Deposition gradients across mangrove fringes

Observations in a mangrove in the Whangapoua Harbour, New Zealand, have shown that deposition rates are greatest in the fringing zone between the tidal flats and the mangrove forest, where the vegetation is dominated by a cover of pneumatophores (i.e. pencil roots). Current speeds and suspended sediment concentrations dropped substantially across this zone. Near-bed turbulence within the fringe was substantially lower where the pneumatophore canopy was denser, facilitating the enhanced deposition in this zone. However, the near-bed conditions were not the primary control on the instantaneous sediment concentrations at this site. The total deposition across the different zones was the combined result of the reduced near-bed turbulence inside the vegetation and the larger-scale dynamics over the spatially variable vegetation cover, along with other confounding factors such as changing sediment inputs

Quantifying macrodetritus fluxes from a small temperate estuary

Empirical measurements of estuary-to-coast material fluxes usually exclude the fraction of primary production that is exported as macrodetritus (marine plant litter), potentially leaving a gap in our understanding of the role of estuaries as outwelling systems. To address this gap, we sampled water and suspended material seasonally from the mouth of Pepe Inlet, Tairua Estuary, New Zealand. From samples collected hourly over 24 h, we calculated the lateral tidal fluxes (import, export, net flux) of macrodetritus, particulate and dissolved forms of nitrogen (N) and phosphorus (P). Annually, the inlet was a net exporter of N and P (5145 and 362 kg respectively). However, macrodetritus accounted for 87%). Nevertheless, seasonal pulses in the source and supply of macrodetritus may have consequences for the temporal scales over which this resource subsidy affects receiving ecosystems (e.g. intertidal sandflats). These mensurative investigations are useful to inform estuarine nutrient budgets that quantify the ecosystem services provided by temperate estuaries (e.g. contribution to fisheries food webs)

Mercury Bay Coastal Processes Study: Data Report for 2014 & 2015.

Two month-long hydrographic and sedimentation field campaigns were conducted from July 15th, 2014 to August 13th, 2014 and from April 9th, 2015 to May 8th, 2015 within the Mercury Bay-Whitianga Inlet-northern Whitianga Estuary system to fulfill the field data needs of the Waikato Regional Council towards the goal of producing a hydrodynamic model of Mercury Bay. The purpose of this data report is to summarize the instrument deployment locations,durations, and settings that were used during the field campaign. This document also aims to aid the modeler (or other Waikato Regional personnel) in the locating of the desired instrument data files from the accompanying data discs. In all cases, the raw data files have been provided for each instrument. In the instance that a raw data file is in a data format that requires proprietary software from the instrument manufacture to process, an export of the data files to a universally-readable file format has been included (e.g., .txt, .dat, .csv, .etc.) or to a MATLAB file format when necessary. Clarification has been provided for instruments that export data files without headers

Crustal structure across the Grand Banks–Newfoundland Basin Continental Margin – II. Results from a seismic reflection profile

