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

https://doi.org/10.1016/j.ecss.2006.04.012Get rights and content

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 Niño, 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 (δ13COC < −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 δ13COC compositions (−25.5 ± 1.1‰). The radioisotopic compositions of OC from a selected set of seabed samples (Δ14COC 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, mineral-bound 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.

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

References (103)

  • L.A. Cifuentes et al.

    Significance of carbon isotope discrimination between bulk carbon and extracted phospholipid fatty acids in selected terrestrial and marine environments

    Organic Geochemistry

    (2001)
  • L.A. Cifuentes et al.

    Isotopic and elemental variations of carbon and nitrogen in a mangrove estuary

    Estuarine, Coastal and Shelf Science

    (1996)
  • P. Davies

    Nutrient processes and chlorophyll in the estuaries and plume of the Gulf of Papua

    Continental Shelf Research

    (2004)
  • A.I. Dittel et al.

    Carbon and nitrogen sources for shrimp postlarvae fed natural diets from a tropical mangrove system

    Estuarine, Coastal and Shelf Science

    (1997)
  • T. Dittmar et al.

    Molecular evidence for lignin degradation in sulfate-reducing mangrove sediments (Amazonia, Brazil)

    Geochimica et Cosmochimica Acta

    (2001)
  • M.A. Goni et al.

    Stable carbon isotopic analyses of lignin-derived CuO oxidation products by isotope ratio monitoring-gas chromatography-mass spectrometry (irm-GC-MS)

    Organic Geochemistry

    (1996)
  • M.A. Goni et al.

    Lignin dimers: structures, distribution and potential geochemical applications

    Geochimica et Cosmochimica Acta

    (1992)
  • M.A. Goni et al.

    A reassessment of the sources and importance of land-derived organic matter in surface sediments from the Gulf of Mexico

    Geochimica et Cosmochimica Acta

    (1998)
  • M. Goni et al.

    Distribution and sources of organic biomarkers in arctic sediments from the Mackenzie River and Beaufort Shelf

    Marine Chemistry

    (2000)
  • M. Goni et al.

    Sources and distribution of organic matter in a river-dominated estuary (Winyah Bay, SC, USA)

    Estuarine, Coastal and Shelf Science

    (2003)
  • M.A. Goni et al.

    The supply and preservation of ancient and modern components of organic carbon in the Canadian Beaufort Shelf of the Arctic Ocean

    Marine Chemistry

    (2005)
  • M.E. Gonneea et al.

    Tracing organic matter sources and carbon burial in mangrove sediments over the past 160 years

    Estuarine, Coastal and Shelf Science

    (2004)
  • E.S. Gordon et al.

    Sources and distribution of terrigenous organic matter delivered by the Atchafalaya River to sediments in the northern Gulf of Mexico

    Geochimica et Cosmochimica Acta

    (2003)
  • E.S. Gordon et al.

    Controls on the distribution and accumulation of terrigenous organic matter in sediments from the Mississippi and Atchafalaya river margin

    Marine Chemistry

    (2004)
  • E.S. Gordon et al.

    Organic matter distribution and accumulation on the inner Louisiana shelf west of the Atchafalaya River

    Continental Shelf Research

    (2001)
  • S.G. Haberle

    Late Quaternary vegetation change in the Tari Basin, Papua New Guinea

    Palaeogeography Palaeoclimatology Palaeoecology

    (1998)
  • S.G. Haberle et al.

    Biomass burning in Indonesia and Papua New Guinea: natural and human induced fire events in the fossil record

    Palaeogeography Palaeoclimatology Palaeoecology

    (2001)
  • P.T. Harris et al.

    A preliminary study of sedimentation in the tidally dominated Fly River Delta, Gulf of Papua

    Continental Shelf Research

    (1993)
  • P.T. Harris et al.

    Sediment transport in distributary channels and its export to the pro-deltaic environment in a tidally dominated delta: Fly River, Papua New Guinea

    Continental Shelf Research

    (2004)
  • J. Hedges

    Global biogeochemical cycles: progress and problems

    Marine Chemistry

    (1992)
  • J.I. Hedges et al.

    Sedimentary organic matter preservation: an assessment and speculative synthesis

    Marine Chemistry

    (1995)
  • R.G. Keil et al.

    Mineralogical and textural controls on the organic composition of coastal marine sediments: Hydrodynamic separation using SPLITT-fractionation

    Geochimica et Cosmochimica Acta

    (1994)
  • R.G. Keil et al.

    Loss of organic matter from riverine particles in deltas

    Geochimica et Cosmochimica Acta

    (1997)
  • H. Kennedy et al.

    Organic carbon sources to SE Asian coastal sediments

    Estuarine, Coastal and Shelf Science

    (2004)
  • G.C. Kineke et al.

    Fluid-mud processes on the Amazon continental shelf

    Continental Shelf Research

    (1996)
  • G.C. Kineke et al.

    Sediment export from the Sepik River, Papua New Guinea: evidence for a divergent sediment plume

    Continental Shelf Research

    (2000)
  • S.A. Kuehl et al.

    A long, square-barrel gravity corer for sedimentological and geochemical investigation of fine-grained sediments

    Marine Geology

    (1985)
  • E. Lallier-Verges et al.

    Relationships between environmental conditions and the diagenetic evolution of organic matter derived from higher plants in a modern mangrove swamp (Guadeloupe, French West Indies)

    Organic Geochemistry

    (1998)
  • E. Leithold et al.

    Watershed control on the carbon loading of marine sedimentary particles

    Geochimica et Cosmochimica Acta

    (2001)
  • S. Nagao et al.

    Combined use of delta C-14 and delta C-13 values to trace transportation and deposition processes of terrestrial particulate organic matter in coastal marine environments

    Chemical Geology

    (2005)
  • C.A. Nittrouer et al.

    Geological significance of sediment transport and accumulation on the Amazon continental shelf

    Marine Geology

    (1995)
  • C.A. Nittrouer et al.

    The geological record preserved by Amazon shelf sedimentation

    Continental Shelf Research

    (1996)
  • A.S. Ogston et al.

    Observations of storm and river flood-driven sediment transport on the northern California continental shelf

    Continental Shelf Research

    (2000)
  • F.G. Prahl et al.

    Terrestrial organic carbon contributions to sediments on the Washington margin

    Geochimica et Cosmochimica Acta

    (1994)
  • P.A. Raymond et al.

    Controls on the variability of organic matter and dissolved inorganic carbon ages in northeast US rivers

    Marine Chemistry

    (2004)
  • A.I. Robertson et al.

    Pelagic biological processes along a salinity gradient in the Fly delta and adjacent river plume (Papua New Guinea)

    Continental Shelf Research

    (1993)
  • A.I. Robertson et al.

    The influence of fluvial discharge on pelagic production in the Gulf of Papua, Northern Coral Sea

    Estuarine, Coastal and Shelf Science

    (1998)
  • L.H. Sorensen

    Carbon-nitrogen relationships during the humification of cellulose in soils containing different amounts of clay

    Soil Biology and Biochemistry

    (1981)
  • J.R. Todorov et al.

    Molecular analysis of microbial communities in mobile deltaic muds of Southeastern Papua New Guinea

    FEMS Microbial Ecology

    (2000)
  • J.J. Vogel et al.

    Catalyst and binder effects in the use of filamentous graphite for AMS

    Nuclear Instruments and Methods in Physics Research

    (1987)
  • Cited by (92)

    View all citing articles on Scopus
    View full text