A total of six holes were drilled at Site 1200 on the summit of South Chamorro Seamount. Among the six holes, Holes 1200D, 1200E, and 1200F were drilled with the APC/XCB and yielded good recovery of serpentine mud in the depression on the summit. These three holes were examined for calcareous nannofossils. To further assess the nature of the nannofossil-bearing deposits, mud samples were also processed and examined for their foraminiferal contents.
In order to have a comprehensive account of the nature of the microfossil-bearing muds recovered at this site, this report includes a section documenting the foraminifers found in selected samples. A discussion of the depositional significance of both microfossil groups follows.
Rare but diversified and well-preserved calcareous nannofossils were found in the top 32 cm (0.32 mbsf) of Core 195-1200D-1H. The remainder of Core 195-1200D-1H is barren, except for traces of Gephyrocapsa in a poor state of preservation in Sample 195-1200D-1H-1, 51 cm (0.51 mbsf) (Table T4). Abundant Emiliania huxleyi is present continuously from the top of this hole to 0.32 mbsf, assigning the interval to Zone NN21 (<0.26 Ma). Besides E. huxleyi, Gephyrocapsa oceanica, Umbellosphaera spp., and Umbilicosphaera sibogae are the most common species. Other minor elements include Syracosphaera spp., Helicosphaera carteri, Rhabdosphaera clavigera, small Reticulofenestra spp., Pontosphaera spp., Oolithotus fragilis, small Gephyrocapsa spp., Ceratolithus cristatus, Calcidiscus leptoporus, and Neosphaera coccolithomorpha (roughly in descending order of abundance). These species are characteristic of upper water (0-100 m) communities of the subtropical Pacific (Okada and Honjo, 1973). The high abundance of umbelliforms (Umbellosphaera tenuis, Umbellosphera irregularis, and R. clavigera) and fewer placoliths indicates that the overlying surface water at this site is oligotrophic with low productivity (Young, 1994). The preservation of these dissolution-susceptible umbelliforms, together with the frequent occurrence of Syracospheara attests to the lack of differential dissolution and, therefore, good preservation of the assemblages. The absence of Gephyrocapsa caribbeanica in these samples further constrains the assemblages to Zone NN21, with an age younger than 0.27 Ma (Pujos, 1988).
Well to poorly preserved trace calcareous nannofossils were sporadically observed in the core catchers of Cores 195-1200D-1H to 6H (6.85-23.41 mbsf). The presence of Coccolithus pelagicus, Reticulofenestra pseudoumbilicus, and Cyclicargolithus floridanus appears to suggest that these specimens originated from pre-Quaternary sediments. However, the extreme rarity of nannofossils and the virtual absence of more definite and robust age-diagnostic markers (such as Discoaster and Sphenolithus, both common in low latitudes) hindered us from assigning firm ages to these samples.
Traces of well-preserved nannofossils were found in the core (Sample 195-1200E-1H-1, 0-10 cm). The assemblage is comparable to that found in the top of Hole 1200D and can be assigned to Zone NN21.
The rest of Core 195-1200E-1H and the entire Core 2H are virtually barren of nannofossils. In contrast, diversified but moderately preserved nannofossils in various degrees of abundance were recorded from several intervals of Cores 195-1200E-3H and 4H (11.11-13.07 mbsf) (Table T4). It is not clear whether the nannofossils persist throughout the entire 2-m section or are present sporadically in several separated intervals. These samples are characterized by a common and persistent occurrence of Umbellosphaera spp., Umbilicosphaera sibogae, and Gephyrocapsa spp., with less frequent H. carteri, Syracosphaera spp., and R. clavigera. Of great significance is the presence of abundant (~10%-20%) G. caribbeanica, which allows us to assign these samples to Zone NN20, with an age of 0.27-0.46 Ma. A high abundance of G. caribbeanica prior to the first occurrence of E. huxleyi has been documented quantitatively in the North Atlantic and the northeast Pacific (Su, 1996; Su et al., 2000). The acme of G. caribbeanica marks 0.268 Ma (Pujos, 1988).
Except for the great abundance of G. caribbeanica at the expense of E. huxleyi, the makeup of the assemblages is comparable to that found in the top sections of Holes 1200D and 1200F. Most of the specimens are overgrown with calcite, and thus, some of the specific diagnostic fine structures are obscured. The heavy encrustation of calcite on placoliths makes it difficult to identify whether E. huxleyi is present in the samples. Nevertheless, Pseudoemiliania lacunosa is still absent, constraining these samples to younger than 0.46 Ma in age.
