BIOSTRATIGRAPHY

Site 1169 is situated beneath present-day subantarctic waters on the western part of the STR (Hole 1169A is located at latitude 47°03.9159´S) in upper abyssal water depths (3568 m). A high degree of sediment flow and disturbance was encountered within the recovered cores for Hole 1169A. Soft-sediment deformation and downhole caving complicated the biostratigraphy, therefore only last occurrence (LO) events are used in the final age model (Fig. F4). Seven major microfossil groups recovered from core-catcher samples are used to constrain the base of Hole 1169A to no older than 13.6 Ma (middle Miocene) and to construct an age-depth model. The youngest core (189-1169A-1H) is assigned to the middle Pleistocene on biostratigraphic and paleomagnetic information (see also "Paleomagnetism"). The biostratigraphic data suggest the presence of two hiatuses; the younger spans the Miocene/Pliocene boundary and the older straddles the middle Miocene/upper Miocene boundary. Reconstructed linear sedimentation rates imply enhanced calcareous nannofossil production during the early Pliocene, demonstrated for the first time in the subantarctic. In addition, Hole 1169A contains the most southerly late Neogene dinoflagellate cyst (dinocyst) record discovered to date. There are few subantarctic sites where such a broad range of microfossil groups are present, affording a unique opportunity for a highly integrated multiple-group biostratigraphy.

Fluctuations in the abundance of cold- and warm-water dinocysts and siliceous microfossils (diatoms and radiolarians) in the Pliocene-Pleistocene signify possible meridional shifts in the position of the Subtropical Convergence during this time. Deposition at abyssal paleodepths throughout the sequence is inferred from the benthic foraminifers.

Calcareous Nannofossils

All core-catcher samples as well as additional samples from some critical intervals were examined for calcareous nannofossils at Site 1169. Calcareous nannofossils are generally abundant and well to moderately preserved throughout the cored interval (Table T2). Poor coring conditions at this site resulted in Cores 189-1169A-1H through 21H being highly disturbed and Cores 189-1169A-22X through 26X having low recovery (see "Lithostratigraphy"). The nannofossil biostratigraphy (Table T3) for the middle Miocene through the Pleistocene sediments should be considered with these limitations in mind.

Sample 189-1169A-1H-CC contains Pseudoemiliania lacunosa but not Emiliania huxleyi. Toothpick samples were taken through Core 189-1169A-1H in an effort to establish the first occurrence (FO) of Emiliania huxleyi (0.26 Ma) and the LO of P. lacunosa (0.45 Ma). Analysis of these samples revealed successive events taking place in a haphazard order indicating core disturbance or strong bioturbation. The FO of E. huxleyi and the LO of P. lacunosa could not be located within Core 189-1169A-1H to any degree of confidence.

The LO of Discoaster brouweri (1.95 Ma) and the LO of Discoaster surculus (2.55 Ma) are found between Samples 189-1169A-2H-CC and 3H-CC (Table T3). Toothpick samples taken from Core 189-1169A-2H showed similar disturbance to Core 1H, and these datums could not be separated. The Pliocene/Pleistocene boundary, which is marked by the LO of D. brouweri in low latitudes or approximated by the LO of Calcidiscus macintyrei (1.67 Ma) in nontropical areas where discoasters are rare, is therefore placed between Samples 189-1169A-1H-CC and 2H-CC based on the latter datum.

The LO of Reticulofenestra pseudoumbilicus (3.75 Ma) is found between Samples 189-1169A-3H-CC and 4H-CC, marking the lower Pliocene/upper Pliocene boundary. The base of nannofossil Zone CN11 at 4.6 Ma is marked by the LO of Amaurolithus primus between Samples 189-1169A-4H-CC and 5H-CC. Ceratolithus acutus and Discoaster quinqueramus are not encountered, and the Miocene/Pliocene boundary can not be identified by nannofossil biostratigraphy.

Samples 189-1169A-7H-CC through 22X-CC yield abundant, well-preserved nannofossil assemblages that do not provide useful age information. These assemblages are dominated by Calcidiscus leptoporus, C. macintyrei, and Coccolithus pelagicus with a few reticulofenestrids.

