Unexpected findings in a paleontological investigation of the continental shelf sea bed off southern California revealed the ecosystem was once gravel beds hosting scallops and shelled marine brachiopods, still found from Catalina Island to Alaska.
Complete Collapse
A complete collapse of the gravelly-bed, brachiopod-scallop ecosystem occurred suddenly, and the collapse correlated with the 18th century introduction by the Spanish of range grazing cattle, horses and sheep to southern California, as reported by the University of Chicago. Scallops and brachiopods have low tolerance for soft, muddy sea beds, and grazing livestock create an increase in sea bed sediment due to disturbances to root and soil stability leading to muddy silt deposits.
The section of the California Palos Verdes Continental Shelf under investigation spans the 250 miles from San Diego to Santa Barbara. The submerged shoulders of continents form steeply sloped unique environments that drop off to deeper ocean expanses.
Die-Off
The die-off of the brachiopod-scallop ecosystem due to silt deposition — leading to the transformation from gravelly to muddy sea bed suitable for burrowing clams, worms and mollusks — was complete by the end of the 1860s. No indication of the 4,000-year-old brachiopod-scallop ecosystem was left other than the newly discovered paleontological fossils, as seen in Phys.org.
Paleobiological Dating
Paleontologists Adam Tomašových of the Slovak Academy of Sciences and Susan Kidwell of the University of Chicago applied a paloebiological fossil dating technology — pioneered by Kidwell and her University of Chicago associates — combining paleontological methodology with a biological molecular dating technology called amino acid racemization. As explained in Science Direct, amino acid racemization measures the rates of known molecular changes in amino acids, providing a dating timetable that is independent of, but correlatable with, geological dating technologies such as carbon dating and paleomagnetism.
Tomašových and Kidwell determined that the gravelly sea bed, brachiopods and scallops had been absent from the shelf for between 100 to 200 years, some fossils dating at more than 100 years old and most dating at more than 200 years old. The collapse and die-off of the ancient ecosystem had taken “less than a century” to complete.
Adam Tomašových, head of the Department of Paleoecology and Organismal Evolution at the Slovak Academy, said: “The methods applied here provide crucial information on ecosystem response to natural and human pressures over otherwise inaccessible timescales.”
Coastal Land Use
Tomašových and Kidwell’s results strongly suggest the value of investigating the impact of coastal land-use practices on all the world’s continental shelf ecosystems. Results obtained using paleobiological molecular dating technology align the beginning of the brachiopod-scallop collapse with the introduction in 1796 of cattle, horses and sheep to the pasture lands of southern California. Free-range feeding livestock contributed to an increase in “silt deposition on the continental shelf,” negatively impacting ecosystem habitability for brachiopod-scallop populations.
Susan Kidwell, the William Rainey Harper Professor in Geophysical Sciences, said: “The disappearance of these abundant filter-feeding animals coincided with the rise of lifestock and cultivation in coastal lands, which increased silt deposition on the continental shelf, far beyond the lake and nearshore settings where we would expect this stress to have an impact.”
Study Reported in Royal Society Proceedings B
Paleontologists Adam Tomašových of the Slovak Academy of Sciences and Susan Kidwell of the University of Chicago examine the lost ecosystem in a study published online June 7 in the Royal Society Proceedings B.
Adam Tomašových, Susan M. Kidwell. “Nineteenth-century collapse of a benthic marine ecosystem on the open continental shelf.” Proceedings of the Royal Society B: Biological Sciences, 7 June 2017. DOI: 10.1098/rspb.2017.0328.
