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70
PAGES News • Vol.17 • No 2 • June 2009
Science Highlights: Paleo Sea Level
Figure 2: A) A sedimentary sequence including a paleosol (fossil soil) that caps the Pleistocene substrate, basal
peat and lagoonal mud from the Mississippi Delta. Note the sharp contact (arrow) between the ~2 cm thick peat
layer and the overlying lagoonal mud, which represents an abrupt sea level rise at ca. 8.2 cal ka BP. B) Stratigraphic
signature of the abrupt sea level rise at ca. 8.2 cal ka BP at Bayou Sale, Mississippi Delta (Törnqvist et al., 2004). The
occurrence of Rangia cuneata, a brackish water clam characteristic of estuarine and lagoonal environments, is
also shown.
Coastal vegetation evidence for sea level changes
associated with Heinrich Events
Ca T a l i n a Go n z á l e z a n d ly d i e M. du p o n T
MARUM - Centre for Marine Environmental Sciences, Geosciences Department, University of Bremen, Germany; catalina@uni-bremen.de
A Cariaco Basin pollen record shows the development of tropical salt marshes during marine isotope stage
3 and suggests that millennial sea level changes during the periods encompassing Heinrich Events followed
Antarctic climate variability.
The timing of sea level changes during
marine isotope stage 3 (MIS 3; 60-25 ka)
is a key issue in understanding the role
of ice sheets in millennial-scale climate
variability. The available reconstructions
of sea level changes during this interval
greatly rely on oxygen isotope records
from deep-sea cores (since coral-based
data are sparse and chronologies less pre-
cise), and consistently show four cycles of
similar amplitude of sea level change in
the order of 20-30 m (Siddall et al., 2008
and references therein). However, there
is little agreement on the exact timing of
these changes or on the relative roles of
the Southern and Northern Hemisphere
ice sheets in global sea level scenarios.
The ecological response of sensitive
terrestrial ecosystems can provide inde-
pendent information that complements
the almost exclusively marine body of
evidence of millennial sea level change.
For this purpose, intertidal tropical eco-
systems can be particularly useful, since
they are very sensitive to environmental
gradients in the sea-continent interface.
In tidal salt marsh plant communities,
species composition varies with eleva-
tion, usually in a banded pattern parallel
to the shore. Its variation often reects
environmental gradients that result from
the interaction between tidal regime, local
topography, freshwater input, and biota. It
has been proposed that the zonation is a
spatial expression of successional changes
over time and has potential to be recon-
structed for the past by pollen analysis. If
patterns of pollen deposition follow zona-
tion and succession patterns, these can be
Barber et al., 1999). This rapid sea level rise
serves as an example of how the amount
and source of meltwater can be inferred
from sea level records by the ngerprint-
ing method; a technique that capital-
izes on the distinct spatial pattern of the
global sea surface due to the gravitational
attraction of large ice and/or water masses
(Mitrovica et al., 2001; Clark et al., 2002;
Kendall et al., 2008). However, more de-
tailed records are required to rene the es-
timate of the water volume impounded in
these glacial lakes. To this end, our ongo-
ing high-resolution sea level work in the
Mississippi Delta aims to rene the timing
and amplitude of the rapid sea level rise
corresponding to the “8.2 ka event” by de-
tailed stratigraphic studies. With regard to
the “7.6 ka event”, its extent still remains a
matter of debate. Some records suggest a
ca. 3 m rapid rise that occurred at about
7.5 cal ka BP or slightly later (e.g., Siddall et
al., 2003; Liu et al., 2004; Bird et al., 2007),
while others indicate a smooth rise of sea
level during this time window (e.g., Van de
Plassche, 1982; Törnqvist et al., 2006). The
causes of such spatial contrasts are at pres-
ent unknown but may in part be related to
the location of the associated meltwater
sources and their sea level ngerprints.
We therefore conclude that our under-
standing of rapid sea level rise during the
early Holocene is still in its infancy. Many
more high-resolution sea level records for
this critical time interval are needed. Com-
bined with “ngerprint modeling”, they
could serve to rene the timing, amplitude
and origin of such abrupt events.
