Timothy M. Dellapenna

Ephemeral deposition, seabed mixing and fine-scale strata formation in the York River estuary, Chesapeake Bay

Abstract A process-oriented sedimentary facies model is developed for the York River estuary, a sub-estuary of the lower Chesapeake Bay. This facies model was based on 210Pb and grain-size profiles, as well as X-radiographs taken from kasten cores and box cores collected in a series of across-river transects. Throughout most of the energetic microtidal York River, the seabed is characterized by physical mixing to depths of 25–200 cm. A strong cross-estuary gradient in processes is observed with one side, including channel, flank and shoal, dominated by frequent deep erosion and redeposition (physical mixing), while physical mixing is reduced on the other side, resulting in a greater preservation of biological mixing signatures. Within the physically dominated side of the river, the mixed layer is characterized by ‘stair-stepped’ 210Pb profiles with one or more segments (∼25–200 cm thick) of nearly uniform excess activity. X-radiographs reveal that, although a record of limited biogenic sediment modification is preserved, sedimentary structures within the mixed layer are dominated by centimeter to decimeter scale units of finely to coarsely laminated strata bounded by hiatal surfaces. This demonstrates that mixing results primarily from erosion, resuspension and deposition. Reduced salinity limits the number of benthic species in the York River. Physical disturbance leads to an impoverishment of this community, which is composed primarily of small, opportunistic species with a paucity of larger macrofauna. As a result, mixing in the biologically dominated side of the river is generally on the order of a few centimeters, but may be as deep as 40 cm, and 210Pb geochronology yields low biodiffusion rates (0.43–3.35 cm2 yr−1). X-radiographs reveal the presence of some laminations which suggest that although the mixing is controlled by biological processes the mixing intensity is relatively low. Based on 210Pb geochronologies, residence time estimates for particles within the mixed layer are on the order of centuries. Residence time calculations based on the sediment mass in the physically mixed layer is equivalent to 70 yr of river sediment yield, consistent with century-scale residence times from core data. The frequency and intensity of seabed mixing appears to differ between the lower and upper river. The lower York River is wider and deeper, and is more susceptible to large storms and sea surges, which we suspect drives much of the recorded seabed mixing. Within the upper river, longer-term events (storms) may cause the deepest mixing, but much of this record is destroyed by shorter-term, high-frequency events which produce shallow to mid-depth (

The effects of Hurricanes Katrina and Rita on seabed polycyclic aromatic hydrocarbon dynamics in the Gulf of Mexico

To assess the extent to which Hurricanes Katrina and Rita affected polycyclic aromatic hydrocarbons (PAH) in the Gulf of Mexico (GOM), sediment cores were analyzed in late 2005 from: a shallow shelf, a deeper shelf, and a marsh station. Sediment geochronology, fabric, and geochemistry show that the 2005 storms deposited approximately 10cm of sediment to the surface of a core at 5-12A. Bulk carbon geochemistry and PAH isomers in this top layer suggest that the source of sediment to the top portion of core 5-12A was from a relatively more marine area. Particulate PAHs in the marsh core (04M) appeared unaffected by the storms while sediments in the core from Station 5-1B (deeper shelf) were affected minimally (some possible storm-derived deposition). Substantial amounts of PAH-laden particles may have been displaced from the seabed in shallow areas of the water column in the GOM by these 2005 storms.

Transient, Longitudinal, Sedimentary Furrows in the York River Subestuary, Chesapeake Bay: Furrow Evolution and Effects on Seabed Mixing and Sediment Transport

Sedimentary furrows in fine-grained sediments have been observed in a variety of settings ranging from the deep ocean and deep lake bottoms to shallow estuaries and are commonly described as persistent, long-term features of the seabed. A series of 12 sidescan sonar surveys over the course of three years reveal that transient, longitudinal sedimentary furrows regularly form and then occasionally dissipate within the middle portion of the York River. Varying furrow morphologies were observed depending on current conditions, ranging from large regularly space (0.7–7 m) linear furrows during low current conditions to large patches of meandering furrows as the mean current increases or no bed forms during the higher current conditions. Based on210Pb and137Cs profiles of kasten cores, differences in physical mixing depths of ∼25 cm between cores collected <2 m apart indicate a high degree of small-scale spatial heterogeneity within the seabed. By documenting the position of kasten cores using a digital sidescan sonar system, we showed that a core taken within a furrow had a mixing depth 15 cm shallower than an adjacent core taken between furrows. A time-series of mixing depths over the 35 mo of the study reveals that, along with the ∼25 cm scale differences in mixing depths due to the formation and destruction of furrows, there is a longer temporal signal of mixing producing 100-cm-scale changes in mixing depths on the annual to interannual time frame. Although the formation and destruction of the furrows appear to be a significant process contributing to decimeter-scale seabed mixing, there is a longer-term unknown process which is controlling the meter-scale seabed mixing.

Impact of Hurricanes Katrina and Lili on the Inner Shelf of the Mississippi-Atchafalaya Delta

Box cores collected on the Louisiana continental shelf immediately following Hurricanes Lili and Katrina allow comparison of sediment event layer formation on the actively accreting Mississippi delta adjacent to Southwest Pass (Katrina) and the Atchafalaya delta (Lili). Post-Katrina box core sites in water depths of 25 m, post-Katrina cores display event layer deposition 1–25 cm thick above the incision surface: post-Lili cores from the Atchafalaya show similar thicknesses dependent on proximity to the storm path, but also extending into shallower water (shallowest station sampled was 4 m).

