Site 1140, on the northern tip of the Kerguelen Plateau, penetrated 234.5 m of Miocene to lowermost Oligocene sediments similar to those encountered at Site 1139, plus a small amount of upper Eocene chalk intercalated with basalt. The major biostratigraphic objectives were to obtain minimum ages for basalts (presumed Cenozoic) and to determine the paleoceanographic history of the NKP.
Quaternary and upper Neogene sediments were not recovered. The uppermost 46.5 m consists of middle Miocene, diatom-bearing nannofossil ooze containing common planktonic foraminifers. Well-preserved diatoms and silicoflagellates provide age control in the diatom-bearing nannofossil ooze but are less abundant and highly fragmented in the lower Miocene and Oligocene chalks. Nannofossil and planktonic foraminifer assemblages in middle Miocene Sample 183-1140A-5R-CC contain species typical of lower latitudes that have not been previously found on the Kerguelen Plateau, indicating warmer water conditions on the NKP during parts of the middle Miocene. Unfortunately, recovery in this interval is very poor.
Beneath the light-colored ooze lies an apparently complete succession of lower middle Miocene to lower Oligocene foraminifer-bearing nannofossil ooze and chalk. Preservation of microfossils in these sediments is variable and diminishes downhole, particularly in dolomitized chalks in the vicinity of the sediment-basalt contact (Core 183-1140A-26R). Minor dolomitized chalk beds within submarine basalts at the bottom of Hole 1140A yielded nannofossils and microfossils of late Eocene age (planktonic foraminifer Zone AP12). Sediments directly above the basalts are early Oligocene in age. Paleomagnetic and biostratigraphic evidence suggest an unconformity in the basal Oligocene, but poor recovery of the uppermost basalts underlying the upper Eocene intercalated chalks reduces the possibility of confirming an unconformity or of identifying the Eocene/Oligocene boundary using paleomagnetic methods.
An age-depth summary based on examination of core-catcher samples shows a mean sedimentation rate for the section of 12 m/m.y (Fig. F5). We attribute higher sedimentation rates in chalks of similar age at Site 1139 (~16 m/m.y. in the Miocene and 20 m/m.y. in the Oligocene) to the input of significant amounts of clay derived from nearby Skiff Bank.
Nannofossils and diatoms were well to moderately well preserved throughout the sequence at Site 1140, except near the bottom, where they were absent in smear slides. Because the site was drilled at ~46°S at the northern edge of the Kerguelen Plateau, some nannofossil assemblages reflect a warmer environment than nannofossils of similar age recovered from more southerly domains of the plateau. For that reason, this is the first finding of some taxa on the Kerguelen Plateau. The downslope, downcurrent position of the site, however, made it more vulnerable to the deposition of reworked microfossils, a complication that has not been a factor at the previous sites drilled during Leg 183.
Sample 183-1140A-1R-CC contains common Discoaster exilis, few Cyclicargolithus floridanus, Cyclicargolithus abisectus, Sphenolithus moriformis, and Calcidiscus leptoporus/macintyrei, and abundant Coccolithus pelagicus and Reticulofenestra perplexa. The latter taxon constitutes ~90% of the assemblage. We assign the sample to the combined CN5a-CN3 Zone. Rare to few Reticulofenestra bisecta and Chiasmolithus altus are reworked from the Oligocene. This is the first record of D. exilis from the Kerguelen Plateau. Diatoms include Denticulopsis hustedtii, Actinocyclus ingens, and Crucidenticula kanayae but Nitzschia denticuloides is absent. These diatoms belong to the D. hustedtii-Nitzschia grossepunctata Zone (= 13.5-14 Ma or mid-middle Miocene in age). We also assigned the next three core-catcher samples (183-1140A-2R-CC through 4R-CC) to these zones. Coccolithus pelagicus constitutes 90% of the assemblage in Sample 183-1140A-3R-CC, a dominance reversal that is characteristic of assemblage changes induced by climatic/paleoceanographic cycles in this region (Wei and Wise, 1992). Sample 183-1140A-4R-CC contains abundant D. hustedtii and the silicoflagellates Bachmannocena paulschulzii and B. sp. cf. B. diadon.
