The nannofossil and foraminiferal biostratigraphy, complicated by numerous turbidites, is shown in Figure F62. Estimates of paleobathymetry were interpreted from assemblages of benthic foraminifers.
Nannofossils are abundant and well preserved in samples from Sections 180-1109A-1H-CC; 180-1109B-1H-CC and 2H-CC; and 180-1109C-1H-CC through 8H-CC. From Cores 180-1109C-9H through 15X, nannofossils range in abundance from few to abundant (mostly common), with generally moderate preservation. Sample 180-1109C-17X-CC contains rare, poorly preserved specimens. Nannofossils in Samples 180-1109C-20X-CC through 41X-CC range from few to abundant (37X-CC is barren), with moderate to good preservation.
In Hole 1109D, nannofossils are mostly abundant from Samples 180-1109D-1R-CC down to 8R-CC, common to abundant from 9R-CC through 26R-CC, and few to abundant in 27R-CC through 35R-CC. Nannofossils are rare in Sample 180-1109D-36R-CC and occur in only trace amounts in 37R-CC. Sediments in Cores 180-1109D-38R through 43R are barren of nannofossils. Scrapings were taken from the sandstones and conglomeratic matrix in Sections 180-1109D-44R-1, 44R-2, 45R-1, and 45R-2; all samples were barren of nannofossils. Preservation is moderate to very good from Samples 180-1109D-1R-CC down to Sample 23R-CC, moderate to poor from Sample 24R-CC to Sample 32R-CC, and poor from Sample 33R-CC to Sample 37R-CC.
Calcareous nannofossil zonal assignments for samples taken from Holes 1109A, 1109B, 1109C, and 1109D are shown in Figure F62. A range chart showing the distribution of the index species and other selected species is shown in Table T6A and T6B. This table was prepared during shipboard investigations and does not list all species present in a given sample.
We assign Samples 180-1109A-1H-CC, 180-1109B-1H-CC to 2H-CC, and 180-1109C-1H-CC to 2H-CC to calcareous nannofossil Zone NN21 (late Pleistocene to Holocene) on the basis of the presence of Emiliania huxleyi. Samples 180-1109C-3H-1, 95-97 cm, to 4H-3, 18-19 cm, lack E. huxleyi and indigenous Pseudoemiliania lacunosa, indicating Zone NN20. Sample 180-1109C-4H-4, 105-106 cm, contains the stratigraphic last occurrence (LO) of P. lacunosa, which marks the upper boundary of Subzone NN19F. The stratigraphic first occurrence (FO) of Gephyrocapsa omega in Sample 180-1109C-4H-CC indicates the lower boundary of Subzone NN19F. Samples 180-1109C-5H-1, 95-97 cm, to 6H-3, 95-97 cm, lack both G. omega and Discoaster brouweri; this interval is in Subzones NN19A-E. Samples 180-1109C-6H-6, 95-97 cm, through 31X-CC are in Zone NN17 through Subzone 19E, based on lack of Discoaster surculus. The LOs of D. brouweri and D. pentaradiatus (Zone NN18/19 and Zone NN17/18 boundaries, respectively) are not assigned because of large numbers of apparently displaced specimens. D. surculus has its LO in Sample 180-1109C-32X-CC (Zone NN16/NN17 boundary).
Martini's (1971) Zone NN16 was subdivided into Subzones 16A and 16B by Rio et al. (1990), based on the LO of D. tamalis, a distinctive event in the Mediterranean area where the subdivision was first established. The LO of D. tamalis is less distinctive elsewhere. D. tamalis is rare throughout its range at Site 1109. We have not subdivided Zone NN16 here because the species is too rare to place the LO with certainty.
The Zone NN15/NN16 boundary is defined by the LO of Reticulofenestra pseudoumbilica. The difficulty of determining an accurate position for this boundary, especially in low latitudes where the index species is rare, is well known (see Perch-Nielsen, 1985). The tendency for specimens of this large, robust species to be reworked above its true extinction level is an additional complicating factor. Some biostratigraphers (e.g., Perch-Nielsen, 1985; Siesser and de Kaenel, 1999) have, therefore, recommended using the LO of Sphenolithus abies/S. neoabies to approximate this zonal boundary, because Sphenolithus spp. become extinct just above the LO of R. pseudoumbilicus (Perch-Nielsen, 1985). However, Sphenoliths are present up through Sample 180-1109C-37X-1, 93-95 cm, which is anomalously high in the section when compared to the FO of the foraminifer Globorotalia tosaensis and to the Gauss/Gilbert Chron boundary. Therefore, we assign Zone NN15/NN16 undifferentiated down through Sample 180-1109C-41X-CC and from Sample 180-1109D-1R-CC to 23R-2, 21-22 cm.
