The sedimentary sequence recovered at Site 1241 consists of a 447.06-m-thick interval of Quarternary-lower upper Miocene sediments containing calcareous microfossils throughout and well-preserved diatoms in the Miocene interval (Figs. F24, F40; Table T9). Calcareous nannofossils are abundant and generally well to moderately well preserved throughout the sequence. Planktonic foraminifers are abundant to common in the 0- to 217-mcd interval and generally rare below ~217 mcd. The percentage of benthic foraminifers relative to total foraminifers is low (as low as ~1%) in the upper interval and reaches as high as ~99% in the lower interval. Diatoms are rare to few and poorly preserved at depths above 184 mcd. Diatom abundance increases and preservation improves below ~195 mcd.
The index calcareous nannofossil species, Emiliania huxleyi, was not observed at this site, suggesting that the 0- to 0.26-Ma interval may be missing. However, planktonic foraminifers suggest that the 0- to 0.12-Ma interval has been recovered. Except for this discrepancy, which should be resolved in postcruise research, the biostratigraphies of the three planktonic microfossil groups document a relatively continuous sequence of early late Miocene-late Pleistocene age (Table T10). The biostratigraphies also show an increase in sedimentation rate (~60 m/m.y.) for the 5- to 7-Ma interval, which may be considered a biogenic bloom interval. Calcareous nannofossils and planktonic foraminifers constrain the basal age of the site at ~11.2-11.6 Ma.
Calcareous nannofossils are abundant, and their preservation is generally good to moderate in all the samples examined from Hole 1241A (Table T9). Overgrowth of nannofossils was not apparent in any of the samples, but strong dissolution was noted in a number of samples below ~365 mcd. Reworking of nannofossils is minimal at the site. Most of the well-known nannofossil datums for the Pleistocene-late Miocene have been determined within a sample spacing of ~1.5 m. Nannofossil biostratigraphy suggests that the basal sediment age is 10.8-11.6 Ma. Of taxonomic interest is the new observation of a transitional form between Discoaster bellus and Discoaster berggrenii at the site. This fills in the missing link between the two species and has implications for biostratigraphy and evolutionary studies.
Samples 202-1241A-1H-1, 0 cm, and 1H-1, 10 cm (0-0.1 mcd), contain abundant and well-preserved late Pleistocene nannofossils, including abundant medium-sized Gephyrocapsa spp. and common to abundant Florisphaera profunda. E. huxleyi and Pseudoemiliania lacunosa were not found. This indicates an age between 0.26 and 0.46 Ma for these samples. If this interpretation is correct, it means that the record of the most recent 0.26 m.y. or more is missing from Hole 1241A. Alternatively, if one assumes that the sediment section is complete, it means that E. huxleyi has been largely excluded from the site as a result of some unknown anomalous environmental conditions, even though the species has been present in the world's oceans almost universally for the last 0.26 m.y. and abundant for the last 0.08 m.y. Detailed shore-based studies of additional samples from this site and nearby sites should help clarify the problem.
The placements of all the nannofossil datums are listed in Table T10. All the Pleistocene-late Miocene nannofossil datums were well determined without any apparent complications, thanks in part to the virtual absence of reworked nannofossils at the site. The deepest sediment sample (Sample 202-1241A-43X-CC; 446.7 mcd) just above the basement contains abundant nannofossils, including Reticulofenestra pseudoumbilicus and Coccolithus miopelagicus. The presence of these species in the absence of Cyclicargolithus floridanus and Calcidiscus premacintyrei indicates an age of 10.8-11.6 Ma.
Planktonic foraminifers are abundant to common in the upper part of Hole 1241A (mudline to Sample 202-1241A-21H-CC; 0-216.69 mcd). Abundance decreases markedly downhole, and the proportion of radiolarians in the >63-µm coarse fraction increases significantly. Planktonic foraminifers are rare in the lower part of Hole 1241A (Samples 202-1241A-33H-CC to 43X-CC; 349.56-446.67 mcd). Preservation is initially poor at the top of Hole 1241A (mudline to Sample 202-1241A-1H-CC; 0-4.10 mcd), and planktonic tests appear strongly dissolved within this interval. Between Samples 202-1241A-2H-CC and 32H-CC (14.48-337.16 mcd), planktonic tests show minor or moderate evidence of dissolution (etching and fragmentation in 5%-30% of tests). Preservation deteriorates again in the lower part of Hole 1241A (Samples 202-1241A-33H-CC to 43X-CC; 349.56-446.67 mcd).
