OS51B-1298 0800h
Sediment dispersal and deposition on the Waiapu River Shelf, N.Z., implications for sediment transport mechanisms and event preservation
Sediment dispersal off an extremely high-yield river, the Waiapu River, New Zealand, is examined using a suite of gravity cores collected on the adjacent highly-energetic continental shelf. High $^{210}$Pb accumulation rates are observed, coincident with high surficial clay content, on the mid-to outer-shelf. The nearshore region sediments are composed of discrete physically mixed layers, trending to high (1.5 to 3 cm yr$^{-1}$) $^{210}$Pb accumulation rates on the mid-shelf and lower accumulation rates ($<$1 cm yr$^{-1}$) on the outer-shelf and submarine canyon area. X-radiographs indicate that sediments generally either bypass the nearshore or are partially preserved in event layers. These event layers are observed less frequently on the mid- and outer-shelf interspersed with homogenous sediments or sediments that have been bioturbated. The presence of bioturbation in the sediment cores seems to correlate with distance from the Waiapu River mouth, not bathymetry, indicating that the area around the river mouth is too physically disturbed to allow benthic communities to flourish. This may also indicate that riverine deposition and dispersal mechanisms may have more impact on the shelf than wave energy. Seasonal variations in sediment depositional patterns are evident from $^7$Be geochronology. Surface sediments collected during an annual period of high river discharge (June through September) reveal that sediment deposition on the entire shelf is sparse and ranges from the nearshore to the shelf break. A contrasting pattern of deposition was observed during the stormy season (April through June) when $^7$Be was found on most of the shelf and even beyond the shelf break. High sediment accumulation rates and the energetic hydrodynamic regime of the Waiapu River shelf imply that a variety of sediment transport mechanisms deliver sediments to the shelf and slope. Furthermore, these high accumulation rates and the low observed benthic activity may allow event layers to be preserved in the sediments.
OS51B-1299 0800h
Distribution and Composition of Terrigenous Organic Matter in Surface Sediments from the Fly River Delta and Clinoform (Papua New Guinea)
The distribution of terrigenous organic matter in surface sediments from the delta and inner clinoform region off the Fly River was investigated by measuring organic carbon:nitrogen (C/N) and stable carbon (d13C) isotopic ratios, as well as the concentrations and stable isotopic compositions of lignin-derived CuO oxidation products. The average C/N and d13C ratios of the sediments from this region ranged from 11 to 70 and from -23 to -27 per mil, respectively. Lignin phenol yields ranged from 0.2 to over 9 mg/100 mg OC, with an average of 3.4 mg/100 mg OC. Most of the differences in C/N ratios and lignin yields occur within a narrow range of d13C compositions (-27 to -25 per mil), which suggest that most of the variability encountered within the surfaces sediments is due to variations in the relative abundance of vascular plant detritus (high C/N ratios and high lignin yields) and soil organic matter (lower C/N ratios and lower lignin yields). Our results suggest negligible contributions from marine organic carbon in surface sediments of the northeast region of the delta. In contrast, the most enriched d13C values and lowest lignin yields, both indicative of marine inputs, were measured in coarse sands collected in the southwest region of the Fly River delta and inner clinoform. The relative abundances of different lignin phenol classes are consistent contributions from woody and non-woody angiosperm sources. The stable carbon isotopic compositions of individual lignin phenols indicate a predominant C3 source, with no evidence for contributions from C4 vascular plant sources. Overall, these results point to C3 angiosperm vegetation from the tropical forest in the drainage basin and delta region of the Fly River as the predominant sources of terrigenous organic matter in this part of the Gulf of Papua. The spatial distribution of these two pools of organic matter will be interpreted in the context of sediment texture, mineral surface area and particle distribution processes in this region of the gulf.
