Download Warm deep-water ocean conveyor during

Warm deep-water ocean conveyor during Cretaceous time
Bernd J. Haupt
Dan Seidov
Earth and Mineral Sciences Environment Institute, Pennsylvania State University, University Park, Pennsylvania 16802-6813, USA
ABSTRACT
Because the deep water is associated with its high-latitude sources, a warm deep ocean
during Mesozoic-Cenozoic time is a challenge; there is no feasible physical mechanism
that could maintain warm subpolar surface oceans in both hemispheres. The goal of this
study is to explore a hypothesis that a warm deep ocean can coexist with relatively cool
subpolar (high latitude) sea surface in one hemisphere and a warmer subpolar sea surface
in the other. A series of numerical ocean circulation experiments confirms that the ocean
meridional circulation can keep the abyssal ocean warm despite the northern subpolar
surface water staying relatively cool.
Keywords: Cretaceous, Mesozoic, Cenozoic, paleo-oceanography, thermohaline circulation, numerical model.
INTRODUCTION
Observations and ocean models suggest
that the ocean circulation during warm Mesozoic-Cenozoic climates was dramatically
different from its present-day pattern (Barron
and Peterson, 1989; Bice et al., 1997; Gordon,
1973; Hay et al., 1981; Kennett, 1977; Kutzbach and Ziegler, 1993; Poulsen et al., 1998;
Seidov, 1986; Shackleton and Kennett, 1975;
see also Barrera and Johnson, 1999). The
warm, ice-free Cretaceous Period (65–135
Ma) presents perhaps the most challenging
problem, because no consensus exists on what
climatic mechanisms could maintain a warm
polar climate with very small meridional and
vertical thermal gradients in the world ocean.
Although meridional oceanic heat transport
can be called upon to explain a warm sea surface in the high latitudes (e.g., Schmidt and
Mysak, 1996), keeping the high-latitude sea
surface at ;10–15 8C in both hemispheres
would necessitate substantial equatorially
symmetric oceanic poleward heat transport.
The assumption of equatorially symmetric
high-latitude sea surface temperatures (SSTs)
is often used in atmospheric modeling (Sloan
and Barron, 1992) and implicitly in data interpretation. However, there are indications
that the southern subpolar ocean was warmer
than the northern oceans (e.g., Huber and
Sloan, 2000). Sloan et al. (1995) argued that
increased poleward heat transport is difficult
to accomplish in the case of reduced oceanic
thermal contrasts. They suggested that atmospheric feedbacks, in conjunction with increased greenhouse gases, might be responsible for warming the poles. A higher CO2 level
during the Cretaceous is often chosen to explain the warm equable Cretaceous-Eocene
climate (Allen, 1997; Barron et al., 1995;
Sloan and Rea, 1995; Sloan et al., 1995;
Thomas et al., 2000). Although substantial increase of CO2 is the signature of warm climates (e.g., Barron and Washington, 1985),
this increase could not be too strong without
overheating the tropics, as the CO2-induced
warming occurs everywhere (e.g., Sloan and
Rea, 1995). The equatorial regions, however,
either were as warm as today or even cooler
(Crowley and Zachos, 2000; see also a review
by Valdes, 2000). Even an eightfold CO2 increase appears to be insufficient to raise the
Earth temperature to the Cretaceous mean values (e.g., Valdes, 2000).
The warm deep water is usually associated
with high-latitude deep-water sources (see review in Valdes, 2000). However, because the
seawater density depends on both temperature
and salinity, it may be questioned whether the
deep-ocean water temperature (direct geologic
evidence) reflects the warm polar surfaceocean regions (a deduced supposition). Dedensification of surface waters shuts off convection and reduces meridional overturning.
Reducing the overturning leads to reducing
poleward heat transport and vice versa; cooling and/or increase of salinity initiates or
strengthens convection and leads to stronger
overturning and stronger poleward heat transport. Thus, a compromise can be found in a
scenario with a relatively cool high-latitude
surface at least in one hemisphere that would
allow for a reasonable poleward heat transport
and warm enough deep water being produced
in the opposite hemisphere.
