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AGU/ASLO SPECIAL PROJECT TROPICS SESSION, 24-28 JANUARY 2000 SAN ANTONIO, TEXAS This session highlights processes controlling riverine sediment, water, and chemical fluxes and fate in the tropical coastal ocean. About half the contributions reflect research efforts off the island of New Guinea, where massive material fluxes and contrasting oceanographic conditions affect a variety of oceanographic phenomena such as carbon preservation, chemical budgets of the Western Pacific, and continental margin development. Studies in other tropical and subtropical settings demonstrate the anthropogenic influence on material fluxes and mechanisms of near shore sediment trapping and offshore productivity. The present importance of tropical rivers in contributing to global material budgets is contrasted to that during the last glacial maximum.
Material Flux and Fate in The Tropical Coastal Ocean Abstracts Early Diagenetic Processes in Gulf of Papua Inner Shelf Muds Robert C. Aller
(1-516-632-8746; raller@notes.cc.sunysb.edu) Marine Sciences Research Center, State University of New York, Stony Brook, NY. 11794-5000, United States Mobile mud belts represent a major class of diagenetic systems characterized by extensive and frequent physical reworking of bottom deposits underlying oxygenated waters. These systems function as fluidized bed reactors, and often maintain suboxic, nonsulfidic conditions while continuously incorporating reactive organic matter and oxidized sedimentary debris. Previous studies indicated that surface sediments (~20 cm) over large regions of the Gulf of Papua are suboxic despite high rates of remineralization and a general lack of bioturbation (Alongi, 1995). Subsequent sampling of sediment over the upper (~1 m at three sites (~10-50 m depth) demonstrate properties consistent with the mobile mud belt diagenetic model for the Gulf of Papua inner shelf. Oxygen penetrates only a few mm into surface sediment. Remineralization rates are relatively high (CO2 ~0.1-0.3 mM d-1) and show little attenuation with depth in deposits. Excess 234Th is present to ~10 cm. Although dissolved Fe is not particularly high (<100 m M) in cores, short-term incubation of sediment indicates the potential for rapid release and build-up of Fe+2. Pyrite represents only ~10-20% of diagenetically reduced Fe and C/S ratios are ~5-6, reflecting generally nonsulfidic suboxic conditions despite high rates of remineralization. There is little evidence of macrofaunal or meiofaunal activity. Bacterial populations have high cellular rRNA contents with a large proportion of cells dividing (5-50%) throughout the upper ~1 m with little depth attenuation. This suggests high rates of bacterial production indicative of abundant reactive substrate supply and metabolite flushing. The Gulf of Papua inner shelf apparently represents a tropical diagenetic system comparable to mobile muds along the northeast coast of South America.
Onshore Sediment Flux in the Ganges-Brahmaputra Lower Delta Plain: Is it Significant? Mead A Allison
1 (504-862-7386; malliso@mailhost.tcs.tulane.edu) 1Tulane University, Department
of Geology, New Orleans, LA 70118, United States Vibracores and auger samples were collected from the lower (tidal) delta plain of the Ganges-Brahamaputra River in Bangladesh to examine whether the area is a significant sink for riverine sediments and organic carbon. While the modern river mouths are known to be a site of shoal-island accretion, older areas to the west have been thought to be moribund due to the ongoing eastward shift of distributary channels. This 300-km-wide (15,000 km2) zone of low-elevation (0.9-2.1 m) reclaimed land and mangrove swamp (Sunderbans) has accreted since maximum sea level transgression at 7,000 yBP. Sediments are laterally homogenous and show an environmental succession upcore from cross-bedded, subaqueous shoal sandy silts to tidally bedded silts to bioturbated (mangrove rooted) clayey silts. Radiocarbon ages demonstrate that the mangrove sequence has formed area-wide since about 3,000 yBP. Modern sediment accumulation is indicated by the presence of Cs-137 in the mangrove layer to depths suggesting rates of 2-12 mm/yr. Radiocarbon dating of buried mangrove stumps and peaty layers suggest these rates have been present throughout the period of mangrove forestation. The relative temporal stability of the mangrove environment indicates that accumulation may be keeping pace with regional subsidence. The area is only inundated during the May-August period of onshore monsoonal winds that generate coastal setup, and by tropical cyclones. We hypothesize that the primary mechanism of sediment delivery is by onshore flux during these events from turbid river water entrained in the westward-flowing coastal current.