The base of the late Miocene calcareous nannofossil Subzone CN9b is placed between Samples 189-1169A-23X-3, 44 cm, and 23X-4, 25 cm, based on the LO of Discoaster loeblichii (6.8 Ma). The LO of Amaurolithus delicatus is recorded between Samples 189-1169A-23X-CC and 24X-1, 120 cm. Although this species is rare in Hole 1169A, its LO at 6.9 Ma in this interval is in good agreement with bioevents above and below this interval. The FO of D. loeblichii (8.7 Ma) between Samples 189-1169A-24X-1, 120 cm, and 24X-2, 60 cm, may have been shifted downhole in this disturbed section by caving or other means, and this datum has not been included in the construction of the age-depth model for this site (see "Age Model and Sedimentation Rates"). The LO of Cyclicargolithus floridanus (11.9 Ma) is present between Samples 189-1169A-24X-3, 10 cm, and 24X-4, 35 cm. This datum generally lies below the upper Miocene/lower Miocene boundary (Gartner, 1992).

The last three datums are present within 7.9 m and span 5.0 m.y. This indicates a condensed section or hiatus between Sample 189-1169A-23X-CC and 24X-4, 35 cm. The break in sedimentation appears to be between Sample 189-1169A-23X-CC and 24X-1, 120 cm, indicated by 120 cm of sediment representing 1.8 m.y. (0.07 cm/k.y. sedimentation rate). An even less optimistic interpretation of the nannofossil data, ignoring the FO of D. loeblichii as possibly being reworked and using the LO of C. floridanus at 11.9 Ma, would result in 5.0 m.y. being represented by 6.4 m of sediment (0.12 cm/k.y. sedimentation rate).

A distinct color change (Fig. F5) is observed in Core 189-1169A-23X (see "Lithostratigraphy"). The nannofossil ooze gradually changes from white (N8) to light greenish gray (5GY 8/1) in Section 189-1169A-23X-3, and a change in color reflectance data trends is seen at Sample 169-1169A-23X-3, 124 cm. Analysis of toothpick samples from this core suggests a change in paleowater temperatures across this interval. The overlying white nannofossil ooze is interpreted as representing cooler waters as indicated by the lack of discoasters in addition to the low diversity and high abundance of placoliths. The light greenish gray nannofossil ooze is interpreted to represent warm-water conditions based on the sudden influx of numerous, well-preserved specimens of discoasters. The LO of D. loeblichii between Samples 189-1169A-23X-3, 44 cm, and 23X-4, 25 cm, at 6.8 Ma provides good age control for this event. The LO of the diatom Actinocyclus fryxellae (6.7 Ma) between Sections 189-1169A-20H-CC and 21H-CC indicates that this water-mass change was not accompanied by a recorded hiatus in sedimentation.

Analysis of abundant Neogene nannofossils from Site 1169 indicates a well-preserved "mixed" assemblage. Despite the subantarctic location of Site 1169 neither warm-water nor cold-water assemblages dominate the record. At least one hiatus or condensed section is indicated by the nannofossil biostratigraphy. Further study of Cores 189-1169A-7H through 22X may yield additional biostratigraphic datums, but the quality of the cores recovered at Site 1169 may preclude higher resolution results.

Planktonic Foraminifers

In general, the planktonic foraminiferal assemblages (>250 µm) are temperate in composition, dominated by species belonging to the Globoconella plexus. Other species common throughout the section are Globigerina bulloides, Globigerina falconensis, Globigerina quinqueloba, Neogloboquadrina pachyderma, Orbulina universa, Globorotalia crassaformis, and Globigerinita glutinata (see Table T4). Though the diversity of the assemblages is low, the populations are large. Characteristics of the populations of the various species seem to change with time and are probably related to the changing environmental conditions associated with movements in the Subtropical and Subantarctic Fronts. Future isotopic studies may provide an insight into these faunal variations. The zonal scheme as discussed in "Biostratigraphy" in the "Explanatory Notes" chapter seems to be readily applied to these assemblages. The distribution of species in these samples is given in Table T4.

Quaternary (Pleistocene/Holocene)

Samples 189-1169A-1H-CC and 2H-CC both contain Globorotalia truncatulinoides, indicating Zone SN14. Thus, the G. truncatulinoides FO (1.96 Ma) is restricted to the interval between 15.04 and 27.90 mbsf. Planktonic foraminifers are abundant and well preserved.

