Sediment-starved sand ridges on a mixed carbonate/siliciclastic inner shelf off west-central Florida

High-resolution side-scan mosaics, sediment analyses, and physical process data have revealed that the mixed carbonate/siliciclastic, inner shelf of west-central Florida supports a highly complex field of active sand ridges mantled by a hierarchy of bedforms. The sand ridges, mostly oriented obliquely to the shoreline trend, extend from 2 km to over 25 km offshore. They show many similarities to their well-known counterparts situated along the US Atlantic margin in that both increase in relief with increasing water depth, both are oriented obliquely to the coast, and both respond to modern shelf dynamics. There are significant differences in that the sand ridges on the west-central Florida shelf are smaller in all dimensions, have a relatively high carbonate content, and are separated by exposed rock surfaces. They are also shoreface-detached and are sediment-starved, thus stunting their development. Morphological details are highly distinctive and apparent in side-scan imagery due to the high acoustic contrast. The seafloor is active and not a relict system as indicated by: (1) relatively young AMS ¹⁴C dates (<1600 yr BP) from forams in the shallow subsurface (1.6 meters below seafloor), (2) apparent shifts in sharply distinctive grayscale boundaries seen in time-series side-scan mosaics, (3) maintenance of these sharp acoustic boundaries and development of small bedforms in an area of constant and extensive bioturbation, (4) sediment textural asymmetry indicative of selective transport across bedform topography, (5) morphological asymmetry of sand ridges and 2D dunes, and (6) current-meter data indicating that the critical threshold velocity for sediment transport is frequently exceeded. Although larger sand ridges are found along other portions of the west-central Florida inner shelf, these smaller sand ridges are best developed seaward of a major coastal headland, suggesting some genetic relationship. The headland may focus and accelerate the N–S reversing currents. An elevated rock terrace extending from the headland supports these ridges in a shallower water environment than the surrounding shelf, allowing them to be more easily influenced by currents and surface gravity waves. Tidal currents, storm-generated flows, and seasonally developed flows are shore-parallel and oriented obliquely to the NW–SE trending ridges, indicating that they have developed as described by the Huthnance model. Although inner shelf sand ridges have been extensively examined elsewhere, this study is the first to describe them in a low-energy, sediment-starved, dominantly mixed siliciclastic/carbonate sedimentary environment situated on a former limestone platform.     

Preliminary attempt to characterize the rotation of seafloor in the Pito Deep area of the Easter Microplate using a submersible magnetometer

Several mechanisms have been proposed to account for the rotation of the nearly north-south abyssal hill fabric formed on the East Pacific Rise north of the Easter Microplate to the nearly east-west trends in the northern microplate interior. Proposed mechanisms include rigid microplate rotation, transform fault – parallel shear, and bookshelf faulting during the transfer of lithosphere from the Nazca Plate to the microplate. We used a submersible magnetometer on a NAUTILE dive program to measure the magnetic vector rotation of a pillow basalt and dike spur near Pito Deep, the present location of the tip of the propagating rift system that created the microplate. Our results, although too limited to draw strong conclusions from, suggest clockwise rotations of the seafloor magnetic vectors inconsistent with the transform-parallel shear model, and larger than can be explained solely by rigid microplate rotation.     

