Distribution and sources of particulate organic matter in the water column and sediments of the Fly River Delta, Gulf of Papua (Papua New Guinea)

Estuarine, Coastal and Shelf Science 69 (2006) 225e245 www.elsevier.com/locate/ecss

Distribution and sources of particulate organic matter in the water column and sediments of the Fly River Delta, Gulf of Papua (Papua New Guinea) Miguel A. Goni a,*, Natalie Monacci a, Rachel Gisewhite a, Andrea Ogston b, John Crockett b, Charles Nittrouer b a

Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA b School of Oceanography, University of Washington, Seattle, WA 98195, USA Received 30 December 2005; accepted 6 April 2006 Available online 6 June 2006

Abstract Suspended particles from surface and bottom waters and surficial sediments from the seabed were collected throughout the Fly River subaqueous delta region during the monsoon season in January 2003. Because of the unusually low river discharge associated with a strong El Nin˜o, water-column salinities were relatively high (10 to 32) throughout most of the delta, with brackish salinities (<10) only measured within the main distributary channel of the Fly River. The concentration and composition of particulate organic matter (POM) in these samples showed distinct spatial differences and marked contrasts between water column and seabed samples. Overall, relatively low concentrations of total suspended solids (TSS), particulate organic carbon (POC) and particulate nitrogen (PN) were measured in surface (27
50, 0.83
1.2, and 0.05
0.05 mg/L, respectively) and bottom (400
743, 7.1
7.3, and 1.7
7.2 mg/L, respectively) waters throughout the delta. Particles in both surface and bottom waters displayed elevated organic carbon contents (%OC > 4 wt.%), relatively high organic carbon:nitrogen molar ratios (OC:N > 20 mol:mol) and quite depleted stable isotopic compositions of organic carbon (d13COC < 27&). In contrast, surface sediments in the seabed displayed spatially uniform compositions that were characterized by markedly lower %OC contents (1.1
0.8 wt.%), lower OC:N ratios (17
9 mol:mol) and relatively enriched d13COC compositions (25.5
1.1&). The radioisotopic compositions of OC from a selected set of seabed samples (D14COC of 408
82&) indicate OM in surface sediments is old (14C ages of 2800 to over 6000 years before present). The ratios of organic carbon to mineral surface area exhibited by these sediments are within the typical (mono-layer equivalent) ranges characteristic of shelf sediments and do not reflect the preferential removal of terrigenous OM. Overall, these compositions indicate that while water-column POM appears to be derived mainly from terrigenous vascular plant debris and riverine/estuarine phytoplankton, the source of most of the sedimentary POM is aged, soil OC ultimately derived from C3 vegetation. We speculate that the concentrations and sources of suspended POM in the water column of the Fly River delta region reflect the conditions of low river discharge, low wave energy and neap tides encountered at the time of sampling. In contrast, POM compositions in surface sediments are consistent with the transport and deposition of old, mineralbound OC most likely eroded from the upland regions of the Fly River watershed, which is characterized by steep slopes, high precipitation and C3 tropical forests and grasslands. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Fly River; Papua New Guinea; Gulf of Papua; delta; clinoform; continental margin; organic carbon; organic matter sources; carbon isotopes

1. Introduction

* Corresponding author. College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA. E-mail address: [email protected] (M.A. Goni). 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.04.012

Over 80% of the global burial of organic carbon (OC) in the oceans (w0.1  1015 g C/y) occurs in margins adjacent to rivers (Berner, 1982; Hedges, 1992; Hedges and Keil, 1995). The elevated sediment accumulation rates and the input of

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recalcitrant organic matter from terrigenous sources both contribute to the efficient sequestration of carbon in these regions (e.g., estuaries, deltas, continental shelves). Recent studies indicate that although river-dominated ocean margins act as net sinks for carbon, they are also sites where both terrigenous and marine organic matter are actively recycled (e.g., Aller, 1998; Aller et al., 2004; Blair et al., 2003, 2004; Goni et al., 2005; Gordon and Goni, 2004; Stein and Macdonald, 2004). Understanding the mechanisms that control OC cycling in riverdominated margin systems requires multidisciplinary fieldbased studies to investigate the relationships and feedbacks among the diverse processes that affect the fate of organic matter (OM). In the past few years, we have started to appreciate the importance that the modes of sediment delivery and deposition (e.g., Kineke et al., 1996, 2000; Nittrouer et al., 1996; Ogston et al., 2000; Wheatcroft, 2000; Wheatcroft and Sommerfield, 2005) have on the ultimate fate of OC in margin sediments (e.g., Goni et al., 2005; Gordon et al., 2001; Leithold and Blair, 2001). Factors such as the timing of sediment input by rivers in relation to the dispersal forces acting on shelves (tides, waves, currents) have a critical, but as of yet, poorly understood effect on the efficiency of OC burial. Similarly, new insights into the highly heterogeneous composition of the OM exported by rivers show that these materials range from relatively reactive freshwater algae and discrete vascular plant debris, to much more resistant OM eroded from soils and sedimentary rocks (e.g., Bianchi et al., 2002; Blair et al., 2003; Goni et al., 2000; Goni and Hedges, 1992; Gordon and Goni, 2003; Masiello and Druffel, 2001; Prahl et al., 1994). The relative abundances of these various OM sources, which can vary significantly depending on the geological and climate characteristics of each drainage basin (e.g. bedrock composition, soil types, vegetation, rainfall, temperature, weathering rates), have the potential to affect the net carbon burial at each site. The characteristics that make deltaic and siliciclastic depositional systems the most globally significant environments in terms of long-term carbon sequestrationdrapid and abundant delivery of sediments and entrainment of recalcitrant organic materialsdare most pronounced in rivers systems draining wet mountainous regions. These fluvial systems, such as the ones found in the islands of Oceania, deliver a disproportionately high fraction (25 to 40%) of the global material fluxes to the ocean (e.g., Milliman and Syvitski, 1992; Nittrouer and Kuehl, 1995), potentially making tropical river-dominated ocean margins significant sites of carbon burial. With this rationale in mind, several field campaigns were conducted in 2003 and 2004 to investigate sediment and carbon dynamics in the Gulf of Papua as part of the ‘‘Source to Sink’’ MARGINS program funded by the US National Science Foundation (http://www.margins.wustl.edu/S2S/S2S.html). The objective of this paper is to evaluate the distribution of particulate OM (POM) in the water column and surface sediments from the Fly River Delta. Specifically, we investigate the provenance of the POM in the shallow, inshore regions of this humid, tropical river margin and assess its transport, deposition and

cycling. On-going studies are examining the distribution and composition of OM in the deeper regions of the clinoform system extending farther offshore into the Gulf of Papua. 2. Background of study area The Fly River delta system is located in the southwestern region of Papua New Guinea (PNG) along the western margin of the Gulf of Papua (Fig. 1). This wet, tropical river-dominated ocean margin has been the site of several studies investigating the export and dispersal of materials from land (e.g., review by Brunskill, 2004). The delta receives discharge from the Fly River and its major tributary, the Strickland River. Together these rivers have a combined drainage basin of w79,000 km2 that includes the southwestern highlands of PNG with peak elevations of over 4000 m. Approximately 30% of the drainage basin consists of very wet mountainous regions with steep slopes and a history of tectonic instability (Pickup and Springs, 1984). Abundant rainfall characterizes the drainage basin, with rates of 5e10 m/y in the high altitude regions and 2e3 m/y in the lowland coastal areas (Pernetta and Osborne, 1990; Harris et al., 1993). Rainforests (C3 plants) dominate the drainage basin, ranging from the subalpine to lowland regions (Bird et al., 1994). Alpine grasslands are found above tree line (w3900 m above sea level), but also within the subalpine and lowland regions, where C4 grasses tend to dominate (Bird et al., 1994). The natural vegetation has been affected in localized areas by cultivation of crops such as sweet potato, taro and sugarcane and the replacement of forests by grasslands due to anthropogenic activities (Haberle, 1998; Haberle et al., 2001). Water and sediment discharges by the Fly/Strickland River are quite high, averaging 6000 m3/s and 4 tonnes/s, respectively (Harris et al., 1993, 2004). Because of consistently elevated precipitation, river discharge to the coast is rather constant. The major exceptions are El Nin˜o periods, when extensive droughts can dramatically reduce river levels (Dietrich et al., 1999). A relatively narrow (10e15 km wide) floodplain, which is constrained in the south by the Orinomo Plateau and in the north by the Fly-Digoel Plateau, characterizes the low gradient reaches (w300 km long) of the Fly River system (Pernetta and Osborne, 1990; Dietrich et al., 1999). There are no dams or dikes along the river, and there is no dredging or development in the floodplain and the delta. Copper mining activities on the Ok Tedi River (a tributary of the Fly) have resulted in a measurable increase (w15%) of the pre-mining sediment discharge (85 million tonnes/y) by the Fly River system (Day and Dietrich, 1997). The Fly River delta system includes a subaerial delta and a subaqueous-delta clinoform (Harris et al., 2004; Walsh et al., 2004). The subaerial portion of delta has three major distributary channels (southern, middle, northern distributaries; Fig. 1), which are separated by sand-mud islands with lush mangrove vegetation (Robertson et al., 1991; Wolanski et al., 1998). Several channels also dissect the subaqueousdelta clinoform, with the Umuda channel being the most prominent. The delta is macrotidal in nature, with mean spring

