interface between the north-central European and Mediterranean climate systems. Ostracod valves are present only during interglacial intervals, during the Marine Isotope Stage (MIS) 5 and 1. The ostracod oxygen isotope values (d 18 O) quantitatively reflect changes in the oxygen isotope signal of the lake water (d 18 O L ). The interpretation of this record however, is far from straight forward. d 18 O L variations throughout MIS 5/6 transition (TII), MIS 5 and MIS 1 appear to be controlled by site specific hydrological processes as shown by modern isotope hydrology. The d 18 O L trends at TII, MIS 5 and MIS 1 match the timing and the main structural feature of the major regional climate records (Corchia cave d 18 O, Iberian margin Sea Surface Temperature) suggesting that the Ohrid d 18 O L responded to globalscale climate changes, although it seems certain that the lake experienced a significant degree of evaporation and varying moisture availability.

The Mg and Sr isotopic compositions (δ 26 Mg and 87 Sr/ 86 Sr) of pore fluids and bulk carbonates from Ocean Drilling Project Site 1171 (South Tasman Rise; 2148.2 m water depth) are reported, in order to evaluate the potential of diagenesis to alter carbonate-based geochemical proxies in an open marine system. Given the trace amounts of Mg in marine carbonates relative to coexisting pore fluids, diagenesis can alter carbonate δ 26 Mg, a promising proxy for seawater δ 26 Mg that may help elucidate long-term changes in the global Mg cycle. Constraints on the effect of diagenetic recrystallization on carbonate δ 26 Mg are therefore critical for accurate proxy interpretations. This study provides context for assessing the fidelity of geochemical proxyreconstructions using the primary components (i.e., foraminiferal tests and nannofossils) of bulk carbonate sediments. We find that pore fluid δ 26 Mg values (on the DSM3 scale) at Site 1171 increase systematically with depth (from-0.72‰ to-0.39‰ in the upper ~260 m), while the δ 26 Mg of bulk carbonates decrease systematically with depth (from-2.23‰ to-5.00‰ in the upper ~260 m). This variability is ascribed primarily to carbonate recrystallization, with a small proportion of the variability due to down-hole changes in nannofossil and foraminiferal species composition. The inferred effect of diagenesis on bulk carbonate δ 26 Mg correlates with downcore changes in Mg/Ca, Sr/Ca, Na/Ca, and 87 Sr/ 86 Sr. A depositional reactive-transport model is employed to validate the hypothesis that calcite recrystallization in this system can generate sizeable shifts in carbonate δ 26 Mg. Model fits to the data suggest a fractionation factor and a partition coefficient that are consistent with previous work, assuming calcite recrystallization rates of ≤7%/Ma constrained by Sr geochemistry. In addition, either partial dissolution or a distinctly different previous diagenetic regime must be invoked in order to explain aspects of the elemental chemistry and 87 Sr/ 86 Sr of relatively deep sediments from Holes A and C. This study indicates that the dynamics of a given sedimentary system can significantly alter bulk carbonate geochemistry, and presents a framework for considering the potential impact of such alteration on picked archives such as foraminiferal tests and nannofossils. Ultimately, this study contributes to the development of δ 26 Mg as a proxy for seawater δ 26 Mg by quantifying the susceptibility of carbonate δ 26 Mg to diagenetic alteration, particularly in sediments in open marine systems. This study suggests that because of the sensitivity of carbonate δ 26 Mg to diagenetic recrystallization, it can, in certain systems, be used to quantify the impact of diagenesis on carbonate-based geochemical proxies.

