Site 1131 is the middle site (332.4 m water depth) of a downslope transect from 200 m (Site 1129) to 480 m water depth (Site 1127). The site includes two biostratigraphic units (Fig. F4): (1) an expanded Quaternary interval more than 510 m thick, underlain by a thin and conformable Pliocene? interval and (2) a middle-lower Miocene section that also appears continuous, although poor recovery and poor preservation degraded biostratigraphic resolution of this interval. These units are separated by a disconformity at 532 mbsf that spans ~10 m.y. The disconformity coincides with a prominent change in lithology from bioclastic packstone above to lithified chert and grainstone below, coinciding with the base of seismic Sequence 2 (see "Seismic Stratigraphy" in the "Site 1127" chapter). The top and bottom boundaries of an interval of poor nannofossil preservation (at 347.86 and 463.52 mbsf) appear to coincide with seismic Sequence 2 Horizons B and C. Foraminifers throughout the section below ~60 mbsf are strongly affected by calcite reprecipitation, consistent with the high alkalinity reported in pore waters (see "Inorganic Geochemistry").
An environmental crisis produced an unusual nannofossil assemblage dominated by Braarudosphaera bigelowii near the base of Zone NN19 at ~522 mbsf. A similar event was observed in the same stratigraphic interval at Sites 1127 and 1130.
Benthic foraminifers are relatively abundant and well preserved in the upper part of Hole 1131A (Cores 182-1131A-1H through 11X). However, abundance decreases significantly below Core 182-1131A-11X, where preservation deteriorates markedly with tests being heavily recrystallized. One main Pleistocene upper bathyal assemblage is recognized. This assemblage also contains a large proportion of small neritic tests (63-150 mm), probably redeposited from the shelf. Poor preservation prevented detailed faunal analysis below Core 182-1131A-11X.
At Site 1131, as at Site 1127, an expanded Pleistocene section (~ 532 m) overlies a shorter (~85 m) lower-middle Miocene section. Almost all of the samples are rich in nannofossils, with the exception of those between Samples 182-1131A-58X-CC, 0-3 cm (532.04 mbsf), and 62X-CC, 0-3 cm (568.80 mbsf), which contain rare to few nannofossils. There was no recovery in Core 182-1131A-61X. Preservation of nannofossils is generally good to moderate in the upper 330 mbsf and generally poor over the next ~120 m, with improvement within the lower 120 m of the hole. One hiatus spanning ~10 m.y., indicated by the absence of Zones NN18 through NN6, occurs between Samples 182-1131A-58X-CC, 0-3 cm (532.04 mbsf), and 59X-CC, 19-22 cm (540.39 mbsf).
The upper 195 mbsf of Hole 1131A, as seen in Samples 182-1131A-1H-CC, 16-19 cm (3.36 mbsf), through 22X-CC, 13-16 cm (194.60 mbsf), and all of Hole 1131B (104.64 mbsf total depth), is characterized by common to abundant occurrences of Gephyrocapsa caribbeanica and/or Gephyrocapsa oceanica, and common small Gephyrocapsa spp., Helicosphaera carteri, and Calcidiscus leptoporus but lacks Pseudoemiliania lacunosa. These sections are placed into the combined Zones NN21-NN20. The presence of P. lacunosa in Samples 182-1131A-23X-CC, 13-16 cm (204.33 mbsf), through 55X-CC, 30-33 cm (506.14 mbsf), indicates a Zone NN19 assignment. We observed increasing downhole abundances of small Gephyrocapsa spp., decreasing abundances of G. caribbeanica, and few to common occurrences of Calcidiscus leptoporus and Helicosphaera carteri. The highest stratigraphic occurrence of Calcidiscus macintyrei is in Sample 182-1131A-49X-CC, 33-36 cm (451.62 mbsf). Sample 182-1131A-57X-CC, 43-46 cm (521.97 mbsf), contained dominant B. bigelowii, indicating a change in surface-water conditions. This braarudosphaerid chalk was also noted in the lower part of Zone NN19 at Sites 1127 and 1130.
