Southern Ocean mixing, seasonal sea ice, and glacialinterglacial CO2 variation.
Abstract
Several lines of evidence indicate that a change in the rate of stratification and mixing
in the Southern Ocean, either near the surface or at depth (or both), is a key factor in
explaining glacial-to-interglacial atmospheric CO2 change. Here I discuss how the
Southern Ocean is ventilated today, and then propose a new idea for why this may
have been different in glacial time. South of the Polar front, the region is largely
ventilated from below, with rapid mixing apparently driven by interaction of deep
currents with topography driving high rates of mixing well up into the water column.
Between the surface and ~1500m depth, the water column is ventilated from above,
stabilized by a halocline that is due in part to sea ice formation and brine rejection,
and probably dominated by the effects of storms. I propose that in glacial time, more
copious sea ice formation towards the Antarctic continent, together with substantial
seasonality and melting further out, resulted in denser bottom water formation and
more fresh water near the surface. The greater stratification at depth caused lower
mixing rates there while greater winter-time sea ice cover reduced mixing towards the
surface. The greater stratification in the glacial deep ocean led to reduced ventilation
of the deep ocean as a whole, allowing the build up there of biologically-transported
carbon. This scenario is consistent with most proxies, including those on the extent of
sea ice, the productivity and nutrient distribution in the Southern Ocean, and the
distribution of 13C. I use a box model, similar to that of Toggweiler (1999) to
illustrate that, (in conjunction with other mechanisms known to have influenced
atmospheric CO2), this scenario can reconcile CO2 variation with current proxies.
Specific tests are suggested that would help distinguish this “Southern Ocean seasonal
sea ice” mechanism from others that have been suggested.
Introduction
It has been clear that a change in the rate at which the deep sea is ventilated could
lead to changes in atmospheric CO2 since 1984, when three classic papers were
published pointing to the possible role of the high-latitude oceans as controllers of
natural CO2 concentrations (Knox and McElroy 1984; Sarmiento and Toggweiler
1984; Siegenthaler and Wenk 1984). The box models on which these these
“Harvardton Bears” papers were based highlighted the dependence of atmospheric
CO2 on a balance between biological productivity and ventilation between the surface
and the deep in the Southern Ocean. Today, water at the surface of the Southern
Ocean (and the North Pacific) contains non-zero mineral macro-nutrients (nitrate and
phosphate), and correspondingly has a higher pCO2 than would be the case if these
nutrients were more fully utilized. Much of the water in the the deep sea is ventilated
from this region, and its “preformed” nutrient and CO2 is set at the surface of the
Southern Ocean. Either increasing biological productivity, or decreasing the exchange
of water between the surface and depth in this region, can cause atmospheric CO2 to
be lower in this kind of model.
The results of these “Harvardton Bears” papers were initially interpreted in terms
of increased biological export productivity in the Southern Ocean. A fundamental
problem with this interpretation is however, that proxy evidence does not in general
support the idea of an increased Southern Ocean productivity in glacial time. There is
room for doubt because not all proxies seem to tell the same story, but a recent
“multiproxy” compilation of LGM export production estimates (Bopp, Kohfeld et al.
2003) suggests a coherent picture. While there is an increase in productivity in the
Subantarctic region, particularly in the Atlantic sector, the Southern Ocean south of
the present day polar front seems to have had less export production than today during
peak glacial time.
An alternative possible explanation is that the rate of ventilation of the deep ocean
was less at the LGM. Recently, (Toggweiler 1999) has proposed this as an important
mechanism for causing lower atmospheric CO2. In fact, given the constraints put on
the problem by our present knowledge, I will argue in this paper that the deep ocean
must have been more slowly ventilated in glacial time. In view of this, it seems
important to fully understand how the deep, particularly the Southern Ocean is
ventilated today. Accordingly, the first section of this paper is entirely concerned with
observations of the modern ocean. I then put forward a suggestion for why, and how,
ventilation was different in glacial time. Some calculations using a box-type model
are used to show that this mechanism, acting in combination with other effects that we
know occurred, has the potential to change atmospheric CO2 by the right amount.
Mine is not the first suggestion for a mechanism to change ocean ventilation, and I
therefore end by comparing it with other recently proposed mechanisms and
proposing tests that may help to distinguish between the competing theories.
Our uncertainty about the mechanisms causing glacial-interglacial change in
atmospheric CO2 is often cited as an illustration of our ignorance of fundamental
processes in the Earth system. However, here I will emphasize how much the viable
theories are convergent. Though they may look rather different, plausible theories for
lower glacial atmospheric CO2 all share many common characteristics. In particular,
there is an interaction between lower ventilation rates, higher sea ice cover and more
efficient nutrient utilization due to higher atmospheric iron fluxes in glacial time, all
of which serve to partition biologically fixed carbon into the deep sea and away from
the atmosphere.
Mixing and ventilation of the modern-day Southern Ocean
A variety of
Role of: Temperature, salinity, growth of terrestrial biosphere,
Proxy evidence: salty deep water near the freezing point, low Antarctic productivity
Del13C
The need for a ventilation-related mechanism
The need for slower ventilation of the deep ocean. – a puzzle
Energy-limited mixing of the deep ocean ventilation.
How can deep ocean in glacial
References
Bopp, L., K. E. Kohfeld, et al. (2003). "Dust impact on marine biota and atmospheric
CO2 during glacial periods." Paleoceanography 18(2): art. no.-1046.
Knox, F. and M. B. McElroy (1984). "Changes In Atmospheric Co2 - Influence Of
the Marine Biota At High- Latitude." Journal Of Geophysical ResearchAtmospheres 89: 4629-4637.
Sarmiento, J. L. and J. R. Toggweiler (1984). "A New Model For the Role Of the
Oceans In Determining Atmospheric Pco2." Nature 308: 621-624.
Siegenthaler, U. and T. Wenk (1984). "Rapid Atmospheric Co2 Variations and Ocean
Circulation." Nature 308: 624-626.
Toggweiler, J. R. (1999). "Variation of atmospheric CO2 by ventilation of the ocean's
deepest water." Paleoceanography 14(5): 571-588.