The Pleistocene-latest Miocene assemblage is a diverse tropical assemblage characterized by Globigerinoides fistulosus, Globorotalia exilis, Globorotalia limbata, Globorotalia menardii, Globorotalia tumida, Globigerinoides sacculifer, Globigerinoides ruber, Globigerinoides trilobus, Neogloboquadrina acostaensis, Neogloboquadrina dutertrei, Orbulina universa, and Sphaeroidinella dehiscens. Standard marker species are present throughout the succession (Tables T10, T11) and can be used to establish a relatively detailed preliminary biostratigraphy for the Pleistocene-uppermost Miocene interval recovered at this site. Thus, Site 1241 offers an excellent opportunity for a well-constrained biostratigraphy and has potential to provide an orbitally tuned low-latitude timescale that will serve as a much-needed reference for the East Pacific.
The preliminary shipboard planktonic foraminifer biostratigraphy is based on core catcher samples from Hole 1241A and some additional samples from Hole 1241C in order to constrain the upper part of the sedimentary sequence. The Pleistocene-latest Miocene biostratigraphy is relatively straightforward. However, the early late Miocene biostratigraphy (Samples 202-1241A-33H-CC to 43X-CC; 349.56-446.67 mcd) is not so well constrained because of poorer preservation and scarcity of planktonic foraminifers (Table T11).
The last occurrence (LO) of G. ruber (pink) between the mudline and Sample 202-1241A-1H-CC (0-4.10 mcd) indicates an age younger than 0.12 Ma for the overlying interval. The LO of G. ruber (pink) was constrained further at Hole 1241C, between Samples 202-1241C-1H-2, 80-81 cm, and 1H-3, 80-81 cm (2.35-3.86 mcd). The standard marker Globorotalia tosaensis is extremely rare at Site 1241 and could not be used to define the boundary between Subzones Pt1a and Pt1b of Berggren et al. (1995). The LO of representatives of the benthic genus Stilostomella (~0.65 Ma), between Samples 202-1241A-2H-CC and 3H-CC, provides an alternative datum to approximate this boundary. However, this extinction event was probably diachronous at low and high latitudes (Hayward, 2001), and detailed shore-based studies from Site 1241 and from the southern Leg 202 sites are needed to clarify the timing of this event in the eastern Pacific.
The percentage of benthic foraminifers relative to total foraminifers is initially low (typically ~1%) in the upper part of Hole 1241A (mudline to Sample 202-1241A-21H-CC; 0-216.69 mcd). The proportion of benthic foraminifers increases markedly below this depth, reaching ~99% between Samples 202-1241A-33H-CC and 39X-CC (349.56-409.21 mcd). Benthic foraminifers, overall, exhibit better preservation than planktonic foraminifers but start to show dissolution damage in sediments from the lowermost part of Hole 1241A (Samples 202-1241A-33H-CC to 43X-CC; 349.56-446.67 mcd) (Fig. F24).
The relatively diverse, but sparse, Pleistocene-latest Miocene assemblage in Samples 202-1241A-1H-CC to 21H-CC characterizes a mesotrophic middle bathyal environment. It typically contains Eggerella bradyi, Eubulimina exilis, Globocassidulina subglobosa, Globobulimina affinis, Globobulimina pyrula, Gyroidinoides soldanii, Gyroidinoides orbicularis, Karreriella bradyi, Laticarinina pauperata, Melonis affinis, Melonis pompilioides, Neugeborina longiscata, Oridorsalis umbonatus, Pleurostomella brevis, Pullenia bulloides, Pullenia quinqueloba, Pyrgo murrhina, Pyrgo serrata, Stilostomella subspinosa, Uvigerina peregrina, and Uvigerina proboscidea. The late Miocene assemblage in Samples 202-1241A-22H-CC through 43X-CC generally shows much higher abundance, and assemblage composition overall reflects shallower upper bathyal water depths or increased carbon fluxes at the seafloor. Marked variations in composition (particularly in the proportions of Bolivina, Bulimina, Globobulimina, and Uvigerina) may relate to changes in productivity or circulation. However, it was not possible to evaluate from shipboard study to what extent these changes indicate fluctuations in productivity and/or circulation or stem from a preservation bias.
All core catcher samples from Hole 1241A, as well as smear slides from some additional layers of the split cores, were analyzed. Diatoms are rare to few, and preservation is poor from the mudline down to Sample 202-1241A-18H-CC (184 mcd), but abundance increases and preservation improves below Sample 19H-CC (195 mcd) (Table T12; Fig. F24). Between Samples 202-1241A-32H-CC and 37X-6, 38 cm (337-386 mcd), diatom abundance increases significantly. In this interval, diatom oozes dominated by highly fragmented Thalassiothrix spp. are present, interbedded with the nannofossil ooze.
Diatoms recovered from Site 1241 indicate a continuous stratigraphic interval from the Holocene Fragilariopsis doliolus Zone to the Actinocyclus moronensis Zone. The diatom flora present provides 20 stratigraphic events for the 447 mcd recovered from this site. However, accurate placement of some of these events was difficult in intervals with lower abundance and poorer preservation of diatoms, such as the top 18 cores (0-184 mcd). Diatom events recognized are presented in Table T10.