Reply to comment on “dynamic topography in South America” by Hechenleitnera,Fiorelli, Larrovere,Grellet-Tinnera,and Carignano

This is a Reply to Hechenleitner and collaborators Comment, who proposed a Cretaceous age for the whole Llanos Formation (central Argentina, Sierras Pampeanas Province), based on neosauropod fossils, instead of Miocene as originally proposed by Ezpeleta et al. (2006) and Dávila et al. (2007). However, red beds that underlay the thick paleosoils of the Llanos Formation provided nine detrital U–Pb Paleogene (62 Ma, earliest Cenozoic) ages on zircon grains (Astini et al., 2009, Ezpeleta, 2009). On the base of this evidence, and other mammal remnant within the Sierras Pampeanas (where the Llanos Formation develops), we proposed this is a condensed unit with Mesozoic ages at the bottom and Mio-Pliocene (likely younger) to the top.     

Influence of Mussel Culture on the Vertical Export of Phytoplankton Carbon in a Coastal Upwelling Embayment (Ría de Vigo,NW Iberia)

The goal of this paper is to find out whether suspended mussel culture affects the vertical fluxes of biogenic particles in the Ría de Vigo on a seasonal scale. With this aim, vertical fluxes of particulate organic carbon (POC) and the magnitude and composition of vertical export of phytoplankton carbon (Cphyto) collected in sediment traps were examined by comparing data obtained inside a mussel farming area (RaS) with those found at a reference station (ReS) not affected by mussels. Our results indicate that mussel farming has a strong impact on sedimentation fluxes under the rafts, not only increasing POC flux but also altering the magnitude and composition of Cphyto fluxes. Average POC flux at RaS (2564?±?1936 mg m^?2 day^?1) was four times higher than at ReS (731?±?276 mg m^?2 day^?1), and much of this increase was due to biodeposit fluxes (Cbiodep) which accounted for large proportion of POC flux (35–60 %). Indeed, because of this high Cbiodep flux, only a small proportion of the POC flux was due to Cphyto flux (3–12 %). At the same time, we observed an increased sedimentation of phytoplankton cells at RaS that could be explained by a combination of mechanisms: less energetic hydrodynamic conditions under mussel rafts, ballast effect by sinking mussel feces, and diatom aggregates. Moreover, mussel farming also altered the quality of the Cphyto flux by removing part of the predatory pressure of zooplankton and thus matching diatom composition in water column and sediment traps.     

Groundwater flow in an ‘underfit’ carbonate aquifer in a semiarid climate: application of environmental tracers to the Salt Basin,New Mexico (USA)

The Salt Basin is a semiarid hydrologically closed drainage basin in southern New Mexico, USA. The aquifers in the basin consist largely of Permian limestone and dolomite. Groundwater flows from the high elevations (～2,500 m) of the Sacramento Mountains south into the Salt Lakes, which are saline playas. The aquifer is ‘underfit’ in the sense that depths to groundwater are great (～300 m), implying that the aquifer could transmit much more water than it does. In this study, it is speculated that this characteristic is a result of a geologically recent reduction in recharge due to warming and drying at the end of the last glacial period. Water use is currently limited, but the basin has been proposed for large-scale groundwater extraction and export projects. Wells in the basin are of limited utility for hydraulic testing; therefore, the study focused on environmental tracers (major-ion geochemistry, stable isotopes of O, H, and C, and ¹⁴C dating) for basin analysis. The groundwater evolves from a Ca–HCO₃ type water into a Ca–Mg (Na) – HCO₃–Mg (Cl) water as it flows toward the center of the basin due to dedolomitization driven by gypsum dissolution. Carbon-14 ages corrected for dedolomitization ranged from less than 1,000 years in the recharge area to 19,000 years near the basin center. Stable isotopes are consistent with the presence of glacial-period recharge that is much less evaporated than modern. This supports the hypothesis that the underfit nature of the aquifer is a result of a geologically recent reduction in recharge.     

Reply to comment on: “A densely feathered ornithomimid (Dinosauria: Theropoda) from the Upper Cretaceous Dinosaur Park Formation,Alberta, Canada”

We confirm the presence of pigmented keratinized integumentary structures attributable to feathers in the Late Cretaceous Ornithomimus specimen UALVP 52531. We falsify the hypothesis that these features represent collagen fibers and address additional criticisms of our paper made by Lingham-Soliar (2016).     

Reply to the comment by J. R. Ali on “Illawarra Reversal: the fingerprint of a superplume that triggered Pangean breakup and the end-Guadalupian (Permian) mass extinction” by Yukio Isozaki

The following four major questions were raised about my recent proposal for the possible link between the end-Guadalupian extinction and a unique geomagnetic event called the Illawarra Reversal (Isozaki, 2009a); 1) timings of extinction, cooling, and the Illawarra Reversal (end of the Kiaman Superchron), 2) geomagnetic intensity during superchrons, 3) ascent rate of mantle plume, and 4) age constraints of LIP volcanism in east Pangea. The latest research results on the Permian biodiversity change, numerical modeling of plume, and single-crystal measurement of geomagnetism support that the timings of extinction and the Illawarra Reversal, high field intensity during the Kiaman superchron, and ascent rate of plume are reasonably explained in accordance with the integrated “plume winter” scenario (Isozaki, 2009b). The onset ages of LIP volcanism need further refinement for identifying the impingement of a plume head.     