Sample 183-1140A-5R-CC contains few D. exilis and abundant D. veriabilis, some of which are overgrown. As a group, discoasters, like sphenoliths, prefer warmer waters. No D. hustedtii are present among the diatoms, but A. ingens v. nodus is, rendering an age assignment to the zone of the same name (14.18 Ma).
The warm water-loving Sphenolithus heteromorphus, another nannofossil observed for the first time on the plateau, is common in Sample 183-1140A-6-CC; it ranges from Zones CN4 to CN3 of the early middle Miocene. Helicosphaera granulata is rare, and Discoaster variabilis exhibit strong overgrowths, causing it to resemble Discoaster deflandrei. Among the diatoms, we found rare Eucampia antarctica and Coscinodiscus lewisianus. Sample 183-1140A-7-CC exhibits a similar assemblage, except S. heteromorphus is absent. S. moriformis is common and we noted C. kanayae among the diatoms.
Well-preserved D. deflandrei in Sample 183-1140A-8-CC indicates an age of 16.2 Ma or greater. A few Ch. altus are present as a reworked Oligocene taxon.
In-place C. leptoporus/macintyrei are absent in Sample 183-1140A-9R-CC, which we assign to the combined nannofossil Zone CN2-CN1 with a minimum age of about 18 Ma. Discoaster deflandrei are abundant and pristine in preservation. Zyghrablithus bijugatus are common and well-preserved R. bisecta represent reworked Oligocene taxa, indicating continued downslope or along-slope transport of eroded older material.
We also assign Samples 183-1140A-9R-CC to 12R-CC to the lower Miocene Zones CN2-CN1. Discoaster deflandrei is common to abundant, with Coronocyclus nitescens, S. moriformis, and H. granulata found in Sample 183-1140A-12R-CC, along with a few reworked R. bisecta. The central area of H. granulata is so thin that it appears absent in phase contrast light, showing up only in cross-polarized light; in this way, it superficially mimics Helicosphaera ampliaperta, a form not seen in this material.
The nannoflora is similar to the above in Samples 183-1140A-13R-CC to 15R-CC except for more reworked Oligocene taxa. For instance, well-preserved R. bisecta, including large forms (up to 15 µm), are common in Samples 183-1140A-13R-CC and 14R-CC but are rare to few in 15R-CC. Other taxa we consider reworked are Discoaster tanii, Ch. altus (no central cross preserved), and rare Reticulofenestra umbilica (lower Oligocene). Among the taxa considered to be in place, nannofloras in Samples 183-1140A-14R-CC to 15R-CC are dominated by C. abisectus and C. floridanus (about 60%) plus C. pelagicus (about 35%). Among the diatoms, we noted Coscinodiscus rhombicus (Oligocene to lower Miocene) in Sample 183-1140A-13R-CC, and, in the silicoflagelates, Distephanus crux longispinus was found in Sample 183-1140A-14R-CC.
Sample 183-1140A-16R-CC is somewhat problematic in that it could be dated as Oligocene considering the number of older nannofossil taxa in it, but we consider all of them to be reworked. They include rare to few Ch. altus (many intact), few R. bisecta, and rare to few R. umbilica and Reticulofenestra samodurovii (both lower Oligocene). Forms considered indigenous are common D. deflandrei and Coccolithus miopelagicus (up to 18 µm). Diatom floras include Bogorovia veniamini and C. rhombicus (Oligocene to lower Miocene).
There is no uncertainty as to the Oligocene age of Sample 183-1140A-17R-CC because large R. bisecta and D. deflandrei are abundant. Bogorovia veniamini fragments are present among the diatoms, and Distephanus speculum pentagonus among the silicoflagellates.
