DIATOM
ZONATION
Thalassiosira
lentiginosa Partial Range Zone
- Author: McCollum
(1975), renamed by Kellogg and Kellogg (1986)
- Definition of
top: LO of Thalassiosira lentiginosa (still extant)
- Definition of
base: LO of Actinocyclus ingens
- Age: 0-0.64
Ma
- Discussion
and remarks: Gersonde and Bárcena (1998) determined the first
abundant occurrence (FAO) and last abundant occurrence (LAO) of Hemidiscus
karstenii to be 0.19 and 0.42 Ma, respectively. These ages agree with
those observed by Harwood and Maruyama (1992). In the material from this
leg, H. karstenii was observed in low numbers in Holes 1095C,
1096A, 1096B, and 1101A. The LAO of this species was noted to be at 0.15
Ma (Hole 1095C) and 0.16 Ma (Holes 1096A and 1096B), and the FAO occurred
at 0.29 Ma (Hole 1095C) and 0.26 Ma (Holes 1096A and 1096B). As the upper
portions of all the holes exhibit noncontinuous diatom abundance and
barren intervals, these ages should be considered with this in mind. H.
karstenii was rarely observed in two samples from Hole 1101A, making
age determinations in this hole imprudent. Site 1095 was the only one to
include the last occurrence datum of Actinocyclus ingens, thereby
definitely placing the lower boundary of the T. lentiginosa Zone
at this site.
Actinocyclus
ingens Partial Range Zone
- Author:
Gersonde (1990), modified by Harwood and Maruyama (1992), further modified
in this zonation
- Definition of
top: LO of Actinocyclus ingens
- Definition of
base: LO of Thalassiosira kolbei
- Age: 0.64-1.85
Ma
- Discussion
and remarks: This zone is a combination of two from Harwood and
Maruyama (1992). The LO of Fragilariopsis barronii was used to
define a boundary between their A. ingens and F. kerguelensis
Zones. As F. barronii was not consistently observed in large
numbers, we combined the two into one. The last occurrences of Thalassiosira
elliptipora and Thalassiosira
torokina were observed for this zone at Site 1095. This zone was not
identified at Site 1096. The LO of T. torokina was also observed
at Site 1101, as were F. barronii (LO) and Thalassiosira
fasciculata (LO). The species datum indicating the base of this zone,
the LO of T. kolbei, was not seen at any of the three
sites; this taxon was only rarely observed in the present study.
Thalassiosira
kolbei Partial Range Zone
- Author:
Ciesielski (1983), renamed by Baldauf and Barron (1991)
- Definition of
top: LO of Thalassiosira kolbei
- Definition of
base: LO of Thalassiosira vulnifica
- Age: 1.85-2.4
Ma
- Discussion
and remarks: The base of this zone was defined at all three sites
with the LO datum of T. vulnifica. The LOs of T. complicata
and T. inura were also noted at both Sites 1095 and 1096, whereas
the LO of Actinocyclus karstenii was the only datum observed
within this zone at Site 1101.
Thalassiosira
vulnifica Partial Range Zone
- Author: Ciesielski
(1983), renamed by Fenner (1991)
- Definition of
top: LO of Thalassiosira vulnifica
- Definition of
base: LO Thalassiosira insigna
- Age:
2.4-2.6 Ma
- Discussion
and remarks: The LO datums of F. interfrigidaria and T.
insigna were not observed at Site 1095, but some well-preserved
specimens of these two species were observed in Holes 1095A and 1095D.
Sites 1096 and 1101 did preserve the LO datums for both of these species.
Sample 178-1096B-29X-CC (239.6 mbsf) contained an abundance of T.
insigna poorly preserved central areas. These were often noted, in
lower numbers, in other samples as well.
Thalassiosira
insigna-Thalassiosira vulnifica Concurrent Range Zone
- Author:
Harwood and Maruyama (1992)
- Definition of
top: LO of Thalassiosira insigna
- Definition of
base: LO of Thalassiosira vulnifica
- Age: 2.6-3.23
Ma
- Discussion
and remarks: The LO of T. vulnifica was found to occur
at all three sites. Another datum within this zone, the LO of Thalassiosira
striata, also occurred in Hole 1101A.