Pliocene

The uppermost planktonic foraminifer zone of the Pliocene (Globorotalia inflata Zone; SN13) was not recognized in Hole 1169A. Therefore, the G. inflata Zone is either present within a highly condensed interval between 15.04 and 27.90 mbsf or absent altogether. Samples 189-1169A-3H-CC through 6H-CC contain Globorotalia puncticulata without either G. inflata or Globorotalia pliozea, indicative of Subzone SN12b. This subzone, the G. puncticulata Subzone, is highly expanded in Hole 1169A, spanning the interval bounded by Samples 189-1169A-3H-CC and 21H-CC, roughly 130 m of section. Between Sections 189-1169A-7H-CC and 21H-CC, samples contain a typical Subzone SN12b assemblage, but many specimens of G. inflata are present in most of the assemblages, indicating contamination from above. In general, planktonic foraminifers are abundant and well preserved throughout this interval.

Sample 189-1169A-22X-CC yielded diminutive specimens ascribed to G. pliozea, indicating Subzone SN12a. The absence of G. pliozea in Sample 189-1169A-21H-CC and its presence in Sample 22X-CC restricts the LO of this taxon (4.6 Ma) to between 198.19 and 202.06 mbsf. Planktonic foraminifers are common within Sample 189-1169A-22X-CC but are unusually small. Preservation within the G. pliozea Subzone is variable with many specimens exhibiting differing degrees of dissolution and abrasion.

Middle Miocene

Sample 189-1169A-22X-CC, which is assigned to the lower Pliocene, is underlain by sediments in Sample 189-1169A-23X-CC that contain specimens of Paragloborotalia mayeri, indicating an age of middle Miocene (Zone SN7). The presence of P. mayeri indicates a minimum age of 11.4 Ma for Sample 189-1169A-23X-CC. Thus, a significant stratigraphic gap (~6.8 m.y.) is inferred between Samples 189-1169A-22X-CC and 23X-CC. Preservation in Sample 189-1169A-23X-CC is good with abundant planktonic foraminifers.

Mixed Assemblages of Indeterminant Age

Sample 189-1169A-24X-CC contains a mixed assemblage; however, the presence of Paragloborotalia continuosa restricts the age to a minimum of Zone SN9 (8.0 Ma). Given its stratigraphic position, Sample 189-1169A-24X-CC should be relatively older than Sample 189-1169A-23X-CC, yet the age of this mixed assemblage remains equivocal. It is suspected that another hiatus separates Samples 189-1169A-23X-CC and 24X-CC.

Sample 189-1169A-25X-CC contains an enigmatic assemblage characteristic of the Pleistocene, consisting only of small specimens of G. quinqueloba and G. glutinata. Assemblages contained within Sample 189-1169A-26X-CC also consist of upsection contaminants. These two lowermost samples are assigned a general Neogene age. Preservation throughout this part of the record varies from good (Section 189-1169A-25X-CC) to poor (Section 189-1169A-26X-CC) with planktonic foraminifers being common. Age-significant events are summarized in Table T5.

Benthic Foraminifers, Ostracodes, and Bolboforma

Benthic foraminifers are generally well preserved, highly diverse, and abundant at this site (Fig. F6). Faunal assemblages suggest a paleodepth of 2000-4000 m (abyssal). Sample 189-1169A-1H-CC contains the low-oxygen indicator Chilostomella oolina. Deposition under moderate- to well-oxygenated bottom-water conditions is inferred from Samples 189-1169A-2H-CC through 23X-CC. The presence of Melonis barleeanus and Melonis pompilioides suggests a high flux of organic carbon in these samples. Samples 189-1169A-24X-CC through 26X-CC lack M. barleeanus and M. pompilioides, suggesting a lower flux of organic carbon in this interval. The transition from samples containing Melonis spp. to those which do not contain Melonis spp. coincides with the middle/upper Miocene hiatus/condensed interval (see Figs. F4, F6).

Other microfossils recorded include ostracodes, which are present throughout the drilled section. Their carapaces are mostly disarticulated, suggesting some degree of water turbulence. Additionally, bolboformids are present in Hole 1169A. Bolboforma aculeata is identified in Samples 189-1169A-22X-CC through 26X-CC. This species has a range from the base of Zone N14 to the top of Zone N17 at the Miocene/Pliocene boundary (Spiegler and von Daniels, 1991). Notable is the high abundance of bolboformids in Sample 189-1169A-26X-CC.