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227

Fig. 1. Map of the study area showing transects (in bold letters) and stations occupied during the Western Venturer Cruise. Samples taken from different regions within the study area are categorized according to the sample key. The locations of specific stations discussed in the text are indicated in the map. The bathymetry is based on data made available by P. Harris (Geoscience Australia).

tidal ranges that vary from 3.5 m at the mouth of the distributaries to over 5 m in the apex (Wolanski et al., 1995, 1997; Harris et al., 2004). Tides are mixed semi-diurnal and can result in currents of >1 m/s along the distributary channels (e.g., Wolanski et al., 1995). The fluxes of materials through the distributaries vary seasonally and are controlled primarily by the balance between tidal and wave energy (e.g., Harris et al., 1993, 2004; Wolanski et al., 1995, 1997), the latter of which varies according to seasonal wind patterns. During the austral summer (DecembereMarch), NW monsoon winds blow southeastward off the land (w5 m/s) and relatively calm seas (0.3 m significant wave heights) predominate (McAlpine and Keig, 1983; Thom and Wright, 1983). During the winter (MayeOctober), SE trade winds average 5 to 8 m/s and blow northwestward and northward, leading to much more energetic conditions (1.3 m average significant wave heights). The large-scale ocean circulation is dominated by a clockwise gyre in the northern Coral Sea, causing a predominant eastward transport along the shelf. Overall, these conditions facilitate the transport of fine sediments along and across the delta and their deposition in the offshore regions, leading to the formation of a well-developed clinoform between 20 and 70 m of water depth (Walsh and Nittrouer, 2003; Walsh et al., 2004). Distinct sedimentary facies characterize the seabed deposits from different regions of the Fly River delta system (e.g., Alongi et al., 1992; Baker et al., 1995; Dalrymple et al.,

2003; Harris et al., 1993). Within the subaerial delta, the high tidal currents result in deposits along the distributary channels that are characterized by well-sorted fine sands devoid of benthic infauna (Alongi et al., 1992). In the topset region of the subaqueous-delta clinoform (<15 m depth), sediments are characterized by interbedded layers of wellsorted fine sands and poorly sorted muds composed primarily of silts with less than 30% clay (Baker et al., 1995; Harris et al., 1993). These interbedded laminae are thought to result from the seasonal regimes of contrasting energy. The typical thickness of a sand-mud couplet is about 2 cm. Sediment accumulation rates in this region of the subaqueous-delta clinoform average 1.7 cm/y. Accumulation rates in the foreset region of the subaqueous-delta clinoform (20 to 35 m depth) are significantly higher than in the topset, averaging 4
1 cm/y (Harris et al., 1993; Walsh et al., 2004). Most of these deposits are composed of massively interbedded finely laminated deltaic muds made up of fine-grained, kaolinite rich sediment supplied from the estuary and delta. Benthic macrofauna are mostly absent from these deposits due to the high sediment deposition, which results in significant rates of progradation (w6 m/y). Terrigenous OM from the river and the delta islands dominate the carbon cycle in the vicinity of the Fly River delta (Bird et al., 1995; Robertson and Alongi, 1995). Algal production peaks in the more distal locations off the delta as turbidity

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decreases due to the settling of river suspended sediments (e.g., Alongi et al., 1992; Ayukai and Wolanski, 1997; Davies, 2004; Robertson et al., 1993, 1998). Both autochthonous and allochthonous OM fuels active microbial activity in the water column and sediments of the Gulf of Papua (e.g., Aller and Aller, 2004; Alongi, 1995; Alongi et al., 1992; Todorov et al., 2000), which is enhanced in the physically mixed mobile mud belts that occur in some areas of the inshore regions west of the subaqueous-delta clinoform system (e.g., Aller and Blair, 2004; Aller et al., 2004). Marine OM contributions to the sedimentary OC become progressively more important in the more distal offshore locations of the Gulf (e.g., Bird et al., 1995). 3. Sampling and methods The cruise to the Fly River delta region (Fig. 1) took place during the monsoon season on January 2003, aboard the OK Tedi Mining Co. vessel Western Venturer. Stations were occupied across several transects over the study area. For the purpose of data presentation and discussion, we have divided the Fly River delta study area into three regions (Fig. 1): the northeast region (composed of stations along transects A0 , AA, BB, CC, DD, EE, FF), the southwest region (composed of stations along transects C, D, E, G) and the river channel (composed of stations along transects A an B). An 8-h anchor station was completed at a location 40 km east of the apex along transect A (Fig. 1). The stations occupied during this cruise were for the most part located on the topset of the subaqueous-delta clinoform system, covering water depths that ranged from 3 to 20 m. Due to logistical reasons, transect A0 and B were occupied using a small inflatable craft, which limited the sample collection to surface waters. At each station, the water column was profiled using a boundary-layer in-situ profiler (BLISP) equipped with a programmable CTD/logger, optical backscatter sensor (OBS), and pump system designed to collect samples near the seabed (as close as 10 cm above the bed). The BLISP orients the sensors and pump nozzle into the flow to minimize the influence of the instrument on the flow and suspended sediment regime. Salinity (measured using the Practical Salinity Scale) and temperature profiles in transects A0 and B were performed by manually lowering a small CTD/OBS system from the inflatable craft. Seabed sediments were sampled using a kasten corer equipped with a 3-m  12.5-cm  12.5-cm barrel (Kuehl et al., 1985). In several stations where the sandy bottom precluded the use of the kasten corer, we used a Shipek grab sampler to collect surface sediments. The locations of the kasten core and grab stations are indicated in Fig. 1. Once collected, the kasten cores were opened and sub-sampled on the ship at 1-cm intervals for the top 50 cm of sediment and at 2-cm intervals below that depth. In the case of the grab samples, the top 2 cm of sediment was sub-sampled. All sediment samples were stored frozen in sealed Kpak bags until analyses. Splits from the surface horizons of all stations, which are the focus of this paper, were processed onboard the ship by oven-drying the sediments at 50  C and grinding them with a mortar and pestle.

The vertical structure of the current at each station was measured using a ship-mounted downward-looking Acoustic Doppler Current Profiler (ADCP, 600 kHz Workhorse, RD Instruments) equipped with a bottom tracking capability. The three components of flow were recorded continuously in 25cm vertical bins while the vessel was on station and were subsequently time-averaged over 10 min, the approximate period of sampling operations. The horizontal current velocities were transformed into the geographical orthogonal coordinate system (i.e., north and east current components). In this paper we present current velocities from the near surface and near the bed intervals, determined to be 1 meter above bed (mab) and 2 meters below the surface (mbs), respectively. These intervals ensured minimal noise in the data and correspond to the depths of particle sampling. No current data were collected along transects occupied using the small boat. Wave heights were measured by an instrumented tripod located at station CT (Fig. 1) using a pressure sensor that recorded at 4 Hz over 10 min every hour. The data were converted to significant wave height through spectral methods and linear wave theory to correct for pressure attenuation with depth. Bottom shear stresses were estimated from the nearbed current speed obtained at approximately 1 mab at each station and wave height data that was measured at CT, assuming that the wave heights were relatively consistent over the sampling region. We used the Grant-Madsen model (Grant and Madsen, 1979) to estimate the shear velocity (u*), where the shear stress, tb ¼ r  u*2. Bottom-water samples were collected within 1 mab into collapsible bags using the BLISP-mounted pump. At each station, samples from surface waters were also collected within 2 mbs using a clean, acid-rinsed bottle. Only surface-water samples were collected at stations occupied by small boat (transects A0 and B). Total suspended solids (TSS) concentrations were determined by filtering known volumes of samples through pre-weighted filters with 47-mm diameter and 0.45mm nominal pore size. A similar approach was used for geochemical analyses but we used 13-mm diameter glass fiber (GF) filters, which were pre-combusted to remove organics. Four filters were collected from each water sample in order to carry out elemental and stable isotope analyses. Filtration was carried out aboard the Western Venturer immediately after collection and all filters were stored at 20  C until returned to the laboratory, where they were oven dried at 50  C for at least 48 h. Particulate organic carbon (POC) and particulate nitrogen (PN) concentrations were determined by high-temperature combustion on a Thermo Quest EA2500 Elemental Analyzer of acid-treated 13-mm filters according to established procedures (e.g., Goni et al., 2003, 2005). The stable isotopic compositions of organic carbon (d13COC) in pre-acidified suspended particles were determined using a ThermoQuest EA interfaced with a Finnigan Mat Delta Plus-XLS by a Conflo-III system (Goni et al., 2003, 2005). The weight percent organic carbon (%OC) and nitrogen (%N) contents of sediments were determined using the same approach as for POC and PN. In addition, we determined the weight percent of total carbon

