Dramatic reductions of calcium carbonate weight percent, lower calcium carbonate mass accumulation rates (MARs), and poorer preservation of calcium carbonate microfossils characterize the interval at the middle to late Miocene transition (12-10 Ma) in the Caribbean (Sigurdsson, Leckie, Acton, et al., 1997). This interval has been identified as the Caribbean "carbonate crash" at three Ocean Drilling Program (ODP) Leg 165 sites. Such significant changes in preservation of carbonate sediments are related to processes that affect the global carbonate and carbon budgets. Studying the nature, extent, and timing of these intense fluctuations in the burial of carbonate sediments should result in a better understanding of the changes in global thermohaline circulation and the establishment of the modern ocean circulation.
Similar occurrences of carbonate dissolution at the middle to late Miocene transition have been recorded previously in other parts of the world. Vincent (1981) reported unusually low carbonate weight percent at the Deep Sea Drilling Project (DSDP) Site 310 on the Hess Rise (north central Pacific) and referred to the interval as the mid-Epoch 10 event. Epoch 10 is now correlated with the 4a Chron interval spanning between 9.2 and 9.6 Ma, based upon the geomagnetic polarity time scale of Cande and Kent (1992). The interval between 11.2 and 8.6 Ma cored during ODP Leg 138 and other DSDP sites in the eastern equatorial Pacific is characterized by very low carbonate MARs (Fig. 1). This interval was referred to as the "carbonate crash" by Lyle et al. (1995) and was interpreted by these authors as a 1200-m shoaling of the lysocline. A long-term shoaling of the lysocline occurred in the middle Miocene from 14.0 to 11.5 Ma followed by a lysocline deepening at 10.5 Ma as recorded in several ODP Leg 154 sites drilled on the Ceara Rise in the western tropical Atlantic (King et al., 1997).
The comparable nature and partially overlapping timing of the carbonate reductions in the Pacific, Atlantic, and Caribbean suggest a common cause associated with changing oceanic circulation. The opening and closing of gateways could have changed global ocean thermohaline circulation and, as a result, triggered the carbonate crash at the middle to late Miocene transition. Since the Cretaceous, the global ocean has evolved from a circum-tropical to circum-Antarctic surface circulation and from a halothermal to the thermohaline deep-water circulation of today (Kennett and Barker, 1990). The middle Miocene was a critical time of paleoceanographic reorganization during which time the oceanic circulation became more similar to that of today. Wright et al. (1992) and Wright and Miller (1996) place the initiation of a North Component Water (NCW), a precursor to the modern North Atlantic Deep Water (NADW) and a primary component of deep-water convection, in the late early Miocene. Other authors have suggested that the initial production of NADW occurred in the late middle Miocene. Woodruff and Savin (1989) proposed that NADW began approximately at 13.5 Ma (originally published ages translated to those of Raffi and Flores, 1995). More recently, Wei (1995) and Wei and Peleo-Alampay (1997) further constrained NADW initiation to 11.5 Ma. This age falls between the 13.2- and 10.4-Ma coccolith datums according to the biostratigraphy of Raffi and Flores (1995).
Two gateways within the Caribbean, the Central American Seaway and the Pedro Channel/Walton Basin on the northern Nicaraguan Rise, are thought to have been closing and opening, respectively, during the middle Miocene (Fig. 2, Fig. 3, Fig. 4, Fig. 5) (Duque-Caro, 1990; Droxler et al., 1998, and references therein). Closure and opening of these gateways have initiated the Caribbean and Loop Currents, directly strengthened the Gulf Stream, triggered and/or re-established the NADW production, and, therefore, changed the global distribution of deep-water masses and affected the preservation of carbonate sediment in the major oceans.
The semi-enclosed nature of the Caribbean acts as a discriminating valve for inflowing water masses. The Caribbean's connection to the deep Atlantic Ocean is restricted by sills extending from Venezuela to the Greater Antilles. Today, the uppermost part of NADW (the upper NADW [UNADW]) can enter the Caribbean through the deepest sills, the Windward Passage at 1540 m and the Anegada-Jungfern Passage at 1800 m (Fig. 3). Antarctic Intermediate Water (AAIW) flows at depths of 800-1400 m, overriding the UNADW, and mixes with the UNADW upon entering the Caribbean just above sill depth (Fig. 6A) (Haddad, 1994; Haddad and Droxler, 1996). This mixture then fills the lower reaches of the Caribbean basins. The Caribbean physiography provides a unique setting where sediment at abyssal depths in the Caribbean basins is being influenced by water masses of intermediate origins (a mixture of AAIW and UNADW). The Caribbean ODP Sites 998, 999, and 1000 cover a range of paleodepths from abyssal to upper bathyal waters. Moreover, these three sites are also located on both sides of the northern Nicaraguan Rise (which contains the Pedro Channel) (Fig. 4) and are in the vicinity of the Central American Seaway (Fig. 3).