Dolomite is a common feature in organic carbon–rich hemipelagic sediments deposited beneath upwelling zones. Dolomite was recovered during several Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) cruises in different upwelling systems around the world, such as the California margin (DSDP Leg 63), the Gulf of California (DSDP Leg 64), the Oman margin (ODP Leg 117), the Japan Sea (ODP Legs 127/128), and the Namibia margin (ODP Leg 175). This is consistent with observations made in outcrops, such as the Miocene Monterey Formation (California) (e.g., Murata et al., 1969). Because dolomite is usually associated with high organic matter content, an organic dolomite model was proposed (Pisciotto and Mahoney, 1981; Kelts and McKenzie, 1982; Baker and Burns, 1985). The observation that even low amounts of sulfate inhibit the formation of dolomite in high-temperature experiments led to the formulation of the sulfate inhibition model of dolomite formation (Baker and Kastner, 1981). Because pore water sulfate is efficiently removed at sites with increased bacterial sulfate reduction, dolomite can precipitate in association with organic carbon–rich sediments.
Nadson (1928) and Neher and Rohrer (1958) reported precipitation of dolomite in bacterial culture experiments. Vasconcelos and McKenzie (1997) studied a hypersaline coastal lagoon (Lagoa Vermelha, Brazil), a site of modern dolomite precipitation. They formulated a microbial dolomite model, where the presence of active living microbes is essential to overcome the kinetic barrier of dolomite precipitation. Using sulfate-reducing bacteria cultured from Lagoa Vermelha, Vasconcelos et al. (1995) and Warthmann et al. (2000) successfully precipitated dolomite in low-temperature experiments.
The applicability of the sulfate inhibition vs. microbial dolomite model for the formation of deep-sea dolomite remains unresolved, as methods to trace this process in natural environments are limited. A microbial origin may be indicated by crystal morphologies that are similar to round and dumbbell shape crystals produced in culture experiments. Such morphologies were described by Vasconcelos and McKenzie (1997) and van Lith et al. (2003) in sediments from Lagoa Vermelha. Also in this context, it is of interest if dolomite is precipitated as primary precipitate or if a dissolution step of the precursor carbonate is involved (cementation vs. replacement). Bernoulli and Gunzenhauser (2001) and Bernoulli et al. (2004) observed perfect replica structures of diatom frustules in a dolomitized Miocene diatomite layer (Gonfolite Lombarda Group, southern Switzerland) and in pelagic limestone and diatomite of the Romanche Fracture Zone (equatorial Atlantic). These studies showed that deep-sea dolomite is often a primary precipitate, which was formed at an early stage in uncompacted sediment. The petrographic relationships of dolomite with sedimentary particles or other diagenetic minerals (e.g., pyrite) provide information about the relative timing of the dolomite precipitation and, therefore, allow correlation with a particular biogeochemical environment.
In this study, we describe and investigate the occurrence, petrography, and mineralogy of dolomite sampled during Leg 201 in deep-sea sediments from the Peru margin. A comprehensive sampling strategy with good depth control of the dolomite occurrences enabled correlation with lithostratigraphic context, pore water chemistry, and microbiology. Leg 201 was the first deep-sea drilling leg dedicated to investigation of a deep subseafloor biosphere and was thus an ideal program to study the relation between dolomite formation and deep biosphere activity. The data set produced on board the JOIDES Resolution during Leg 201 provides a framework in which to discuss active dolomite precipitation on the Peru margin and to evaluate the environmental factors associated with this process.
The Peru margin is a classic site for deep-sea dolomite formation. In the Nazca Plate Project, dolomite was discovered during deep-sea dredging (Kulm et al., 1981b, 1984). These dredge samples provided a good overview of the lateral distribution and occurrence of dolomite. Thornburgh and Suess (1990) studied selected samples of dolostone in order to reconstruct the origin of the pore waters in different parts of the Peru margin.
During Leg 201, four sites (all of them redrilled sites of ODP Leg 112) were drilled on the Peru margin (Fig. F1). Sites 1228 (Leg 112 Site 680) and 1229 (Leg 112 Site 681) are located on the Peru shelf at 250 and 150 meters below sea level (mbsl), respectively. Site 1227 (Leg 112 Site 684) was drilled at 430 mbsl on the upper slope of the Peru margin, and Site 1230 (Leg 112 Site 685) was drilled on the lower slope of the Peru Trench (5086 mbsl).
The oceanography of the Peru margin is strongly influenced by southeasterly tradewinds, which cause strong upwelling along the coast of Peru. High productivity leads to an oxygen minimum zone between 150 and 400 mbsl (Suess, von Huene, et al., 1988). Glacial–interglacial sea level variations highly affected sedimentation at the shelf sites. During sea level lowstands, increased rainfall on land delivered higher amounts of siliciclastic material to the basin, upwelling cells migrate seaward, and the oxygen minimum zone impinged farther distal on the seafloor (Suess, von Huene, et al., 1988). Glacial–interglacial cycles are well expressed as variations in sediment composition, total organic carbon (TOC) (Wefer et al., 1990; Meister et al., this volume), and color reflectance (Sites 1228 and 1229) (D'Hondt, Jørgensen, Miller, et al., 2003). Large-scale variations in sediment composition (diatom ooze vs. siliciclastic sediment) correlate with paleobathymetry, based on benthic foraminifers (Resig, 1990), are nonperiodic, and are probably related to tectonic activity. More continuously TOC-rich sediments are present at the lower slope (Site 1230) (Meister et al., this volume).
The tectonic history of the Peru margin was mainly interpreted from the seismic lines acquired during the Nazca Plate Project (Kulm et al., 1981a; Suess, von Huene, et al., 1988). A major hiatus indicates erosion and uplift in the middle Miocene. During this time, hypersaline brine formed under evaporative conditions. Since the late Miocene, the Peru margin has strongly subsided, forming basins and ranges in a forearc basin–type tectonic regime. On the topographic highs, sediment by-pass (nondeposition) or erosion occurred, facilitating the recovery of dolomite samples of Miocene age with dredging (Kulm et al., 1981b, 1984), whereas the basins, such as the Salaverry, Lima, and Trujillo Basins, were infilled by continuous sedimentation (Fig. F1). Today, these ancient basins form the shelf and upper slope of the Peru margin. The sediments recovered during Leg 201 on the shelf and upper slope are younger than the middle Miocene hiatus. Paleobathymetry reconstructed by Resig (1990) indicates lower neritic to upper bathyal conditions throughout the Pliocene to Holocene. Therefore, the recovered dolomite is not related to the middle Miocene hypersaline evaporative environment. Tectonic activity at the lower slope is dominated by backthrusting along the accretionary prism, where fluids can be transported upward along the fault zones.
High-resolution pore water chemistry profiles produced during Leg 201 (D'Hondt, Jørgensen, Miller, et al., 2003) indicate different microbial activity in the sediments of the different sites. At the shelf sites, strongly variable TOC contents appear to be related with penetration depths of sulfate to >30 meters below seafloor (mbsf) (Meister et al., this volume), whereas continuously high TOC of the trench site is reflected in removal of sulfate at 7 mbsf and the presence of gas hydrates. Evaporative brine, present at the shelf sites, delivers electron acceptors from beneath. Different microbial activities as well as cell concentrations (D'Hondt, Jørgensen, Miller, et al., 2003) are the result of these special biogeochemical conditions, but microbial activity also influences the pore water chemistry and, therefore, may control the formation of dolomite at the different sites.