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(%TC) of sediments that were not acidified prior to analysis. Weight percent inorganic carbon (%IC) contents were determined by subtraction the %TC and %OC concentrations (e.g. %IC ¼ %TC  %OC). The d13COC compositions of seabed sediments, which were pre-treated with 10% aqueous HCl prior to analyses in order to remove carbonates, were determined using the same approach as for the suspended particles. The surface area (SA) of bulk sediments was determined using a Coulter SA3100 Surface Area Analyzer according to the BET technique (Mayer, 1994; Goni et al., 2005). Grain-size analyses of selected surface sediments were carried out using wet sieving, to separate the sand fraction, and the pipette technique (Folk, 1968). Radiocarbon measurements were made on decalcified bulk sedimentary organic matter. Samples were combusted in individual quartz tubes and the resulting CO2 cryogenically purified and distilled. CO2 was reduced in the presence of a stoichiometric excess of hydrogen and iron catalyst in a manner similar to Vogel et al. (1987). The resulting graphite-iron matrix was pressed into aluminum cathode targets and analyzed by 14C-AMS at the Center for Accelerator Mass Spectrometry in University of California/Lawrence Livermore National Laboratory. Results are reported according to international convention (Stuiver and Polach, 1977) and include a background subtraction based on 14C-free acid-only charcoal and a sample specific d13C correction. The variability associated with parameters derived from individual measurements, such as POC:PN, POC:TSS, OC:SA ratios, were calculated using propagation of error (Taylor, 1997). Standard deviations (sx) and standard errors (s.e. ¼ sx/n1/2, n is the number of samples) were used to summarize the variability among different sample sets. ANOVA tests were used to assess statistically significant differences among samples. Unless specifically stated, all statistically significant differences reported are above the 95% confidence interval (P value of 0.05). All data are available in table format upon request. 4. Results 4.1. Characteristics of the water column Hydrographic and oceanographic conditions changed throughout the sampling period (8e17 January 2003), leading to contrasts in the physical forcings responsible for sediment delivery and transport. In this section, we briefly describe the conditions encountered as the ship moved from the northeast region, to the southwest region and into the river channel. Because of the prevailing El Nin˜o conditions, river discharge during January 2003 was lower than the normal average flow (Dietrich, personal communication). Tidal conditions during the cruise changed from neap to spring (Fig. 2). Neap tides characterized the first five days of the cruise (7e11 January 2003), during which we sampled the C transect and most of northeast transects (AA to FF; Fig. 1). The shift to spring tides started on January 12, when we sampled the B transect (no current data were recorded at this time). The rest of the southwest transects (D, E, G), as well as transects A and A0 ,

229

were sampled during spring tides (Fig. 2). During the neaptide period, current velocities at the surface ranged from 10 cm/s up to 70 cm/s while current velocities at the bottom ranged from 1 cm/s to 40 cm/s, both varying in magnitude and direction depending on location and tidal stage (Fig. 2). Current velocities increased significantly after January 14, when we measured speeds up to 120 cm/s in the surface and up to 100 cm/s at the bottom. Winds during the whole cruise were relatively light, with significant wave heights that ranged from 0.1 m to 0.3 m (Fig. 2). There was a spike in wave height to 0.5 m during the morning hours of January 12, a period when we sampled transect B. Overall, the water depths for most stations were two orders of magnitude higher than the wind-driven wave heights. Surface salinities ranged from 20 to 30 throughout most of the Fly River delta transects at the time of sampling (Fig. 2). We did not encounter brackish water (salinity <10) until we entered the distributary channels of the subaerial delta. Because of the unusually low river discharge, freshwater was constrained to the apex region along the main Fly River channel (Fig. 3a,b). Relatively high salinity values were measured in surface waters at the entrances of all the major distributary channels of the Fly Delta, ranging from low values of 18 in the northern distributary to 26 in the southern distributary channel. Bottom-water salinities were consistently elevated at all stations relative to the salinities measured in surface waters. 4.2. Concentration and composition of suspended particles TSS and POC (and PN) concentrations in surface and bottom waters displayed significant spatial differences (Fig. 3cef). In surface waters, for example, TSS concentrations ranged from 1 to 245 mg/L with the highest concentrations measured in the westernmost stations within the Fly River channel (Fig. 3c). In contrast, surface POC concentrations ranged from 0.1 to 7 mg/L with the highest values observed across the entrance of the northern distributary (Fig. 3e). Elevated POC concentrations were also measured in several stations from the northeast region, as well as in the stations along the western portion of the river channel. Surface PN concentrations (plots not shown) ranged from <0.01 mg/L to over 0.2 mg/L and were highest in the western part of the Fly River channel, with moderately elevated concentrations at the inshore stations of the northeast region. Low TSS, POC and PN concentrations characterized the surface waters of all southwest stations. TSS and POC (and PN) concentrations in samples from bottom waters were much higher relative to their surface counterparts (Fig. 3). Overall, TSS concentrations in bottom waters ranged from 20 to over 1000 mg/L (Fig. 3d). Concentrations well above these levels (>10,000 mg/L) were measured in three stations along the AA transect in the northeast region (not shown in Fig. 3d). These peak concentrations are within the ranges of fluid mud (e.g., Kineke et al., 1996), indicating that conditions favorable for the occurrence of fluid mud

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W. Venturer Cruise

(a) C

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Fig. 2. Hydrographic parameters measured in the Western Venturer stations. (a) Expected tidal height in cm above United Kingdom Hydrographic Office Datum (UKHO) and measured tidal excursion (m); (b) wave height (m) at the tripod location and surface salinity at specific stations; (c) current speeds at the surface (2 mbs) and bottom (1 mab) of the water column during the time of sampling in each of the stations. The periods when stations from different transects were occupied are indicated in (a). Tidal and wave data were not recorded from 15e17 January 2003 as the tripod was recovered prior to heading up the river channel (transects A and A0 ).

were constrained to stations directly offshore from the northern distributary. POC concentrations in bottom waters ranged from 0.6 to over 16 mg/L, with elevated values for the river channel and northeast stations (Fig. 3f). PN concentrations in bottom waters (plot not shown) ranged from 0.02 to over 2.5 mg/L and displayed a similar distribution to that of POC. The fluid-mud samples displayed very high POC and PN concentrations (>90 mg/L and >8.5 mg/L, respectively) that were consistent with the extremely high TSS contents. As was the case for surface samples, TSS, POC and PN concentrations in bottom waters were consistently the lowest in the southwest region. In addition to the trends in particulate concentrations described above, there were marked contrasts in the composition of the particulate material throughout the water column of the Fly Delta (Fig. 4). For example, in both surface and bottom waters, the organic carbon content (weight %OC) of

suspended particles ranged from low values of w1.0% to over 10%. The lowest %OC contents were consistently displayed by particles from both surface and bottom waters of the river channel above the subaqueous delta. The highest %OC values in surface waters were displayed by particles collected at the mouth of the northern distributary (Fig. 4a). In the case of bottom waters, the highest %OC concentrations were measured in the most offshore stations of several southwest and northeast transects (Fig. 4b). The distribution of %OC is in stark contrasts to that of TSS and POC concentrations (Fig. 3), and suggests significant differences in the nature of the particles in suspension throughout the Fly River delta. Atomic OC:N ratios of suspended particles in surface waters ranged from values <10 mol:mol along several inshore stations in the northeast region to >50 mol:mol, across the entrance of the northern distributary (Fig. 4c). Most of the

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(a) -8.0

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Fig. 3. Spatial distribution of salinity, total suspended solids (TSS) and particulate organic carbon (POC) in surface and bottom waters throughout the Fly Delta region. The sampling locations are indicated by open diamonds.

suspended particles in surface waters displayed OC:N ratios of 10e30 mol:mol. The distribution of OC:N ratios in bottomwater particles was quite different, with low values (w10 mol:mol) detected along the river channel and inner stations of the northeast region. Elevated ratios (>50 mol:mol) were measured in offshore stations of several southwest and

northeast transects (Fig. 4d). All of the suspended particles throughout the Fly River Delta displayed relatively depleted d13COC values that in surface samples ranged from 23 to 31& (Fig. 4e), while in bottom samples d13COC values ranged from 27 to 31.5& (Fig. 4f). In surface waters, the most enriched compositions (23& > d13COC > 25&)

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-8.4

Bottom OC:N

50 45 35 30

-8.8

25 20 15

-9.0

45 40 35 30

-8.8

25 20 15

-9.0

10

10

5

5

142.6

143

143.4

143.8

144.2

(e)

OC:N (mol:mol)

40

50

-8.6

OC:N (mol:mol)

-8.6

-8.0

%OC (wt.%)

10 9 8 7 6 5 4 3 2 1 0

-8.6

Bottom %OC

142.6

(f) Surface d13Coc

-8.0

-8.2

-8.2

-8.4

-9.0

142.6

-8.6 -8.8 -9.0

143

143.4

143.8

144.2

142.6

-21 -22 -23 -24 -25 -26 -27 -28 -29 -30 -31 -32

d13Coc (per mil)

-8.8

-8.4 -21 -22 -23 -24 -25 -26 -27 -28 -29 -30 -31 -32

d13Coc (per mil)

-8.6

Bottom d13Coc

Fig. 4. Compositions of suspended POM, including organic carbon content (%OC), molar organic carbon:nitrogen ratios (OC:N) and stable isotopic composition of OC (d13COC) in the surface (2 mbs) and bottom (1 mab) waters throughout the Fly River delta region. The sampling locations are indicated by open diamonds.

were displayed by samples from the B transect and several offshore stations along the northeast transects (Fig. 4e). In the case of particles from bottom waters, all samples displayed depleted signatures independent of location (d13COC < 27&; Fig. 4f).

