Marine Micropaleontology 40 (2000) 67–81
www.elsevier.nl/locate/marmicro
Biochronology and evolutionary implications of Late Neogene
California margin planktonic foraminiferal events
M. Kucera*, J.P. Kennett
Department of Geological Sciences and Marine Science Institute, University of California, Santa Barbara, CA 93106, USA
Received 2 December 1999; accepted 27 January 2000
Abstract
The biochronology of eight events (first or last occurrences) among species of the planktonic foraminifer Neogloboquadrina
plexus have been examined in six Pliocene to Quaternary deep-sea sequences drilled during ODP Leg 167 off the coast of
California. The sites form a meridional transect along the California margin from 31⬚N to 41⬚N, covering an area under the
direct influence of the California Current and associated upwelling system. Using age models based on combination of
calcareous nannofossil and radiolarian data and magnetostratigraphic chron boundaries, ages were assigned to the events,
thus allowing investigations of their spatial and temporal distribution. Three of the events have been identified as clearly
diachronous; two of these seem to represent examples of latitudinal immigrations. The evolutionary activity in the neogloboquadrinid clade on the California margin appears centered around 2.1 Ma, with most of the events occurring between 2.5 and
1 Ma. This interval was a time of major climate change. No events have occurred after 0.7 Ma supporting the notion that the
extreme climate variations of the Late Pleistocene inhibited speciation. Assignment of ages to events defining boundaries of the
California margin (CM) zones of Kennett and others has enabled us to compare this zonation with the standard planktonic
foraminiferal zonal schemes. Seven of the neogloboquadrinid events occurred during the last 2.5 myr, suggesting that the time
resolution of the CM zones (including the coiling dominance zones of Lagoe and Thompson) during this interval is at least
twice that of the standard planktonic foraminiferal zonation. The remaining Pliocene is divided only into two CM zones
compared to six standard planktonic foraminiferal zones. One of the Pleistocene neogloboquadrinids is formally described
as a new species. 䉷 2000 Elsevier Science Ltd. All rights reserved.
Keywords: planktonic foraminifera; biochronology; evolution; Pliocene; Pleistocene; northeast Pacific
1. Introduction
Correlating Late Neogene deep-sea sedimentary
sequences of the California margin with standard
biostratigraphic zonal schemes has remained a major
challenge. During the last 5 myr the area has been
persistently influenced by the California Current and
* Corresponding author. Tel.: ⫹ 1-805-893-3103; fax: ⫹ 1-805893-2314.
E-mail address: [email protected] (M. Kucera).
associated upwelling (e.g. Ingle, 1967; Ravelo et al.,
1997), creating a unique environmental setting with
planktonic communities made up of unusual combinations of taxa.
Earlier studies of sediments from this region (Ingle,
1967) indicated that the composition of planktonic
foraminifer faunas in these sediments differs from
assemblages found in open-ocean regions at similar
latitudes. The absence of many low-latitude taxa used
as markers of zonal boundaries, and the presence of
taxa endemic for this cold-water, high-productivity
0377-8398/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved.
PII: S0377-839 8(00)00029-3
68
M. Kucera, J.P. Kennett / Marine Micropaleontology 40 (2000) 67–81
Fig. 1. Location of the six sites along the California margin drilled during ODP Leg 167. Data from these sites were used to investigate the
biochronology of the eight late Neogene planktonic foraminiferal events. Also shown is the general pattern of surface water circulation along the
margin.
environment preclude the use of standard biostratigraphic
schemes based on planktonic foraminiferal events.
The continuous effort to develop a viable planktonic foraminifer biostratigraphy for the California
margin has been marked by shift of interest towards
the use of species and morphotypes of a group of
planktonic foraminifera typical of the California
margin deposits—the neogloboquadrinids. Pioneered
by Ingle (1973) and Keller (1978a,b), this approach
led Lagoe and Thompson (1988) to the first compre-
hensive compilation of planktonic foraminiferal data
in California margin sediments and the establishment
of a new zonation based on shifts in the dominant
coiling direction of Neogloboquadrina pachyderma.