Preliminary Carbon Mass Balance in the Gulf of Papua, Papua New Guinea, Project TROPICS, 1991-1999. G. J.
Brunskill, K. A. Burns, D.Alongi, I. Zagorskis, and J. Pfitzner Australian Institute of Marine Science, PMB 3, Townsville, Queensland 4810, Australia The Fly, Purari, and Kikori Rivers discharge large amounts of water, sediment, and organic matter from wet tropical catchments into the broad continental shelf of the Gulf of Papua. Most of this water, sediment, and organic matter is retained on the inner shelf (<60 m water depth), and large amounts of organic matter must be imported from the Coral Sea to balance the large respiration rate on the shelf. Our carbon mass balance incorporated annual river inputs of organic and inorganic carbon, primary production on the shelf, water column and benthic respiration, burial rates in sediments, and sediment trap carbon fluxes at the base of the continental slope. Annual river inputs of dissolved and particulate organic matter, plus annual primary production on the continental shelf, were only 50% of measured water column and benthic respiration rates across the shelf. Sediments on this shelf are a sink for water column DOC, and this carbon is retained in sedimentary microbial biomass. This mass balance suggests that advection of organic matter in Coral Sea water onto the continental shelf of the Gulf of Papua must be equivalent to shelf photosynthetic carbon fixation and river inputs. This continental shelf appears to be strongly heterotrophic, with P/R = 0.5. This large microbial respiratory engine affects the salinity gradient of alkalinity, and provides a flux to the atmosphere of about 12 Moles CO2 m-2 yr-1. Some refractory terrestrial & mangrove organic matter (about 2.5% of river inputs plus primary production) escapes this combustion engine and is buried in intertidal mangrove mud and a crescentic band of inner shelf mud in the 20-60 m depth zone. Burial rates of carbonate carbon on the shelf require about 24% of river alkalinity inputs, and the remainder is available for export to the atmosphere or oceanic circulation. Carbon fluxes to sediment traps at the base of the continental shelf slope and trough appear to be dominantly marine and pelagic.
Temporal and Spatial Variations in Storm Derived Material Fluxes from Small Subtropical Watersheds: Natural and Anthropogenic Signatures Eric Heinen De Carlo
(808-956-6473; edecarlo@soest.hawaii.edu) School of Ocean and Earth Science and Technology, University of Hawaii , Honolulu, HI 96822, United States High-intensity but often short duration orographic rainstorms result in rapidly changing hydrographs in streams of subtropical islands. Because the rugged topography of the principal Hawaiian islands leads to substantial erosion, rainstorms often deliver significant pulses of terrestrial material to the nearshore ocean. We have established a network of stations in streams of high-relief watersheds of O'ahu, Hawaii, and in their receiving waters, to evaluate the short term and annual variability in material transport to estuaries and coastal waters. Because lower reaches of the streams pass through urbanized areas (i.e., Honolulu and Kaneohe), we examine spatial as well as temporal variations in the composition and abundance of dissolved and suspended solids in the waterways. The primary objectives of our study are to quantify fluxes of material delivered to the nearshore ocean by streams, to evaluate the variability in the intensity and duration of pulsed events, and to resolve natural and anthropogenic contributions to the material flux associated with such events. Companion studies are evaluating effects of material delivery on coral reefs. The isotopic composition and concentration of Pb and other dissolved and suspended heavy metals vary during rapidly changing hydrographic conditions. Variations are thought to reflect rapidly changing contributions from pulsed input of easily eroded soils followed by rapid mobilization of anthropogenic material accumulated since the previous rainstorm on impervious urbanized surfaces in the watershed. Thus, material fluxes derived from natural processes are overprinted by anthropogenic activity in the watershed. Nonetheless, natural soils originating in the relatively unimpacted mountainous portions of the watersheds comprise the bulk of the suspended material flux during storms. Anthropogenic contributions to the flux of Pb are easily resolved from natural inputs, but are less obvious for other metals such as Cu and Zn. Data on the distribution, abundance and fluxes of several other metals during individual storms will also be presented.