We noted the top of the nannofossil mid-Oligocene Ch. altus Zone in Sample 183-1140A-18R-CC, where the nominate taxon is very abundant; about 60% have complete central crosses. Cyclicargolithus are abundant and Clausicoccus fenestratus is present. R. samodurovii is reworked.
The last occurrence (LO) of Z. bijugatus is noted in Sample 183-1140A-19R-CC in about the same stratigraphic position as at Site 1139. Discoaster tanii and C. fenestratus are few, as is the diatom C. rhombicus.
Samples 183-1140A-20R-CC to 23R-CC are all included in the Ch. altus Zone. Bachmannocena apiculata prominent among the silicoflagellates. Sample 183-1140A-24R-CC is somewhat problematic in that it contains few Reticulofenestra hillae and R. umbilica (both up to 16 µm), very abundant C. fenestratus, two specimens of Coccolithus formosus, and a single specimen of Isthmolithus recurvus. We presume that the latter two taxa are reworked, and we tentatively assign the sample to the Reticulofenestra daviesii Zone. The sample includes abundant Blackites spinosus, common D. deflandrei, few Discoaster tanii nodifer (with large central bosses as in Dictyocha ornata), abundant Ch. altus, rare Chiasmolithus oamaruensis (reworked?), common R. bisecta, and few S. moriformis. We observed no siliceous microfossils.
Isthmolithus recurvus, D. tanii nodifer, and R. umbilica/hillae are abundant and C. formosus is common in the last recovered sediment (Sample 183-1140A-25R-5, 90 cm) above the first basalt encountered in the hole. Otherwise, the assemblage is similar to that in the superjacent core catcher. The possible age range represented by the last occurrence of C. formosus (32.8 Ma) and the first occurrence of I. recurvus (35.7-36.3 Ma at these latitudes according to Wei, 1992) spans the Eocene/Oligocene boundary. The high abundance of C. fenestratus, however, would suggest an earliest Oligocene age (~CP16a/b) when compared with the Eocene/Oligocene sequence at Deep Sea Drilling Project (DSDP) Site 511 (Leg 71) on the Falkland Plateau and Ocean Drilling Program (ODP) Site 737B (Leg 119) on the SKP (Wise, 1983, table 1A; Wei and Thierstein, 1991, table 3).
Beneath the first basalt encountered, some subjacent flows contained sediments apparently trapped between or within the flows (see "Lithostratigraphy" and "Physical Volcanology"). Among these are a piece of greenish chalk in interval 183-1140A-31R-1, 135-139 cm, and 1-m-thick orangish, partly dolomitized nannofossil chalk found in Samples 183-1140A-32R-3, 61-63 cm, and 32R-3, 115 cm. Both intervals contained essentially the same nannofloral assemblage with excellent preservation in the greenish chalk (better than at the base of the sedimentary sequence overlying basalt in Core 183-1140A-25R) and moderate preservation in orange chalk below. As both assemblages from the intercalated chalks have essentially the same compositions, we describe here the better preserved assemblage in the greenish chalk (Sample 183-1140A-31R-1, 135-139). This assemblage consists of many of the same forms as in Core 183-1140A-25R, which overlies the basalt; however, there are some important differences. The assemblage is composed of abundant I. recurvus, C. oamaruensis, Ch. altus, C. formosus, C. pelagicus, R. umbilica/samodurovii, Z. bijugatus, and B. spinosus; common Discoaster tanii nodifer (five- and six-rayed, with large central area knobs), and D. deflandrei, few S. moriformis, R. daviesii, and Coccolithus eopelagicus (up to 21 µm); and rare Helicosphaera compacta (a single small [9-µm] specimen). Noticeably absent are C. fenestratus, which are abundant in Core 183-1140A-25R. Other differences include a higher percentage of C. oamaruensis (approximately equal in number to its evolutionary descendant, Ch. altus), a larger number of C. formosus, and a smaller size range among the R. bisecta (maximum size = 12 vs. 14 µm in Core 183-1140A-25R). Of these differences, the larger percentage of the ancestral form C. oamaruensis vs. Ch. altus indicates an older assemblage. This, plus the absence of C. fenestratus, indicates that the assemblage is latest Eocene (~CP15b) rather than earliest Oligocene (cf. Eocene/Oligocene boundary sequence on the Falkland and southern Kerguelen plateaus [Wise, 1983, table 1A; Wei and Thierstein, 1991, table 3]).