Further developments of any biostratigraphic
framework of general validity for the area have been
hampered by the paucity of suitable sections. Before
1995, only a handful of scattered DSDP and ODP
Holes had been drilled in the area. Land outcrops,
such as those studied by Lagoe and Thompson
M. Kucera, J.P. Kennett / Marine Micropaleontology 40 (2000) 67–81
69
Table 1
ODP Leg 167 drill sites used in this paper. These sites were used to develop the California margin planktonic foraminiferal zonation of Kennett
et al. (2000)
Site (hole)
Location
Latitude
Longitude
Water depth
(m)
Distance from shore
(km)
1011 (B)
1012 (A)
1013 (A, B)
1014 (A)
1018 (A)
1020 (B)
Animal Basin
East Cortes Basin
San Nicolas Basin
Tanner Basin
Guide Seamount
Eastern flank of Gorda Ridge
31⬚16.8 0 N
32⬚17.0 0 N
32⬚48.0 0 N
32⬚50.0 0 N
36⬚59.3 0 N
41⬚00.1 0 N
117⬚38.0 0 W
118⬚23.0 0 W
118⬚53.9 0 W
119⬚58.9 0 W
123⬚16.7 0 W
126⬚26.1 0 W
2033
1783
1575
1177
2476
3050
85
105
115
155
76
167
(1988), are limited and often discontinuous. More
importantly, the land-exposed sediments were deposited in relatively nearshore environments and may not
completely reflect the general succession of faunas in
the entire California Current system.
Ocean Drilling Program Leg 167 for the first time
provided a meridional transect of offshore sequences
with apparently continuous sedimentation and good
preservation of planktonic foraminifera (Lyle et al.,
1997). Consequently, six of the ODP Leg 167 holes
(Fig. 1, Table 1), covering the entire length of the
California margin, were used by Kennett et al.
(2000) to develop for this area the first comprehensive
Late Neogene planktonic foraminiferal zonation. The
zonation comprises a sequence of eight zones; all but
one based on evolutionary changes within the Neogloboquadrina plexus (Fig. 2). While most other planktonic foraminiferal species occur throughout the entire
studied interval without notable changes in
morphology, species and morphotypes of the Neogloboquadrina plexus provide a suite of evolutionary
events recognizable throughout the region.
This contribution places the new planktonic foraminiferal zonation of Kennett et al. (2000) within a
chronological framework. Age models, independent
of planktonic foraminiferal data, for all the six ODP
holes (Lyle et al., 1997; Fornaciari, 2000; Koizumi et
al., 2000) were used to assign ages to eight planktonic
foraminiferal events which define boundaries of either
the California margin (CM) zones of Kennett et al.
(2000) or the coiling dominance (CD) zones of Lagoe
and Thompson (1988). In doing so, we were able to
explore the biochronology of these events and
correlate the new California margin zonation
with standard biostratigraphic and chronological
schemes.
2. Material and methods
Our study is based on a compilation of data on the
positions of eight planktonic foraminiferal events in
Late Neogene sequences from six sites drilled during
the Ocean Drilling Program Leg 167 (Table 1, Fig. 1).
The positions of these events were taken from Kennett
et al. (2000), with minor modifications for Site 1014.
To assign ages to individual events, the age models
constructed for all sites by Koizumi et al. (2000) have
been used. These age models (Table 2) are based on
the combination of magnetostratigraphy and biostratigraphy, the latter including calcareous nannofossil
(Fornaciari, 2000) and diatom (Maruyama, 2000)
data. In addition, we have also used one radiolarian
event at Site 1018 (Lyle et al., 1997). The LO of
Discoaster pentaradiatus was not included in our
calculations; its age is almost identical to the LO of
D. surculus, which is apparently less diachronous
(Fornaciari, 2000).
The composite age models constructed by Koizumi
et al. (2000) did not reveal any unconformities in the
late Neogene sections from these sites. Thus, the ages
of individual samples were calculated by means of
linear interpolation between the nearest lower and
higher age-model control points (Table 2). To demonstrate the precision of identification of each of the
events, we have calculated ages of the two samples
between each particular event. The age of every event
has been defined as the average of the ages of these
two samples. One half of the age difference between
the two samples has been used as a measure of the
time resolution.