Author Posting. © Blackwell, 2006. This is the author's version of the work. It is posted here by permission of Blackwell for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 167 (2006): 157-170, doi:10.1111/j.1365-246X.2006.02989.x.New multi-channel seismic (MCS) reflection data were collected over a 565km transect covering the non-volcanic rifted margin of the central eastern Grand Banks and the Newfoundland Basin in the northwestern Atlantic. Three major crustal zones are interpreted from west to east over the seaward 350-km of the profile: (1) continental crust; (2) transitional basement; (3) oceanic crust. Continental crust thins over a wide zone (~160 km) by forming a large rift basin (Carson Basin) and seaward fault block, together with a series of smaller fault blocks eastward beneath the Salar and Newfoundland basins. Analysis of selected previous reflection profiles (Lithoprobe 85-4, 85-2 and Conrad NB-1) indicates that prominent landward-dipping reflections observed under the continental slope are a regional phenomenon. They define the landward edge of a deep serpentinized mantle layer, which underlies both extended continental crust and transitional basement. The 80-km-wide transitional basement is defined landward by a basement high that may consist of serpentinized peridotite and seaward by a pair of basement highs of unknown crustal origin. Flat and unreflective transitional basement most likely is exhumed, serpentinized mantle, although our results do not exclude the possibility of anomalously thinned oceanic crust. A Moho reflection below interpreted oceanic crust is first observed landward of magnetic anomaly M4, 230 km from the shelf break. Extrapolation of ages from chron M0 to the edge of interpreted oceanic crust suggests that the onset of seafloor spreading was ~138Ma (Valanginian) in the south (southern Newfoundland Basin) to ~125Ma (Barremian-Aptian boundary) in the north (Flemish Cap), comparable to those proposed for the conjugate margins.This work was funded by NSF grants OCE-9819053 and OCE-0326714 to Woods Hole Oceanographic Institution, NSERC (Canada) and the Danish Research Council. B. Tucholke also acknowledges support from the Henry Bryant Bigelow Chair in Oceanography at Woods Hole Oceanographic Institution

Crustal structure across the Grand Banks–Newfoundland Basin Continental Margin – I. Results from a seismic refraction profile

Author Posting. © Blackwell, 2006. This is the author's version of the work. It is posted here by permission of Blackwell for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 167 (2006): 127-156, doi:10.1111/j.1365-246X.2006.02988.x.A P-wave velocity model along a 565-km-long profile across the Grand Banks/Newfoundland basin rifted margin is presented. Continental crust ~36-kmthick beneath the Grand Banks is divided into upper (5.8-6.25 km/s), middle (6.3- 6.53 km/s) and lower crust (6.77-6.9 km/s), consistent with velocity structure of Avalon zone Appalachian crust. Syn-rift sediment sequences 6-7-km thick occur in two primary layers within the Jeanne d’Arc and the Carson basins (~3 km/s in upper layer; ~5 km/s in lower layer). Abrupt crustal thinning (Moho dip ~ 35º) beneath the Carson basin and more gradual thinning seaward forms a 170-km-wide zone of rifted continental crust. Within this zone, lower and middle continental crust thin preferentially seaward until they are completely removed, while very thin (<3 km) upper crust continues ~60 km farther seaward. Adjacent to the continental crust, high velocity gradients (0.5-1.5 s-1) define an 80-km-wide zone of transitional basement that can be interpreted as exhumed, serpentinized mantle or anomalously thin oceanic crust, based on its velocity model alone. We prefer the exhumed-mantle interpretation after considering the non-reflective character of the basement and the low amplitude of associated magnetic anomalies, which are atypical of oceanic crust. Beneath both the transitional basement and thin (<6 km) continental crust, a 200-kmwide zone with reduced mantle velocities (7.6-7.9 km/s) is observed, which is interpreted as partially (<10%) serpentinized mantle. Seaward of the transitional basement, 2- to 6-km-thick crust with layer 2 (4.5-6.3 km/s) and layer 3 (6.3-7.2 km/s) velocities is interpreted as oceanic crust. Comparison of our crustal model with profile IAM-9 across the Iberia Abyssal Plain on the conjugate Iberia margin suggests asymmetrical continental breakup in which a wider zone of extended continental crust has been left on the Newfoundland side.This research was supported by National Science Foundation (NSF) grants OCE-9819053 and OCE-0326714, by the National Sciences and Engineering Research Council of Canada (NSERC), and by the Danish National Research Foundation. B. Tucholke also acknowledges support from the Henry Bryant Bigelow Chair in Oceanography from Woods Hole Oceanographic Institution