OS51B-1300 0800h
Sedimentological Records of Organic Matter Accumulation in the Inshore Region of the Fly River Clinoform (Papua New Guinea)
Thirteen kasten cores collected from the northeast and southwest regions of the clinoform topset off the Fly River Delta and from the Umuda Channel were selected for elemental (organic carbon, nitrogen), mineral surface area (SA), stable isotope (d13C and d15N) and biomarker (lignin phenols) analyses. The depth of the cores ranged from 30 to over 250 cm and represent variable periods of depositional history. The average organic carbon content (OC) of sediments ranged from 0.6 to 2 wt. percent, depending on the site location. The highest OC levels were observed in the northeast region off the Fly River Delta, while the lowest levels were found in cores located in the southern offshore region. Average molar organic carbon:nitrogen ratios ranged from 10 to 18, with the lowest C/N ratios found in the sites located to the south of the delta. Average d13C and nitrogen d15N signatures ranged from -26 to -25 per mil and from 1.4 to 2.5 per mil, respectively. Average OC/SA ratios for the sediments in these cores ranged from 0.4 to 1.7 mg C/m2, with the lowest values measured in samples from the Umuda Channel and the most distal site in the northeast region of the topset. Overall, these compositions indicate organic matter accumulation in all these sites is dominated by the burial of terrigenous derived soil organic matter. Down core peaks in C/N and OC/SA ratios indicate the presence of discrete layers enriched in vascular plant detritus. Low OC/SA ratios indicative of extensive decay were only measured in selected sites and in specific horizons. In contrast, most samples were characterized by "normal" organic carbon loadings that are consistent with the active burial and effective preservation of terrigenous organic matter in this region of the Fly River Delta and clinoform. There is little evidence for the preservation of marine-derived organic matter in these deposits. These data will be further interpreted in the context of additional sedimentological data (X-rays, grain size, radioisotope activities).
OS51B-1301 0800h
Sediment Flux to the Gulf of Papua Continental Slope Using Pb-210 Geochronology, Source to Sink Papua New Guinea Focus Area
The Gulf of Papua is a mixed siliciclastic/carbonate environment where terrigenous sediments from New Guinea rivers are supplied to shelf and slope where they mix with carbonate sediments from pelagic and platform sources. During a March-April 2004 cruise of the R/V Melville, we recovered multicores from a range of depositional settings on the upper and middle slope of the Gulf of Papua. Cores were navigated using dynamic positioning, and a multibeam bathymetric mosaic collected during the cruise. These multicores have been examined using Pb-210 radiochemistry, x-radiographs and granulometry, in order to constrain carbonate and terrigenous sediment flux to the slope over century timescales. Preliminary results support the hypothesis that terrigenous sediments are reaching the slope from the northeast Gulf of Papua shelf break, where a core from a shelf-edge clinoform revealed an apparent Pb-210 accumulation rate of 0.4 cm/y. Southwestwards along the upper slope, toward the northern edge of the Great Barrier Reef, sedimentation rates appear to be lower, and carbonate platform influence appears to increase. In the Moresby Trough, a major sediment conduit from the upper Gulf of Papua slope to the Coral Sea Basin, we have documented modern turbidite sedimentation through x-radiographs and Pb-210 analyses of one multicore. The depositional sedimentary fabric of the Moresby Trough turbidite contrasts sharply with the intensely bioturbated fabric of hemipelagic sediments identified elsewhere.
OS51B-1302 0800h
Petrology and Provenance of Turbidite Sands From the Pandora and Moresby Troughs, Source to Sink Papua New Guinea Focus Area
The Moresby and Pandora Troughs of the northern Coral Sea are components of the deep-sea depositional system that is the ultimate sink for the Source to Sink Papua New Guinea Focus Area. Cores collected from the R/V Melville during March-April 2004 reveal abundant and varied turbidites deposited in these troughs during the Quaternary. Constraining the source regions for these terrigenous sands through mineralogical and textural analysis is the primary focus of this study. Two jumbo piston cores were analyzed for this study, MV25-0403-22JPC from the Moresby Trough (southeast of the Gulf of Papua), and MV26-0403-66JPC from the Pandora Trough (south-central slope of the Gulf of Papua). Cores were analyzed using a multi-sensor core logger onboard ship. Point counts of lithic grains as well as standard Quartz-Feldspar-Lithic (QFL) percentages in thin section were prepared for six sand samples from comparable depths in each of these cores. For comparison to terrigenous source areas, identical analyses will be conducted on samples from the Fly and Strickland rivers. Cathodoluminescence (CL), energy dispersive spectroscopy (EDS), and standard petrographic techniques were used for grain identification. Both cores contain thin-bedded sandy turbidite packages, although Moresby Trough core JPC22 appears to be generally finer-grained. QFL percentages from 22JPC are 13%-65%-23%, respectively, and plagioclase/ total feldspar ratios are near .90. These mineralogical characteristics are generally typical of back-arc settings. QFL percentages from Pandora Trough core JPC66 are 69%-14%-17%, and plagioclase / total feldspar ratios are near .47, characteristic of trailing-edge, passive-margin basins. The concentration of brown and green hornblende is 2 to 3 times greater in 22JPC than in 66JPC. Isotropic volcanic glass is present in both cores. Based on these preliminary observations, we suggest that the apparent contrast in sand mineralogy between 66JPC and 22JPC reflects contrasting source areas and fluvial delivery systems. If this is the case, then the Moresby Trough receives turbidite sands derived mostly from volcanic/collision margin highlands of SW PNG, and the Pandora Trough receives more quartzose sands from the Fly/Strickland system, more akin to a trailing-edge margin.