The recognition of the Neogloboquadrina pachyderma coiling dominance (CD) zones of Lagoe and
Thompson (1988) in the studied sites has been accom-
70
Event a
Age
(Ma)
LO P. lacunosa (N)
Brunhes (o)
Jaramillo (t)
Jaramillo (o)
Cobb Mountain (t)
Cobb Mountain (o)
LO large Gephyrocapsa spp. (N)
FO large Gephyrocapsa spp. (N)
FO G. oceanicas.l. (N)
Olduvai (t)
Olduvai (o)
LO D. surculus (N)
LO D. tamalis (N)
LO A. pliocenica (R)
LO R. pseudoumbilicus (N)
0.46
0.78
0.99
1.07
1.20
1.21
1.24
1.46
1.69
1.77
1.95
2.53
2.97
3.36
3.82
1011 B
Depth (mcd) b
21.98
1012 A
Depth (mcd)
102.45
108.45
37.08
61.85
75.95
83.18
87.64
89.75
90.97
105.68
114.62
124.57
132.57
183.03
204.32
147.45
269.46
60.63
1013 A,B
Depth (mcd)
13.57
56.64
65.97
69.27
76.37
78.65
84.41
(98.99) c
92.44
104.81
132.12
138.33
1014 A
Depth (mcd)
1018 A
Depth (mcd)
1020 B
Depth (mcd)
39.21
65.66
81.58
87.62
96.78
116.24
49.98
84.83
100.68
108.81
95.83
113.92
123.08
177.57
193.18
224.97
123.51
148.17
165.60
201.34
249.77
341.58
376.99
393.19
242.43
255.76
332.41
289.07
For biostratigraphic data, LO ˆ last occurrence; FO ˆ first occurrence: Calcareous nannofossil events are marked (N), ages and position of these events were taken from
Fornaciari (2000). Ages specific for California margin were used wherever the magnetostratigraphy-calibrated age reported by Fornaciari (2000) deviated from the standard age for
the event by more than 100 kyr. The radiolarian event, marked (R), and its age were taken from Lyle et al. (1997). For paleomagnetic chron boundaries, o refers to onset, t to
termination (from Koizumi et al., 2000).
b
Meters composite depth (see Lyle et al., 1997).
c
This event was not used for the age model, since it has been recognized in the core with an uncertainty of 12 m, far higher than that of the other events (Fornaciari, 2000).
a
M. Kucera, J.P. Kennett / Marine Micropaleontology 40 (2000) 67–81
Table 2
Control points used to construct age models for the investigated sites. Magnetostratigraphic horizons identified in the studied cores are shown in bold typefaces
M. Kucera, J.P. Kennett / Marine Micropaleontology 40 (2000) 67–81
71
Table 3
Summary of estimated ages for planktonic foraminiferal events in California margin sediments
Event
LO N. inglei n.sp.
FO N. sp. B
FO N. inglei n.sp.
LO N. sp. C
LO N. kagaensis
LO N. asanoi
FO N. sp. C
FO N. asanoi
Zone a
CM1
CM2
CM3
CD 10
CD 11
CM4
CM5
CD 14
CM6
Diachrony b
Age (Ma)
Mean
Res d
Min
Max
Range
Sd
0.712
1.232
1.543
1.881
1.915
2.216
2.362
3.583
0.021
0.018
0.023
0.021
0.019
0.015
0.017
0.034
0.569
1.163
1.240
1.704
1.813
1.980
2.230
3.284
0.774
1.278
1.945
1.984
2.003
2.390
2.475
3.897
0.205
0.115
0.705
0.280
0.190
0.410
0.245
0.613
0.077
0.039
0.270
0.096
0.063
0.164
0.089
0.253
Latitudinal
?
Incoherent
?
Latitudinal
Previous estimate (Ma) c
–
1.30
–
1.80
1.60
1.85
2.50
–
a
Events define bases of the zones. CM refers to California margin planktonic foraminiferal zones by Kennett et al. (2000); CD refers to N.
pachyderma coiling dominance zones by Lagoe and Thompson (1988).
b
Following the nomenclature of Spencer-Cervato et al. (1994).
c
From Lagoe and Thompson (1988).
d
Average time-resolution due to sampling frequency. Calculated as one half of the difference in ages of the two samples between which the
event occurred averaged for the six holes.
plished by identification of two intervals in the cores
with virtually no sinistral specimens of this form.
These two intervals correspond to Zones CD15 and
CD11 of Lagoe and Thompson (1988) (see also
Table 3). Zones based on relatively short-lived
changes in the preferred coiling direction, such as
Zones CD12-14 and CD1-10 have not been differentiated in this study. The stratigraphic value of these
zones is not clear. Sinistrally coiled N. pachyderma
appear to have been migrating substantially within the
California Current system in response to millennialscale climate oscillations during at least the last 60 kyr
(e.g. Hendy and Kennett, 1999), i.e. on a time-scale
much finer that the CD zonation would suggest.
3. Taxonomy
Ever since the original establishment of the genus
Neogloboquadrina by Bandy et al. (1967), its basic
taxonomic concept as well as the phylogenetic relationships among its species remained unclear. The
lack of diagnostic features on shells of this group
of planktonic foraminifera has further contributed
to difficulties in definition of individual species
attributed to the genus. Thus, with the exception of
the multichambered forms in the Neogloboquadrina
dutertrei lineage, all Pliocene and Pleistocene neogloboquadrinids in northern Pacific sediments have been
originally lumped into a single species—Neogloboquadrina pachyderma (Ingle, 1967, 1973; Bandy, 1972).
