The following section sets forth a description of the silicoflagellate assemblages encountered in the Miocene section of the holes considered in this study. It is accompanied by a qualitative interpretation of the history of surface waters through the time interval represented by these phytoplanktonic micro-organisms. The latter is based on the analysis of variations in generic (and sometimes specific) abundance through time, using the affinities of the different silicoflagellate groups for both water temperature and nutrient concentrations. Locker and Martini (1989) and Ciesielski and Case (1989) provided a paleoenvironmental analysis for ODP Leg 104 sites, applying the criteria given in previous literature for the use of silicoflagellate groups for such purposes (e.g., Bukry, 1986) without considerable modifications. Most of their criteria are followed here: in this way, dictyochids and corbisemids are warm-water adapted (Martini, 1971, and Bukry, 1981b, respectively); hexagonal distephanids are cool-water adapted (Ciesielski, 1975); quadrate ("tetragonal") distephanids are temperate adapted (Bukry, 1985); and bachmannocenids are representative of nutrient-rich waters (Bukry, 1986).
One important aspect to consider, however, is that Locker and Martini (1989) place bachmannocenids within three separate genera, all with different temperature affinities. In this way, septamesocenids are warm adapted, mesocenids are warm to temperate adapted, and paramesocenids are cool to temperate adapted. Even though all bachmannocenids are placed within the genus Bachmannocena for this study (see "Taxonomy" section, this chapter) following the most accepted view on these morphotypes, there seem to be differences in the environmental tolerances of different species within the genus. In the discussion that follows, B. apiculata ssp. (the "septamesocenids" of Locker and Martini, 1989) are considered to be warm adapted, B. circulus apiculata (a "paramesocenid" of these authors) is considered to be temperate adapted, and B. diodon nodosa (a "mesocenid") is considered to be cool to temperate adapted. Only the latter interpretation differs from that of Locker and Martini (1989), who considered mesocenids to be warm to temperate adapted.
The other group in which temperature affinity is not clear-cut is the caryochids. Ciesielski and Case (1989) recognize this group but comment on the poorly established paleoenvironmental affinity of these taxa. Locker and Martini (1989) include most of these morphotypes in the genus Cannopilus, which they consider to be warm adapted. As discussed below, their occurrence together with quadrate distephanids identifies them as more temperate adapted than warm adapted. They do not, however, persist into the late Miocene as do the quadrate distephanids. Their true affinity, therefore, might be more temperate to warm adapted, as opposed to quadrate distephanids, which could be more temperate to cool adapted.
Site 907 is located on the Iceland Plateau, at the center of a planned transect from Norway to Greenland, with ODP Leg 104 Vøring Plateau sites at the eastern end and Leg 162 Site 987 at its western end. Hole 907A was drilled during Leg 151, whereas Holes 907B and 907C were drilled during Leg 162. Hole 907B is positioned at 69º14.989'N and 12º41.898'W at a water depth of ~1800 m. Twenty-three cores were recovered using the advanced hydraulic piston corer (APC) system, totaling 211.7 m of sediment. Four lithologic units were described by shipboard scientists, of which only Units III and IV contain Miocene sediments. Only Unit III (63.1-196.1 meters below seafloor [mbsf], Cores 162-907B-8H-3 through 22H-3) yielded biosiliceous microfossils. The primary lithology of this unit is silty clay, and biosiliceous components increase from <5% at the upper boundary to 5%-20% in the lower portion of the unit (Jansen, Raymo, et al., 1996).
The oldest sample with biosilica is Sample 162-907B-22H-3, 109-110 cm. Together with the next oldest, Sample 162-907B-22H-2, 109-110 cm, the upper Corbisema triacantha Zone is represented. In both samples, preservation is low (see Table 2), and the assemblage is characterized by Distephanus crux lockerii n. ssp. and Bachmannocena apiculata ssp.
