"Carbonate crash" is the term applied by Lyle et al. (1995) and Farrell et al. (1995) to a major middle/late Miocene carbonate shift in tropical regions characterized by a dramatic reduction of calcium carbonate content in sediments and poor preservation of calcareous microfossils. This phenomenon has been widely documented in the equatorial Pacific, Atlantic, and Indian Oceans (Lyle, 2003; King et al., 1997; Peterson et al., 1992; respectively), as well as in the Caribbean Sea (Roth et al., 2000), and provides a seismic reflector for long-range correlation (Mayer et al., 1986; Bloomer et al., 1995).
Several hypotheses have been advanced to account for the carbonate crash. Mechanisms recently proposed attributed it to enhanced dissolution (rather than reduced surface water productivity of calcite-secreting organisms) associated with changes in deepwater circulation and shoaling of the carbonate compensation depth (CCD) and/or lysocline (e.g., Farrell et al., 1995; Lyle et al., 1995; Roth et al., 2000). Alternatively, the crash may have resulted from a biologic bloom (Theyer et al., 1985) (i.e., decomposition of organic matter from increases in surface productivity can result in enhanced dissolution of carbonate through acid production where the ratio of organic carbon to carbonate is high in falling debris) (Emerson and Bender, 1981; Archer, 1991a, 1991b). Dilution by terrigenous or other noncarbonate sediments could also have caused carbonate reduction in the sediments (Keller and Barron, 1983; Diester-Haass et al., 2004). This scenario involves changes in sedimentation patterns probably driven by climate change and/or tectonic changes in basin configurations or closure of oceanographic pathways. A contemporary global sea level drop (Haq et al., 1987) was excluded because it is believed that the middle/late Miocene sea level drop is associated with a deepening of the CCD and enhanced carbonate preservation (Berger, 1970; Peterson et al., 1992).
The carbonate crash recorded in the Caribbean and major world oceans, however, seems to point to a common cause as indicated by the comparable nature and time overlap of its occurrences, a cause related to processes affecting the global carbonate and carbon budget. A determination of the role that carbonate dissolution, production, and dilution have played would help unravel the cause of the crash.
We present here an oxygen and carbon stable isotopic record and its correlation with carbonate mass accumulation rates (MARs) in sediments from Ocean Drilling Program (ODP) Site 1256 located in the eastern equatorial Pacific (Fig. F1). We then discuss the possible cause of the middle/late Miocene carbonate crash in light of these data.