The significant morphological variability within
this cumulative taxon was first systematically
exploited by Mayia et al. (1976) and Keller (1978a).
Based on material from land sections in Japan, the
former work established two new species, Neogloboquadrina asanoi and Neogloboquadrina kagaensis,
which could be primarily differentiated from Neogloboquadrina pachyderma by having substantially
larger shells (Mayia et al., 1976). These two species,
limited in their distribution to late Pliocene and early
Pleistocene, approximately correspond to Keller’s
(1978a) N. pachyderma form 3. Lagoe and Thompson
(1988) for the first time recognized the presence of
both species in California margin sediments. These
authors also provided the first account on the phylogenetic relationship among these two forms, considering N. kagaensis the ancestor of N. asanoi. This
concept was adopted by Kennett et al. (2000), who
found further evidence for such phylogenetic relationship (see Fig. 2). However, Kennett et al. (2000) noted
a presence of another “large neogloboquadrinid”
species—N. pachyderma A, with a short stratigraphic
range in the early late Pleistocene. This species is
stratigraphically isolated from N. asanoi and N.
kagaensis, and is slightly different in general
morphology. It was probably included in N. pachyderma form 2 of Keller (1978a). Following the taxo-
72
M. Kucera, J.P. Kennett / Marine Micropaleontology 40 (2000) 67–81
Fig. 2. Schematic sketch of the distribution and relationships among the members of the neogloboquadrinid plexus, as seen in California margin
sediments. Question marks indicate uncertainties in evolutionary relationships. The nomenclature as well as the definition of the California
margin (CM) zones follows Kennett et al. (2000). Arrows indicate events used in the CM zonation. The ranges of individual species correspond
to those observed at Site 1014. SEM illustrations of typical specimens (all from Site 1014) are shown on the same scale (× 45).
nomic concept of N. pachyderma A of Kennett et al.
(2000), this species is formally described in this
contribution as Neogloboquadrina inglei n.sp. (see
Appendix A).
Unlike the “large neogloboquadrinids” which
appear to have predominantly dextrally coiled
shells, the coiling directions of the smaller morphospecies, Neogloboquadrina pachyderma, is more
variable. The polar and subpolar waters of modern
oceans are dominated by sinistrally coiled populations of N. pachyderma, while dextrally coiled
populations of the species inhabit temperate waters
M. Kucera, J.P. Kennett / Marine Micropaleontology 40 (2000) 67–81
73
Fig. 3. Positions of the eight neogloboquadrinid events in the six investigated sites. The sites are ordered from left to right according to
increasing latitude (Table 1). The shaded area in each plot depicts a 200 kyr interval centered on the mean age of the event for all sites (marked
by solid line). Arrows indicate latitudinal gradients in the ages of two of the events.
74
M. Kucera, J.P. Kennett / Marine Micropaleontology 40 (2000) 67–81
sinistral populations by an interval of about 0.4 myr
duration (Table 3) when only dextrally coiled N.
pachyderma occurred. In addition, the two populations differ in many respects by the forms of their
shells. This difference, first reported by Olson
(1974), was noted by Kennett et al. (2000) and used
to distinguish the Pliocene sinistral N. pachyderma C
from the late Pleistocene sinistral N. pachyderma B
(Fig. 2). Thus, the Pliocene species is almost certainly
different from the form occurring in the late Pleistocene. This clearly casts doubts on the use of the coiling ratio in “N. pachyderma” as a climatic signal
beyond the late Pleistocene.
4. Results
Fig. 4. Intersite correlation of the California margin (CM) zones of
Kennett et al. (2000) and Coiling Dominance (CD) zones of Lagoe
and Thompson (1988). The ages of the zonal boundaries are from
Fig. 3. The sites are ordered from left to right according to increasing latitude (Table 1).
(see Bandy, 1972; Kennett, 1976 for review). Thus,
coiling ratio of N. pachyderma has been extensively
exploited to trace sea-surface temperature variations
in the Late Neogene (Bandy, 1972; Kennett, 1976;
Keller, 1978b). Until recently, however, the exact
meaning of the variation in the coiling direction of
this species remained unclear. Although numerous
earlier studies have indicated differences between
the two coiling types (e.g. Reynolds and Thunell,
1986), Darling et al. (1998), using DNA analyses,
first provided conclusive evidence that the two
modern coiling varieties represent two distinct
genetic types. Thus, coiling direction differences in
modern N. pachyderma seem to reflect phylogenetic
relations rather than being a manifestation of
ecophenotypic variability. This implies that the
discontinuous occurrence of the sinistrally coiled
N. pachyderma through the Pliocene and Quaternary
(Fig. 2) must be interpreted in terms of repeated
extinction and origination.