In Sample 162-907B-22H-1, 109-110 cm, the first occurrence of Bachmannocena circulus apiculata is recorded, which marks the bottom of the overlying Caryocha spp. Subzone of the Bachmannocena circulus apiculata Zone. This subzone extends to Sample 162-907B-17H-3, 109-110 cm, and is characterized by B. c. apiculata throughout. Preservation is very good in the middle part of the subzone and decreases toward the upper and lower parts. Bachmannocena apiculata ssp. are found only in the subzone's lowermost part, where an acme of a morphotype compared to Dictyocha rhombica is observed (Sample 162-907B-21H-6, 109-110 cm). The disappearance of the Bachmannocena apiculata ssp. (of warm surface-water affinities, according to this study) is followed by a sudden increase in Caryocha ernestinae and quadrate distephanids. These latter taxa are considered to have more temperate affinities (Bukry, 1985, and this study) and mark a decline in surface-water temperatures. Toward the middle part of the subzone, there is an acme of Dictyocha fibula (with concomitant decreases in caryochids and quadrate distephanids) marking a brief return of warmer surface temperatures (Martini, 1971), followed by a sudden increase in Bachmannocena circulus apiculata, caryochids, and quadrate distephanids. This signals a return to temperate conditions together with an increase in surface-water fertility (Bukry, 1986). The upper part of the subzone is marked by rare occurrences of the aforementioned taxa, together with some reworked Paleogene specimens (Corbisema hastata, C. inermis, and Naviculopsis constricta).
The first occurrence of Bachmannocena diodon nodosa in Sample 162-907B-17H-2, 109-110 cm, defines the beginning of the overlying Distephanus crux stradneri Subzone of the B. c. apiculata Zone, which straddles the middle/late Miocene boundary. Because preservation in the samples below this first occurrence is poor, this bioevent might actually be slightly older. The interval spanned by this subzone is characterized by good preservation, except for a small section (the bottom of Core 162-907B-15H and most of Core 16H). The abundant bachmannocenids point toward continuing high-productivity surface waters (Bukry, 1986), but the temperature signal differs from that of the subzone below. The persistence of temperate quadrate distephanids (Bukry, 1985), in spite of growing numbers of cool-water hexagonal distephanids (Ciesielski, 1975), suggests cool-temperate conditions instead of the warm-temperate conditions characteristic of the underlying subzone's assemblage. In addition, even though all bachmannocenids are considered to be upwelling indicators (Bukry, 1986), the fact that increases of B. c. apiculata go hand-in-hand with decreases of B. diodon nodosa (and vice versa) implies that they have different temperature affinities. Also, a peak abundance of B. d. nodosa within the middle part of the subzone (Sections 3 and 4 of Core 162-907B-15H) correlates with a marked decrease in quadrate distephanids. For these reasons, B. d. nodosa is considered herein to be a cool- to temperate-surface-water indicator, as opposed to B. c. apiculata, which would be more characteristic of temperate to somewhat warm surface waters.
Most quadrate distephanids have their last occurrence in Sample 162-907B-14H-1, 109-110 cm. This bioevent is extremely reliable because it occurs within an interval of excellent biosiliceous preservation and marks the top of the D. c. stradneri Subzone. Since all younger assemblages are characterized by hexagonal distephanids, this point would mark the beginning of cool surface-water conditions in the area. Species diversities decrease considerably as from this time, and surface-water productivity remains high, as evidenced by the continued presence of bachmannocenids throughout the overlying Bachmannocena diodon nodosa Subzone. The last occurrence of B. c. apiculata is recorded in Sample 162-907B-12H-2, 109-110 cm, but preservation above this sample is extremely poor, and an unconformity has been suggested at this depth (Jansen, Raymo, et al., 1996). This means that most probably the uppermost part of this subzone is missing, as well as the lowermost part of the overlying Bachmannocena diodon Zone. All of the B. diodon Zone in this hole has poor preservation, and its top is defined by the last occurrence of B. diodon nodosa (Sample 162-907B-11H-4, 109-110 cm). Most of the overlying Distephanus boliviensis Zone, which straddles the Miocene/Pliocene boundary, is also characterized by poor preservation, but a few samples with excellent preservation provide windows into this time period, showing a dominance of cool-water hexagonal distephanids.
Site 982 is located on the Rockall Plateau, a shallow platform that lies roughly between Iceland and Ireland, underlain by continental crust, which separated from Greenland sometime during the Paleocene (Laughton, Berggren, et al., 1972). Four holes were drilled during Leg 162 (A through D), the deepest being Hole 982B, with a recovery of 614.9 m. The latter is positioned at 57º31.002' and 15º51.993'W at a water depth of 1134 m. The first 26 cores were obtained with the APC system, and 39 further cores with the extended core barrel system. Two lithologic units were described by shipboard scientists, of which Unit II (57.4-614.9 mbsf, Cores 162-982B-7H-3 through 65H) contains Miocene sediments (Core 18H and older). Although the primary lithology of Unit II is a nannofossil ooze (with calcium carbonate percentages averaging around 90%), most samples from this unit yielded enough biosiliceous components to allow a detailed silicoflagellate analysis.
