Size-related isotopic trends in Maastrichtian planktic foraminifera of DSDP Core 44-390A (Appendix B)

Stable isotope analyses were performed on ontogenetic dissections of four taxa of low latitude Late Cretaceous planktonic foraminifera from DSDP Hole 390A. The species studied include Planoglobulina acervulinoides, Planoglobulina multicamerata, Pseudoguembelina palpebra, and Racemiguembelina fructicosa. Delta18O and delta13C data indicate a deeper surface water paleohabitat for P. multicamerata than the other three taxa, and ontogenetic increases in delta18O values suggest all these taxa underwent vertical migrations from shallow to deeper surface waters. Changes in delta13C values through ontogeny include sharp increases in delta13C composition in the juvenile size intervals, a decrease in the rate of delta13C change through intermediate size intervals, and reversals to a negative trend in delta13C values in terminal size intervals. The intermediate and terminal growth changes in delta13C signals are similar to ontogenetic trends observed in some extant and Paleogene planktonic foraminifera and may result from decreasing metabolic rates through ontogeny or endosymbiont digestion prior to gametogenesis. The ontogenetic delta13C increases of 1.04?, 0.76?, 0.83?, and 0.77? in R. fructicosa, P. palpebra, P. acervulinoides, and P. multicamerata, respectively, may indicate the presence of photosymbionts. However, our review and critique of the current literature discussing photosymbiont effects on stable isotope values in living and fossil planktonic foraminifera suggests that conclusions regarding the presence of photosymbionts in fossil taxa may be more equivocal than previously thought.

Delta 44/40 Ca, Mg/Ca and Sr/Ca record of Globigerinoides sacculifer and calculated temperatures and salinity changes of ODP Hole 165-999A

The delta18O values of planktonic foraminifera increased in the Caribbean by about 0.5? relative to the equatorial East Pacific values between 4.6 and 4.2 Ma as a consequence of the closure of the Central American Gateway (CAG). This increase in delta18O can be interpreted either as an increase in Caribbean sea surface (mixed layer) salinity (SSS) or as a decrease in sea surface temperatures (SST). This problem represents an ideal situation to apply the recently developed paleotemperature proxy delta44/40Ca together with Mg/Ca and d18O on the planktic foraminifer Globigerinoides sacculifer from ODP Site 999. Although differences in absolute temperature calibration of delta44/40Ca and Mg/Ca exist, the general pattern is similar indicating a SST decrease of about 2-3 8C between 4.4 and 4.3 Ma followed by an increase in the same order of magnitude between 4.3 and 4.0 Ma. Correcting the delta18O record for this temperature change and assuming that changes in global ice volume are negligible, the salinity-induced planktonic delta18O signal decreased by about 0.4? between 4.4 and 4.3 Ma and increased by about 0.9? between 4.3 and 4.0 Ma in the Caribbean. The observed temperature and salinity trends are interpreted to reflect the restricted exchange of surface water between the Caribbean and the Pacific in response to the shoaling of the Panamanian Seaway, possibly accompanied by a southward shift of the Intertropical Convergence Zone (ITCZ) between 4.4 and 4.3 Ma. Differences in Mg/Ca- and delta44/40Ca-derived temperatures can be reconciled by corrections for secular variations of the marine Mg/Ca[sw] and delta44/40Ca, a salinity effect on the Mg/Ca ratio and a constant temperature offset of ~2.5 °C between both SST proxy calibrations.