Siliceous deep-sea sponge Monorhaphis chuni: A potential paleoclimate archive in ancient animals

摘要

The deep-sea sponge Monorhaphis chuni forms giant basal spicules, which can reach lengths of 3. m; they represent the largest biogenic silica structures on Earth that is formed from an individual metazoan. The spicules offer a unique opportunity to record environmental change of past oceanic and climatic conditions. A giant spicule collected in the East China Sea in a depth of 1110. m was investigated. The oxygen isotopic composition and Mg/Ca ratios determined along center-to-surface segments are used as geochemical proxies for the assessment of seawater paleotemperatures. Calculations are based on the assumption that the calculated temperature near the surface of the spicule is identical with the average ambient temperature of 4. °C. A seawater temperature of 1.9. °C is inferred for the beginning of the lifespan of the Monorhaphis specimen. The temperature increases smoothly to 2.3. °C, to be followed by sharply increased and variable temperatures up to 6-10. °C. In the outer part of the spicule, the inferred seawater temperature is about 4. °C. The lifespan of the spicule can be estimated to 11,000 ± 3000. years using the long-term trend of the inferred temperatures fitted to the seawater temperature-age relationships since the Last Glacial Maximum. Specimens of Monorhaphis therefore represents one the oldest living animals on Earth. The remarkable temperature spikes of the ambient seawater occurring 9500-3100. years. B.P. are explained by discharges of hydrothermal fluids in the neighborhood of the spicule. The irregular lamellar organization of the spicule and the elevated Mn concentrations during the high-temperature growth are consistent with a hydrothermal fluid input.