OS44A-01 16:00h
Rates and spatial patterns of sediment dispersal across the lower Strickland River floodplain, Papua New Guinea
As part of a NSF Margins Source to sink study, we are exploring the rates and patterns of floodplain sediment vertical accretion on the Middle Fly River and the lower Strickland River, Papua New Guinea. The rivers join at 6 m above sea level where tidal influences are currently weak (except at exceptionally low flow). In response to post-glacial sea level rise both rivers aggraded, but the Strickland, carrying 7 times the sediment load, twice the discharge, and transiting through a much shorter lowland reach, has built a roughly 10 times steeper slope and a much coarser bed than the Fly. We have hypothesized that as a consequence of this difference the rate of sediment loss to the floodplain will be much lower than on the Fly (which is currently about 40% for the Middle Fly Reach). Near the gravel-sand transition, the Strickland floodplain rapidly widens downstream to an average width of approximately 10 km, and the channel has built an elevated meander belt across the roughly 100 km long plain. Recently, sediment laden flood flows have occasionally spilled out of the Strickland through an oxbow and traveled along a small channel into Lake Murray. Lake Murray is directly connected to the Strickland via the Herbert River, a large, deep channel that occasionally reverses and directs Strickland waters to the Lake. To document the spatial pattern of sediment deposition, sediment cores were collected at 11 transects across the floodplain. In addition, samples were collected along the outflow channel, in Lake Murray, and in a separate oxbow fed only by a tie channel. Duplicates and, in places, triplicate core samples were taken to enable independent analysis of sedimentation rates using two methods. Mine tailings introduced into the headwaters of the Strickland since 1990, although of relatively minor importance to the total load of the lower Strickland; provide a distinct elevated Pb and Ag signature in the recently deposited sediment. In over 30 cores, 210Pb analyses have been performed to document sedimentation rates over longer periods. Analyses are still underway, but findings to date show that the two methods of estimating sedimentation rates give roughly similar rates. Across the approximately 2 km active portion of the floodplain, vertical accretion rates on average are on the order of 1 cm/yr, with rates being highest near the channel. Applying this rate uniformly across the entire active floodplain gives an annual sedimentation rate of about 2 million tonnes, which is less than 3% of the estimated annual load of 70 Mt/yr. It is unlikely that further refinement of our analyses will increase this percent to a value similar to the 40% found on the Fly. Although the current sediment loss rate to the Strickland floodplain appears to be relatively low, sediment laden flood pulses from the uplands are substantially damped out by transient flow storage in the main channel and floodplain.
OS44A-02 16:15h
Seasonal Trends in Sedimentation on the Fly River Margin: Shelf, Channel, and Open Clinoform
Recent multibeam mapping and sedimentary studies on the Fly River margin, Papua New Guinea, have revealed across-shelf heterogeneity that likely impacts sediment-transport processes on the margin. Several ancient river valleys with up to 50 m of relief have been discovered and their sedimentary history provides insights to the mechanisms of channel infilling and sedimentation on this margin. Umuda channel, which is located adjacent to the Northern Entrance of the Fly River, exhibits the greatest extent of infilling, and will be a focus of this paper. Kasten ($\sim$2 m) and box ($\sim$50 cm) cores were collected during the energetic Trade-wind season (June to November), the relatively quiescent Monsoon season (December to March), and the Transition from Monsoon to Trade-wind conditions (April/May). $^{210}$Pb, x-radiograph, and grain-size analysis reveal the patterns of sedimentation within Umuda channel, and on the open clinoform to the north. The surface 150 cm of sediment in Umuda channel has relatively uniform excess $^{210}$Pb activity that abruptly decreases to background levels below $\sim$150 cm. These data suggest that the surface 150 cm were deposited relatively quickly within the last $\sim$100 y. Over the course of sampling seasons, a decrease of $\sim$10 cm was observed in the elevation of the seabed within Umuda channel. During the Trade-wind season, seabed elevation was at a maximum. Surface sediments were removed between the Trade-wind and Monsoon seasons, and again between the Monsoon and Transition seasons. This pattern is similar on the open clinoform to the north. We hypothesize that this represents a seasonal transfer of sediment from shallow to deeper water.
