Cindy M. Palinkas

Inner-Shelf Sedimentation in the Gulf of Papua, New Guinea: A Mud-Rich Shallow Shelf Setting

Abstract Each year, approximately 4 × 108 tons of sediment from multiple river sources is discharged to the Gulf of Papua. This large supply enters a system where physical energetics vary seasonally and accumulation of mud and sand occurs on the inner shelf. Sediment supplied by the largest of these fluvial sources, the Fly River, is advected northeastward by the prevailing currents, where it joins with sediment supplied from other rivers. Three sedimentary deposits are identified along the 20-meter isobath: (1) slowly accumulating (∼0.6 cm y−1) massive sandy mud in the west; (2) interbedded mud and sand with higher accumulation rates (1.9–2.7 cm y−1) in the central region; and (3) slowly accumulating (∼0.7 cm y−1) bioturbated muds in the east. Trends in the preserved strata reflect the interaction of fluvial, tidal, and wave processes. In the tidally energetic western region, massive sandy units dominate, whereas interbeds of mud and sand are observed in the central region. The interbeds (centimeters thick) are hypothesized to be related to seasonal changes in the hydrodynamics of the Gulf: deposition and erosion of sediment associated with changes between monsoon and trade-wind conditions. On finer scales (millimeters), the sedimentary structures observed in the Gulf are typical of those found in tide-dominated deltaic settings. Bioturbated sedimentary structures characterize the eastern Gulf, as a result of slow accumulation by fine material and less energetic physical processes. Several important relationships are recognized for the Gulf of Papua: (1) the large supply of fine-grained sediment allows it to accumulate at significant rates on the inner shelf; (2) quiescent monsoon conditions probably aid sediment retention on the inner shelf; and (3) coalescence of supply from multiple river sources causes maximum accumulation rates to occur in the central Gulf of Papua.

The Timing of Floods and Storms as a Controlling Mechanism for Shelf Deposit Morphology

Abstract Deltas and clinoforms are accretionary deposits with subaerial and subaqueous topsets, respectively. Various factors have been proposed that may control their geometry, especially the interaction of sediment supply and physical oceanographic energy. A conceptual model of how this interaction affects shelf sedimentation indicates that sediment from most rivers should form deltaic features in quiescent environments. In more energetic settings, most sediment from small rivers (sediment load of 106–107 t/y) should be dispersed, limiting significant shelf accumulation, but the increased supply from larger rivers (sediment load of 108–109 t/y) could allow for sediment retention on the shelf, leading to clinoform development. However, in the Adriatic Sea, a river delta and a shelf clinoform (i.e., adjacent to the Po River and the Apennine rivers, respectively) have developed from similar sediment supply and oceanographic energy. This suggests that other factors are likely important, particularly differences in the shelf gradient and the timing of floods and storms. The shelf gradient is lower near the Po River, favoring retention of sediment in shallower water as compared to the Apennine rivers. In addition, floods and storms are uncorrelated on the Po River shelf due to the large size of the drainage basin, enhancing sediment deposition in shallow water. For the Apennine rivers, the drainage basin of individual rivers is small, and floods and storms are generally correlated, facilitating offshore deposition of sediment and leading to the development of a shelf clinoform.

Spatial patterns of plant litter in a tidal freshwater marsh and implications for marsh persistence

The maintenance of marsh platform elevation under conditions of sea level rise is dependent on mineral sediment supply to marsh surfaces and conversion of above- and belowground plant biomass to soil organic material. These physical and biological processes interact within the tidal zone, resulting in elevation-dependent processes contributing to marsh accretion. Here, we explore spatial pattern in a variable related to aboveground biomass, plant litter, to reveal its role in the maintenance of marsh surfaces. Plant litter persisting through the dormant season represents the more recalcitrant portion of plant biomass, and as such has an extended period of influence on ecosystem processes. We conducted a field and remote sensing analysis of plant litter height, aboveground biomass, vertical cover, and stem density (collectively termed plant litter structure) at a tidal freshwater marsh located within the Potomac River estuary, USA. LiDAR and field observations show that plant litter structure becomes more prominent with increasing elevation. Spatial patterns in litter structure exhibit stability from year to year and correlate with patterns in soil organic matter content, revealed by measuring the loss on ignition of surface sediments. The amount of mineral material embedded within plant litter decreases with increasing elevation, representing an important tradeoff with litter structure. Therefore, at low elevations where litter structure is short and sparse, the role of plant litter is to capture sediment; at high elevations where litter structure is tall and dense, aboveground litter contributes organic matter to soil development. This organic matter contribution has the potential to eclipse that of belowground biomass as the root:shoot ratio of dominant species at high elevations is low compared to that of dominant species at low elevations. Because of these tradeoffs in mineral and organic matter incorporation into soil across elevation gradients, the rate of marsh surface elevation change is remarkably consistent across elevation. Because of the role of plant litter in marsh ecosystem processes, monitoring and assessment of these dynamic geomorphic marsh landscapes might be streamlined through the measurement of plant litter structure, either via LiDAR technologies or field observation.

The Influence of Breakwaters on Nearshore Sedimentation Patterns in Chesapeake Bay, USA

ABSTRACT Palinkas, C.M.; Barth, N.; Koch, E.W., and Shafer, D.J., 2016. The influence of breakwaters on nearshore sedimentation patterns in Chesapeake Bay, USA. This study describes nearshore Chesapeake Bay sedimentation at sites adjacent to and landward of 24 segmented breakwaters, varying in age (1–19 years) and physical setting. Grain-size and organic-content profiles are examined at the breakwater-protected sites to assess potential changes induced by breakwater installation as well as at the adjacent-exposed sites to establish historical trends. Sedimentation rates at all sites are calculated with 210Pb (half-life 22.3 years). At the breakwater-protected sites, these rates largely reflect preconstruction sedimentation because of the long half-life of 210Pb relative to breakwater ages. Determining the postconstruction sedimentation rate can be more difficult because the signature of breakwater influence in the sedimentological record can be obscured. For example, if the source of sediment is not affected dramatically by construction, down-core profiles may not have obvious changes. The depth of breakwater influence, however, can be interpreted by considering all the sedimentological evidence at a given location, and the postconstruction rates are calculated from this depth. In general, the sedimentological response to breakwater construction is fairly unique for each location but depends on such factors as breakwater age and geometry, shoreline sediment composition, and construction technique.

Sedimentation adjacent to naturally eroding and breakwater-protected shorelines in Chesapeake Bay

Sediment characteristics, especially grain size and organic content, in nearshore Chesapeake Bay environments show significant temporal and spatial variability. This can impact benthic organisms, particularly submerged aquatic vegetation (SAV), which are important components of the ecosystem. In order to better understand how these changes are reflected in the stratigraphic record, the radiochemical and textural properties of sediment at four sites are examined. Fine and organic material are observed to increase at some nearshore locations, whereas others have experienced a shift toward lower-organic, coarser sediments. These changes are likely related to local variations in sedimentary processes. Other, more recent, perturbations are due to breakwater construction, which can trap fine and organic material in the protected area. Accumulation rates inshore of the breakwater are ~2–4 times higher than in adjacent exposed locations, and this change is coincident with breakwater construction. Thus, because sedimentary processes vary according to physical setting, local trends must be discerned to determine whether a given site may be suitable for SAV restoration.