At all three sites, there is a high concentration of CaCO3 with lower concentrations of terrigenous matter and dispersed ash (Fig. 2). The concentration of CaCO3 varies between 0 and 80 wt% of the bulk sediment, with an average ~70 wt%. The extremely low concentrations of carbonate at Sites 998 and 999 at ~12 Ma define the "carbonate crash" discussed elsewhere (Farrell et al., 1995; Lyle et al., 1995; Roth et al., Chap. 17, this volume). Terrigenous matter concentrations average ~20 wt%, yet increase in the last 10 m.y. to ~35 wt%. This increase does not appear to be solely caused by the decrease in the amount of dispersed ash (i.e., is not caused solely by dilution), because the terrigenous accumulation rates also show a marked increase in this time interval (as discussed below). Overall there is a significant amount of dispersed ash, reaching a maximum value of 45 wt% of the bulk sediment. This is especially true before 10 Ma, where dispersed ash averages 18 wt%; after 10 Ma, dispersed ash only averages 5 wt% (i.e., near the detection limit of the normative calculation).
Site 999 generally shows the greatest mass accumulation rates of all three sediment components with two exceptions (Fig. 3). During the mid-Oligocene (between 36 and 24 Ma), CaCO3 and terrigenous accumulation rates are greater at Site 998 than at Site 999 and reach to more than three times greater in the terrigenous component and 1.5 times greater in the CaCO3 component. Also, between 60 and 62 Ma, Site 1001 has greater CaCO3 and dispersed ash accumulation rates than Site 999 by ~1.5 and two times, respectively. Details of the variations in these components, as well as their interrelationships and tectonic significance, are discussed below.
Sites 999 and 1001 record maxima in terrigenous matter accumulation rates at ~59 Ma (Fig. 4). At Site 999 these relatively high accumulation rates are maintained until ~50 Ma, whereas at Site 1001 the terrigenous accumulation reaches a maximum. Notwithstanding the caution needed in interpreting single data points, it appears that even at Site 1001 several points define an increase up to the maximum at 59 Ma and the decrease down from it. Site 999 shows low accumulation rates between 48 and 28 Ma, with essentially zero terrigenous accumulation between 46 and 32 Ma. Much of this time period of ultra-low terrigenous accumulation at Site 999 corresponds with the hiatus at Site 1001, which may reflect a regional depositional pattern. Between 28 and 6 Ma, Site 999 shows a steady increase in terrigenous accumulation (with the exception of the "carbonate crash"). Site 998 also shows a period of low accumulation rates in the Oligocene through the middle Miocene except for an increase in terrigenous accumulation at ~34 Ma. This event may reflect uplift in Guatemala (Weyl, 1980) that began during the Oligocene and increased through middle Miocene orogeny (Morris et al., 1990). Such uplift may be associated with the left-lateral offset between North America and the Caribbean as evidenced by the Oligocene-aged initial opening of Cayman Trough (Mann et al., 1990). At Sites 998 and 999, there is a significant maximum in terrigenous accumulation at ~12 Ma, which is not seen at Site 1001, and is related to the carbonate crash.
Most significantly, all three sites show a broad pattern of increased terrigenous matter (Fig. 2) and in terrigenous accumulation (Fig. 4) in sediment younger than 10 Ma. Sites 998 and 999 also record a maximum in terrigenous accumulation in the Pliocene at ~5 Ma and again at 1 Ma. In general, Sites 998 and 999 seem to be responding to the same input(s), especially since the late Miocene. The high terrigenous matter accumulation rates over the last 10 m.y. correspond with several orogenic events. From the late Miocene to the present, the final postorogenic phase in Guatemala, southern Central America, and northwestern South America has been characterized by uplift, deposition, and the advent of volcanism (Escalante, 1990; Morris et al., 1990; Mann et al., 1990; Mann and Burke, 1984).
Increases in terrigenous input over the past 10 m.y. are also recognized at Sites 925 and 929 located on the Ceara Rise (Dobson et al., 1997). Only carbonate and terrigenous matter (i.e., not ash) were studied at these Ceara Rise sites. Dobson et al. (1997) determined mass accumulation rates for the terrigenous component by using the calculated weight percent values determined from an operationally derived sequential extraction, shipboard discrete density measurements, and a biostratigraphic time scale. Even with the difference in techniques between those used by Dobson et al. (1997) and those used in this paper and the lower temporal resolution of the Dobson et al. (1997) data set, a marked increase in terrigenous accumulation rate for this same time interval at Ceara Rise is apparent and compares reasonably with the patterns we observe at Site 999 (Fig. 5). For the Ceara Rise, the increase is thought to be associated with Andean uplift and increased Amazon River flow (Dobson et al., 1997). Sites 998 and 999, located in the western Caribbean, are likely too far away to be reflecting an Amazon source. However, Site 999 is in close proximity to the Magdelena Fan, which drains the northern Andes (Sigurdsson, Leckie, Acton, et al., 1997). It is difficult to resolve whether the increase in terrigenous accumulation at Site 999 (beginning at 38 Ma) precedes the increase at Ceara Rise, since the Ceara Rise data set is poorly resolved from 30 to 42 Ma (Fig. 5). Regardless, both the Atlantic and the Caribbean are apparently showing the same broad depositional pattern from both the Amazon and Magdelena drainage basins. Thus, these two locations serve as good circum-Andean tracers of tectonism.