The present-day deep-ocean circulation is
driven by two deep-water sources—North Atlantic Deep Water (NADW) and Antarctic
Bottom Water (AABW). Both water masses
are characterized by their own distinct temperature and salinity. The NADW, which is
slightly warmer, flows southward above the
northward-flowing, cooler AABW, forming
the well-known layered structure of the abyss.
The structure of water masses during warm
climates is less well known. However, it may
be assumed that the deep-ocean currents are
driven similarly to their present-day analogue,
as required by the Stommel and Arons (1960)
theory. Therefore, it may also be assumed that
past ocean circulation was sensitive to highlatitude salinity distribution (e.g., Bryan,
1986; Manabe and Stouffer, 1988; Stocker et
al., 1992; see extended references in Seidov
and Haupt, 1999). Asynchrony of some of the
glacial cycles of the Pleistocene implies that
both the southern and northern deep-water
sources could be affected by asynchronous
meltwater events (Antarctica and North Atlantic show mixed record indicating that either
synchrony, or asynchrony could occur, e.g.,
Blunier et al., 1998; Bond et al., 1997). In
some instances, the meridional overturning
might have behaved as a bipolar seesaw with
a periodicity of hundreds to a thousand years
or more (e.g., Broecker, 1998; Seidov and
Maslin, 2001; Stocker, 1998).
In cold climates, low sea-surface salinity
(SSS) in the high latitudes is due mainly to
melting of sea ice or icebergs; poleward water
vapor transport from the tropics is an important but secondary factor. A high-salinity signal is due to freezing seawater that leads to
salt brine rejection (e.g., Gill, 1982). In warm,
ice-free climates, poleward water vapor transport or river runoff can be the only causes of
a low-salinity signal, whereas increased evaporation (unlikely in high latitudes) might be a
cause of increased high-latitude surface salinity. However, in our approach, we can model
an impact of temperature increase by changing
salinity, because our concern is sea-surface
density rather than temperature or salinity. A
q 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400.
Geology; April 2001; v. 29; no. 4; p. 295–298; 4 figures.
295
Figure 1. A: Reconstruction
of land-sea distribution for
mid-Cretaceous time. Crosshachured area around Antarctica is circumglobal band
of surface ocean with salinity anomalies added in sensitivity tests (see text). Solid
line shows position of sections in Figure 4. B: Zonally
averaged sea-surface temperature (SST) representing
two surface climates: intermediate (solid line) and
warm (dashed line) Cretaceous climate scenarios.
useful rule of thumb is that the same density
increase can be achieved by either increase of
salinity by ;1 psu (practical salinity unit) or
by decrease of temperature by about 25 8C
(Pond and Pickard, 1983).
NUMERICAL EXPERIMENTS
The Modular Ocean Circulation Model,
version 2.2 (e.g., Cox, 1984; Pacanowski,
1996), is used here with a grid resolution of
48 latitude 3 48 longitude and 16 unevenly
spaced vertical layers with annual mean SST,
SSS, and wind stress. All runs are 2000 model
years long, with five-fold acceleration in the
deep layers (this means that the deep ocean is
effectively run for 10 k.y.). A complete steady
state is reached in all numerical experiments.
The thermohaline ocean circulation is determined by density distribution and thus is
controlled by surface density, which depends
on both temperature and salinity. In the Cretaceous scenarios we specify the sea-surface
thermal conditions. This requires that we
make specific assumptions about the equatorto-pole SST gradients that cannot be changed
in sensitivity tests. Here, in order to vary seasurface density, we keep SST unchanged and
vary SSS.
To test the role of hemispheric asymmetry
of the Cretaceous climate, we explore two different climatic scenarios described by Poulsen
et al. (1998) and Poulsen (1999). Two different types of Cretaceous surface climates (comprising SST, SSS, and wind stress) were computed in ocean model numerical experiments
driven by the GENESIS atmospheric model
(Thompson and Pollard, 1997) in Poulsen
(1999); model output was provided to us by
Poulsen (1999, personal commun.). The landsea distribution (Fig. 1A) and bottom topography are from Poulsen et al. (1998).