Fluvial Discharge to the Global Ocean: Importance of Tropical Rivers Katherine L Farnsworth
1
(804-684-7267; farnswor@vims.edu) 1School of Marine Science College of William and Mary, Virginia Institute of Marine Science, Gloucester Point, VA 23062, United States Rivers presently discharge approximately 35,000 km3 of freshwater to the global ocean; another 3000 km3/yr are assumed to be contained in reservoirs behind dams. By virtue of high runoff, monsoonal climate, and large total drainage area, the rivers draining southeast Asia and the high-standing islands of Oceania (including New Zealand, Indonesia and the Philippines) collectively contribute about 30% of the global freshwater flux, with northeastern South America and equatorial west Africa totaling another 30%. Suspended sediment discharge to the oceans is much more difficult to estimate, since it depends on a number of highly variable factors, such as basin area, topography, climate, geology, landuse, etc. Tropical and sub-tropical rivers are particularly susceptible to high rates of erosion due to high-standing geologically young mountains, heavy seasonal rains, and many small drainage basins that result in greater response to periodic events and relatively little sediment storage. A first-order estimate suggests that rivers draining southern Asia and Oceania account for about 75% of the sediment discharged annually to the oceans (estimated to be about 18.6 x 109 t/yr). Climate, geological framework, and anthropogenic activities also play important roles in determining dissolved sediment flux. While south Asia accounts for about 35% of the 3.9 x 109 t/yr global dissolved flux, European and eastern North American rivers play much greater roles (collectively, about 25\%) than they do for either water or suspended solid discharge. Regional differences in fluxes of various dissolved species, however, are significant. For example, Europe and eastern North America collectively account for about 10% and 35% of the silicate and chloride export, respectively, whereas tropical rivers discharge more than 65% of the global silicate but less than 40% of the chloride.
Physical Oceanography of the Seaward Mamberamo River Estuary (Northern Irian Jaya, Indonesia) in May 1999, Project IndoTROPICS. Abdul Gani Ilahude Research and Development Center for Oceanology, Indonesian Institute of Sciences, Jakarta. Twenty eight current and CTD stations were occupied adjacent to the Mamberamo Estuary in May 1999. The currents were measured with a Doppler Sonar Current Indicator, and a CTD recorded the temperature, salinity, depth, together with oxygen, transparency, turbidity and phosphorescence of sea water. The results showed that currents at 5, 10, and 15 m depth in the seaward estuary varied between 0.5 and 1.5 knots with the direction predominantly northwestward, indicating that the southeast monsoon wind was already in the offing, as is truly supported by local wind data. The effect of fresh and turbid water coming out of the Mamberamo River is quite strong, as indicated by the lowering of salinity and transparency values and by the increase of the turbidity values.
The Exacerbation of Erosion Induced by Human Perturbation in a Subtropical Mountainous Watershed in Taiwan: Evidence from Historical Records of Sediment Load Shuh-Ji Kao (011-886-2-23636040
ex 316; sjk@keep.oc.ntu.edu.tw
) National Taiwan University, Institute of Oceanography, National Taiwan University, Taipei 106, Taiwan Taiwan, a typical Oceania island, has a very high sediment yield (14400 t/km2/yr), which is two orders of magnitude greater than the world mean value (150 t/km2/yr). We analyzed long-term data sets (1950-94) of suspended sediment loading from two gauges in Lanyang-Hsi watershed, a small mountainous river in northeastern Taiwan. Prior to major road construction that began in 1957, the annual sediment export was in the range of 0.54 to 4.12 Mt/yr, which gave a mean sediment yield of 2600 t/km2/yr. This value is about 17 times higher than the global mean sediment yield, but consistent with revised estimates for the Oceania islands. Following two massive road construction periods during 1957-60 and 1975-80, there occurred surges of sediment export with more than 10 fold increase, indicating exacerbation of erosion in the watershed induced by human activities. Such conditions lasted for 2-4 years before returning to normal. By comparing the sediment loads obtained at the gauge at an altitude of 450 m with those from the gauge near the river mouth, we concluded that the extra sediment loads mainly originated from the upper reach. Some of the sediments apparently resided in the middle reach until a major flood with enough strength sweeping the sediments to the sea. From 1950 to 1994 the total sediment export was 353 Mt, yet 70% of which was probably attributable to human perturbation. Newly obtained C-14 dating results for suspended sediments collected during a major typhoon event in 1994 indicated that more than 65% of the exported particulate organic carbon was fossil carbon stored in poorly weathered coarse fragments of bedrock, suggesting the effect of human perturbation. Therefore, great care must be taken to differentiate natural from perturbed conditions, when assessing sediment and particulate organic carbon yields on Oceania small islands.