Geochemical proxy-based reconstructions widely utilize the trace elemental and isotopic compositions of marine biogenic carbonates to interpret past climatic and oceanographic conditions. However, such proxy-based reconstructions are often challenged by marine diagenesis as carbonates are highly susceptible to diagenetic alteration, especially to partial dissolution and calcite recrystallization. Thus, quantifying diagenetic effects is a prerequisite for the development of any carbonate-based geochemical proxy. Among the metal isotopes, Mg isotopic composition (δ 26 Mg) of foraminiferal carbonates is a promising proxy to reconstruct secular seawater δ 26 Mg variability, which is useful in understanding long-term changes in the Mg geochemical cycle. However, Mg in marine carbonates is particularly susceptible to diagenetic alterations due to its low abundance in carbonates compared to pore fluids. Hence, the development of δ 26 Mg of marine carbonates as a proxy demands a thorough understanding of the isotopic systematics of Mg in marine sedimentary systems, which to date has not been accomplished. The primary goal of this dissertation is to quantify the effect of diagenesis on metal isotopes, in particular, the δ 26 Mg of marine carbonates from various depositional settings. Multiple marine sites are investigated to evaluate the effect of advection, diffusion, and lithological variation within the sedimentary column on the extent of calcite diagenesis. The first two chapters are focused on comparing the extent of diagenetic alteration among various marine sites and quantifying the impact of such diagenetic reactions on carbonate-based proxies. In the first study, the effect of diagenetic recrystallization on bulk carbonate δ 26 Mg is quantified from an open marine site (ODP Site 1171, Hole A), which is influenced by upward advection. Trace elemental (e.g., Mg/Ca and Sr/Ca) and isotopic compositions (δ 26 Mg and 87 Sr/ 86 Sr) of pore fluids and bulk carbonates are analyzed from Hole 1171A using an ICP-AES and a Neptune Plus MC-ICP-MS, respectively. A systematic decrease in bulk carbonate δ 26 Mg (-4 to 0.40 •10-4 mol/m 2 /d). The increase in [Ca 2+ ], [Mg 2+ ], and [Sr 2+ ] in the fluid of a set of parallel non-tracer experiments indicates that dissolution-precipitation is the dominant mechanism of the atom exchange in foraminiferal calcite. Interestingly, in the presence of aqueous silica in some of the reactors the exchange rate of modern foraminifera decreases (3.70•10-4 to 0.13•10-4 mol/m 2 /d). This observation indicates that the presence of dissolved silica in the system can inhibit calcitefluid exchange, which enhances the preservation of foraminiferal tests and minimizes the extent of alteration of foraminiferal proxies. vi

The study of past biotic interactions is important not only to understand the paleoecological history of a community, but also to test the evolutionary role of such interactions. Drill holes in invertebrate exoskeletons provide one of the very few scenarios where biotic interaction could be studied directly and the related hypotheses could be tested with statistical rigor. Hence, the documentation and interpretation of the spatiotemporal patterns of drill hole frequencies has been the subject of extensive paleontological research. The two main gastropod groups responsible for the drilling predation in modern marine environment arose in Cretaceous as supported by their body fossil. However, the drill holes have been reported from the fossils of as far back as Precambrian age. The trend shows an overall low but variable intensity in Paleozoic and Mesozoic and a significant increase in Cenozoic. There are few reported cases of drilled bivalves from the Mesozoic, although in those instances, frequencies are fairly low. Most of the previous records of drilling predation during Paleozoic and Mesozoic come from North America and Western Europe. Here we report 148 drilled bivalve specimens of a single species from the Upper Jurassic horizon in western India. This is highest in number for any taxon ever recorded since Precambrian to Cretaceous. The frequency of drilling constitutes 30% of shells of the same species examined. This drilling frequency is highest when compared to all the reported Mesozoic drilling frequencies in Bivalves. The shapes of the drill holes are indicative of gastropod predation. The drilling gastropods responsible for these lethal attacks are also similar to their modern counterparts in terms of their highly selective prey choice and site-specificity. These results suggest that (1) Mesozoic bivalves were preyed upon by drilling gastropods, often with high intensities, and (2) the specialized characters of modern drilling predators were also present in Mesozoic. This largely unexplored record of bivalve drill holes from the Middle Mesozoic contradicts the general trend of "Mesozoic quiescence" as claimed by most researchers.

