The species considered in this study are mainly included within the family Noelaerhabdaceae (placoliths including the genera Emiliania, Pseudoemiliania, Gephyrocapsa, and Reticulofenestra) (Thierstein et al., 1977; Pujos-Lamy, 1977; Wei, 1993; Raffi et al., 1993; Weaver and Thomson, 1993). The taxonomy of this group is complex and confusing, mainly because of a proliferation of species names and morphotypes. Here, we adopted the ideas of Raffi et al. (1993) and Flores et al. (2000) for the morphological features of gephyrocapsids, using readily identifiable features under cross-polarized light such as coccolith diameter, bridge angle, and so on.
The first occurrence (FO) of E. huxleyi was dated by Thierstein et al. (1977) at 268 ka (late in marine isotope Stage [MIS] 8). At Sites 1237, 1238, 1240, and 1241, this species is very scarce and sometimes difficult to identify. Dissolution may also make identification difficult, especially at Site 1242.
The last common occurrence of G. caribbeanica is close to the FO of E. huxleyi, proposed as an alternative event for midresolution biostratigraphic studies when E. huxleyi is scarce or absent (Flores and Marino, 2002). A clear reduction in the abundance of G. caribbeanica occurs in all the studied sites just after the FO of E. huxleyi, although it is possible to find some specimens of the morphotype G. caribbeanica until the most recent samples, especially in samples from Sites 1240, 1241, and 1242.
Thierstein et al. (1977) identified the globally synchronous last occurrence (LO) of P. lacunosa within MIS 12. This event is clearly identified in all the studied sites. To be consistent, we considered the LO when the record of P. lacunosa is <1% in abundance. It is common to find specimens of this species close to the FO of E. huxleyi, interpreted here as reworked.
A clear increase in the abundance of G. caribbeanica can be identified within MIS 13 (Matsuoka and Okada, 1990; Pujos and Giraudeau, 1993; Bollmann et al., 1998; Flores et al., 1999), although prior to this dominance, this species increased progressively from MIS 15. This event is easy to identify at Sites 1237 and 1238, recorded just before the LO of P. lacunosa. It is difficult to see the signal of this species at Sites 1240, 1241, and 1242; the low sampling resolution contributed to this fact.
The LO of R. asanoi is known to be a synchronous event in low and relatively high latitudes and occurred late in MIS 22 (Sato and Takayama, 1992; Wei, 1993). In this study, we considered under the denomination R. asanoi specimens larger than 6 µm, and the signal is very well identified in all the studied sites.
Raffi et al. (1993) reported the reentry of so-called medium Gephyrocapsa between MIS 29 and 16, with a marked degree of diachronism. This event was identified at all the studied cores, although it is difficult to identify at Sites 1241 and 1242.
The FO of R. asanoi is not easy to identify owing to the occurrence of intermediate forms between this species and small reticulofenestrids. Wei (1993) observed the FO of the species between MIS 35 and 29. Sato and Takayama (1992) dated the event at 1.06 Ma (~MIS 30) in the northeast Atlantic (ODP Leg 94) and the Boso Peninsula (west Pacific). Flores and Marino (2002) identified this event in the Atlantic Southern Ocean at the top of Subchron C1r.2r. We clearly observed this event at all the studied cores, considering specimens with maximum diameters >6 µm, following the original description of Sato and Takayama (1992).
The LO of the large Gephyrocapsa morphotype (large Gephyrocapsa sp. B-Matsuoka and Okada, 1990; LG, Raffi et al., 1993) is a globally synchronous event that has been recorded in MIS 37 (Raffi et al., 1993; Wei, 1993). In our material, this event is identified at all sites, although the abundance is sometimes <1%.
The FO of large Gephyrocapsa is a diachronous event. Raffi et al. (1993) placed this event between MIS 46 and 49, and Wei (1993) observed the same event from MIS 47 to 51. As in the mentioned FO of this morphotype, their identification in our material is easy, although maximum abundance is habitually <1%.
Raffi et al. (1993) and Wei (1993) dated this event between 1.59 and 1.64 Ma (MIS 55–57), although it is sometimes absent because of ecological restrictions (Flores and Marino, 2002). In our material this species is always scarce but present enough to identify the event, which occurs always immediately after the LO of Discoaster broweri, close to the Olduvai/Matuyama boundary (Wei, 1993).