Three Pleistocene, five Pliocene, and thirteen late and middle Miocene calcareous nannofossil datums have been identified in the Leg 170 cored sequences. A summary table of these datums, their ages, and their locations in the Leg 170 cores is presented in Table T10. Whereas other datums were identified, the datums listed in this table are considered to be the most reliable.
What appears to be a complete—or nearly complete—late Pleistocene through early middle Miocene section (see below) is recognized in the calcareous nannofossil record. The oldest assemblages, observed at Sites 1039 and 1040, are latest early Miocene in age, nannofossil Zone NN4, with an absolute age assignment between 15.6 and 18.2 Ma. These assemblages are associated with gabbro intrusions into the basal sediments, indicating an age for the intrusion event between 15.6 and 18.2 Ma at both Sites 1039 and 1040. One contact metamorphic hornfels sample contains relict nannofossils. Reference Site 1039, located on the Cocos plate, provides the best-preserved sequence of sediments from late Pleistocene to latest early Miocene in age. The sediments cored in the prism sections at Sites 1040, 1041, 1042, and 1043 all indicate that the age of nannofossil assemblages in the prism sediments, including the toe, wedge, and apron, are all of Pleistocene age with a considerable amount of upper Miocene reworking.
A period of low sediment accumulation rates is recorded for Pliocene sediments at Sites 1039, 1040, and 1043. Differentiating Pliocene Zones NN18–NN16 (equivalent to nannofossil Subzones CN12c, CN12b, and CN12a) could not be accomplished at any of these sites. Pliocene nannofossil Zones NN15, NN14, NN13, and NN12, which occur in the top of the early Pliocene and bottom of the late Pliocene, could not be resolved at any site either. In late Miocene sediments, nannofossil Zone NN11 was easily identified at all three sites; however, the four subzones within nannofossil Zone NN11 could not be resolved. Within the Miocene, nannofossil zones older than NN11 (equivalent to nannofossil Zones CN8–CN5b) were difficult to distinguish individually until Zone NN6 (equivalent to Subzone CN5a). These intervals, where the nannofossil zones were not resolved or were difficult to resolve, lie in an ~8.5 m.y.-long (2.5–11 Ma) low sediment accumulation rate time interval (Figs. F8, F9, F10). This interval has a sediment accumulation rate of ~5.3 m/m.y. Resolution of nannofossil, planktonic foraminifer, and diatom zones within this long section all suffer from the low sedimentation rates.
Interestingly, at DSDP Site 155 in the Panama Basin, Bukry (1973a) encountered many of the same difficulties encountered here in resolving the early Pliocene and late Miocene nannofossil Zones. Coccolith assemblages recovered from Site 155 also range in age from Pleistocene to late early Miocene. At DSDP Site 155, nannofossil Zones NN15– NN12 could not be resolved because of the absence of Discoaster tamalis, D. surculus, the amauroliths, Ceratolithus amplificus, C. rugosus, and Triquetrorhadulus rugosus.
Nannofossil Zones NN8 (equivalent to Zone CN 6) and NN7 (equivalent to Zone CN5b) are recognized at Site 155 in only 1 m of sediment. These zones were not resolved at any Leg 170 site because of the absence of C. coalitus. In fact, some of the zones inferred by Bukry (1973a) at Site 155 were determined with secondary markers.
It is noted that the closure of the Central American Isthmus (CAI) may have played a significant role in the plankton productivity of the overlying Leg 170 study area. The formation of the isthmus has been a complex process over the last 15 m.y. (Keigwin, 1982; Ruddiman and Raymo, 1988; Coates and Obando, 1996; Cronin and Dowsett, 1996), creating a total marine barrier by ~3.1–2.8 Ma. The closure of the isthmus has been tied to the initiation of Northern Hemisphere glaciation at 2.5 Ma and a revamping of both Atlantic and Pacific Ocean current flow regimes. The nannofossil data and conclusions drawn from this study will become the basis for a more comprehensive study into the effects of the formation of the CAI on Eastern Equatorial Pacific calcareous plankton.