Sediment dispersal from the Sepik River, Papua New Guinea, via surface and subsurface plumes Gail C. Kineke
1 (617-552-3655;
kinekeg@bc.edu ) 1Dept of Geology and Geophysics Boston College, 140 Commonwealth Ave, Chestnut Hill, MA 02467 2School of Oceanography University of Washington, Box 357940, Seattle, WA 98195 Measurements of suspended sediments and water properties have been made in the vicinity of the Sepik River mouth in April 1996, May 1997, and April 1999. The Sepik River empties directly into a steep submarine canyon that cuts across the narrow shelf from a minimal depth of 6 m at a shallow bar 1 km upstream of the river mouth to a depth of 800 m over a distance of 15 km. The combined observations cover a range of discharge conditions and suggest that suspended sediment delivered by the river follows distinct dispersal pathways via a surface buoyant plume and subsurface plumes near the bottom and in the intermediate water column. Maximum velocities in the surface plume exceed 100 cm/s and the water column is strongly sheared over the top 6 m with velocities <10 cm/s at the base of the halocline. The freshwater signal can be traced beyond the ~ 2,000 km2 study area. However, suspended-sediment concentrations (SSCs) decrease rapidly within ~240 km2 of the river mouth from approximately 200 mg/l in freshwater to background SSC of 5 mg/l or less. The surface plume supplies sediments to the adjacent shelf and slope. It appears that large quantities of fine sediments are being trapped just seaward of the shallow river mouth bar, still well within the river mouth, forming a dense near-bottom plume that supplies sediments down the axis of the canyon. Measured near-bottom SSCs can be 10's of g/l (as high as 228 g/l). These near-bottom suspensions can be several m thick, and contain a mix of grain sizes from fine sand to silt and clay. In water depths of several hundred meters in the canyon, distinct layers of increased turbidity tens of meters thick are observed. These intermediate turbid layers can have concentrations twice those observed in the surface plume and may supply sediments to the deep ocean. The dispersal pathways of the Sepik River may serve as an analogue to sediment transport across the continental margin to the deep sea during sealevel lowstands for other systems with deep submarine canyons that cut across the continental shelf.
Slope Sedimentation off the
Sepik River, Papua New Guinea: Steven A. Kuehl (804-684-7118;
kuehl@vims.edu ) Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, VA 23062, United States Sedimentological and geochemical studies of kasten core and grab samples from the shelf and slope off the Sepik River provide clues regarding sediment dispersal and accumulation for this moderate-size, high-relief river system discharging into an active margin setting. The shelf off the Sepik River is narrow (<10 km), and the river empties directly into a submarine canyon, making this system a potential analog for rivers during low sea-level conditions. Sediment accumulation rates derived from 210Pb geochronology are high (~0.5 cm/y) on the open slope east and west of the river mouth, indicating significant input from the Sepik and perhaps other local rivers. Seabed evidence supports the idea of a decoupled dispersal system, with a surface plume which moves in response to coastal currents and wind forcing, and a hyperpycnal plume carrying sediments down the canyon. A corridor of silt-rich sediment floors the canyon with a progressive fining along the slope. Anomalously low 210Pb activities in the clayey-silt canyon corridor could be produced by sediment transported downslope by hyperpycnal flows, which deliver 210Pb-poor river sediment directly down slope with little mixing (and hence scavenging) with 210Pb-rich offshore waters. Sediment transported in surface or mid-water plumes along slope scavenge additional 210Pb from offshore waters, yielding an order of magnitude higher seabed surface activities, despite only a doubling of clay content. Contrasts among the Sepik margin and other well-studied river/margin systems suggest that our understanding of margin sedimentation processes has be strongly biased by the previous focus on passive-margin settings.
Osmium Supply to the Oceans From New Guinea Candace E.
Martin1, Bernhard
Peucker-Ehrenbrink2, Gregg J.
