J.P. Walsh

Contrasting styles of off-shelf sediment accumulation in New Guinea

Abstract The accumulation of river sediment (and associated materials) off the continental shelf is widely acknowledged to have been significant during the last glacial maximum, when shelf accommodation space was reduced in many areas. However, the importance of off-shelf sinks during modern sea-level conditions is largely unknown. This study examined off-shelf sediment accumulation for two dispersal systems, the Gulf of Papua and the Sepik margin, using radionuclide (210Pb, 234Th), sediment texture and structure data. These study areas were selected because they have a similar source of sediment (the Papuan Fold Belt) with dramatically different margin morphologies. The Gulf of Papua is part of a foreland basin that lies in the northern Coral Sea, and receives over 3×108 tons of sediment annually from numerous rivers. The majority of this load accumulates on the shelf, and a small fraction (

Sediment slump likely caused 1998 Papua New Guinea tsunami

Two major marine surveys off northern Papua New Guinea (PNG) earlier this year now suggest, when survivors reports are taken into account, that last summers disastrous tsunami there was caused by a sediment slump 25 km offshore. The slump was probably the result of seabed shaking from an earthquake. Not only was a sediment slump, or submarine landslide, responsible for the tsunami, according to the data, but the magnitude and wave-height distribution of the tsunami along the coast were the result of focusing by local seabed morphology. n nThe conclusions are based on new off-shore bathymetry, remote operated vehicle (ROV) dive investigations, the time delay between the source earthquake and when the tsunami struck, computer simulation models, and earthquake aftershock distribution. The most critical evidence is in survivors accounts of the timing of the tsunami relative to the initially felt earthquake and aftershock [see Davies, 1998a].

Observations of sediment flux to the Eel continental slope, northern California

Abstract Sediment flux on the northern California slope off the Eel River was observed using time-series sediment traps between September 1995 and January 1997. A mooring that held three sediment traps (at 60, 220 and 435 m) and three current meters/transmissometers was placed in 450 m water depth. Thirty-three sampling periods of 10–16 days in length covered 394 days. All samples were analyzed for total mass, calcium carbonate, combustible matter and biogenic silica, and from these, lithogenic fluxes were determined. Total sediment-trap fluxes ranged from 0.1 to 24.2 g m −2 d −1 , and annually averaged 1.1, 4.5 and 11.8 g m −2 d −1 at the top, middle and bottom traps, respectively. Lithogenic fluxes were 53, 70 and 83% at the top, middle and bottom traps. The middle trap experienced the greatest absolute variability as a result of intermediate-nepheloid-layer activity; 6 of the 33 intervals accounted for more than 50% of the flux. River, wind, wave, current, transmissometer and satellite data reveal that the magnitude of sediment flux was controlled by a variable mix of shelf sediment resuspension, river discharge, and margin circulation.

The effect of bathymetry on tsunami characteristics at Sisano Lagoon, Papua New Guinea

The 1998 Papua New Guinea tsunami was greater than expected from its earthquake magnitude. The area of significant impact was small, approximately a 30 km stretch near the mouth of Sissano Lagoon, Papua New Guinea. To explain the localized nature of the event, a submarine landslide has been conjectured to be responsible. Our study indicates that offshore bathymetry is critical to predicting tsunami coastal behavior. Model runs with newly obtained bathymetric data indicate that an earthquake fault source combined with the existing seafloor geometry may also explain the concentrated tsunami. Although the definitive cause of the Papua New Guinea tsunami remains uncertain, local bathymetry had a notable effect on the wave behavior.

Mississippi Delta mudflow activity and 2005 Gulf hurricanes

Gravity-driven sediment flows can be important mechanisms for transporting sediments and solutes rapidly across continental margins, and therefore may have important impacts on benthic ecosystems and geochemical cycling. Also, infrastructure damage can result from these events, as was the case when mudflow activity during Hurricane Ivan in fall 2004 caused pipeline damage (U.S. Minerals Management Service (MMS) press release on 8 October 2004; http://www.mms.gov/ooc/press/2004/press I 008a.htm).

A record of recent change in terrestrial sedimentation in a coral-reef environment, La Parguera, Puerto Rico : A response to coastal development?

