Scientists have discovered that some deep sea sponges have growth bands that, like tree rings, can reveal past environmental conditions. The bands show that, far from being a constant environment, the deep ocean experiences changes akin to seasons.

Scientists from New Zealand’s National Institute of Water & Atmospheric Research (NIWA) and New Caledonia’s Institut de Recherche pour le Developement (IRD) found seasonal fluctuations in levels of carbon and trace metals in a lithistid (‘rock’) sponge aged over 135 years. The research was published in the September issue of the journal Limnology & Oceanography.

The large bowl-shaped sponge was collected at a depth of 540 m from Norfolk Ridge (between New Caledonia and New Zealand) by NIWA sponge expert Dr Michelle Kelly onboard IRD research vessel Alis. “Rock sponges grow very slowly and this one was huge, about 30 cm by 20 cm, so I knew it had to be old,” says Dr Kelly, who identified the specimen. “It also had these intriguing banding patterns on the surface. I thought maybe they could tell us something about past environmental conditions, in the way that bands of aragonite in corals do, or growth rings in trees.”

Rock sponges grow their hard-as-rock skeletons from silica filtered out of the water.  Levels of silica and other trace elements in their skeletons therefore reflect conditions in the surrounding water at the time that the layers or bands were formed.

X-ray analysis of two sections of the sponge’s skeleton revealed about 140 pairs of alternating light and dark bands. “We think these are like annual growth rings in trees, with the light and dark layers representing seasonal variation in silica deposition,” says Dr Michael Ellwood, who led the analytical aspects of the research while at NIWA.  “The light layers are areas of denser silica.”

“Carbon dating techniques put the sponge at between about 135 and 160 years old, which tallies with the 140 band pairs each representing a year of growth,” says Dr Ellwood, now at The Australian National University in Canberra. “This is the first time sponges have been accurately aged.”

Analysis of the bands showed that levels of trace elements and carbon near the seafloor fluctuate twice-yearly in a pattern akin to seasons. The fluctuations are at least partly linked to changes in food supply at the ocean’s surface. “The results shatter the common perception that conditions remain constant in the deep ocean and tell us that the deep ocean is not disconnected from the surface,” says Dr Ellwood.

“These sponge skeletons offer important archives of past oceanic and climatic conditions over timescales of tens or hundreds of years,” adds Dr Kelly.  “But because rock sponges are such an ancient group, dating back 145 million years, they can also offer us information about changing conditions over much longer timescales.” For instance, the nearest living relatives of some fossil rock sponges found near Oamaru and the Chatham Islands are today restricted to warmer and deeper waters to the north of New Zealand, suggesting that southern waters were once much warmer and more silica-rich than they are today.

“Rock sponges are living fossils that have survived through to modern times using amazing chemicals produced by symbiotic microbes,” says Dr Kelly. Scientists are investigating potential uses of these chemicals, for instance, in fighting human and veterinary diseases, and in the production of agrochemicals and antifoulants.