The final Miocene carbonate crash in the Atlantic: Assessing carbonate accumulation, preservation and production
Introduction
The carbonate crash events (CC events) were first recognised as distinct drops in carbonate content in Miocene sediments of the South Atlantic (DSDP Leg 3 — Shipboard Scientific Party). Subsequently, more cores in the time period from 12 to 9 Ma were retrieved that frequently showed carbonate minima. The most distinct carbonate crashes are recorded in sites located at both sides of the Panamanian Isthmus, which is the Equatorial East Pacific (ODP Leg 138) and the Caribbean (ODP Leg 165). Here carbonate accumulation dropped to zero several times between 12 and 9 Ma (e.g. Lyle et al., 1995, Roth et al., 2000). A comprehensive overview of previous studies dealing with CC events is given in Table 1. A rise in CCD level of 800 m occurred at about 10 Ma and the CCD remained shallow east of the East Pacific Rise (EPR), while carbonate accumulation recovered at the western sites (Lyle et al., 1995). A variety of explanations were proposed: (1) Carbonate dissolution occurred as a response to the constriction of the CAS, which resulted in a significant reduction in the flow of carbonate saturated water entering the Pacific (Lyle et al., 1995, Table 1). (2) The productivity based hypothesis relates the CC-events in the Pacific to modern-like alternating El-Nino/La-Nina stages that are evidenced by consistent shifts in opal and carbonate production (Jiang et al., 2007, Nathan and Leckie, 2009; Table 1). This involves establishment of a Proto-West Pacific Warm Pool during La Nina like conditions. Due to high production of opal and carbonate, carbonate mass accumulation in the EEP at 11.6 to 10 Ma remained comparably high despite bottom waters may have been undersaturated (Nathan and Leckie, 2009). (3) A specific observation on the CC events in the Caribbean is, that in addition to the main carbonate minima between 12 and 9 Ma precursor events are recognised between 13.8 Ma to 12 Ma (Roth et al., 2000; Table 1). These events are ascribed to the influx of carbonate corrosive AAIW (or its precursor) replacing sinking waters in the northern hemisphere in times of re-established NCW formation (Roth et al., 2000). (4) Another hypothesis postulates the influx of corrosive Pacific intermediate waters triggering dissolution in the Caribbean (Newkirk and Martin, 2009; see Table 1). (5) Carbonate preservation and accumulation pattern in the SE Atlantic are partly explained by shifts in coccolith production rates in the Benguela upwelling system in the middle to late Miocene. Furthermore, local influences of terrigenous material from the Orange River caused lowered carbonate contents (Diester-Haass et al., 2004, Kastanja et al., 2006, Krammer et al., 2006; Table 1). (6) Several CC events are also recognised in the Ceará Rise sediments where a long-term shoaling of the lysocline took place from 14 Ma to 11.5 Ma (King et al., 1997).
Taking all evidences together, the middle to late Miocene ocean seems to be influenced by a larger re-organisation of the ocean circulation and chemistry, which might be associated with regional to global climate changes. However, the processes behind have still to be disentangled, and with the given study we try to contribute to this effort studying a transect of drill sites of the Ceará Rise, located in the western Equatorial Atlantic within the deep water masses of the global ocean conveyor.
Sediments from the Ceará Rise depth transect (ODP Sites 926, 927 and 928) were investigated in the time period from 10.5 Ma to 9.0 Ma coinciding with the occurrence of CC events in the Miocene ocean. Carbonate silt preservation proxies will be used to record preservation changes during the final phase of the CC events in order to distinguish between carbonate production, dissolution, and dilution at Ceará Rise.
Section snippets
Study area
Ceará Rise is an aseismic ridge located 700 km to the north-east of the Amazon River delta, ranging between 2600 and 4500 m water depth within the transition from North Atlantic Deep water (NADW) to Antarctic Bottom water (AABW) today. The chemical lysocline for calcite is located at a water depth of 4500–4600 m, and the foraminiferal lysocline is centred at 4400 m (Curry and Cullen, 1997; Fig. 1B). Since the sites are located in the oligotrophic subtropical West Atlantic gyre (Fig. 1A), their
Methods and materials
We sampled ODP Sites 927 (5°28′N, 44°29′W, 3314 m water depth), 926 (3°43′N, 42°54′W, 3533 m water depth) and 928 (5°27′N, 43°45′W, 4011 m water depth) in the interval from 10.5 to 9 Ma. Samples of 10 cm3 volume were taken every 10 ky according to the orbitally tuned age model (Shackleton and Crowhurst, 1997, Backman and Crowhurst personal communication). Sediments consist of nannofossil oozes with foraminifers and variable contributions of clay. Careful inspection of the sedimentary sections during
Carbonate and coarse fraction content
Maximum carbonate contents are in the order of 80% and similar at all sites (see Fig. 2). Site 927 shows rather stable carbonate contents between 10.4 and 10.1 Ma accompanied by a decreasing coarse fraction content. Minima in carbonate content (i.e. carbonate content 40–65%) occurred at Site 926 at 10.1 Ma, 9.9 Ma and 9.6 Ma. After 9.9 Ma the carbonate content of Site 928 is approaching that of Site 926 until it reaches the same level at 9.7–9.6 Ma. Coarse fraction contents are in the order of 5–30%
Controlling mechanisms on carbonate sedimentation
During the investigated period (10.5 to 9.0 Ma), three intervals of lowered carbonate accumulation (marked by grey shading in Fig. 2) are assigned to the final phase of the carbonate crash. The first two intervals, i.e. 10.5 Ma to 10.0 Ma, and around 9.9 Ma, reveal strongest dissolution as indicated by the preservation proxies, whereas in the last event at 9.6 Ma according to both, WTF and coarse fraction, dissolution is weaker (Fig. 2). However all events are accompanied by a minimum in MARcalc.
Conclusions
The investigation of preservation proxies, mass accumulation rates and calcareous grain sizes from the investigated Atlantic sites and comparison to Site 999 in the Caribbean led to the following conclusions:
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Models from previous studies concentrate on dissolution as control due to changing deep water circulation patterns. The CC event at and before 9.9 Ma appear to be predominantly controlled by carbonate dissolution. However, this study has shown that variation of carbonate production in
Acknowledgement
We thank H. Heilmann and B. Kockisch, for laboratory assistance and technical support. We also thank R. Roters, K. Stolz and G. de Lange as well as two anonymous reviewers for thorough reviews and suggestions. This research used samples provided by the Ocean Drilling Program (ODP). The ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI). This study was funded by the Deutsche Forschungsgemeinschaft
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