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Calcite preservation—implications for the robustness of geochemical proxies

Determining the preservation of benthic and planktonic foraminiferal specimens and coccoliths from Site U1338 is important, as it helps to quantify the robustness of the geochemistry on the archives.

The foraminifer BSE-TOPO SEM images show that both the benthic and planktonic foraminifer specimens have undergone some alteration, which was not easily identifiable using RLM. The benthic foraminifers have only slightly been affected (Fig. F2). Out of the eight imaged specimens, six showed good preservation with only very minor overgrowth. The other two specimens showed considerable overgrowth on the umbilical side. Considering Edgar et al. (2013) work on the influence of recrystallization and overgrowth on the δ18O and δ13C of benthic foraminiferal calcite, stable isotope measurements on benthic foraminifers from this site are probably only minimally affected by diagenesis.

The BSE-TOPO images show that the planktonic foraminifer specimens have undergone far more alteration, with preservation ranging from fair to poor (Figs. F3, F4). Preservation is highly variable, with all specimens showing evidence of some overgrowth and recrystallization. The variability of preservation is interesting, particularly when comparing specimens that are only separated by ~1 m in depth. Between these specimens (Fig. F3H compared to Fig. F3I and Fig. F3J to F3K), the difference in recrystallization and overgrowth is large. A recent study of planktonic foraminifers in the mid-Miocene section of Site U1338 shows that the preservation of the planktonic foraminifers investigated is generally good (Fox and Wade, 2013). Generally, the specimens have not undergone substantial recrystallization, with the original submicrometer microgranular wall texture remaining intact (Fox and Wade, 2013). The difference in preservation between the foraminifers investigated by Fox and Wade (2013) and the foraminifers investigated in this study is unexpected, as the specimens in Fox and Wade (2013) have experienced greater burial depths (an additional 200 m or more) than the specimens in this study. This suggests that burial depth was not the main control on diagenetic alteration at this site. Because of the poor to fair preservation of the planktonic foraminifers, stable isotope measurements on this species should be treated cautiously, particularly δ18O records, although δ13C records do not seem to be much affected by recrystallization (Sexton et al., 2006).

The coccoliths investigated only show moderate or poor to moderate preservation. The SEM study could not resolve sufficiently high spatial resolution to determine whether any of the large or complete heterococcolith fragments had undergone recrystallization, although recrystallization of the heterococcolith calcite is unlikely because of the large crystal size of the individual calcite crystals. Many heterococcoliths show evidence for small amounts of secondary calcite overgrowth (Fig. F5F). In addition, slight to moderate etching must have occurred because none of the heterococcoliths have centrally preserved structures. Overall, both these processes would have reduced the proportion of surface water coccolith calcite present in the sample, which should be taken into account when using this archive for geochemistry.

The high-resolution SEM and EDS images (Figs. F5, F6) show that in addition to the main two components (biogenic SiO2 and CaCO3), some marine barite is also present (Fig. F6). The small size (1–2 µm) and euhedral to subspherical shape of crystals indicates that the barite is marine barite, rather than hydrothermal or diagenetic barite (Paytan et al., 2002). Marine barite is precipitated in the upper water column during the degradation of organic material, potentially aided by zooplankton and/or bacteria (Griffith and Paytan, 2012). X-ray fluorescence measurements on the Site U1338 core splice indicate that BaSO4 is present throughout the late Miocene and early Pliocene (ranging from 0.5 to 2 wt%) (Lyle et al., 2012).