DISCUSSION

It can be seen from the stratigraphic charts (Tables T1–T12) and the correlation summary (Fig. F2) that certain taxa used in the most recent global Upper Cretaceous UC biozonation of Burnett (1998) were either absent or unreliable at Shatsky Rise or that certain intervals were either condensed or contained hiatuses, resulting in combination of biozones and reduced stratigraphic resolution. The low-resolution problem is particularly, and predictably, apparent in the Campanian, which is the interval of greatest nannofossil paleobiogeographic differentiation (e.g., Lees, 2002). The preponderance of condensed section was generally due to the lack of ooze material in chert intervals and was a problem restricted to the pre-Campanian.

In the upper Maastrichtian, the FOs of M. murus and L. quadratus appear to be reversed in Holes 1209C, 1211A, and 1212B but not in 1210B, although this latter effect may be a result of bioturbation (it should be noted that bioturbation was not recorded in the core description for Core 198-1210B-26H, although it may not have been apparent in these pure white oozes, and so this occurrence remains somewhat enigmatic) (Bralower, Premoli Silva, Malone, et al., 2002). This reversal means that the base of Subzone UC20a^TP cannot be recognized. This appears to be a real signal since both taxa have consistent occurrences, species concepts were rigorously applied, and there are no deleterious changes in preservation to explain the possible loss of L. quadratus from lower samples. It is possible that M. murus has a diachronous FO; Self-Trail (2001) noted this phenomenon in the western Atlantic region, where M. murus was shown to occur first in a deep-ocean setting and later in a near-shore environment. However, this diachroneity occurred above the FO of L. quadratus. Further study is required to clarify this situation. For the time being, the FO of M. murus was used as the more reliable datum since this lower-latitude-preferring species was relatively more abundant and conspicuous in the studied sections.

Two previously unused biostratigraphic events were noted from the Shatsky Rise region that may prove to have utility across the Pacific in subdividing the upper Maastrichtian: the FO of C. ultimus in Subzone UC20d^TP and the LO of Tegumentum stradneri in Subzone UC20b^TP (see Fig. F2).

Lower in the Maastrichtian, Zones UC18 and UC19 could not be differentiated because the LO of R. levis could not be determined. This species, common in Subantarctic to Temperate Paleobiogeographic Zones (PBZs) in the Indian Ocean (Lees, 2002), is extremely rare and sporadic at Shatsky Rise, present only in Hole 1210B and questionably in Hole 1212B but absent from Holes 1209C and 1211A. Its distribution and utility is therefore definitely affected by biogeographic factors. It is possible that the biostratigraphic resolution (lost through the nonutility of both R. levis and L. quadratus in the lower Maastrichtian) could be supplemented by a number of events identified here, namely (in reverse stratigraphic order), the FOs of Micula praemurus and consistent Cribrocorona gallica, the LOs of Z. bicrescenticus and C. echinus, and the FO of C. echinus.

Subzones UC15d^TP and UC15e^TP in the upper Campanian could not be differentiated because the marker species, E. parallelus, was either not present or could not be reliably determined at Shatsky Rise because the distinguishing crossbars that span the central area were missing from many eiffellithid specimens. The species is present in the region, occurring in Holes 1209A, 1209C, 1210A, 1210B, 1211A, and 1212B, but was recorded only sporadically and with its FO always occurring well above the expected stratigraphic level. It is probable that we have included specimens of this species without bars in Eiffellithus turriseiffelii, since E. parallelus was shown by Lees (2002) to be present in all PBZs in the Indian Ocean although, until further study is completed (Lees, unpubl. data), we cannot assume that the FO of E. parallelus is a reliable marker in the Pacific Ocean.

Some problems were experienced in reliably determining the FO of U. sissinghii, since this species seems to represent the end-member of a morphological continuum from Uniplanarius gothicus through specimens identified here as Uniplanarius cf. U. sissinghii. Consequently, in Hole 1212B, U. sissinghii apparently first occurs above the FO of Uniplanarius trifidus. In that instance, it is possible that we have included early forms of U. sissinghii with U. gothicus since in this hole we did not log intermediate forms.

The boundary between Subzones UC14d^TP and UC15a^TP could not be determined. According to Lees (2002, table 4), the marker species M. pleniporus does not occur in the Tropical PBZ. Thus, this is the most likely reason for its absence from Shatsky Rise. Although B. hayi is possibly restricted to the Tropical PBZ (Lees, 2002, table 4), it also is not present at Shatsky Rise. Consequently, the base of Subzone UC14c^TP cannot be recognized. This suggests that during the early Campanian Shatsky Rise lay in a situation at the biogeographic limits of these two species, possibly straddling the (Sub)tropical/Temperate PBZ divide (see "Biogeography" below).