Pliocene sinistral Neogloboquadrina pachyderma
populations are separated from the late Pleistocene
Ages of the two samples defining each event at all
six holes are shown in Fig. 3. On average, all events
were located in the holes with a time resolution
between 15 and 23 kyr, with the exception of the
FO of Neogloboquadrina asanoi where the average
time resolution was only ^34 kyr (Table 3). In four
cases the calculated ages deviate from previous estimates by Lagoe and Thompson (1988) by as much as
0.15–0.4 myr (Table 3). Based on the distribution of
their ages throughout the region, the events can be
divided into two groups. Five of the events seem to
have occurred within less than 0.28 myr along the
entire California margin, while the ages of three
events (FO and LO of N. asanoi, and FO of
Neogloboquadrina inglei n.sp. ( ˆ N. sp. A of Kennett
et al., 2000)) varied across the six holes by some 0.4–
0.7 myr (Fig. 3, Table 3).
In some cases, the distribution of the ages of the
events shows a strong correlation to latitude. Thus,
Neogloboquadrina inglei n.sp. clearly appeared first
in the northernmost site 1020 and last in the southernmost site 1011 (Fig. 3). Neogloboquadrina asanoi, on
the other hand, clearly first appeared in the southern
sites 1011 and 1012, compared to its much later first
appearance in the remaining sites (Fig. 3). Similar
patterns, although on a much finer time-scale and in
a much less pronounced manner can be seen in the LO
of N. pachyderma (sin) C (increase in ages of the
event from south to north) and in the LO of Neogloboquadrina kagaensis (increase in ages from north to
south) (Fig. 3, Table 3).
M. Kucera, J.P. Kennett / Marine Micropaleontology 40 (2000) 67–81
75
Fig. 5. Comparison of late Neogene California margin planktonic foraminiferal zonations with the standard zonal scheme of Berggren et al.
(1995). Oblique lines in the scheme indicate diachrony.
The California Margin zonation of Kennett et al.
(2000) was established without detailed knowledge
of the chronology of the individual neogloboquadrinid
data. The establishment of such chronology in this
study is significant for the validation of the original
zonal scheme. Thus, the apparent time-transgressive
nature of FO of Neogloboquadrina inglei n.sp. indicates that Zone CM3 is a northern equivalent of Zone
CM4 (Fig. 4). Similarly, the diachrony observed in
both FO and LO of Neogloboquadrina asanoi
implies that Zone CM5 is chronologically less useful
than the approximately coeval Zone CD12-14 of
Lagoe and Thompson (1988). Also, the boundary
between zones CM7 and CM6 corresponds only
generally with the Early–Late Pliocene transition
(Figs. 4 and 5). Seven of the events occurred during
the last 2.5 myr, suggesting that the time resolution
of the CM and CD zones during this interval is at
least twice that of the standard (low-latitude) planktonic foraminiferal zonation (Berggren et al., 1995)
(Fig. 5).
5. Discussion
5.1. The quality of age models
The quality of the age models used to produce the
biochronology of the neogloboquadrinid events is
essential for the accuracy of the biochronology and ultimately for the overall validity of any conclusions drawn
from this data set. The models may be considered fairly
reliable in the interval between 0.7 and 1.2 Ma, where
magnetostratigraphic data are available for most
sections (Table 2). The age of the highest event in
Hole 1011B, LO Neogloboquadrina inglei n.sp.,
appears to be significantly younger than in the other
sections (Fig. 3). This ⬃150 kyr difference may be
caused by the lack of magnetostratigraphic control for
Hole 1011B, together with only few biostratigraphic
data being recognized in this part of the section (Table
2). The same may apply for the explanation of the
slightly younger age of the LO of Neogloboquadrina
pachyderma (sin) C observed in Hole 1018A (Fig. 3).
76
M. Kucera, J.P. Kennett / Marine Micropaleontology 40 (2000) 67–81
Fig. 6. Temporal distribution of the eight California margin neogloboquadrinid events. The data were generated by moving a 500 kyr interval
across the last 5 myr in 100 kyr steps. For each step, the number of events that fell within the 500 kyr interval was recorded. The right panel
shows benthic foraminiferal oxygen isotope record combined from data from ODP sites 677 and 846, and Core V1930 (Shackleton, 1995). This
record reflects the evolution of climate variability as the global cryosphere expanded. Vertical bars indicate intervals dominated by either 41 kyr
(obliquity) or 100 kyr (eccentricity) climate cycles. Also shown is the period of high mass accumulation rate (MAR) of CaCO3 along the
California margin, caused by higher primary productivity (Ravelo et al., 1997).