Brunskill3, and
Ron Szymczak4 [Greg Ravizza will be
presenting the paper] 1Dept. of Geology and Res. Sch
Earth Sci., ANU, Canberra ACT 0200 Australia Osmium isotopic budgets for the oceans have previously considered that rivers provide Os with a low concentration and an isotopic composition similar to or more radiogenic than estimates for the bulk upper continental crust. In order to balance this source, a high concentration, low isotopic composition meteoritic or hydrothermal influx has been suggested. However, the influence of the weathering of young crust from the region of Oceania and other parts of the Pacific rim have not been considered in this scenario. We have analyzed sediments from the Fly and Sepik Rivers, two of the largest rivers draining the island of New Guinea and among the world's large rivers in discharge and sediment yield. Sediment from the upper Fly River and its delta have low 187Os/188Os ratios of 0.510 to 0.560, and sediments from the upper and lower Sepik River have even lower ratios of 0.294 to 0.417. The difference in isotopic composition between the rivers is consistent with the presence of a larger proportion of unradiogenic ultramafic rocks in the drainage basin of the Sepik compared to the Fly. The sediment supplied by both these rivers is remarkably unradiogenic compared to the very high values usually contributed to the oceans by rivers draining regions of old continental crust (187Os/188Os 1.2). An acidic peroxide leach of a Sepik river sediment has a 187Os/188Os ratio lower than the bulk sediment, suggesting that the dissolved load supplied to the oceans from New Guinea is also very unradiogenic. This is further confirmed by the analysis of a filtered water sample taken from the northern Coral Sea (TROPICS Leg 3 station 17). This sample, with a salinity of 34.938, has an 187Os/188Os of 0.844, much lower than the accepted value for open ocean waters of 1.06 (Levasseur et al., 1998, Science 282:272-274). These results indicate that New Guinea is a significant source of nonradiogenic Os to the oceans. Considering the large fluxes of material emanating from Oceania, an unradiogenic riverine source may rival hydrothermal and meteoritic sources in balancing the Os isotope budget of the oceans.
Relict Shelf Ooids: Late Quaternary Sea Level and Tropical Aridity John D. Milliman (01-804-684-7112; milliman@vims.edu) School of Marine Science, College of William and Mary, 124 Northoint Dr. 23185, Gloucester Pt., VA 23062, United States Relict ooid-rich deposits occur on many low-latitude outer shelves (water depths generally between 70 and 120 m), including the Indian subcontinent, northeastern Australia-southern Papua New Guinea, northeastern South America and the southeastern U.S. Their depth distributions and C-14 ages compare closely with the Barbados, New Guinea and Tahiti eustatic seal-level curve, but their greater depth distribution allows us to extend the maximum depth of sea-level lowering to about -130 m, at about 18 ka C-14 B.P., during the peak of the last glacial maximum (LGM). These relict shelf ooids also raise a number of questions. Their wide distribution and shelf-edge location suggest a different mode of formation than the present-day Bahamian model, and their mineralogy is predominatly magnesian calcite, not aragonite. Most curious is the occurrence of ooid-rich deposits occur off many large rivers (Amazon, Ganges-Brahmaputra, Godavari, Indus, Fly), the exact antithesis of where we would expect to find ooid formation. This suggests that tropical climate during the LGM and early Holocene may have been considerably more arid than at present; in southern Asia this presumably resulted from a weakened Southwest Monsoon, but the case for reduced precipitation in NE South America is more problematic.