The topmost interval of Core 195-1200F-1H (0-20 cm) contains rare, well to moderately preserved late Quaternary calcareous nannofossils (Table T4). E. huxleyi dominates the topmost two samples (195-1200F-1H-1, 0-0.5 cm, and 1H-1, 8.5 cm [0-0.085 mbsf]), probably because of the good preservation. Also common are members of Umbellosphaera, Umbilicosphaera, Gephyrocapsa, and Syracosphaera. Species of H. carteri and C. leptoporus are rare. With the addition of subordinate O. fragilis and Pontosphaera spp., the assemblages are similar to those found in the topmost section of Hole 1200D. Nannofossils occur continuously in the next two samples downsection (Samples 195-1200F-1H-1, 14 cm, and 1H-1, 20 cm), but the preservation deteriorates significantly. E. huxleyi is not identifiable, and dissolution-susceptible species, such as Syracosphaera spp. and O. fragilis, are absent. Except for the change induced by the poor preservation, the assemblage is comparable to that of the topmost samples of the section. Traces of nannofossils appear once again in Samples 195-1200F-1H-1, 66 cm, and 1H-1, 121 cm, consisting mainly of poorly preserved U. sibogae and Umbellosphaera. The absence of P. lacunosa once again suggests a late Pleistocene age (<0.46 Ma) for these samples.
Abundant and well-preserved planktonic foraminifers were found in the topmost core (Sample 195-1200D-1H-1, 0-5 cm) (Table T5). Typical subtropical Globigerinoides plexus such as Globigerinoides ruber, Globigerinoides sacculifer, Globigerinoides quadrilobatus, and Globigerinoides immaturus dominate the faunal assemblage. Other common tropical-subtropical species are also present: Orbulina universa, Globorotalia menardii, and Globigerinella aequilateralis, with less common Neogloboquadrina dutertrei, Sphaeroidinella dehiscens, and Zeaglobigerina rubescens. The sample also contains rare (Table T5) Globorotalia truncatulinoides, which is age diagnostic of Zone N22 (Quaternary). Benthic foraminifers were also found in small quantities. The benthic foraminiferal assemblage shows a wide range of diversity; for instance, members of Cibicidoides, Melonis, Fissurina, Lagena, Gyroidina, and Ehrenbergina were found.
Comparable assemblages of planktonic foraminifers were found between 0.27 and 0.29 mbsf in the same section, but the abundance is much less and the preservation state also deteriorates. Only a few benthic foraminifers were spotted. The presence of G. truncatulinoides indicates that this sample belongs to Zone N22 (Quaternary). The rest of the hole is barren of foraminifers.
Only a few nannofossils were spotted in the smear slides of the topmost core (Sample 195-1200E-1H-1, 0-0.5 cm); however, a diversified planktonic foraminiferal assemblage was recovered. Despite lower abundance and poorer preservation, the assemblage is very similar to that of the top of Hole 1200D. Globigerinoides dominates the assemblage in association with other typical subtropical dwellers such as G. menardii and O. universa (Table T5). The abundance and diversity of benthic foraminifers decreases significantly compared to those from Hole 1200D. The rest of Cores 195-1200E-1H and 2H are barren of foraminifers.
Beyond our expectation, abundant planktonic foraminifers were found in Core 195-1200E-3H: Samples 195-1200E-3H-1, 12-17 cm (11.12 mbsf), 35-38 cm (11.35 mbsf), 77-82 cm (11.77 mbsf), and 3H-CC (12.06 mbsf). Foraminifers in the uppermost sample (195-1200E-3H-1, 12-17 cm [11.12 mbsf]) are more common and better preserved. This sample contains a few specimens of Candeina nitida and G. truncatulinoides, as well as a few well-preserved, orange, tiny Z. rubescens. The lower three samples show increasing degrees of differential dissolution. The lowermost sample (195-1200E-3H-CC [12.06 mbsf]) is so badly preserved that there is no sign of Z. rubescens; instead, Globorotalia tumida, the most resistant species, becomes common.
All four samples contain G. truncatulinoides, indicating a Quaternary age. A detailed examination of the makeup of the assemblages suggests changes related to surface-water conditions. All four samples contain more G. menardii/G. tumida and G. truncatulinoides and fewer Pulleniatina obliquiloculata than the other samples. The ratio of G. menardii to P. obliquiloculata has been used as an indication of the westward extent of eastern tropical Pacific water along the equator (Parker and Berger, 1971). Presumably, the western extension was strengthened during glacial periods. Indeed, it was found that increased abundances of G. menardii relative to P. obliquiloculata occurred during glacial periods on the Ontong Java Plateau in the western equatorial Pacific (Yasuda et al., 1993). The increase of globorotalids in our record suggests that sediments in Core 195-1200E-3H were deposited during a cooler period than today.
