Detecting, sourcing, and age-dating dredged sediments on the open shelf, southern California, using dead mollusk shells

• Published in: Marine pollution bulletin Vol. 114; no. 1; pp. 448 - 465
• Main Authors: Bizjack, Matthew T., Kidwell, Susan M., Velarde, Ronald G., Leonard-Pingel, Jill, Tomašových, Adam
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 15-01-2017
• Subjects: Animal Shells - anatomy & histology; Animals; Bays; Benthic response; Bivalvia; California; Dead shell assemblages; Environmental Monitoring - methods; Geologic Sediments - analysis; Live-dead discordance; Marine; Mollusks; Ocean disposal; Paleoecology; Taphonomy; INE, USA, California, San Diego Bay; Benthic response; Ocean disposal; Live-dead discordance; Mollusks; Paleoecology; Dead shell assemblages; Taphonomy
• ISSN: 0025-326X; 1879-3363
• DOI: 10.1016/j.marpolbul.2016.10.010

Molluscan shell debris is an under-exploited means of detecting, sourcing, and age-dating dredged sediments in open-shelf settings. Backscatter features on the Southern California shelf are suggestive of dredged sediment hauled from San Diego Bay but deposited significantly inshore of the EPA-designated ocean disposal site. We find that 36% of all identifiable bivalve shells >2mm (44% of shells >4mm) in sediment samples from this 'short dump' area are from species known to live exclusively in the Bay; such shells are absent at reference sites of comparable water depth, indicating that their presence in the short-dump area signals non-compliant disposal rather than natural offshore transport or sea level rise. These sediments lack the shells of species that invaded California bays in the 1970s, suggesting that disposal preceded federal regulations. This inexpensive, low-tech method, with its protocol for rejecting alternative hypotheses, will be easy to adapt in other settings.
•Novel use of molluscan debris to determine the provenance of dredge spoil•Absence of non-native species in spoil on shelf suggests disposal prior to regulations.•Broadens the scope for environmental and ecological applications of skeletal debris•Applies explicit protocol for rejecting alternative explanations of ‘live-dead’ discordance•Inexpensive and low-tech method, will be easy to adopt in other settings.