Radiolarians

Sediments from Hole 1169A contain Pliocene to middle Miocene "non-antarctic" and antarctic radiolarians. Abundant, well-preserved radiolarians were recovered from Samples 189-1169A-1H-CC through 17H-CC. Samples 189-1169A-18H-CC through 20H-CC commonly contain radiolarians, but with poor preservation. Samples 189-1169A-21H-CC through 25X-CC yield few to common radiolarians, whereas Sample 189-1169A-26X-CC is almost barren of radiolarians.

Samples 189-1169A-1H-CC through 11H-CC yield typical antarctic faunas. An abrupt faunal change to non-antarctic assemblages is present between Samples 189-1169A-11H-CC and 12H-CC. This fauna lacks or rarely contains species of Antarctissa, Cycladophora, and Spongoplegma. However, previous studies have reported rare to common occurrence of these taxa in the subantarctic near the Subtropical Convergence.

Pliocene

Radiolarian evidence suggests that the interval between Samples 189-1169A-1H-CC through 22X-CC should be assigned to the Pliocene. Sample 189-1169A-1H-CC is younger than the LO of Eucyrtidium calvertense. This sample is older than 1.9 Ma as indicated by the presence of E. calvertense. Five events are recognized for age assignment in the Pliocene interval (viz., the LO of Pseudocubus vema between Samples 189-1169A-4H-CC and 5H-CC (2.4 Ma), the LO of Lampromitra coronata between Samples 189-1169A-6H-CC and 7H-CC (3.35 Ma), the FO of P. vema between Samples 189-1169A-7H-CC and 8H-CC (4.5 Ma), the LO of Lychnocanoma grande between Samples 189-1169A-16H-CC and 17H-CC (5.0 Ma), and the LO of Dictyophimus splendens between Samples 189-1169A-21H-CC and 22X-CC (5.2 Ma). Samples 189-1169A-1H-CC through 4H-CC are correlated to the E. calvertense Zone, Samples 189-1169A-5H-CC through 7H-CC to the Upsilon Zone, and Samples 189-1169A-8H-CC through 11H-CC to the Tau Zone, respectively. The interval between Samples 189-1169A-12H-CC through 23X-CC cannot be correlated to any radiolarian zonal schemes, owing to the absence of zonal marker species.

Published records show that the FO of Cycladophora davisiana is found in the Upsilon Zone, but this species is present consistently from Samples 189-1169A-1H-CC through 11H-CC. This inconsistent stratigraphic distribution probably resulted from disturbance of sediments during coring.

Radiolarian assemblages from Samples 189-1169A-1H-CC through 11H-CC are characterized by the presence of antarctic or cold-water species, such as C. davisiana, Cycladophora humerus, Cycladophora pliocenica, Cycladophora spongothorax, Spongoplegma antarcticum, and Triceraspyris antarctica. The assemblage within this interval also contains species living in mid- to high-latitudes of the North Pacific such as Sphaeropyle langii and its ancestor Sphaeropyle robusta.

Radiolarian faunas change abruptly between Samples 189-1169A-11H-CC and 12H-CC. Antarctic or cold-water species become less abundant in Samples 189-1169A-12H-CC through 20H-CC, whereas spumellarians such as Thecosphaera become more abundant. These faunas are comparable to those in Samples 1168A-12H-CC through 20X-CC, with the exception that 20X-CC lacks any artiscinid species. The assemblages of Samples 189-1169A-12H-CC through 20H-CC occasionally contain a few specimens of antarctic or cold-water dwelling species.

Between Samples 189-1169A-20H-CC and 21H-CC there is a sharp decrease in species diversity. Radiolarians in Sample 189-1169A-21H-CC through 23X-CC consist mainly of Stylacontarium acquilonium, Druppatractus irregularis, Hexacontium spp., and Thecosphaera spp. Sample 189-1169A-22X-CC is marked by the common occurrence of reworked Miocene species such as Cyrtocapsella japonica and Cyrtocapsella tetrapera.