4.3. Observations from the anchor station Sixteen casts over an 8-h period were conducted near the apex of the Fly River delta (Fig. 1). Hydrographic data and particle concentrations and compositions from surface

M.A. Goni et al. / Estuarine, Coastal and Shelf Science 69 (2006) 225e245

120

5

80

4

0

Flood

-40

2 1

10

6

8

5

%POC (wt.%)

0

6 4

4 3

2

2

0

1

35

5

400

4

300

3

200

2

100

1

0

0

10

6

60

-24

5

50

4

40

3

30

2

20

1

10

0

0 14

15

16

17

18

Hours on Jan-16-2003

19

20

21

Bottom POC (mg/L)

13

[POC:PN]a (mol:mol)

500

δ13COC (‰)

Salinity

3

-120

Bottom TSS (g/L)

Surface TSS (mg/L)

u* (cm/s)

40

-80

Surface POC (mg/L)

friction with the bottom, the magnitude of current generally was smaller near the bed than at the surface. Bottom shear velocity, u*, ranged from less than 1 cm/s near slack water to over 4 cm/s during periods of maximum ebb current (Fig. 5b). Salinities were at their highest (w8) in the first two hours of the anchor station, decreasing steadily as the ebb tide progressed to minimum values of w1. Salinities at the surface were slightly lower than at the bottom by 1e2

Ebb

u (cm/s)

(2 mbs) and bottom (1 mab) waters are presented in Fig. 5. We started the anchor station just after high tide (13:00 h), when currents were near slack and salinity was high. Along-channel ebb current velocities reached maximum values of about 120 cm/s around 16:00 h (Fig. 5a). Following the peak in ebb current, velocities decreased, with flood currents measured during the last two hours of the anchor station. The surface and bottom velocities were well correlated and, as expected due to

233

30 25 20 15

-26

-28

-30

-32 13

14

15

16

17

18

19

20

21

Hours on Jan-16-2003

Fig. 5. Anchor station data, including along channel current velocity (u, cm/s), salinity, total suspended solid (TSS) concentrations (mg/L) and particulate organic carbon POC concentrations (mg/L) in surface (open symbols) and bottom (closed symbols) waters. Also plotted are the calculated shear velocities near the seabed (u*, cm/s), as well as the compositions of suspended POM, including organic carbon content (%OC), organic carbon:nitrogen ratios (OC:N) and stable isotopic composition of OC (d13COC) in the surface (open symbols) and bottom (closed symbols) waters collected throughout the anchor station. The x-axis represents local time on January 16, 2003. The location of the anchor station is highlighted in Fig. 1.

M.A. Goni et al. / Estuarine, Coastal and Shelf Science 69 (2006) 225e245

234

during high tide and as the flood tide started at 20:30 h. During the falling stages of the tide following peak ebb (16:00 to 20:00 h), identical salinities near the surface and at the bottom were consistent with well mixed conditions. TSS and POC (and PN, plot not shown) concentrations displayed similar distributions throughout the anchor station (Fig. 5). In surface waters, both TSS and POC, concentrations peaked (>400 mg/L and >4 mg/L, respectively) during low tide. In contrast, TSS and POC concentrations in bottom waters were highest (>1000 mg/L and >20 mg/L, respectively) during high tide, especially near slack water. The lowest TSS and POC concentrations at the surface (<100 mg/L and <1 mg/L, respectively) were measured at this time. In bottom waters, the lowest TSS and POC concentrations (<250 mg/L and <10 mg/L, respectively) coincided with peak ebb current. Compositionally, most particles displayed similar %OC contents (1e2%) although there were some significant contrasts. For example, at maximum ebb, the particles near the seabed were considerably enriched in %OC (up to 5%) relative to all others. Additionally, surface particles collected during low tide displayed %OC contents (w1%) that were half those

4.4. Compositions of surface sediments The distributions of SA, %OC, OC:N and d13COC in surface sediments from the seabed are illustrated in Fig. 6. For these graphs, we plotted the average compositions from the 0e1 cm and 1e2 cm intervals of the kasten core samples to match the 0e2 cm intervals from the grab samples. In subsequent graphs, the compositions of these sedimentary horizons are presented separately. Surface area values ranged from 5 to 30 m2/g, with low SA values (<10 m2/g) measured in several stations from the northeast and southwest regions. Most of these samples were sandy sediments that were collected using

(b)

(a) -8.0

of bottom particles (Fig. 5). Both surface and bottom particles displayed atomic OC:N ratios that ranged from 15 to 20 mol:mol, with slightly higher values (up to 30 mol:mol) measured in a few of the samples collected after peak ebb. The d13COC values of both surface and bottom particles ranged from 26 to 31&, with the most depleted values being measured during peak ebb while the most enriched values generally coincided with slack water.

0-2 cm SA

-8.0

-8.2

0-2 cm %OC

-8.2 27

-8.4

1. 2

-8.4

30

-8.6

18

1.6

-8.8

1.2

14

-9.0

0.8

-9.0

10

0.4

6

0.0

142.6

143.4

143

143.8

142.6

144.2

(c) -8.0

%POC (wt.%)

-8.8

2.0

SA (m2/g)

22

2.4

-8.6

26

143

143.4

143.8

144.2

143.4

143.8

144.2

(d) 0-2 cm OC:N

-8.0

-8.2

-8.2 13

-8.4

20 15

-9.0

-8.6

-23

-8.8

-24 -25

-9.0

10 5

142.6

-22

-26

d13Coc (per mil)

-8.8

-21

30 25

-26

-8.4

OC:N (mol:mol)

-8.6

0-2 cm d13Coc

-27 -28

143

143.4

143.8

144.2

142.6

143

Fig. 6. Compositions of sediments, including mineral surface area (SA), organic carbon content (%OC), molar organic carbon:nitrogen ratios (OC:N) and stable isotopic composition of OC (d13COC), from the top 0e2 cm of the seabed throughout the Fly River delta region. The sampling locations are indicated by open diamonds. The values for the one sediment sample collected from the river channel are highlighted in the graphs.

M.A. Goni et al. / Estuarine, Coastal and Shelf Science 69 (2006) 225e245

the grab sampler (see Fig. 1). Intermediate values of surface area (10e25 m2/g) were measured throughout most of the other stations. These values are consistent with the predominance of silt-size mineral grains and low clay content (e.g., Baker et al., 1995; Harris et al., 1993; see below). The %OC contents of surface sediments ranged from 0.2 to over 2.5% (Fig. 6b). Most sediments displayed %OC contents comparable to the overall average (%OC ¼ 1.2
0.66%), with several stations in the southwest region and two stations in the northeast regions displaying lower values (%OC < 0.5%). These latter samples were sandy sediments recovered using the grab sampler. The highest %OC content was found in a station located in southernmost part of the southwest region along transect G. The %N contents of surface sediments ranged from 0.02% to 0.15% and for the most part displayed similar spatial distributions to %OC (plot not shown). The major difference was that, in contrast to %OC, the %N values of northeast sediments were slightly elevated relative to those from the southwest region. All of the surface sediments analyzed yielded very low levels of inorganic carbon (IC), with %IC contents ranging from undetectable to less than 0.3% (average for whole data set 0.00
0.014 wt.%, plot not shown). The contrasts in atomic OC:N ratios of surface sediments (Fig. 6c) illustrate the spatial differences in the sources/composition of OM throughout the Fly River delta region. Sedimentary OC:N ratios ranged from 11 to over 30 mol:mol, with northeast sediments generally displaying lower values than the rest of the data set. As was the case with %OC, the OC:N ratios of sediments from the southwest region were highly variable, with the highest ratios measured along the southernmost G transect. The d13COC compositions of surface sediments also displayed spatial contrasts within the study area, ranging from 23.4 to 27.2& (Fig. 6d). In spite of some variability within each of the regions, the d13COC signatures of most northeast sediments were slightly depleted in 13C relative to those measured in the samples from the southwest region. In selected samples, we conducted grain-size analyses (Table 1) that showed silt-sized particles dominate the sedimentary compositions throughout the Fly River subaqueous delta region. Silts accounted for over 60% of the materials in most analyzed samples (Table 1). Overall, clays accounted for most of the remaining sediment, with typical abundances

235

Table 1 Grain-size distributions for selected sediments. SA, mineral surface area; D50, median particle diameter; n.m., not measured Station

Horizon (cm)

SA %Sand %Silt %Clay Median D50 (m2/g) f (mm)

Northeast region (<10 m water depth) AA5 0.0e1.0 23 1 AA5 1.0e2.0 22 1 BB5 0.0e1.0 19 2 BB5 1.0e2.0 21 1 CC5 0.0e1.0 23 2 DD5 0.0e1.0 16 4 DD5 1.0e2.0 18 3 EE5 0.0e1.0 20 4 FF3 0.0e1.0 26 0 FF3 1.0e2.0 24 0