Sepik Submarine Canyon: Evidence for Modern Sedimentation Charles A. Nittrouer and J.P. Walsh School of Oceanography, University of Washington, Seattle, WA 98195 The Sepik River discharges approximately 100 million tons of sediment to the north coast of New Guinea. The continental shelf adjacent to the river mouth is very narrow (<10 km wide), and a submarine canyon penetrates the mouth. The magnitude of discharge and the proximity of the canyon suggest that substantial amounts of fluvial sediment could be transported to the deep sea. In shallow portions (<100 m) of the canyon thalweg, overconsolidated gray mud or lithified rock are present at the seafloor. These hard substrates are covered typically by layers of soft brown sandy mud (<10-30 cm thick), and in some cases by sand and gravel. The brown mud has homogenous or physically stratified sedimentary structure, and contains excess Th-234 (half-life 24 days). The thalweg in deeper water ( 100 m) consists of thick deposits ( 3 m) of soft brown sediment. Cores from these areas have distinct sandy laminations (<1 cm thick and separated by ~10 cm of brown mud), which have erosional bases and grade upward to finer sand and to mud. Excess Th-234 extends as much as 25 cm into cores (at 650 m water depth). Pb-210 (half-life 22 years) profiles indicate accumulation rates 1 cm/y, integrated over a century. Canyon cores reveal little evidence of bioturbation. The observations were obtained in April (1999), which is the seasonal period of peak discharge. The presence of excess Th-234 in physically stratified sediment indicates rapid deposition during the proceeding ~3 months ( 8 cm/mo, in some areas). Therefore, during periods of large discharge, some modern sediment is temporarily placed on the seabed of the upper Sepik canyon. Much material is transported to depths of at least 700 m, and leads to rapid sediment accumulation. The likely mechanisms transporting sediment through the canyon are frequent turbidity currents (several times per months) or other gravitational flows that lead to graded bedding. These conditions make the Sepik dispersal system a potential analog for conditions that occurred commonly on continental margins during low stands of global sea level.
Siliceous Phytoplankton off Mauretania (NW Africa): Results From a Four-Year Sediment Trap Study Oscar Romero
1
(49-421-218-77-59; oromero@uni-bremen.de) 1Department of Geosciences,
University Bremen, Postfach 33 04 40, Bremen 28334, Germany Four years of observations (1988-1991) of downward fluxes of diatoms and silicoflagellates at a trap site off Cape Blanc (CB) off Mauritania, NW Africa, are presented. Significant intrannual flux variations, and a strong decrease in export production from 1988 through 1991 are observed. This diminution can be explained by the interplay of various coupled processes, including seasonal and year-to-year variations in the offshore transport of chlorophyll filaments, and decreased productivity of coastal surface waters. Diatoms parallel total mass fluxes, and dominate (as valves m-2 d-1) the biogenic opal flux at all the times. The specific composition of the trapped diatom assemblage differs in accordance with the seasonal pattern; however, on an annual basis, no significant qualitative variations were observed. The dominance of neritic diatoms reflects the almost continuous offshore spreading of the coastal upwelling reaching the CB site through chlorophyll filaments (with stronger intensity in spring/summer), while occurrence of the pelagic diatoms and high silicoflagellate fluxes suggests inshore transport of more oceanic waters (mainly in winter). With the exception of some fragile, pelagic diatoms, most of the species found in the water column also occurred in the underlying sediments off Cape Blanc, with predominance of neritic forms. The good correspondence between the trapped and preserved diatom assemblages records the persistent offshore spreading of the high-production coastal surface waters, with some imprint of the low productivity season.
The estuarine chemistry of rare earth elements: comparison of the Amazon, Fly, Sepik, and the Gulf of Papua systems. Edward Sholkovitz & Ronald Szymczak By comparing and contrasting the estuaries of the Amazon, Fly and Sepik Rivers (Papua New Guinea) and the Gulf of Papua, we will explore the degree to which the estuarine chemistry of the rare earth elements (REE) is shaped by different hydrographic, morphologic and sedimentological processes. There are two distinct processes operating on dissolved REE in these estuaries. There is removal in the low salinity region and release to the mid to high salinity region. Fractionation of the dissolved REE during these two processes operate in different directions. The order of removal follows LREE MREE HREE, and the order of release follows HREE MREE LREE where L, M and H refer to light (La, Ce, Nd), middle (Sm, Eu, Gd, Tb and Dy) and heavy (Er, Yb and Lu) REE respectively. Large scale removal and fractionation in the low salinity region is the result of salt-water induced coagulation of river borne colloids. The increase of dissolved REE in the mid to high salinity waters of the Amazon, Fly and Gulf of Papua estuaries implies that there is a source of dissolved REE to the outer regions of these estuaries. Resuspended sediment and/or bottom sediment are the mostly likely sources. The lack of REE increases in the Sepik River estuary helps to strengthen this explanation as the mixing zone of the Sepik River estuary is located over a deep shelf where there is little physical contact between bottom sediment and estuarine waters. In marked contrast, the other three estuaries are dominated by the resuspension and deposition of bottom sediments. Release and its accompanying fractionation, we speculate, results from the complexation of dissolved REE with carbonate ions during the reaction of seawater with suspended particles and/or bottom sediment. Diagenetically-mobilized dissolved REE in pore waters may also contribute to the release of REE to the mid and high salinity waters. Estuarine reactions markedly modify the absolute and relative abundance of dissolved REE reaching the oceans. In tandem, fractionation, associated with the removal (LREE MREE HREE) and release of REE (HREE MREE LREE) during estuarine mixing, leads to an ‘effective’ river REE composition (that reaching the ocean after modification in estuaries) which is evolved toward the REE composition of seawater which is HREE enriched with respect to most rivers and the earth’s upper continental crust. Fluxes of dissolved Nd from estuarine and shelf sediments may be the means of maintaining the inter-ocean differences in the Nd isotopic composition of seawater by reducing the residence time of Nd in the oceans.