Fossilized foraminiferal tests are widely used as proxies to constrain paleoenvironments, including past sea surface and bottom water temperatures and the chemical composition of ancient seawater. However, relatively little is known about the extent to which calcareous foraminiferal tests exchange with seawater prior to burial. In this study, we used a 45 Ca radiotracer approach to quantify the extent and rate of foraminiferal calcite-fluid exchange over a three-month period. Modern foraminiferal tests exhibited faster exchange with a 45 Ca-spiked solution (2.6•10 −4 to 2.0•10 −5 mol/m 2 /d) than fossil foraminiferal tests (7.6•10 −5 to 5.3•10 −6 mol/m 2 /d) from ODP Site 807. The impact of dissolved Si on the exchange rate of modern foraminifera was minimal (2.6•10 −4 to 9.2•10 −6 mol/m 2 /d). Unlike foraminiferal calcite, inorganic calcite exchanged at remarkably slow rates (6.6•10 −6 to 5.9•10 −7 mol/m 2 /d) suggesting that biogenic calcite is more reactive than inorganic calcite. Increased Ca 2+ , Mg 2+ , and Sr 2+ concentrations in fluids from parallel tracer-free reactors indicated preferential dissolution of tests, though there was no overt change in test morphology during experiments (as imaged by scanning electron microscopy and micro-CT scanning). Time-dependent box model simulations of 45 Ca, as well as aqueous elemental chemistry, support the hypothesis that calcite-fluid exchange occurred via dissolution-reprecipitation of foraminiferal tests that was induced by intra-test chemical heterogeneity. The short-term exchange reactions investigated in this study are comparable to the reactions that can occur in the seawater column and at the sediment-seawater interface. This work suggests that dissolution-reprecipitation in such settings has the potential to overprint the chemistry of foraminiferal tests without overt physical alteration, which can lead to a potential bias in paleoenvironmental reconstructions. Model-based estimates suggest a maximum of 2-3 • C alteration of Mg/Ca-based temperatures over experimental time scales. Model efforts also suggest that the diagenetic trajectories in Mg/Ca-δ 18 O space, for instance, are impacted by the partition coefficient of Mg in calcite (λ Mg) as well as by the mechanism of alteration. This is demonstrated for (i) the ratio of exchange to dissolution and (ii) the relative rate of high-Mg-calcite to low-Mg-calcite reaction. We have illustrated that the mechanism of alteration, in addition to waterrock ratios, determine diagenetic trajectories. Finally, surface reaction has implications for our ability to constrain fractionation factors in experimental and natural systems. The experimental observations suggest that exchange between tests and solution is nominal over ∼100 days. Experiments aimed at deducing fractionation factors rely on the equilibration of bulk solid and fluid, since these are the readily analyzed reservoirs. We quantify the fraction of carbonate (>5%) that must react so that surface reaction does not impact the interpretation of Ca isotopic fractionation factors, and discuss the implications for more "fluid-dominated" elements such as Mg and O.

Autoregressive statistical models of monthly salinity variations in the mainstem of Chesapeake Bay are developed for use in climate change applications. Observations of salinity and Susquehanna River flow from 1984 to 1994 are used to calibrate the models. Up to 93% of the variance in salinity is captured by these models, with the best fits occurring in the middle of the bay, where submonthly fluctuations due to river flow and oceanic currents are damped. Our salinity models use output from a climate-forced hydrological model under a doubling of present-day atmospheric CO 2 . This coupling scheme predicts salinity changes between ϩ3.5% and Ϫ27.5% near the mouth of the Susquehanna River, to between ϩ0.1% and Ϫ0.7% near the ocean. These ranges demonstrate the uncertainty in climate model predictions, although three of the four scenarios used indicate increased Susquehanna River flow and therefore decreased salinity. In the highest flow scenario (a 32% annual increase) our results show that the bay's isohalines may recede by approximately 6.3 km (about 2% of the length of the bay) near the Susquehanna River, to as much as 55 km (about 17% of the length of the bay) near the middle bay. This shift implies that climate change may have consequences for organisms with low-salinity thresholds, including oysters and crabs.

