Deep ocean nutrients during the Last Glacial Maximum deduced from sponge silicon isotopic compositions

Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 292 (2010): 290-300, doi:10.1016/j.epsl.2010.02.005.The relative importance of biological and physical processes within the Southern Ocean for the storage of carbon and atmospheric pCO2 on glacial-interglacial timescales remains uncertain. Understanding the impact of surface biological production on carbon export in the past relies on the reconstruction of the nutrient supply from upwelling deep-waters. In particular, the upwelling of silicic acid (Si(OH)4) is tightly coupled to carbon export in the Southern Ocean via diatom productivity. Here, we address how changes in deep-water Si(OH)4 concentrations can be reconstructed using the silicon isotopic composition of deep-sea sponges. We report δ30Si of modern deep-sea sponge spicules and show that they reflect seawater Si(OH)4 concentration. The fractionation factor of sponge δ30Si compared to seawater δ30Si shows a positive relationship with Si(OH)4, which may be a growth rate effect. Application of this proxy in two down-core records from the Scotia Sea reveals that Si(OH)4 concentrations in the deep Southern Ocean during the Last Glacial Maximum (LGM) were no different than today. Our result does not support a coupling of carbon and nutrient build up in an isolated deep-ocean reservoir during the LGM. Our data, combined with records of stable isotopes from diatoms, are only consistent with enhanced LGM Southern Ocean nutrient utilization if there was also a concur rent reduction in diatom silicification or a shift from siliceous to organic walled phytoplankton.Cruise NBP0805 was funded by NSF Office of Polar Programs (OPP) Antarctic Sciences (grant number ANT-0636787). Data from the Palmer LTER data archive were supported by Office of Polar Programs, NSF grants OPP-9011927, OPP-9632763 and OPP-0217282. The work was funded by the Natural Environment Research Council (NERC) grant NE/F005296/1 and an Antarctic Science Bursary