Researching Ancient Oceans

Joe Proudman
(4 min 40 sec)

Everybody has a story to tell. Even a sea urchin or a clam. The tale 5,400 fossilized sea creatures told UC Davis marine ecologist Sarah Moffitt was one of abundance, trauma and recovery wrought by thousands of years of climate change, some of it quite abrupt.

In a recent study, Moffitt showed that while climate change and oxygen loss in the sea can alter ocean ecology very quickly — within decades — recovery can take thousands of years, not the 100-year scale previously thought.

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“For me, looking forward for my kid, people in the future are not going to have the same ocean that I have today,” Moffitt said. “That’s actually a place of personal heartbreak. To know that, if we go down a path of unchecked climate warming, these places that are so beautiful, these organisms that are so fascinating, and so bizarre and alien, are not going to be there for my grandkids.”

Tree rings of the sea

‘For me, looking forward for my kid, people in the future are not going to have the same ocean that I have today. That’s actually a place of personal heartbreak.’

— Sarah Moffitt

Moffitt, a scientist from UC Davis Bodega Marine Laboratory and Coastal and Marine Sciences Institute, analyzed invertebrate fossils found within a sediment core from offshore Santa Barbara. The core, essentially a tube of sediment that covers a period between 3,400 and 16,100 years ago, read much like the tree rings of the sea.

It provided a before-and-after snapshot of what happened during the last major deglaciation — a time of abrupt climate warming, melting polar ice caps and expansion of low oxygen zones in the ocean.

The study, published March 30 in the Early Edition of the journal PNAS, is the first record of disturbance and recovery of seafloor ecosystem biodiversity in response to abrupt climate change. It documents how long it took for recovery of ecosystems to begin, after a dramatic episode of climate change.

What the invertebrate fossils might say

Previous studies reconstructing Earth’s climatic history through marine sediments primarily relied upon single-celled organisms called foraminifera, and less so on multicellular life, like invertebrates. Moffitt, whose background is in marine ecology, wondered what the invertebrate fossils within the sediments could tell her about the structure of the ocean ecosystem during past periods of climate change.

“After the initial sampling at sea, I took the entire core, which was about 30 feet long,” Moffitt said. “I cut it up like a cake, and I sampled the whole thing. Because of that, I had the whole record.”

The history lesson shown by the sediment core is one of initially abundant, diverse and well-oxygenated seafloor ecosystems, then a period of warming and oxygen loss in the oceans, followed by a rapid loss of diversity. The fossils nearly disappeared from the record during those times of low oxygen.

The study found that oceanic oxygen levels fell by between 0.5 and 1.5 mL/Lover a period of less than 100 years, showing that relatively minor changes in oxygen levels could result in dramatic changes and reorganizations for seafloor communities.

A unique climate record

‘It basically has not ever been done at this scale — across time scales like this, never in the open ocean.’

— Tessa Hill

Co-author and UC Davis professor Tessa Hill from the Department of Earth and Planetary Sciences and Bodega Marine Laboratory, said such a close examination of the fossilized ocean fauna is rare.

“It basically has not ever been done at this scale — across time scales like this, never in the open ocean,” she said. “This is really a unique climate record from offshore California where you can see in exquisite details what was happening in terms of the ocean temperatures and of the ocean climate system. Then you see the response of these very complex, deep-sea ecosystems to those changes. There’s really no other record like that.”

‘Gritty reality’

‘I took the entire core, which was about 30 feet long. I cut it up like a cake, and I sampled the whole thing. Because of that, I had the whole record.’

— Sarah Moffitt

The results suggest that future global climate change may result in similar 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,” Moffitt said. “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. 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’s co-authors include Hill, Peter Roopnarine, curator of Invertebrate Zoology and Geology from the California Academy of Sciences, and professor James Kennett from UC Santa Barbara Department of Earth Science.