This award will provide funds to obtain new proxy records of the Atlantic Multidecadal Oscillation (AMO) over time periods between 100 to 1000 years using sclerosponge aragonite geochemistry (18O/16O and Sr/Ca), and to investigate possible artifacts in stable oxygen and carbon (13C/12C) isotope data due to alteration during the sampling of aragonitic skeletal material. Specific objectives include 1) the generation of sclerosponge geochemical data, combined with improved dating, to confirm a correlative link between salinity and AMO at multiple locations in the Bahamas and Caribbean both prior to and after the instrumental record (~1850), 2) improvement of the geochemical calibrations between temperature and Sr/Ca and d18O based on a new collection of ~ 24 sclerosponge specimens collected between 1986 and 1996 and 3) investigation of additional minor and trace elements as recorders of environmental conditions in the tropical subsurface environment. The Broader Impacts include the societal relevance of research directed at understanding a fundamental question of the climate system that could have a synergistic impact on human-contributed forcings of the climate system and support of research by undergraduate and graduate students.
The north Atlantic Ocean is an integral cog in in the global climate system. Recent changes in the temperature and salinity of this ocean has generated concern that we are witnessing the effects of human-induced climate change. However, we require longer-term records of these water mass properties if we are to be able to attribute these changes to greenhouse gas concentrations in the atmosphere. Presently our instrumental record extends back less than 100 years and at only a few locations. Our goal was to use the skeletons of calcifying sponges (sclerosponges) – in particular the chemistry of these skeletons – to develop long-term, multi-centennial records of temperature and salinity at seasonal to monthly resolution. These records would allow us to place the recent changes that have been measured in situ by oceanographic instruments into the context of variability in these variables over the last several hundred years. The analytical approach that we have used for determining the concentrations of several elements in the sclerosponge skeletons relies on a short pulse (<10 ns) 193nm excimer laser coupled to an inductively coupled plasma mass spectrometer (ICP-MS). The laser allows us to sample the skeletons at extremely small spatial scales (approximately 50 ?m) which is essential in this application because sclerosponges grow very slowly. This slow growth is beneficial because it means that even a small sclerosponge can be quite old. Strontium (Sr) values in sclerosponges is related to water temperature, and we measured Sr, magnesium (Mg), barium (Ba) and lead (Pb) along the growth axis of the skeletons. We sampled a high-resolution record from a sclerosponge collected adjacent to Lee Stocking Island in the Bahamas. From Pb values in the skeleton, the chronology represents approximately 200 years based on the Pb peak that was generated by the introduction of leaded gasoline in the early 1920’s in the U.S. We also have assayed a high-resolution elemental from a sclerosponge from Acklins Island in the Bahamas. The first item of note is that, based on the Pb record, this sclerosponge grew considerably faster than the sclerosponge from Lee Stocking Island. The Sr data from both specimens provided strong evidence of a significant warming trend at this site over the last 60-100 years. The magnitude of the changes matches those from instrumental records in the north Atlantic Ocean, but expands the area of warning well into the sub-tropics. This finding is important because it adds significantly to the amount of heat that the north Atlantic Ocean has assimilated during this time period. This heat presumably would have remained in the atmosphere if not taken up by the ocean. Finally, we have made significant advances in our understanding of the biomineralization in sclerosponges and the use of elemental data in sclerosponges and corals as oceanographic proxies. For instance, we see clear correlations between Sr, Mg, Ba and Pb in our records. Crystal growth rate appears the most likely mechanism for generating these correlations, and therefore it is important to understand the effect of growth rate of skeletal composition. We are hoping that stable isotope analyses performed by our collaborators at RSMAS will help us refine our understanding of the processes generating variability in elemental composition of sclerosponge skeletons.