Habitat associations of sea otters (Enhydra lutris) in a soft- and mixed-sediment benthos in Alaska

Abstract We investigated the habitat associations of sea otters (Enhydra lutris) during resting and feeding in an area with a predominately soft- and mixed-sediment benthos supporting infaunal prey populations in a fjord in Alaska during the summer months of 2001–2003. Water depth and benthic sediments were sampled, analyzed, and mapped throughout the bay. Sea otter locations and behavior were determined during boat surveys, and water depth, benthic sediment type, and position in the bay (peripheral compared to central) were determined for each animal location. We used logistic regression analysis to determine whether the use of habitat by sea otters was nonrandom according to these variables. Water depth was the most significant habitat association for feeding behavior, with 39% of feeding dives occurring in water 0–10 m deep. Feeding behavior was not strongly associated with sediment type. Position in the bay was the most significant habitat association for resting behavior, with the majority (63%) of otters resting in the central areas of the bay. Overall, habitat associations were nonrandom, a possible reflection of selective pressure to maximize energy intake, minimize energy expenditure, and avoid terrestrial predators.

Geological Responses to Urbanization of the Naples Bay Estuarine System, Southwestern Florida, USA

The Naples–Dollar Bay Estuarine System (NDBES), situated in southwestern Florida, has undergone extensive modifications caused directly and indirectly by anthropogenic influences. These alterations include: (1) the substitution of mangrove-forested shorelines with concrete bulkheads and installation of residential canals; (2) installation of a regionally extensive navigational channel; and (3) canalization of the watershed, resulting in annexation of a heavily altered drainage basin ten times the size of the pre-alteration basin and with a significantly different soil and bedrock. The NDBES consists of northern Naples Bay, southern Naples Bay, and Dollar Bay, whose shorelines range from highly developed to undeveloped, respectively. This project explored the geological response of the system to these alterations using data from side-scan sonar, sediment grab samples, and vibracores. In highly urbanized northern Naples Bay, benthic substrates consist primarily of muddy sand with few oyster reefs. Southern Naples Bay and Dollar Bay, however, consist of coarser sediment, and are characterized by extensive mangrove shorelines and numerous fringing oyster reefs. The impact of anthropogenic alterations has significantly shifted sediment distributions in northern Naples Bay from a relatively coarser to a relatively fine grained substrate; to a lesser degree in southern Naples Bay, and Dollar Bay, this transition has not taken place due to the general lack of anthropogenic modifications made to this part of the system.

Recent paleoseismicity record in Prince William Sound, Alaska, USA

Sedimentological and geochemical investigation of sediment cores collected in the deep (>400 m) central basin of Prince William Sound, along with geochemical fingerprinting of sediment source areas, are used to identify earthquake-generated sediment gravity flows. Prince William Sound receives sediment from two distinct sources: from offshore (primarily Copper River) through Hinchinbrook Inlet, and from sources within the Sound (primarily Columbia Glacier). These sources are found to have diagnostic elemental ratios indicative of provenance; Copper River Basin sediments were significantly higher in Sr/Pb and Cu/Pb, whereas Prince William Sound sediments were significantly higher in K/Ca and Rb/Sr. Within the past century, sediment gravity flows deposited within the deep central channel of Prince William Sound have robust geochemical (provenance) signatures that can be correlated with known moderate to large earthquakes in the region. Given the thick Holocene sequence in the Sound (~200 m) and correspondingly high sedimentation rates (>1 cm year−1), this relationship suggests that sediments within the central basin of Prince William Sound may contain an extraordinary high-resolution record of paleoseismicity in the region.

Centennial record of anthropogenic impacts in Galveston Bay: Evidence from trace metals (Hg, Pb, Ni, Zn) and lignin oxidation products

During the 20th century the impacts of industrialization and urbanization in Galveston Bay resulted in significant shifts in trace metals (Hg, Pb, Ni, Zn) and vascular plant biomarkers (lignin phenols) recorded within the surface sediments and sediment cores profile. A total of 22 sediment cores were collected in Galveston Bay in order to reconstruct the historical input of Hg, Pb, Ni, Zn and terrestrial organic matter. Total Hg (T-Hg) concentration ranged between 6 and 162 ng g-1 in surface sediments, and showed decreasing concentrations southward from the Houston Ship Channel (HSC) toward the open estuary. Core profiles of T-Hg and trace metals (Ni, Zn) showed substantial inputs starting in 1905, with peak concentrations between 1960 and 1970s, and decreasing thereafter with exception to Pb, which peaked around 1930-1940s. Stable carbon isotopes and lignin phenols showed an increasing input of terrestrial organic matter driven by urban development within the watershed in the early 1940s. Both the enrichment factor and the geoaccumulation index (Igeo) for T-Hg as a measure of the effectiveness of environmental management practices showed substantial improvements since the 1970s. The natural recovery rate in Galveston Bay since the peak input of T-Hg was non-linear and displayed a slow recovery during the twenty-first century.