The Zone NN14/NN15 boundary is defined by the LO of Amaurolithus tricorniculatus. This species is often very rare, and we did not find it in any of the samples from this hole. The rarity or absence of this zonal marker has been dealt with elsewhere by either combining Zones NN14 and NN15 into one zone (e.g., Rio et al., 1990) or by using a secondary nannofossil event to approximate the boundary (e.g., Siesser and de Kaenel, 1999). The LO of A. tricorniculatus is known to occur just below the last common occurrence (LCO) of R. pseudoumbilicus (Siesser and de Kaenel, 1999). Specimens of R. pseudoumbilicus decrease in abundance and in overall size rather abruptly at this level. This decrease occurs between Samples 180-1109D-23R-2, 21-22 cm, and 23R-CC in this hole, which approximates the Zone NN14/NN15 boundary.
The first common occurrence (FCO) of D. asymmetricus marks the next lower zonal boundary (NN13/NN14). The marker species occurs rarely, but fairly consistently, down to Sample 180-1109D-24R-CC, and we consider that sample to be the base of Zone NN14. Zone NN13 continues down to at least Sample 180-1109D-34R-CC, based on the presence of Ceratolithus rugosus in that sample.
An unusual oceanographic event may have occurred in the time interval represented between Samples 180-1109C-7H-CC and 8H-3, 94-96 cm. Large specimens of Braarudosphaera bigelowi occur in this interval in unusually large numbers (3%-5% of the sediment). Braarudosphaerids are present, although rare, in samples from Sections 180-1109C-8H-1 and 8H-3, but in Sample 180-1109C-7H-CC they are few to common in abundance. Moreover, the braarudosphaerids are abnormally large. Coccoliths of this species are normally less than ~10 µm wide, whereas these specimens average 17 µm in width and range up to 21 µm. We did not find braarudosphaerids in any sample above or below this interval.
Braarudosphaera bigelowi is a long-ranging species (Jurassic to Holocene). In today's oceans it is found only in low-salinity coastal waters (e.g., Gulf of Maine and Gulf of Panama) and rarely in the open ocean (see Siesser et al., 1992). In fossil assemblages, B. bigelowi is also found in sediments interpreted as having been deposited in nearshore environments. It is generally assumed that the same environmental control(s) (salinity?) influencing the modern distribution also controlled the occurrence of this species in the past. Thus, it has been difficult to explain the occurrence of the Oligocene "Braarudosphaera Chalk" deposited under apparently open-ocean conditions in the South Atlantic and other open-ocean braarudosphaerid-rich layers drilled during Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) cruises. The South Atlantic "Braarudosphaera Chalk" is more than 1 m thick in places and may be composed of >90% braarudosphaerid coccoliths.
There has been considerable speculation as to the oceanographic conditions that triggered such enormous blooms of this genus. A low-salinity event, either alone or combined with the availability of selective nutrients, are the solutions most frequently offered (see Parker et al., 1985, and Siesser et al., 1992, for reviews of the Braarudosphaera problem). Heavy regional rainfall (Bukry, 1974) increased meltwater from the Antarctic (Bukry, 1978), and various mechanisms for upwelling (Berger, 1979; Parker et al., 1985; Siesser et al., 1992) have all been suggested to explain possible low-salinity episodes in the open ocean.
Because Site 1109 is so close to shore, the possibility of downslope transport from a nearby coastal bay must be considered as a possible explanation for the braarudosphaerid-enriched interval at this site (unlike the South Atlantic and other occurrences of braarudosphaerid-rich sediments, which are long distances from shore). Alternatively, Site 1109 is also probably close enough to shore to have been affected by a period of prolonged rainfall and freshwater runoff during this interval in the late Pliocene, which could have temporarily lowered the salinity of surface water at this site.
In Holes 1109A, 1109B, and 1109C, planktonic foraminifers were abundant, and their preservation was good in Samples 180-1109A-1H-CC, 180-1109B-1H-CC to 2H-CC, and 180-1109C-1H-CC through 15X-CC, 180-1109C-18X-CC, 19X-CC, 27X-CC through 30X-CC, and 180-1109C-38X-CC through 41X-CC. Abundance was few to rare and the preservation moderate to poor, or the samples were barren of planktonic species as in Samples 180-1109C-16X-CC, 17X-CC, 20X-CC through 25X-CC, 36X-CC, and 37X-CC.
In Hole 1109D, planktonic foraminifers were abundant and their preservation was good in Samples 180-1109D-1R-CC through 7R-CC, 18R-CC through 24R-CC, and in 26R-CC. Abundance was few to common and the preservation was moderate in Samples 180-1109D-8R-CC through 17R-CC. In Samples 180-1109D-25R-CC and 27R-CC through 34R-CC specimens were rare or few and their preservation was poor, whereas Samples 180-1109D-36R-CC through 39R-CC were barren of planktonic foraminifers.
The planktonic foraminiferal zonation for Site 1109 is summarized in Figure F62. Table T7 shows the stratigraphic distribution of species.
Samples 180-1109A-1H-CC, 180-1109B-1H-CC to 2H-CC, and 180-1109C-1H-CC to 2H-CC are placed in late Pleistocene-Holocene planktonic foraminiferal Zone N23, based on the occurrence of Bolliella adamsi and Bolliella calida, whose first appearance datum (FAD) marks the basal zonal boundary.