Dispersed ash accumulation rates (Fig. 3, Fig. 6) show three distinct periods of increased accumulation. Two of these periods are in the Paleocene (at 62-60 and 58-54 Ma) and are recorded at both Sites 999 and 1001. The younger (58-54 Ma) is through the Paleocene/Eocene boundary and may be causally related to the late Paleocene thermal maximum climatic event (Bralower et al., 1997).
These periods of increased deposition of dispersed ash occurred somewhat earlier at Site 999 than at Site 1001; however, Site 1001 has higher absolute accumulation rates (Fig. 3). The difference in timing probably is not caused by variations in age models; at both sites, the ages during these periods are tightly constrained (Sigurdsson, Leckie, Acton, et al., 1997). Also, the sedimentation rates at both sites were approximately the same during these times, especially between 62 and 60 Ma.
The major source of dispersed ash accumulation during both Paleocene maxima appears to be the Late Cretaceous Greater Antilles/Cayman Volcanic Arc (Montgomery, 1998; Pindell and Barrett, 1990; Stykes et al., 1982; Wadge and Burke, 1983; White and Burke, 1980; see also discussion in Sigurdsson, Leckie, Acton, et al., 1997). This arc evolved as a result of a reversal in subduction polarity between 120 and 110 Ma (Draper et al., 1996; Draper and Gutierrez-Alonso, 1997), but volcanism did not cease until the early Eocene in some of the Greater Antilles islands (Pindell and Barrett, 1990; Lewis and Draper, 1990). The Central American arc is not likely to be a major source of these Paleocene maxima because Site 999, located closest to Central America during this time (Pindell and Barrett, 1990), records lower dispersed ash accumulation (Fig. 3). Both Sites 998 and 1001 were closer than Site 999 to the Late Cretaceous Greater Antilles Arc (Pindell and Barrett, 1990), Site 1001 shows greater dispersed ash accumulation rates during these episodes than Site 999, and Site 998 also records a maximum in discrete ash layers at ~52 Ma.
The second volcanic episode was in the Eocene, between 50 and 40 Ma. The record of this event is only seen at Site 999 because of the hiatus at Site 1001 and the fact that samples older than 39 Ma were not analyzed for dispersed ash from Site 998. The source of the dispersed ash deposited between 50 and 40 Ma is most likely the Central American arc (Sigurdsson, Leckie, Acton, et al., 1997), which probably formed a continuous string of volcanic islands between South America and the Chortis block of Nicaragua during the Paleocene to middle Eocene (Maury et al., 1995).
The third episode of volcanism occurred in the Miocene between 22 and 14 Ma. The greatest accumulation by far is at Site 999, but the episode is also recorded at Site 998. The source is thought to be the Central American arc, similar to the source of the Eocene event (Sigurdsson, Leckie, Acton, et al., 1997) The volcanic activity was concentrated in the middle of Central America within the Chorotega block and gave rise to intensive volcanism (Escalante, 1990).
These dispersed ash records of volcanic activity agree well with the tectonic history of the region. Eruptive and intrusive calc-alkaline units are found in the Greater Antilles, such as the Mal Paso Formation and Palma Escrita Formation of Puerto Rico (Jolly et al., 1998), and in northern South America and Central America as seen in the volcanic agglomerate from the Tuira-Chucunaque Basin (Escalante, 1990; Frost et al., 1998). Crustal movement, which started in the early Tertiary and reached a climax in the early Eocene, caused uplift in northern Central America and throughout the northern Caribbean margin (Lewis and Draper, 1990). Arc volcanism in the eastern Greater Antilles commenced in the Cretaceous and terminated throughout the Eocene (depending on location), progressing from initial collision in Cuba to younger events of lesser effect to the west (Montgomery, 1998; Lewis and Draper, 1990; Pindell and Barrett, 1990).
