Holocene paleoclimate change in the Antarctic Peninsula: evidence from the diatom, sedimentary and geochemical record F. Taylor a, * , J. Whitehead b , E. Domack a a Department of Geology, Hamilton College, Clinton, NY 13323, USA b Antarctic Co-operative Research Centre/Institute of Antarctic and Southern Ocean Studies, GPO Box 252-80, Hobart 7001, Tasmania, Australia Received 7 December 1999; revised 2 July 2000; accepted 10 July 2000 Abstract Holocene, marine deposition in Lallemand Fjord, Antarctic Peninsula, is reinterpreted using statistical analyses (cluster analysis, analysis of variance, nonmetric multidimensional scaling and multiple regression) to compare diatom assemblages and the primary sedimentological proxies. The assemblages have been deposited in a variable sea ice zone over the last ca. 10,500 yr BP in response to a climate change. In the Late Pleistocene/early Holocene (10,580–7890 yr BP), a sea ice diatom assemblage was deposited in the presence of a retreating ice shelf at the head of the fjord. In the mid Holocene (7890–3850 yr BP), an open water assemblage was deposited and sea ice cover was at a minimum. We associate the assemblage with climatic warming, which characterizes much of the Antarctic Peninsula during this time. A second sea ice assemblage, different from that deposited in the early Holocene, has been deposited in Lallemand Fjord since the late Holocene (,3850 yr BP). The assemblage reflects Neoglacial cooling, an increase in sea ice extent and/or an advance of the Mu ¨ ller Ice Shelf. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Holocene; Antarctica; paleoclimate; diatoms; sedimentology; statistical analysis 1. Introduction Sedimentary diatom assemblages have been used successfully in numerous studies as a proxy for Antarctic marine paleo-reconstructions (e.g. Truesdale and Kellogg, 1979; Pichon et al., 1987; Leventer et al., 1996; Cunningham et al., 1999). Many of these studies include statistical techniques to reconstruct Quaternary glacial history, but few incorporate a subjective, multi-disciplinary approach. In the present study, we incorporate diatom, sedimentological and geochemical data with classification and indirect ordination analyses to interpret the Holocene paleo- environment of Lallemand Fjord on the western Antarctic Peninsula. Climate records from both Hemispheres demonstrate increasingly that the Holocene (,11,500 yr BP, after Roberts, 1998) has been a period of rapid and variable climate change (Domack and Mayewski, 1999; Rosqvist et al., 1999). Marine sediment cores from the Antarctic Peninsula revealed multi-century and millennial-scale variations in primary production (Leventer et al., 1996; Shevenell et al., 1996; Marine Micropaleontology 41 (2001) 25–43 0377-8398/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0377-8398(00)00049-9 www.elsevier.nl/locate/marmicro * Corresponding address. Antarctic Co-operative Research Centre/Institute of Antarctic and Southern Ocean Studies, GPO Box 252-80, Hobart 7001, Tasmania, Australia. Tel.: 161-3- 6226-7888; fax: 161-3-6226-2973. E-mail addresses: fi[email protected] (F. Taylor), [email protected] (J. Whitehead), [email protected] (E. Domack).
19
Embed
Holocene paleoclimate change in the Antarctic Peninsula ... · from the diatom, sedimentary and geochemical record ... bAntarctic Co-operative Research Centre/Institute of Antarctic
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Holocene paleoclimate change in the Antarctic Peninsula: evidencefrom the diatom, sedimentary and geochemical record
F. Taylora,*, J. Whiteheadb, E. Domacka
aDepartment of Geology, Hamilton College, Clinton, NY 13323, USAbAntarctic Co-operative Research Centre/Institute of Antarctic and Southern Ocean Studies, GPO Box 252-80, Hobart 7001,
Tasmania, Australia
Received 7 December 1999; revised 2 July 2000; accepted 10 July 2000
Abstract
Holocene, marine deposition in Lallemand Fjord, Antarctic Peninsula, is reinterpreted using statistical analyses (cluster
analysis, analysis of variance, nonmetric multidimensional scaling and multiple regression) to compare diatom assemblages
and the primary sedimentological proxies. The assemblages have been deposited in a variable sea ice zone over the last ca.