Petrology of the Easter microplate region in the South Pacific

Submersible investigations along the East Rift segments, the Pito Deep and the Terevaka transform fault of the Easter microplate eastern boundary, and on a thrust-fault area of the Nazca Plate collected a variety of basalts and dolerites. The volcanics consist essentially of depleted (N-MORB), transitional (T-MORB) and enriched (E-MORB) basalts with low (0.01−0.1, < 0.7), intermediate (0.12–0.25, 0.7–1.2) and high (> 0.25, > 1.2–2) K/Ti and(La/Sm)_N ratios, respectively. The Fe-Ti-rich ferrobasalt encountered among the N-MORBs are found on the Pito Deep Central volcano, on the Terevaka intra-transform ridge, on the ancient (< 2.5 Ma) Easter microplate (called EMP, comprising the East Rift Inner pseudofaults and Pito Deep west walls) and on thrust-fault crusts. The most enriched (T- and E-MORB) volcanics occur along the East Rift at 25 °50′–27 °S (called 26 °S East Rift) and on the Pito seamount located near the tip of the East Rift at 23 °00′–23 °40′S (called 23 °S East Rift). The diversity in incompatible element ratios of the basalts in relation to their structural setting suggests that the volcanics are derived from a similar heterogenous mantle which underwent variable degrees of partial melting and magma mixing. In addition the Pito seamount volcanics have undergone less crystal fractionation (< 20%) than the lavas from the other Easter microplate structures (up to 35–45%). The tectonic segmentation of the East Rift observed between 23 and 27 °S corresponds to petrological discontinuities related to Mg# variations and mantle melting conditions. The highest Mg# (> 61) are found on topographic highs (2000–2300 m) and lower values (Mg# < 56) at the extremities of the East Rift segments (2500–5600 m depths). The deepest area (5600 m) along the East Rift is located at 23 °S and coincides with a Central volcano constructed on the floor of the Pito Deep. Three major compositional variabilities of the volcanics are observed along the East Rift segments studied: (1) the 26 °S East Rift segment where the volcanics have intermediate Na₈ (2.5–2.8%) and Fe₈ (8.5–11%) contents; (2) the 23 °S East Rift segment (comprising Pito seamount and Pito Deep Central volcano) which shows the highest (2.9–3.4%) values of Na₈ and a low (8–9%) Fe₈ content; and (3) the 25 °S (at 24 °50′–26 °10′S) and the 24 °S (at 24 °10′–25 °S) East Rift segments where most of the volcanics have low to intermediate Na₈ (2.6–2.0%) and a high range of Fe₈ (9–13%) contents. When modeling mantle melting conditions, we observed a relative increase in the extent of partial melting and decreasing melting pressure. These localized trends are in agreement with a 3-D type diapiric upwelling in the sense postulated by Niu and Batiza (1993). Diapiric mantle upwelling and melting localized underneath the 26, 25 and 23 °S (Pito seamount and Central volcano) East Rift segments are responsable for the differences observed in the volcanics. The extent of partial melting varies from 14 to 19% in the lithosphere between 18 and 40 km deep as inferred from the calculated initial (P_o=16kbar) and final melting (P_f=7kbar) pressures along the various East Rift segments. The lowest range of partial melting (14–16%) is confined to the volcanics from 23 °S East Rift segment including the Pito seamount and the Central volcano. The Thrust-fault area, and the Terevaka intra-transform show comparable mantle melting regimes to the 25 and 26 °S East Rift segments. The older lithosphere of the EMP interior is believed to have been the site of high partial melting (17–20%) confined to the deeper melting area (29–50 km). This increase in melting with increasing pressure is similar to the conditions encountered underneath the South East Pacific Rise (13–20 °S).     

A Benthic Terrain Classification Scheme for American Samoa

Coral reef ecosystems, the most varied on earth, continually face destruction from anthropogenic and natural threats. The U.S. Coral Reef Task Force seeks to characterize and map priority coral reef ecosystems in the U.S./Trust Territories by 2009. Building upon NOAA Biogeography shallow-water classifications based on Ikonos imagery, presented here are new methods, based on acoustic data, for classifying benthic terrain below 30 m, around Tutuila, American Samoa. The result is a new classification scheme for American Samoa that extends and improves the NOAA Biogeography scheme, which, although developed for Pacific island nations and territories, is only applicable to a maximum depth of 30 m, due to the limitations of satellite imagery. The scheme may be suitable for developing habitat maps pinpointing high biodiversity around coral reefs throughout the western Pacific.     

Field tests of seasat bathymetric detections

Knowledge of the locations and sizes of seamounts is of great importance in applications such as inertial navigation and ocean mining. The quality and density of bathymetry data in the equatorial regions and the southern hemisphere are, unifortunately, highly variable. Our present knowledge of bathymetry, and in particular of seamount locations and characteristics, is based upon ship surveys, which are both time-consuming and expensive. It is likely that a significant number of uncharted seamounts exist throughout the oceans, and remote-sensing techniques may be the most effective means of locating them.     