OS51B-1303 0800h
Uppermost Pleistocene Sea-Level Transgression across a Last Glacial Maximum Mixed Carbonate/Siliciclastic Coastline, Modern Gulf of Papua Shelf Break in the Northern Ashmore Trough
This research focuses on the analyses of an 8.5 m-long piston core, MV26-0403-73JPC, recovered during a Spring 2004 R/V Melville cruise, in the Gulf of Papua. The core was taken at 138 m of water depth on the modern shelf break of the northern Ashmore Trough. Sediments in the core represent a deepening upward sequence deposited in a mixed carbonate/siliciclastic setting during the time interval from 17,680 $\pm$ 370 to 10,980 $\pm$ 360 calendar years BP based on six calibrated $^{14}$C dates. Four main units are identified based upon the downcore sedimentologic, mineralogic, and micropaleontologic variations. The lowermost Unit D (8.5 to 6.0 mbsf) is approximately 17.5-17 cal. kyr BP in age and consists of a fining upward sequence ranging from bioclastic gravel with abundant ooids to fine sand with other coated grains, bioclasts, and angular quartz grains. The carbonate content of Unit D average 80% whereas the $<$ 63 $\mu$m fraction is rich in quartz. The benthic foraminiferal assemblage is dominated by {\it Calcarina} spp. indicative of a shallow water open marine environment. The overlying Unit C, (6.5 to 3.0 mbsf) consists of silty sand with increased quartz and lower carbonate contents (15%-40%). The age of the top is approximately 16.5 cal. kyr BP. Unit B (3.0 to 0.5 mbsf), which spans the interval up to 12 cal. kyr BP, records an important transition marked by the disappearance of coated grains, an increase of the silt-mud fraction, and the appearance of planktonic foraminifers. The uppermost 0.5 m, Unit A, mostly indurated and ranging in age from 12 to 11 cal. kyr BP, consists of bioclastic packstone-wackestone with {\it Halimeda} segments, red algae, more abundant planktonic foraminifers, and quartz grains. The carbonate content increases to 75% and high-Mg calcite and quartz are the main components of the fine fraction. Calcarinids become a minor component among the benthic foraminifers whereas {\it Amphistegina} spp. (flattened form) dominates, pointing to an outer shelf depositional setting. These observations indicate that the coastline reached the present-day shelf break during the LGM and that the sediments record two distinct pulses of sea-level rise. The first pulse, occurring between 16.5 and 12 cal. Kyr BP and probably representing MWP 1A, corresponds to a backstepping of the shoreline and the first major reflooding of the modern outer shelf, with the shoreface deposits of Unit C overlain by the inner shelf sediments of Unit B. The second melt water pulse (MWP 1B) is recorded in this core ca. 11 cal. kyr BP as a deepening of depositional facies to the outer shelf deposits of Unit A. Holocene sediments have been bypassing the shelf edge at this location.