10,500 yr BP in response to a climate change. In the Late Pleistocene/early Holocene (10,580±7890 yr BP), a sea ice diatom
assemblage was deposited in the presence of a retreating ice shelf at the head of the fjord. In the mid Holocene (7890±3850 yr
BP), an open water assemblage was deposited and sea ice cover was at a minimum. We associate the assemblage with climatic
warming, which characterizes much of the Antarctic Peninsula during this time. A second sea ice assemblage, different from
that deposited in the early Holocene, has been deposited in Lallemand Fjord since the late Holocene (,3850 yr BP). The
assemblage re¯ects Neoglacial cooling, an increase in sea ice extent and/or an advance of the MuÈller Ice Shelf. q 2001 Elsevier
Fig. 3. Dendrogram illustrating sample af®nities, based on diatom abundance. Cluster group 1� dense sea ice assemblage; cluster group 2�seasonally open water assemblage; cluster group 3� loose sea ice assemblage.
00.
10.
20.
30.
4
Axis
Str
ess
1 432
Fig. 4. Nonmetric multidimensional scaling (NMDS) ordination axis versus stress. Two axes were selected as best ®tting the data, based on the
point of maximum change in direction of the curve (from Kruskal and Wish, 1978).
taxa are present: Actinocyclus actinochilus, Fragi-
laria spp. and F. vanheurckii.
Two NMDS ordination axes were chosen as best
summarizing the data (Fig. 4). Stress values
converged after 10 iterations at a value of 0.1202,
indicating a good ®t with the original data (Hosie,
1994). The NMDS results are illustrated in Fig. 5.
There is good agreement between NMDS and cluster
analysis, visible when the cluster groups are circled on
the NMDS plot (Fig. 5). Using the ordination scores,
multiple regression analysis compared the diatom data
with the eight core variables. Three variables are
signi®cantly correlated with the data (Table 2), and
the direction of maximum correlation for each (Table
3) is illustrated in Fig. 5. (In Fig. 5, arrow length
represents the signi®cance of the correlation between
the diatom data and core variable, i.e. the longer the
arrow the greater the correlation with that variable,
and arrow direction indicates the direction in which
the variable is most correlated to the data.) Percent
TOC accounts for 54.9% of the variance observed in
the data; MS for 38.6% of the variance, and percent
®ne-medium silt for 30.7% of the variance. d 13C is
slightly less signi®cant, accounting for 22.3% of the
variance. The signi®cant variables and cluster groups
2 and 3 are clearly separated. Arrow length and
direction (Fig. 5) indicate the direction of maximum
correlation between the core variables and cluster
groups. Cluster group 2 is closely associated with
high TOC values, a high abundance of ®ne-medium
F. Taylor et al. / Marine Micropaleontology 41 (2001) 25±4332
Fig. 5. Ordination plot of samples, based on diatom species abundance (%). Arrows indicate direction of maximum correlation for signi®cant
multiple regressions between ordination scores and core variables. Cluster groups identi®ed in Fig. 3 are superimposed. Cr silt� coarse silt;
FM silt� ®ne-medium silt.
Table 2
Multiple regression analysis between dependent variables (physical
properties) and NMDS scores for a two-dimensional ordination.
Degrees of freedom� 2.48. ANOVA P values as for Table 1. Radj2 �
adjusted coef®cient of determination, which gives the fraction of
variance accounted for by the explanatory variable (Jongman et al.,
1987)
Dependent
variable
Direction cosine Radj2 F P
x y
d13C 1.778 21.213 0.223 8.190 **
MS 31.048 390.505 0.386 17.331 ***
TOC 0.740 20.420 0.549 33.247 ***
Clay 5.480 7.633 0.059 2.655 *
FM Silt 21.230 214.868 0.307 12.719 ***
Coarse silt 1.603 5.389 0.058 2.644 *
Sand 25.853 1.846 0.003 1.072 n.s.
Mean Grain Size 28.017 0.749 0.006 1.153 n.s.
silt and high d 13C; group 3 is closely associated with
high MS (Fig. 5). There is little or no signi®cant
association between the cluster groups and percent
clay, coarse silt, sand or mean grain size (Table 2).
5. Discussion
5.1. Chaetoceros abundance
Chaetoceros spores are the dominant taxa in GC1,
forming .50% of frustules counted in all samples
(Fig. 2). Abundance is high, but variable, in the
upper 520 cm of the core, and ranges from 66.8 to
95.1%. High concentrations of Chaetoceros spores
in Antarctic sediment are considered to be indicative
of high primary production in the water column
(Donegan and Schrader, 1982; Leventer, 1992;
Leventer et al., 1996). During spring diatom blooms,
surface waters can become so nutrient-depleted that
diatom growth is limited (Nelson and Smith, 1986;
McMinn et al., 1995) and spore formation is induced
(Davis et al., 1980). They remain dormant at the
sediment±water interface until favorable conditions
induce germination. Chaetoceros spore abundance
decreases relative to other diatom taxa below
520 cm, reaching a minimum of 53.1%. The decrease
suggests reduced primary production. The abundance
of Chaetoceros spores may also have been diluted by
higher siliciclastic deposition, although this is less
likely as there is no increase in gravel abundance in
this section of the core (Shevenell et al., 1996).
