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Stable isotope and trace element data for planktonic foraminifera from IODP Site U1338 (NERC Grant NE/I528750/1)
Oxygen and carbon isotope ratios (δ18O, δ13C) were measured on 10 to 12 shells of mixed-layer dwelling species Globigerinoides subquadratus from the 250 to 315 μm size fraction from 425 meters composite depth (mcd) until its extinction at 390 mcd. Analyses then continue with Globigerinoides spp. until 350 mcd. In a few samples, where foraminiferal density was low, only 5–7 specimens were analyzed. Analyses were made with a VG Optima mass spectrometer with multi prep device at the British Geological Survey, Keyworth, UK. When picking shells, care was taken to exclude individuals with broken or missing chambers, although preservation of specimens was generally excellent (Fox & Wade, 2013). The external reproducibility of our measurements is ±0.07‰ and ±0.05‰ for δ18O and δ13C respectively. To examine the reproducibility of the results, duplicate measurements were made on 35 samples (5%), which indicate mean reproducibility better than ±0.12‰ and ±0.14‰ for δ18O and δ13C, respectively. Oxygen isotope data are reported as per mil on the VPDB scale (Table S1) calibrated through laboratory and international standards. At ODP Site 1146, δ18O and δ13C were measured by Holbourn et al., (2010) on the mixed-layer dwelling planktonic foraminifera Globigerinoides obliquus or Gs. subquadratus, using 10 to 20 well-preserved tests from the size fraction 250-350 μm. Paired measurements in 51 samples indicate no significant offset in δ18O and δ13C between Gs. obliquus and Gs. subquadratus. Detailed methods are outlined in Holbourn et al., (2010). δ13C data are not used here. For Mg/Ca analyses, we selected 25–35 specimens of Trilobatus quadrilobatus (140–550 μg) from the 250 to 315 μm size fraction; the same size fraction as used for δ18O analysis, to minimize size-related intraspecific elemental variation (Elderfield et al., 2002). Analyses were performed on 86 samples over the studied interval. The tests were gently crushed and subsequently cleaned according to the protocol of Martin & Lea (2002) to remove clays. Cleaning included a reductive step with hydrazine to remove Mn-(hydr)oxides. Samples were measured on an ICP-AES device at Christian-Albrechts-Universität zu Kiel, Germany. Analytical precision is ~1.1%, based on measurements of an internal laboratory standard. Replicate Mg/Ca measurements revealed an average standard deviation of ~0.08 mmol/mol. Adequate cleaning is indicated by very low Fe/Ca, Al/Ca and Mn/Ca ratios.