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doi:10.2204/iodp.proc.307.207.2009

Results

Petrographic and X-ray diffraction analyses

Carbonate composition of the mound sediments was characterized using X-ray diffraction (Fig. F3) and transmitted light microscopy (Fig. F4) (see XRD_DATA in “Supplementary material”). Smear slide descriptions (made by J.M. Gregg and other observers on board the JOIDES Resolution during Expedition 307) are available in “Core descriptions.” Smear slides descriptions made by Gregg from Hole U1317E are available as .RTF files (see SMEARSLD in “Supplementary material”). XRD analyses are available as tab delineated .TXT files (see XRD_DATA in “Supplementary material”).

Carbonate components of the mound sediments include low-Mg calcite nannoplankton (primarily coccoliths), aragonite skeletal (primarily coldwater coral) debris, and minor high-Mg calcite skeletal debris (foraminifers). Dolomite is petrographically characterized by micrometer- to decimicron-sized limpid, rhombic crystals (Fig. F4A, F4B, F4D). Noncarbonate components of the mound sediments mainly include continent-derived detrital quartz and feldspar, clay minerals, and biogenic sponge spicules. Mound sediments typically contain >50% bulk carbonate; however, occasional meter-scale intervals were observed to be dominantly siliciclastic. Off-mound and submound sediments are similar in composition to mound sediments but are more dominantly composed of detrital siliciclastics, although frequent meter-scale intervals are dominantly carbonate in composition.

The ratio of dolomite to calcite in the carbonate component of the sediments was estimated by comparing the dolomite and calcite 104 line intensities. Aragonite content of most of the samples that were X-rayed is minor and is not considered in this estimate. Low-Mg calcite (coccoliths) is the dominant carbonate phase observed in most samples analyzed. Dolomite commonly is present in cored sediments from trace amounts (<10% of the carbonate based on XRD 104 line intensities) to >50% of the total carbonate component (Fig. F5).

Dolomite and calcite stoichiometry (percentage magnesium composition) was calculated using the shift of their respective 104 lines from ideal stoichiometric composition (Lumsden and Chimahusky, 1980). Trace dolomite observed in the uppermost part of the Pliocene–Pleistocene strata displays near ideal stoichiometry, typical of Paleozoic dolomites, indicating a likely detrital origin and possible derivation from Paleozoic sediments in nearby Ireland. Lower in the section dolomite is calcium rich, which is typical of most Neogene dolomites (Budd, 1997). Corrections were not made for potential FeCO₃ content in calcite or dolomite, so it must be assumed that part of the 104 line shift is due to Fe composition.

The 015 dolomite ordering line (Goldsmith and Graf, 1958) is present but slightly attenuated in all cases where dolomite was abundant enough in the sample for its observation. This attenuation indicates that the samples are mineralogically dolomite (not high-Mg calcite or “protodolomite”) but display cation disorder. Cation disordering is also typical of Neogene dolomites (e.g., Gregg et al., 1992; Budd, 1997).

Distribution of dolomite

X-ray diffraction analysis of >400 samples from cores taken at Expedition 307 sites indicates that significant dolomitization (in some places >50% of the total carbonate component of the sediment) has occurred or is occurring throughout the Miocene section with lesser amounts of dolomite within the Pliocene–Pleistocene sections in the off-mound areas (Fig. F5). Probable trace detrital dolomite was observed in the upper part of the mound. Trace authigenic dolomite was observed in the lower portion of the mud mound facies. Observation of dolomite in several lithified intervals of the Miocene section lead to the initial hypothesis that dolomitization was a factor in diagenetic lithification in these sediments. However, observation of lithified sections containing only trace and minor amounts of dolomite indicate that the two diagenetic processes likely are not related. Indeed, sections where dolomite exceeds 50% of the total carbonate minerals present typically are unlithified.

The distribution of partially dolomitized intervals shown on Figure F5 is likely biased by the random nature of sampling for petrographic and X-ray analysis during the initial core descriptions. It is very probable that a systematic sampling of the cores would result in identification of more dolomitized intervals.

SEM and electron probe analyses

SEM and electron probe analysis of selected dolomitized samples from Sites U1317 and U1318 confirm that dolomite crystals are well-formed rhombs that appear to be replacing adjacent calcite, which appears to be undergoing dissolution (Fig. F6A, F6B, F6C). Backscattered electron imagery of sectioned dolomite crystals display distinct compositional zoning with respect to iron (Fig. F6D). Based on wavelength dispersive spectrometer analysis, it is estimated that dolomite composition ranges between <1 to >5 mol% FeCO₃. Dolomite does not display cathodoluminescence (CL) under an electron beam, which is typically the case of ferroan dolomite, as iron quenches the CL response (Machel, 1985).

Of considerable interest is the observation of bacteria and associated filamentous organisms in several unconsolidated, partly dolomitized sediment samples (Fig. F7). These filaments have the characteristics of nannowires, which are electrically conductive structures that recently have been described in terrestrial sediments and have been cultured in the laboratory (Ntarlagiannis et al., 2006; E. Atekwana and Y. Gorby, pers. comm., 2007; Gramling, 2006). Possible relationships of these structures to dolomitization or other diagenetic processes ongoing in these sediments are a mater of speculation.

Geochemical analysis of interstitial waters

Figure F8 displays pore water saturation indexes for calcite, aragonite, and dolomite in core from Holes U1317A and U1317D through Challenger Mound and into the underlying Miocene sediments. According to these data, dolomite is the most stable of the three carbonate minerals in the Neogene section. Pore waters in the mud mound facies range from undersaturated to supersaturated with respect to dolomite. All pore waters analyzed in the submound section are supersaturated with respect to dolomite (Fig. F8). Dolomitization is especially predicted to occur above 25 meters below seafloor (mbsf) (Hole U1317A) and below ~125 mbsf (Hole U1317B). These downcore geochemical trends are consistent with the observed occurrences of dolomite (Fig. F5) at Site U1317.

Similarly, incomplete analysis of water data predicts the distribution of dolomite as follows: at Site U1316 below 50 mbsf and at Site U1318 below ~150 mbsf. These predicted distributions of dolomite match well with petrographic observations and XRD results throughout the study area (Fig. F8).

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