59 70 72 74 60 66 75 66 61 62

40 29 26 25 38 30 23 30 39 38

7.7 7.5 7.3 7.0 7.7 7.1 6.6 7.1 7.7 7.6

4.7 5.5 6.4 7.7 4.8 7.4 10.4 7.4 4.9 5.1

Northeast region (>10 m water depth) BB9 0.0e1.0 18 6 BB9 1.0e2.0 n.m. 25

74 62

20 13

6.9 7.2

8.2 6.9

Umuda Channel CT 0.0e1.0 CT 1.0e2.0 C2-A11 0.0e1.0

73 73 55

24 23 44

6.8 6.7 8.0

8.9 9.4 4.0

21 16 26

3 4 0

of 20e40%. The sand-size fraction was the least abundant, typically representing less than 6% of the mass of total particles. The major exception was the 1e2 cm horizon in BB9, in which sand accounted for 25% of the mass of particles, while clay represented 13%. Overall, these grain-size distributions are consistent with previous studies that showed silts dominated the sediment compositions in the Fly River subaqueous delta (e.g. Baker et al., 1995; Harris et al., 1993). The sediments analyzed for grain-size compositions displayed mineral SA values that ranged from 20 to 30 m2/g in samples dominated by silts, with lower values (w10 m2/g) characterizing sediments rich in sands (Table 1). As expected, in those samples where we performed grain-size and surface-area analyses, we observed a significant negative correlation between the median particle diameter (D50) and SA values (SA ¼ 1.31  D50 þ 29.8; r2 ¼ 0.70, a ¼ 0.05, n ¼ 14; plot not shown). The radiocarbon compositions of organic matter in the surface horizons from selected sediments are presented in Table 2. All the seabed sediments displayed relatively depleted D14COC values that ranged from 300 to 540& (Table 2). The

Table 2 Compositions of surface (0e1 cm) sediments from selected locations of the Fly River delta. SA, mineral surface area; %OC, weight percent organic carbon content; [OC:N]a, atomic organic carbon:nitrogen ratio; d13COC, stable isotopic ratio of organic carbon; D14COC, radiocarbon isotope ratio of organic carbon; Age, radiocarbon age in years before present (ybp; Stuiver and Polach, 1977); f-modern, radiocarbon concentration as fraction modern Parameter

Units

SA %OC [OC/N]a d13COC D14COC Age f-modern

m2/g wt.% mol:mol & & ybp fraction

Inshore northeast

Offshore northwest

Southwest

CC5

EE5

FF3

BB9

CC9

C2-A11

D2

E15

22.9 1.6 14 26.5 340.6 3290 0.66

20.2 1.3 14 26.4 343.1 3325 0.66

25.6 1.2 13 26.4 301.2 2825 0.70

17.7 1.2 14 26.2 401.5 4070 0.60

15.0 0.82 17 25.8 462.8 4925 0.54

25.7 1.3 12 26.0 388.8 3900 0.62

10.4 1.1 17 26.5 538.9 6165 0.46

14.6 1.2 17 26.3 492.9 5400 0.51

236

M.A. Goni et al. / Estuarine, Coastal and Shelf Science 69 (2006) 225e245

corresponding ages for the sedimentary OM on the seabed surface ranged from 2800 to >6000 years before present (ybp). The fraction of modern carbon in these samples ranged from 0.46 to 0.7. The samples from the inshore northeast region displayed significantly enriched D14COC values and younger ages than the rest of the samples analyzed. Overall, the compositions presented in Table 2 indicate surface sediments from the Fly River delta-clinoform system contain OM of advanced age (4238
1159 year) that is characterized by relatively elevated OC:N ratios (15
2 mol:mol) and depleted d13COC signatures (26.3
0.3&). The D14COC values and 14C ages measured in these study are comparable to those measured by Aller and Blair (2004), who analyzed sediments collected farther east along the clinoform region located in the central part of the Gulf of Papua. 5. Discussion 5.1. General oceanographic conditions Because salinity profiles were recorded over 10 days of contrasting wave and tide climate, it is difficult to quantitatively interpret their distribution (Figs. 2 and 3). However, it appears that during the conditions of low river discharge and low wave energy that characterized the January 2003 period, the supply of freshwater from the Fly River to the inner shelf was relatively small and predominantly constrained to the northeast region. The surface salinity distributions during this El Nin˜o period were similar to those measured by Robertson et al. (1993), who occupied stations within the Fly River delta system in February 1990 during similar conditions of low river discharge, neap tides and low wave activity. Lower salinities are much more prevalent during periods of higher discharge and spring tides (Harris et al., 2004; Wolanski et al., 1997). For example, Wolanski et al. (1997) measured salinities of 10e20 in the southern distributary channel during spring tides in April, and June 1995, while salinities ranged

from 15 to 30 during neap tides. The effects of tidal stage on salinity were also evidenced by the study of Harris et al. (2004), who measured salinities ranging from 5 to 15 in the northern distributary channel during spring tides in January 1994, whereas salinities ranged from 20 to 30 during neap tides in the same month. These salinity contrasts reflect large differences in the flux of fresh water through the delta distributaries between neap- and spring-tide conditions. In January 2003, we sampled the northeast and southwest regions during periods of low tidal energy, when seaward transport may have been low. On the other hand, the sampling along the western part of the river channel region (transect A) took place during a spring tide and may be reflective of more energetic dispersal. Overall, excluding the samples of fluid mud, the highest TSS, POC and PN concentrations in both surface and bottom waters were found in stations along the Fly River channel, followed by samples from the northeast region (Table 3). The stations with near freshwater salinities (<5) displayed significantly higher TSS, POC and PN concentrations than stations with brackish salinities (>5) (Table 3). The trends in TSS, POC and PN concentrations measured throughout the Fly River delta during the January 2003 cruise were similar to those measured during other periods of relatively low river discharge and low wave energy. For example, TSS concentrations in surface waters of the delta in February 1990 and January 1993 ranged from w5 to w300 mg/L (Robertson et al., 1993, 1998). In contrast, TSS concentrations during the more energetic periods of the year (e.g. trade winds) were significantly higher, ranging from 100 to over 2000 mg/L in July/August 1989 and September 1991 (Robertson et al., 1993, 1998). Davies (2004) observed similar seasonal contrast in TSS concentrations, with relatively low values (<160 mg/ L) during January 1999 and elevated concentrations (up to 350 mg/L) during August-September 1997. Thus, it appears particle export from the Fly River into the subaqueous delta during the January 2003 cruise was quite low and probably unrepresentative of periods with more typical high river

Table 3 Average suspended particulate concentrations. Sal, salinity; s.d., standard deviation; s.e., standard error; n, number of samples Sample type Surface waters Whole data set (n ¼ 82) River channel (Sal < 5) (n ¼ 8) River channel (Sal > 5) (n ¼ 11) River channel (all) (n ¼ 19) Southwest (n ¼ 27) Northeast (n ¼ 36) Bottom watersa Whole data set (n ¼ 63) River channel (Sal < 5) (n ¼ 5) River channel (Sal > 5) (n ¼ 4) River channel (all) (n ¼ 9) Southwest (n ¼ 20) Northeast (n ¼ 34) a

Excluding fluid mud samples.

TSS (mg/L) 26.6 169 28.4 86.2 8.8 9.8

400 1769 397 1254 149 340

s.d.

49.5 56.8 20.0 80.7 4.6 6.6

743 1325 268 1236 173 669

s.e.

POC (mg/L)

s.d.

s.e.

PN (mg/L)

s.d.

s.e

5.7 21.5 6.3 19.6 0.9 1.1

0.83 2.18 0.75 1.35 0.32 0.94

1.24 0.48 0.65 0.92 0.41 1.63

0.14 0.17 0.19 0.21 0.08 0.27

0.05 0.18 0.06 0.11 0.02 0.04

0.05 0.04 0.05 0.08 0.02 0.03

0.01 0.01 0.02 0.02 0.00 0.00

98 593 155 437 38.6 122

7.11 13.6 9.43 11.5 4.85 7.30

7.3 19.0 6.2 13.3 6.14 5.02

0.99 9.51 3.11 4.70 1.45 0.95

1.66 0.91 0.43 0.67 0.13 0.40

7.27 1.26 0.36 0.89 0.13 0.36

1.00 0.63 0.18 0.32 0.03 0.07

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discharge. Sediment delivery to the shallow nearshore regions of the subaqueous system during El Nin˜o periods appears to be diminished, resulting in TSS concentrations that are low relative to non-El Nin˜o years.

of total suspended solids. However, there was some variability in the relationship between POC and TSS. For example, among surface samples (Fig. 7a), elevated POC concentrations were measured at several northeast stations with relatively low TSS values (<30 mg/L). In the case of bottom water samples, several samples from the river channel, northeast and southwest region displayed low POC contents relative to TSS (Fig. 7b). Also, at least one sample from the southwest region displayed elevated POC concentrations relative to TSS. Such variability in the relationship between POC and TSS suggests significant spatial and temporal heterogeneity in the composition of particles at the time of sampling (see discussion below).

5.2. POM distribution 5.2.1. Water column Based on the general positive correlation between POC and TSS concentrations (Fig. 7), the amount of suspended POM in the water column throughout the Fly River delta during January 2003 appeared to be controlled primarily by the concentrations

(a)

237

Surface Samples 8 Y = 0.0123 X + 0.210 n = 85; r2 = 0.95

*

7 6

*

POC (mg/L)

5

*

4

1 0.8

*

3

0.6

*

2

0.4 0.2

1

0 0

20

40

0

200

60

0 0

100

200

300

400

500

TSS (mg/L)

(b)

Bottom Samples

50 Y = 0.0136 X + 2.89 n = 64; r2 = 0.87

POC (mg/L)

40

30

*

10 8

20

*

6 4

10

2 0

* 0 0

1000

2000

* 3000

400

600

800

4000

TSS (mg/L) River Channel

Anchor Station

Northeast

Southwest

Fig. 7. Relationships between total suspended solids (TSS) and particulate organic carbon (POC) concentrations in (a) surface-water samples and (b) bottom-water samples throughout the Fly Delta. Indicated in the graphs are the linear relationships between these two parameters. Data points marked with * were not used to calculate regressions.

