James Kennett, a geologist at the University of California, Santa Barbara and his colleagues at the University of California, Davis and the California Academy of Sciences, used a sediment core to examine seafloor ecosystem biodiversity in response to climate change.
The Santa Barbara Basin presents an ideal natural laboratory for researchers studying the global climate record. Exploring this data-rich area off the coast of Southern California, Kennett and his colleagues found that abrupt climate changes caused small decreases in seawater oxygenation, leading to extensive seafloor ecosystem reorganizations. The recovery from this reorganization took up to 1,000 years.
The research team analyzed over 5,400 invertebrate microfossils, including sea urchins, clams, and crustaceans, from the offshore Santa Barbara sample. The sediment core, a tube of material that covers a period between 3,400 and 16,100 years ago, provided a snapshot of what happened during global warming that occurred at the end of the last glacial period. This was a time of abrupt climate warming, expansion of low oxygen zones, and melting polar ice caps. The study showed how long it took for ecosystems to begin recovering from the dramatic climate change.
“We were surprised to learn that this microfossil record is richer and more useful than first expected,” said co-author Kennett, UCSB professor emeritus of earth and marine science. “As a result, we were able to test the response time of different members of the bottom-dwelling ecosystem to both abrupt warming and cooling episodes. We were also surprised to discover just how long some took to recover.”
The sediment core was initially an abundant, well-oxygenated, and diverse seafloor ecosystem before it experienced a period of warming and oxygen loss in the oceans and then rapid loss of diversity. Microfossils nearly disappeared from the record during those times of low oxygen.
“Our analysis demonstrates that ocean sediments harbor metazoan fossil material that can be used to reconstruct the response of seafloor biodiversity to global-scale climate events,” Kennett explained. “We show that the last deglaciation—the most recent episode of climate warming—was accompanied by abrupt reorganizations of continental margin seafloor ecosystems through expansions and contractions of the subsurface low-oxygen zones.”
The researchers found that oceanic oxygen levels fell between 0.5 and 1.5 mL over a period of less than 100 years, demonstrating that relatively minor changes in oxygen levels could result in dramatic shifts and reorganizations for seafloor communities. These results suggest that future global climate change may result in ecosystem-level effects with millennial-scale recovery periods.
“These past events show us how sensitive ecosystems are to changes in Earth’s climate; it commits us to thousands of years of recovery,” said lead author Sarah Moffitt, a postdoctoral scholar at UC Davis in its Bodega Marine Laboratory and Coastal and Marine Sciences Institute.
“It shows us what we’re doing now is a long-term shift; there’s not a recovery we have to look forward to in my lifetime or my grandchildren’s lifetime,” said Moffitt. “It’s a gritty reality we need to face as scientists and people who care about the natural world and who make decisions about the natural world.”
The study appears in the online Early Edition of the Proceedings of the National Academy of Sciences and is the first to quantitatively examine broad ecosystem responses in the deep-sea sediment record.