Increased sediment flux to the coastal ocean due to coastal development is considered a major threat to the viability of coral reefs. A change in the nature of sediment supply and storage has been identified in a variety of coastal settings, particularly in response to European colonization, but sedimentation around reefs has received less attention. This research examines the sedimentary record adjacent to a coastal village that has experienced considerable land-use change over the last few decades. Sediment cores were analyzed to characterize composition and sediment accumulation rates. Sedimentation rates decreased seaward across the shelf from 0.85 cm y(-1) in a nearshore bay to 0.19 cm y(-1) in a fore-reef setting. Data reflected a significant (up to 2x) increase over the last approximately 80 years in terrestrial sediment accumulating in the back-reef setting, suggesting greater terrestrial sediment flux to the area. Reef health has declined, and increased turbidity is believed to be an important impact, particularly when combined with additional stressors.

Demise of a submarine canyon? Evidence for highstand infilling on the Waipaoa River continental margin, New Zealand

[1]xa0Submarine canyons are major geomorphologic features on the Earths surface. Their formation has received considerable debate, but their demise has received less attention. Research of modern canyons with cores and moorings has documented active sediment transport and deposition, but extrapolation of these local observations over larger areas is precluded by complex canyon geomorphology. High-resolution multibeam and chirp data presented here provide convincing evidence of an infilling canyon head on the Waipaoa River margin of New Zealand. Tens of meters of Holocene sediment have accumulated on the outer shelf and in Lachlan canyon as a result of off-shelf sediment transport. Regardless of the ultimate fate of this system over geological time scales, this research demonstrates highstand sedimentation as a possible mechanism for canyon burial and cause of canyon demise, which has important implications for the evolution of canyons globally.

Shoreline Change along Sheltered Coastlines: Insights from the Neuse River Estuary, NC, USA

Coastlines are constantly changing due to both natural and anthropogenic forces, and climate change and associated sea level rise will continue to reshape coasts in the future. Erosion is not only apparent along oceanfront areas; shoreline dynamics in sheltered water bodies have also gained greater attention. Additional estuarine shoreline studies are needed to better understand and protect coastal resources. This study uses a point-based approach to analyze estuarine shoreline change and associated parameters, including fetch, wave energy, elevation, and vegetation, in the Neuse River Estuary (NRE) at two contrasting scales, Regional (whole estuary) and Local (estuary partitioned into eight sections, based on orientation and exposure). With a mean shoreline-change rate of –0.58 m yr−1, the majority (93%) of the NRE study area is eroding. Change rates show some variability related to the land-use land-cover classification of the shoreline. Although linear regression analysis at the Regional Scale did not find significant correlations between shoreline change and the parameters analyzed, trends were determined from Local Scale data. Specifically, erosion rates, fetch, and wave exposure increase in the down-estuary direction, while elevation follows the opposite trend. Linear regression analysis between mean fetch and mean shoreline-change rates at the Local Scale provide a first-order approach to predict shoreline-change rates. The general trends found in the Local Scale data highlight the presence of underlying spatial patterns in shoreline-change rates within a complex estuarine system, but Regional Scale analysis suggests shoreline composition also has an important influence.

Analyzing Estuarine Shoreline Change: A Case Study of Cedar Island, North Carolina

Abstract Continued climate change, sea-level rise, and coastal development have lead to concern about shoreline dynamics beyond oceanfront areas, encompassing more sheltered coastal water bodies such as estuaries. Because estuaries are critically important ecosystems, understanding coastline changes in these areas is necessary to evaluating resource risks. A transect-based approach is commonly used to quantify shoreline change on linear (i.e., ocean) shorelines; however, due to the complex morphology of the study area, a point-based approach was developed and applied in this study. Shoreline-change rates and additional parameters (i.e., wave energy and shoreline composition) were determined using 1958 and 1998 aerial photography and available datasets. From this data, the average shoreline change in the study area is −0.24 m yr−1, with 88% of the shoreline eroding. Of the parameters analyzed, shoreline composition appears to have an important control on shoreline erosion, whereas wave energy is not significantly correlated with shoreline-change rates.

Understanding sediment transfer from land to ocean

A new research program focusing on sediment dispersal across the active margin of the New Zealand east coast has provided the foundation for a holistic understanding of the transport and fate of terrestrial materials in the coastal ocean. Field studies began in January 2005 with two acoustic mapping and shallow seabed sampling expeditions to the shelf and slope off the Waipaoa River (Figure l), and in February 2006, the specially designed French research vessel (R/V) Marion Dufresne II collected seven long (up to 25 meters) piston cores from the study area for stratigraphic control. Both the 2005 and 2006 expeditions are part of the U.S. National Science Foundation (NSF) MARGINS Source-to-Sink (S2S) initiative.