M.A. Goni et al. / Estuarine, Coastal and Shelf Science 69 (2006) 225e245

5.2.2. Seabed surface The amount of OC present in surface sediments from the Fly River subaqueous-delta clinoform is loosely correlated to the mineral surface area (Fig. 10). In fact, the majority of samples displayed SA-normalized OC loadings between 0.5 and 1.0 mg C/m2 of sediment, which is the typical range of most

(a)

Surface Samples 500

400

TSS (mg/L)

Overall, the general relationship between PN and TSS (not shown) was similar to the trends between POC and TSS. In surface waters, the highest TSS concentrations were consistently measured in low-salinity samples, with decreased concentrations (<25 mg/L) detected as salinities increased over 15 (Fig. 8a). Similar relationships with salinity were observed for POC and PN (plots not shown). These results are consistent with the removal of particles from the buoyant river plume as it mixes with ocean water (Ayukai and Wolanski, 1997). The distributions of salinity and TSS suggest that under the low discharge conditions encountered during the cruise this process occurred within the delta distributary channels. An overall similar trend between TSS and salinity was evident in bottom waters (Fig. 8b). However, there was much higher variability in the TSS and salinity data near the seabed than those measured at the surface, suggesting other factors in addition to ionic strength and estuarine convergence regulated the concentrations of suspended particles in the benthic boundary layer of the water column. Using the data from the anchor station, we investigated the relationship between shear stress velocities in the bottom boundary layer and TSS concentrations (Fig. 9), as well as POC and PN (plots not shown). Within the river channel, TSS concentrations in bottom waters increased as shear stress decreased (Fig. 9). Notably, surface-water TSS concentrations decreased during this period of the anchor station (Fig. 5d). Combined, these measurements indicate particles settled towards the bed during slack periods while they were distributed throughout the water column during maximum current. The fact that at the highest shear velocities, suspended particles near the seabed were characterized by elevated %POC contents (Fig. 5f) and the most negative d13COC values (Fig. 5h) is consistent with the presence of large-sized, vascular plant fragments (which tend to be enriched in POC and depleted in 13C) during peak ebb. Outside the distributary channels, however, the relationship between wave and current-induced shear stress and TSS (as well as POC and PN) is not as clear. It is likely that during the low-energy conditions encountered during much of the cruise, localized advection from proximal sources (e.g. river mouth, shallow tidal flats and bars) affected TSS concentrations to a much higher degree than wave/current induced resuspension from the seabed. Under these quiescent conditions, particle resuspension was minimal. During spring tides, resuspension was probably more significant, leading to measurements of TSS and POC concentrations that were generally low but highly variable. More detailed examination of the relationship between physical forcings and particulate concentrations are explored in a separate publication (Ogston et al., in preparation).

ln(Y) = -1.16 ln(X) + 5.92 n = 88; r2 = 0.72

300

200

100

0 0

5

10

15

20

25

30

35

Salinity

(b)

Bottom Samples 4000

* 3000

TSS (mg/L)

238

ln(Y) = -0.766 ln(X) + 7.54 n = 72; r2 = 0.38

2000

1000

0 0

10

20

30

40

Salinity River Channel

Anchor Station

Northeast

Southwest

Fig. 8. Relationships between total suspended solids (TSS) concentrations and salinity in (a) surface-water samples and (b) bottom-water samples throughout the Fly Delta. Indicated in the graphs are the exponential relationships between these two parameters. Data points marked with * were not used to calculate regressions.

marine shelf sediments (Keil et al., 1994; Mayer, 1994; Hedges and Keil, 1995). Only a few samples had OC:SA values that were lower than the typical shelf sediments and most of these were relative coarse samples collected via grab and characterized by high sand content. Three samples from the southwest region (specifically from the E and G transects) plot above the typical OC:SA relationship. All three of these

M.A. Goni et al. / Estuarine, Coastal and Shelf Science 69 (2006) 225e245

4000

TSS (mg/L)

3000

ln(Y) = -0.838 ln(X) + 6.89 n = 16; r2 = 0.47 2000

1000

0 0

1

2

3

4

5

Bottom Shear Velocity (u*, cm/s) River Channel

Anchor Station

Fig. 9. Relationship between total suspended solids (TSS) concentrations in bottom waters and shear velocity near the seabed (u*) for samples collected within the main distributary channel of the Fly River, including anchor and non-anchor stations.

samples displayed [OC:N]a ratios > 20, suggesting the presence of elevated amounts of vascular plant fragments likely contributed to the high OC:SA ratios (>1.4 mg C/m2). Most surface sediments from the Fly delta system display higher SA-normalized OC loadings than those from other river-dominated ocean margins, such as those associated 30

25

OC (mg/g)

20

239

with the Amazon, Mississippi-Atchafalaya and Huange He rivers (Keil et al., 1997; Gordon et al., 2001; Gordon and Goni, 2004). The OC:SA ratios of these samples are also higher than those measured in deeper horizons of gravity and kasten cores collected in the central part of the Gulf of Papua, which ranged from 0.25 to 0.5 mg C/m2 (Aller and Blair, 2004). The low OC:SA ratios measured in sediments from these deltaic systems were interpreted to be the result of post-depositional removal of terrigenous OM. Unlike these previous studies, the surface sediments from the Fly River delta retain relatively high OC loadings and show little evidence for the preferential removal of terrigenous OC from particles prior to their deposition on the seabed. In that respect, Fly River delta sediments are similar to systems such as the Eel River shelf, Winyah Bay and the Mackenzie River shelf, which display terrigenous OC loadings between 0.5 and 1.0 mg C/m2 (e.g. Blair et al., 2003; Goni et al., 2003, 2005). The ‘‘monolayer-equivalent’’ OC-loadings (Mayer, 1994) of Fly River deltaic sediments may be related to the geomorphologic characteristics of the Fly/Strickland river basin, mainly its high relief and relatively narrow flood plain (Dietrich et al., 1999; Pernetta and Osborne, 1990; Pickup and Springs, 1984). Because of these characteristics, particles are likely transported more directly by the Fly River to its delta than in other rivers with extensive lowland flood plains (such as the Amazon and Mississippi). As part of their coupled watershed-seabed model, Blair et al. (2004) invoked the more direct export of POC by rivers lacking extensive flood plains (such as the Eel River) as a key factor that minimizes degradation during dispersal and may contribute to the relatively elevated carbon loadings present in margin sediments. Our results from the Fly River delta are consistent with this conceptual model of POC transport and burial. The nature of the OM exported by the Fly River can also contribute to the high OC:SA ratios, especially if POM includes large amounts of recalcitrant fractions such as highly altered soil carbon and/ or fossil carbon (e.g. Blair et al., 2003; Goni et al., 2005). Below we discuss in more detail the sources and provenance of OM in this system. 5.3. POM compositions

15

10

5 Grab Samples

0 0

5

10

15

20

25

30

SA (m2/g) River Channel

Northeast

Southwest

Fig. 10. Relationship between mineral surface area (SA) and organic carbon content (OC) in surface sediments collected throughout the Fly River delta via kasten core (0e1 cm, 1e2 cm) and grab (0e2 cm). The area between the two dashed lines illustrates the ranges typical of shelf sediments (‘‘monolayerequivalent’’ OC loadings).

The compositions of POM associated with suspended and seabed samples in the Fly River Delta are summarized in Table 4. Based on the average %OC values, suspended particles from surface waters as a whole appeared to be only slightly more enriched in OM than their bottom-water counterparts. In contrast, surface sediments yielded significantly lower %OC values consistent with diminished OM content. Suspended particles in surface waters from the northeast region were the most enriched in OM (%OC of 8.2
9.2%) whereas the surface and bottom particles from the river channel were the most OM-poor (%OC of 21
1.4%). Within the main stem of the river channel, surface and bottom samples collected at low salinities (<5) were characterized by significantly lower %OC contents (1.4
0.3%) than their counterparts collected at higher salinities (2.6
1.7%). The

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Table 4 Average particle compositions. Sal, salinity; s.d., standard deviation; s.e., standard error; n.m., not measured; n.a., not applicable; n, number of samples Sample type

d13COC (per mil)

s.d.

s.e.

2.2 1.2 2.4 1.5 1.2 5.1

27.9 29.7 27.2 28.1 27.4 28.2

1.8 0.9 2.5 2.4 1.4 1.8

0.2 0.4 0.8 0.6 0.3 0.4

16.3 3.1 23.3 16.7 16.9 14.9

2.3 1.5 11.7 5.9 4.5 2.9

29.6 30.3 29.8 29.9 29.5 29.8

1.2 n.a. 0.1 0.3 1.2 1.2

0.2 n.a. 0.1 0.2 0.3 0.2

16.7 18.7

2.9 5.3

0.9 0.9

29.4 30.2

2.0 0.9

0.7 0.2

17.0 12.9 18.5 14.6

9.2 0.2 11.3 2.1

1.4 0.2 2.2 0.5

25.5 26.4 25.1 26.1

1.1 0.1 1.1 0.9

0.2 0.1 0.2 0.2

%OC (wt.%)

s.d.

s.e.