Within Zone NN19, preservation degrades from moderate in Sample 182-1131A-37X-CC, 27-30 cm (333.34 mbsf), to poor in Sample 38X-CC, 20-23 cm (347.86 mbsf), and remains poor to Sample 50X-CC, 42-45 cm (463.52 mbsf), below which preservation improves again to moderate. These levels coincide with prominent seismic stratigraphic reflectors at 336 and 466 mbsf within Sequence 2 but do not seem to coincide with any lithologic contacts or changes (see "Lithostratigraphy," also see "Seismic Stratigraphy" in the "Site 1127" chapter). The interval of poor preservation does, however, correspond to a drop in the percentage of aragonite in sediments at ~330 mbsf (see "Inorganic Geochemistry").
Sample 182-1131A-59X-CC, 19-22 cm (540.39 mbsf), contains Cyclicargolithus floridanus and Calcidiscus macintyrei, an association that indicates an assignment of Zone NN6. This indicates that Zones NN18-NN7 are missing, which suggests a ~10 m.y. hiatus. Also present in this sample are Sphenolithus abies, Calcidiscus leptoporus, B. bigelowii, Discoaster variabilis, and Cyclicargolithus abisectus. This hiatus coincides with a change in lithology from a partially lithified bioclastic packstone to lithified chert and grainstones (see "Lithostratigraphy").
Samples 182-1131A-63X-CC, 0-3 cm (578.40 mbsf), through 66X-CC, 6-9 cm (607.36 mbsf), contain Sphenolithus heteromorphus, Helicosphaera euphratis, B. bigelowii, Cyclicargolithus floridanus, C. abisectus, and Helicosphaera carteri. This floral association indicates an assignment to Zones NN5-NN4 for this interval. It is interesting to note that Cyclicargolithus abisectus is present in these samples and is found in common abundances associated with the short-ranging S. heteromorphus. Cyclicargolithus abisectus should not be seen above its extinction in Zone NN1. It is possible that the specimens of Cyclicargolithus abisectus were reworked, although there is no direct evidence of reworking, (i.e., overgrowths or fragmentation).
A thick Quaternary to upper? Pliocene section (~532 m) occurs unconformably above a middle-lower Miocene section (~85 m), separated by a major disconformity of ~10 m.y. at 532 mbsf. The biostratigraphic succession at Site 1131 is similar to that at Site 1127, except that the lower Pleistocene section is somewhat expanded. The quality of preservation is also different between these sites. Preservation of planktonic foraminifers is quite poor at Site 1131 because of thick overgrowths, cementation, and recystallization below ~65-100 mbsf. In contrast, preservation is quite good at Site 1127, especially in the upper Neogene section.
Preservation of planktonic foraminifers declines from the surface to 60-70 mbsf (Sections 182-1131A-7H-CC and 182-1131B-8X-CC) and remains poor throughout the remainder of the section, principally because of calcite overgrowths and cementation on test surfaces and sporadic recrystallization of the tests themselves. This interval of poor preservation coincides with high pore-water alkalinity (see "Inorganic Geochemistry"), and the depth where overgrowths become prominent coincides with the onset of sediment lithification (see "Lithostratigraphy").
Planktonic foraminifers are common in the upper ~40 mbsf but are somewhat diluted in this interval by other sand-sized bioclasts such as bryozoans, gastropods, benthic foraminifers, and ostracodes. Abundance decreases markedly below ~65 mbsf, the depth below which tests are poorly preserved.
