Seafloor hits bottom fast, bounces back slowly after climate change

Seafloor ecosystems are likely to take a millennium or more to recover from climate change, according to research published yesterday in the Proceedings of the National Academy of Sciences.

That conclusion comes from an analysis of over 5,400 fossil snails, sea stars, and other invertebrates in a sediment core collected off the coast of Santa Barbara, California. The core is like a 30-foot-long layer cake that records the evolution of the seafloor ecosystem between 16,100 and 3,400 years ago.

Analysis of sediment and ice cores is a common technique for reconstructing past climate fluctuations and their effects. The results of such studies both feed into and complement predictions about climate based on computer simulations.

Scientists know that when the climate warms, oxygen levels in seawater tend to drop and low-oxygen zones in the oceans expand. This effect is predicted to occur with current climate change as well, and in fact is already underway.

Previous studies of sediment cores have documented the ebb and flow of single-celled marine organisms called foraminifera, which provide hints that the biotic community changes along with shifts in oxygen concentration.

But as the biotic community goes, foraminifera are the tip of the proverbial iceberg, and scientists have lacked a picture of how the structure and composition of the community as a whole changes. The new study is the first to document the effects of past climate change on a broad suite of marine organisms.

About 15,300 years ago, near the end of the last glaciation, the seafloor teemed with brittle stars, heart urchins, tiny crustaceans known as ostracods, and a variety of molluscan deposit feeders, suspension feeders, and carnivores, the researchers found.

Then, about 14,700 years ago, the climate rapidly warmed. Glaciers receded on land and oceanic oxygen levels dropped slightly, by about 0.5 to 1.5 mL/L. But this relatively small shift in ocean chemistry abruptly reorganized the seafloor community.

Within just 12 centimeters of sediment, equivalent to about 130 years, invertebrates all but disappeared. In fact, the change may have happened even faster than it appears in the core, because marine organisms can stir up the top layers of seafloor sediment and “smear” the record of an event.

The new seafloor community that came into being during the period that followed, an era known as the Bølling-Allerød interstadial, was dominated by foraminifera; invertebrates were few and far between.

A 1,300-year cold snap called the Younger Dryas came on suddenly about 12,800 years ago. During this period a diverse invertebrate community gradually reassembled itself. But in contrast to the community’s sudden collapse, its recovery was slow, occurring through 107 centimeters of sediment or about 1,070 years.

The results increase by an order of magnitude scientists’ estimates of the time required for seafloor communities to recover after climate-related disturbance. Previously, such processes were believed to take place over the span of more like 100 years.

Past climate fluctuations are imperfect analogues of present-day climate change, says study leader Sarah Moffitt, a postdoctoral researcher at the Bodega Marine Laboratory of the University of California, Davis. But that fact shouldn’t reassure us. For one thing, the current warming begins with a planet that is already relatively warm, in contrast to the rapid climate change that took place at the end of the last glacial period as recorded in the sediment core.

As a result, “How this [recovery] will operate in the future is definitely unclear and will most likely not behave in the same way,” Moffitt says. One thing is for sure: “it will absolutely take a very long time.” – Sarah DeWeerdt | 31 March 2015

Source: Moffitt S.E. et al. 2015 Response of seafloor ecosystems to abrupt global climate change. PNAS DOI:10.1073/pnas.1417130112

Header image: A fossil brittle star, one of the groups of invertebrates examined in the study. Credit: Mark A. Wilson (Department of Geology, The College of Wooster), via Wikipedia.