Planktonic foraminifer assemblages consist of species characteristic of the Southern Ocean, and, in places, species associated with the low latitudes. The Miocene fauna can be mostly characterized in terms of the Neogene Kerguelen (NK) zonal scheme (see "Biostratigraphy" in the "Explanatory Notes" chapter), but in intervals where warmer water species are present, we refer to the Transitional Miocene (Mt) zonation of Berggren et al. (1995).
We examined planktonic foraminifers in core-catcher samples from the 234.5-m pelagic succession (Cores 183-1140A-1R to 25R) and from the dolomitized chalk beds that occur between submarine basalt flows of Unit III (Cores 183-1140A-31 and 32R). We examined additional samples within cores to locate major stratigraphic boundaries. Preservation is good in the Miocene but diminishes downhole in Paleogene cores because of dissolution and, in Core 183-1140A-25R, because of dolomitization of carbonate. Minor carbonate laminae between lavas of Unit III occasionally show signs of thermal alteration.
Planktonic foraminifers are generally sparse in the biosiliceous sediments of the middle Miocene (Cores 183-140A-1R to 5R), becoming more common in the early Miocene (Cores 183-140A-6R to 14R) as the biogenic carbonate content increases. Foraminifers are never abundant in the >63-mm size fraction of Unit II greenish gray, foraminifer-bearing nannofossil chalk and ooze, because of dilution by carbonate grain aggregates that did not break down during sample processing. Planktonic foraminifers are extremely rare in Sample 183-1140A-25R-CC and absent in Sample 183-1140-32R-CC as a result of abundant sand-sized, rhombic dolomite crystals that dominate the >63-µm size fraction. In places, reworked Oligocene forms are in low abundance in the Miocene section.
The youngest planktonic foraminifers found at Site 1140 are of late middle Miocene age. Samples 183-1140A-1R-CC and 2R-CC contain occasional Globigerina bulloides, Globigerina falconensis, Globorotalia scitula, Globorotalia miozea, and Neogloboquadrina nympha, an assemblage characteristic of upper middle Miocene Zone NK5. Planktonic foraminifer abundance is variable in the subjacent few cores downhole. Examination of Sample 183-1140A-3R-CC reveals that the Last Appearance Datum (LAD) of N. nympha is in Core 183-1140A-3R. This sample contains Globigernia woodi, Globigerina praebulloides, common tenuitellids, G. miozea, and G. praescitula, in addition to N. nympha, the nominate taxon of lower middle Miocene Zone NK4, to which we assign this sample.
A similar fauna occurs in Samples 183-1140A-4R-CC and 5R-CC. In addition to globorotalids and globigerinids characteristic of Zone NK4, the latter sample also contains low numbers of Orbulina spp., indicating a maximum age for this sample of 15.1 Ma (First Appearance Datum (FAD) Orbulina) and confining it to the upper part of NK4. This species is usually restricted to tropical and temperate waters and has never been recorded previously from the Kerguelen Plateau. Its occurrence at Site 1140, the most northerly Kerguelen Plateau locality drilled, suggests that this area experienced warmer conditions than other regions of the Kerguelen Plateau during the middle Miocene. In Sample 183-1140A-6R-CC, G. praescitula occurs with less angular globorotalids such as Globorotalia zealandica and Paragloborotalia incognita, species associated with Zones NK3-NK4.
Catapsydracids appear for the first time in radiolarian-rich Sample 183-1140A-7R-CC and are abundant in the subjacent sample. They occur in this interval with P. incognita, G. zealandica, G. praescitula, and common tenuitellids. In the absence of G. miozea, we assign this assemblage to Zone NK3.