Age determinations for the interval before
1.2 Ma rely solely on biostratigraphic data, with
the exception of two sites (1012 and 1013),
where the Olduvai chron (1.77–1.95 Ma; Berggren
et al., 1995) has been identified (Table 2). Thus, it
may be argued that the observed latitudinal
diachrony (⬃200 kyr) of LO N. kagaensis and
FO Neogloboquadrina pachyderma (sin) C (Fig.
3, Table 3), may be an artifact of diachrony
amongst nannofossil events upon which the age
models for this part of the Pliocene rely. On the
other hand, the age differences across the sites for
FO and LO of Neogloboquadrina asanoi are so
large that they cannot be explained solely by
imperfections of the age models. The nannofossil
events used to construct the age models for all
holes have been calibrated to magnetostratigraphic
data at sites 1010 and 1021 (Fornaciari, 2000),
and are clearly synchronous along the California
margin.
5.2. Diachronous events
By definition, every evolutionary event must be
diachronous. If the resolution of biochronological
studies were unlimited, diachrony would be the
pattern observed for every evolutionary event. What
would differ, however, are the magnitude and the
spatial pattern of the diachrony. The notion of
synchronous versus diachronous events commonly
applied in biostratigraphy needs to be viewed merely
as a matter of practice which has proven useful in
classification of events on the basis of their suitability
for biostratigraphic correlation. An event is
M. Kucera, J.P. Kennett / Marine Micropaleontology 40 (2000) 67–81
considered synchronous if the differences within a
region in its timing overlap with the sampling resolution and/or are within the assumed error of the age
models employed. Thus, our study has revealed that
three (i.e. 38%) of the California margin neogloboquadrinid events are clearly diachronous (Fig. 3,
Table 3). The percentage of synchronous vs. diachronous events thus seems to be higher when compared
with the results of the global survey by SpencerCervato et al. (1994) who found 60% diachrony
among investigated Neogene planktonic foraminiferal
events. This difference may be explained by the much
smaller geographical scope of our study.
Spencer-Cervato et al. (1994) calculated the standard deviation for Neogene planktonic foraminiferal
event ages as being 880 kyr for FOs and 680 kyr for
LOs; in addition, they have noted that the most
common pattern in age distribution for these events
is that of geographical (hemispherical or latitudinal)
diachrony. The same pattern can be observed in the
FO of Neogloboquadrina inglei n.sp and FO of
Neogloboquadrina asanoi (Fig. 3). The diachrony of
such events can be explained either by gradual migration due to changing environment or gradual evolutionary adaptation (see also Spencer-Cervato et al.,
1994). Irrespective of the mechanisms that caused
migration of these two species, the direction of the
migrations provides useful criteria as to their ecology.
N. asanoi apparently entered the California margin
from the south and reached the northernmost sites of
the studied transect some 700 kyr later. Thus, the
species may be viewed as adapted to conditions
originally prevailing in lower latitudes of the Eastern
Pacific. Similarly, N. inglei n.sp. entered the region
from the north and was therefore initially adapted to
environment associated with the northern part of the
California Current System.
The incoherent pattern of diachrony (using the terminology of Spencer-Cervato et al. (1994)) observed in LO
of Neogloboquadrina asanoi is more difficult to explain.
Perhaps, it reflects some taxonomic bias in the identification of the species (N. asanoi is very similar to
Neogloboquadrina kagaensis), or the low abundance
of the species prior to its final extinction in the region.
5.3. Synchronous events
The majority of the studied events appear to be
77
synchronous given the sampling resolution and age
model limitations of this investigation (Fig. 3, Table
3). The apparent synchrony of these events may be
explained by rapid migrations of the species as a
response to abrupt environmental changes. Alternatively, the species may have evolved in the California
Current System, or be primarily adapted to conditions
prevailing in the region at one particular time. This
would allow them to rapidly inhabit the entire area
after initial immigration or evolution, or force them
to rapid retreat or extinction once environmental
conditions changed. At any rate, these events provide
useful biostratigraphic horizons that can be used for
robust stratigraphic correlation in California margin
sediments.
Whether these synchronous events reflect true
originations and extinctions of species or whether
they largely record their migrations into the Eastern
Pacific region is not clear. Circumstantial evidence,
such as that based on biostratigraphic data of Maiya et
al. (1976), Keller (1978a,b, 1979) and Lagoe and
Thompson (1988), suggest that at least some of
these data may be synchronous over a large part of
the northern Pacific and thus represent true evolutionary events. Nevertheless, the ages of these events and
observations related to their diachrony or synchrony
should be extrapolated with caution to areas outside
the California margin.