OS44A-03 16:30h
Factors Leading to the Spatial Heterogeneity of Sediment-Transport Processes on the Fly River Clinoform, Gulf of Papua
As part of the NSF Source-to-Sink program in the Gulf of Papua, the relationships between sediment transport processes, development of short-term sedimentary deposition, subsequent burial and long-term accumulation are being investigated. Sediment-transport processes were evaluated using both time-series observations from bottom-mounted tripods and repeated water-column profiling throughout the study area during three seasonal cruises. These observations are being evaluated in relation to the morphology of the clinoform that is offshore and to the northeast of the Fly River mouth. Channels across the clinoform feature off the river distributary mouths have been identified as active transport pathways delivering sediment to the deeper portions of the shelf. Tidal currents are focussed in the channels and there is potentially a direct supply of sediment from the river mouth. During the trade wind season, gravity-driven (fluid mud) flows were observed in this environment when waves added a critical component to the shear stress. Processes on the open clinoform differ from those in the channels. The open clinoform to the northeast appears to be a relatively smooth feature, yet measurements show that dramatically different sediment flux rates exist along the feature. On the open clinoform in the central part of the gulf, gravity flows were observed during trade-wind conditions, but were not seen closer to the Fly River mouth under similar energetic forcing. Factors that influence this heterogeneity include convergence of flow due to regional circulation and fresh water input, and variable sediment supply from a number of rivers entering the broad, shallow tidal region.
OS44A-04 16:45h
Characterization of Particulate Organic Matter in the Water Column and Sediments of the Fly River Delta and Clinoform, Gulf of Papua (Papua New Guinea)
Suspended particles and surface sediments were collected from the delta and inner topset region of the Fly River clinoform in order to investigate the sources, transport and fate of particular organic matter (POM) in this region. Total suspended sediment concentrations ranged from 1 to 500 mg/L, with the highest values observed near the sea bed and in the shallow region adjacent to the north channel of the Fly Delta. Suspended particles in surface waters displayed organic carbon contents (OC) of 6.1 +/- 1.3 wt. percent, atomic carbon:nitrogen ratios (C/N) of 23 +/- 6.2 and stable carbon compositions (d13C) of -28.0 +/- 0.4 per mil. The suspended particles collected from bottom waters had similar compositions, although the average POC and PN concentrations were almost two orders of magnitude higher within the benthic boundary layer than in the surface plume. Surface seabed sediments from the northeast region of the delta and inner shelf displayed OC contents of 1.0 to 1.4 wt. percent, C/N ratios of 14 to 24 and d13C signatures of -26 to -27 per mil. In contrast, sediments from the southwest region of the delta and inner shelf displayed OC values of 0.4 to 0.8 wt. percent, C/N ratios of 12 to 24 and d13C values of -24 to -25 per mil. Significant contrasts in average lignin phenol yields were also observed between the sediments from the northeast region of the Fly River delta/inner topset (4.8 +/- 0.3 mg/100 mg OC) and those from the southwest region (3.0 +/- 0.4 mg/100 mg OC). Overall these data indicate that the majority of POM in the water column and sediments of this region of the Gulf of Papua originates from terrigenous C3 plant sources, including vascular plant fragments from the delta and soil organic matter from the Fly River drainage basin. The observed spatial contrasts suggest an efficient export of terrigenous POM to the northeast region of the study area and much inputs to the southwest. These results will be discussed in the context of physical forcings and factors, including particle transport dynamics, sediment deposition and river discharge.