The Iceland Microcontinent and a continental Greenland-Iceland-Faroe Ridge

The breakup of Laurasia to form the Northeast Atlantic Realm was the culmination of a long period of tectonic unrest extending back to the Late Palaeozoic. Breakup was prolonged and complex and disintegrated an inhomogeneous collage of cratons sutured by cross-cutting orogens. Volcanic rifted margins formed, which are blanketed by lavas and underlain variously by magma-inflated, extended continental crust and mafic high-velocity lower crust of ambiguous and probably partly continental provenance. New rifts formed by diachronous propagation along old zones of weakness. North of the Greenland-Iceland-Faroe Ridge the newly forming rift propagated south along the Caledonian suture. South of the Greenland-Iceland-Faroe Ridge it propagated north through the North Atlantic Craton along an axis displaced ~ 150 km to the west of the northern rift. Both propagators stalled where the confluence of the Nagssugtoqidian and Caledonian orogens formed a transverse barrier. Thereafter, the ~ 400-km-wide latitudinal zone between the stalled rift tips extended in a distributed, unstable manner along multiple axes of extension that frequently migrated or jumped laterally with shearing occurring between them in diffuse transfer zones. This style of deformation continues to the present day. It is the surface expression of underlying magma-assisted stretching of ductile mid- and lower continental crust which comprises the Icelandic-type lower crust that underlies the Greenland-Iceland-Faroe Ridge. This, and probably also one or more full-crustal-thickness microcontinents incorporated in the Ridge, are capped by surface lavas. The Greenland-Iceland-Faroe Ridge thus has a similar structure to some zones of seaward-dipping reflectors. The contemporaneous melt layer corresponds to the 3–10 km thick Icelandic-type upper crust plus magma emplaced in the ~ 10–30-km-thick Icelandic-type lower crust. This model can account for seismic and gravity data that are inconsistent with a gabbroic composition for Icelandic-type lower crust, and petrological data that show no reasonable temperature or source composition could generate the full ~ 40-km thickness of Icelandic-type crust observed. Numerical modeling confirms that extension of the continental crust can continue for many tens of Myr by lower-crustal flow from beneath the adjacent continents. Petrological estimates of the maximum potential temperature of the source of Icelandic lavas are up to 1450 °C, no more than ~ 100 °C hotter than MORB source. The geochemistry is compatible with a source comprising hydrous peridotite/pyroxenite with a component of continental mid- and lower crust. The fusible petrology, high source volatile contents, and frequent formation of new rifts can account for the true ~ 15–20 km melt thickness at the moderate temperatures observed. A continuous swathe of magma-inflated continental material beneath the 1200-km-wide Greenland-Iceland-Faroe Ridge implies that full continental breakup has not yet occurred at this latitude. Ongoing tectonic instability on the Ridge is manifest in long-term tectonic disequilibrium on the adjacent rifted margins and on the Reykjanes Ridge, where southerly migrating propagators that initiate at Iceland are associated with diachronous swathes of unusually thick oceanic crust. Magmatic volumes in the NE Atlantic Realm have likely been overestimated and the concept of a monogenetic North Atlantic Igneous Province needs to be reappraised. A model of complex, piecemeal breakup controlled by pre-existing structures that produces anomalous volcanism at barriers to rift propagation and distributes continental material in the growing oceans fits other oceanic regions including the Davis Strait and the South Atlantic and West Indian oceans

Rip current observations on a low-sloping dissipative beach

Rip currents are the main cause of beach rescues and fatalities. Key drivers of rip current hazard are: (1) fast current speeds; and (2) the exit rate of floating material from inside to outside of the surf zone. Exit rates may vary temporally, such as due to Very Low Frequency (VLF) motions, which have a period on the order of 10 minutes. However, there is little field data to determine the driver(s) of exit rate. Therefore, the aim of this research was to determine rip current circulation patterns, and specifically, determine their relationship to surf zone exits, on a high-energy dissipative beach. Three days of field measurements were undertaken at Ngarunui Beach, New Zealand. Three daily surf zone flow patterns were found: (1) alongshore; (2) surf zone eddy with high exit rate; and (3) surf zone eddy with no exits. There were strong infragravity peaks in energy within the surf zone, at 30-45s, although none at VLF (~10 minute) frequencies. Further research is underway to determine what drove the high surf zone exit rate observed at Ngarunui Beach

