, defined by the total range of P. eugubina; and (4) the lower part of the lower Paleocene Subzone P1a of Zone P1, defined by the LO of P. eugubina to the FO of Subbotina triloculinoides (see Olsson et al., 1999).
The interval investigated spans four foraminiferal biozones, (1) the uppermost Maastrichtian Abathomphalus mayaroensis Zone, defined by the first occurrence (FO) of the nominate taxon to the last occurrence (LO) of most Cretaceous taxa; (2) the lowermost Paleocene Zone P0, defined by the LO of most Cretaceous taxa to the FO of P. eugubina; (3) the lower Paleocene Zone P, defined by the total range of P. eugubina; and (4) the lower part of the lower Paleocene Subzone P1a of Zone P1, defined by the LO of P. eugubina to the FO of Subbotina triloculinoides (see Olsson et al., 1999).
Total interval studied: Samples 198-1209C-15H-4, 5–6 cm, to 15H-2, 70–71 cm (235.48–233.2 meters below seafloor [mbsf]). Based on planktonic foraminiferal distribution, few intervals can be identified (from bottom to top).
Top Sections 198-1209C-15H-4 to 15H-3, 88 cm (235.48–234.88 mbsf): in the uppermost Maastrichtian white ooze, the assemblage is highly diversified and belongs to the A. mayaroensis Zone with large (>250 µm) specimens of Globotruncanita stuarti, Globotruncanita stuartiformis, Globotruncanella havanensis, Globotruncanella petaloidea, Pseudoguembelina excolata, Pseudoguembelina hariaensis, Pseudotextularia elegans, Racemiguembelina fructicosa, H. holmdelensis, and H. monmouthensis, and common Guembelitria occur in the small-sized fraction (<150 µm). Faunal preservation through this interval is very variable, ranging from fair to progressively poorer approaching the top of the Cretaceous; faunas, although unevenly preserved from layer to layer, become chalky in aspect, tend to dissolve, and specimens are highly fragmented.
Pale orange burrows extend up to 10 cm into the irregular surface of the white uppermost Cretaceous ooze; washed residues of carefully taken samples from the bottom of the deepest burrows contain minute (<75 µm), well-preserved planktonic foraminiferal assemblages consisting of almost 100% Guembelitria and rare, very small sized hedbergellids (H. holmdelensis and H. monmouthensis) attributable to the lowermost Paleocene Zone P0. However, in washed residues from U-channel samples, this minute fauna is mixed with the poorly preserved, chalky broken specimens from the surrounding uppermost Maastrichtian white ooze.
Foraminiferal assemblages from the top part of the burrows and/or small depressions on the white ooze irregular surface comprise the same species described above along with very rare, minute five-chambered P. eugubina. At the same level, rare yellow-brown spherules are first noted and range up to Sample 198-1209C-15H-3, 83–84 cm (234.82 mbsf) in Zone P.
In Sample 198-1209C-15H-3, 87–88 cm (234.86 mbsf), P. eugubina is still rare, and then it rapidly increases in size and abundance. Important components of the assemblage include hedbergellids, Guembelitria, Chiloguembelina morsei, Chiloguembelina midwayensis, and Woodringina hornerstownensis, whereas Woodringina claytonensis is slightly subordinate. Eoglobigerina eobulloides, Eoglobigerina edita, Eoglobigerina fringa, and Globoconusa daubjergensis are very rare. Guembelitria and chiloguembelinids display an opposite trend in abundance.
In the interval from Sample 198-1209C-15H-3, 83–84 cm (234.82 mbsf), upward, P. eugubina becomes very abundant and reaches the maximum abundance in Sample 198-1209C-15H-3, 81–82 cm (234.80 mbsf), whereas hedbergellids decrease in abundance abruptly and disappear in Sample 198-1209C-15H-3, 73–74 cm (234.72 mbsf). Important components of the assemblages include C. midwayensis and woodringinids.
In Sample 198-1209C-15H-3, 72–73 cm (234.71 mbsf), P. eugubina starts to decrease in abundance and is last recorded in Sample 198-1209C-15H-3, 51–52 cm (234.50 mbsf). In this interval the faunal assemblage is dominated by the genera Guembelitria, Chiloguembelina, and Woodringina. Eoglobigerina eobulloides and Globanomalina archeocompressa are very rare and occur sporadically.
In Sample 198-1209C-15H-3, 59–60 cm (234.58 mbsf), Parasubbotina pseudobulloides appears first and is followed by Praemurica pseudoinconstans slightly above, even though their presence is sporadic across the P/P1a zonal boundary.
Subzone P1a starts in Sample 198-1209C-15H-3, 50 cm (234.49 mbsf); the record of P. pseudobulloides and P. pseudoinconstans is still scattered, and only few specimens are present. Guembelitria shows a marked increase in abundance in Sample 198-1209C-15H-3, 49–50 cm (234.48 mbsf), which is balanced by a decrease of C. midwayensis. However, the other biserial heterohelicids remain more or less stable in abundance.
The presence of P. pseudobulloides and, to a minor extent, P. pseudoinconstans, even rarer still, becomes more constant in Sample 198-1209C-15H-3, 37–38 cm (234.36 mbsf and above). From Samples 198-1209C-15H-3, 25–26 cm (234.24 mbsf), to 15H-3, 9–10 cm (234.08 mbsf), E. eobulloides is also better represented in the assemblages.