OS44A-05 17:00h
Isotopic Stage 3 Deposition and Stage 2 Erosion of a Clinoform in the Gulf of Papua: Regional Tectonics Versus Eustatic Sea-Level Change
Preliminary analysis of 21 piston cores, 24 gravity cores, 1300 nmi of towed CHIRP and more than 2500 nmi of hull-mounted CHIRP seismic data in the Gulf of Papua (GoP) shows that beyond the modern clinoform lies an older, partially eroded clinoform comprising the middle shelf. Approximately 30-40 m of generally sub-parallel reflectors lie beneath a relatively hard acoustic surface and above a set of obliquely prograding reflectors. Based on stratal geometry, their stiff character when cored, and their present elevation, we deduce that this mid-shelf clinoform prograded two-thirds of the way across the pre-existing shelf in response to a relative sea-level rise during Stage 3, when the rate of tectonic subsidence outpaced the rate of eustatic sea-level fall. Deflection of the transgressive surface from horizontal yields a differential subsidence of about 1 to 2 mm/yr from the peripheral bulge in the south to near the basin depocenter in the north. On the basis of the Chappel and Shackleton eustatic sea level curve, eustatic sea level fell from ~-40 to ~-80 m between 45,000 and 25,000 years BP. During this period, tectonic subsidence is interpreted to have outpaced eustasy to create the observed 30-40 m of accommodation. In the central Gulf mid-shelf region, northeast of the Fly River mouth, this clinoform has been dissected by six major valleys ranging in width from 10 to 15 km, cut into the sub-parallel strata and separated from one another by "mesas" that stand 10-20 m higher than the valleys. The mesas also dip gently to the north, presumably reflecting regional subsidence. Cut into both the valleys and mesas are numerous channels, mostly less than ~200 m wide but as much as 30-40 m deep. We interpret this erosion to be the product of rapid eustatic fall commencing ~25,000 years BP (Stage 2) that lowered sea level to -125 m. As eustatic sea level rose from -120 to +3 m about 7,000 years BP, the former mid-shelf river channels within the river valley were partially filled, presumably by fluvial and estuarine facies. The transgression was so rapid, however, that the valleys themselves remained unfilled except for minimal input from eroding interfluves. During this rise the river mouths backstepped landward and carbonate sediments began to be deposited on the outer shelf, blanketing the former alluvial plain.
OS44A-06 17:15h
Development of the Holocene Clinoform in the Gulf of Papua
The Gulf of Papua (GoP) off the southeastern coast of Papua New Guinea, is a modern example of a marine foreland basin in mid-life. Loading of the Australian plate by New Guinea is creating accommodation space that is being filled by clastic sediment delivered by rivers draining the central mountains. Collectively, the Fly, Turama, Kikori, and Purari rivers discharge $>200 x 10^{6}$ t/yr into the basin. Filling is both longitudinal (Fly and Turama Rivers) and transverse, creating a crescentic shelf, similar to ancient shelves such as the Miocene shelf of the Alpine foreland. To develop a model for GoP-type shallow-marine shelf processes and products we have collected 21 piston cores, 24 gravity cores, 4000 nmi of CHIRP and 1000 nmi of multi-electrode sparker profiles in the GoP, and conducted numerical experiments with NCOM, the US Navy Coastal Ocean Model. Morphologically the GoP shelf is characterized by three treads and two risers, which define two clinoforms; here we focus on the upper Holocene clinoform whose face extends from ~20 to ~80 m water depth, where it downlaps onto an erosional surface etched into topsets of an older clinoform. The face of the Holocene clinoform undulates along strike in a series of subtle promontories and reentrants. Geometrical relationships of stratal packages suggest that the undulations are caused by progradation of sand/mud lobes across the roll-over and oblique transport down the clinoform. Intercalated on- and off-lapping wedges suggest a multi-stage clinoform growth, with upslope sand-rich build-outs alternating with mud-rich toe of slope build-outs. The latest sediments consist of a subjacent heterolithic facies, deposited during a phase of oblique clinoform growth, overlain everywhere by an acoustically transparent facies that forms an on- and down-lapping toe of the wedge. We assume that this transparent facies represent sediment winnowed from topset strata, suggesting that the rate of clinoform growth has slowed. The causes of these variations in clinoform growth and stratal geometry are presently uncertain but may reflect temporal variations in climate, coastal circulation, or longer-term Holocene changes in sea level. Computed annual circulation of the GoP in response to trade wind and monsoon conditions shows that the flow fields are significantly different. During trade winds sediment particle paths on the clinoform top are obliquely offshore to the east. A zone of convergence lies near the 25-m isobath along the clinoform face, where offshore-directed waters on the shelf meet onshore-directed bottom waters climbing the clinoform face, possibly localizing sediment deposition there. During monsoon conditions average bottom flow is landward on the modern clinoform top and minimal over much of the slipface, suggesting that variations in sediment type at the bed level may be circulation related and seasonal.