Although it is called an intermediate Cretaceous scenario, the scenario with a relatively
cool subpolar ocean is still a very warm climate, if compared to today’s. The principal
296
difference in this scenario from the ‘‘warm
Cretaceous’’ scenario, with temperatures to
20 8C in the subpolar regions, is that the intermediate scenario bears a noticeable southnorth SST asymmetry, the northern subpolar
ocean SST being only 6 8C, whereas the
southern ocean subpolar SST is 12 8C. The
equatorial SST is ;28 8C in the intermediate
and 31 8C in the warm scenario (Fig. 1B
shows zonally averaged SSTs, and therefore
depicts a slightly lower maximum value).
To test sensitivity to the density changes in
the high latitudes that are known to be the
strongest regulator of the ocean global thermohaline conveyor, salinity anomalies are
added to the SSS field as circumglobal bands
between Antarctica and 608S (crosshachured
area in Fig. 1A), the amplitude being varied
in different runs from 21 to 11 psu. The perturbed salinity was smoothed in 88 latitudinal
bands to the north of the anomaly edge by use
of a low-pass filter (Shapiro, 1971) to blend
the modified SSS to the unchanged field. The
wind stress and SSTs (different in the two scenarios) were held the same as in the control
runs in the corresponding scenarios in order
to modify only one variable at a time. Note,
however, that we do not specify what kind of
high-latitude impact could change salinity.
Our only goal is to test whether the Cretaceous ocean circulation is sensitive to perturbations of the sea-surface density, i.e., to examine how robust the deep-ocean circulation
patterns are, and whether they favor a warm
or cold abyssal ocean if one of the poles is
relatively cool. Possibly an equivalent temperature change would produce the same
changes of the sea-surface density. As mentioned here, we keep SSTs unchanged. Thus,
we choose SSS to be the variable controlling
density in our idealized perturbation
experiments.
RESULTS
The oceanic heat transport (Fig. 2), if added
to other impacts, such as increased CO2 and
increased water vapor (e.g., Sloan et al.,
1995), seems to be consistent with this moderately warm subpolar sea surface. This is because even moderately cooler high-latitude
ocean surface in the intermediate scenario
leads to substantial changes of the thermohaline conveyor operation, if compared to the
warm case, with strongly intensified southern
overturning (see following).
Figure 2 shows noticeable cross-equatorial
heat transport in the intermediate scenario,
whereas there is almost no cross-equatorial
heat transport in the warm scenario. It is important that in this scenario the southern and
northern deep-water sources trade places in
driving the conveyor. The warmer Southern
Ocean is saltier and therefore denser than the
northern subpolar oceans, and the Drake Passage is nearly closed (200 m deep in this geometry; Poulsen, 1999). At the same time, the
northern basins are colder and fresher, and the
Tethys passageway prevents formation of an
analogue to the present-day Gulf Stream in the
Northern Hemisphere (compare the land-sea
distribution in Fig. 1A with the present-day
ocean geometry).
Meridional temperature sections (Fig. 3)
along 1208W in the Pacific Ocean (along the
meridional arc in Fig. 1A) show the thermal
structure in the two scenarios. The intermediate scenario, although less sensitive to additional density changes in the subpolar surface oceans, provides a realistic combination
of relatively warm bottom water and warm
southern but cold northern subpolar surface
ocean. The warm Cretaceous scenario provides even warmer bottom water, but at the
expense of unrealistically warm surface ocean
in the high latitudes. Thus, the intermediate
scenario is consistent with the Southern Hemisphere being warmed by southward crossequatorial oceanic heat transport, and with the
warm deep ocean coexisting with a reasonable
oceanic heat transport amount. In other words,
we argue that a combination of warm deep
GEOLOGY, April 2001
Figure 2. Global northward heat flux in PW (1 PW 5 1015 W): intermediate Cretaceous scenario (control run and experiment with low-salinity anomaly [21 psu] in
Southern Hemisphere); warm Cretaceous scenario (control run and experiment
with low-salinity anomaly [21 psu] in Southern Hemisphere).
