the Late Pleistocene Glacial Cycles
Robert F. Anderson,1 Katherine A. Allen,2 Jimin Yu,3 and Julian P. Sachs4
With 2 Figures
The nutrient-deepening hypothesis (Boyle 1988) posits that the restructuring of ocean wa-ter masses under ice-age environmental conditions led to the downward displacement and broadening of the zone of minimum oxygen concentrations in the ocean. A greater ice-age ef-ficiency of the ocean’s biological pump in response to a combination of enhanced ocean strat-ification and fertilization of the Southern Ocean with iron supplied by dust may have worked synergistically to lower the oxygen concentration in the deep sea (Anderson et al. 2014, Lamy et al. 2014, Martínez-García et al. 2014, Sigman et al. 2010). Acidification of the deep ocean by increased storage of respiratory CO2 under ice-age conditions would have in-duced dissolution of sedimentary CaCO3, raising the ocean’s alkalinity and further increasing the storage of CO2 by carbonate compensation (Broecker and Peng 1987). None of these conditions in isolation is sufficient to account for the observed reduction of atmospheric CO2 during the late Pleistocene ice ages, by amounts ranging between 80 and 100 ppm relative to interglacial periods (Archer et al. 2000). However, in combination, these processes are thought to be capable of storing sufficient CO2 in the deep sea to balance the loss of carbon from the atmosphere and the terrestrial biosphere (Peacock et al. 2006).
Any combination of physical and biogeochemical conditions that increases the efficiency of the biological pump (Volk and Hoffert 1985), so as to sequester more carbon in the deep ocean, will also reduce the dissolved O2 concentration in the deep ocean (Sigman et al.
2010). A growing body of qualitative evidence points to lower dissolved O2 levels in the deep ocean during the peak of the last ice age (~18 –28 ka BP, Bradtmiller et al. 2010, Jaccard and Galbraith 2012, Jaccard et al. 2009). While lower deep-sea O2 levels during the last ice age are now generally accepted, as yet it has not been possible to quantify the amount of additional CO2 storage in the deep sea, or the contribution of this storage to ice-age atmos-pheric CO2 levels.
Although quantitative constraints on dissolved oxygen concentrations in the past are lack-ing, here we illustrate the evolution of deep Pacific Ocean carbonate chemistry by making reasonable assumptions that are consistent with qualitative proxy evidence. For the central
1 Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA.
2 Department of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901, USA.
3 Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia.
4 School of Oceanography, University of Washington, Seattle, WA 98195, USA.
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24 Nova Acta Leopoldina NF 121, Nr. 408, 23 –27 (2015)
equatorial Pacific Ocean (140°W) where we have a 500 ka record of CaCO3 burial (see be-low), we take an ice-age bottom water oxygen concentration of 35 µmol/kg, which is con-sistent with the Pacific oxygen concentrations estimated by Jaccard et al. (2009). Oxygen concentrations are unlikely to have been much lower than this because we are unaware of any evidence that the deep Pacific became anoxic during the last glacial period, or even suf-ficiently low in dissolved oxygen to allow the proliferation of species of benthic foraminifera that thrive under low-oxygen conditions. Further assuming that bottom water was 2 °C colder than today (Elderfield et al. 2010) and that salinity increased by the global average of about 3 % due to storage of freshwater in continental ice sheets, we calculate that the bottom water would have had an initial O2 concentration of 371 µmol/kg if it formed in equilibrium with the atmosphere. Under these assumptions, the apparent oxygen utilization of Pacific bottom water would have been 153 µmol/kg greater than today, which is equivalent to 108 µmol/kg greater storage of respiratory CO2 using a respiratory quotient of 1.415 (Anderson 1995).
Bottom water in the central Pacific during the Last Glacial Maximum had a carbonate ion concentration that was not significantly different from that which exists today (Yu et al.
2013). The conditions that we reconstruct for the Last Glacial Maximum, with carbonate ion
Fig. 1 Graphical illustration of the calculation of total dissolved inorganic carbon (TCO2) and alkalinity in cen-tral Pacific bottom water during the peak of the last glacial period assuming a dissolved oxygen concentration of 35 µmol/kg (see main text for details and assumptions).