5.4. Evolutionary turnover
The neogloboquadrinid events are not distributed
evenly throughout the studied interval, implying that
the evolutionary turnover rate in the clade was not
constant with time (Fig. 6). The highest concentration
of events is found around 2.1 Ma, with elevated
numbers between 2.6 and 0.9 Ma (Fig. 6). Apparently,
the initial increase in the turnover rate is caused by
three LO and one FO events occurring between 2.4
and 1.9 Ma; it is then followed by two FO events
between 1.5 and 1.2 Ma (Fig. 6).
Although we are well aware that our limited database cannot be taken as a comprehensive representation of biotic changes in this region, the apparent
coincidence between the “neogloboquadrinid evolutionary burst” and major climatic changes is striking. The initial “burst” seems to have started shortly
after the north Pacific began to experience the
78
M. Kucera, J.P. Kennett / Marine Micropaleontology 40 (2000) 67–81
M. Kucera, J.P. Kennett / Marine Micropaleontology 40 (2000) 67–81
consequences of the intensification of the Northern
Hemisphere glaciation (Rea et al., 1993; Maslin et
al., 1995; Prueher and Rea, 1998). At the same time,
diatom floras of the north Pacific exhibited a series
of stepwise changes (Sancetta and Silvestri, 1986).
Possibly changes in climate state during the glacial
transition promoted evolution in the high-latitude
biota. Some evolutionary theories, such as the
“Plus ça change” model (Sheldon, 1996) predict
that evolution occurs during changes in the mode
of climate variability. Perhaps, the Pliocene neogloboquadrinids that became extinct shortly after the
major cooling step at 3–2.5 Ma (Fig. 6) included
species not adapted to the environment of glacial
oceans.
In the latest Quaternary, on the other hand, no
evolutionary activity within the California margin
members of the neogloboquadrinid plexus occurred
(Fig. 6). Similar observations have been made
among other groups of organisms providing further
evidence for the notion that broad segments of the
world’s biota are adapted to the large-magnitude
millennial climate shifts of the 100 kyr world
(Roy et al., 1996; Cannariato et al., 1999). Indeed,
the ability to migrate in the ocean in pace with the
rapid, large-magnitude climate shifts seems to be
the trademark of the modern high-latitude Pacific
neogloboquadrinids (e.g. Hendy and Kennett, 1999).
6. Conclusions
Using age models based on the combination of
calcareous nannofossil and radiolarian events and
magnetostratigraphic chron boundaries, ages were
assigned to eight neogloboquadrinid events (LOs or
FOs) in the six sites studied. In four cases the calculated ages deviate from previous estimates by Lagoe
and Thompson (1988) by as much as 0.15–0.4 myr.
Given the average sampling resolution of 15–
79
34 kyr, and the general quality of the employed age
models, three of the eight investigated California
margin neogloboquadrinid events were found clearly
diachronous. The magnitude of the diachrony varied
between 0.4 and 0.7 myr.
In two cases, the FO of Neogloboquadrina inglei
n.sp and FO of N. asanoi, the events seem to represent
examples of latitudinal immigrations, where the
species in question invaded the California margin
from the north or south, respectively. The diachrony
of the LO of N. asanoi does not show any coherent
spatial pattern.
The remaining six events occurred at all sites within
0.12–0.28 myr, and are thus considered synchronous
within to the resolution of this study. These events are
useful in correlating latest Neogene rocks of the California margin.
By assigning ages to events defining boundaries of
the California margin (CM) zones, we were able to
compare this zonation with the standard planktonic
foraminiferal zonal scheme of Berggren et al.
(1995). While the temporal resolution of CM zones
is almost twice that of the standard zonation for the
last 2 Myr, the remaining Pliocene is divided into only
two CM zones compared with six standard planktonic
foraminiferal zones.
Most of the investigated events occurred between
2.5 and 1 Ma. We suggest that the elevated evolutionary turnover among the neogloboquadrinids occurred
as a consequence of major changes in the mode of
climate variability associated with ice-age development at the end of the Pliocene. No events have
occurred after 0.7 Ma supporting the notion that the
extreme climatic variability of the past 1 myr inhibited speciation.