OS44A-07 17:30h
Quaternary Turbidite Sedimentation in the Moresby Trough, Preliminary Observations from the Papua New Guinea S2S Study Area
Multicores and jumbo piston cores were collected from the Moresby Trough from the R/V Melville in March-April 204 to study patterns of off-shelf sediment flux from the Gulf of Papua to the deep sea over Pleistocene-Holocene timescales. The Moresby Trough represents the primary deep-sea sediment conduit linking the Source to Sink Gulf of Papua study area to the north with the Coral Sea Basin to the south. Core locations were navigated from the surface using dynamic positioning and multibeam bathymetric mosaics collected during the cruise. Selected cores have been analyzed using a multi-sensor core logger, X-radiography, granulometry, and Pb-210 geochronology. Most piston cores collected from the Moresby Trough contain abundant mafic sandy turbidites overlain by fine-grained hemipelagic deposits 1-2 m thick. X-radiographs of multicores reveal mostly bioturbated hemipelagics. However, one core, MV25-0403-24MC, was collected from the axis of a primary channel, and contains at least two fine-grained turbidites within the upper 0.5 m of sediment. The uppermost turbidite contains low but uniform activities of excess Pb-210, and may have been 15-20 cm thick at the time of deposition (i.e., before subsequent disruption by bioturbation), based on fabric and radiochemical evidence. Application of a simple box-model for physical mixing (i.e., during the flow) and subsequent decay suggests that the uppermost turbidite was deposited 70-120 years before present. Bioturbation has since reworked about 60% of the bed, suggesting that such beds will only escape destruction by bioturbation if they are $>$ 10 cm thick, and are deposited more frequently than once per century. Based on these observations, we suggest that gravity flows are presently active in the Moresby trough, and may account for a significant fraction of the hemipelagic drape present in channel systems. However, fabric evidence for these thin, fine-grained beds may be obscured by subsequent bioturbation. Also, at some point in the past, possibly at the onset of the Holocene Transgression, the character of sediment flux to the Moresby Trough shifted from sandy mafic-rich sandy turbidity currents to much finer sediments, delivered downslope by both benthic flows and vertical flux through the water column.
OS44A-08 17:45h
Neogene Evolution of the Mixed Carbonate/Siliciclastic Margin of the Gulf of Papua: Preliminary Results of Spring 2004 PANASH Cruise on the R/V {\it Melville}
A series of NNE trending ridges and troughs, probably Eocene in age, influenced the establishment and development of carbonate platforms during the Oligocene and Miocene on parts of the topographic positive relief in the northeastern part of the Australian plate. This overall physiography of NNE trending troughs, separated by isolated carbonate platforms, can still be observed today east of the modern Gulf of Papua (GOP) shelf edge and as imaged in seismic lines and gravity maps in the sub seafloor of the GOP shelfal areas. Earlier drilling for oil and gas exploration showed that most of these platforms burried under the GOP shelf flourished until the early Miocene and had already drowned by the late early Miocene or the middle Miocene. Those drowned platforms perhaps after being exposed for sometime, subsided through time but remained topographic positive features without being buried by hemipelagic sediments for most of the late Miocene and early Pliocene. Preliminary results of spring 2004 PANASH cruise on the R/V {\it Melville} in the Ashmore, Pandora, and Moresby troughs show that some of the carbonate platforms in the offshore areas had also drowned probably by the late part of the early Miocene, upper (uTe) zone based upon the occurrence of {\it Lepidocyclina (Nephrolepidina)} and {\it Spirockypena margaritatus}, and remained unburied for more than 15 My. Others, however, have remained active, back stepped, and aggraded until today (i.e. Ashmore, Boot, Portlock, Eastern Field Reefs). Its only since the late Pliocene that the majority of siliciclastic sediments, products of erosion of the newly formed PNG mountains, began to quickly infill the different troughs from west to east and at the same time to bury the drowned platforms by more than 1.5 -2.0 km of sediments. The thick late Pliocene and Quaternary sedimentary section, observed under the GOP shelf, represents a vertical succession of sedimentary deposits including at their base deep basin fans which probably underlie some slope depositional environments. These are in turn capped by shelf edge lowstand deltas and aggrading shelf deposits. In the Moresby Trough, the eastern most and deepest basin, the basin floor deposits represent a poorly developed fan, whereas in the northern part of Pandora Trough adjacent to the GOP shelf edge, thick muddy slope deposits were encountered and are currently partially covering an isolated carbonate platform. In the northern part of the Ashmore Trough, the westernmost and shallowest basin, what was likely a lowstand coastal system and shelf edge delta deposited during the Last Glacial Maximum is observed. In conclusion, a lateral east-west distribution in the modern troughs, of basin, slope, and shelf edge environments was encountered that corresponds to the vertical stack of similar successions imaged and drilled in the sub-seafloor of the GOP shelf itself.