The next interval of cores (Cores 183-1140A-9R to 12R) is characterized by common P. incognita, catapsydracids, Tenuitella munda, Globigerina brazieri, and a large, evolute globigerinid with a distinctive rimmed aperture comparable to Berggren's (1992) Globoturborotalia sp. cf. Globigerina labiacrassata. We do not find large globorotalids below Core 183-1140A-8R; therefore, we assign the assemblage to early Miocene Zone NK2.
Quality of preservation and abundance of planktonic foraminifers diminish in the greenish gray nannofossil clay of cores downhole as a result of the increasing effects of dissolution and dilution by abundant carbonate grain aggregates. Preservation is particularly poor in Samples 183-1140A-9R-CC to 15R-CC. In these cores, we recognize an early Miocene fauna composed of small, dissolution-resistant forms, including P. incognita, Paragloborotalia opima, T. munda, catapsydracids, and small indeterminate globigerinids that are not zone-specific. In addition, Sample 183-1140A-14R-CC contains a large angular form of possible globoquadrinid affinities. Rare and sporadic Chiloguembelina cubensis in these cores are probably reworked from older sediments.
The first indisputable Oligocene sediments are indicated by Globigerina euapertura in Sample 183-1140A-20R-CC. Paragloborotalia opima, Globorotaloides suteri, Catapsydrax dissimilis, Catapsydrax unicavus, and Tenuitella spp., species characteristic of the upper Oligocene Zones AP15 and AP16, are also present in this and the subjacent sample. We located the downhole LAD of C. cubensis in Core 183-1140A-20R. In the absence of Subbotina angiporoides, the marker for Zone AP13, we assign this sample to the AP14-AP15 zonal range. Poor preservation of foraminifers in overlying Sample 183-1140A-20R-CC prevents delineation of these two zones. This sample contains a rather impoverished fauna composed of rare paraglobrotalids, catapsydracids and T. munda, and lacks G. labiacrassata, the AP14-AP15 boundary maker. We also assign the core catcher of Core 183-1140A-21R, containing a similar assemblage of foraminifers, to the same range of biozones.
The downhole LAD of S. angiporoides occurs in Sample 183-1140A-22R-CC. In addition to S. angiporoides, this sample and the sample below (Sample 183-1140A-23R-CC) contain C. cubensis, T. munda, and Tenuitellinata angustiumbilicata species characteristic of the early Oligocene Zone AP13 age. Subbotina utilisindex is also present in these samples, although, as noted in the Site 1137, 1138, and 1139 chapters, this species is often difficult to distinguish from the Eocene species Subbotina linaperta. Subbotina linaperta is normally confined to the middle and upper Eocene, so its possible occurrence in lower Oligocene sediments at Sites 1140 and 1139 is noteworthy. The Leg 119 Shipboard Scientific Party (1989) recorded S. linaperta ranging into the early Oligocene at Kerguelen Plateau Site 737 (Leg 120).
Core-catcher samples from the final two cores of Unit II, which overlie submarine basalts (Cores 183-1140A-24R and 183-1140A-25R), contain increasing amounts of dolomite with only sparse planktonic foraminifers. Preservation and abundance is still moderate to good in Sample 183-1140A-24R-CC, but deteriorate rapidly within Core 183-1140A-25R. Sample 183-1140A-25R-4, 135-139 cm, is the oldest sample examined above the first basalts that contains recognizable planktonic foraminifers. A fauna composed of S. angiporoides, C. cubensis, C. unicavus, and S. utilisindex is present, suggesting these sediments are also of early Oligocene (Zone AP13) age. The basalt-sediment contact occurs at the base of Core 183-1140A-25R.
