Chemical and Sr isotopic compositions of 3 step leachates and residues of bulk samples, zeolites, and smectites from Lower Miocene sediments at ODP Site 166-1007

Massive clinoptilolite authigenesis was observed at about 1105 meters below sea floor (mbsf) in lower Miocene wellcompacted carbonate periplatform sediments from the Great Bahama Bank [Ocean Drilling Program, ODP Leg 166, Site 1007]. The diagenetic assemblage comprises abundant zeolite crystallized within foraminifer tests and sedimentary matrix, as well as Mg smectites. In carbonate-rich deposits, the formation of the zeolite requires a supply of silica. Thus, the objective of the study is to determine the origin of the silica supply, its diagenetic evolution, and consequently the related implications on interpretation of the sedimentary record, in terms of local or global paleoceanographic change. For lack of evidence for any volcaniclastic input or traces of Si-enriched deep fluids circulation, an in situ biogenic source of silica is validated by isotopic data and chemical modeling for the formation of such secondary minerals in shallow-water carbonate sequences. Geochemical and strontium isotopic data clearly establish the marine signature of the diagenetic zeolite, as well as its contemporaneous formation with the carbonate deposition (Sr model ages of 19.6-23.2 Ma). The test of saturation for the pore fluids specifies the equilibrium state of the present mineralogical assemblage. Seawater-rock modeling specifies that clinoptilolite precipitates from the dissolution of biogenic silica, which reacts with clay minerals. The amount of silica (opal-A) involved in the reaction has to be significant enough, at least 10 wt.%, to account for the observed content of clinoptilolite occurring at the most zeolite-rich level. Modeling also shows that the observed amount of clinoptilolite (~19%) reflects an in situ and short-term reaction due to the high reactivity of primary biogenic silica (opal-A) until its complete depletion. The episodic occurrence of these well-lithified zeolite-rich levels is consistent with the occurrence of seismic reflectors, particularly the P2 seismic sequence boundary located at 1115 mbsf depth and dated as 23.2 Ma. The age range of most zeolitic sedimentary levels (biostratigraphic ages of 21.5-22 Ma) correlates well with that of the early Miocene glaciation Mi-1 and Mi-1a global events. Thus, the clinoptilolite occurrence in the shallow carbonate platform environment far from volcanogenic supply, or in other sensitive marine areas, is potentially a significant new proxy for paleoproductivity and oceanic global events, such as the Miocene events, which are usually recognized in deep-sea pelagic sediments and high latitude deposits.

Composition of hardgrounds and of pore-water parameters in ODP Holes 166-1008 and 166-1009

Mineralogic, petrographic, and geochemical analyses of sediments recovered from two Leg 166 Ocean Drilling Program cores on the western slope of Great Bahama Bank (308 m and 437 m water depth) are used to characterize early marine diagenesis of these shallow-water, periplatform carbonates. The most pronounced diagenetic products are well-lithified intervals found almost exclusively in glacial lowstand deposits and interpreted to have formed at or near the seafloor (i.e., hardgrounds). Hardground cements are composed of high-Mg calcite (~14 mol% MgCO3), and exhibit textures typically associated with seafloor cementation. Geochemically, hardgrounds are characterized by increased d18O and Mg contents and decreased d13C, Sr, and Na contents relative to their less lithified counterparts. Despite being deposited in shallow waters that are supersaturated with the common carbonate minerals, it is clear that these sediments are also undergoing shallow subsurface diagenesis. Calculation of saturation states shows that pore waters become undersaturated with aragonite within the upper 10 m at both sites. Dissolution, and likely recrystallization, of metastable carbonates is manifested by increases in interstitial water Sr and Sr/Ca profiles with depth. We infer that the reduction in mineral saturation states and subsequent dissolution are being driven by the oxidation of organic matter in this Fe-poor carbonate system. Precipitation of burial diagenetic phases is indicated by the down-core appearance of dolomite and corresponding decrease in interstitial water Mg, and the presence of low-Mg calcite cements observed in scanning electron microscope photomicrographs.