Microstructural changes induced by the microbial reduction of Fe(III) in nontronite by Shewanella oneidensis were studied using environmental cell (EC)-transmission electron microscopy (TEM), conventional TEM, and X-ray powder diffraction (XRD). Direct observations of clays by EC-TEM in their hydrated state allowed for the first time an accurate and unambiguous TEM measurement of basal layer spacings and the contraction of layer spacing caused by microbial effects, most likely those of Fe(III) reduction. Non-reduced and Fe(III)-reduced nontronite, observed by EC-TEM, exhibited fringes with mean d 001 spacings of 1.50 nm (standard deviation, s = 0.08 nm) and 1.26 nm (s = 0.10 nm), respectively. In comparison, the same samples embedded with Nanoplast resin, sectioned by microtome, and observed using conventional TEM, displayed layer spacings of 1.0À1.1 nm (non-reduced) and 1.0 nm (reduced). The results from Nanoplast-embedded samples are typical of conventional TEM studies, which have measured nearly identical layer spacings regardless of Fe oxidation state. Following Fe(III) reduction, both EC-and conventional TEM showed an increase in the order of nontronite selected area electron diffraction patterns while the images exhibited fewer wavy fringes and fewer layer terminations. An increase in stacking order in reduced nontronite was also suggested by XRD measurements. In particular, the ratio of the valley to peak intensity (v/p) of the 1.7 nm basal 001 peak of ethylene glycolated nontronite was measured at 0.65 (non-reduced) and 0.85 (microbially reduced).

Using a multicomponent inverse model, we quantified the rates of organic matter (OM) remineralization and the relative importance of major terminal electron acceptors [Fe(III)-(oxy)hydroxides and SO ] in salt marsh sediments 2Ϫ 4 with varying degrees of bioturbation and vegetation at Skidaway Island, Georgia. The model determined the rates of OM diagenesis by seeking simultaneous agreement between measured and model-calculated depth-concentration profiles of multiple major redox species while using the biological transport parameters determined from direct field observations. The OM degradation rates are found to be greater and penetrate deeper in sediments with vegetation or bioturbation than in sediments with limited macrobenthic components, in which organic matter degradation is restricted to the immediate vicinity of water-sediment interface. The results also confirmed previous observations that Fe(III)-(oxy)hydroxides are much more important than sulfate as terminal electron acceptors in the heavily bioturbated station and provided the numerical values to the depth-dependent relative importance. Solid-phase Fe is recycled as a terminal electron acceptor Ն30 times in the bioturbated station, because biological mixing repeatedly moves reduced Fe(II) back into the aerobic environment, where it is reoxidized to form Fe(III)-(oxy)hydroxides and is reused as a terminal electron acceptor. Vegetation appears to have little influence on net solute transport, but it stimulates microbial activities and significantly enhances the remineralization of OM at depths.

We investigated interactions between ferruginous smectite (NAu-1) and facultative dissimilatory iron-reducing bacteria, Shewanella oneidensis, in order to determine whether the bacteria are directly respiring structural Fe(III) or respiring Fe(III) after it has been dissolved from the structure of the clay. Anaerobic flow-through experiments were used to facilitate the microbial reduction under controlled solution chemistry and to monitor the time-dependent concentrations of Fe, Al, and Si, which are major structural components of NAu-1. Complimentary batch experiments were conducted to allow for time series examinations of the reacted solid and extensive control experiments. The control experiments revealed that NAu-1 dissolution occurred under anaerobic, aerobic, biotic, and abiotic conditions even when Fe remained oxidized due to the presence of O 2 or the absence of viable bacteria. Thus, this study shows that NAu-1 dissolution precedes microbial reduction of Fe. The dissolution products of this dynamic system are present not only in solution, but also as secondary precipitates and/or adsorbates. Thus, S. oneidensis in our systems are primarily respiring Fe(III) in the aqueous phase or associated with secondary solids, rather than Fe(III) that is still a part of the crystalline NAu-1 structure. Published by Elsevier B.V.