5.2. Diatom assemblages
Excluding Chaetoceros spores, cluster analysis and
NMDS identify three cluster groups (representing
diatom assemblages). The assemblages are interpreted
to have been deposited within the complex SIZ, where
studies have indicated that the different sea ice types
contain different algal assemblages (e.g. Garrison et
al., 1986; Scott et al., 1994; Leventer and Dunbar,
1996). Based on the dominant and indicator taxa
with known ecological af®nities in the present study
(Fig. 6 and Plate 1), the diatom assemblages discussed
are interpreted to represent different sub-environ-
ments within the SIZ.
5.2.1. Cluster group 3 (sea ice associated)
Cluster group 3 dominates the lower third of GC1
(550±420 cm; Fig. 7). Fragilariopsis curta and
Eucampia antarctica are the most abundant species;
Thalassiosira antarctica T2 is subdominant (Table 1).
Based on the known ecology of the abundant and
indicator taxa in cluster group 3, the diatom assem-
blage is described as sea ice associated.
Fragilariopsis curta occurs commonly in ice edge
and within-ice algal assemblages (Scott et al., 1994;
Leventer and Dunbar, 1996) and in meltwater-
strati®ed surface water layers associated with retreat-
ing sea ice (Garrison et al., 1987). Leventer and
Dunbar (1996) hypothesized that the high abundance
of F. curta within the water column and surface
sediment of the Ross Sea is due to its being seeded
into the water column from fast ice during the spring
ice recession. In surface sediment diatom assemblages
from Prydz Bay and the Ross Sea, F. curta occurs
in high abundance where sea ice often persists
throughout summer (Taylor et al., 1997; Cunningham
and Leventer, 1998).
Eucampia antarctica is also widely considered to
F. Taylor et al. / Marine Micropaleontology 41 (2001) 25±43 33
Table 3
Cosine and acosine values (using NMDS axes 1 and 2 coef®cient values) used to determine direction of maximum correlation in Fig. 5
Variable Coeff1 Coeff2 Cos1 Cos2 Acos1 Acos2
d13C 1.778 21.213 0.826 20.564 34.303 124.303
MS 31.048 390.505 0.079 0.997 85.454 4.546
TOC 0.740 20.420 0.870 20.494 29.578 119.578
Clay 5.480 7.633 0.583 0.812 54.324 35.676
FM Silt 21.230 214.868 20.082 20.997 94.729 175.271
Coarse Silt 1.603 5.389 0.285 0.958 73.434 16.566
Sand 25.853 1.846 20.954 0.301 162.495 72.495
Mean Grain Size 28.017 0.749 20.996 0.093 174.663 84.663
F. Taylor et al. / Marine Micropaleontology 41 (2001) 25±4334
Fig. 6. Relative abundance (%) of key indicator taxa in GC1.
F.
Ta
ylor
eta
l./
Ma
rine
Micro
paleo
nto
logy
41
(2001)
25
±43
35
Fig. 7. Core log, core variables, sedimentary units (dashed horizontal line) and diatom assemblages from GC1. Diatom assemblages in relation to cluster groups are: dense sea ice
(cluster group 1); seasonally open water (cluster group 2); loose sea ice (cluster group 3). Sedimentary units from Shevenell et al. (1996). December solar insolation from Berger
(1978).
be a sea ice diatom (Burckle et al., 1990), although
Zielinski and Gersonde (1997) suggest that it should
not be de®ned as such. They have noted that E.
antarctica is most abundant where surface waters
are 22±08C and 2.5±5.58C, indicating that it is
related to surface waters in the Antarctic and the
Polar Front Zone of the Southern Ocean (Zielinski
and Gersonde, 1997). This discrepancy in distribution
may be attributable to the two varieties of Eucampia
that Fryxell and Prasad (1990) identify. One variety is
a truly Antarctic, ice edge organism (E. antarctica
var. recta), and the other is subpolar (E. antarctica
var. antarctica). Abundance of E. antarctica in
surface sediment assemblages has been interpreted
to indicate reworking and/or current winnowing
(Truesdale and Kellogg, 1979; Taylor et al., 1997;
Cunningham and Leventer, 1998). The high abun-
dance (up to 46%) of E. antarctica in cluster group
3 is not attributed to reworking mechanisms, however.