Effect of a broad shallow sill on tidal circulation and salt transport in the entrance to a coastal plain estuary (Mira—Vila Nova de Milfontes, Portugal)

We describe the tidal circulation and salinity regime of a coastal plain estuary that connects to the ocean through a flood tide delta. The delta acts as a sill, and we examine the mechanisms through which the sill affects exchange of estuarine water with the ocean. Given enough buoyancy, the dynamics of tidal intrusion fronts across the sill and selective withdrawal (aspiration) in the deeper channel landward appear to control the exchange of seawater with estuarine water. Comparison of currents on the sill and stratification in the channel reveals aspiration depths smaller than channel depth during neap tide. During neap tide and strong vertical stratification, seawater plunges beneath the less dense estuarine water somewhere on the sill. Turbulence in the intruding bottom layer on the sill promotes entrainment of fluid from the surface layer, and the seawater along the sill bottom is diluted with estuarine water. During ebb flow, salt is effectively trapped landward of the sill in a stagnant zone between the aspiration depth and the bottom where it can be advected farther upstream by flood currents. During spring tide, the plunge point moves landward and off the sill, stratification is weakened in the deep channel, and aspiration during ebb extends to the bottom. This prevents the formation of stagnant water near the bottom, and the estuary is flooded with high salinity water far inland. The neapspring cycle of tidal intrusion fronts on flood coupled with aspiration during ebb interacts with the sill to play an important role in the transport and retention of salt within the estuary.     

The Franciscan Complex (California,USA) – The model case for return-flow in a subduction channel put to the test

The model of “Return-Flow in a Subduction Channel” (RFSC) is put to the test, by opposing key-assumptions of the model to an integrated review of field data, petrology, geochronology and structures of high-P mélanges of the Franciscan Subduction Complex (FSC), to which RFSC has been first applied. Key-assumptions are: (i) The subduction channel (SC) is a low-viscosity zone of finite width restrained by the rigid, subducted and overriding plates. (ii) Forced (return) flow in the SC is associated with formation and km-scale displacements of small tectonic blocks (< 1 km) in a highly deformable matrix. (iii) This results in apparently random distributions and close associations of blocks exhumed from different depths. Hence, the block-in-matrix structures and “exotic” blocks in high-P mélanges would reflect the structural state at depth of the SC. On the basis of the predictions ensuing from the key assumptions of the RFSC model on the rock record, it is detailed why the actual record of the high-P mélanges of the FSC does not comply with this perception. The record rather reflects recurring subduction, underplating-associated décollement, erosion and mingling of high-P rocks by sedimentary processes, in the trench, all occurring in an 80 m.y.-lasting period. Distributed deformation of the mélange matrix is unrelated to formation of apparently chaotic block-in-matrix structures since: (i) creation, subduction, ascent and cooling of “exotic” blocks on counterclockwise P-T paths predates deposition of the bulk of the metasediments constituting the matrix by ~ 20–40 m.y.; (ii) deformation of the matrix occurs during a second loop of high-P metamorphism postdating creation and ascent of the “exotic” blocks by at least ~ 20–40 m.y.; (iii) the magnitudes of distributed deformation of the matrix are too low to cause juxtaposition of blocks different in P-T paths against each other from km-scale vertical distances. Instead, narrow (few tens of metres-wide) fault/shear and décollement zones formed at locally high pore fluid pressures accumulate the major tectonic transport during high-P metamorphism. They bound several 100s of metres wide tabular tectonic bodies that are coherent. Sedimentary processes at the active continental margin formed the bulk of the mélanges. Tectonically formed block-in-matrix structures within the shear and décollement zones are unrelated to juxtaposition of “exotic” blocks. Universal diagnostic criteria are developed to identify return-RFSC as a mechanism that potentially amalgamates heterogeneous metamorphic terrains by using field data petrology and geochronology.     

Bottom-water Hypoxia Effects on Sediment–Water Interface Nitrogen Transformations in a Seasonally Hypoxic,Shallow Bay (Corpus Christi Bay,TX, USA)

Bottom-water hypoxia effects on sediment–water interface nitrogen (N) transformations in Corpus Christi Bay (TX, USA) were examined using continuous-flow intact sediment core incubations. Sediment cores were collected from three sites in August 2002 (summer hypoxia) and April 2003 (normoxia). Oxygen (O₂) and hydrogen sulfide (H₂S) depth profiles were generated with microelectrodes. Membrane inlet mass spectrometry was used to measure sediment O₂ demand and net N₂ flux and combined with isotope pairing to determine potential denitrification and N fixation. Potential dissimilatory nitrate reduction to ammonium (DNRA) was measured using high-performance liquid chromatography. Sediment O₂ penetration depths ranged from 5 to 10 mm. H₂S ranged from being present in overlying water and throughout the sediment column in August to not detectable in overlying water or sediment in April. Sediment O₂ demand was higher during bottom-water normoxia conditions versus hypoxia. Sediments were a significant source of \textNH_\text4^{\text +} {\text{NH}}_{\text{4}}^{\text{ + }} to overlying water during hypoxia but not during normoxia. Net N₂ fixation was observed at one station in August and all stations in April. Denitrification rates were significantly higher during hypoxia at two of three sites. Potential DNRA was observed during both oxic states, but rates were significantly higher during hypoxia, which may reflect sulfide enhancement and absence of cation exchange with ^\text14 \textNH_\text4^{\text +} ^{{\text{14}}} {\text{NH}}_{\text{4}}^{\text{ + }} . DNRA may contribute to formation and maintenance of bottom-water hypoxic events in this system. These results show that N transformation pathways and rates change when bottom-water O₂ concentrations drop to hypoxic levels. Since south Texas is a semiarid region with few episodic runoff events, these results indicate that Corpus Christi Bay sediments are a N source most of the year, and denitrification may drive N limitation between episodic runoff events.