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concentration similar to today but a respiratory CO2 concentration at least 100 µmol/kg high-er, requires the dissolution of substantial amounts of CaCO3 to raise the alkalinity of seawater and thereby maintain a constant carbonate ion concentration ({CO32–} ~ DIC – ALK), where DIC is the total dissolved inorganic carbon concentration and ALK is alkalinity. With these constraints, we reconstruct the DIC and ALK of Central Pacific bottom water during the Last Glacial Maximum by working stepwise through each of the factors that would have had a significant impact on carbonate system (Fig. 1). The combined effect of greater accumulation of respiratory CO2 and carbonate compensation would have created a DIC concentration
~217 µmol/kg greater than that which exists today.
Taking the illustration a step further, if half the global ocean (~6.5 × 1020 liters; i.e., the deep ocean) contained 108 µmol/kg more respiratory CO2 than today, then this would amount to a total of 846 gigatons of carbon (GtC), sufficient to account for the carbon lost from the atmosphere (~200 GtC) and from the terrestrial biosphere (~600 GtC, Peterson et al. 2014).
Of course, these values need to be refined with more detailed models, but they illustrate the potential to close the ice-age carbon budget with increased storage of CO2 in the deep ocean under the combined effects of greater stratification and greater iron fertilization.
Recent success in application of B/Ca ratios as a proxy for carbonate ion concentration (Yu et al. 2013, 2014) have confirmed that CaCO3 accumulation in sediments throughout the deep Indo-Pacific region is regulated primarily by changes in carbonate ion concentration (i.e., by CaCO3 preservation, Anderson et al. 2008, Hodell et al. 2001). Building on this,
Fig. 2 A 500 ka record from TT013-PC72 (0.11°N, 139.4°W, 4298 m) in the central equatorial Pacific Ocean. The oxygen isotope composition of benthic foraminifera (Murray et al. 2000) was used to provide an age model, and to illustrate changes in global ice volume (red curve). The accumulation rate of CaCO3 (blue curve) reflects past chang-es in deep water carbonate ion concentration (Anderson et al. 2008, Yu et al. 2013). Note the inverted axis scale;
the sense is that “up” is equivalent to low carbonate ion concentration. A 6000-year lag has been added to the CaCO3 record to illustrate the (potential) time scale for the deep-sea carbonate system to respond to changes in ice volume.
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26 Nova Acta Leopoldina NF 121, Nr. 408, 23 –27 (2015)
we use the 500 ka record of CaCO3 accumulation in central equatorial Pacific sediments (TT013 PC72, 0.1°N, 140°W, Anderson et al. 2008) to compare past changes in deep-sea {CO32–} to global ice volume, as inferred from the δ18O of C. wuellerstorfi in the same core.
We find a good correlation between CaCO3 accumulation and δ18O, which is improved by imposing a 6000-year lag on the CaCO3 record (Fig. 2), likely reflecting the time scale for adjustment of the ocean carbonate system to external forcing. In viewing records like this one must be mindful of the caveat that “everything correlates with everything else on 100-ka time-scales” (colloquially referred to by some as the Labeyrie principle). That is, one cannot infer cause and effect from correlations like this. Nevertheless, we can be confident that deep-sea {CO32–} is systematically linked to global ice volume, at least throughout the Late Pleisto-cene. Taking a lesson from the last glacial period, we can further infer that these changes in {CO32–} reflect past changes in ocean stratification, the efficiency of the biological pump, and carbonate compensation.
One must also keep in mind that the carbonate ion record consists of a transient signal that follows the accumulation and release of respiratory CO2 as well as a quasi-steady-state signal tied to changes in whole-ocean ALK. Future work should seek higher resolution records to better define the phase relationship between ice volume and deep-sea carbonate chemistry, to-gether with models to resolve the transient from steady state aspects of changes in carbonate chemistry.
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Prof. Robert F. Anderson, Ph.D.
Lamont-Doherty Earth Observatory Columbia University
Earth Institute 61 Route 9W P.O. Box 1000 Palisades, NY 10964 USA
Phone: +1 845 3658508 Fax: +1 845 3658155 E-Mail: boba@ldeo.columbia.edu
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