Understanding the Impact of Recent Ocean Acidification on Bio-Calcification

The rate of increase in atmospheric carbon dioxide (CO2) since the industrial revolution has been unprecedented. Over the last two decades, only half of the CO2 released by anthropogenic activities has remained in the atmosphere and about one third has been taken up by the oceans. Knowledge about the impact of recent rises in atmospheric CO2 on our oceans is growing through recently funded national and international programmes on Ocean Acidification (OA) but is by no means complete. The proposed project will provide insights into the impact of such changes during the past 25 years on marine calcifying plankton (foraminifera) using sediment trap time series samples available from the Ocean Flux Program (OFP, 1984-2008) off Bermuda (31degree 50'N, 64degree10'W). The results will be compared with seafloor surface sediments at nearby locations collected during the Challenger (1872-76) and Discovery (1901-04) expeditions. These comparisons will provide a pre-industrial baseline with which to evaluate the impact of recent OA. In addition, the student will have an opportunity to carry out additional sampling of time-integrated seafloor sediment at the same location for comparative study during a planned internship. The time series sediment trap and seafloor sediment samples will collectively form a unique sample set with which to quantify the ongoing impact of anthropogenic CO2 on surface ocean carbonate chemistry and plankton calcification. The project will focus on answering two major questions: (1) how has the recent rise in anthropogenic CO2 impacted bio-calcification (e.g., in this case planktonic foraminifera)? This question will be addressed by studying changes in size normalized shell mass and dimensions (diameter, area and maximum and minimum length of surface dwelling species) along with species composition and size abundance patterns within a species. These shell calcification parameters from a seasonally resolved time series will be compared against depth-stratified (0-500 m) seawater carbonate chemistry datasets at the sampling location after reconstructing species' habitat depths (using Mg/Ca & oxygen isotopes to calculate calcification temperatures). (2) How has seawater carbonate ion concentration ([CO32-]) changed as a consequence of OA? A better understanding of this issue will be gained using existing proxies of [CO32-] such as the boron/calcium ratio of foraminiferal shells and shell mass. The project will generate annually resolved records of [CO32-] using the unique sample set (time series and seafloor surface sediments). The fortnightly samples from an annual seasonal cycle of the time series samples, combined with measured depth-stratified carbonate ion concentration (calculated form measured total alkalinity and dissolved inorganic carbon) and hydrography data (from the Bermuda Atlantic Time Series Study (31 degrees 43' N, 64 degrees 10' W) and Hydrostation 'S' (32 degrees 50' N, 64 degrees 10' W)), will provide additional empirical calibration for the foraminiferal B/Ca proxy. We will exploit our refined calibration of B/Ca vs. [CO32-] to infer pre- and post-industrial changes in [CO32-] and, consequently, the scale of recent OA. The sediment trap samples will be obtained through project partner (Dr. Conte) at Bermuda Institute of Ocean Sciences, (BIOS)/Marine Biological Laboratory (MBL), Woods Hole, USA and the sea floor surface sediment samples from the Challenger and Discovery expeditions will be obtained through collaborating institution (Natural History Museum, London (support letter available)). The student will benefit from training of shipboard sample collection off Bermuda through internship, training at University of Cambridge (UK) for trace element/Ca work and co-supervision from Professor Jelle Bijma at Alfred Wegner Institute (Germany), core-member of the European Project on OCean Acidification (EPOCA) and Biological Impacts of Ocean ACIDification (BIOACID) programmes.