Orogeny in Central America and western Columbia likely reached its peak during the late Miocene (Escalante, 1990). This activity gave rise to intense volcanism that is associated with the volcanic Aguacate, Rio Pey, and Paso Real Formations in Costa Rica, and other volcanic rocks in western Panama (Escalante, 1990). The broad timing of tectonic activity corresponds with the large Miocene accumulation of both dispersed ash and discrete ash observed at Sites 999 and 998. As described in Carey and Sigurdsson (Chap. 5, this volume), the extremely large volcanic eruptions responsible for the ash in the Caribbean Sea resulted in ejected material reaching stratospheric elevations, where wind flows from west to east (i.e., in the opposite direction of the lower level trade winds).
The record of discrete ash layers (Carey and Sigurdsson, 1998; Carey and Sigurdsson, Chap. 5, this volume) shows that the major volcanic episodes occurred in the Paleocene (65-60 Ma), Eocene (47-37 Ma), and Miocene (~17 Ma). Grain-size and shape analyses, along with modeling of ash transport mechanisms, indicate that these discrete ash layers most likely represent eruptions that exceeded 40 km in height and are associated with large volume ignimbrites (Carey and Sigurdsson, Chap. 5, this volume). The timing of these peaks in comparison with those for the dispersed ash accumulation rates indicate that the discrete layers consistently lag temporally behind the dispersed ash accumulation. This is especially apparent at Site 999 (Fig. 7). At this site, dispersed ash maxima are seen between 52 and 48 Ma and again between 42 and 41 Ma, whereas discrete ash layers peak between 42 and 40 Ma and between 36 and 34 Ma. The lead-lag relationship is also seen between 22 and 10 Ma with the dispersed ash leading by ~2-4 Ma. At Site 998, the relationship is less pronounced, but can be observed in a dispersed ash accumulation rate peak at ~14 Ma followed by a 12 Ma peak in the ash layer accumulation (Fig. 7).
Various causes can be proposed for these relationships. One explanation is that perhaps there is not a true temporal relationship being observed, but rather that the discrete ash layers reflect volcanism at close proximity and the dispersed ash accumulation rates reflect volcanism at further distances in the circum-Caribbean region. For example, the maximum in ash layer accumulation at Site 998 at ~50 Ma corresponds with identically timed peaks in dispersed ash observed at Site 999 (Fig. 7). Similarly, the maxima in dispersed ash accumulation rates at Site 998 at 18 and 34 Ma correspond with ash layer accumulations at Site 999. However, even though the accumulation of the ash layers (in centimeters per million years) is comparable (~250 cm/m.y.) at Site 998 at 50 Ma and at Site 999 at 34 and 18 Ma, the corresponding dispersed ash maxima are greater at Site 999.
If the discrete ash at one site and dispersed ash at another are contemporaneous and receiving ash from the same source, this provides information regarding the transport mechanism and the distribution of volcanic sediments. To form discrete ash layers at these sites would require high intensity eruptions with column heights in excess of 40 km (Carey and Sigurdsson, Chap. 5, this volume). Particles in the umbrella region of such an eruption would be transported by the prevailing high-altitude wind direction (Carey and Sigurdsson, Chap. 5, this volume; Sigurdsson et al., 1980). Thus, the discrete ash was most likely transported in the stratosphere and the transport is stronger in the northward direction.
Another potential explanation is that the region experienced long periods of eruptions of small magnitude followed by intense large-scale explosive volcanic activity. According to Sigurdsson et al. (1980), small magnitude eruptions such as the 1979 eruption of Soufriere in St. Vincent do not form megascopic ash layers; they only contribute to the dispersed ash content of the sediment. Experience in the Lesser Antilles indicates that visible layers are formed from eruptions that produce total volumes much greater than 1 km3 (Sigurdsson et al., 1980).
In this context, the dispersed ash may represent erosion of smaller terrestrially deposited ash sediments, which was then followed by associated massive volcanism (Noble et al., 1974) that resulted in discrete ash layers. If this is the source of dispersed ash, we should expect that even though the percent terrigenous component is small, there would be some coincidence between the terrigenous and the dispersed ash accumulation rate peaks because production and transport of both the dispersed ash and terrigenous components are caused by the same physical mechanism (i.e., erosion). This is in fact observed at all three sites. At Sites 999 and 1001, the terrigenous accumulation rate peak at 58-60 Ma corresponds with peaks in the dispersed ash. At Site 999, from 50 to 52 Ma a peak in terrigenous accumulation rate coincides with a peak in dispersed ash accumulation rate. The erosional event referred to earlier at Site 998 at 34 Ma shows a dispersed ash accumulation rate increase occurring at the same time as the increase in the terrigenous accumulation rate. Thus, in addition to recording obvious volcanism, the dispersed ash maxima may also be indicators of erosional events and could be proxies for uplift in the area.