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suggest that climatic nonlinearities can translate high frequency variations into longer period geochemical flux anomalies by mechanisms still unknown. One consequence of the climate–sediment nonlinearities may be to transfer depositional anomalies of carbon and nutrient storage from time scales shorter than the residence times of these elements to 100-kyr, 400-kyr, and even longer scales, which could well affect their mean oceanic inventories. We also suspect, although we do not know yet, that the terrigenous input of carbon to the Cretaceous oceans may have also varied in phase with the clay accumulation cycles observed in the deep sea.
Are There Common Threads?
It would be simplistic to assume that all variations in carbonate and organic carbon time series from the deep sea represent precisely the same climatic response to orbital forcing. The spectral mix of frequencies observed in sediment records in fact varies on time scales of millions of years. It does appear, however, that variations in the deposition of both carbon phases have always been climatically sensitive at the orbital frequencies. If the Pleistocene–Pliocene precedent holds for earlier times, spectra of carbonate time series in marine records give a good representation of the frequency mix of paleoclimatic energy in the entire climatic system. Precessional forcing appears to be a constant in the accumulation of organic and inorganic carbon in the deep sea, and in the influx of terrigenous materials. Obliquity signals appear strongly only after the cooling of the high latitude Southern Hemisphere began.
The persistence of periodicities associated in some way with eccentricity forcing is one of the major surprises that come from looking at older records. Energy at about 100-, 400-, and in some cases, 2,400-kyr wavelengths is especially clear before the onset of large-scale Southern Hemisphere glaciation in the earliest Oligocene, but it also appears in many later carbon records. The dominance of the 41-kyr obliquity cycle in long Oligocene, early Miocene, and early Pleistocene carbon records make it clear that 100-kyr cyclicity is not inherent in the carbonate recorder itself. Rather, eccentricity-related cycles in carbonate content and in carbon isotopic composition most likely reflect real changes in ocean and atmospheric circulation at these frequencies. One of the unresolved puzzles of late Pleistocene climate, the dominance of an orbital term with little direct insolation forcing, therefore grows in importance with a longer time perspective.
What we do not know yet is the extent to which anomalies in marine carbon deposition related to climate change represent not only passive tracers of changes in ocean circulation but also active feedback responses that could modify climate sensitivity. As tracers, the aspects of the carbon system that can be monitored geologically include changes in the path and sources of deep-water formation, changes in wind stress and nutrient inventories to the surface biota, and transfers of carbon between terrigenous and marine reservoirs. Feedbacks implied by ancient carbon cycles range from variable uptake by the ocean of atmospheric carbon dioxide due to changes in temperature and circulation, to anomalies in the global rates of organic and carbonate carbon deposition rate that could affect the long-term greenhouse gas content of the atmosphere. It is particularly important to understand the origin of long-wavelength marine carbon cycles as these could potentially alter factors such as sedimentary storage of nutrient elements and the carbon dioxide content of the atmosphere for long enough periods of time to affect weathering cycles on land and evolution and biogeochemical cycling in the ocean. If the data presented above are representative of ocean history for the past 125 Myr, the gap between the relatively short duration (˜20 and 41 kyr) orbital insolation and tectonic perturbations to climate and biogeochemical cycles may be bridged by nonlinear climate–carbon connections. These may cause changes in the oceanic output of carbon and associated elements on time scales normally considered to fall within the tectonic window (cf. ref. 60).
If we are to look for common threads over time, it is necessary to look for robust mechanisms that do not depend, for example, on the existence of extensive glaciation (for example, see ref. 24), or on other boundary conditions not appropriate for the span of late Mesozoic and Tertiary time. A simple model should also explain why different variables might be coupled in a response to orbital forcing. I suggest that the interaction between radiative forcing, marine sea surface temperatures, and the hydrological cycle may give a rich array of climatic responses at Milankovitch frequencies over long periods of geological time. For the carbon cycle, small changes in the density structure of deep waters can modulate the basin–basin fractionation of alkalinity and nutrients (cf. ref. 57) and explain dissolution patterns imposed on many sediment carbonate records. The existence of carbonate dissolution cycles in basinal deposits during most of the Tertiary and late Mesozoic suggests that there have been at least two regions of deep-water formation, with different initial dissolved inorganic carbon characteristics, throughout this time.
The density differences between potential deep-water-forming areas may be extremely small and would be sensitive to changes in both sea surface temperature and salinity. The Mediterranean may be a small-scale example of this phenomenon. It seems likely that the present evaporative water balance shifted at a number of time in the past to fresher conditions in response to enhanced African monsoons, those regulated by the precessional cycle ( 58 ). During such periods, the exchange of waters between the Atlantic and Mediterranean may have been reversed, leading to a nutrient-trap configuration and the development of anoxic bottom waters and organic-carbon-rich sediment layers ( 61 ).
Because water vapor is such a potent greenhouse gas, changes in sea surface temperature could amplify global temperature anomalies. On land, changes in the water balance could promote or disfavor the storage of organic carbon in soils and biomass. The signs of terrestrial climate change may be imprinted in cycles in the carbon isotopic composition of sea water, and in the variations in deposition rate of terrigenous dust in deep-sea sediments. Geological records hint that all of these responses happen commonly, and that they have important nonlinear aspects that enhance modest initial forcings.
I thank S. D’Hondt, E. Erba, A. G. Fischer, C. D. Keeling, J. Park, and I. Premoli Silva for many helpful discussions, and two anonymous reviewers who helped improve the final manuscript. Portions of this work were funded by the Petroleum Research Foundation of the American Chemical Society and by grants from the National Science Foundation.
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