%N (wt.%)

s.d.

s.e.

[OC:N] (atomic)

s.d.

Surface waters Whole data set (n ¼ 82) River channel (Sal < 5) (n ¼ 8) River channel (Sal > 5) (n ¼ 11) River channel (all) (n ¼ 19) Southwest (n ¼ 27) Northeast (n ¼ 36)

5.04 1.35 2.63 2.10 3.01 8.18

6.70 0.30 1.68 1.43 1.45 9.21

0.78 0.11 0.53 0.35 0.29 1.63

0.34 0.10 0.20 0.16 0.23 0.54

0.37 0.01 0.13 0.11 0.15 0.49

0.05 0.00 0.04 0.03 0.03 0.09

20.6 15.7 18.1 17.1 18.4 24.5

18.8 3.2 7.7 6.2 6.0 28.2

Bottom waters Whole data set (n ¼ 63) River channel (Sal < 5) (n ¼ 5) River channel (Sal > 5) (n ¼ 4) River channel (all) (n ¼ 9) Southwest (n ¼ 20) Northeast (n ¼ 34)

4.21 1.39 3.39 2.24 5.38 4.31

4.91 1.17 1.16 1.51 7.23 3.55

0.66 0.58 0.67 0.57 1.70 0.67

0.15 0.09 0.13 0.11 0.09 0.21

0.14 0.06 0.04 0.06 0.05 0.17

0.02 0.03 0.02 0.02 0.01 0.03

27.0 17.7 29.7 23.7 37.0 23.9

Anchor station Surface particles (n ¼ 15) Bottom particles (n ¼ 15)

1.50 2.13

0.30 1.00

0.07 0.24

0.11 0.14

0.03 0.11

0.01 0.03

Surface sediments (0e2 cm) Whole data set (n ¼ 43) Channel (n ¼ 1) Southwest (n ¼ 27) Northeast (n ¼ 15)

1.08 1.12 1.03 1.17

0.78 n.a. 0.93 0.44

0.12 n.a. 0.18 0.11

0.07 0.11 0.06 0.09

0.04 0.00 0.03 0.04

0.01 0.00 0.01 0.01

surface samples from the seabed were uniformly characterized by low %OC contents (1.1
0.8%) and displayed none of the spatial differences evident among the suspended particulates. Overall, these trends are comparable to those measured by Robertson et al. (1993), who detected %OC contents of 2e12% in suspended particles collected during monsoon and trade-wind conditions in the Fly River Delta. We speculate that while low %OC contents (1 < %OC < 3) are consistent with the presence of OM-bearing mineral particles (e.g. clays and silts), elevated %OC values (w10%) indicate significant contributions from OM-rich detritus (e.g. plant fragments, plankton). Hence, while OM detritus appears to be relatively important in the water column during the low river-discharge conditions in January 2003, the contributions from these materials are significantly diluted in the seabed. The preferential degradation of OM detritus prior to its incorporation into surface sediments likely contributes to these contrasts. However, the suspended particles collected during January 2003, a period of low discharge due to El Nin˜o conditions, are probably not representative of the bulk of the particulate load exported by the Fly/Strickland River system. Instead, the compositions of surface sediments from the seabed may reflect particles exported during previous periods when river discharge was much higher. This caveat applies to the other compositional parameters discussed below. The average OC:N ratios indicate suspended particles in surface waters were significantly more N-rich (P ¼ 0.1) than their bottom counterparts (Table 4). Among suspended particles from surface waters, those collected from the river channel and southwest regions consistently displayed moderately

s.e.

elevated OC:N values of 16 to 18 (mol:mol), while the particles form the northeast region displayed more variability and markedly higher averages (up to OC:N w40 mol:mol; Table 4). In the case of bottom waters, suspended particles from the southwest regions displayed the most elevated OC:N ratios (w55 mol:mol) of the whole data set, followed by those from the northeast (Table 2). Particles from the low-salinity region of the river channel displayed the lowest OC:N ratios (w18 mol:mol) of all the bottom water samples, which were comparable to those measured at the surface in the river channel and southwest region. Seabed samples were characterized by relatively low OC:N ratios (17
9 mol:mol), with the lowest values (13e14 mol:mol) measured within the river channel and the northeast region. Samples from the southwest region were characterized by higher OC:N ratios (19
11 mol:mol). The compositional ranges we observed were comparable to those reported by Robertson et al. (1998), who measured molar C:N ratios in suspended particles during monsoon and trade-wind seasons that ranged from 7 to 30 mol:mol. Suspended particles from bottom waters yielded d13COC compositions that were significantly depleted (29.5
1.2&) relative to their surface counterparts (27.9
1.8&). Among surface samples, those collected from the main stem of the river channel and the southwest region were the most enriched (27.2& and 27.4&, respectively), whereas all of the bottom particles displayed more depleted d13COC signatures (<29&) independent of location. Seabed samples displayed enriched d13COC signatures (25.5
1.1&) relative to the water column particles. These data are comparable with previously determined d13COC signatures of surface sediments from the Fly

M.A. Goni et al. / Estuarine, Coastal and Shelf Science 69 (2006) 225e245

(a)

Surface Waters River Channel Northeast Southwest

0.18 0.16 0.14

[ON:OC]a (mol:mol)

River delta, which ranged from 25 to 28&, and are consistent with a predominant terrestrial C3 plant source (Bird et al., 1995). The compositions observed in the water column and surface of the seabed overlap with the 13COC signatures of OM-rich humus layers from forest soils (27.8
0.9&) and from high altitude (>3500 m above sea level) grasslands (25.0
1.1&) throughout Papua New Guinea (Bird et al., 1994). In contrast, they are quite different from the compositions obtained from low (0e1500 m) and mid-altitude (1500e 3500 m) grasslands (15.9
3.9& and 19.5
2.5&, respectively), which are dominated by C4 vegetation (Bird et al., 1994). They also differ from marine phytoplankton sources, which typically range from 19& to 21& (Goericke and Fry, 1994).

241

Marine Algae

Riv. & Est. Algae

0.12 Forest Soil Organic Matter

0.10 0.08 0.06 0.04 0.02

C4 Vascular Plants

C3 Vascular Plants

5.4. POM sources 0.00

(b)

Bottom Waters River Channel Northeast Southwest

0.18 0.16

[ON:OC]a (mol:mol)

0.14

Marine Algae

Riv. & Est. Algae

0.12 Forest Soil Organic Matter

0.10 0.08 0.06 0.04 0.02

C4 Vascular Plants

C3 Vascular Plants

0.00

(c)

Surface Sediments

0.18 0.16 0.14

[ON:OC]a (mol:mol)

All of the suspended and seabed samples analyzed in this study display linear relationships between OC and N that show positive carbon intercepts at zero N values (plots not shown). Such observations indicate the N in these samples appears to be organic (e.g., Goni et al., 2003). Thus, we can combine the organic carbon:organic nitrogen atomic ratios ([OC:ON]a) with the d13C compositions to constrain the provenance of the OM present in water column and sediments of the Fly River delta (Fig. 11). In this case, we chose to graph ON:OC ratios rather than OC:ON ratios because we can plot two carbon-normalized parameters (i.e. ON:OC and 13C:12C ratios) and at the same time help constrain the elemental ratios of N-depleted samples (e.g. ON:OC w0 rather than OC:ON wN). This approach takes advantage of the distinct signatures of the different types of OM typically present in river-dominated margins (e.g., Goni et al., 1998, 2003; Hedges et al., 1986, 1988). It is difficult to obtain quantitative estimates of OM abundances from these plots because diagenesis can alter the original C:N and d13COC compositions (e.g., Benner et al., 1987; Rice and Hanson, 1984). However, they can be used to illustrate major trends in the provenance of coastal organic materials (e.g., Cifuentes et al., 1996; Dittel et al., 1997; Dittmar and Lara, 2001; Goni et al., 2003, 2005; Gonneea et al., 2004; Gordon et al., 2001; Kennedy et al., 2004; LallierVerges et al., 1998). For example, the suspended particles from surface waters plot within the mixing region of two endmembers, terrigenous C3 plants and fresh water/estuarine phytoplankton (Fig. 11a). In contrast, the suspended particles from bottom waters plot in the region representative of C3 vascular plants (Fig. 11b). Hence, at the time of our sampling, there was very little contribution from C4 terrestrial plant sources and from marine phytoplankton to the suspended POM in the water column of the Fly River delta. Instead, it appears most of the material in suspension was ultimately derived from C3 terrigenous vegetation sources (mainly vascular plant detritus and soil OM) with some contributions from freshwater/estuarine algae. The d13COC signatures of many surface particulate samples and of most bottom-water POM samples (Fig. 11) are relatively depleted in 13C (32& > d13COC > 28&). In many

Marine Algae

Riv. & Est. Algae

River Channel Northeast (kc) Northeast (gb) Southwest (kc) Southwest (gb)

0.12 Forest Soil Organic Matter

0.10 0.08 0.06 0.04 0.02

C4 Vascular Plants

C3 Vascular Plants

0.00 -32 -30 -28 -26 -24 -22 -20 -18 -16 -14 -12 13C OC

(‰)

Fig. 11. Plots of the stable carbon isotopic composition of particulate organic carbon (d13COC) versus the molar nitrogen:organic carbon ratios (N:OC) for (a) suspended POM from surface waters, (b) suspended POM from bottom waters and (c) sedimentary POM from the seabed surface throughout the Fly River delta. The compositional ranges of different end members are included in the plot and are discussed in the text.