Pulsations in surf zone currents on a high energy mesotidal beach in New Zealand

The exchange of material between the surf zone and continental shelf can be driven by pulsations in rip current velocities. However, there is a poor understanding of the relationship of these pulsations to surf zone morphology and material exchange. Moreover, understanding of rip current dynamics has focused mainly on single-barred beaches in an intermediate state, and there have been few studies on high energy beaches. Therefore, this paper undertakes preliminary research on surf zone current velocity pulsations, on a high energy beach in New Zealand. This initial analysis presents results from two days of measurements using Acoustic Doppler Velocimeters and Lagrangian GPS drifters. Drifters revealed pulsations in current velocities on the order of ~0.5-2 m s⁻¹ throughout the surf zone, whether inside a rip current circulation cell or not. More infragravity wave energy was associated with constant pulsations in current velocity, and lower infragravity energy with pulsation bursts, lasting 5-10 minutes, interspersed with periods of relatively constant velocity lasting 15-25 minutes. However, higher wave conditions also reduced the exit rate from the surf zone.5 page(s

Rip current circulation and surf zone retention on a double barred beach

Rip currents have an important control on the exchange of water and advected materials such as sediment and pollutants, between the surf zone and inner shelf. Concurrent in situ Eulerian and Lagrangian (GPS drifter) data of surf zone waves and currents were combined with video data on wave breaking patterns over the inner and outer bars on a high energy, double-barred beach. The data collectively show how the occurrence of wave breaking over the outer bar changes the behavior of a channel rip current, and the exchange process. On both days, there was a prominent clockwise eddy in the surf zone, for which the seaward-heading portion formed a rip current in a well-defined channel rip, incised into the inner bar. Exit rate (measured with drifters) from the surf zone to inner shelf decreased significantly with increased wave breaking over the outer bar, from 71% exits to 6% over the two days. Exit rate appears to be driven by the balance between wave breaking over the inner and outer bars and pulsing of currents within the surf zone. Under higher wave conditions, there were stronger pulsations in surf zone currents and more surf zone exits. However, higher wave conditions caused wave breaking over the outer bar. This breaking increases vorticity around the outside of the surf zone eddy, which increases surf zone retention. This is in contrast to previous studies showing that vorticity is highest at the center of surf zone eddies. Under such conditions, drifter exits were rare, and occurred due to vortex shedding. During lower incident wave conditions, eddy vorticity was lower, and drifters could relatively freely exit the surf zone. This is one of the few studies that investigate surf zone circulation on a high energy, double-barred beach

Regulation of phosphorus bioavailability by iron nanoparticles in a monomictic lake

Dissolved reactive phosphorous (DRP) in lake systems is conventionally considered to predominate over other dissolved P species, however, this view neglects an important set of interactions that occurs between P and reactive iron hydroxide surfaces. This study addresses the coupling of P with dispersed iron nanoparticles in lakes, an interaction that may fundamentally alter the bioavailability of P to phytoplankton. We used difusive gradients in thin flms (DGT) and ultrafltration to study Fe-P coupling in the water column of a monomictic lake over a hydrological year. Fe and P were predominantly colloidal (particle diameters>~5nm<~20nm) in both oxic epilimnetic and anaerobic hypolimnetic waters, but they were both DGT-labile under sub-oxic conditions, consistent with difusion and dissolution of Fe-and-P-bearing colloids within the DGT difusive gel. During peak stratifcation, increases in Fe and P bioavailability were spatially and temporally coincident with Fe nanoparticle dissolution and the formation of a deep chlorophyll maximum at 5–8m depth. These results provide a window into the coupling and decoupling of P with mobile iron colloids, with implications for our understanding of the behaviour of nutrients and their infuence on phytoplankton community dynamics