The lack of high biostratigraphic resolution through the Campanian is possibly insurmountable in a global biozonation scheme due to the peak in nannofossil endemicity experienced at that time, but there are a number of biostratigraphic events that are useful in subdividing this interval on Shatsky Rise and that may have potential applicability to the wider Pacific region. These untested datums comprise the LO of Rucinolithus? magnus in Zone UC16, the FO of R.? magnus in Subzone UC15b^TP, the FO of Perchnielsenella stradneri in Subzones UC15a^TP or UC15b^TP, and the FO of Ceratolithoides indiensis in undifferentiated Subzones UC14d^TP–UC15d^TP.

Note that we have declined to erect formal subzones using the proposed potential marker taxa listed above until further studies reveal their broad utility across the Pacific (Lees, unpubl. data).

Work is in progress to determine Upper Cretaceous nannofossil paleobiogeography and consequent climate change for 5-m.y. time intervals in the wider Pacific region (Lees, unpubl. data). However, some preliminary broad paleobiogeographic inferences can be drawn for the Shatsky Rise sites using the deductions of Lees (2002, table 4) from Indian Ocean data and including a summary of the biogeographic observations from various oceans of numerous authors, most importantly Worsley (1974), Thierstein (1976, 1981), Roth (1978), Wind (1979), Roth and Bowdler (1981), Shafik (1990), Huber and Watkins (1992), Mutterlose (1992), Watkins et al. (1996), and Street and Bown (2000). It should be noted that a tropical to subtropical influence is expected, since the sites lay between 10°N and 20°N during the Late Cretaceous (Bralower, Premoli Silva, Malone, et al., 2002).

In the pre-Campanian Upper Cretaceous, the taxa that dominated assemblages at Sites 1207 and 1212 are mainly ones that Lees (2002, table 4) determined as dominant in the Austral to Temperate PBZs of the Indian Ocean. These include Biscutum ellipticum (abundant), Biscutum cf. B. ellipticum, Discorhabdus ignotus, Manivitella pemmatoidea, Prediscosphaera columnata, Prediscosphaera ponticula, and Tetrapodorhabdus decorus (all common). Common Eiffellithus gorkae was also found at Site 1212. Lees (2002) indicated that this was indicative of the Austral PBZ, although it is known to be dominant in Temperate sediments (J.M. Self-Trail, pers. comm., 2004); additionally, Lees (2002) lacked low-latitude data for this interval, so this indicator is likely misleading. It is important to note the absence of Seribiscutum primitivum and Repagulum parvidentatum from Shatsky Rise. S. primitivum was only recorded at temperate and austral latitudes in the Indian Ocean, even during times when tropical data were available, and is only ever abundant in the Austral PBZ (Lees, 2002, and others). R. parvidentatum is also only abundant in the Austral PBZ of the Indian Ocean but does have a rare record into the Tropical PBZ (Lees, 2002). Consequently, we can say that there was no austral (or boreal, since these taxa are bipolar) influence on Shatsky Rise. Nannofossils indicative of a Tropical PBZ over Shatsky Rise include common Rotelapillus crenulatus at Site 1212 (conforming to Lees' deduction [Lees, 2002]) and common Hayesites, a genus that Street and Bown (2000) showed to have a low-latitude distribution, at Site 1207 (and all mid-Cretaceous sites). The absence of Hayesites from the Indian Ocean Temperate and Austral PBZs at this time is a major assemblage difference, and so it can be confirmed that Shatsky Rise was tropically situated in the pre-Campanian, but it may also have come under the occasional influence of cooler, temperate waters.

