18O
exhibit some coincidence with these volcanic episodes, suggesting that some
connection may exist between the intensified volcanic activity and climate
change in the Cenozoic, particularly during the late Eocene and Oligocene.A principal discovery from Leg 165 drilling in the western Caribbean was the identification of three major episodes of explosive volcanism that occurred during the last 55 m.y. on the margins of the evolving Caribbean Basin. These episodes can be clearly related to regional sources, but how do these events compare with the global record of explosive volcanism? Kennett and Thunnell (1975) first pointed out that the frequency of volcanic ash layers in deep-sea sediments is episodic with pronounced peaks in the Quaternary (last 2 m.y.) and the middle Miocene (14-16 Ma). They suggested that these episodes were global in nature and related to large-scale plate tectonic processes. Subsequent studies have also shown evidence for broad global synchronism in volcanic ash deposition in the ocean basins (Kennett and Thunnell, 1977; Kennett, 1981). These observations were further supported by later results from DSDP sites with better core recovery (Cadet and Fujioka, 1980; Cadet et al., 1982a, 1982b; Cambray and Cadet, 1996). Compared to these studies, however, the Caribbean sites are characterized by a conspicuous lack of a late Quaternary peak and an offset between the global mid-Miocene peak and the early- to mid-Miocene peak seen at Sites 998, 999, and 1000.
Comparison with tephra layer distributions in a number of other ODP and DSDP sites (Cambray and Cadet, 1996) indicates that explosive volcanic activity also increased globally during the Eocene, and thus the Caribbean Eocene volcanic episode also has contemporaneous equivalents in many other arcs. Data on the global volcanic ash distribution during the Paleogene are much poorer at sites outside the Caribbean, however, as many drill sites do not penetrate to this depth.
Thus the Caribbean episodes discussed above, and evidence from earlier compilations of tephra episodes from ODP and DSDP sites in other ocean basins, indicate two major global pulses of explosive volcanic activity. We note, however, that the drilled marine record does not as yet include adequate sampling of volcanic ash falls from the greatest ignimbrite flare up on Earth: the Sierra Madre Occidental in Mexico. On the basis of normal atmospheric transport and fallout patterns, it is likely that tephra layer deposits from this volcanic episode are to be found in the Gulf of Mexico, a region that has not experienced scientific drilling since DSDP Leg 10 (Worzel et al., 1973). Because of very poor core recovery, information on tephra distribution from Leg 10 drilling in the Gulf of Mexico is scanty, but lower Oligocene tephra layers were observed at DSDP Site 94, and Sites 86, 94, 95, 96, and 97 also contain common ash layers (Worzel et al., 1973). The Sierra Madre Occidental activity is quite extensive in space and time, ranging from 47 to 27 Ma (McDowell and Mauger, 1994), and its northward continuation, the San Juan and the Mogollon-Datil volcanic fields, extend the record of large-scale explosive volcanism in this region up to 25 Ma, or spanning all of the Oligocene. When taken together, the combined land and marine evidence shows that the Paleogene ignimbrite activity of the Central American and Mexican arcs thus spans the mid- to late Eocene and most of the Oligocene (Fig. 8).
As shown by a comparison
of the major volcanic episodes with the oxygen isotope evidence in Figure
8, major oxygen isotope enrichments in benthic 18O
exhibit some coincidence with these volcanic episodes, suggesting that some
connection may exist between the intensified volcanic activity and climate
change in the Cenozoic, particularly during the late Eocene and Oligocene.
The concept of global synchroneity of explosive volcanism is difficult to reconcile with the paradigm of plate tectonics. At any one time, the various subduction zones on Earth experience a spectrum of the parameters that may affect the rates of subduction zone volcanism, such as the rates and direction of plate motion, and age and composition of the subducting slab. No unifying mechanism is obvious that could lead to a simultaneous increase in global eruption rates. A possible exception to this may be the rate of sediment subduction. The high global rate of explosive volcanism in the Quaternary could possibly be related to increased availability of sediment for subduction (von Huene and Scholl, 1991). Such a hypothesis does not account, however, for the Miocene and Eocene explosive volcanic episodes documented here.
It has been proposed (Sigurdsson, 1990a) that the global episodic ash layer frequency in deep-sea sediments may be influenced by a transport function, such as major climate-related variations in the global atmospheric circulation and tephra dispersal. Evidence from marine sediments supports the idea that the observed variations in deposition rate of some deep-sea volcanic ash layers may be influenced by transport processes (i.e., the vigor of atmospheric circulation). For example, data from DSDP Site 284 show peaks in ash abundance coinciding with glacial episodes during the latest Miocene, late Pliocene and the Quaternary (Shackleton and Kennett, 1975; Kennett et al., 1979). Similarly, early studies showed that the peak periods of eolian transport and strong atmospheric circulation coincide with the episodes of volcanic ash deposition (Leinen and Heath, 1981; Leinen, 1985; Rea et al., 1985). This coincidence may indicate that volcanic ash frequency may not simply be an indicator of variations in the volcanic source function, but may also be influenced by the rate of transport (i.e., climate-related variations in the vigor of atmospheric circulation). Conversely, the relationship between the intensity of volcanism and climate may result from crustal dynamics affected by changes in sea level and glacier loading. However, in reviewing the marine eolian deposition on the basis of evidence from a sediment core from the central gyre of the North Pacific, Rea (1994) concludes that dust size increases progressively from 50 Ma to present, with some oscillations, but no distinct peaks during the mid-Miocene or Quaternary. Similarly, the dust flux in this core shows a dramatic increase at 3 Ma (onset of Northern Hemisphere glaciation), but this is primarily an aridity change, not a wind intensity shift.
Thus the causes of the global episodes in deep-sea tephra deposition remain poorly understood, and the relative importance of the variable intensity of both source and transport mechanisms needs to be unraveled. On land, however, we have clear indication that volcanism in the source regions is highly episodic, and a tectonic forcing function is a most likely explanation.
Do these silicic magmas represent true addition of new continental crust, or are they largely recycled or reworked continental material, with minor addition of mantle-derived material? Two contrasting models have been proposed: (1) crustal melting by influx of basaltic magma from the mantle (Verma, 1984; Ruiz et al., 1988); and (2) fractional crystallization from primary basaltic magma (Cameron et al., 1980; Smith et al., 1996). Intermediate models propose crustal magmatism as a result of assimilation fractional crystallization (AFC) between basaltic magma and the crust (Smith et al., 1996). In the case of crustal melting, the basalt volume may be approximately equal to the amount of melted crust. In the case of fractional crystallization, the fraction of basaltic melt would be considerably higher than the derived silicic magma. A central implication of these models is that a large volume of basaltic magma was available, either supplying heat for crustal melting (Reiners et al., 1995), or as primary magma for fractionation to generate voluminous silicic differentiates. The requirements for basaltic magma are truly large. In the Sierra Madre Occidental volcanic arc, for example, the volcanic extrusion rate of silicic magmas was probably of the order 20 km3/m.y./km arc during the Eocene-Oligocene episode. By comparison, the modern volcanic flux of the Central American arc is only 0.1-1 km3/m.y./km (Carr et al., 1990). This silicic magma flux would require a supply rate of basaltic magma of the order of 100 km3/m.y./km for fractionation and AFC scenarios, and no less than 50 km3/m.y./km for dominantly crustal melting scenarios, as compared to 1-9 km3/m.y./km magma extrusion rates in modern arcs (Sugimura et al., 1963; McBirney, 1978; Sigurdsson et al., 1980; Bloomer et al., 1989; Sigurdsson, 1990b).