References
Carlson, A.E., Legrande, A.N., Oppo, D.W., Came, R.E., Schmidt, G.A.,
Anslow, F.S., Licciardi, J.M. and Obbink, E.A., 2008: Rapid early
Holocene deglaciation of the Laurentide ice sheet, Nature Geosci-
ence, 1: 620-624.
Kendall, R.A., Mitrovica, J.X., Milne, G.A., Törnqvist, T.E. and Li, Y., 2008:
The sea level fingerprint of the 8.2 ka climate event, Geology, 36:
423-426.
Törnqvist, T.E., Bick, S.J., Gonzalez, J.L., van der Borg, K. and de Jong,
A.F.M., 2004: Tracking the sea level signature of the 8.2 ka cool-
ing event: New constraints from the Mississippi Delta, Geophysi-
cal Research Letters, 31: L23309.
Törnqvist, T.E., Bick, S.J., van der Borg, K. and de Jong, A.F.M., 2006: How
stable is the Mississippi Delta? Geology, 34: 697-700.
Yu, S.-Y., Berglund, B.E., Sandgren, P. and Lambeck, K., 2007: Evidence for
a rapid sea level rise 7600 yr ago, Geology, 35: 891-894.
For full references please consult:
www.pages-igbp.org/products/newsletters/ref2009_2.html
71
PAGES News • Vol.17 • No 2 • June 2009
Science Highlights: Paleo Sea Level
Figure 1: Left - main ecological preferences of 3 salt marsh taxa. R ight - schematic representation of salt marsh
community dynamics in a changing sea level environment according to the Cariaco Basin pollen record (González
and Dupont, 2009). Thicker black lines indicate areas of soil hypersalinity. SL 1 to SL4 denote different sea levels
reconstructed from the pollen record and correspond with phases indicated in Fig. 2. a) Establishment of salt
marshes when arid conditions promote extensive hypersaline environments; b) rapid sea level rise causes erosion;
only pioneer species tolerate the change; c) sea level rise decelerates, and accretion of sediments and autochthonous
organic material takes place; more competitive species take advantage of favorable conditions; d) sea level drops,
sediment accumulation constrains the tidal influence to the seaward edge.
reconstructed back in time by establishing
a time-depth relationship with the fossil
evidence, this then enables past sea level
to be reconstructed. Here, we present new
palynological evidence from the marine
core MD03-2622 collected from the Ca-
riaco Basin that reconstructs the history
of intertidal plant communities during
intervals associated with Heinrich Events
(HEs), linking them to the well-constrained
North Atlantic signal of millennial- to sub-
millennial-variability.
The Cariaco Basin is located on the
northern shelf of Venezuela and is particu-
larly sensitive to the seasonal shifts of the
Intertropical Convergence Zone (ITCZ),
which deeply inuence the hydrology and
oceanographic features of the basin. Dur-
ing MIS 3, the Cariaco Basin record displays
a clear North Atlantic climatic variability,
shifting from dry conditions during cold
stadials to wet conditions and increased
river runo during warm interstadials. This
hydrological pattern is reected by varia-
tions in the input of terrestrial materials
and has been explained by the latitudinal
migration of the ITCZ (Peterson et al., 2000;
Peterson and Haug, 2006; González et al.,
2008). The chronology used in this study
was established by linking similar features
of sediment reectance prole of Cariaco
site MD03-2622 with that of the nearby
ODP Site 1002D, which has an extremely
high-resolution age model for the past 60
ka (Hughen et al., 2004; 2006).
Tropical salt marsh response to
millennial climate and sea level
changes
During glacial periods, when sea level was
80-120 m lower than today, a broad shal-
low shelf became exposed south of the
Cariaco Basin. Periods of extremely dry
atmospheric conditions might, therefore,
have resulted in hypersaline coastal en-
vironments (Medina et al., 1989). These
extreme conditions could have been tol-
erated only by a limited number of plant
species. Chenopodiaceae, Poaceae and
Cyperaceae belong to the most common
representatives of salt tolerant plants in
tropical and subtropical wetlands (Adam,
2002) (Fig. 1).
