CONCLUSIONS

The middle to late Miocene Caribbean carbonate crash consists of a time interval that included several massive carbonate-dissolution episodes. From 12 to 10 Ma, five distinct intervals of dissolution were recorded in the Yucatan Basin, Colombian Basin, and on the northern Nicaraguan Rise. The timing and periodicity of four of the five carbonate-dissolution episodes in the Caribbean basins appear to correspond to the peaks of NCW (equivalent to NADW) production suggested by Wright and Miller (1996) based upon the variations of the benthic ¹³C gradient between the North Atlantic, the southern, and eastern Pacific oceans. These findings suggest that the carbonate crash in the Caribbean was caused by a reorganization of the global thermohaline circulation induced by the re-establishment and intensification of the NADW production and concomitant influx into the Caribbean of corrosive southern-sourced intermediate waters (analogous to the modern AAIW). Because of its semi-enclosed nature below intermediate-water depths, the waters entering the Caribbean over sill depth (part of the thermohaline return flow) fill the deep Caribbean basins and affect carbonate sediment accumulation therein. At the time of the late middle Miocene carbonate crash, the Caribbean became—and remains—an important pathway for the return flow of the global thermohaline oceanic circulation.

Tectonic activity on the northern Nicaraguan Rise in the early middle Miocene led to the establishment of a connection between the southern and northern Caribbean basins by opening two new main seaways, the Pedro Channel and the Walton Basin. Once established, this connection triggered the initiation of the Caribbean Current. The gradual closing of the Central American Seaway, simultaneous to the opening of seaways along the northern Nicaraguan Rise, disrupted, temporarily shut down the low-latitude connection between the Atlantic and the eastern Pacific, and, as a direct consequence, strengthened the Caribbean Current. In the late middle Miocene, the newly developed and strengthened Caribbean Current transported warm, saline waters of the Caribbean to the northern North Atlantic via the Loop Current, the Florida Current, and the Gulf Stream. Several lines of evidence show that these different currents strengthened as the Caribbean Current was initiated.

Before the seaway opening along the northern Nicaraguan Rise, the northern Caribbean was largely isolated from the southern Caribbean basins, explaining the differences in calcareous coccolith assemblages and the absence of the Caribbean carbonate-crash precursor at Site 998 in the Yucatan Basin. At this time, well-ventilated, carbonate-rich waters entered the northern Caribbean Basin and promoted the preservation and the accumulation of carbonate sediments. In contrast, the Caribbean carbonate-crash precursor is well developed at Sites 999 and 1000, the two sites in the southern Caribbean basins. In the middle Miocene interval prior to 12 Ma when the NADW was not fully developed (Wright and Miller, 1996), the timing of several carbonate-dissolution episodes during the carbonate-crash precursor in the Colombian Basin correspond relatively well to some punctuated episodes of lysocline shoaling on the Ceara Rise. This finding suggests that the deep and intermediate waters in the equatorial Atlantic during these dissolution episodes probably had a southern source, were corrosive to carbonates, and were connected to the waters filling in the southern Caribbean basins, at least at intermediate water depths.

The re-establishment and intensification of the NADW at the middle to late Miocene transition modified the global thermohaline circulation pattern, which became comparable to that of today. The reorganization of the global oceanic circulation is well recorded in the contrasting carbonate-preservation pattern observed in the Caribbean basins, the Ceara Rise, and the eastern equatorial Pacific basins. An oxygen carbonate-rich, nutrient-poor, northern-sourced deep-water mass (NCW equivalent to the NADW) bathed the deep equatorial Atlantic (Ceara Rise) and preferentially preserved the carbonate sediments on the rise. In contrast, a southern-sourced, nutrient-rich, corrosive intermediate-water mass (similar to the modern AAIW) filled in the Caribbean basins by flowing over the sills of the Lesser Antilles and causing dissolution of the Caribbean sediments. Although the pattern of carbonate preservation between the equatorial Atlantic (Ceara Rise) and the Caribbean basins during the late middle and early late Miocene interval is not as clearly out of phase as the model would have predicted, it is still interesting to note that intervals characterized by maximum carbonate dissolution occurred in the equatorial Atlantic before 11 Ma and most of them prior to 12 Ma. The interval between 12 and 11 Ma, corresponding to the first half of the Caribbean carbonate-crash interval, was a transition period when the pattern of carbonate preservation became out of phase between the equatorial Atlantic and the southern and northern Caribbean basins. For instance, the two most intense episodes of carbonate dissolution in the Caribbean basins (Episodes I and III) correspond to times of good carbonate preservation in the deep equatorial Atlantic. During the second half of the Caribbean carbonate-crash interval, from 11 to 10 Ma, the equatorial Atlantic displays an overall pattern of good carbonate preservation and it appears to remain out of phase with the Caribbean. It is also well established that the eastern equatorial Pacific sustained considerable carbonate reductions at the middle to late Miocene transition. Intervals of intense carbonate dissolution in the eastern equatorial Pacific coincide with the younger three, possibly four, dissolution episodes of the Caribbean carbonate crash. The synchronous nature of carbonate dissolution on both sides of the Isthmus of Panama after the uplift of the isthmus to upper bathyal depths supports global causes for the carbonate crash at the middle to late Miocene transition and can be explained by the establishment of a global ocean circulation similar to the one we observe today. The postponed recovery of the carbonate system in the eastern equatorial Pacific until 9-8.5 Ma, as opposed to 10 Ma in the Caribbean basins, appears to be linked to the contemporaneous temporary complete closure of the Central American Seaway, most likely related to a major eustatic fall at ~10-9.5 Ma. Although the carbonate recovery occurred significantly earlier in the Caribbean basins than in the eastern equatorial Pacific, the benthic ¹³C values in the Caribbean basins remained depleted during the nadir of the carbonate crash in the eastern equatorial Pacific. The benthic ¹³C records of the Caribbean basins, therefore, probably better establish the temporal and global conveyor links between the basins on either side of the Isthmus. Alternatively, the sea-level fall contemporaneous to the nadir of the carbonate crash in the eastern equatorial Pacific may be the cause for the light ¹³C values observed from ~10 to 9.5 Ma in the Caribbean basins rather than another pulse of NCW production. The observation that the NCW production was gradually decreasing rather than increasing during the same time interval would favor this alternative scenario.