Reworked and current winnowed assemblages typi-
cally contain a high abundance of other heavily
silici®ed, robust, often extinct, taxa, and lack small
and fragile taxa that have been removed by dissolution
or strong water currents. In contrast, the assemblage in
GC1 contains fragile taxa, such as Fragilariopsis
cylindrus and the Chrysophyte Pentalamina corona,
in comparable, if not greater, abundance to the other
diatom assemblages observed in the core.
Less abundant, but statistically signi®cant, indica-
tor taxa in cluster group 3 are underlined in Table 1.
Planktic pennates, such as Fragilariopsis obliquecos-
tata and F. vanheurckii, are considered to be ice asso-
ciated (Garrison and Buck, 1985; Medlin and Priddle,
1990). F. obliquecostata has been observed in sub-ice
microalgal strands under coastal fast ice (Watanabe,
1988), but Cunningham et al. (1999) report it to be
open water associated. The planktic, centric Actino-
cyclus actinochilus is considered a typical Antarctic,
neritic species (Kozlova, 1966) that occurs in the ice
edge zone (Medlin and Priddle, 1990). It is one of the
characteristic species found in the ªSouth Weddellº
diatom assemblage described in surface sediment by
Pichon et al. (1987), which is associated with an area
where sea ice is absent for only ,2 months.
Odontella spp. are also important statistically in
cluster group 3 and reach a maximum abundance of
10.2%. There is little documentation of the ecology
of this spore-forming genus, although Odontella
weis¯oggii (Janisch) Grunow is considered endemic
to the Southern Ocean and occurs in Antarctic near-
shore regions where water temperatures are between
22 and 58C (Zielinski and Gersonde, 1997).
Froneman et al. (1997) report it to be a temperate,
neritic species that probably gets transported into
Antarctic waters by unusual, southern intrusions of
subantarctic surface waters. The ecology of Thalas-
siosira gracilis var. expecta and the benthic taxa
(Cocconeis, Fragilaria, Navicula) are also amongst
the less-well-documented taxa. Zielinski and
Gersonde (1997) observe that T. gracilis (no variety
speci®ed) reaches maximum abundance in Antarctic
surface sediment that occurs below relatively warm
waters with a temperature 20.5±28C, but should be
considered a taxon with no de®nitive relation to envir-
onmental parameters. Due to the generally low abun-
dance of the above taxa and ecological uncertainties,
these species have not been used to interpret the
assemblage's paleoecology.
5.2.2. Cluster group 2 (seasonal open water)
Cluster group 2 characterizes the mid-section of the
core (Fig. 7). The most abundant taxon is Thalassio-
sira antarctica T2 (54.4%). T. antarctica T1 and
Eucampia antarctica are subdominant. Fragilariopsis
curta is relatively common, but it is signi®cantly less
abundant compared to cluster groups 1 and 3. The
presence of species such as Fragilariopsis kergue-
lensis and Thalassiosira lentiginosa suggests that the
diatom assemblage was deposited in a seasonally
open water environment (discussed below). F.
kerguelensis attains a maximum abundance of 6.5%
in cluster group 2. Whilst this does not rank it amongst
the most common species in the group, the abundance
is signi®cantly higher compared to groups 1 and 3,
and it forms a unique indicator of the assemblage.
Fragilariopsis kerguelensis is a valuable paleo-
indicator, used to identify open marine deposition.
Today it is dominant between 52 and 638S (Burckle
et al., 1987), where summer surface water tempera-
tures are .08C (Krebs et al., 1987). Abundance is also
known to be negatively correlated with sea ice
(Burckle et al., 1987), and to increase with distance
from the Antarctic continent in both surface water
(Kozlova, 1966) and sedimentary assemblages
(Leventer, 1992; Harris et al., 1998). Similar observa-
tions have been made of Thalassiosira lentiginosa,
F. Taylor et al. / Marine Micropaleontology 41 (2001) 25±4336
suggesting that it is an equally valuable indicator of
open water (Zielinski and Gersonde, 1997). In cluster
group 2, T. lentiginosa is numerically rare (maximum
abundance 2.5%), but its abundance is signi®cantly
high and it forms a unique indicator in the assemblage.
