Ocean Drilling Program, United States

Carbonate content and mass accumulation rate (MAR) were determined for Holes 998A, 999A, and 1000A recovered during the Ocean Drilling Program (ODP) Leg 165 in the Yucatan Basin (3101 m), the Colombian Basin (2839 m), and the Pedro Channel (927 m), respectively, for an interval spanning most of the middle Miocene and the early part of the late Miocene. Aragonite MAR was analyzed in Hole 1000A to detect dissolution of metastable carbonates at subthermocline depths in addition to sea-level variations. Oxygen and carbon stable isotope composition of benthic foraminifers are used as a proxy for sea-level fluctuations and as a record for the chemistry variations of deep and intermediate water, respectively. The middle to late Miocene transition in the Caribbean was characterized by massive increase of carbonate dissolution. Five well-defined dissolution episodes between 12 and 10 Ma are characterized by dramatic reductions in carbonate content and MAR. This interval is referred to as the Caribbean carbonate crash. The term "carbonate crash" was borrowed from ODP Leg 138 published results (Lyle et al., 1995). The timing and periodicity of four of the five carbonate-dissolution episodes in the Caribbean basins appear to correspond to the late middle Miocene production peaks of the North Component Water (Wright and Miller, 1996), equivalent to the modern North Atlantic Deep Water (NADW). These findings suggest that the carbonate crash in the Caribbean and in the eastern equatorial Pacific was caused by a reorganization of the global thermohaline circulation induced by the re-establishment and intensification of the NADW production and concomitant influx of corrosive southern-sourced intermediate waters (analogous to the modern Antarctic Intermediate Water Mass) into the Caribbean. 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 and foundering along the northern Nicaraguan Rise in the middle Miocene led to the establishment of a connection between the southern and northern Caribbean basins by the opening of two new main seaways, the Pedro Channel and the Walton Basin (Droxler et al., 1998). Once established, this connection triggered the initiation of the Caribbean/Loop Currents. The gradual closing of the Central American Seaway, simultaneous to the opening of seaways along the northern Nicaraguan Rise, disrupted the low latitude connection between the Atlantic and the eastern Pacific and, as a direct consequence, further strengthened the Caribbean Current. Based upon the observation of different coccolith assemblages present on either side of the Central American Seaway at some point during the middle to late Miocene transition, the Pacific-Atlantic connection might have been completely closed at that time. This full closure would explain the observed contemporaneous intermingling of terrestrial fauna between North and South America. 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. These conditions were favorable for the contemporaneous re-establishment of NADW. This reorganization of the global oceanic circulation at the middle to late Miocene transition is well recorded in the contrasting carbonate preservation pattern observed in the Caribbean basins, the eastern equatorial Pacific, and the Ceara Rise (equatorial Atlantic).