Deep-sea Corals as Archives of Past Ocean Acidification and Changes in the Ocean Minimum Zone

The extent of the ocean oxygen minimum zone (OMZ) varies with sea surface water temperature and ocean circulation. In addition, increasing CO2 in the atmosphere leads to increased seawater CO2. This combination leads to the dissolution of CaCO3, which many organisms depend on for their shells. Developing a proxy for past ocean CO2 and [O2] is crucial to understanding oceanic response to future natural and anthropogenic environmental changes.Deep-sea bamboo corals contain annual growth bands in calcite internodes and may provide high- resolution paleo-oceanographic records of environmental conditions. We examined the response of U/Ca incorporated into modern bamboo coral internodes collected from intermediate water depths (800-2000 m) in the eastern Pacific Ocean to CO2 and [O2] in ambient seawater. Seawater CO2 and [O2] was determined from the World Ocean Circulation Experiment (WOCE). Three samples were collected from each of the study corals: exterior, middle, and interior of the specimen. Corals were drilled in 1.50 millimeter wide samples, integrating ~15 years of coral growth.U/Ca in corals were analyzed by inductively coupled plasma-mass spectrometry. U/Ca ratios varied both between corals and along the growth axis within individual corals. The U/Ca ratios ranged from 0.01 to 0.03 μmol/mol. U/Ca ratios varied from .012 to .021 μmol/mol within individual corals, suggesting considerable variation through time. The exterior coral sample U/Ca ratios measured on the ICP-MS were compared to WOCE data and found to positively correlate; thus, U/Ca ratios of deep-sea corals may serve as an effective archive of oceanic environmental conditions over the past several hundred years