Intratest oxygen isotope variability in the planktonic foraminifer N. pachyderma (sin.)

1. Intratest oxygen isotope variability in the planktonic foraminifer N. pachyderma (sin.) R. Kozdon, T. Ushikubo, N.T. Kita, M. Spicuzza, J.W. Valley • Conventional analytical techniques require the measurement of multiple pooled foraminiferal tests which are homogenized in order to obtain a single oxygen isotope value. • Using Secondary Ion Mass Spectrometry (SIMS), measurements of d18O in cross-sections of foraminiferal chamber walls provide access to a new proxy archive with a temporal resolution of several days or less, which can be regarded as a “flight recorder” of a specimens’ life cycle. (A-C). SEM images of polished foraminiferal cross sections (N. pachyderma (sin.), N. Atlantic core top) showing locations of 2x3 µm analysis pits for d18O. (D). The corresponding d18O values, that are arranged by distance from the inner chamber wall, vary systematically by 3‰ with location and reflect growth history. Kozdon, R., Ushikubo, T., Kita, N.T., Spicuzza, M. and Valley, J.W., (2009). Intratest oxygen isotope variability in the planktonic foraminifer N. pachyderma: Real vs. apparent vital effects by ion microprobe. Chem. Geol., 258: 327-337.

2. Analytical capabilities • SEM image showing SIMS analysis pits in UWC-3 calcite standard. These measurements were performed to determine the minimum distance between SIMS analysis pits without affecting analytical precision. • Spot-to-spot precision of d18O measurements with ~3 µm (dia.) pits typically better than 0.7‰ (2 SD), and 0.3‰with 10 µm analysis pits. • Sample volume ~10 µm3 = ~30 pico grams  1/200,000 of the mass of a singe foraminiferal test.

3. The cool tropic paradox: Reassessing aberrant δ18O in foraminifera by SIMS R. Kozdon, D.C. Kelly, N.T. Kita, J.W. Valley Oxygen isotope records derived from conventional analysis of the shells of tropical planktonic foraminifera suggest that tropical sea surface temperatures (SST) peaked at only ~25°C during the Paleogene greenhouse climate. This is substantially lower than SSTs (30°C) predicted by climate models and may be caused by diagenetic recrystallisation of fossil foraminiferal shells. SIMS analysis reveals that the original d18O is commonly preserved in calcareous outgrowths from the chamber walls (muricae) of tropical morozovellid species (left SEM images). Muricae have a lower porosity than the remaining test and are less sensitive to diagenesis. Right: Muricae d18O (red diamonds) indicate that tropical open-ocean SSTs during the Early Paleogene greenhouse climate varied between 24–34°C at Site 865, which is 2-8°C higher than paleo-temperatures inferred from previously reported d18O records for this Site (blue open circles; Bralower et al., 1995). Kozdon R., Kelly D.C., Kita N.T. and Valley J.W. (2009). The cool tropic paradox: Reassessing aberrant δ18O in foraminifera by SIMS. Geochim. Cosmochim. Acta Suppl. 73, A693.