Fragilariopsis
interfrigidaria Partial Range Zone
- Author: McCollum
(1975), modified in Weaver and Gombos (1981) and Harwood and Maruyama
(1992)
- Definition of
top: FO of Thalassiosira vulnifica
- Definition of
base: FO of Fragilariopsis interfrigidaria
- Age:
3.23-3.8 Ma
- Discussion
and remarks: The base of this zone was clearly defined at Sites
1095 and 1096 with the first occurrence of F. interfrigidaria.
The base of Hole 1101A ended within this zone; F. interfrigidaria
was included in the assemblage through the deepest sample. The FO of T.
insigna, a datum occurring within this zone, was observed in Hole
1101A. This same species datum also occurred at Site 1096, but not at Site
1095. The FO of Thalassiosira lentiginosa
was seen only at Site 1095.
Fragilariopsis
barronii Partial Range Zone
- Author:
Weaver and Gombos (1981), renamed by Gersonde and Burckle (1990)
- Definition of
top: FO of Fragilariopsis interfrigidaria
- Definition of
base: FO of Fragilariopsis barronii
- Age:
3.8-4.46 Ma
- Discussion
and remarks: The
first occurrence of F. barronii
was observed at both Sites 1095 and 1096. Each site also had an additional
datum from within this zone. The FO of Thalassiosira striata
was noted at Site 1095 and the LO of Fragilariopsis
praeinterfrigidaria
at Site 1096.
Thalassiosira
inura Partial Range Zone
- Author:
Gersonde and Burckle (1990)
- Definition of
top: FO of Fragilariopsis barronii
- Definition of
base: FO of Thalassiosira inura
- Age:
4.46-4.89 Ma
- Discussion
and remarks: The base of this zone occurs near the bottom of Hole
1096C. The FO datum of T. inura, was observed at both Sites 1095
and 1096. The FO datums of T. fasciculata and T. complicata
also occur within this zone at Site 1095.
Thalassiosira
oestrupii Partial Range Zone
- Author:
Baldauf and Barron (1991)
- Definition of
top: FO of Thalassiosira inura
- Definition of
base: FO of Thalassiosira oestrupii
- Age:
4.89-5.59 Ma
- Discussion
and remarks: Three datums within this zone were observed at Site
1095: the FOs of T. oestrupii, F. praeinterfrigidaria, and Thalassiosira
tetraoestrupii.
Nitzschia
reinholdii Partial Range Zone
- Author:
Harwood and Maruyama (1992), modified in this study
- Definition of
top: FO of Thalassiosira oestrupii
- Definition of
base: FO of Thalassiosira oliverana
- Age:
5.59-6.42 Ma
- Discussion
and remarks: The FO of Thalassiosira oliverana was
substituted for the FO of Thalassiosira miocenica (the species
used in the original zonal definition) (Harwood and Maruyama, 1992). The
stratigraphic age of this boundary is not compromised with this
substitution, as their previously observed ages are very close. Even
though this change was made to facilitate zonal determination, the FO of T.
oliverana was not observed at Site 1095. The LO of A. ingens
var. ovalis was observed in this zone in Hole 1095B, which is
well constrained above and below by several paleomagnetic reversals.
Actinocyclus
ingens var. ovalis Partial Range Zone
- Author:
Harwood and Maruyama (1992)
- Definition of
top: FO of Thalassiosira oliverana
- Definition of
base: FO of Actinocyclus ingens var. ovalis
- Age: 6.42-8.68
- Discussion
and remarks: The FO of A. ingens var. ovalis
marks the base of this zone and occurs in Hole 1095B, bounded by
paleomagnetic reversals that are interpreted with the ages of 6.57 and
6.94 Ma. The LO of Thalassiosira mahoodii occurs upcore from A.
ingens var. ovalis, and is also well constrained with
paleomagnetic ages.
Thalassiosira
torokina Partial Range Zone
- Author:
Baldauf and Barron (1991), modified in Harwood and Maruyama (1992)
- Definition of
top: FO of Actinocyclus ingens var. ovalis
- Definition of
base: FO of Thalassiosira torokina
- Age: 8.68-8.97
Ma
- Discussion
and remarks: This zone is the lowermost one in Hole 1095B. There
are three diatom datums and three paleomagnetic reversals within this
zone. The diatom datums, in order of increasing age, are the FO of T.
mahoodii, the LO of Denticulopsis crassa, and the FO of T.
torokina.