At the top of Section 198-1209C-15H-3 (233.99 mbsf), the abundance of Guembelitria decreases markedly and the assemblage is dominated by heterohelicids (mainly Chiloguembelina and woodringinids and by a lesser amount of Guembelitria) representing almost 100% of the total specimens. The trochospiral taxa (P. pseudobulloides and P. pseudoinconstans) represent only 3%–6%, and E. eobulloides is absent.
Section 198-1209C-15H-2, sampled every 5 cm (233.9–233.5 mbsf), yields a fauna similar to the top of Section 15H-3, dominated by biserial taxa, whereas Guembelitria decreases significantly upcore.
Above Sample 198-1209C-15H-2, 100–101 cm (233.50 mbsf), chiloguembelinids, especially C. morsei, further increase in abundance.
From Sample 198-1209C-15H-2, 95–96 cm, to 15H-2, 70–71 cm (233.45–233.20 mbsf), P. pseudobulloides and P. pseudoinconstans significantly increase in abundance (>25%); in addition, the assemblages comprise ~45% of C. morsei alone and ~15% woodringinids, whereas Guembelitria is almost absent.
The faunal assemblage from the few samples investigated in Holes 1209A, 1209B, and 1210A supports the distribution and evolution of planktonic foraminiferal taxa recorded in Hole 1209C across the K/P boundary.
Total interval studied: Samples 198-1211C-15H-5, 22–23 cm, to 15H-3, 80–81 cm (135.02–132.61 mbsf). Samples 198-1211C-15H-3, 150 cm, to 15H-3, 81 cm (133.3–132.62 mbsf), were taken in 1-cm intervals using the U-channel sampling device; Section 198-1211C-15H-4 was sampled every 10 cm. Data from a few samples studied on board were also incorporated.
Shore-based study of the interval across the K/P boundary in Hole 1211C revealed that the succession is much more disturbed and complex with respect to that recovered in Hole 1209C.
In Hole 1211C, Cretaceous faunas are poorly preserved and strongly affected by dissolution and well-preserved, recognizable specimens are very rare or even missing in some samples.
Moreover, characteristic features of the succession in Hole 1211C are
As shown in Table T2, the interval in which P. eugubina was observed is twice as long as the range of P. eugubina in Hole 1209C and extends well below the level where the K/P boundary was previously placed (Bralower, Premoli Silva, Malone, et al., 2002). Moreover, Cretaceous taxa are scattered over a longer interval but are concentrated in the middle of the P. eugubina range, whereas very few of them were found in the samples thought to belong to the top of the Cretaceous section.
Sample 198-1211C-15H-5, 22–23 cm (135.02 mbsf), corresponds to the uppermost sample investigated that definitely belongs to the uppermost Cretaceous A. mayaroensis Zone.
Sample 198-1211C-15H-4, 9–10 cm (133.39 mbsf), yields a lowermost Paleocene P. eugubina fauna that already contains common to abundant P. eugubina (including multiple morphotypes), common woodringinids, few Guembelitria, rare E. eobulloides, and very rare fragments of Cretaceous taxa. A similar assemblage is also present in the underlying Sample 198-1211C-15H-4, 25–26 cm (133.55 mbsf).
Based on these data, the base of the U-channel sample set at the bottom of Section 198-1211C-15H-3 lies well above the oldest Paleocene faunas. Moreover, comparing faunal composition from the lower part of the section in Sample 198-1211C-15H-4, 80–81 cm (134.10 mbsf), with that from Hole 1209C, it looks likely that the K/P boundary in Hole 1211C was probably lying in the unrecovered lower part of Section 15H-4 (below 134.10 mbsf).
The top of Zone P is also difficult to place. The LO of P. eugubina recorded in Sample 198-1211C-15H-3, 88–89 cm (132.69 mbsf), may correspond to its true extinction level, as P. eugubina co-occurs with P. pseudobulloides and P. pseudoinconstans in the upper part of its range. However, the latter species are much more common than in the corresponding interval in Hole 1209C. Also anomalous is the presence of common hedbergellids along with common P. eugubina (including multiple morphotypes) in Samples 198-1211C-15H-3, 110–111 cm (132.91 mbsf), to 15H-3, 97–98 cm (132.78 mbsf), as the hedbergellids should disappear in the lower half of the range of P. eugubina.
From the anomalous distribution of both uppermost Cretaceous and lowermost Paleocene taxa, we can conclude that the succession across the K/P boundary in Hole 1211C is affected by significant reworking or, alternatively, by intensive bioturbation. The latter possibility, however, may apply to the upper part of Section 198-1211C-15H-3 but is unlikely for the lower Section 15H-4, where burrows are not evident. We also considered the possibility that one or more layers were repeatedly slumped downslope into the normal succession. However, the faunal record does not show any repeated trend, as the assemblages are very mixed. Apparently, reworking, or mixing by bioturbation, extends up at least to Sample 198-1211C-15H-3, 96–97 cm (132.77 mbsf), as suggested by the highest occurrence of hedbergellids.