Sample 180-1109C-2H-CC contains the pink variant of Globigerinoides ruber, which disappears from the Pacific at 120 ka; thus, the sample is no younger than that date. Sample 180-1109C-3H-CC, which contains Globorotalia truncatulinoides without G. tosaensis, lies in the upper part of Zone N22, whereas Samples 180-1109C-4H-CC through 30X-CC, which have the concurrence of those two species, are assigned to the lower part of Zone N22, older than 650 ka. Samples 180-1109C-5H-CC through 8H-CC contain abundant Globigerinoides fistulosus, but in the interval between Samples 180-1109C-9H-CC and 29X-CC, rare to few specimens of G. fistulosus occur only in Samples 180-1109C-14X-CC, 24X-CC, 26X-CC, and 28X-CC, before continuous downhole distribution sets in at Section 30X-CC. Paleomagnetic data suggests massive displacement of late Pliocene sediment through this interval. The top of Zone N21 is placed below the FO of G. truncatulinoides in Sample 180-1109C-30X-CC and the base of Zone N21 is defined by the absence of Globorotalia tosaensis (FAD 3.35 Ma) below Sample 180-1109C-38X-CC. Samples 180-1109C-39X-CC through 41X-CC are placed in Zone N20, based on the presence of Globorotalia crassaformis in the absence of G. tosaensis (early middle Pliocene).
Sample 180-1109D-1R-CC, which contains Globorotalia tosaensis, is placed in Zone N21. Samples 180-1109D-2R-CC through 20R-5, 70-72 cm, are assigned to Zone N20, based on the presence of Globorotalia crassaformis in the absence of its descendant, G. tosaensis. These two species are quite similar in appearance initially. We differentiated G. tosaensis by its more circular periphery, gentle curve of the dorsal sutures, and sharp projection of the final chamber from the umbilical area.
Samples 180-1109D-21R-CC through 23R-CC are placed in Zone N20, although Globorotalia crassaformis is absent. These samples contain Pulleniatina, which undergoes the sinistral to dextral coiling change in Sample 180-1109D-23R-CC that has been dated at 3.95 Ma (562.7 mbsf). The samples below, Samples 180-1109D-24R-CC through 30R-CC, are in either Zone N20, Zone N19, or older, based on the presence of sinistral coiling in Pulleniatina primalis. The reduction in planktonic diversity in these samples, as well as those from Samples 180-1109D-31R-CC through 39R-CC, is because of a progressive shallowing to inner neritic facies in which planktonic foraminifers are rarely found.
The stratigraphic sequence of assemblages of benthic foraminifers suggests that Site 1109 has subsided progressively from subaerial exposure to its present lower bathyal depth of 2211 m. Some of the species defining the assemblages are given below.
Surface sediments in Sample 180-1109B-1H, 0-1 cm, contained a large agglutinated component that was almost entirely lacking deeper in the section, probably because of decomposition of the organic cement binding the grains. The agglutinated and calcareous taxa found at the surface are
Cyclamina Ceratobulimina pacifica,
Recurvoides Globocassidulina moluccensis,
Reophax Hoeglundina elegans,
Rhabdammina Nuttallides umbonifera,
Saccammina Parrelloides bradyi,
Trochammina Pseudoparrella exigua, and
Martinottiella Triloculina tricarinata auct.
From Samples 180-1109C-1H-CC through 30X-CC (Pleistocene through late Pliocene) the assemblage is indicative of middle bathyal (500-2000 m) depths and includes
Ceratobulimina pacifica,
Fontbotia wuellerstorfi,
Parrelloides bradyi,
Pseudoparrella exigua, and
Uvigerina asperula.
From Samples 180-1109C-31X-CC through 1109D-26R-CC (middle and late Pliocene) the assemblage is indicative of upper bathyal (150-500 m) depths, with the following species:
Anomalinoides colligerus,
Bolivinita quadrilatera,
Osangularia culter,
Rectobolivina columellaris (striate),
Siphonina tubulosa/australis, and
Uvigerina schwageri.
From Samples 180-1109D-27R-CC through 34R-CC (early Pliocene) a neritic environment (0-150 m) is indicated by the following species:
Amphistegina radiata,
Asterorotalia gaimardi,
Elphidium craticulatum,
Elphidium spp.,
Loxostomina limbata, and
Spiroplectammina arenosuturata.
From Samples 180-1109C-35R-CC through 38R-CC (age undetermined) a brackish water environment is indicated by the presence of abundant ostracodes in Section 35R-CC, as well as the presence of only two types of small benthic foraminifers:
Ammonia beccarii and
Elphidium spp. Gr.
Sample 180-1109D-39R-CC contains no benthic foraminifers, but the oogonia of charophytic algae are present, suggesting a freshwater environment. Sands coated with iron oxide suggest subaerial deposition in Samples 180-1109D-40R-CC through 43R-CC.
We estimated the sedimentation curve based on nannofossil and foraminifer datum events and magnetic chron and subchron boundaries at Site 1109 (Fig. F63).