Mangrove-bank sedimentation in a mesotidal environment with large sediment supply, Gulf of Papua

Abstract

The most extensive mangrove forests on Earth are associated with major river systems. The Indo-Pacific region has numerous large rivers that discharge onto broad continental shelves, and mangroves occur widely along these coastlines. In the Gulf of Papua (GOP) alone, there are >3350 km 2 of mangrove forests, the majority of which (2900 km 2) are associated with the Fly, Kikori, and Purari River deltas. This study examines data from 28 vibracores to describe preserved sedimentary strata and to determine rates of sediment accumulation in this dynamic coastal environment. The observations suggest a common stratigraphy is contained in prograding mangrove banks of the mesotidal western GOP, and that it is similar to mud-dominated and physically stratified stratigraphy recorded by other intertidal areas (with and without mangroves) experiencing large tidal range (>2 m) and sediment supply (e.g., Ganges–Brahmaputra and Yangtze River deltas). Grain-size, sedimentary-structure, radionuclide and remotely sensed data are analyzed, and four stratigraphic facies are identified: (1) supratidal to high-tidal muds; (2) mid-tidal sandy muds; (3) low-tidal sand and mud; and (4) subtidal channel sands. Supratidal and high-tidal sediments of GOP mangrove forests are typically muddy, but can have high sand contents (>30%) in areas exposed regularly to ocean waves. These muds can be homogenous, but commonly have physical structures (laminations and bedding), probably reflecting high rates of sediment accumulation relative to bioturbation. Mangrove muds may have low water contents due to infrequent inundation and, because of their cohesive nature and abundant roots, can form steep intertidal banks and erosional scarps. Mid-tidal and low-tidal sediments are thinly laminated (<2 mm), but the latter also contain thick (>2 mm) laminations, likely deposited from fluid-mud transport. Channel sediments have thin and thick mud and sand laminations, but are identified by thick (>5 cm) sand beds. A conceptual model of mangrove-bank progradation is created using sedimentological and radionuclide data. Maximum sediment accumulation rates were determined using steady-state 210Pb profiles, and greatest mean rates are found in mid-tidal areas (4.4 cm/year; 3.9 g/cm 2/year). Slower accumulation occurs in high-tidal and low-tidal areas (1.8 and 1.5 cm/year, respectively; 1.2 and 1.0 g/cm 2/year, respectively). Remotely sensed data indicate that areas of erosion and accretion co-exist in the Kikori-Purari region. Very large 210Pb accumulation rates were measured in areas of significant accretion, supporting the validity of the calculated rates. In areas of rapid accretion, cores can have non-steady-state or event-related 210Pb profiles, reflecting episodic sedimentation or complex radionuclide scavenging dynamics. Most cores from the Fly River delta are of this type. Sediment trapping in western GOP mangroves is estimated to account for 2–14% of the total GOP sediment load.