Planktonic foraminiferal assemblages comparable to those in the top of Hole 1200E are also present in the top of Hole 1200F (Sample 195-1200F-1H-1, 14-19 cm [0.14 mbsf]), but in greater quantities (Table T5). A similar, moderately preserved assemblage was found in Sample 195-1200F-1H-1, 121-126 cm (1.21 mbsf); otherwise, downcore sections are barren. The Quaternary diagnostic species, G. truncatulinoides, is present in all foraminifer-bearing samples.
The distribution of calcareous microfossils in the three examined holes is summarized in Figure F41. It appears that calcareous nannofossils and foraminifers are ubiquitous on the summit of the seamount, indicating that the surface of the summit is blanketed with recent pelagic biogenous sediments diluted by varying amounts of serpentine sediments. Holes that are farthest away from the active mud volcano conduit show less dilution with serpentine-bearing sediments and contain more abundant and diversified benthic foraminifers, such as those at the top of Hole 1200D. Apparently, the outflow of spring water and/or the outpouring of serpentine mud results in a hostile environment for benthic foraminifers.
Except for a few isolated nannofossils found in deep sections in Hole 1200D, all of the planktonic foraminiferal and nannofossil assemblages at Site 1200 are indicative of a late Quaternary age. Most samples contain good quantities of orange-pigmented Z. rubescens (except Sample 195-1200E-3H-CC because of strong dissolution/recrystallization), which indicates the freshness and young age of the samples (Parker and Berger, 1971). Nannofossils found in Core 195-1200E-3H appear to indicate an age older than 0.268 Ma, belonging to the upper part of Zone NN20, as indicated by the elevated abundance of G. caribbeanica (Fig. F42). The planktonic foraminifers in these samples consist of cooler subtropical dwellers, such as G. truncatulinoides and G. menardii, than samples from the first core and the uppermost cores of Holes 1200D, 1200E, and 1200F, implying cooler sea-surface conditions.
Detailed census data on nannofossil assemblages from Core 195-1200E-3H (Fig. F41) show a clear floral succession. The relatively high percentage of G. caribbeanica and decreased abundance of Umbellosphaera spp. in Sample 195-1200E-3H-1, 77-82 cm (11.77 mbsf), seems to indicate a cooler interval relative to others. Correspondingly, the planktonic foraminiferal assemblage also shows a relatively high frequency of G. menardii and G. truncatulinodes and an absence of P. obliquiloculata, indicative of a dominance of subtropical rather than tropical fauna. The succession shown by both foraminiferal and nannofossil assemblages attests strongly that these microfossils are in situ and represent a paleosurface deposit. At the same time, samples immediately below the top of Holes 1200D and 1200F (i.e., Samples 195-1200D-1H-1, 27-29 cm, and 195-1200F-1H-1, 14-19 cm) contain more warm-water dwellers such as P. obliquiloculata and fewer globorotalids (Table T5). Such faunal and floral successions are evidence that these fossil horizons are in their original depositional sequence, reflecting climatic fluctuation during the late Quaternary. These fossil-bearing intervals were deposited when the mud volcano was more or less quiet. During these quiet periods, only small amounts of serpentine mud were brought to the seafloor surface; thus, the microfossil assemblages were less diluted and were preserved in their original depositional sequence.
The existence of paleosurfaces, where abundant calcareous microfossils are present, explains the high carbonate concentrations in the sediments at these intervals (see "Geochemistry"). However, preservation deteriorates rapidly downhole. Pore fluids apparently play an important role in dissolving the most susceptible delicate microfossils and in precipitating calcite on other robust ones. The tiny, orange Z. rubescens and other small, thin foraminiferal tests are considered to be the first victims of dissolution (Adelseck, 1978), whereas other less-susceptible forms were beneficiaries, exhibiting obvious signs of overgrown calcite. Microfossils in Core 195-1200E-3H display dramatic dissolution and overgrowth. For instance, pristine umbelliforms usually show faint shadows under crossed nicols, but the outgrowth of calcite causes them to show bright yellow birefringence. Planktonic foraminifers also show such distinct overgrowth to various degrees. Increased dissolution and overgrowth is manifested by the increasing dominance of Globigerinoides conglobatus/G. immaturus over G. ruber and the appearance of thick-tested G. tumida at the expense of Z. rubescens and C. nitida. Furthermore, in those intervals where foraminifers show moderate preservation (e.g., Samples 195-1200E-3H-1, 77-82 cm, and 195-1200F-1H-1, 121-126 cm), microscopic aragonite crystals are present, indicating oversaturation of Ca2+ and CO32- in pore water and subsequent in situ precipitation. Nannofossils in these samples show significant recrystallization with excessive calcite on the surface as well as in the void spaces.
In summary, calcareous microfossils at Site 1200 suggest the following: