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Climate in Earth History: Studies in Geophysics
net heat loss to propel the atmospheric circulation that has led to modern climates.
The Arctic Ocean sediment record for the past approximate 5 million years has been studied in detail. Some 13 lithostratigraphic units have been defined for the late Miocene to Holocene (Clark et al., 1980) (Figure 14.4). These sedimentary units, correlatable over 400,000 km2, show variations on a single theme, ice rafting. The 13 lithostratigraphic units indicate alternating times of more and less ice rafting but no significant deviation from this mode of sedimentation. Whether times of greater ice rafting correlate with thinner or thicker ice cover has not been answered definitely (Clark, 1971).
Recently, a few coccoliths have been reported from late Pliocene and Pleistocene central Arctic sediment (Worsley and Herman, 1980). Although this is interpreted to indicate episodic ice-free conditions for the central Arctic, the occurrence of ice-rafted debris with the sparse coccoliths is more easily interpreted to represent transportation of coccoliths from ice-free continental seas marginal to the central Arctic. The sediment record as well as theoretical considerations make strong argument against alternating ice-covered and ice-free condi
FIGURE 14.4 Lithostratigraphy of the central Arctic Ocean. Units A-M consist of glacial-marine sediment, suggesting alternating times of relatively rapid and slow rates of deposition. Magnetic stratigraphy correlation indicated. Modified from Clark (1977a).
tions (Donn and Shaw, 1966; Clark et al., 1980). Although the time of development of the pack ice for the central Arctic Ocean is unknown, to date there is no evidence that precludes a Miocene origin.
Geologically significant invertebrate faunas developed in this glacial-marine environment. This includes a predominantly arenaceous (textularid) biofacies in the late Miocene to Pliocene and a calcareous biofacies (milioids and rotalids) in the Pliocene and Pleistocene (O’Neill, 1981). There is no evidence for other than normal salinity Arctic bottom water since the late Miocene.
Productivity of Arctic water, as measured by foraminifera abundance, has increased to the present condition, and at no time during the Late Cenozoic was productivity any greater than that of the present (Clark, 1971). Reported high-bottom-water productivity during the late Miocene is grossly overstated (Margolis and Herman, 1980). Modern productivity is not measured by the standard of other oceans. Nonetheless, it is high compared with productivity for any other part of the Late Cenozoic for which there is a record. Modern productivity has been achieved under year-round ice-covered conditions.
Correlation of pre-Pleistocene climate events of lower latitudes with that of the central Arctic is difficult. For example, waxing and waning of continental glaciers occurred while the Arctic ice-cover remained relatively constant. Perhaps, thickening and thinning of ice may have been the only central Arctic response to glacial and interglacial climates in lower latitudes. The Ewing-Donn (1956) theory suggested that the Arctic Ocean played a central role in northern hemisphere glaciation, with an ice-free Arctic producing glaciation and an ice-covered Arctic (such as that of the present) producing interglacial times. The evidence available no longer supports this model. Rather, Late Cenozoic glacial and interglacial stages developed with a constant, central Arctic ice cover. It appears that the Arctic ice cover, while responsible in part for Pleistocene atmospheric circulation patterns, was only a participant in the drama of the recent ice ages; certainly it did not play the leading role.
The conclusion, that the Arctic Ocean had an important role in pre-Pleistocene climates appears justified. The open-water Late Cretaceous-Paleocene Arctic was one of the factors contributing to worldwide climate uniformity. The probable Middle Cenozoic development of an ice cover, accompanied by Antarctic ice development and a late shift of the Gulf Stream to its present position, were important events that led to the development of modern climates.
The record suggests that altering the present ice cover would have profound effects on future climates. The technology is available to melt the Arctic Ocean ice cover (e.g., Arnold, 1961). In addition, there have been proposals by the Soviets to divert major rivers from their normal courses toward the Arctic Ocean, which would significantly alter the Arctic Ocean water-mass density structure. A change in the Arctic water budget could affect the ice cover. Yearly ice-free conditions could develop because of more vertical convection and release