Duncan, R. A., Backman, J., Peterson, L. C , et al., 1990 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 115 29. PLIOCENE-PLEISTOCENE ARAGONITE CYCLIC VARIATIONS IN HOLES 714A AND 716B (THE MALDIVES) COMPARED WITH HOLE 633A (THE BAHAMAS): RECORDS OF CLIMATE-INDUCED CaCO 3 PRESERVATION AT INTERMEDIATE WATER DEPTHS 1 Andre W. Droxler, 2 Geoffrey A. Haddad, 2 David A. Mucciarone, 2 and James L. Cullen 3 ABSTRACT Ocean Drilling Program (ODP) Leg 115 post-cruise research was focused on two Maldives sites, more precisely on the top 108 m of Hole 716B (water depth, 540 m), equivalent to the past 3.5 m.y., and the top 19.5 m of Hole 714A (water depth, 2195 m), equivalent to the past 0.55 m.y. These sediments consist of mostly unaltered and undisturbed, turbidite-free, periplatform ooze. Results of our research are compared with existing data on Hole 633A (water depth, 1681 m), drilled in the Bahamas during ODP Leg 101, using age/depth models built on the basis of oxygen isotope, nannofossil, and magnetic stratigraphies. Climate-induced, long-term (roughly 0.5 m.y.) aragonite cycles, superposed on short-term (roughly 0.04 and 0.1 m.y.) aragonite cycles, have been established at least during the past 2.0 m.y., in the Maldives and the Bahamas. Our most interesting result is the clear correlation among the aragonite long-term cycles in the Maldives and the Bahamas and the carbonate-preservation, long-term cycles from the open Pacific, Indian, and North Atlantic oceans. The mid- Brunhes dissolution interval, corresponding to the youngest preservation minima of the carbonate-preservation, long- term cycles, is clearly defined by fine aragonite minimum values in the deep periplatform sites, and by maximum frag- mentation of pteropod tests in the shallow sites. Aragonite and planktonic 5 18 O records, usually in phase during the late Pleistocene, display, further back in time, discreet intervals where the two records do not match with one another. Ma- jor mismatches between both records occur synchronously in the Maldives and Bahamas periplatform sites and seem to correspond to extreme events of either carbonate-preservation or dissolution in the deep pelagic carbonate sites of the equatorial Pacific Ocean. Based on our findings, short- and long-term aragonite cycles can no longer be explained only by variations of arago- nite input from the nearby shallow carbonate banks, in response to their alternate flooding and exposure through cyclic sea-level fluctuations. The aragonite long-term cycles in the periplatform environments are interpreted as carbonate- preservation cycles at intermediate-water depths. Their occurrence shows, therefore, that the carbonate chemistry of the entire water column has been influenced by long-term (0.5 m.y.) cyclic variations during the past 2.0 m.y. These major changes of the water-column carbonate chemistry are linked to the climate-induced carbon cycling among the different atmospheric, oceanic, and sedimentary carbon reservoirs. INTRODUCTION Deep-sea carbonate sediments play an important role in the global carbon cycle by storing or releasing large volumes of car- bon through time. Because of the chemical interactions at the seafloor between sediments and the overlying water masses, deep-sea carbonate sediments have also recorded, at least parti- ally, the carbonate chemistry variations of different water masses. In addition, since the general deep-ocean circulation is modu- lated and intimately linked to global fluctuations of Quaternary climate, deep-sea sediments contain unique records of the inter- actions between the atmosphere and the ocean. Below the calcite lysocline (3500-4000 m), deep-sea sedi- ments have recorded cyclic changes of the ocean carbonate chemistry over the Milankovitch orbital time scale (10 4 -10 5 yr) in deep- and bottom-water masses. Above the calcite lysocline, in intermediate-water depths, little or no dissolution is generally observed in planktonic calcitic sediments, which consist mainly of coccolith and foraminifer tests. Aragonite, a less stable car- bonate phase and therefore more sensitive to minor chemical 1 Duncan, R. A., Backman, J., Peterson, L. C , et al., 1990. Proc. ODP, Sci. Results, 115: College Station, TX (Ocean Drilling Program). 2 Department of Geology and Geophysics, Rice University, P.O. Box 1892, Houston, TX 77251, U.S.A. 3 Department of Geological Sciences, Salem State College, Salem, MA 01970, U.S.A. changes than calcite, has been used to study the carbonate chemistry variations at intermediate-water depths. Previous stud- ies have used the occurrence and depth distribution of ptero- pods (planktonic micro-mollusks with aragonitic tests) to define the aragonite lysocline and aragonite compensation depth (ACD) at intermediate-water depths (Chen, 1964; Herman, 1971; Berner, 1977, 1981; Berger, 1978; Berner et al., 1976; Droxler et al., 1988a). Chen (1968) and Herman (1971) are among a few au- thors who have related the downcore pteropod distribution to late Pleistocene paleoclimatic fluctuations. To our knowledge, however, no studies based on the downcore occurrence and dis- tribution of pteropod tests have demonstrated cyclic changes of the ocean carbonate chemistry within Milankovitch frequencies (10 th - 100 th k.y.) in oceanic intermediate-water masses. These cy- cles would be analogous to the cyclic carbonate variations well established in the three major oceans that have mainly recorded carbonate chemistry variations of deep- and bottom-water masses. The late Pleistocene aragonite cycles observed in the peri- platform ooze, deposited in the deep surroundings of the car- bonate Bahamas Banks, are well established and have been the subject of several studies (Supko, 1963; Kier and Pilkey, 1971; Lynts et al., 1973; Droxler et al., 1983; Boardman et al., 1986; Haddad, 1986; Burns and Neumann, 1987; Slowey et al., 1989). The detailed correlation between the aragonite cycles and the oxygen isotope planktonic record (Droxler et al., 1983; Board- man et al., 1986) shows the clear tie between the aragonite cy- cles and the late Pleistocene climatic fluctuations. The origin of 539