The high abundance of Eucampia antarctica in
cluster group 2 also supports the hypothesis that this
species is not solely a sea ice indicator (Zielinski and
Gersonde, 1997). We have not distinguished between
the two varieties of Eucampia identi®ed by Fryxell
and Prasad (1990), and demonstrated to have different
geographical distributions and to inhabit different
environments (Fryxell and Prasad, 1990; Kaczmarska
et al., 1993), but suggest this in future analyses. E.
antarctica var. antarctica has a subpolar distribution,
whilst E. antarctica var. recta is restricted to cold,
polar waters associated with sea ice cover (Fryxell
and Prasad, 1990; Kaczmarska et al., 1993).
The genus Thalassiosira is widespread in Antarctic
waters, where it generally occurs in open water. It is
uncommon in sea ice (Fryxell and Kendrick, 1988;
Leventer and Dunbar, 1996; Zielinski and Gersonde,
1997), which Fryxell et al. (1987) attribute to its
inability to survive the low light intensity beneath,
and within, sea ice. The observation that Thalassiosira
antarctica is a member of some sea ice samples (e.g.
Villareal and Fryxell, 1983; Leventer and Dunbar,
1996), however, has led to the suggestion that it
may be associated with coastal sea ice and zones of
lose platelet ice (Cunningham and Leventer, 1998).
Indeed, some Thalassiosira species are sea ice related.
A bloom of Thalassiosira tumida, for example, is
reported in slush ice forming near the Ronne Ice
Shelf (El-Sayed, 1971), and Thalassiosira australis
Peragallo 1921 is observed amongst the dominant
species beneath snow-free fast ice in Ellis Fjord
(Vestfold Hills, East Antarctica), and McMurdo
Sound (McMinn, 1996; McMinn, 1999). Taylor
(1999) suggests that the formation of T. antarctica
spores could be triggered by the low light intensities
that occur beneath developing pack and platelet ice.
Reduced wind mixing below the sea ice may also
induce spore formation.
As in cluster group 3, the ecology of many of the
rarer, but statistically signi®cant diatoms is ambigu-
ous. Stellarima microtrias, for example, is reported as
being restricted to the Antarctic Zone south of the
Polar Front, in waters 22±18C (Zielinski and
Gersonde, 1997). Hasle et al. (1988) ®nd S. microtrias
benthic on or in sea ice and planktic in waters in¯u-
enced by sea ice Ð a paradox that they suggest may
be explained by its ability to produce spores. Along
with the benthic taxa (Achnanthes and Cocconeis), the
species whose ecology is poorly documented and
whose ecological af®nity is uncertain are not used
for paleoenvironmental interpretation.
5.2.3. Cluster group 1 (sea ice associated)
Cluster group 1 is present in the upper 200 cm
of GC1 (Fig. 7). Thalassiosira antarctica T1 is
signi®cantly more abundant (up to 47.6%)
compared to that in cluster groups 2 or 3. T.
antarctica T2 and Fragialriopsis curta are subdo-
minant members of the assemblage (with an aver-
age of 28.3 and 15.7%, respectively), but both are
signi®cantly less abundant compared to abundance
in cluster group 2. The diatom assemblage of
cluster group 1 is interpreted to represent deposi-
tion in a sea ice-associated environment, but is
statistically different from the sea ice diatom
assemblage of cluster group 3. Each probably
represents deposition within a different zone of
the seasonal sea ice zone, but we ®nd it dif®cult
to distinguish these differences based on diatoms
alone.
The subdominant and common taxa in cluster
group 1 are of mixed ecological preference. As
discussed previously, the genus Thalassiosira tends
to be associated with open water deposition, but rest-
ing spore formation may be induced by sea ice. Fragi-
lariopsis curta is a member of sea ice assemblages
where ice retreat has created a melt-water, strati®ed
surface water layer. Fragilariopsis cylindrus has been
observed amongst the dominant taxa in pack and fast
ice (Garrison and Buck, 1989; Scott et al., 1994) and
ice edge blooms (Kang and Fryxell, 1992), and has
been also found in open water (Garrison et al., 1987;
Leventer et al., 1993). The ecology of the Chrysophyte,
Pentalamina corona, is not well known, although
evidence suggests that Parmales inhabit ice edge and
pack ice environments. Silver et al. (1980) report
ªsiliceous cystsº (now known to be Parmales) in low
abundance in sea ice samples from the Weddell Sea, and
Brandon (1998) has found one species in water column
samples from in front of the MuÈller Ice Shelf. Three
of the ®ve indicator species in the cluster group
F. Taylor et al. / Marine Micropaleontology 41 (2001) 25±43 37