Pito and Orongo fracture zones: the northern and southern boundaries of the Easter microplate (southeast Pacific)

The Easter (Rapanui) microplate is a case example of a large dual spreading center system in a region where the fastest seafloor spreading on Earth is occurring today. Recent theoretical models of the tectonic evolution of dual spreading center systems have explored the effects of shear and rigid rotation on the boundaries and internal structure of microplates but the models must be critically constrained by improved relative motion and structural fabric data sets.During the January 1987 Rapanui expedition on the N/O “Jean Charcot” we conducted a Sea Beam/magnetics/ gravity survey of a portion of the microplate boundaries. The method that was used was to fully map selected portions of the boundaries in order to establish precise structural relationships. The northern terminus of the East Rift or eastern boundary of the microplate is expressed as a series of parallel NW-SE trending valleys including what appears to be, with 5890 m depth, the deepest active rift axis mapped in the Pacific today (Pito Rift).The northern end of the Pito Rift merges with an E-W to 083° narrow band of linear faults interpreted to be a transform fault between the Nazca and Easter (Rapanui) plates.The northern triple junction between the Easter (Rapanui), Nazca and Pacific plates is a RFF type with the two transform faults colinear along an approximately E-W direction.The southwestern boundary of the Easter (Rapanui) microplate is marked by a series of en-echelon offsets, outlined by depressions, which merge into an approximately E-W zone where shear must be predominant.The southern triple junction is a RRF junction with an overlapping ridge system.The structural data acquired during the survey provide strong constraints for kinematic models of the microplate. The structural data need to be combined with crustal age determinations in order to derive a model for the evolution of the microplate.     

Multibeam Observations of Mine Burial Near Clearwater, FL, Including Comparisons to Predictions of Wave-Induced Burial

A Kongsberg Simrad EM 3000 multibeam sonar (Kongsberg Simrad, Kongsberg, Norway) was used to conduct a set of six repeat high-resolution bathymetric surveys west of Indian Rocks Beach (IRB), just to the south of Clearwater, FL, between January and March 2003, to observe in situ scour and burial of instrumented inert mines and mine-like cylinders. Three closely located study sites were chosen: two fine-sand sites, a shallow one located in 13 m of water depth and a deep site located in 14 m of water depth; and a coarse-sand site in 13 m. Results from these surveys indicate that mines deployed in fine sand are nearly buried within two months of deployment (i.e., they sunk 74.5% or more below the ambient seafloor depth). Mines deployed in coarse sand showed a lesser amount of scour, burying until they present roughly the same hydrodynamic roughness as the surrounding rippled bedforms. These data were also used to test the validity of the Virginia Institute of Marine Science (VIMS, Gloucester Point, VA) 2-D burial model. The model worked well in areas of fine sand, sufficiently predicting burial over the course of the experiment. In the area of coarse sand, the model greatly overpredicted the amount of burial. This is believed to be due to the presence of rippled bedforms around the mines, which affect local bottom morphodynamics and are not accounted for in the model, an issue currently being addressed by the modelers. This paper focuses specifically on two instrumented mines: an acoustic mine located in fine sand and an optical instrumented mine located in coarse sand.     

A Benthic Terrain Classification Scheme for American Samoa

Coral reef ecosystems, the most varied on earth, continually face destruction from anthropogenic and natural threats. The U.S. Coral Reef Task Force seeks to characterize and map priority coral reef ecosystems in the U.S./Trust Territories by 2009. Building upon NOAA Biogeography shallow-water classifications based on Ikonos imagery, presented here are new methods, based on acoustic data, for classifying benthic terrain below 30 m, around Tutuila, American Samoa. The result is a new classification scheme for American Samoa that extends and improves the NOAA Biogeography scheme, which, although developed for Pacific island nations and territories, is only applicable to a maximum depth of 30 m, due to the limitations of satellite imagery. The scheme may be suitable for developing habitat maps pinpointing high biodiversity around coral reefs throughout the western Pacific.     

Coastal Bathymetry from Hyperspectral Remote Sensing Data: Comparisons with High Resolution Multibeam Bathymetry

We present a large-scale quantitative test of a hyperspectral remote-sensing reflectance algorithm. We show that coastal bathymetry can be adequately derived through model inversions using data from the Airborne Visible-Infrared Imaging Spectrometer instrument. Data are analyzed from a shore-perpendicular transect 5 km offshore Sarasota, Florida at water depths ranging from 10 m to 15.5 m. Derived bottom depths are compared to a high-resolution multibeam bathymetry survey. Model-derived depths are biased 4.9% shallower than the mean of the multibeam depths with an RMS error of 7.83%. These results suggest that the model performs well for retrieving bottom depths from hyperspectral data in subtropical coastal areas in water depths ranging from 10 m to 15.5 m.