Distribution and sources of particulate organic matter in the water column and sediments of the Fly River Delta, Gulf of Papua (Papua New Guinea)
Introduction
Over 80% of the global burial of organic carbon (OC) in the oceans (∼0.1 × 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 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, Blair et al., 2004, Goni et al., 2005, Gordon and Goni, 2004, Stein and Macdonald, 2004). Understanding the mechanisms that control OC cycling in river-dominated margin systems requires multidisciplinary field-based 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, Kineke et al., 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 sequestration—rapid and abundant delivery of sediments and entrainment of recalcitrant organic materials—are 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.
Section snippets
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 ∼79,000 km2 that
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 A′, AA, BB, CC, DD, EE, FF), the southwest region (composed of stations along
Characteristics of the water column
Hydrographic and oceanographic conditions changed throughout the sampling period (8–17 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 Niño conditions, river discharge during January 2003 was lower than the normal average flow (Dietrich, personal
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 (Fig. 2, Fig. 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 Niño
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)
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
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