Zooplankton of the Mamberamo River Estuary, Irian Jaya, Indonesia, Project IndoTROPICS Agustinus B. Sutomo Research and Development Center for Oceanology, Indonesian Institute of Sciences, Jakarta. Twenty eight zooplankton samples were collected during the beginning of east monsoon at the end of May 1999. The average count of total zooplankton was 444 individuals/m3, standard deviation 251, and coefficient of variation 56%. Copepods were the dominant group on all stations, followed by chaetognaths and oikopleuras. Chaetognaths were found on the stations far from the coast while oikopleuras were found closer to the coast. Siphonophores and appendicularians were found almost on all stations.
The Effect of First-Order Physiographic Features on Continental-Margin Sedimentation Patterns: Insights From Coastal New Guinea Walsh, J.P. and Nittrouer, C.A. The position of a continental margin relative to the Earth’s plate boundaries has a primary control on its first-order physiography. Modern sediment dispersal as well as the development of higher-order features are influenced strongly by the resulting margin form. Three major classes of coasts have been recognized: collision, trailing-edge and marginal-sea. Margin characteristics (e.g., shelf width) can be compared with global rivers to establish a framework on which to gage first-order attributes at mouths of major rivers. This demonstrates that rivers discharge onto a wide range of margin shapes, and their classification into tectonic categories may be misleading in regard to sedimentation. To examine the influence of first-order physiography, modern sediment dispersal is considered in New Guinea coastal waters. The Sepik River on the north coast and the Fly, Kikori and Purari Rivers on the south coast are contrasted. The Sepik River is situated adjacent to a subduction zone and discharges into a submarine canyon, an area with essentially no continental shelf (<5 km). Much ( 50%) of its load is rapidly transported to the deep-sea, probably in gravity-driven flows. The rivers on the south coast are located in a foreland basin with a margin of variable width. Fly, Purari and Kikori sediments are advected clockwise around the Gulf of Papua along a channelized, mangrove-covered coastline. A significant portion ( 50%) of their loads accumulates in large coastal and shelf clinoform deposits. It is evident that despite similar climate, vegetation and river-source areas, sedimentation patterns differ markedly between north and south coast Papua New Guinea rivers, highlighting the importance of first-order physiography in regulating modern sedimentation.
Material Flux and Fate in The Tropical Coastal Ocean Wet tropical environments are recognized as extremely important sites of particulate and dissolved inputs to the coastal ocean. The many unique characteristics of tropical settings lead to important contrasts in the ultimate fate of these materials relative to middle- and high-latitude settings. The purpose of this session is to highlight ongoing studies of project TROPICS regarding the biogeochemical, geological, and physical oceanographic processes operating to control the trapping, bypassing, and cycling of solutes and sediments from a wet tropical area of high relief (Island of New Guinea) on contrasting broad and narrow coastal shelves. Contributions are encouraged from other studies of river-ocean interaction in wet tropical environments. Conveners: Gregg J. Brunskill,
Australian Institute of Marine Science, PMB No. 3, Townsville,
Queensland 4810 Australia, Kathy Burns, Australian Institute of
Marine Science, PMB No. 3,
Townsville, Queensland 4810 Australia,
Gail C. Kineke, Department of Geology and Geophysics, Boston
College, Chestnut Hill, MA 02167, Steven A. Kuehl,
Virginia Institute of Marine Science, College of
William and Mary, Gloucester Point, VA 23062, Charles A. Nittrouer, School of Oceanography, University
of Washington, Seattle, WA 98195,
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