The pollen record
Five high-amplitude vegetation shifts were
recorded in the pollen record during MIS
3 (60-25 ka), indicating rapid oscillations
of environmental conditions in north-
ernmost South America. Recurrent salt-
tolerant vegetation expansions (i.e., the
development of salt marshes) were shown
to correlate with HEs 3-6. Within single HE
intervals, a recurrent and directional suc-
cession of pollen taxa was observed in
the following order: Abrupt increases in
saltbush (Chenopodiaceae) followed by a
dominance of grasses (Poaceae), which in
turn were replaced by sedge (Cyperaceae)
(Fig. 2). Once interstadial conditions re-
turned, salt marshes were replaced by
mangroves and other arboreal species.
In this sequence, salt marshes started
to develop under extremely arid stadial
conditions (Peterson and Haug, 2006;
González et al., 2008) when intertidal habi-
tats became hypersaline due to extended
periods of strong evaporation and reduced
rainfall. The salt marshes were most likely
restricted to narrow intertidal areas be-
cause under strongly seasonal conditions
they are usually fringed on the landward
side by extensive bare salt pans (Fig. 1a, b;
Adam, 2002). Early colonizing species of
salt marshes, like the annual Atriplex and
Salicornia (Chenopodiaceae), rst colonize
bare zones of lower and middle marsh ar-
eas, with a high incidence of waves and
prolonged inundation regimes (Ranwell,
1972). Thus, intervals of maximum pol-
len representation of Chenopodiaceae
are interpreted as periods of direct tidal
inuence and sediment relocation. Fre-
quent tidal ooding under accelerated sea
level rise would result in ooding of the
marsh surface, transforming it into a new
seaoor, with the later landward accretion
of new, low marsh sediments (Fig. 1b).
By comparing our high-resolution
pollen data with sea level reconstructions
from the Red Sea (Arz et al., 2007; Siddall
et al., 2008 and references therein) and
the independently dated fossil corals from
the Huon Peninsula (Thompson and Gold-
stein, 2006) for the period between 40.5-38
ka, we found that the phase dominated by
Chenopodiaceae corresponds closely with
an interval of accelerated sea level rise (Fig.
2). This conrms that only early succes-
sional plants, with high colonizing abilities
(e.g., rapid growth, annuals or short-lived
perennials) were capable of surviving the
stressful high rates of change (Fig. 1a).
Moreover, the erosion of low marsh sedi-
ments would wash out and transport the
pollen produced in situ (Fig. 1b).
As soon as sea level rise decelerated
(ca. 1 ka after the Chenopodiaceae peak),
some vegetation was able to establish per-
manently. In low marsh areas, sediment
accretion greatly depends on vegetation
cover, which limits erosion, and enhances
sediment and organic matter trapping.
Thus, areas covered with vegetation ex-
perienced higher marsh heights. The
72
PAGES News • Vol.17 • No 2 • June 2009
Science Highlights: Paleo Sea Level
Figure 2: Comparison of the high-resolution palynological record from core MD03-2622 (Cariaco Basin) with sea
level reconstructions from Red Sea marine sediment cores and Huon Peninsula (Papua New Guinea) fossil corals
during HE 4 (González and Dupont, 2009). Top to bottom: Reflectance data from core MD03-2622 (Laj, 2004). Sea
level data; dark blue line - central Red Sea (Siddall et al., 2003; 2008), light blue line - northern Red Sea (Arz et
al., 2007), and dotted pink line - Huon Peninsula (Thompson and Goldstein, 2006). Pollen % of Chenopodiaceae,
Poaceae, and Cyperaceae indicating the directional alternation of salt marsh species during HE4. Dotted gray
lines SL1 to SL4 denote different sea levels reconstructed from the Cariaco Basin pollen record, which correspond
to phases explained in Figure 1.
build-up of middle and high marsh envi-
ronments favored the expansion of more
competitive perennial grasses (Poaceae),
thus replacing Chenopodiaceae pioneer
species (Figs. 1c, d and 2). In contrast, the
presence of Cyperaceae indicates less sa-
line conditions, since sedges do not toler-
ate salinity excess. Thus, since there is no
evidence of increased freshwater input
during HEs, Cyperaceae pollen maxima
might reect an expansion of elevated
marsh areas (Fig. 1c, d).