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cases, these negative d13C values coincide with low [ON:OC]a ratios, which indicate vascular plants are the major source. Woody tissues from C3 tropical vegetation, which tend to be low in N (ON:OC < 0.02), typically yield d13COC compositions that range from 25 to 28& (e.g., Goni and Eglinton, 1996; Hedges et al., 1986). In contrast, tropical leaf-detritus is more N-rich (0.05 < ON:OC < 0.02) but can yield highly depleted d13C signatures relative to their woody counterparts, ranging from 29 to 32& (Hedges et al., 1986; Goni and Eglinton, 1996). Plant tissues from low altitudes tend to be depleted in d13COC relative to high-altitude counterparts because of differences in gas partial pressure and temperature (Korner et al., 1988, 1991). Most significantly for our study, mangrove leaves can be significantly depleted in 13C, with bulk d13COC values that range from 26 to 32& (e.g., Bouillon et al., 2003; Dittmar and Lara, 2001; Fry and Smith, 2002). Factors such as salinity, humidity and temperature affect the stomatal conductance of mangrove leaves, which in term largely control their internal of CO2 concentrations and d13COC compositions (e.g., Wooller et al., 2003). Because mangrove vegetation dominates many of the islands in the Fly River delta, we speculate that a large fraction of the POM with ON:OC ratios <0.05 and d13COC values <28& is probably derived from local mangrove and/or lowland forest sources. Most of the surface sediments and some of the suspended particles from surface waters display intermediate ON:OC ratios between 0.06 and 0.10 (e.g. 16 > OC:ON > 10) relatively depleted d13C signatures (28& > d13COC > 24&). These samples plot midway between the terrigenous C3 vascular plant and river/estuarine phytoplankton end-member compositions (Fig. 11) possibly suggesting their OM may be mixture of these sources. However, the depleted D14COC values (409
83&) and old OC ages (4238
1159 ybp) (Table 2) suggest that a more likely explanation for these trends is the significant input of OM from older, mineral soils eroded from the drainage basins of the Fly and Strickland Rivers. Multiple studies have shown OM in soils to be enriched in N relative to vascular-plant detritus as the result of N-incorporation into the soil matrices during the decomposition process (e.g., Hsieh, 1988; Melillo et al., 1989; Sorensen, 1981; Tiessen et al., 1984). Similarly, in many instances, OM in soils is enriched in 13C relative to vegetation through the addition of microbial biomass (e.g., Ehleringer et al., 2000; Gleixner et al., 1993; Hobbie et al., 1999) and isotopic changes associated with soil organic matter formation such as decay, desorption and sorption (e.g., Burke et al., 2003; Cifuentes and Salata, 2001; Quideau et al., 2003; Wynn et al., 2005). Therefore, we propose that a large fraction of the OC present in the surface sediments of the Fly River subaqueous-delta is derived from aged soil OM that is enriched in N but still contains the C3-plant d13C signature characteristic of the Fly River drainage basin (e.g., Bird et al., 1994). Mineral soils are typically low in OC content, consistent with our observation that most of the seabed samples display %OC values of 0.5% to 2%. Mineral soils are also enriched in recalcitrant carbon fractions with turnover times of >1000 years (e.g., Trumbore, 1993), which helps explain the old ages determined in

this study. At this point it is not possible to delineate the specific provenance of the mineral-bound soil OM in the delta sediments, but it is likely to originate from the upland watersheds of the Fly and Strickland Rivers, which are characterized by C3 vegetation (both forests and grasslands; Bird et al., 1994) and high levels of erosion (Pickup and Springs, 1984). In the absence of C:N data for local Papua New Guinea soils, we have included the compositional range for temperate forests (Goni and Thomas, 2000) as an example of where soil OM with predominant C3 vegetation would plot in this graph (Fig. 11). It is also possible that the old ages for the OM on the seabed surface are partially due to the presence of fossil carbon associated with sedimentary rocks, which are important sources of sediment to the rivers through erosion and landslides (Pickup and Springs, 1984). Old organic matter present in soils and sedimentary rocks can make up a significant fraction of the particulate load delivered by rivers to land-ocean margins (e.g., Goni et al., 1998, 2005; Gordon and Goni, 2004; Masiello and Druffel, 2001; McCallister et al., 2004; Nagao et al., 2005; Ogrinc et al., 2005; Raymond et al., 2004). This aged OM appears to be most important in the upper reaches of streams (Mayorga et al., 2005; Townsend-Small et al., 2005), in mountainous rivers (Leithold and Blair, 2001; Blair et al., 2003) and in some major Arctic rivers (Goni et al., 2005). All of these systems have in common a more direct linkage between erosion, transport and lower dilution/exchange of old soil OC by modern carbon sources (e.g., Blair et al., 2004). We propose that the geomorphologic characteristics of the Fly/Strickland River drainage basin (i.e. steep slopes, high physical erosion rates, relatively narrow floodplain) are reflected in the nature of the OM deposited in its subaqueous delta-clinoform region. On-going studies on the biogeochemical composition of sediments from the Fly River delta-clinoform system should help further constrain the composition and ultimate sources of the organic materials deposited in this wet and mountainous land-ocean margin. 6. Summary Soil organic matter, debris from vascular plants, and riverine/estuarine plankton are the predominant sources of organic materials in the Fly River subaqueous delta-clinoform system. During the condition of low river discharge and low wind energy that characterized the January 2003 study period, there was little evidence for the active seaward transport of these land-derived materials. Suspended particulate concentrations throughout the subaqueous delta-clinoform region were low (<1 g/L) relative to those measured during periods of higher river discharge. Most of the suspended POM appeared to be derived from local vascular plants and fresh/brackish water plankton sources. In contrast, surface sediments from the seabed contained relatively old OM that was highly diluted by mineral materials. The bulk composition of this sedimentary OM was consistent with a soil OC origin, mainly in the form of aged organic materials associated to inorganic matrices, derived from the erosion of the steep upland regions of the

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Fly/Strickland river watershed. The OC loading levels measured in surface sediments throughout the Fly River delta region suggest most of this soil OC is relatively unreactive and has not undergone the preferential losses observed in other deltas. It is likely that the unique characteristics of the drainage basin, which facilitate the erosion and direct export of upland soils, are responsible for the recalcitrant nature and efficient dispersal of terrigenous OC in this system. Acknowledgments The authors thank Wayne McCool, David Shelley, Joel Rowland, Debbie Nittrouer and Marie Bera for their help in the collection of data and samples during the January 2003 cruise. The RV Western Venturer and her crew provided a unique opportunity to study the Fly River delta. We acknowledge the generous logistical help provided by Jim Veness (PNG Interior Ministry) and Ok Tedi Mining Limited. We also would like to recognize Hugh Davies, and Sioni Sioni of the University of Papua New Guinea for their invaluable help and hospitality during our work in PNG. This research was supported by funds from the National Science Foundation through the Chemical Oceanography (OCE-0220600 grant to M.G.) and Margins Programs (OCE-0203351 and OCE0504616 grants to C.N. and A.O.). Radiocarbon analyses were performed under the auspices of the U.S. Department of Energy by the University of California/Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. Funding for the radiocarbon analyses was provided to T. Guilderson by UC/LLNL 04-ERD-060. This paper benefited from comments by two anonymous reviewers. References Aller, R.C., 1998. Mobile deltaic and continental shelf muds as suboxic, fluidized bed reactors. Marine Chemistry 61, 143e155. Aller, J.Y., Aller, R.C., 2004. Physical disturbance creates bacterial dominance of benthic biological communities in tropical deltaic environments of the Gulf of Papua. Continental Shelf Research 24, 2395e2416. Aller, R.C., Blair, N.E., 2004. Early diagenetic remineralization of sedimentary organic C in the Gulf of Papua deltaic complex (Papua New Guinea): net loss of terrestrial C and diagenetic fractionation of C isotopes. Geochimica et Cosmochimica Acta 68, 1815e1825. Aller, R.C., Hannides, A., Heilbrun, C., Panzeca, C., 2004. Coupling of early diagenetic processes and sedimentary dynamics in tropical shelf environments: the Gulf of Papua deltaic complex. Continental Shelf Research 24, 2455e2486. Alongi, D.M., 1995. Decomposition and recycling of organic matter in muds of the Gulf of Papua, northern Coral Sea. Continental Shelf Research 15, 1319e1337. Alongi, D.M., Christoffersen, P., Tirendi, F., Robertson, A.I., 1992. The influence of freshwater and material export on sedimetary facies and benthic processes within the Fly Delta and adjacent Gulf of Papua (Papua New Guinea). Continental Shelf Research 12, 287e326. Ayukai, T., Wolanski, E., 1997. Importance of biologically mediated removal of fine sediments from the Fly River Plume, Papua New Guinea. Estuarine, Coastal and Shelf Science 44, 629e639. Baker, E.A., Harris, P.T., Keene, J.B., Short, S.A., 1995. Patterns of sedimentation in the macrotidal Fly River delta, Papua New Guinea. In: Flemming, B.W., Bartoloma, A. (Eds.), Tidal Signatures of Modern and

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