In the Campanian, for which there are low-latitude data from the Indian Ocean, although only from one site (Lees, 2002), there seems to be a mixture of tropical and temperate influences over Shatsky Rise. Note that S. primitivum and R. parvidentatum are again absent from Shatsky Rise, and so no austral influence is inferred. Tropical indicators include Ceratolithoides brevicorniculans and U. sissinghii (Lees, 2002). The former species occurs frequently to rarely at Sites 1210 and 1212 but is absent from Sites 1207 and 1208, whereas the latter occurs frequently to rarely at all sites. Intriguingly, Lapideacassis asymmetrica was noted as endemic in the Indian Ocean Temperate PBZ (Lees, 2002) but here is restricted to Sites 1210 and 1212, which occupy a more southerly position on Shatsky Rise. Common Rotelapillus (Cylindralithus) biarcus, used to define the early Campanian Temperate PBZ in the Indian Ocean (Lees, 2002), is only found at Site 1207; however, L. grillii, the common occurrence of which was also used to define the Temperate PBZ, is very poorly represented at all sites. Common D. ignotus and Broinsonia parca constricta, used to denote the Austral to Temperate PBZs in the Indian Ocean (Lees, 2002), were here found commonly only at the northerly Site 1207 in the Campanian. A number of other taxa noted as common in the Indian Ocean Austral to Temperate PBZs (Lees, 2002) occur commonly in most if not all of Sites 1207, 1208, 1212, and 1210 and include Eiffellithus eximius, M. pemmatoidea, Microrhabdulus decoratus, Microrhabdulus undosus, P. ponticula, Prediscosphaera stoveri, and T. decorus. This cursory and temporally broad examination of the nannofloras seems to suggest subtle paleobiogeographic differences emerging between the different sites on Shatsky Rise during the Campanian.

In the Maastrichtian, S. primitivum and R. parvidentatum are again absent, suggesting a lack of austral influence at Shatsky Rise. Commonly occurring Tropical PBZ indicators include M. murus (common at Sites 1209, 1210, and 1211), Ceratolithoides pricei, Lithraphidites praequadratus, L. quadratus, and M. praemurus (all present at Sites 1209, 1210, 1211, and 1212 but not common at any). Ceratolithoides kamptneri and C. ultimus were identified as endemic Tropical PBZ taxa by Lees (2002), and these occur at all sites of this age except for C. ultimus, which does not occur at Site 1212 (probably because the uppermost Maastrichtian was not examined). However, again there is some evidence of a temperate influence over Shatsky Rise, based on common occurrences of Cylindralithus nudus, D. ignotus, M. decoratus, M. undosus, Placozygus fibuliformis, P. ponticula, P. stoveri, and T. decorus. According to Lees (2002), a possible austral influence is, however, indicated by common Corollithion? madagaskarensis at Site 1210; Retecapsa schizobrachiata at Sites 1209, 1210, and 1211; and also possibly Z. bicrescenticus at Site 1212. However, C.? madagaskarensis and Z. bicrescenticus have apparently both been recorded commonly in temperate-subtropical assemblages (J.M. Self-Trail, pers. comm., 2004). Once again, there appear to be some distinct differences in the nannofloras between the sites.

Some taxa that are absent or occur rarely on Shatsky Rise and that are worthy of note include the holococcoliths, especially Acuturris, Calculites obscurus, and Lucianorhabdus cayeuxii. All of these genera have been found commonly to abundantly in both shelfal and oceanic situations in the Indian Ocean but had a preference for the Austral PBZ (Lees, 2002). However, they also occurred, albeit only rarely to frequently, down to the Tropical PBZ (Lees, 2002), so their absence from Shatsky Rise is enigmatic. Nephrolithus, which is a renowned high-latitude (subantarctic–austral/boreal) genus, is absent at Shatsky Rise, although it did reach down to the Tropical PBZ in the late Maastrichtian of the Atlantic (Self-Trail, 2001) and Indian Oceans (Lees, 2002). This may be further evidence that there was no high-latitude influence over Shatsky Rise in the Maastrichtian. Alternatively, since it is present in the low-latitude Atlantic and Indian Oceans, this may indicate unconformity at Shatsky Rise. Reinhardtites, with a preference for Austral to Temperate (R. anthophorus) and Subantarctic to Temperate (R. levis) PBZs in the Indian Ocean but with paleobiogeographic ranges down into the Tropical PBZ (Lees, 2002), occurred only rarely and very sporadically on Shatsky Rise. Similarly, Gartnerago and Kamptnerius had only rare and very sporadic distributions on Shatsky Rise; both preferred higher latitudes in the Indian Ocean (Lees, 2002). M. pleniporus, which only occurred down to the Temperate PBZ in the Indian Ocean (Lees, 2002), is missing from Shatsky Rise.

As described above, the MME and layers of isolated inoceramid prisms (Frank et al., 2005) occur around the level of the LO of Z. bicrescenticus in Holes 1209C, 1210B, 1211A, and 1212B, following the LOs of C. echinus and T. orionatus. There is currently no evidence to support a change in nannofloral productivity levels through this interval (Frank et al., 2005).