Hypothesis of the
100 Ka cycle
Emporia State University
December 1, 2008
ES 767 Quaternary Geology
The source of the 100,000 year (100 Ka) glacial-interglacial cycle present throughout the Quaternary (Shackleton, 2000) continues to remain unknown. It has been attributed to effects of insolation expressed by orbital variation of the Milankovitch Cycle. While it seems this may be responsible for some aspect of the glacial interglacial cycle, especially given the astonishing regularity, the effect exceeds the cause for the total climate variability seen in the 100 Ka cycle (Zeng, 2007). Only when CO2 change is included can the climate variability seen in Antarctic ice core sampling begin to be accounted for.
Introducing CO2 variability as a cause of climate change presents the issue of cause. Measurements from ice core samples show CO2 levels to have been lower during the last four glacial periods than during the intervening interglacial periods. Here I introduce two complementary hypotheses regarding atmospheric CO2 change.
The largest carbon reserve in the carbon-cycle system is by far the sediment and crustal aspect. The release and exchange rate of this pool however is far too slow to effect any cycle resonating with the 100 Ka cycle. The remaining pools consist of the atmospheric reserve and the oceanic surface which are both approximately comparable in size at about a quarter to a third of the terrestrial carbon reserve. These three pools together however only represent ten percent of the carbon contained within the deep ocean pools. This disproportionate volume of carbon being contained within a single reservoir system suggests that the source of any major fluctuation of carbon throughout the global system may be attributed to changes within the deep ocean.
Carbon uptake in the deep ocean is driven by phytoplankton populations. In low latitude oceans, these populations are dependent on nutrient availability, primarily phosphate and nitrate. The Redfield ratios refer to the 1:16:106 ratio at which phosphate, nitrate, and inorganic carbon are taken up in conversion to biomass and eventually are sequestered in the deep ocean. Using the Redfield ratio, it follows that if more nutrients were added to the system, then more carbon could be processed and sequestered in the deep ocean reservoir given the relatively large amount of free carbon. High latitude oceans have constraints on carbon uptake other then nutrient level and so the conversion of available carbon with nutrients is not complete. Excluding the high latitude oceans, models indicate a 30% rise in nutrient availability could decrease CO2 levels in the atmosphere by 30-45 ppm depending on the rate of CaCO3 production and its effect on ocean alkalinity. If CaCO3 production increases proportionally with the production of organic carbon then the net decrease in the atmospheric concentration would be blunted. If CaCO3 production remains constant with the increased organic carbon export, then atmospheric CO2 would drop even more than predicted.
“Paleoceanographic proxy data suggest that Antarctic export production was lower during the last ice age, we infer that more complete nitrate utilization in the Antarctic was due to a lower rate of nitrate supply from the subsurface, implying that the fundamental driver of the CO2 change was an ice age decrease in the ventilation of deep waters at the surface of the Antarctic.” (Sigman & Boyle, 2000). Accompanying this change is a decreased utilization of silicates in the Antarctic during this time, another nutrient used by phytoplankton. These differences point to a fundamental change in nutrient dispersal and water circulation. Sigman & Boyle proposed that the origin of these changes is a northward shift in the circumpolar winds that drive upwelling and northward surface flow. This northward shift resulted in reduced nutrient exchange between the surface and deep ocean in the Antarctic and therefore CO2 outgassing. The lack of upwelling permitted a stable, fresh, surface layer to develop cutting off deep ocean ventilation. Export production continued utilizing increasing proportions of available nutrients, as well as the iron and other dust particles common during glacial periods, lowering atmospheric CO2.
This hypothesis suggests that a northward shift in the circumpolar wind belts is brought about from a cooling episode. This reduces the upwelling and CO2 ventilation from the deep ocean. At the same time carbon continues to be utilized at the surface and sequestered in the deep ocean environment reducing total atmospheric CO2. When a sufficiently warm period occurs, the winds shift southward and the upwelling restarts outgassing large amounts of CO2.
Subglacial Burial Carbon
Based on marine 13C, pollen-based vegetation reconstruction and terrestrial carbon models, the terrestrial carbon storage was smaller during glacial periods than ensuing interglacial periods. This is due to the smaller land area available for vegetation due to the large amount of land covered by ice as well as the generally unfavorable climate for vegetative growth. Zeng (2003) proposed that quite the opposite is true. He proposed the idea that instead of a smaller carbon storage, a larger one actually existed due to the trapping of large amounts of vegetative material underneath the ice sheets. The ice sheets in effect lock up a large enough amount of vegetative matter and prevent the escape of decay gasses. Model simulations demonstrate this effect locking up 500 Pg C under the terrestrial glaciers and releasing this carbon during deglaciation.
This hypothesis complements the oceanic biological pump hypothesis in that when taken together they can begin to account for the difference in atmospheric CO2 concentration between the glacial and interglacial periods. However a drastic rethinking of the mechanism of continental glaciation is required. Previous models have all considered glaciation to act like today’s mountain glaciers, pushing vegetative matter ahead of the glacier allowing the carbon matter to slowly decompose in the terminal moraine or ablation zone. Zeng proposed that we consider the inception process of glaciation a freezer, one that begins with snow covering vegetable matter and carbon soils year round and progressively building the glacier rather than highland formation spilling out across the plains.
Both of these complementary hypotheses require fundamental changes in both the processes and causes underlying our current understanding of Quaternary glaciation. Neither of these approaches seem to adequately explain the triggering mechanism that might begin the glacial process. If orbital forcing were to be the initiating factor one would expect the cycle to much older and more stable. While these models begin to accomplish plausible mechanisms to account for the variation in CO2 concentration it seems neither have at this time a significant weight of evidence.
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