Middle Miocene

The upper Miocene is apparently missing from Hole 1169A based on radiolarian evidence. The interval represented by Samples 189-1169A-23X-CC through 25X-CC is assigned to the middle Miocene. The fauna is similar to that from the mid-latitude North Pacific with respect to the occurrence of C. tetrapera, C. japonica, Lychnocanoma nipponica nipponica, and Theocorys redondoesnsis. These species are dominant in the mid-latitude oceans.

One bioevent, the last abundant occurrence (LAO) of C. tetrapera is recognized between Samples 189-1169A-23X-CC and 24X-CC. This event occurred during 12.5 Ma in the North Pacific. Sample 189-1169A-26X-CC lacks age-diagnostic radiolarians.

Diatoms, Silicoflagellates, and Sponge Spicules

All core-catcher material from Hole 1169A was analyzed for diatoms, silicoflagellates, and sponge spicules. Smear slides were examined to assess overall relative abundance, and additional material was cleaned of the carbonate component for full assemblage analysis. Diatoms are present in common to high abundance in Cores 189-1169A-1H through 17H with good to moderate preservation. Notable, however, is Sample 189-1169A-14H-CC, which is almost completely barren of diatoms and sponge spicules. Sample 189-1169A-18H-CC and below contain rare to few diatoms in a poor to moderate state of preservation. Relative abundance data for diatoms, sponge spicules, and silicoflagellates are presented in Table T6.

Most biostratigraphic diatom markers (see "Biostratigraphy" in the "Explanatory Notes" chapter) are not present in core-catcher material from Hole 1169A. Some reworking is evident, notably in Sample 189-1169A-24X-CC, where upper Oligocene to lower Miocene material is inferred to be reworked into middle Miocene sediments by the common presence of the well-preserved late Oligocene to early Miocene marker Rocella gelida var. gelida and the variety schraderi. No other samples contain these taxa. Robust diatom Actinocyclus ingens var. nodus may also be reworked into Sample 189-1169A-24X-CC.

The LO of Proboscia barboi (1.8 Ma) is confidently placed within Core 189-1169A-1H. This defines the youngest bioevent for this site. Similarly, the LO of Fragilariopsis weaveri (2.6 Ma) and Fragilariopsis lacrima (2.9 Ma) are positioned between the two upper cores and Cores 189-169A-4H and 5H, respectively. An apparent FO of Fragilariopsis barronii (4.44 Ma) is encountered between Cores 189-1169A-6H and 7H, but this event is discarded from the final age model for reasons explained above. The LO of A. fryxellae (6.7 Ma) is present between Cores 189-1169A-20H and 21H. This robust taxon is observed in trace amounts in samples above Core 189-1169A-20H and is assumed to be reworked. The LO of the distinctive middle-late Miocene marker Denticulopsis dimorpha (10.7 Ma) is confidently placed between Cores 189-1169A-23X and 24X. Diatom datums are presented along with other microfossil events in Table T7.

Paleoceanographic information is evident from the initial analyses of diatom floras at Site 1169. Downhole floristic and diversity changes suggest fluctuations in the dominance of different water masses. Samples 189-1169A-1H-CC through 4H contain a relatively diverse, mixed flora of temperate-warm taxa (e.g., Hemidiscus cuneiformis) and endemic subantarctic-antarctic taxa (e.g., Thalassiosira lentiginosa). Samples 189-1169A-5H-CC and 8H-CC contain a comparatively lower diversity flora of dominantly temperate-warm species, whereas Samples 189-1169A-7H-CC and 13H-CC contain dominantly subantarctic-antarctic taxa. The remaining samples contain a mixed flora (viz., Samples 189-1169A-1H-CC through 4H; see above). Noteworthy, however, is the distinctive change from Samples 189-1169A-8H-CC (temperate-warm signal) to 7H-CC (subantarctic-antarctic signal) occurring at (inferred) 4.0 Ma in the late early Pliocene. Such changes may herald meridional shifts in the position of the Subtropical Convergence. Radiolarian and dinocyst assemblage changes observed at this site (see "Radiolarians" and "Palynology") also imply surface-water variations over this site.

Palynology

Onboard palynological analysis included approximately half of the core-catcher samples taken from Hole 1169A. Recovery of palynomorphs was good down to Sample 189-1169A-9H-CC, assigned to the earliest Pliocene on nannofossil evidence. Unfortunately, below this horizon, samples are barren or palynomorphs (notably dinoflagellate cysts) are present only in trace amounts. Such occurrences are considered the result of downhole contamination. Dinoflagellate cysts (dinocysts) are the most abundant palynomorphs in Hole 1169A; these occurrences represent the southernmost late Neogene dinocyst record ever found. Foraminifer organic linings and sporomorphs are the other quantitatively important categories of palynomorphs present in Hole 1169A (Table T8).