The well-preserved samples above 60 and 70 mbsf in Holes 1131A and 1131B, respectively, contain a typical warm temperate Quaternary assemblage comprised of abundant Globorotalia inflata and few Globigerina bulloides, Globigerina falconensis, Globigerina quinqueloba, Globigerinita glutinata, Globigerinoides ruber, Globigerinoides tenellus, Globorotalia hirsuta, Globorotalia scitula, Globorotalia truncatulinoides, Orbulina universa, and Neogloboquadrina pachyderma (dextral). The poorly preserved Quaternary sections below 60 and 70 mbsf in Holes 1131A and 1131B contain a depauperate assemblage of consistently recognizable Globorotalia inflata, G. ruber, N. pachyderma (dextral), and G. truncatulinoides. Other Quaternary species are recognized sporadically, and many specimens are not recognizable through the cement and recrystallized overgrowths. The section is placed in the subtropical Zone Pt1, based on the presence of G. truncatulinoides. The zone could not be divided into Subzones Pt1a and Pt1b because Globorotalia tosaensis was recognized in only two samples near the base of the interval (Samples 182-1131A-51X-CC, 35-38 cm [468.54 mbsf], and 56X-CC, 28-31 cm [512.50 mbsf]), ~200 m below the Brunhes/Matuyama boundary, which is older than the true LO of G. tosaensis. The base of Zone Pt1 as defined by Berggren et al. (1995) could not be applied because Globigerinoides fistulosus and G. extremus were not observed. We placed the base at the first appearance of G. truncatulinoides (between 512.22 and 521.97 mbsf at Hole 1131A), following the definition of Jenkins (1993) and Chaproniere et al. (1995) for the estimated regional base of the Pleistocene. However, we suspect that the base of this zone in the Great Australian Bight lies in the lower Pleistocene.
The poorly preserved lower? Pleistocene-upper? Pliocene assemblage includes essentially the same species as those of the Quaternary. The upper Pliocene fits the definition of Zone SN13, the Globorotalia inflata Zone of Jenkins (1985, 1993) because it contains Globorotalia inflata without G. truncatulinoides. In this volume, the interval is informally called the Globorotalia crassaformis interval, in part because its age and stratigraphic relationships to zones defined in other regions needs clarification. Samples 182-1131A-57X-CC, 43-45 cm, to 58X-1, 89-91 cm (521.97-531.49 mbsf), are placed in this interval.
The uppermost sample of this interval (Sample 182-1131A-58X-CC, 0-3 cm; 532.04 mbsf) occurs in a lithified layer that marks the top of lithostratigraphic Unit II (see "Lithostratigraphy"), corresponding to the top of Sequence 3 (see "Seismic Stratigraphy" in the "Site 1127" chapter). The assemblage contains very badly overgrown and recrystallized tests with affinities to Globorotalia puncticulata, a species restricted to the Pliocene, and Globigerina bulloides, Globorotalia conoidea, and Globoquadrina dehiscens, all of which occur together in the upper and middle Miocene. The fossils could be reworked from the middle Miocene unit below the lithified layer or could be a downhole contamination; hence, the sample is tentatively assigned to the Pliocene-Mio-cene. The underlying samples from 540.39 to 588.00 mbsf (core-catcher samples of Cores 182-1131A-59X to 64X) are assigned to the middle Miocene based on the occurrence of Fohsella peripheroronda, Globigerinoides sicanus, Globorotaloides suteri, Neogloboquadrina continuosa, Praeorbulina glomerosa, Tenuitella minutissima, and Zeaglobigerina druryi. The bottom two samples are assigned to the lower Miocene based on the occurrence of Zeaglobigerina connecta. The fossils in these samples are sparse and badly preserved; therefore, zonal assignments are not possible without further work and good luck.
Benthic foraminifers were studied from every core-catcher sample in Cores 182-1131A-1H through 7H and from every fourth core-catcher sample below Core 7H. Benthic foraminifers are relatively abundant and well preserved in the upper part of Hole 1131A (Cores 1H through 11X). However, abundance decreases significantly below Core 182-1131A-11X, where preservation deteriorates markedly because tests are heavily recrystallized. Between 100 and 300 benthic foraminifers were picked from the >63-µm fraction in Samples 182-1131A-1H-CC through 11X-CC. Below Core 182-1131A-11X, benthic foraminifers were picked from the >150-µm fraction. A check of the >63- to 150-µm fraction was also carried out to evaluate the relative proportion of small specimens present in the samples. In the interval below Core 182-1131A-11X, poor preservation impaired taxonomic determination and prevented a detailed faunal analysis. The following benthic foraminifer assemblages are recognized in the Neogene succession of Hole 1131A.
This Pleistocene assemblage is characterized by the common to abundant occurrence of small (63-150 µm) Triloculina spp., Spiroloculina spp., Patellina corrugata, Spirillina spp., Palliolatella spp., and Rosalina spp. Also present as rare to frequent constituents of the assemblage are Hoeglundina elegans, Sphaeroidina bulloides, Cibicides refulgens, Bigenerina nodosaria, Bulimina marginata, Uvigerina hispidocostata, Textularia spp., Elphidium spp., Cibicidoides spp., Loxostomum spp., Loxostomoides spp., and various nodosariids. Upper bathyal paleodepths are suggested by the presence of the depth-indicative species Hoeglundina elegans, S. bulloides, B. nodosaria, Bulimina marginata, and U. hispidocostata. However, the dominance of small shallow-water taxa (Triloculina spp., Spiroloculina spp., P. corrugata, Spirillina spp., Palliolatella spp., and Rosalina spp.), typically found in shallower neritic depths, suggests grain size sorting and redeposition of a large component of the assemblage in an upper bathyal setting. Sample 182-1131A-6H-CC contains a large proportion (~20%) of dark stained tests, principally miliolids, Elphidium spp., and Cibicidoides spp., which suggest extensive reworking before deposition.
Site 1131 is situated a few miles away from Site 1127 in a slightly more proximal position and at a slightly shallower water depth (332.4 m at Site 1131 compared to 479.3 m at Site 1127). Sedimentation rates are very high at both sites: 246 m/m.y. at Site 1131 (Fig. F5) and 280-350 m/m.y. at Site 1127 (Fig. F6 in the "Site 1127" chapter). The Pleistocene assemblage at Site 1131 is similar in composition to the coeval assemblage recorded at Site 1127. Both assemblages show similarity to shelf assemblages described from Lakes Entrance in southeastern Australia by Li and McGowran (in press). They include a high proportion of cosmopolitan taxa but also contain more geographically restricted species. Postcruise studies will be necessary to fully document the composition of the benthic foraminifer assemblages at these two sites and to relate faunal changes to climatic, sea level, and/or circulation fluctuations during the Pleistocene.
This striking assemblage was found only in two core-catcher samples that also contained abundant and extremely well-preserved bryozoans. The assemblage includes a few large miliolid and spirillinid tests (>1 mm) and a remarkable proportion of B. nodosaria (large) and Textularia specimens (>1 mm). Also present are S. bulloides, Sigmoilina obesa, Cancris auriculus, Hoeglundina elegans, Martinottiella communis, U. hispidoco-stata, Loxostomum spp., Loxostomoides spp., Sigmoilina spp., Anomalinoides spp., and some miliolids. The assemblage is extremely well preserved, exhibiting no obvious evidence of sorting or reworking. The presence of S. bulloides, Hoeglundina elegans, and numerous B. nodosaria indicates upper bathyal paleodepths, providing further evidence that this assemblage was not transported from the shelf. This assemblage appears to have been associated with a flourishing but transitory in situ bryozoan community. No similar benthic foraminifer assemblage was encountered at Site 1127, where only poorly preserved, presumably reworked bryozoan fragments were found in all cores.
Sediment accumulation rates shown in Figure F5 were calculated based on preliminary biostratigraphic and paleomagnetic results from Site 1131 (see "Paleomagnetism"). The biostratigraphic datum levels and relevant paleomagnetic data used to calculate sedimentation rates are listed in Table T2.
A very high sedimentation rate averaging 240 m/m.y. is recorded for the Pleistocene to upper? Pliocene section. Interestingly, the paleomagnetic data suggest low sedimentation rates (43 m/m.y.) between the Brunhes/Matuyama boundary and the termination of the Jaramillo, suggesting that short intervals of low rates are likely imbedded amid the long Pleistocene intervals of high rates. The underlying middle Mio-cene section registered a significantly lower sedimentation rate of ~22 m/m.y. based on poorly constrained datum levels. A hiatus of >10 m.y. occurred at this sharp change in sedimentation rate (Fig. F5).