The final Miocene carbonate crash in the Atlantic: Assessing carbonate accumulation, preservation and production

Highlights

• We examined carbonate preservation at the Ceará Rise during Carbonate Crash events.

• Carbonate preservation differences are best portrayed by foraminiferal fragmentation.

• Carbonate accumulation and preservation at Ceará Rise and the Caribbean were in phase.

Abstract

In many palaeoceanographic studies low carbonate contents in Neogene deep-sea sediments are interpreted as dissolution events related to prominent shifts in intermediate and deep-water circulation. In the middle to late Miocene such carbonate crash events were a widespread phenomenon throughout the world's ocean basins. However, low carbonate contents may also derive from terrigenous dilution and carbonate production changes in surface waters as shown for late Miocene records of the South Atlantic (Diester-Haass et al., 2004, Westerhold et al., 2005). In order to contribute to solve this question we investigated the youngest of the Miocene crash events (10.5 Ma to 9.0 Ma) at a prominent key location of global deep water circulation, i.e. the Equatorial Atlantic Ceará Rise depth transect (ODP Sites 926, 927 and 928). By analysing different carbonate preservation proxies like carbonate content, silt grain size parameters, sand content, fragmentation index and mass accumulation rates we were able to quantify dissolution, dilution and production, and to assess previous conflicting interpretations. Critical evaluation of the different dissolution proxies and comparison to former studies revealed that silt preservation proxies do not entirely reproduce the depth dependant dissolution in the Miocene sediments, as indicated by foraminifer fragmentation. In the investigated late Miocene time slice three minima of carbonate accumulation were detected. The predominant control on these events changed from dissolution of carbonate to lowered production. The last event at 9.6 Ma is accompanied by an increased preservation but reduced carbonate accumulation. During this time the first prominent pulses of Northern Component Water (NCW) occurred, sourced most probably from the Labrador Sea. This may be understood as a first short term precursor of long time persistent and much stronger NCW production that is characteristic for younger periods and marks the turnover to the modern type of basin-to-basin fractionation between the Atlantic and Pacific. Furthermore, preservation trends of Caribbean and Ceará Rise records were in phase in contrast to former hypotheses that suggested antithetical preservation during carbonate crash events. The similarity of the preservation records in the studied ocean basins in conjunction with the inconsistency in timing of the CC-events and proposed NCW estimates indicates that changes in the carbon cycle other than dissolution controlled the CC events prior to ~ 9.9 Ma.

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.