 Shallow marine carbonate sediments near the Dry Tortugas undergo extensive biogeochemical diagenesis upon deposition, resulting in postdepositional fabric comprised of micritic aggregates of clay-sized particles, a matrix of peloidal mud, and intraparticular cementation. Freshly deposited primary skeletons, mostly Halimeda fragments, yield micritic aggregates of clay-sized particles, which then form peloidal mud fabric upon compaction. The cryptocrystalline particles produced by micritization are later replaced by high magnesian calcite. Cementation is observed in the intra-particular microenvironment. However, no inter-particular, grain-binding cement was detected throughout the gravity core samples investigated.

Burrow irrigation by benthic infauna affects chemical mass transfer regimes in marine and estuarine sediments. The bioirrigation facilitates rapid exchange of solutes between oxygenated overlying water and anoxic pore water, and thus promotes biogeochemical reactions that include degradation of sedimentary organic matter and reoxidation of reduced species. A comprehensive understanding of chemical mass transfer processes in aquatic sediments thus requires a proper treatment of bioirrigation. We investigated bioirrigation processes during early diagenesis using laboratory benthic mesocosms. Bioirrigation was carried out in the mesocosms by Schizocardium sp., a funnel-feeding enteropneust hemichordate that builds and ventilates a U-shaped burrow. Interpretation of the laboratory results was aided by a two-dimensional multicomponent model for transport and reactions that explicitly accounts for the depth-dependent distribution of burrows as well as the chemical mass transfers in the immediate vicinity of burrow walls. Our study shows that bioirrigation signi cantly affects the spatial distributions of pore water solutes. Moreover, bioirrigation promotes burrow walls to be the site of steep geochemical gradients and rapid chemical mass transfer. Our results also indicate that the exchange function, a, widely used in one-dimensional bioirrigation modeling, can accurately describe the bioirrigation regimes if its depth attenuation is coupled to the depth-dependent distribution of burrows. In addition, this study shows that the multicomponent 2D reaction-transport model is a useful research tool that can be used to critically evaluate common biogeochemical assumptions such as the prescribed depth dependenciesof organic matter degradation rate and C/N ratio, as well as the lack of macrofaunal contribution of metabolites to the pore water.

Biologically enhanced pore water irrigation affects the course of early diagenesis in shallow marine sediments, as illustrated here for the carbonate sediments from North Key Harbor, Dry Tortugas National Park, Florida. Whereas macrofaunal activity at the study site extends approximately 15 cm below the water-sediment interface, measured O 2 micropro les only show O 2 penetration to depths of a few mm. This apparent discrepancy can be explained by considering the 3-D O 2 distribution in the burrowed sediments. Calculations based on an idealized tube model for burrow irrigation show that measureable O 2 concentrations are limited to the immediate vicinity of burrows. Given the observed burrow density (705 6 15 m 2 2 ), a randomly positioned O 2 micropro le has a high probability (. 90%) to fall outside the reach of radial O 2 diffusion from burrows. Hence, the shallow penetration depths recorded at the site do not exclude a much deeper supply of O 2 in the sediment via the burrows. Other characteristic features observed in the upper 15-20 cm of the sediments, in particular, the absence of SO 4 22 depletion and the presence of subsurface maxima in the pro les of NH 4 1 and TCO 2 , are also the result of pore water irrigation. These features are reproduced by the multicomponent reactive transport model STEADYSED1. Results of the model simulations indicate that bacterial SO 4 22 reduction is the dominant pathway of organic carbon degradation, but that consumption of SO 4 22 in the sediments is compensated by its enhanced transport by irrigation. Thus, depth pro les of SO 4 22 may be poor indicators of the importance of SO 4 22 reduction in irrigated sediments.