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The Pleistocene Precedent
Geologically measurable aspects of the marine carbon cycle trace important perturbations in the Earth’s climate; these measurements also suggest that the marine carbon cycle may have feedbacks that amplify climatic variance. One of the first convincing links between marine sediments and global climate change of the Pleistocene came from Arrhenius’ ( 16 ) study of the calcium carbonate content of sediment cores from the equatorial Pacific Ocean. Arrhenius correctly connected the variations in deep-sea sediment composition with ice age cycles known from the land and also noted the cyclical nature of the changes over time. It is now clear that over the past 2.5 My, nearly all aspects of the Earth’s climate have been paced by variations in the Earth’s orbital elements ( 17 , 18 and 19 ). In the deep sea, these changes include rearrangement of the deep circulation such that present differences in carbon chemistry between the abyssal Atlantic and Pacific nearly vanished during glacial stages ( 6 , 20 ). This restructuring is evidenced by changing lateral gradients in carbon isotopes recorded by benthic foraminifera ( 4 , 6 ), and by compensating changes in calcium carbonate content in Pacific and Atlantic sediments ( 21 ). During ice ages, carbonate microfossils tend to be better preserved in the Pacific and to show poorer preservation in the Atlantic. The carbonate variations trace important modulations in deep-water sources over time and, therefore, in the heat and salinity budgets of the global ocean circulation.
Carbon isotopic records measured from the skeletal remains of foraminifera indicate major changes in the carbon budget of the glacial world as compared with the present. A global shift to lighter isotopic compositions is best explained as a transfer of terrestrial organic matter (with mean isotopic composition of -25‰ to the ocean during ice ages after the destruction of forests and soils in the colder and drier glacial world ( 22 ). The mean oceanic shift in d13C of -0.4‰ ( 22 , 23 ) corresponds to the addition of some 450 × 1015 g of carbon to the ocean during glacial times. By itself, such a transfer would have tended to promote a higher CO2 content of the atmosphere. Ice core records, however, indicate the reverse; the paradox must be resolved by changes in the carbon cycle in the ocean that favor increasing carbon storage in the ocean during glacial periods. To date, hypotheses involving changes in the production of marine organic carbon in the glacial world ( 24 , 25 and 26 ) and in the alkalinity structure of the glacial ocean ( 27 ) have been proposed but have not produced a satisfying resolution.
As we look to time scales of millions of years, it becomes clear that the spectral content of Pleistocene carbon records mirrors that of other paleoclimatic indices. For example, the transition in ice volume frequency at ˜1 Myr ago from an earlier regime of 41 kyr (obliquity) to the late Pleistocene rhythm of large-amplitude, 100-kyr cycles, shows clearly in carbonate time series as well as in changes in oxygen isotope ratios ( 28 , 29 ). In both regimes, carbonate variations record in large measure the dance of deep-water sources over time. The faithfulness of marine carbonate records in replicating the mid-Pleistocene switch observed in oxygen isotopic data from 41- to 100-kyr-dominated spectra is significant for it indicates that nonlinearities in carbonate preservation and production are not so inherently large as to inevitably produce 100-kyr and longer cycles from precessional climate forcing alone (we will return to this point later). If changes in ocean alkalinity and nutrient structure are to be inferred for late Pleistocene, 100-kyr ice age cycles ( 27 ), then the early Pleistocene regime saw largely 41-kyr repeat cycles of high latitude origin in the oceanic carbon pump.
Organic carbon contents of marine sediments also vary at Milankovitch frequencies in the late Pleistocene ( 30 , 31 , 32 and 33 ). If these can be read as paleo-productivity records, then there is evidence for substantial modifications in the export of carbon from surface waters in soft form. It appears, however, that the patterns of organic carbon accumulation obtained from sediments are highly regional, and it is difficult at present to reconstruct a global mass balance for the export of organic matter during glacial periods.
Signatures of Orbital Cycles
A general understanding of the “fingerprint” of orbitally driven cycles is necessary if we are to follow the sensitivity of the climate system in general, and the carbon cycle in particular, to orbital perturbations in the more distant past. Direct insolation forcing comes from the obliquity cycle, with a current mean period of 41 kyr, and from precessional variations, with periods of 23 and 19 kyr. Considerations of tidal friction indicate that the periods of these orbital cycles should be shorter in the past; Berger and colleagues ( 34 ) estimate that the obliquity repeat time would be 38.75 kyr by 100 Ma (mid-Cretaceous) and that the precessional periods would lie at about 22 and 18 kyr, respectively. Eccentricity forcing is likely to remain at similar frequencies to the present day, that is, at a short cycle of ˜109 kyr actually comprised of components near 95 and 125 kyr, and nearly “line” frequencies of 413 and 2,400 kyr ( 35 , 36 ). These frequencies should primarily enter the climate system as modulations (the variance around the mean) of precession, as the direct radiative forcing due to eccentricity variations is minuscule. However, the importance of 100-kyr energy in the late Pleistocene record of ice volume tells us that Milankovitch climate theory may be missing a major component of climate variance.
Geologists have a number of tools to use as they examine older marine records for continuing evidence of orbitalclimatic forcing. By using multiple constraints, a convincing case can often be made that orbital frequencies are correctly identified. Criteria include repeat periods estimated by paleomagnetic and paleontological data consistent with orbital cycles, continuity of cyclic sedimentation, statistical evidence for periodicity from spectral analyses, successful correlation of patterns between different study locations, and internal consistency of the data viewed with the orbital model of sedimentation (for example, evidence of the expected amplitude modulating frequencies of precession when a precessional signal is identified). In most cases, the repeat times of carbon cycles can be estimated to a relative error of 10–20% over the past 84 Myr, the period of frequent, well-dated magnetic reversals. Good matches over long time periods between the repeat times of sedimentary cycles and expected orbital periods are powerful indications that a cause and effect connection has been found. Difficulties in dating older marine sediments accurately mean that other consistency arguments become more important in fingerprinting an orbital-climatic connection. One can also compare the character of sedimentary cycles to the expected forcing. For example, obliquity cycles are nearly sinusoidal in nature, whereas the precessional rhythm is strongly modulated by eccentricity cycles at about 100, 400, and 2,400 kyr. Such diagnostic features can often be observed in sedimentary time series.
As we perform time series analyses of pre-Pleistocene deep-sea sediments, some of the questions we ask are as follows. (i) Is there evidence of continuing paleoceanographic sensitivity to orbital forcing since the Mesozoic, as seen through carbon records? (ii) If so, how does the climatic response to the possible mix of orbital components evolve over time? (iii) Is there evidence that Northern and Southern hemispheres continue to be synchronized to orbital-climatic cycles in the distant past? (iv) Do long-wavelength, eccentricity-driven cycles appear in pre-Pleistocene carbon records? (v) Can we deduce some features common to orbitally driven changes in the carbon cycle over the long late Mesozoic and Tertiary marine record?