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doi:10.2204/iodp.proc.303306.205.2009

Results and discussion

Our study shows the evolution of relative abundances of some planktonic foraminifer species through time with special emphasis on the first or last occurrences of these species (Figs. F1, F2). The interval of study extends from the early Pliocene to the present. See Table T1 for quantitative data.

Early Pliocene events

The latest Miocene to early Pliocene interval is dominated by the presence of G. margaritae, which is relatively abundant from the base of this site (Fig. F1). We recognized the last occurrence (LO) of this species between 185.29 and 182.25 meters composite depth (mcd). It never reached percentages above 10% of the assemblage. Because of the low resolution of our study for this part of the record, we are not able to accurately locate this event.

We also analyzed the early Pliocene abundance of G. puncticulata. The first occurrence (FO) of this species, which is a very useful event in the North Atlantic, was located between 213 and 207 mcd, and the LO was located between 114.6 and 115.02 mcd (Fig. F1). This species shows high percentages during the Pliocene, reaching >30% of the total assemblage immediately above its FO. An interval in which this species is completely absent was observed in the middle part of its time range (Fig. F1). This absence allowed us to define two bioevents: (1) the temporary disappearance of G. puncticulata located between 168.92 and 164.97 mcd, and (2) its reappearance located at 153.8 mcd. Immediately after its reappearance, again G. puncticulata abundance reached very high percentages (Fig. F1).

Late Pliocene–early Pleistocene events

We analyzed the late Pliocene and Pleistocene quantitative changes of G. inflata, G. truncatulinoides, and N. pachyderma sinistral (Figs. F1, F2). These three species appear for the first time near the Pliocene/Pleistocene boundary.

After the LO of G. puncticulata, the Globorotaliids are almost absent from this site until the FO of G. inflata, located at 106.17 mcd, although only one specimen was found at this level. A short peak in abundance of this species appears at 101.73 mcd, followed by very low abundances. The sharp increase in abundance observed at 98.23 mcd was used to define the first abundant occurrence (FAO) of G. inflata (Fig. F1). From this level uphole G. inflata abundance varies greatly, reaching percentages >40% of the assemblage during some intervals.

N. pachyderma sinistral is present at low percentages in the Pliocene and earliest Pleistocene and becomes very abundant between 82.43 and 83.92 mcd (Fig. F2). We used this change in abundance to define the FAO of this species. In addition, we studied the abundance of N. pachyderma dextral to calculate the percentage of N. pachyderma sinistral with respect to the total number of Neogloboquadrinids. The percentage of sinistral Neogloboquadrinids is very low during the Pliocene and earliest Pleistocene and then suddenly changes at 82.43 mcd because of the large increase in abundance of N. pachyderma sinistral (Fig. F2).

We recognized a relatively long interval in which N. pachyderma sinistral is rare or absent that was used to define two new bioevents (Fig. F2). The temporary reduction in abundance of the species was observed between 66.4 and 64.85 mcd, whereas the increase in abundance was located between 56.35 and 57.85 mcd. In this interval with very low values of N. pachyderma sinistral, the dextral Neogloboquadrinids become dominant (Fig. F2).

Finally, we identified the FO of G. truncatulinoides between 94.81 and 96.31 mcd. This species does not reach percentages above 4% of the assemblage and is absent in many samples after its FO. Consequently, the accurate location of this bioevent should be analyzed with caution.

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