Minor interbeds of nannofossil chalk were recovered in the basalt cores and were apparently deposited between basalt flows. A small (>10 cm) piece of greenish-gray nannofossil chalk occurs in interval 1140A-31R-1, 134-139 cm. It contains very well-preserved planktonic foraminifers including C. cubensis, Paraglobrotalia nana, S. angiporoides, C. unicavus, S. linaperta/utilisindex, possible Praetenuitella insolita, and common Globigerinatheka index, which are characteristic of middle to late Eocene Zones AP11-AP12. The Eocene/Oligocene boundary would appear to occur between Cores 183-1140A-25R and 31R. Foraminifers were extremely sparse within 1.03 m of rusty brown and pale brown dolomitized chalk in Section 183-1140A-32R-3. Rare and poorly preserved specimens of G. index, C. cubensis, and P. nana in the pale brown chalk, Sample 183-1140A-32R-3, 111-114 cm, indicate a similar middle to late Eocene age. Low recovery in intervening Cores 183-1140A-26R to 30R makes it impossible to locate the position of the Eocene/Oligocene boundary more precisely using biostratigraphic evidence alone.
At high latitudes, in the absence of Hantkenina spp. (the LAD of which denotes the Eocene/Oligocene boundary at low latitudes [Coccioni et al., 1988; Berggren et al., 1995]), the LAD of G. index provides a proxy for the Eocene/Oligocene boundary. This useful index species and zonal maker for the top of Zone AP12 is apparently absent from sediment samples above the basalt. Preservation problems and low foraminifer abundance may account for the absence of G. index, although this seems unlikely as it is usually a fairly robust species. An early Oligocene, rather than a latest Eocene age, is also supported by the nannofossils (see "Calcareous Nannofossils and Diatoms"). We therefore estimate from foraminifers a minimum age of 34.3 Ma for the first basalts encountered in Hole 1140A based on the maximum sediment age.
Biostratigraphic evidence for an Oligocene age of strata above the basalt contact and a late Eocene age of sediments below, is consistent with the paleomagnetic reversal record in the cores (see "Paleomagnetism"). Sediments below interval 183-1140A-25R-4, 0-30 cm, which are assigned to lower Oligocene foraminiferal Zone AP13 and nannofossil Zone ~CP16a/b, are of a reversed polarity that can probably be correlated with C13r of the geomagnetic reversal time scale (GRTS) in the earliest Oligocene. Above interval 183-1140A-25R-4, 0-30 cm, sediments exhibit normal polarity, possibly C13n.
The first basalts beneath the chalk succession are of a normal polarity, whereas the intercalated chalks beneath the contact (Sample 183-1140A-31R-1, 134-139 cm) lie within basalts of a reversed polarity. As noted above, we assigned these sediments to late Eocene, high-latitude Biozone AP12, which, based on approximate correlations to the Berggren et al. (1995) GRTS, spans C13-C17.
Diverse silicoflagellates are common in nearly all core-catcher samples from Site 1140A, and are sometimes abundant and well preserved. Beginning in Sample 183-1140A-2R-CC with two species of the genus Distephanus Stohr and ending in Sample 1140A-22R-CC with the genus Corbisema Hanna, we found that the amount of debris at the bottom of the sequence was higher than the number of undamaged skeletons. In the last few sediment cores, intact silicoflagellates appear to be absent.
Preliminary results show that the silicoflagellates range in age from late Miocene to late Oligocene. The data support the foraminifer and the calcareous nannofossil biostratigraphy.
Among the age-diagnostic species, we observed the last occurrence of Corbisema tricantha tricantha (Ehrenberg) Hanna together with a high number of Distephanus crux crux (Ehrenberg) Haeckel in Sample 183-1140A-7R-CC. The former seems to be a good marker for the middle Miocene.
Corbisema archangelskiana (Schulz) Frenguelli, which we found only in Sample 183-1140A-21R-CC, is a characteristic upper Oligocene species. Also of importance in this sample is the first uphole occurrence of Naviculopsis bipaculata (Lemmermann) Frenguelli. More detailed results and a correlation with diatom biostratigraphy will be the objective of shore-based studies.