Middle Pleistocene to upper Pliocene Samples 189-1169A-1H-CC and 4H-CC yield relatively well-diversified dinocyst assemblages. Most abundant are taxa indicative of relatively warm, oligotrophic water masses. Also common are species that suggest the influence of distinctly colder and more eutrophic water masses. The latter include taxa endemic to the Antarctic region such as Dalella chathamense and Selenopemphix antarctica. In these "mixed" assemblages, cysts of presumed heterotrophic dinoflagellates (e.g., of Protoperidinium spp.) are also relatively common. Their occurrence may also be caused by the occasional presence of nutrient-rich surface waters, possibly associated with the "cold portion" of the assemblage. The mixed assemblages may indicate shifts of the position of the Subtropical Convergence (see "Diatoms, Silicoflagellates, and Sponge Spicules").

The underlying lower Pliocene Samples 189-1169A-5H-CC and 9H-C, in contrast, yield poorly diversified dinocyst assemblages generally indicative of warm oligotrophic surface waters. Only in Sample 189-1169A-9H-CC can some indication of the influence of colder water masses be found in the occurrences of the arctic species Impagidinium pallidum and the cold-temperate Corrudinium harlandii. The range top of Invertocysta tabulata (2.65 Ma) between Samples 189-1169A-5H-CC and 9H-CC may assist the age assessment of Hole 1169A (Table T9; Fig. F4).

Below Sample 189-1169A-9H-CC, available core-catcher samples are barren or yield a few dinocysts (usually Nematosphaeropsis labyrinthea, Impagidinium aculeatum, and/or Impagidinium paradoxum) that are quite abundant in the overlying interval. These taxa are stratigraphically long ranging and may be present in situ through to early Miocene age or even older sediments. However, given the problems with core recovery at Site 1169, these occurrences are considered a result of downhole contamination, rather than being in place.

Age Model and Sedimentation Rates

Because of the caving throughout the majority of Cores 189-1169A-1H to 21H, FO datums can not be placed with any confidence, therefore only the LO bioevents have been used in the final biostratigraphic assessment of Site 1169. These events total 19 and are presented in Table T7.

The age-depth curve is presented in Figure F4. The curve is refined by four paleomagnetic events in the Pliocene-Pleistocene (see "Paleomagnetism"). Ages may be in error by as much as 1 m.y. at the base of Hole 1169A because of the lack of biostratigraphic data for the bottom three cores. Two hiatuses are inferred—one spanning the interval of 12.5-6.9 Ma (defined by the LAO of C. tetrapera and the LO of A. delicatus, respectively) and a second spanning the interval of 6.7-4.6 Ma (defined by the LO of A. fryxellae and G. pliozea, respectively). Unfortunately, the Miocene/Pliocene boundary falls within the younger hiatus. A short period of sedimentation (200 k.y.) at a rate of 10.9 cm/k.y. is inferred between the two periods of net nondeposition. Unfortunately, no definitive biostratigraphic information was recovered for Cores 189-1169A-8H through 16H. The age-depth model is most sketchy for this interval and should be regarded as such. However, one interpretation suggests a significant increase of nannoplankton production based on an apparently rapid (600 k.y.) and elevated sedimentation rate (23.4 cm/k.y.) in the lower Pliocene (4.6-4.0 Ma) following the younger hiatus. Increased calcareous nannoplankton production resulting in elevated sedimentation rates is observed in the lower Pliocene in tropical and temperate locations (Kennett and Von der Borch, 1986; Nelson, 1986); however, this phenomenon is recorded in the subantarctic for the first time. The average sedimentation rate in the upper Pliocene and Pleistocene falls to just 1.78 cm/k.y. The moderate amount of scatter in the bioevents during this period (Fig. F4) created problems for delineating an average sedimentation rate for this period. The curve, which is defined by the paleomagnetic events, conveniently approximates to an average through the scatter. It is stressed again that the age-depth model should be regarded with caution for intervals where the core is disturbed (see "Lithostratigraphy").

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