Acknowledgements
We thank Paola Kucera for help with age
Plate 1. Neogloboquadrina inglei n. sp. × 120 (Fig. 12 × 650). ODP Leg 167 Hole 1014 A, Figs. 1–3 and 5–8: 12X-02, 120 cm; Figs. 4 and 9–
13: 9X-06, 118 cm. 1: Specimen with shorter, more umbilical aperture. Umbilical view. 2: Kummerform specimen. Umbilical view. 3: Side
view of specimen with the typical rounded shape of axial periphery. 4: Paratype. Umbilical view. 5: Small (juvenile?) specimen with more
umbilical aperture. Umbilical view. 6: Small (juvenile?) specimen with 31/2 chambers in the last whorl. Umbilical view. 7: Spiral view. 8: Side
view of high-spired specimen. 9: Holotype. Umbilical view. 10: Specimen with less developed apertural lip. Umbilical view. 11: Small
(juvenile?) specimen with 31/2 chambers in the last whorl. Umbilical view. 12: Detail of the surface structure of N-1 chamber of the specimen
in Fig. 13. 13: Specimen almost lacking apertural lip. Umbilical view.
80
M. Kucera, J.P. Kennett / Marine Micropaleontology 40 (2000) 67–81
calculations and David Pierce for assistance with
SEM photographs. This research has been supported
by STINT (The Swedish Foundation for International
Cooperation in Research and Higher Education) postdoctoral fellowship to M.K. and an NSF grant (Earth
System History, OCE 9904024) to J.P.K. Samples
were provided through the assistance of the Ocean
Drilling Project.
Appendix A. Taxonomic appendix
Genus Neogloboquadrina Bandy, Frerichs, and
Vincent 1967
Neogloboquadrina inglei n. sp.
Plate I, Figs. 1–13
Neogloboquadrina pachyderma form 2, Keller
(1978a), Pl. 1, Fig. 2; Pl. 3, Figs. 1–9.
Neogloboquadrina pachyderma forma A, Matoba
and Oda (1982), Pl. 4, Figs. 11–13.
Neogloboquadrina pachyderma forma B, Matoba
and Oda (1982), Pl. 4, Figs. 17 and 18.
?non Neogloboquadrina pachyderma forma B,
Matoba and Oda (1982), Pl. 4, Figs. 14–16.
Neogloboquadrina pachyderma A, Kennett et al.
(2000), Pl. 1, Figs. 4, 5, 9, 10.
Holotype: Plate I, Fig. 9, ODP Hole 1014A, sample
9X-06 118 cm, mid Pleistocene.
Paratype: Plate I, Fig. 4, ODP Hole 1014A, sample
9X-06 118 cm, mid Pleistocene.
Repository: Cushman Collection, National
Museum of Natural History, Smithsonian Institution,
Washington, DC, USA. Catalogue numbers: USNM
509369 (holotype) and 509370 (paratype).
Type locality: ODP Hole 1014A, Tanner Basin,
northeastern Pacific.
Etymology: In honor of Prof James Ingle Jr., micropaleontologist and paleoceanographer at Stanford
University, CA.
Diagnosis: Test relatively large (0.2–0.4 mm in
diameter), almost exclusively dextrally coiled,
rounded, lobulate in both axial and equatorial peripheries. Four to three and a half globular chambers in the
last whorl, slowly expanding in size giving the test
somewhat quadrate appearance. Aperture narrow
umbilical–extraumbilical bordered with thin lip.
Surface ultrastructure crystalline with prominent
heavily calcified ridges surrounding pores.
Variability: Specimens of the species with less than
four chambers in the last whorl are rare, often including smaller individuals (Pl. I, Figs. 6, 11.) or kummerforms (Pl. I, Fig. 2). Rare sinistral variants have been
observed at Site 1014, as well as at Site 1018. Apertural lip may vary from distinct elevated rim (Pl. I,
Figs. 1, 3–4, 9) to only faintly represented feature on
the outer apertural face of the last chamber (Pl. I, Figs.
10, 13). Due to dissolution, some specimens appear to
show a relatively open umbilicus with broader, nearly
umbilical aperture.
Remarks: This species differs from Neogloboquadrina pachyderma primarily by its larger test, typical
rounded shape of the axial periphery and inflated
chambers. It is somewhat smaller than both N. asanoi
and N. kagaensis, from which it also differs by
narrower aperture and much slower rate of chamber
expansion in the last whorl.
Occurrence: Neogloboquadrina inglei n. sp. is
generally present in the early and middle Pleistocene
(1.5–0.7 Ma), although it seems to have originated in
the Late Pliocene (⬎1.9 Ma, Table 3). It has been
found in ODP Leg 167 sites along the California
margin (Kennett et al., 2000), in the Gulf of California
(Matoba and Oda, 1982), and in DSDP sites from the
northeastern Pacific (Keller, 1978a,b). It seems to
have been encountered in Japan, where it is referred
to as “small Neogloboquadrina asanoi” (Ibaraki, M.,
pers. comm., 1999).
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