OS51B-1304 0800h
Initial Chronostratigraphic and Paleoclimatic Framework for Sediment Cores From the Mixed Siliciclastic-Carbonate System of the Gulf of Papua New Guinea
The goal of the MARGINS Source-to-Sink (S2S) study in the Papua New Guinea (PNG) focus area is to develop an integrated and quantitative understanding of sediment production, transport, and accumulation in the Gulf of Papua. The PNG S2S focus area was selected for several reasons including large production and rapid transport of sediments in a mixed siliciclastic-carbonate system, clearly defined sources and sinks, variable fluxes over a range of timescales, and a relatively pristine environment. A total of 76 gravity, box, and jumbo piston cores were collected aboard the R/V Melville in spring 2004 from the slope and basin region of the Gulf of Papua. These cores will be used to understand sedimentation dynamics (production, transport, and accumulation) in this region on a range of time-scales and in the context of changes in sea level and climate. The PNG study area is centrally located in the Western Pacific Warm Pool and experiences strong seasonal changes in rainfall associated with the Monsoon which affects sediment delivery to the Gulf. The PNG region is also dramatically affected by the El Nino-Southern Oscillation phenomenon. We report here preliminary stratigraphic results from a number of jumbo piston cores collected from the Eastern Plateau region. This topographically elevated feature was targeted to avoid the rapid siliciclastic sedimentation that dominates the deeper troughs and to sample carbonate-rich pelagic sediments for purposes of generating continuous isotope and microfossil-based records of climate change. Even at sample locations most distant from the major riverine sources and removed from direct transport pathways, the carbonate content of cores rarely exceeds 40%, attesting to the huge input of terrestrial siliciclastic sediments to the Gulf of Papua over the last several glacial-interglacial cycles. Preliminary foraminiferal oxygen isotope stratigraphies indicate sedimentation rates of $\sim$ 6 cm/kyrs at plateau sites expected to have the lowest bulk accumulation rates in the study area. Sedimentation rates should be higher and more variable in slope and trough cores closer to the major siliciclastic sources. The chronostratigraphic record provided by oxygen isotopes in the plateau cores is currently being used to develop the framework by which age models can be applied to the complete set of cores, thus facilitating our overall objectives of assessing the dynamics of sedimentation in this important mixed-sediment region.
OS51B-1305 0800h
Brunhes Evolution of the Belize Mixed Siliciclastic Carbonate Margin: Source to Sink Dynamics Relative to Glacial-Interglacial Sea Level Fluctuations
Mixed siliciclastic carbonate systems are developed on continental margins where significant volumes of terrigenous sand and mud interact with shallow water tropical carbonate barrier reefs and banks. These systems represent highly dynamic environments, directly influenced by sea level fluctuations and climatic changes. In current sequence stratigraphy models, neritic carbonates flourish, exporting sediments to the basins during sea level highstands, while siliciclastic input to the basins is maximum during lowstands. In contrast, this study demonstrates that siliciclastic and carbonate sedimentation is not in phase with late Quaternary glacial-interglacial lowstand/highstand cycles on the central Belize margin. This study is based on the analyses of a well-dated continuous 37.7 m long piston core, MD02 2532, collected 3 km offshore the central Belize Barrier reef in the distal slope of Gladden Basin at 333 m of water depth by the R/V Marion Dufresne (IPEV). A robust chronology is established using high resolution planktic oxygen isotope stratigraphy, anchored by several nannofossil stratigraphic and tephrochronologic markers, as well as radiocarbon ages in the upper part of the core, and demonstrates that the core represents most of the Brunhes (about 0.7 My). The upper 26m of MD02 2532 penetrated five subunits of the distal portion of a sedimentary wedge, likely corresponding to interglacial intervals from Marine Isotope Stage (MIS) 11 to MIS 1 (Holocene), while the lower 11.7m recovered the upper portion of an underling subparrallel reflector seismic unit. Bulk carbonate percent values cyclically vary down the core; low values of 40 to 50% typically occur during the interglacial to glacial transitions, whereas high values of 70 to 80% typically occur at the glacial to interglacial transitions. The Holocene interval is characterized by the highest values observed in the entire core, reaching 82-83%. Calculated siliciclastic fluxes to the slope are greatest during early sea level regressions, whereas the lowest values occur during transgressions and early highstands. In contrast, carbonate fluxes are maximum during transgressions and early highstands. These results show that the late Quaternary is an ideal interval to determine the timing relative to sea level fluctuations of siliciclastic and carbonate sediment maximum fluxes into slope and basinal environments and, therefore, to test some basic sequence stratigraphic principles, because the timing and amplitude of the late Quaternary sea level fluctuations are independently well established.