Foraminifera are sand-sized planktonic organisms that produce a calcium-carbonate shell during their life cycle. Foraminifera shells are widely used to trace variations in past ocean chemistry. Oxygen isotopes in particular have

been used to trace changes in the size of grounded ice-sheets during the Neogene (~3 million years ago to present), to stratigraphically define the timing and magnitude of glacial-interglacial climate fluctuations, and to establish the temperature of the oceans over time. Carbon isotopes of foraminifera shells, which are measured concurrently with oxygen isotopes, have also been used to trace changes in the size of the terrestrial biosphere, deep ocean respired carbon storage, and ocean circulation over thousands to millions of year timescales. A critical consideration in the use of foraminifera to reconstruct past ocean chemistry is to understand where they reside within the marine water column. This information is important because foraminifera shells record the chemistry of the water they grow in. Given that there are vast differences in water chemistry laterally and vertically within the oceans, foraminifera have the potential to record very different chemical conditions depending on where they form their shells- a process called 'calcification. The oxygen isotope composition of marine calcite is a product of several factors: ¹8 Oshell = $¹8 Owater (a product of global ice volume + salinity) + ¹8Otemp + ¹8Ovital Geochemists can calibrate the depth habitats of foraminifera by comparing the oxygen isotope composition of their shells to the predicted isotopic composition of 'equilibrium calcite formed at different depths within the water column. In this exercise, you will carry out this task, and subsequently discuss the pitfalls that might accompany such an approach./nThe exercise consists of two parts: firstly, a data manipulation component; and secondly a written report: 1. You should use the data listed overleaf to calculate the isotopic composition of equilibrium calcite for a water column profile obtained from the South East Atlantic Ocean (~30°S). This will require you to firstly calculate the isotopic composition of the water from the measured salinity, which is provided for you. Salinity scales linearly to the oxygen isotope composition of water, due to the effect of precipitation and evaporation processes on both parameters. However, the slope of this relationship can be different for different areas of the global ocean, due to changes in climatic regimes. You should select what you consider to be an appropriate subset of data from the following database: https://data.giss.nasa.gov/o18data/ Use the relationship you define to convert the measured salinity profile values (tables overleaf) to oxygen isotope values. Secondly, you should calculate the oxygen isotope composition of equilibrium calcite from these water values. The fractionation of calcite from seawater is temperature-dependent, so you will need to use the temperature data provided for this purpose. Suitable equations can be found in the following publications: Beris, B. E., H. J. Spero, J. Bijma, and D. W. Lea (1998), Reevaluation of the oxygen isotopic composition of planktonic foraminifera: Experimental results and revised paleotemperature equations, Paleoceanography, 13(2), 150-160, doi:10.1029/98PA00070. Leng, M.J., Marshall, J.D. (2004), Palaeoclimate interpretation of stable isotope data from lake sediment archives. Quaternary Science Reviews 23, 811-831. Carry out this exercise for both 'summer' and 'annual' datasets. Remember also that oxygen isotopes in water and calcite are referenced to different scales (VSMOW and VPDB respectively) that can be converted to each other. Compare the measured oxygen isotope data provided to you from three species of foraminifera to your calculated profiles in order to estimate their depth of calcification. It will up to you to decide which of the values provided to you are best suited to this comparison. Be sure to justify your treatment of the data. 2. Once you have undertaken the data exercise, you should present your results as part of a written report, where you clearly demonstrate the steps you have taken to arrive at your estimates of calcification depths. You should then fully discuss any sources of uncertainly in these calculations. These sources of uncertainly might, for example, relate to physical aspects of the environment in which the foraminifera are calcifying, or to uncertainties associated with the calculations themselves, e.g. choice of equation. Figures are essential to show your findings, but please include no more than 3. 3. Conclude your report with a discussion of what information you might be able to extract from the three species of foraminifera you have looked at. Do these species all calcify at the same depth? If not, how might these differences be used to infer palaeoceanographic changes in this region? You might be able to support your discussion with reference to the literature.