Figure 4. Sketch of bipolarity of deep-ocean
dynamics. A: Present-day ocean; NADW—
North Atlantic Deep Water, AABW—Antarctic Bottom Water. B: Intermediate Cretaceous ocean; NHW—Northern Hemisphere
water, SHW—Southern Hemisphere water.
C: Warm Cretaceous ocean. Note that NA
present-day overturning is shown, whereas
Cretaceous (B, C) global overturning can be
only estimated. Arrowed lines show
schemes of global meridional overturning.
Figure 3. Meridional temperature sections in Pacific Ocean at 1208W (see Fig. 1). A: Intermediate Cretaceous scenario. B: Warm Cretaceous scenario.
GEOLOGY, April 2001
ocean and cool high-latitude ocean surface in
one of the hemispheres is possible without
sacrificing the main assumption that the Cretaceous ocean was warm in the abyss.
Although in the warm scenario the ocean is
warm in the abyss (18–21 8C bottom water,
Fig. 3B), this scenario presents the major
problem for explaining the warm sea surface
in the polar regions (surface temperature
varies from ;30 8C at the equator to 20 8C at
the poles; Fig 1B). Because the equator-topole and top-to-bottom thermal gradients are
both very small, the meridional overturning
(schematically shown by arrowed lines in Fig.
4), although not that small, produces poleward
oceanic heat transport that is too low, insufficient to maintain polar oceans that warm,
were the subpolar SST allowed to drift from
the specified values (e.g., in an ocean-atmosphere model).
The intermediate scenario has a robust circulation pattern undisturbed by the perturbations of the surface salinity, the heat transport
being virtually unchanged (Fig. 2), whereas
the circulation in the warm scenario indicates
a substantial change of the meridional overturning and shows a northward cross-equato297
rial heat transport. This would have produced
an asymmetry in surface climatology, where
the model would allow SST to drift away from
the specified values (e.g., in a coupled oceanatmosphere model).
DISCUSSION AND CONCLUSIONS
The Cretaceous ocean thermohaline conveyor, although different from its present-day
analogue, might have operated similarly to the
present-day mode, the only difference being
that the amplitude might have been lower than
today (because of smaller equator-to-pole density gradients), and the driving deep-water
sources may have switched roles, the AABW
taking the NADW role in driving the abyssal
ocean currents. However, it could also be that
geometry-induced north-south asymmetry was
a permanent feature responsible for warming
the abyssal ocean (Fig. 4).
We emphasize that our experiments do not
target the details of the Mesozoic-Cenozoic
ocean circulations; they are not designed to
explain how such warm climates could exist.
This far more complex problem can be addressed only by using an advanced coupled
ocean-atmosphere model. There must be some
value of SSS perturbations that would cause
bifurcation and reversing of the conveyor that
would lead to cooling of the deep ocean.
However, within a reasonable interval of SSS
variances, such rebounds were not achieved in
the experiments shown here, implying that
SST variations must be added for the conveyor to be bifurcated and for warm deep water
to be replaced by colder water. That moderately warm deep ocean can easily coexist with
moderately warm, or even relatively cool, subpolar oceans removes the restriction on poleward heat transport and allows for the warm
deep ocean. Thus, it removes the major paradox of too-warm high latitudes having no
plausible physical mechanisms that could
maintain such excessive high-latitude warmth.
ACKNOWLEDGMENTS
We thank Chris Poulsen for providing us with the
Cretaceous surface boundary conditions and paleobathymetric data; Lisa Sloan for discussions and her
help in writing the manuscript; and Eric Barron for
support and valuable comments. The editor, B.A.
van der Pluijm, and two reviewers provided very
useful comments that helped to substantially improve the manuscript. This study is supported by the
National Science Foundation (project 9975107).
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Manuscript accepted December 21, 2000
Printed in USA
GEOLOGY, April 2001