Cenozoic Early

Fig. 4.1. Isotopic paleotemperature analyses of planktonic and benthic foraminifera from the sub-Antarctic Pacific indicating considerably warmer conditions in the Early Cenozoic (Shackleton and Kennett, 1975a).

Kerrick, D.M. and Caldeira, K. (1993) Paleoatmospheric consequences of CO2 released during early Cenozoic regional metamorphism in the Tethyan orogen. Chem. Geol., 108, 205-230. 

Cenozoic is a time of high but variable pCOj. By early Miocene time (ca. 22 Ma) COj levels are comparable to the present (pre-industrial) values. The calculated pCOj for the period from the middle Miocene through the Pliocene comes out lower than the modem value, which does not correspond well with the 5 0 evidence for continued cooling between 15 Ma and the present (Fig. 16c). It may be noteworthy that higher assumed values for 5 correspond to very high levels of atmospheric COj in the early Cenozoic, whereas 5w values in the range -0.5 to -0.6 correspond to the lx to lOx higher COj levels that are typically estimated with other approaches (e.g., Ekart et al. 1999). 

By summarizing the isotope record of sedimentary sulfides throughout the Phanerozoic, StrauB (1997) and (1999) argned that the long-term trend for the entire Phanerozoic broadly parallels the sulfate curve with maximum values in the early Paleozoic, minimum values in the Permian, and a shift back to higher values in the Cenozoic. The isotopic difference between snlfate sulfur and minimum sulfide sulfur varies within -51 8%c. 

Figure 3.49 summarizes the oxygen isotope curve for the last 65 Ma. The most pronounced warming trend is expressed by a 1.5%o decrease in 8 0 and occurred early in the Cenozoic from 59 to 52 Ma, with a peak in Early Eocene. Coinciding with this event is a brief negative carbon isotope excursion, explained as a massive release of methane into the atmosphere (Norris and Rohl 1999). These authors used high resolution analysis of sedimentary cores to show that two thirds of the carbon shift occurred just in a few thousand years, indicating a catastrophic release of carbon from methane clathrates into the ocean and atmosphere. 

Red beds are continental or marine sedimentary rocks with an eye-catching red colour which has been responsible for the interest in them. They are widespread all over the world and belong mainly to the Late Palaeozoic, Early Mesozoic, and Late Cenozoic periods. 

Armstrong R. L. (1988) Mesozoic and early Cenozoic magmatic evolution of the Canadian Cordillera. Geol. Soc. Am. Spec. Pap. 218, 55—91. 

Over the long term, the Cenozoic deep-sea oxygen isotope record is dominated by two important features that relate to major shifts in mean climatic state. The first is a rise in values from 53 to 35. This trend, which is mostly gradual but punctuated by several steps, is an expression of the Eocene transition from greenhouse to icehouse conditions. In the Early Eocene (—53 Ma) the deep sea was relatively warm, —7 °C warmer than present, and there were no ice sheets. Over the next 20 Myr. the ocean cools, and the first large ice sheets appear on Antarctica. The latter event is reflected by the relatively sharp l.2%o increase in 5 0 at 33.4 Ma. This pattern reverses... 

Figure 3 Composite marine strontium isotope record (sources Miller et al., 1991a Oslick et al., 1994 Hodell and Woodruff, 1994 Mead and Hodell, 1995 Farrell et al., 1995 Martin et al., 1999 Reilly et al., 2002). For the Early Cenozoic where data coverage is sparse the hest fit of McArthur et al. (2001) is plotted.

Os/ Os ratio remains relatively constant for the majority of this time interval, with the notable exception of the Eocene-Oligocene transition itself. During the Paleocene and Eocene (35-65 Ma), it appears that the osmium record exhibits considerably more variability than does the strontium record. Neither record shows clear evidence of systematic change to more or less radiogenic isotope compositions during this early part of the Cenozoic. 

Figure 14 Estimates of mixed-layer pH and atmospheric CO2 derived from boron isotope analyses of planktonic foraminifera recovered from drill cores taken in the western equatorial Pacific. Results are replotted from Pearson and Palmer (2000). Note that uncertainty associated with both pH and CO2 estimates amphfies in the Early Cenozoic. This reflects the intrinsic insensitivity of the boron isotope pH proxy at low pH (see Eigure 12). Atmospheric CO2 estimates require additional assumptions about the size of the DIC pool in surface waters.

Further support for the weathering hypothesis for Late Ordovician glaciation comes from the strontium isotope record. A multimillion-year decline in the marine record of Sr/ Sr in limestones is reversed in the early Late Ordovician (Veizer et al., 1999), a likely consequence of collisional tectonics that initiated at this time and continued (as did the rise in Sr/ Sr) through the Silurian (Richter et al, 1992). The increase (from 0.7078 to 0.7088) rivals the Cenozoic rise, which has been attributed to the Himalayan uplift and associated with the progressive cooling leading to Quaternary glaciation (Raymo et al, 1988). 

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