Once interstadial conditions resumed
and the average position of the ITCZ
shifted northwards, the increased avail-
ability of freshwater might have alleviated
salinity stress on soils, allowing a more
complex plant community to develop on
the shelf, and pushing the upper borders
of the salt marsh seawards. Simultaneous
increases in mangrove pollen (González
and Dupont, 2009) conrm that coastal
environments became less saline and in-
creasingly suitable for the establishment
of forests during stadial-interstadial tran-
sitions. In addition to freshening, deceler-
ated sea level rise (or sea level fall) would
be required to allow the establishment of
mangroves, since mangroves do not sur-
vive if sea level rise occurs too rapidly (El-
lison, 1993; Woodroe, 1999).
Comparison
The Cariaco Basin pollen record also shows
a similar relation between salt-marsh ex-
pansion and sea level rise during HEs 3, 5,
5a and 6, in spite of dating uncertainties
and poorer resolution of the vegetation
data (González and Dupont, 2009). In all
ve cycles, maximum values of Chenopo-
diaceae pollen coincide with the onset of
HE stadials in the North Atlantic, and with
warming phases in Antarctica. According
to our palynological evidence, sea level
started to rise before the ice sheet collapse
that caused Heinrich layers in the North
Atlantic, being in agreement with both
Red Sea sea level reconstructions during
the HE 4 (Fig. 2; Siddall et al., 2008; Arz et
al., 2007) and with fossil coral data from
the Huon Peninsula (Thompson and Gold-
stein, 2006). However, a subsequent decel-
erated rise or fall of sea level is needed to
reconcile with the expansion of Poaceae.
In this case, our data supports the timing
of central Red Sea reconstruction (Siddall
et al., 2003; 2008; Rohling et al., 2008),
the independently dated corals from the
Huon Peninsula, and models, which sug-
gest that melting in Antarctica might have
accounted for a rise in sea level of about
20 m (Rohling et al., 2004; 2008; Flückiger
et al., 2006).
Conclusions
Through the palynological reconstruction
of intertidal vegetation in core MD03-2622
we provided indirect evidence of rapid sea
level change during MIS 3. Five intervals
of expanded salt marsh vegetation corre-
sponded to the onset of HEs of the north-
ern high latitudes and suggest periods of
accelerated sea level rise in the tropical
Atlantic. The close relationship between
sea level rise and community dynamics is
consistent with a resource-based mecha-
nism of succession, where soil develop-
ment and salinity gradients are the main
factors determining the vegetation dy-
namics of coastal marshes. In this context,
the Cariaco Basin palynological record is
especially informative on the timing of sea
level changes during MIS 3 and their con-
nection with HEs, supporting the idea that
sea level uctuations followed Antarctica
climate variability.
Acknowledgements
This work was supported by the Programme
Alβan –the European Union Programme of High
Level Scholarships for Latin America (Scholar-
ship E04D047330CO) – and the Deutsche Aka-
demische Austausch Dienst (DAAD)-Colfuturo
Program. Data will be available in PANGAEA
(www.pangaea.de).
References
Adam, P., 2002: Saltmarshes in a time of change, Environmental Conser-
vation, 29: 39-61.
Arz, H.W., Lamy, F., Ganopolski, A., Nowaczyk, N. and Pätzold, J., 2007:
Dominant Northern Hemisphere climate control over millennial-
scale glacial sea-level variability, Quaternary Science Reviews,
26: 312–321.
González, C. and Dupont, L.M., 2009: Tropical salt marsh succession as
sea-level indicator during Heinrich events, Quaternary Science
Reviews, 28: doi: 10.1016/j.quascirev.2008.12.023.
Siddall, M., Rohling E.J., Thompson, W.G. and Waelbroeck, C., 2008:
Marine isotope stage 3 sea level fluctuations: Data synthesis
and new outlook, Reviews of Geophysics, 46: RG4003, doi:
10.1029/2007RG00226.
Thompson, W.G. and Goldstein, S.L., 2006: A radiometric calibration of
the SPECMAP timescale, Quaternary Science Reviews, 25: 3207-
3215.
For full references please consult:
www.pages-igbp.org/products/newsletters/ref2009_2.html
