The sea is home to some 5,000 species of sponges. These multicelled animals first appeared about 800 million years ago. Although they lack muscles, bones, and a nervous system, one particular species has something that scientists want: information on the state of the climate thousands of years ago.
In most species of glass sponges, the spicules consisting of silica are microscopic and can be found littered in sediments underneath where they lived and died. However, Monorhaphis chuni, found in the Pacific Ocean at depths below about 1,000 meters, can live for several millennia and produce a single giant basal spicule that can reach 3 meters in length.
In a new study, Jochum et al. present data collected from five sponges ranging in age from 5,000 to 18,000 years. The M. chuni sponges were collected alive from the depths of the East China Sea, South China Sea, and southwest Pacific. Their spicules are silent sentinels that record changes in the ocean content of dissolved silica, an essential seawater nutrient.
Why is silica in the deep ocean important? Because diatoms, microscopic algae that live in the sunlit surface ocean, make their shells out of dissolved silica. Every year they use up all the available silica in the surface layers of the ocean. The carbon dioxide that diatoms convert to organic carbon during photosynthesis is responsible for about half of all the marine carbon that falls into the deep sea. This sinking of carbon (and silica) out of the surface ocean helps keep this atmospheric greenhouse gas in check. Fortunately, most of it dissolves, and then upwelling brings the nutrients back into the sunlight, so that the diatoms’ annual supply is replenished. How fast this cycle turns and how much silica is supplied for diatom growth are very important to warming or cooling our climate.
However, these fluctuations in the silica content of seawater are difficult to study. Fortunately, deep-sea glass sponges are recorders of the historical fluctuations of these critical climate indicators.
Just like the rings in a cross section of a tree trunk provide information about past wildfires or droughts, cross sections of the sponge spicule reveal rings. Silicon isotope ratios within these rings provide information about the silica concentrations in the seawater that bathed the sponge when the ring formed.
The researchers examined these rings, as well as nearby bottom seawater samples in contact with the modern outer rings of M. chuni spicules. They first compared the outer ring of the spicules with the surrounding seawater to affirm that silica concentrations in the seawater are reflected by the specific silicon isotope ratios of the outer layers of the spicule. After they established that agreement, they analyzed deeper layers in the spicule. Assuming that silicon isotope ratios in those deeper layers also reflect the waters that bathed them when they formed, the authors were able to tease out fluctuations in the concentrations of dissolved silica at each sponge’s location over time.
They found that during the early deglacial period (14,000–18,000 years ago) the concentrations of dissolved silica in the deep Pacific were about 12% higher than current levels. This result suggests that continental sources, such as winds and rivers, supplied more silica to the ocean during the deglacial or that the natural burial of diatom shells in deep-sea sediments was lower, enhancing the deep silica entrained into the upwelling currents. Either of these possibilities would have affected the past global carbon budget. (Geophysical Research Letters, https://doi.org/10.1002/2017GL073897, 2017)
—Mohi Kumar, Scientific Content Editor