The oldest sample with biosilica is Sample 162-982B-57X-2, 99-100 cm, but preservation is poor. Preservation increases as from the next oldest sample (see Table 1) and remains good throughout the Miocene section of this hole's cores, with the exception of two intervals close to the middle/late Miocene boundary. The two above-mentioned samples represent the uppermost part of the lower Corbisema triacantha Zone, of early middle Miocene age. The silicoflagellate assemblage is characterized by warm and temperate taxa, mainly Bachmannocena apiculata curvata and quadrate distephanids, respectively.
The first occurrence of Bachmannocena diodon diodon in Sample 162-982B-56X-1, 99-100 cm, marks the base of the overlying upper Corbisema triacantha Zone. This zone extends to Sample 162-982B-50X-3, 99-100 cm, above which Bachmannocena circulus apiculata is recorded. The zone is characterized by warm and temperate taxa: bachmannocenids such as B. apiculata ssp. and B. elliptica elliptica, together with several dictyochids and Corbisema triacantha, suggest warm surface-water temperatures, whereas common quadrate distephanids and caryochids (which appear for the first time in this zone) point to more temperate conditions. The common to abundant B. diodon diodon indicates highly productive surface waters.
The first occurrence of Bachmannocena circulus apiculata in Sample 162-982B-49X-2, 99-100 cm, denotes the base of the next interval, here defined as the Caryocha spp. Subzone of the Bachmannocena circulus apiculata Zone, which extends to Sample 162-982B-45X-1, 99-100 cm. This subzone is characterized by a silicoflagellate assemblage that suggests warm to temperate surface waters (B. apiculata ssp., dictyochids, caryochids, and quadrate distephanids), though the cool-water hexagonal distephanids begin to be present too. The abundant B. c. apiculata indicates highly fertile surface waters.
The first occurrence of Bachmannocena diodon nodosa in Sample 162-982B-44X-3, 99-100 cm, marks the base of the next interval, the Distephanus crux stradneri Subzone of the Bachmannocena circulus apiculata Zone, which extends to Sample 162-982B-32X-3, 99-100 cm, where all quadrate distephanids have their last occurrence. Both the lower and upper parts of this subzone have poor biosilica preservation, but the analysis of samples within these intervals still allows the identification and counting of taxa in spite of their low abundances. The middle interval, with good preservation, shows common quadrate distephanids (but no caryochids), B. c. apiculata, and B. diodon ssp. As discussed for Hole 907B, the appearance of B. d. nodosa marks a drop in sea-surface temperatures, although some temperate taxa are still part of the assemblage.
The interval above this subzone is characterized by taxa that are cool-water indicators dominating the assemblage (hexagonal distephanids and B. d. nodosa). The continued presence of bachmannocenids points to a high-productivity environment. Of interest is the presence of Dictyocha brevispina ausonia in the middle to upper part of the Bachmannocena diodon nodosa Subzone of the Bachmannocena circulus apiculata Zone, since it marks a brief period of warmth during a time of progressive cooling. This interval might be absent in Hole 907B because of the presence of the cited unconformity, or these warm waters may have not reached the higher latitudes at which Site 907 is located. The boundary between the Bachmannocena circulus apiculata Zone and the Bachmannocena diodon Zone is given by the last occurrence of B. c. apiculata (Sample 162-982B-24H-5, 99-100 cm). There is a marked decline in the preservation of biosilica toward the top of the overlying B. diodon Zone (i.e., above Sample 162-982B-21H-1, 99-100 cm), and no samples from this interval were studied herein. The top of the zone is therefore not recognized within the suite of samples analyzed in this study.
In summary, during the middle Miocene, silicoflagellates with affinities for temperate surface waters begin to form an important part of the assemblages replacing taxa with affinities for warm surface waters. Next, taxa with warm-temperate affinities disappear first, whereas those with cool-temperate affinities persist into the early to middle late Miocene. Last, taxa with cool-water affinities replace all others and become dominant components of the late late Miocene silicoflagellate assemblages.