Contaminant dispersal on the Palos Verdes continental margin: I. Sediments and biota near a major California wastewater discharge

TL;DR: In this article, six cores from the Northern Gulf of Mexico (GOM) were collected from water depths ranging from c700 to 3500 m and processed for radiochemical assays to determine particle reworking (bioturbation) and sedimentation rates in these sediments.

TL;DR: Quensen et al. as discussed by the authors studied the diagenetic fate of organic contaminants in the waste-impacted sediments and found that p,p′-DDE is the dominant DDT metabolite found in shelf sediments, comprising 60% of ΣDDT.

TL;DR: Abundant dead shells of epifaunal suspension-feeding terebratulid brachiopods and scallops on the now-muddy mainland continental shelf of southern California reveal the recent, previously unsuspected extirpation of an extensive offshore shell-gravel ecosystem, evidently driven by anthropogenic siltation.

TL;DR: In this article, the authors integrated existing data with new data of DDTs from different compartments in the coastal zones off southern California to assess the importance of the dispersal mechanism.

TL;DR: In the case of marine sedimentary deposits, the dominant agents of mass transport are often large bottom-dwelling animals that move particles and fluids during feeding, burrowing, tube construction, and irrigation as discussed by the authors.

TL;DR: Thalassinidean shrimp construct species-specific burrows which vary in morphology from simple 'U' or 'Y' shaped tubes to more complex tiers of galleries or reticulate branches, and each burrow type may be indicative of one of the 3 general trophic modes utilized by burrowing shrimp: deposit feeding, drift catching, and filter/suspension feeding.

Abstract: We link specific mechanisms of biogenous sediment mixing with the commonly used bioturbation coefficient (Db) that describes their bulk effects. Using an isotropic, stationary, unbiased random walk model we mechanistically decompose the particulate bioturbation coefficient into the fundamental dimensions of length and time. The result shows that Db depends directly on the square of the distance particles are moved (step length) and inversely on the elapsed time between movements (rest period). This new decomposition in terms of explicit mechanisms (i.e., animal activities), leads to scaling arguments that large, deposit feeding animals will in nearly all cases dominate biogenous mixing. Paradoxically, such animals often transport particles vertically in an advective fashion (e.g., conveyor-belt feeding), making the widespread fit of the diffusion equation to tracer profiles equivocal. Finite-difference simulations reveal that even in the complete absence of vertical diffusion, rapid diffusive horizontal mixing coupled with vertical advection can produce vertical profiles characteristic of diffusion. We suggest that near-surface horizontal mixing rates by animals far exceed vertical mixing rates in the same stratum and that this anisotropy may persist throughout the surface mixed layer. Thus, despite their apparently good kinematic fit, one-dimensional biodiffusion coefficients may not accurately describe the dynamics of sediment displacement, leading to errors in models of early diagenesis.